IB Biology HL - Cell Theory & Origin of Cells PDF

Head to www.savemyexams.com for more awesome resources DP IB Biology: HL Your notes 1.2 Cells: Origin & Ultrastructure Contents 1.2.1 Origin of Cells 1.2.2 Endosymbiotic Theor...

Head to www.savemyexams.com for more awesome resources DP IB Biology: HL Your notes 1.2 Cells: Origin & Ultrastructure Contents 1.2.1 Origin of Cells 1.2.2 Endosymbiotic Theory 1.2.3 Prokaryotic Cell Structure 1.2.4 Eukaryotic Cell Structure 1.2.5 Exocrine Pancreatic & Palisade Mesophyll Cells 1.2.6 Comparison of Prokaryotic & Eukaryotic Cells 1.2.7 Microscopes 1.2.8 Skills: Drawing Cells 1.2.9 Skills: Cell Origin & Ultrastructure Page 1 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.1 Origin of Cells Your notes Spontaneous Generation Pre-existing cells In 1852 Robert Remak made the conclusion that cells divided to form new cells, that is cells came from pre-existing cells His conclusion was reached after studying cells from chicken embryos This discovery is often attributed to Robert Virchow who in 1855 proposed the phrase omnis cellula e cellula (all cells come from cells) Prior to these announcements, it was believed that life arose spontaneously from non-living matter NOS: Testing the general principles that underlie the natural world; the principle that cells only come from pre-existing cells needs to be verified Up until the 17th century, the consensus was that life was spontaneously generated (living organisms arose from non-living matter). This was believed due to: The lack of technology - microscopes were not extensively used Observations being made - Aristotle observing insects forming from dew or van Helmont observing a mouse appearing from a jar containing a sweaty shirt and wheat The idea supporting the cultural and religious beliefs of the time From the 17th century scientists such as Francesco Redi with his maggot and rotting meat experiment began collecting evidence to test and verify that life required life to exist However, it was Louis Pasteur's experiments involving swan-neck flasks that provided sufficient verification to convince scientists that cells could only come from pre-existing cells The universal acceptance that cells come from pre-existing cells also comes from the idea that : The highly complex ultrastructure of cells has not been able to be synthesised by humans All the known examples of growth are a result of cells dividing by mitosis or meiosis Although viruses have a much simpler structure, they can only be produced inside a host cell The universality of the genetic code suggests that all life evolved from the same original cells The translation of the 64 codons produces the same amino acids for nearly all organisms - although there are some rare minor variations that have likely arisen since the common origin of life Page 2 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 3 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 4 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Experiments disproving spontaneous generation Page 5 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Pasteur's Experiments Your notes Louis Pasteur's experiments Louis Pasteur's experiments were designed to verify the principle that cells can only come from pre- existing cells To demonstrate this Pasteur used swan neck flasks (flasks with S-shaped necks) which trapped the microorganisms in the bend of the neck Pasteur added nutrient broth to the flasks then boiled them to sterilise With some of the flasks, Pasteur broke off the necks (leaving no bend) After a long period of time, Pasteur observed that the broth in the flasks with the snapped necks had gone cloudy whereas the broth in the swan neck flasks remained clear Thus Pasteur had shown that the swan necks prevented microorganisms in the air from entering the broth and that no organisms appeared spontaneously Page 6 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The First Cells The Oparin-Haldine hypothesis is that, to create the original first cells from non-living material, the Your notes following four stages occurred: 1. Simple organic compounds needed to be synthesised from inorganic molecules (this was demonstrated by Stanley Miller and Harold Urey) 2. Then assembled into polymers 3. Some of these polymers (it is thought to be RNA) developed the ability to self-replicate (which enables inheritance) 4. Formation of membranes (by lipids) that surrounded the polymers creating packages with internal chemistry different from the surroundings Page 7 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The key stages involved in life arising from non-living materials Miller-Urey experiment Miller and Urey recreated the conditions thought to have existed on Earth prior to life, using a specific piece of apparatus The apparatus allowed them to: Boil water to produce steam reflecting the early primordial soup evaporating in the high temperatures that existed on Earth Page 8 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mix the steam with a mixture of gases (including methane, hydrogen and ammonia) that recreated the atmosphere Add electrical discharges to the gases to stimulate lightning (one of the sources of energy Your notes available at the time) Cool the mixture (representing the condensation of water in the atmosphere) After a week Miller and Urey analysed the condensed mixture and found traces of simple organic molecules including amino acids Page 9 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The apparatus used by Miller and Urey Exam Tip It is important to be able to explain how the experiments that Pasteur and Miller & Urey performed demonstrated the origin of cells. Page 10 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.2 Endosymbiotic Theory Your notes Endosymbiotic Theory Endosymbiosis Endosymbiosis is where one organism lives within another If the relationship is beneficial to both organisms the engulfed organism is not digested For endosymbiosis to occur one organism must have engulfed the other by the process of endocytosis Endosymbiotic theory The endosymbiotic theory is used to explain the origin of eukaryotic cells. The evidence provided for this theory comes from the structure of the mitochondria and chloroplasts Scientists have suggested that ancestral prokaryote cells evolved into ancestral heterotrophic and autotrophic cells through the following steps: Heterotrophic cells: To overcome a small SA:V ratio ancestral prokaryote cells developed folds in their membrane. From these infoldings organelles such as the nucleus and rough endoplasmic reticulum formed A larger anaerobically respiring prokaryote engulfed a smaller aerobically respiring prokaryote (which is not digested) This gave the larger prokaryote a competitive advantage as it had a ready supply of ATP and gradually the cell evolved into the heterotrophic eukaryotes with mitochondria that are present today Autotrophic cells: At some stage in their evolution, the heterotrophic eukaryotic cell engulfed a smaller photosynthetic prokaryote. This cell provided a competitive advantage as it supplied the heterotropic cell with an alternative source of energy, carbohydrates Over time the photosynthetic prokaryote evolved into chloroplasts and the heterotrophic cells into autotrophic eukaryotic cells Page 11 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 12 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The endosymbiotic theory - an explanation for the evolution of eukaryotic cells Evidence to support the endosymbiotic theory The evidence to support the endosymbiotic theory arises from the features that the mitochondria and chloroplasts have in common with prokaryotes: Both reproduce by binary fission Both contain their own circular, non-membrane bound DNA They both transcribe mRNA from their DNA They both have 70S ribosomes to synthesise their own proteins They both have double membranes Page 13 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Exam Tip Your notes Learn how the structure of the mitochondria and chloroplast support the endosymbiotic theory. Page 14 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.3 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 first organisms to evolve on Earth and have been classified 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') Cell structure The cytoplasm of prokaryotic cells is not divided into compartments, it lacks membrane-bound organelles (except for ribosomes) Prokaryotic ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S) Prokaryotes do not have a nucleus, but they do have genetic material. This is generally in the form of a single circular DNA molecule (not associated with proteins) located in the nucleoid and in smaller loops called plasmids 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 In addition, many prokaryotic cells have a few other structures that differentiate the species from others and act as a selective advantage, examples of these are: Plasmids Capsules Flagellum Pili 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) Some prokaryotes (e.g. bacteria) are surrounded by a final outer layer known as a capsule. This is sometimes called the slime capsule Page 15 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism Flagellum (plural = flagella) are long, tail-like structures that rotate, enabling the prokaryote to move Your notes (a bit like a propeller) Some prokaryotes have more than one Pili are shorter and thinner structures than flagella 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 Page 16 of 58 © 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 first living organisms on Earth Exam Tip Make sure you learn the typical structures and organelles found in prokaryotic cells, as well as their functions. Page 17 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Binary Fission Prokaryotes divide by binary fission Your notes Binary fission is a type of asexual reproduction where the parent cell splits into two daughter cells, roughly equal in size During the binary fission process in prokaryotes: The single circular chromosome replicates when signalled The cell elongates resulting in the chromosome copies separating A cross wall (septum) forms in the middle of the cell dividing the cytoplasm (cytokinesis) Two daughter cells are formed As each daughter cell contains an exact copy of the parental circular chromosome they are clones Page 18 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Prokaryotes divide by binary fission Page 19 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.4 Eukaryotic Cell Structure Your notes Eukaryotic Cell Structure Compartmentalized cell structure Eukaryotic cells have a more complex ultrastructure than prokaryotic cells The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles. These compartments are either bound by a single or double membrane The compartmentalization of the cell is advantageous as it allows: Enzymes and substrates to be localised and therefore 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 location of organelles to be altered depending on requirements of the cell Eukaryotic cells have a key compartment called the nucleus Animal and plant cells Animal and plant cells are both types of eukaryotic cells that share key structures such as: Membrane-bound organelles, including a nucleus Larger ribosomes (80S) However, there are key differences: Animal cells contain centrioles and microvilli Plant cells have a cellulose cell wall, large permanent vacuoles and chloroplast Page 20 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The ultrastructure of an animal cell shows a densely packed cell – the ER and RER and ribosomes form extensive networks throughout the cell in reality Page 21 of 58 © 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 have a larger, more regular structure in comparison to animal cells In complex multicellular organisms, eukaryotic cells become specialised for specific functions These specialised eukaryotic cells have specific adaptations to help them carry out their functions For example, the structure of a cell is adapted to help it carry out its function (this is why specialised eukaryotic cells can look extremely different from each other) Structural adaptations include: The shape of the cell The organelles the cell contains (or doesn’t contain) Page 22 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources For example: Red blood cells are biconcave and do not contain a nucleus. This makes more space inside the cell so that they can transport as much oxygen as possible Your notes Cells that make large amounts of proteins will be adapted for this function by containing many ribosomes (the organelle responsible for protein production) Organelles Plasma membrane The structure of the cell surface membrane – although the structure looks static the phospholipids and proteins forming the bilayer are constantly in motion 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 phospholipid bilayer of phospholipids spanning a diameter of around 10 nm Nucleus Page 23 of 58 © 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 Present in all eukaryotic cells (except red blood cells), the nucleus is relatively large and 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 (eg. 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 – these regions are individually termed ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production Rough endoplasmic reticulum Page 24 of 58 © 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) - the attached ribosomes enable this structure to be identified in electron micrographs Found in plant and animal cells Surface covered in ribosomes (80S) Formed from continuous folds of membrane continuous with the nuclear envelope. These flattened membrane sacs are called cisternae Processes proteins made by the ribosomes The proteins synthesised by the ribosomes, move to the cisternae, bud off into vesicles that carry the proteins to Golgi apparatus before being secreted out of the cell Ribosomes Page 25 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells Each ribosome is a complex of ribosomal RNA (rRNA) and proteins. They are constructed in the nucleolus (a region in the nucleus) 80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells Site of translation (protein synthesis) Mitochondrion Page 26 of 58 © 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 mitochondrion is shown – the inner membrane has protein complexes vital for the later stages of aerobic respiration embedded within it The site of aerobic respiration within all eukaryotic cells, mitochondria are just visible with a light microscope Surrounded by double-membrane with the inner membrane folded to form cristae The matrix formed by the cristae 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) Golgi apparatus Page 27 of 58 © 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 Golgi apparatus Found in plant and animal cells Flattened sacs of membrane called cisternae (like the rough endoplasmic reticulum) Modifies proteins and lipids before packaging them into Golgi vesicles The vesicles then transport the proteins and lipids to their required destination Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles Vesicles Page 28 of 58 © 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 Found in plant and animal cells A membrane-bound sac for transport and storage Lysosome The structure of the lysosome Page 29 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down) Break down waste materials such as worn-out organelles Your notes Used extensively by cells of the immune system and in apoptosis (programmed cell death) Chloroplasts Chloroplasts are found in the green parts of a plant – the green colour a result of the photosynthetic pigment chlorophyll Found in plant cells 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 flat 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 Large permanent vacuoles Page 30 of 58 © 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 vacuole A sac in plant cells surrounded by the tonoplast, selectively permeable membrane Vacuoles in animal cells are not permanent and small Cell wall - an extra-cellular component (not an organelle) The cell wall is freely permeable to most substances (unlike the plasma membrane) Found in plant cells but not in animal cells Page 31 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cell walls are formed outside of the cell membrane and offer structural support to cell 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 Additional organelles The below organelles can be found in other specialised cells in eukaryotes Flagella The structure of the flagella Found in specialised cells Similar in structure to cilia, made of longer microtubules Contract to provide cell movement for example in sperm cells Centrioles Page 32 of 58 © 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 centriole Hollow fibres made of microtubules Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division Not found in flowering plants and fungi Microtubules Page 33 of 58 © 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 microtubule Found in all eukaryotic cells Makes up the cytoskeleton of the cell about 25 nm in diameter Made of α and β tubulin combined to form dimers, the dimers are then joined into protofilaments Thirteen protofilaments in a cylinder make a microtubule The cytoskeleton is used to provide support and movement of the cell Microvilli Page 34 of 58 © 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 microvilli Found in specialised animal cells Cell membrane projections Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances Cilia Page 35 of 58 © 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 cilia Hair-like projections made from microtubules Allows the movement of substances over the cell surface Exam Tip In the exam, you could be required to apply your knowledge of organelles to deduce the function of a specialised cell. To answer these questions, just think about what organelles you can see in large numbers, consider the function of that organelle and then think about where this function might need to happen a lot in an organism (e.g. if the cell’s main function is to carry out photosynthesis it will need to contain many chloroplasts)! Page 36 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.5 Exocrine Pancreatic & Palisade Mesophyll Cells Your notes Exocrine Pancreatic & Palisade Mesophyll Cells Exocrine gland cells of the pancreas The pancreas contains two types of gland cells: endocrine and exocrine cells The function of the exocrine gland cells (acinar cells) is to secrete digestive enzymes into the pancreatic ducts. These enzymes then travel to the duodenum where digestion occurs To perform this function the exocrine gland cells have organelles that enable the enzymes (proteins) to be synthesised, processed for secretion, transported to the plasma membrane and released Thus the plasma membrane and the following organelles can be seen in electron micrographs of the exocrine gland cells: Nucleus - where DNA is transcribed into mRNA (that contains the instructions for building the enzymes) Rough endoplasmic reticulum - has ribosomes attached where the enzymes are synthesised Mitochondria - provide the ATP required for all the metabolic processes Golgi apparatus - where the enzymes (proteins) are processed and packaged ready for secretion Vesicles - 'pinch off' the Golgi apparatus and contain the pancreatic digestive enzymes (e.g. pancreatic amylase) that will be released into the ducts (may appear dark in electron micrographs or at least with many dark specks within) Lysosomes - contain hydrolytic enzymes that will digest the unwanted substances in the cell Palisade mesophyll cell The palisade mesophyll cells are located in the leaves of plants and are structured to maximise the efficiency of the leaf's function - photosynthesis The palisade mesophyll cells are situated towards the top of the leaf and are column-like in shape increasing surface area to absorb light, carbon dioxide and water Along with the key organelles mentioned for the exocrine gland cell, the palisade mesophyll cell contains the following organelles: Chloroplasts - the location of light absorption, it provides the energy for producing glucose and oxygen Permanent vacuole - it is large and central pushing the chloroplast to the edge of the cell maximising absorption of light. It also helps maintain water balance The palisade mesophyll cell also contains the extra-cellular structure: Cell wall - it is mainly made of cellulose, is freely permeable (allowing carbon dioxide and water to move through easily) and its strength gives support to the cell (prevents the cell from bursting) Page 37 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.6 Comparison of Prokaryotic & Eukaryotic Cells Your notes Comparison of Prokaryotic & Eukaryotic Cells Animal and plant cells are types of eukaryotic cells, whereas bacteria are a type of prokaryote There are a number of important structural and physiological differences between prokaryotic and eukaryotic cells These differences affect their metabolic processes and how they reproduce Comparison of Prokaryotes & Eukaryotes Table Page 38 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Exam Tip Your notes Become familiar with comparing the differences between prokaryotic and eukaryotic cells. It can be easier to answer comparison questions by drawing a table. Page 39 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.7 Microscopes Your notes Electron & Light Microscopes NOS: Developments in scientific research follow improvements in apparatus; the invention of electron microscopes led to greater understanding of cell structure In scientific research, critical developments often follow improvements in scientific apparatus For example, distant objects in Space often remain undiscovered until a telescope (or some other piece of equipment) powerful enough to detect them is developed The fact that scientific research is often held back by a lack of sufficiently powerful or precise apparatus is a problem that will continue into the future In some ways, this is very exciting, as it suggests that our scientific knowledge and understanding of the universe will continue to expand as new scientific techniques and technologies are developed The discovery of the microscope allowed scientists to discover many things such as: Formulate the cell theory, discover bacteria, see chromosomes, understand fertilisation by witnessing the fusion of gametes and closely examine the complex structure of organs such as the liver Due to constraints in technology (light microscopes cannot produce distinguishable clear images of structures smaller than 0.2 µm) developments in scientific research were limited This was until a different type of the microscope was invented - the electron microscope Electron microscopes enabled scientists to view structures 200 times smaller than light microscopes leading to a better understanding of the ultrastructure of cells The grana of chloroplasts were observed to be constructed of stacks of flattened membrane sacs Ribosomes and endoplasmic reticulum were discovered Improvements to the design of electron microscopes (electron tomography) and the invention of new types of microscopes (fluorescence) are allowing further developments in scientific research to be made Microscopes Microscopes can be used to analyse cell components and observe organelles Magnification and resolution are two scientific terms that are very important to understand and distinguish between when answering questions about microscopy (the use of microscopes): Magnification 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: Page 40 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Optical microscopes (sometimes known as light microscopes) Electron microscopes Your notes 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 magnification 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 magnification of electron microscopes is about ×1,500,000 There are two types of electron microscopes: Transmission electron microscopes (TEMs) Scanning electron microscopes (SEMs) 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 Page 41 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources This makes these denser parts appear darker on the final image produced (produces contrast between different parts of the object being observed) Advantages of TEMs: Your notes 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 off 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 Light microscopes are useful for looking at whole cells, small plant and animal organisms, tissues within organs such as in leaves or skin Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm Page 42 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Electron microscopes fire 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 Your notes light, the magnification and resolution of an electron microscope is much better 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 eg. DNA can be seen replicating and chromosome position within the stages of mitosis are visible 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 Page 43 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Exam Tip Learn the difference between resolution and magnification! Also, learn how the light and electron microscope differ in terms of resolution and magnification. Page 44 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.8 Skills: Drawing Cells Your notes Drawing Cells Drawing the ultrastructure of cells To record the observations seen under the microscope (or from photomicrographs taken) a labelled biological drawing is often made Biological drawings are line pictures that show specific features that have been observed when the specimen was viewed There are a number of rules/conventions that are followed when making a biological drawing Drawing conventions The drawing must have a title The magnification under which the observations shown by the drawing are made must be recorded A sharp HB pencil should be used (and a good eraser!) Drawings should be on plain white paper Lines should be clear, single lines (no thick shading) No shading The drawing should take up as much of the space on the page as possible Well-defined structures should be drawn The drawing should be made with proper proportions Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled Label lines should be kept to one side of the drawing (in parallel to the top of the page) and drawn with a ruler Drawings of cells are typically made when visualizing cells at a higher magnification power, whereas plan drawings are typically made of tissues viewed under lower magnifications (individual cells are never drawn in a plan diagram) Page 45 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Drawing Prokaryotic Cells Due to the size of prokaryotes (0.1 to 5 µm) their ultrastructure can only be seen using an electron Your notes microscope Therefore drawings of prokaryotes are based on electron micrographs When viewing an electron micrograph of a prokaryote there is no distinct dark circular area within the cell, as there is no nucleus and no organelles are visible (apart from ribosomes, but as they are 70 S in size these are difficult to distinguish) Biological drawings should show only visible structures, and should be labelled using the correct labelling conventions Page 46 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Drawing Eukaryotic Cells When viewing a eukaryotic cell under a light microscope it is possible to identify the nucleus and if it is a Your notes plant cell the cell wall and vacuole However, under an electron microscope, more detail of the ultrastructure of the eukaryotic cell can be seen The following organelles should be able to be identified, although it does depend on whether it is a plant or animal cell and the specialisation of the cell: Rough endoplasmic reticulum Golgi apparatus Lysosomes Vesicles Ribosomes Vacuole (plant) Nucleus Mitochondrion Chloroplast The nucleus, mitochondrion and chloroplast all have double membranes The cell wall will be present in plant eukaryotic cells. This is an extra-cellular component Cell structures under an electron microscope Electron microscopes can produce highly detailed images of animal and plant cells The key cellular structures within animal and plant cells are visible within the electron micrographs above The nucleus should be clearly identifiable as it is the largest structure in the eukaryotic cell Page 47 of 58 © 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 the nucleus To identify the mitochondrion look for the crista (the foldings of the inner membrane) which are often visible in electron micrographs Page 48 of 58 © 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 the mitochondrion The rough endoplasmic reticulum (rER) is located next to the nucleus and the attached ribosomes can be used to identify the rER as they make the membrane appear darker Page 49 of 58 © 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 the rough endoplasmic reticulum The chloroplast can be identified by the thylakoid stacks (grana), as they appear as dark lines within the organelle Chloroplasts are large Page 50 of 58 © 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 the chloroplast Golgi apparatus will be located near the endoplasmic reticulum and it: Does not have long membrane sacs The sacs are more curved than the endoplasmic reticulum Does not have ribosomes attached Has many vesicles close by Page 51 of 58 © 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 the Golgi apparatus Vesicles are spherical shapes Page 52 of 58 © 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 the vesicles Free ribosomes appear as dark granules (tiny dark dots) in the cytoplasm Page 53 of 58 © 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 the ribosomes Plant cell electron micrographs Electron micrographs of plant cells, such as palisade mesophyll cells, may show: The chloroplasts along the plasma membrane, as this is where the most light can be absorbed A large vacuole in the centre A cell wall Page 54 of 58 © 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 a plant cell Animal cell electron micrographs An exocrine gland cell of the pancreas may show: Many large secretory vesicles (carrying the digestive enzymes) Many mitochondria Rough endoplasmic reticulum Page 55 of 58 © 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 an exocrine gland cell of the pancreas Exam Tip When producing a biological drawing, it is vital that you only ever draw what you see and not what you think you see. When identifying palisade mesophyll cells, look for the presence of the large central vacuole, cell wall and lots of chloroplasts on the edge of the cell to maximise light absorption. When identifying exocrine pancreatic gland cells, look for the presence of secretory vesicles carrying the digestive enzymes and the large numbers of rough endoplasmic reticulum. Page 56 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 1.2.9 Skills: Cell Origin & Ultrastructure Your notes Interpreting Electron Micrographs When interpreting electron micrographs to deduce the function of the cell it is important to: 1. Identify whether it is a prokaryotic or eukaryotic cell - is a nucleus present 2. Identify which eukaryotic cell it is (plant or animal) by looking for a cell wall or vacuole 3. Identify the organelles present in the cells and consider their function Electron micrograph of cell 1 The cell had a nucleus - it is a eukaryotic cell This cell did not have a cell wall or central vacuole - it is an animal cell The cell has a large u-shape nucleus - it can manipulate itself through small pores There are a large number of lysosomes in the cell - it can digest substances found within the cell There are a large number of mitochondria - it has sufficient energy for the many metabolic reactions Page 57 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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 Your notes Electron micrograph of cell 2 The cell had a nucleus - it is a eukaryotic cell This cell did not have a cell wall or central vacuole - it is an animal cell There are a large number of mitochondria - it requires significant energy for many metabolic reactions The cell has microvilli packed closely together (brush border) - 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 a lot of the substance to be absorbed. This cell is a ciliated epithelium of the small intestine Page 58 of 58 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers