SL IB Biology Water PDF

Head to www.savemyexams.com for more awesome resources SL IB Biology Your notes Water Contents Hydrogen Bonds Physical & Chemical Properties of Water...

Head to www.savemyexams.com for more awesome resources SL IB Biology Your notes Water Contents Hydrogen Bonds Physical & Chemical Properties of Water Page 1 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Hydrogen Bonds Your notes Medium for Life Water as the medium for life The rst cells evolved in a watery environment This is believed to have been in the deep oceans, close to hydrothermal vents in the Earth's crust Some water and solutes got trapped within a membrane Chemical reactions began occurring within the membrane-bound structure This led to the evolution of cells Water in its liquid state allows dissolved molecules to move around, so they are easily able to collide and react with each other Most life processes occur in water The link between water and life is so strong that scientists looking for life on other planets and moons look for evidence of water to suggest that life could have occurred there Page 2 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Hydrogen Bonds Hydrogen bonding plays an important role between many biological molecules Your notes Some key functions include: Dissolving of solutes in water The cohesion and adhesion of water molecules These properties allow water to move up the trunks of really tall trees Base-pairing between the two strands of DNA Structure: Hydrogen bonds help to form part of the secondary and tertiary levels of structure in proteins The hydrogen bonds found between strands of cellulose and collagen give those molecules their tensile strength Interactions between mRNA and tRNA during protein synthesis Surface e ects on membranes between polar phosphate groups and water Hydrogen bonding in water Hydrogen bonding is a fundamental property of water Water is of the utmost biological importance It is the medium in which all metabolic reactions take place in cells Between 70% to 95% of the mass of a cell is water Water is so fundamental to life that astronomers look for signs of water on other planets and moons, as indicators of possible extra-terrestrial life As 71% of the Earth’s surface is covered in water it is a major habitat for organisms Water is composed of atoms of hydrogen and oxygen One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding) Although water as a whole is electrically neutral, the sharing of the electrons is uneven between the oxygen and hydrogen atoms The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a weak negatively charged region on the oxygen atom (δ-) and a weak positively charged region on the hydrogen atoms(δ+), this also results in the molecule's asymmetrical shape This separation of charge due to the electrons in the covalent bonds being unevenly shared is called a dipole When a molecule has one end that is negatively charged and one end that is positively charged it is also a polar molecule Water is therefore a polar molecule Hydrogen bonds in a water molecule diagram Page 3 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The covalent bonds of water make it a polar molecule Hydrogen bonds form between water molecules As a result of the polarity of water, hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming However, when there are large numbers present they form a strong structure Hydrogen bonds cause many of the properties of water molecules that make them so important to living organisms. Hydrogen bonds between water molecules diagram Page 4 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The polarity of water molecules allows hydrogen bonds to form between adjacent water molecules Examiner Tip Familiarise yourself with the formation of hydrogen bonds between two or more water molecules. The delta symbol (δ) indicates that the charge is very small, so the slightly negative (δ-) side of one water molecule will always be attracted to the slightly positive (δ+) side of another water molecule. Page 5 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Physical & Chemical Properties of Water Your notes Cohesion Hydrogen bonds within water molecules allows for strong cohesion between water molecules Allowing columns of water to move under tension (called mass transport) through the xylem of plants Enabling surface tension where a body of water meets the air, these hydrogen bonds occur between the top layer of water molecules to create a sort of lm on the body of water This layer is what allows insects such as pond skaters to move across the surface of water Page 6 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Adhesion Water is also able to bond via hydrogen atoms to other molecules which are polar or charged, such as Your notes cellulose, which is known as adhesion This also enables water to move up the xylem during transpiration Water is drawn up narrow channels in soil, called capillary tubes, by means of capillary action Spaces between cellulose bres in plant cell walls can also draw water from xylem vessels by capillary action and allow water to ow through plant tissue Cohesion and adhesion in xylem diagram Hydrogen bonding results in cohesion and adhesion forces in xylem which allows water molecules to ow through the plant in a continuous stream Examiner Tip COhesion = water particles sticking to each other. ADhesion = water particles sticking to other materials Page 7 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Water as a Solvent Biological molecules can be hydrophilic or hydrophobic (and sometimes both) Your notes Hydrophilic = "water-loving" Hydrophobic = "water-hating" Polar molecules and molecules with positive or negative charges can form hydrogen bonds with water (and dissolve) so are generally hydrophilic Non-polar molecules with no positive or negative charge, cannot form hydrogen bonds with water so are generally hydrophobic These molecules tend to join together in groups due to hydrophobic interactions where hydrogen bonds form between water particles but not with the non-polar molecule Because most biological molecules are hydrophilic and can be dissolved, water is regarded as the universal solvent Water as a solvent diagram Page 8 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Due to its polarity water is considered a universal solvent Solvent properties of water Your notes Di erent solutes behave di erently with water as a solvent Even though water is a universal solvent, di erent metabolites have di erent solubilities in water Di erent solutes have di erent hydrophobic and hydrophilic properties which a ect their solubility in water Highly soluble molecules Some molecules are highly soluble (e.g. sodium chloride, urea) and some are insoluble (e.g. fats) Highly soluble molecules can be easily transported in solution within organisms e.g. salts, glucose, amino acids Even the amino acids with hydrophobic R groups are soluble enough to be freely transported in water Di erent transport mechanisms have evolved to assist in the transportation of the less soluble molecules Insoluble molecules Non-polar, hydrophobic molecules cannot dissolve in water The function of certain molecules in cells depend on them being hydrophobic and insoluble e.g. phospholipids have hydrophobic hydrocarbon tails which forms the hydrophobic core of cell membranes Less soluble molecules A low solubility molecule such as oxygen requires assistance through combining with haemoglobin, to allow more oxygen to be carried than directly in blood plasma Oxygen is less soluble at body temperature (37ºC) than at 20ºC Oxygen is sparingly soluble but soluble enough to allow it to dissolve in oceans, rivers and lakes for aquatic animals to breathe Haemoglobin can bind oxygen to allow su cient oxygen to be transported to all body cells Enzyme action in water Most enzymes require water in order to hold its shape and improve its stability This enables them to catalyse reactions in aqueous solutions Hydrogen bonds will often facilitate the binding of the enzyme active site and its substrate molecule This forms an enzyme substrate complex Page 9 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Physical Properties of Water Speci c heat capacity Your notes Speci c heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1oC Water has a higher speci c heat capacity (4200 J/kg/oC) compared to air (1000 J/kg/°C), meaning a relatively large amount of energy is required to raise its temperature The high speci c heat capacity is due to the many hydrogen bonds present in water It takes a lot of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of water does not uctuate greatly The advantage for living organisms is that it: Provides suitable, stable aquatic habitats since water temperatures will change more slowly than air temperatures Is able to maintain a constant temperature as water is able to absorb a lot of heat without wide temperature uctuations This is vital in maintaining temperatures that are optimal for enzyme activity Artic and sub-artic species, such as the ringed seal (Pusa hispida) are able to survive throughout the year due to stable sea temperatures The density of ice is lower than the density of liquid water, which means that ice oats on water This forms a habitat for the seals both on the oating ice sheets, as well as below the ice Page 10 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources By NOAA Seal Survey, Public domain, Wikimedia A ringed seal (Pusa hispida) in its native habitat Your notes Thermal conductivity Thermal conductivity refers to the ability of a substance to conduct heat The thermal conductivity of water is almost 30 times higher than that of air, which makes air a very good insulator for organisms living in colder climates The black-throated loon (Gavia arctica) is a species of diving bird which spends much time underwater catching its prey Their feathers trap an insulating layer of air, which assists them with regulating their body temperature By Robert Bergman, Public domain, Wikimedia The black-throated loon (Gavia arctica) The seal on the other hand, relies on a layer of fat called blubber to insulate it from the outside air Ice in its environment will also form an insulating layer above the water, since the thermal conductivity of ice is much lower than liquid water This increases the sea temperature below the ice as thermal energy is trapped Buoyancy Page 11 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Buoyancy refers to the ability of an object to oat in water To overcome the problem of buoyancy, the black-throated loon has solid bones, unlike the hollow bones that most bird species have to assist them with ight Your notes This increases the weight of the bird and compresses air out of the lungs and feathers during a dive For the ringed seal, the layer of blubber under its skin will improve the buoyancy of the animal, along with providing a layer of insulation against the cold temperatures of its habitat Viscosity Viscosity refers to the resistance of a uid to ow The viscosity of water is much higher than air, which enables the black-throated loon to y through the air without much friction The body shapes of both the loon and seal makes it easy for them to move through water Both organisms are adapted in their own way for movement through water: The seal has ippers to propel itself The loon uses its webbed feet to push against the water and the lateral location of its feet reduces drag as it moves through water Examiner Tip You may use either the common name or scienti c name for these organisms in an exam Page 12 of 12 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources SL IB Biology Your notes Nucleic Acids Contents DNA & RNA Structure Basis of Genetic Code Nucleic Acid Structure & Function Page 1 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources DNA & RNA Structure Your notes Genetic Material of Life DNA as the genetic material of living organisms Deoxyribose nucleic acid (DNA) carries the genetic code in all living organisms This is the reason why the genetic code is said to be universal, it applies to all forms of life DNA is mainly found in the nucleus where it forms chromosomes It is also found in chloroplasts and mitochondria of eukaryotic cells Ribonucleic acid (RNA) is another type of nucleic acid which is the main component of ribosomes, which play an important role in protein synthesis Some RNA is also found in the nucleus and cytoplasm Certain viruses (such as SARS-CoV-2) contain RNA as their genetic material instead of DNA These viruses cause a variety of di erent diseases, such as COVID-19, Ebola, mumps and in uenza Viruses are not considered to be living organisms, since they are unable to replicate by themselves They are dependent on other living cells for replication and survival Viruses also lack a cellular structure, which is another reason they are not considered to be living Page 2 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Nucleotide Components Components of a nucleotide Your notes Both DNA and RNA are polymers that are made up of many repeating units called nucleotides Each nucleotide is formed from: A pentose sugar (a sugar with 5 carbon atoms) A nitrogen-containing organic base (with either 1 or 2 rings of atoms) A phosphate group (this is acidic and negatively charged) The base and phosphate group are both covalently bonded to the sugar The nitrogenous bases in DNA are: Adenine (A) Guanine (G) Cytosine (C) Thymine (T) RNA share the same nitrogenous bases as DNA except thymine, which is replaced by uracil (U) in RNA The nitrogenous bases can be grouped as either purine or pyrimidine bases: Adenine and guanine are purine bases Cytosine, thymine (in DNA) and uracil (in RNA) are pyrimidine bases Nucleotide structure diagram The basic structure of a nucleotide Drawing simple diagrams of the structure of single nucleotides of DNA and RNA Simple shapes can be used to draw the main building blocks of nucleotides and the DNA double helix Advanced drawing skills are not required! Pentagons can represent pentose sugars Circles can represent phosphates Often shown as a circle with the letter P inside: ℗ Page 3 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Rectangles can represent bases Covalent bonds can be shown with solid lines Hydrogen bonds can be shown with dashed lines Your notes Or with complementary shapes that t together (see diagrams) Components of a nucleotide diagram Simple shapes can be used to represent parts of nucleotide molecules Page 4 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Linking Nucleotides Forming the sugar-phosphate backbone Your notes Nucleotides join together in chains to form DNA or RNA strands The phosphate group of one nucleotide forms a covalent bond to the pentose sugar of the next one This carries on to form a large polymer These polymers form nucleic acids, which are also known as polynucleotides The phosphate group of one nucleotide is linked to the pentose sugar of the next one by condensation reactions This means a molecule of water is released during the formation of each covalent bond This forms a 'sugar-phosphate backbone' with a base linked to each sugar The polymer of nucleotides is known as a strand DNA is double-stranded, RNA is usually single-stranded There are just 4 separate bases that can be joined in any combination/sequence Because the sugar and phosphate are the same in every nucleotide Linking nucleotides together diagram Two nucleotides shown bonded together covalently within a strand Page 5 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources RNA Structure RNA structure Your notes Unlike DNA, RNA molecules are relatively short with lengths of between a hundred to a few thousand nucleotides It usually forms a single-stranded polynucleotide with ribose as the pentose sugar in each nucleotide RNA nucleotides contain the following nitrogenous bases: Adenine Guanine Cytosine Uracil (instead of thymine in DNA) The carbon atoms in nucleotides are numbered from the right in a clockwise direction This makes it easier to identify the bonds in the sugar-phosphate backbone of polynucleotides It also indicates the orientation of the polynucleotide RNA nucleotide diagram The structure of an RNA nucleotide Di erent types of RNA are found in the cells of living organisms: messenger RNA (mRNA), which is formed in the nucleus and transported to the ribosomes in the cytoplasm transfer RNA (tRNA), which is responsible for transporting amino acids to ribosomes during protein synthesis ribosomal RNA (rRNA), which forms part of ribosomes Page 6 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Adjacent RNA nucleotides are linked together by condensation reactions, during which a molecule of water is released This forms a phosphodiester bond between the pentose sugar of one nucleotide and the phosphate Your notes group of the next nucleotide The formation of an RNA polymer diagram Linking RNA nucleotides together by condensation reactions will result in the formation of phosphodiester bonds Examiner Tip Ensure that you are able to draw and recognise diagrams of a single RNA nucleotide, as well as RNA polymers Page 7 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources DNA Structure DNA structure Your notes DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs The nucleic acid DNA is a polynucleotide – it is made up of many nucleotides bonded together in a long chain DNA nucleotide diagram A DNA nucleotide DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel Each DNA polynucleotide strand is made up of alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbone Each DNA polynucleotide strand is said to have a 3’ end and a 5’ end (these numbers relate to which carbon atom on the pentose sugar could be bonded with another nucleotide) Because the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule A single DNA polynucleotide strand diagram Page 8 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A single DNA polynucleotide strand showing 3 nucleotides in a sequence Hydrogen bonding The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases These hydrogen bonds always occur between the same pairs of bases: The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds are formed between these bases The purine guanine (G) always pairs with the pyrimidine cytosine (C) – three hydrogen bonds are formed between these bases This is known as complementary base pairing These pairs are known as DNA base pairs Page 9 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources DNA molecule with hydrogen bonding diagram Your notes A section of DNA – two antiparallel DNA polynucleotide strands held together by hydrogen bonds Double helix DNA is not two-dimensional as shown in the diagram above DNA is described as a double helix This refers to the three-dimensional shape that DNA molecules form DNA double helix formation diagram Page 10 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes DNA molecules form a three-dimensional structure known as a DNA double helix Page 11 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes Make sure you can name the di erent components of a DNA molecule (sugar-phosphate backbone, nucleotide, complementary base pairs, hydrogen bonds) and make sure you are able to locate these on a diagram. Remember that covalent bonds join the nucleotides in the sugar-phosphate backbone, and hydrogen bonds join the bases of the two complementary strands together. Remember that the bases are complementary, so the number of A = T and C = G. You could be asked to determine how many bases are present in a DNA molecule if given the number of just one of the bases. Drawing base-pairing in a DNA molecule When drawing the base pairing, the opposite strand should be antiparallel to the rst. The presence of hydrogen bonding is shown, but the numbers/lengths of bonds is not required Examiner Tip Simple, hand-drawn shapes will su ce in an exam. Expert tip - a large drawing is always easier for an examiner to read (and award marks for) than a small one! Read the question carefully; examiners often want a whole nucleotide to be identi ed in your diagram and to ensure your diagram includes all 4 complementary bases. You don't have to remember the number of hydrogen bonds between the bases. Also, remember to draw DNA strands as antiparallel (one upside-down versus the other) but you don't have to be able to draw a helix shape! Page 12 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Basis of Genetic Code Your notes The Genetic Code Genetic code DNA molecules carry the genetic code as a sequence of nitrogenous bases in the nucleotides These bases are adenine, guanine, cytosine and thymine One of the strands of a DNA molecule will carry the base sequence that will be read by enzymes This strand is known as the coding strand The sequence of bases that form genes on the coding strand will determine the order of amino acids in the proteins that are synthesised The code is read as a triplet of bases, called a codon, with each sequence of three bases coding for one amino acid Remember that there are 20 di erent amino acids that could be coded for The sequence of amino acids will determine the shape and function of the protein that is synthesised from the code From gene to protein diagram The sequence of DNA bases in the genes codes for the production of a speci c protein molecule Page 13 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Conservation of The Genetic Code The genetic code is universal Your notes The genetic code is universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions) The same triplet codes code for the same amino acids in all living things (meaning that genetic information is transferable between species) The universal nature of the genetic code is why genetic engineering is possible This provides evidence for a universal common ancestor from which all living organisms on Earth evolved Over time, mutations have led to changes in some of the base sequences of organisms These base sequences form the genome of an organism Some base sequences form part of regions that code for proteins, called coding sequences, while others are located in regions that do not code for proteins (non-coding sequences) Many of these coding and non-coding sequences have remained unchanged in all organisms and are known as conserved sequences Highly conserved sequences are usually found in the genes that code for proteins involved with transcription and translation, as well as histone proteins which help to package DNA tightly into the nucleus The similarity in these sequences indicate that living organisms share a universal ancestry Page 14 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Nucleic Acid Structure & Function Your notes DNA & RNA: Comparison Di erences between DNA and RNA Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (T) – in place of this they contain the nitrogenous base uracil (U) Unlike DNA, RNA nucleotides contain the pentose sugar ribose (instead of deoxyribose) Comparing DNA and RNA nucleotides diagram Page 15 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes An RNA nucleotide compared with a DNA nucleotide Unlike DNA, RNA molecules are only made up of one polynucleotide strand (they are single-stranded) Unlike DNA, RNA polynucleotide chains are relatively short compared to DNA RNA structure Page 16 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes mRNA as an example of the structure of an RNA molecule Nucleotide Structure Summary Table Properties DNA RNA Pentose sugar Deoxyribose Ribose Bases Adenine (A) Adenine (A) Cytosine (C) Cytosine (C) Guanine (G) Guanine (G) Thymine (T) Uracil (U) Page 17 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Double-stranded (double Your notes Number of strands Single-stranded helix) Examiner Tip You need to know the di erence between DNA and RNA molecules (base composition, number of strands, pentose sugar present). You also need to be able to sketch the di erence between ribose and deoxyribose. Page 18 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Complementary Base Pairing The role of complementary base pairing Your notes Complementary base pairing means that the DNA bases on di erent strands will always pair up in a very speci c way: Adenine (A) will pair up with Thymine (T) Cytosine (C) will pair up with Guanine (G) This is because the hydrogen bonds that hold the two DNA strands together can only form between these base pairs: Two hydrogen bonds form between A and T Three hydrogen bonds form between C and G Complementary base pairing means that the base sequence on one DNA strand determines the sequence of the other strand We say that one strand acts as a template of the other This allows DNA to be copied very precisely during DNA replication which in turn ensures that the genetic code is accurately copied and expressed in newly formed cells Complementary base pairs and hydrogen bonding diagram Page 19 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources A section of DNA showing nucleotide bonding and complementary base pair bonding Your notes DNA: Information Storage Molecule Diversity of DNA base sequences Despite the genetic code only containing four bases (A, T, C, G), they can combine to form a very diverse range of DNA base sequences in DNA molecules of di erent lengths This means that DNA has an almost limitless capacity for storing genetic information in living organisms One way in which this storage capacity can be measured is by the number of genes contained within the DNA of an organism Even the most simplistic forms of life may contain several thousand genes within their DNA Comparing the Number of Genes between Di erent Organisms Table Bacterium Organism Human Dog Water flea Rice plant (E. coli) Approximate number of 20 000 19 000 31 000 4 300 41 500 genes The storage capacity of DNA can also be measured in the number of base pairs contained within the genome of an organism The DNA in the nucleus of a human cell contains about 3.2 gigabases That is about 109 DNA base pairs These base pairs are contained in DNA with a length of about 2 meters, that ts within the nucleus of each human cell Given the fact that a nucleus is microscopic in size, is an indication of how incredibly well packaged this amount of genetic information is This gives DNA an enormous capacity for storing genetic 'data' with great economy Page 20 of 20 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources SL IB Biology Your notes Cell Structure Contents Cell Theory Cell Theory: Skills Microscopes General Cell Structure Prokaryotic Cell Structure Eukaryotic Cell Structure Functions of Life Eukaryotic Cell Structure: Comparisons & Atypical Examples Cell Types & Structures: Skills Drawing Cells: Skills Page 1 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cell Theory Your notes Cell Theory Cells are the basic structural unit of all living organisms Until microscopes became powerful enough to view individual cells, no-one knew for certain what living organisms were made from A scientist called Robert Hooke came up with the term "cells" in the 1660's after examining the structure of cork Matthias Schleiden and Theodor Schwann were two scientists who studied animal and plant cells In 1837, they came up with the idea that all living organisms are made of cells This idea is known as ‘cell theory’ The cell theory is a unifying concept in biology (meaning it is universally accepted) The cell theory includes three main ideas:. All living organisms are made up of one or more cells. Cells are the basic functional unit (i.e. the basic unit of structure and organisation) in living organisms. New cells are produced from pre-existing cells Although cells vary in size and shape they all Are surrounded by a membrane Contain genetic material Have chemical reactions occurring within the cell that are catalysed by enzymes NOS: Deductive reasoning can be used to generate predictions from theories. Based on cell theory, a newly discovered organism can be predicted to consist of one or more cells. Deductive reasoning is an approach where one progresses from general ideas to hypothesis testing to speci c conclusions This is in contrast with inductive reasoning where one starts with speci c observations and then develops theories Cytology, the branch of biology which focuses on cell theory, can be used to demonstrate deductive reasoning Cell theory states that all living organisms are made of at least one cell We can hypothesise that any newly discovered living organisms on Earth will also be made up of at least one cell We can observe living organisms to test this hypothesis Deductive reasoning ow diagram Page 2 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Deductive reasoning can be used to develop speci c hypothesis from existing theories Page 3 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cell Theory: Skills Your notes Skills in Microscopy Many biological structures are too small to be seen by the naked eye Optical (light) microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied For example, the movement of chromosomes during mitosis can be observed using a microscope How optical (light) microscopes work Light is directed through the thin layer of biological material that is supported on a glass slide This light is focused through several lenses so that an image is visible through the eyepiece The magnifying power of the microscope can be increased by rotating the higher power objective lens into place Apparatus The key components of an optical (light) microscope are: The eyepiece lens The objective lenses The stage The light source The coarse and ne focus Other tools that may be used: Forceps Scissors Scalpel Coverslip Slides Pipette A diagram of an optical microscope Page 4 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Image showing all the components of an optical (light) microscope Method Preparing a temporary mount slide using a liquid specimen: Add a few drops of the sample to the slide using a pipette Cover the liquid/smear with a coverslip and gently press down to remove air bubbles Wear gloves to ensure there is no cross-contamination of foreign cells Preparing a temporary mount slide using a solid specimen: Use scissors to cut a small sample of the tissue Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps) Some tissue samples need be treated with chemicals to kill / make the tissue rigid A stain may be required to make the structures visible depending on the type of tissue being examined Gently place a coverslip on top and press down to remove any air bubbles Take care when using sharp objects and wear gloves to prevent the stain from dying your skin Place the microscope slide on the stage, x in place using the stage clips (ensure the microscope is plugged in and on) When using an optical microscope always start with the low power objective lens: It is easier to nd what you are looking for in the eld of view This helps to prevent damage to the lens or coverslip incase the stage has been raised too high Page 5 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Whilst looking through the eyepiece lens move the coarse focusing knob until the specimen comes into focus. The ne focusing knob should be used to sharpen the focus on particular parts (and at higher objective lens only) Your notes To examine the whole slide, move it carefully with your hands (or if using a binocular microscope use the stage adjusting knobs) Once you have focused on the object/structure then carefully move to a higher objective lens (10X and 40X). If resistance is felt do not continue to move the turret At the higher objective powers only use the ne focusing knob Do not move the stage down when moving to higher objective lens Unclear or blurry images: Switch to the lower power objective lens and try using the coarse focus to get a clearer image Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly There could be cross-contamination with foreign cells or bodies Use a calibrated graticule to take measurements of cells A graticule is a small disc that has an engraved scale. It can be placed into the eyepiece of a microscope to act as a ruler in the eld of view As a graticule has no xed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer) By using the two scales together, the number of micrometers each graticule unit is worth can be worked out After this is known the graticule can be used as a ruler in the eld of view The measurements made using these microscope apparatus are a form of quantitative observations Diagram of an eyepiece graticule and stage micrometer Page 6 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The stage micrometer scale is used to nd out how many micrometers each graticule unit represents Magni cation calculations Magni cation is how many times bigger the image of a specimen observed is in comparison to the actual (real-life) size of the specimen The magni cation (M) of an object can be calculated if both the size of the image (I), and the actual size of the specimen (A), is known The magni cation equation triangle An equation triangle for calculating magni cation Page 7 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes An image of an animal cell is 30 mm in size and it has been magni ed by a factor of X 3000. What is the actual size of the cell? Answer: To nd the actual size of the cell: Using the appropriate units The size of cells is typically measured using the micrometer (μm) scale, with cellular structures measured in either micrometers (μm) or nanometers (nm) When doing calculations all measurements must be in the same units. It is best to use the smallest unit of measurement shown in the question To convert units, multiply or divide depending if the units are increasing or decreasing Magni cation does not have units Diagram to show conversion of units Page 8 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes There are 1000 nanometers (nm) in a micrometre (µm) There are 1000 micrometres (µm) in a millimetre (mm) There are 1000 millimetres (mm) in a metre (m) Producing a scale bar A scale bar is a straight line on the drawing or micrograph that represents the actual size before the image was enlarged It can be used to calculate magni cation from biological drawings and images To add a scale bar to a biological drawing of a microscope specimen:. Use the eyepiece graticule and stage micrometer to calculate the distance between two markings on the eyepiece graticule; this is the graticule unit. Remove the stage micrometer and add the specimen to the microscope stage. Measure the length of the specimen using the eyepiece graticule which will be in graticule units. Determine the length of the specimen in micrometers by multiplying the number of graticule units by the length of each unit (calculated in step 1) Your scale bar should represent 20% of the actual length of your specimen. If you specimen is 300µm then your scale bar would represent 60µm. Draw your specimen as directed and measure the length of your drawing in mm; your scale bar should be 20% of the length of your specimen drawing; if your drawing is 150mm then your scale bar should be 30mm long. Add the actual length your scale bar represents underneath your scale bar e.g. 60µm Using a scale bar If the calculation required includes a scale bar on the micrograph or drawing then follow these steps:. Use a ruler to measure the length of the scale bar in millimetres (mm). Convert this measurement into the same units as the number on the scale bar. Insert these numbers into the magni cation formula above (note: the size of the image is the measured length of the scale bar and the actual size is the number on the scale bar) Page 9 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes Calculate the magni cation of the transverse section of the leaf blade. Transverse section of the leaf blade Answer: Step 1: Use a ruler to measure the length of the scale bar in millimetres Using a ruler the length of the scale bar is equal to 20 mm Step 2: Convert this measurement into the same units as the number on the scale bar The units on the scale bar are µm, remember that 1mm = 1000 µm therefore 20 mm = 20 x 1000 = 20 000 µm Step 3: Insert these numbers into the magni cation formula Page 10 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Note: the size of the image is the measured length of the scale bar and the actual size is the number on the scale bar Your notes therefore Magni cation = x 100 Examiner Tip Before doing any calculations make sure that all the measurements have the same units. When doing the calculations it is easier to write the formula, then rearrange it, before you add any measurements, as this helps avoid any possible errors. Note that when you do calculations using a scale bar, the number on the scale bar is informing you how many mm/µm or nm the line actually represents (e.g. if the scale bar has 20 nm above it and the line is 10 mm, then every 10 mm on the diagram is actually 20 nm). NOS: Measurement using instruments is a form of quantitative observation Microscopy can give us accurate quantitative observations about cells Quantitative observations are a collection of data which are focused on numbers and values such as measurements of length, height, volume, or values of quantity and frequency Using instruments such as eyepiece graticules and stage micrometers allow us to take measurements on a small scale such as in micrometers (µm) and nanometers (nm) (using electron microscopes) Data can be collected about cell and organelle sizes Qualitative data is non-numerical data such as colour and presence of structures which can also be determined using microscopes Making observations and taking measurements form the basis for developing new hypotheses in Biology Page 11 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Microscopes Your notes Microscopy: Developments Microscopes can be used to analyse cell components and observe organelles Magni cation and resolution are two scienti c terms that are very important to understand and distinguish between when answering questions about microscopy (the use of microscopes): Magni cation tells you how many times bigger the image produced by the microscope is than the real-life object you are viewing Resolution is the ability to distinguish between objects that are close together (i.e. the ability to see two structures that are very close together as two separate structures) There are two main types of microscopes: Optical microscopes (sometimes known as light microscopes) Electron microscopes Optical (light) microscopes Optical microscopes use light to form an image This limits the resolution of optical microscopes Using light, it is impossible to resolve (distinguish between) two objects that are closer than half the wavelength of light The wavelength of visible light is between 500−650 nanometres (nm), so an optical microscope cannot be used to distinguish between objects closer than half of this value This means optical microscopes have a maximum resolution of around 0.2 micrometres (µm) or 200 nm Optical microscopes can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts They cannot be used to observe smaller organelles such as ribosomes, the endoplasmic reticulum or lysosomes The maximum useful magni cation of optical microscopes is about ×1500 Electron microscopes Electron microscopes use electrons to form an image This greatly increases the resolution of electron microscopes compared to optical microscopes, giving a more detailed image A beam of electrons has a much smaller wavelength than light, so an electron microscope can resolve (distinguish between) two objects that are extremely close together This means electron microscopes have a maximum resolution of around 0.0002 µm or 0.2 nm (i.e. around 1000 times greater than that of optical microscopes) This means electron microscopes can be used to observe small organelles such as ribosomes, the endoplasmic reticulum or lysosomes The maximum useful magni cation of electron microscopes is about ×1,500,000 Page 12 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources There are two types of electron microscopes: Transmission electron microscopes (TEMs) Scanning electron microscopes (SEMs) Your notes Transmission electron microscopes (TEMs) TEMs use electromagnets to focus a beam of electrons This beam of electrons is transmitted through the specimen Denser parts of the specimen absorb more electrons This makes these denser parts appear darker on the nal image produced (produces contrast between di erent parts of the object being observed) Advantages of TEMs: They give high-resolution images (more detail) This allows the internal structures within cells (or even within organelles) to be seen Disadvantages of TEMs: They can only be used with very thin specimens or thin sections of the object being observed They cannot be used to observe live specimens As there is a vacuum inside a TEM, all the water must be removed from the specimen and so living cells cannot be observed, meaning that specimens must be dead. Optical microscopes can be used to observe live specimens The lengthy treatment required to prepare specimens means that artefacts can be introduced Artefacts look like real structures but are actually the results of preserving and staining They do not produce a colour image Unlike optical microscopes that produce a colour image Scanning electron microscopes (SEMs) SEMs scan a beam of electrons across the specimen This beam bounces o the surface of the specimen and the electrons are detected, forming an image This means SEMs can produce three-dimensional images that show the surface of specimens Advantages of SEMs: They can be used on thick or 3−D specimens They allow the external, 3−D structure of specimens to be observed Disadvantages of SEMs: They give lower resolution images (less detail) than TEMs They cannot be used to observe live specimens They do not produce a colour image Comparison of the electron microscope & light microscope Light microscopes are used for specimens above 200 nm Light microscopes shine light through the specimen, this light is then passed through an objective lens (which can be changed) and an eyepiece lens (x10) which magnify the specimen to give an image that can be seen by the naked eye The specimens can be living (and therefore can be moving), or dead Page 13 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Light microscopes are useful for looking at whole cells, small plant and animal organisms, tissues within organs such as in leaves or skin Your notes Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm Electron microscopes re a beam of electrons at the specimen either a broad static beam (transmission) or a small beam that moves across the specimen (scanning) Due to the higher frequency of electron waves (a much shorter wavelength) compared to visible light, the magni cation and resolution of an electron microscope is much higher than a light microscope Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail Electron microscopy requires the specimen to be dead however this can provide a snapshot in time of what is occurring in a cell e.g. DNA can be seen replicating and chromosome position within the stages of mitosis are visible Diagram of the comparison of resolution of microscopes The resolving power of an electron microscope is much greater than that of the light microscope, as structures much smaller than the wavelength of light will interfere with a beam of electrons Light Microscope vs Electron Microscope Table Electron Microscope Light Microscope Page 14 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Large and installation means it cannot be Small and easy to carry moved Your notes Vacuum needed No vacuum needed Complicated sample preparation Simple sample preparation Over x500000 magni cation Up to x2000 magni cation Resolution 0.5nm Resolution 200nm Specimens need to be dead Specimens can be living or dead Examiner Tip Learn the di erence between resolution and magni cation! Also, learn how the light and electron microscope di er in terms of resolution and magni cation. Page 15 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Microscopy: Developments The microscope has undergone many developments since the rst one used in the 1600s by Robert Your notes Hooke Every advancement in microscopy technologies has improved our understanding of cells and their structures Optical (light) microscopes Optical (light) microscopes have made advancements in their ability to to view living cells and their internal structures Condenser lenses have been developed to direct light from the light source through the specimen Light rays pass from the specimen through the objective lens to the eyepiece Di erent types of condensers give di erent features to the microscope The use of uorescent stains and immuno uorescence can be used in optical microscopes which have made it possible to view cellular structures such as RNA Fluorescent dyes and stains are used to combine with speci c cell structures and organelles which, when exposed to UV rays, gives a more detailed view of the specimen Immuno uorescence involves the use of antibodies that have been prepared with uorescent dyes which can bind with target molecules complimentary to the antibody. This allows speci c molecules to be detected such as virus proteins Electron microscopes Electron microscopes bring us many advantages to studying cells High magni cation and resolution meaning that great detail can be seen in a range of cells and structures within cells, and including viruses 3D images can be produced using a scanning electron microscope Electron microscopes have also undergone developments in their abilities Cryogenic electron microscopy This involves ash-freezing solutions containing proteins or other biological molecules The frozen solution is then exposed to electrons to produce images of individual molecules Computer software is used to reconstruct a 3D representation of a cell's proteins using the images of individual molecules Our understanding of virus structure and composition, cell membrane arrangement and protein synthesis have improved thanks to this technique Freeze fracture A sample is rapidly frozen using liquid nitrogen and then physically broken apart (fractured) in a vacuum It can be used to provide a unique planar view of the internal organisation of cell membranes Page 16 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources General Cell Structure Your notes General Cell Structure All living organisms are comprised of cells These cells all have some common unifying features, including DNA as genetic material Cytoplasm A plasma membrane DNA All living cells contain some sort of DNA, this varies between eukaryotic cells and prokaryotic cells The presence of DNA means that a new cell can be formed from an old cell, as genetic material is able to be stored and transferred DNA also controls the production of enzymes and other vital proteins within the cell Cytoplasm Cytoplasm is found within the boundary of a cell It is composed of mainly water with dissolved substances, such as ions The uid is known as cytosol Many of the cell's important reactions take place within the cytoplasm Plasma membrane The plasma membrane surrounds the cell and encloses all the cell contents In all cell types, the plasma membrane has two layers and this is called a bilayer The bilayer consists of lipids; these vary depending on the type of organism The membrane is responsible for controlling the interactions of the cell's interior with the exterior Materials required by the cell are transported into the cell interior Waste substances are exported out of the cell to the surrounding environment The membrane is studded with proteins which have varying functions including: Cell recognition Cell communication Transport into and out of the cell Page 17 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Prokaryotic Cell Structure Your notes Prokaryotic Cell Structure The cell structure of organisms determines whether they are prokaryotic or eukaryotic Prokaryotes have the simplest cell structure, being the rst organisms to evolve on Earth and have been classi ed into two domains: Bacteria or Eubacteria - 'true' bacteria, includes commonly known bacteria such as E.coli and Helicobacter Archaebacteria or Archaea - typically found in extreme environments such as high temperatures and salt concentrations and include methanogens (organisms that exist in anaerobic conditions and produce methane gas) Prokaryotic cells are small, ranging from 0.1µm to 5.0µm Prokaryotes have cells that lack a nucleus (the Greek roots of prokaryote are 'pro' = before and 'karuon' = nut or kernel, relating to 'before the nucleus') Structure of prokaryotic cells The cytoplasm of prokaryotic cells is not divided into compartments, it lacks membrane-bound organelles Structures that are common to most prokaryotes include: 70S ribosomes DNA in a loop Cytoplasm Plasma membrane Cell wall Ribosomes Prokaryotic ribosomes are structurally smaller (70S) in comparison to those found in eukaryotic cells (80S) The function of these ribosomes is the binding and reading of mRNA during translation to produce proteins DNA Prokaryotes do not have a nucleus, but they do have genetic material. This is generally in the form of a "naked" single circular DNA molecule (not associated with proteins) located in the nucleoid and in smaller loops called plasmids Plasmids are small loops of DNA that are separate from the main circular DNA molecule Plasmids contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance) Cytoplasm Prokaryotic cytoplasm is very similar to the cytoplasm of any other cell It is the site of many cellular reactions Page 18 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources This is where the 70S ribosomes are found A major component of the cytoplasm is a gel-like cytosol, a water-based Your notes solution that contains ions, small molecules, and macromolecules Cell membrane The cell membrane of prokaryotes is composed of a lipid bilayer A group of prokaryotes, known as archaea, have their plasma membrane formed as a monolayer as opposed to a bilayer The role of the plasma membrane is to control substances entering and exiting the cell Cell wall Most prokaryotes have a cell wall containing murein/peptidoglycan (a glycoprotein) The cell wall acts as protection, maintains the shape of the cell and prevents the cell from bursting Some bacteria are able to be classi ed because of their cell wall structure Their ability to retain a dye called crystal violet classi es a group of bacteria as Gram positive, they appear blue/violet after exposure to the dye Examples of gram positive bacteria are Bacillus and Staphylococcus Bacteria that do not react with the dye are referred to as Gram negative bacteria Additional structures In addition, many prokaryotic cells have a few other structures that di erentiate the species from others and act as a selective advantage, examples of these are: Plasmids Capsules Flagellum Pili Some prokaryotes (e.g. bacteria) are surrounded by a nal outer layer known as a capsule. This is sometimes called the slime capsule It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism Flagellum (plural = agella) are long, tail-like structures that rotate, enabling the prokaryote to move (a bit like a propeller) Some prokaryotes have more than one Pili are shorter and thinner structures than agella They assist with movement, avoidance of attack by white blood cells, conjugation (the sexual mode for bacteria) and are commonly used to allow bacteria to adhere to cell surfaces A diagram of the structure of prokaryotic cells Page 19 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Prokaryotic cells are often described as being ‘simpler’ than eukaryotic cells, and they are believed to have emerged as the rst living organisms on Earth Examiner Tip Make sure you learn the typical structures and organelles found in prokaryotic cells, as well as their functions. Page 20 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Eukaryotic Cell Structure Your notes Eukaryotic Cell Structure Structure of eukaryotic cells Eukaryotic cells have a more complex ultrastructure than prokaryotic cells The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles The compartmentalisation of the cell is advantageous as it allows: enzymes and substrates to be available at higher concentrations damaging substances to be kept separated, e.g. digestive enzymes are stored in lysosomes so they do not digest the cell optimal conditions to be maintained for certain processes, e.g. optimal pH for digestive enzymes the numbers and locations of organelles to be altered depending on requirements of the cell Animal cell diagram Page 21 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Animal cells contain membrane-bound compartments, e.g. the nucleus and mitrochondria Plant cell diagram Page 22 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Plant cells are also eukaryotic cells, and have additional features when compared to animal cells, e.g. a cell wall and chroroplasts Organelles Plasma membrane All cells are surrounded by a plasma membrane which controls the exchange of materials between the internal cell environment and the external environment The membrane is described as being ‘partially permeable’ The plasma membrane is formed from a bilayer of phospholipids spanning a diameter of around 10 nm Page 23 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The cell surface membrane, or plasma membrane, controls the passage of substances into and out of cells Nucleus Present in all eukaryotic cells (except red blood cells), the nucleus is a large organelle that is separated from the cytoplasm by a double membrane (the nuclear envelope) which has many pores Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes (e.g. DNA polymerases) and signalling molecules to travel in The nucleus contains chromatin (the material from which chromosomes are made) Chromosomes are made of sections of linear DNA tightly wound around proteins called histones Usually at least one or more darkly stained regions can be observed within the nucleus; these regions are individually termed the ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production Page 24 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The nucleus of a cell contains chromatin (a complex of DNA and histone proteins) which is the genetic material of the cell Rough endoplasmic reticulum Found in plant and animal cells Surface covered in ribosomes (80S) Formed from folds of membrane continuous with the nuclear envelope; these attened membrane sacs are called cisternae Processes proteins made by the ribosomes The proteins synthesised by the ribosomes move to the cisternae, bud o into vesicles that carry the proteins to Golgi apparatus before being secreted out of the cell Page 25 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The rough endoplasmic reticulum (RER) has ribosomes on its surface Ribosomes 80S ribosomes are found freely in the cytoplasm or as part of the rough endoplasmic reticulum in eukaryotic cells Each ribosome is a complex of ribosomal RNA (rRNA) and proteins Constructed in the nucleolus Site of translation (which is part of protein synthesis) Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein Page 26 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mitochondria The site of aerobic respiration within all eukaryotic cells, mitochondria (singular mitochondrion) are just Your notes visible with a light microscope Surrounded by a double-membrane with the inner membrane folded to form cristae The matrix contains enzymes needed for aerobic respiration, producing ATP Small, circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication) The inner mitochondrial membrane has protein complexes vital for the later stages of aerobic respiration embedded within it Golgi apparatus Flattened sacs of membrane called cisternae Modi es proteins and lipids before packaging them into Golgi vesicles The vesicles then transport the proteins and lipids to their required destination Proteins that pass through the Golgi apparatus are usually: exported, e.g. hormones such as insulin put into lysosomes, such as hydrolytic enzymes delivered to membrane-bound organelles Page 27 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The Golgi apparatus has a distinctive appearance due to the arrangement of cisternae from which it is formed Vesicles Membrane-bound sacs for transport and storage, e.g. Golgi vesicles transport proteins from the Golgi apparatus around the cell Lysosomes are specialised vesicles that contain hydrolytic enzymes The role of lysosomes is to break down waste materials such as worn-out organelles Lysosomes are used extensively by cells of the immune system and in apoptosis (programmed cell death) Page 28 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The structure of the vesicle Microtubules Make up the cytoskeleton of the cell and are about 25 nm in diameter The cytoskeleton is used to provide support and movement to the cell Made of α and β tubulin proteins combined to form dimers, the dimers are then joined into proto laments Page 29 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Microtubules make up the cell cytoskeleton Plant cell structures Chloroplasts Larger than mitochondria Surrounded by a double-membrane Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana Grana are joined together by lamellae (thin and at thylakoid membranes) Chloroplasts are the site of photosynthesis: The light-dependent stage takes place in the thylakoids The light-independent stage (Calvin Cycle) takes place in the stroma Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis Page 30 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Chloroplasts are found in the green parts of a plant; the green colour is the result of the photosynthetic pigment chlorophyll Large permanent vacuole A sac in plant cells surrounded by the tonoplast, which is a selectively permeable membrane Animal cells can contain vacuoles, but they are temporary and small Plant cells contain large, permanent vacuoles Cell wall Found in plant cells but not in animal cells Formed outside of the cell membrane and o er structural support to the cell Page 31 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cell walls are extra-cellular, so are not technically cellular organelles Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan in most bacterial cells Your notes Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells The cell wall is freely permeable to most substances (unlike the plasma membrane) Page 32 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Functions of Life Your notes Functions of Life Unicellular (single-celled) and multicellular (many cells) organisms must carry out the following functions to stay alive: Metabolism - all the enzyme-catalysed reactions occurring in a cell, including cell respiration Reproduction - the production of o spring. It may be sexual or asexual Homeostasis - the ability to maintain and regulate internal conditions within tolerable limits, including temperature Growth - the permanent increase in size Response - (or sensitivity), the ability to respond to external or internal changes (stimuli) in their environment. Thus improving their chance of survival Excretion - the disposal of metabolic waste products, including carbon dioxide from respiration Nutrition - the acquisition of energy and nutrients for growth and development, either by, absorbing organic matter or by synthesising organic molecules (e.g. photosynthesis) Unicellular organisms have adapted unique ways to carry out these functions compared to multicellular organisms Mitochondria are present to provide energy through respiration The cell membrane controls movement of materials in and out of the cell to maintain homeostasis Ribosomes are present to produce proteins for growth and repair, in addition enzymes for vital cell functions Vacuoles are used for digestion purposes and also to store waste substance Cilia or agella are used for movement of the organism in response to changes in the environment Page 33 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Eukaryotic Cell Structure: Comparisons & Atypical Examples Your notes Cell Structure: Animals, Fungi & Plants Eukaryotic cells exist in four kingdoms The animal kingdom The plant kingdom The fungal kingdom The protist (protoctista) kingdom The cells of each of these possess unique characteristics and structures that contribute to their di erences Di erences in eukaryotic cell structure Cell walls Animal cell do not have a cell wall Plant cell walls are composed of the polysaccharide cellulose Fungal cell walls are made up mainly of glucans, chitin and glycoproteins Vacuoles Vacuoles can be present in animal cells but they tend to be small, temporary and numerous when present with unique functions Plant cells have large permanent vacuoles used for the storage of various substances Like animal cells, fungal cells can contain vacuoles but they are small and non-permanent Chloroplasts Animal cells do not have chloroplasts Plant cells possess many chloroplasts used for the production of carbohydrates through photosynthesis Fungal cells do not have chloroplasts Presence of centrioles Animal cells do contain centrioles used in the role of microtubule organisation during cell division Plant cells do not possess centrioles Fungal cells do not possess centrioles Presence of cilia and agella Page 34 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Animal cells can have cilia and agella, associated with a basal body (a protein structure from which the cilia are assembled), and are used in various Your notes functions such as the movement of an egg cell through the oviduct or the movement of uids in the respiratory tract Plant cells do not contain cilia or agella True fungi do not contain cilia or agella Other di erences Animal and fungal cells store their carbohydrates as glycogen, whereas plants so carbohydrates as starch Animal cells are exible as they lack a rigid cell wall, whereas plant cells have a xed shape. Fungal cells, although they have a cell wall, can be exible and their shape may vary Page 35 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Atypical Cell Structure Some eukaryotic cells have a very unique or atypical structure which enables them to carry out Your notes specialised functions The number of nuclei can be used to illustrate atypical examples Skeletal muscle, aseptate fungal hyphae, red blood cells and phloem sieve tubes are examples of cells/tissue with structures that question the integrity of the cell theory Atypical examples Striated muscle bres Striated muscle bres (fused muscle cells) are: Longer than typical cells (up to 300 mm in length in comparison to a cardiac muscle cell which has a length of 100 - 150 µm) Have multiple nuclei surrounded by a single membrane (sarcolemma) Striated muscle cells are formed from multiple cells which have fused together (which is how they have many nuclei rather than one) that work together as a single unit These features challenge the concept that cells work independently of each other even in a multicellular organism Aseptate fungal hyphae Page 36 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Fungi have many long, narrow branches called hyphae Hyphae have cell membranes, cell walls and some have septa Aseptate fungal hyphae do not have septa, thus these cells are multinucleated with continuous cytoplasm The cells have no end walls making them appear as one cell Red blood cells Page 37 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Red blood cells, a type of animal cell, are unique in that they do not contain a nucleus The reason for this is to enable the cell to carry a large volume of the oxygen binding pigment haemoglobin The biconcave shape of red blood cells means they have maximum surface area to improve their oxygen carrying capacity Phloem sieve tubes Page 38 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes These serve a plant by transporting dissolved substances, such as sucrose, around the plant These unique tissues have no end cell wall and lack many cell organelles such as nuclei, mitochondria and ribosomes Because of the lack of their own organelles, sieve tube elements can only survive due to the presence of companion cells which sit alongside next to the sieve tube elements and help to maintain the cytoplasm of the sieve tubes Page 39 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cell Types & Structures: Skills Your notes Cell Types & Structures: Skills It is important to be able to recognise various organelles from light and electron microscope images When interpreting micrographs to identify and deduce the function of the cell it is important to:. Identify whether it is a prokaryotic or eukaryotic cell - look to see if a nucleus is present or not. Identify which type of eukaryotic cell it is (plant or animal) by looking for a cell wall or vacuole. Identify the organelles present in the cells and consider their function You should be con dent in identifying the following structures and organelles: Nucleoid region in a prokaryotic cell Prokaryotic cell wall Nucleus Mitochondria Chloroplast Sap vacuole Golgi apparatus Rough and smooth endoplasmic reticulum Chromosomes Ribosomes Plant cell wall Plasma membrane Microvilli Some identi able features of key organelles are: Chloroplast Has distinctive stacks of thylakoids Double membrane Has a roughly oval shape Larger than mitochondria Indicates the cell is a plant cell Nucleus Has a nuclear membrane and a dark nucleolus within It has a roughly spherical shape Vacuole Occupies a large space within a cell Often shows up as a very light shade (white) within an electron micrograph Indicates the cell is a plant cell Cell wall Located around the perimeter of the cell Mitochondria Roughly oval-shaped Double membrane Page 40 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sometimes observed with visible cristae (foldings of the inner membrane) An interpretation of an electron micrograph of a cell Your notes Electron micrograph of cell 1 To identify this cell consider the following The cell has a nucleus - therefore it is a eukaryotic cell This cell does not have a cell wall or central vacuole - therefore it is an animal cell The cell has a large u-shape nucleus - so it can manipulate itself through small pores There are a large number of lysosomes in the cell - so it can digest substances found within the cell There are a large number of mitochondria - this means it has su cient energy for its many metabolic reactions The deduction, therefore, is that this cell needs a lot of energy to break down substances that enter the cell and that it can move where it wants. This cell is a macrophage An interpretation of an electron micrograph of a cell Page 41 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Electron micrograph of cell 2 To identify this cell consider the following The cell has a nucleus - therefore it is a eukaryotic cell This cell does not have a cell wall or central vacuole - therefore it is an animal cell There are a large number of mitochondria - so it requires signi cant energy for many metabolic reactions The cell has microvilli packed closely together (brush border) - so it needs to increase the surface area and prevent any substance from crossing into the cell The deduction, therefore, is that this cell needs a lot of energy to control what enters or exits this cell and that the cell requires many of the substance to be absorbed. This cell is a ciliated epithelium of the small intestine Page 42 of 46 © 2015−2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers