Cell Structure.docx

Cell Structure The Cell is the basic unit of life. The simplest organisms consist of just one cell and are described as unicellular. Organisms can be divided into two categories depending on their cell structure – Prokaryotes and Eukaryotes. **Need to Know** Prokaryotic Organisms Eukaryotic Organ...

Cell Structure The Cell is the basic unit of life. The simplest organisms consist of just one cell and are described as unicellular. Organisms can be divided into two categories depending on their cell structure – Prokaryotes and Eukaryotes. **Need to Know** Prokaryotic Organisms Eukaryotic Organisms No nucleus Nucleus is present No membrane-bound organelles Membrane-bound organelles are present (mitochondria/chloroplasts) Mainly unicellular Mainly multicellular Most reproduce asexually Most reproduce sexually Cells are primitive compared to Eukaryotes Cells are bigger and more complex than prokaryotes Includes bacteria i.e., members of the kingdom Monera which can survive in any environment Includes animals, plants, and fungi except Monera Genetic material is in a ring that is folded in many shapes DNA is in the shape of rods as they divide (1,2) Extremophiles are prokaryotes which live in extreme environments. Can be found in deep oceanic hydrothermal vents. Also found a kilometre below the ice in the heart of Antarctica. Microscopes Types of Microscopes - Simple Microscope uses one lens to magnify an object. (Magnifying glass) - A compound microscope uses two or more lenses to magnify an object. - An electron microscope uses accelerated electrons as a source of illumination and this gives much great resolution, which in turn allows much greater magnification. - Transmitting Electron Microscope (TEM): Sends electrons through objects and reveals the most detail. The TEM uses electromagnets as lenses to focus and magnify the image by bending the paths of the electrons. - Scanning Electron Microscope (SEM): Photographs reflected electrons from surfaces and reveals 3D structures. The surface is usually coated with a thin film of gold. **Need to know the differences between light microscope and electron microscope** Light Microscope Electron Microscope Uses light rays & focuses them with more than 2 convex lenses to illuminate an object. Uses a beam of electrons & focuses them with electromagnets to illuminate an object. Magnifies up to 1400X Magnifies up to 500,000 X Low resolution (up to 200 nm) High resolution (up to 1 nm) – beam of electrons has a much smaller wavelength than light. It reveals nucleus, cell organelles, cell walls, vacuoles, and chromatin Reveals details of cell organelles & cell structure such as cilia, flagella & membranes Portable & relatively inexpensive Not portable & very expensive Can examine living tissue (thin) Objects dead (in a vacuum) Image is a photomicrograph – a grainy black & white picture Structure of Compound light microscope **Need to know parts, functions, and diagram** (3) - Eyepiece: This lens magnifies the image e.g. 10X. - Objective lenses: Magnify the image. Low power (x4), medium power (x10), high power (x40). Total magnification = Eyepiece (x10) x objective lens (x40) =. 10 x 40 = 400. - Body tube/barrel: Holds the eyepiece at one end and the revolving nosepiece (objective lenses) at other end. - Revolving nosepiece: Holds and positions the objective lenses. - Coarse focus wheel: Used for initial focussing with low and medium power. - Fine focus wheel: Sharpens the focus after coarse adjustment. Focus the high-power objective with this wheel only. - Stage: Platform on which slide is placed. Slide is kept in place by clips. Keep dry. - Condenser: Focuses light onto slide. - Diaphragm: Controls amount of light passing to the slide. - Light source: Electric bulb or reflecting mirror. - Arm: Joins the body tube to the base of the microscope. Cells viewed through a Light Microscope (4) Difference between animal and plant cells **Know in detail** Plant Animal Plants have a cell wall made of cellulose Animal cells do not have a cell wall Plant cells contain chloroplasts Animal cells do not have chloroplasts Plant cells have a large, permanent vacuole Animal cells only have small, non-permanent vacuoles Cell Ultrastructure Cell ultrastructure are the components of the cell that are only visible using an electron microscope. **Don’t need to know endoplasmic reticulum, Golgi apparatus, lysomes or cytoskeleton** Animal Cell Ultrastructure (5) Cell Membrane Structure (6) The cell membrane is a fluid phospholipid bilayer coated and embedded with protein. Protein gives elasticity and lipid allows fat-soluble molecules to enter. There are temporary pores throughout the membrane. Function: - Holds in cell contents - thus giving shape, support (provided by proteins) and protection. - Controls entry and exit of molecules. It is a semi-permeable barrier i.e., can let small molecules e.g., water (by osmosis), oxygen and carbon dioxide (by diffusion) through but not large molecules e.g., salt, sugar, protein. Proteins assist in the active transport of materials across the membrane (energy needed). Thus, the cell can control the amount of water and salt conc. (osmoregulation). Phospholipids affect the fluidity and permeability of membrane Cytoplasm Structure - Contains the entire contents of the cell inside the cell membrane, except the nucleus. - Consists of cytosol, a gel-like substance where the organelles are suspended. - Contains all the substances needed by the cell to carry out its functions. Function: - The medium in which most chemical reactions of the cell take place - The first stage of respiration, where glucose is split in a process called glycolysis with the release of a small amount of energy, takes place here. Nucleus Structure (7) - Enclosed by a double membrane. - Contains chromatin (genetic material) - becomes arranged into chromosomes during cell division. These are made of protein and DNA. Genes are located along the chromosome. Contains one or more nucleoli. -Nuclear pores allow passage of mRNA, rRNA, nucleotides. - Nucleoplasm = a liquid in nucleus surrounding nucleolus and chromatin Function: - To control all cell activities (by making enzymes). - Contains genetic material. - Involved in cell division. Nuclear Pores Hundreds of gaps in the nuclear envelope which allow substances to enter and leave the nucleus. Proteins, amino acids, and nucleotides bases enter the nucleus, while mRNA, rRNA and the protein subunits that make ribosomes leave through these pores. Nucleolus Darker staining areas within the nucleus. These provide the individual components. rRNA and proteins that combine in the cytoplasm to form ribosomes, which are the structures used by the cell to build proteins. Chromosomes Structure (8) - Chromosomes are made of Deoxyribonucleic Acid (DNA) and proteins. - These macromolecules form the classic double helix, which is like a twister ladder where alternate deoxyribose sugar and phosphate molecules form the sides and base pairs, held together by hydrogen bonds to form the rungs. - In their spread-out form, Chromosomes are referred to as Chromatin which is a combination of DNA and special proteins called histones. Function: - Chromosomes hold the genetic code for making proteins. - Only about 2% of chromosomes are coding structures(genes) that make proteins. The other 98% are non-coding DNA. - Prior to cell division, the DNA coils around the histones many times in succession. Each coiling makes the chromosomes shorter and thicker until they are clearly visible. - Each chromosome consists of two roughly symmetrical strands called chromatids joined by a structure called the centromere. When the coiling is complete, each chromatid is essentially identical to its partner and as soon as they are separated, they are referred to as chromosomes. Mitochondrian Structure (9) - Consists of a smooth outer membrane and a folded inner membrane with a cavity between them. - Inner membrane is folded into structures called cristae. - In the centre of the mitochondrion is a jelly-like substance called the matrix in which are found ribosomes and mitochondrial DNA. Function: - The ‘powerhouses’ of the cell, producing almost all the energy required by the cell. - The second stage of respiration, The Krebs cycle takes place in the matrix. - The final stage of respiration, the electron transport chain, takes place on the very large surface area of the cristae under the influence of an enzyme called ATP synthase. Ribosomes Found in large numbers in the liver. Structure - Small granular structures made of two sub-units. - Made of RNA (ribonucleic acid) and protein. - Found free in cytoplasm or attached to folded membranes Function - Protein synthesis - Free ribosomes make protein used by cell and those on tubes make proteins for export. Plant Cells Chloroplasts Structure (10) - double membrane - contain chlorophyll and DNA - self-duplicating Both mitochondria and chloroplasts have a double membrane and DNA. Having DNA supports the theory that chloroplasts and mitochondria were once independent prokaryotic organisms that lived symbiotically inside large eukaryotic cells. Function - used to make food(carbohydrates) by photosynthesis - light phase in grana and dark phase in stroma Cell wall Secreted by cell membrane. Structure - Made of cellulose. - Adjacent cells are cemented together by the middle lamella of pectin. Function - Gives strength and protection to the cell. - Controls cell growth and shape. - Prevents osmotic bursting of cell membrane. - Fully permeable to gases and water. Vacuoles Usually only one in plants. Very large & permanent. Small, temporary, and more in animals because they excrete their waste (often called vesicles). Structure - Fluid-filled spaces surrounded by a membrane Function - Temporary storage of food (sugars, amino acids, fats), water, salts (help in osmoregulation), pigments, tannins, gases (O2 & CO2) and excretory products. - The cell sap makes the cells turgid. Mandatory Practical – Preparing animal cells for examination using a light microscope Preparing Cheek Cells - Rinse Mouth with water to remove food debris - Rub a clean cotton bud against inside of cheek - Rub the bid against the centre of a clean microscope slide - Repeat with second sample - Add a drop of water to one sample and a drop of methylene blue stain to the other sample - Using a mounted needle, gently lower a clean coverslip at an angle of 45o over the specimen. - Wait 2-3 mins to allow the stain to be absorbed by the second sample - Place a drop of water to one side of the coverslip, and using a piece of tissue on the other side of the coverslip, draw the stain through and in this way remove the excess stain - Slides are ready to view Preparing Onion Cells - Cut an onion in half and separate two layers. You will find a very thin layer of cells attached to one surface. Pull off this layer and use a pair of sharp scissors to cut out two small squares about 0.5cm wide - Place the onion tissue layers into a petri dish of cold water - Using a clean paintbrush, place one on the centre of a clean microscope slide - Repeat with second sample - Add a drop of water to one sample and add a drop of iodine stain to the other sample - Using a mounted needle, gently lower a clean coverslip at an angle of 45o over the specimen. - Wait 2-3 mins to allow the stain to be absorbed by the second sample - Place a drop of water to one side of the coverslip, and using a piece of tissue on the other side of the coverslip, draw the stain through and in this way remove the excess stain - Slides are ready to view Mandatory Practical – Using a light microscope to view plant and animal cells - Place stained sample slide on the stage so that the specimen is over the hole in the centre - Secure the slide in place using the two spring clips - Turn on the light or adjust the mirror - Select the low-power objective lens (x4) - Using the coarse focus knob, bring the specimen into focus - Use the diaphragm to adjust the amount of light passing through to get the clearest view - Draw what you see, label with name of specimen and with the remark ‘low power’ - Select a higher-power objective and using only the fine focus knob, adjust the focus. You may have to use the diaphragm to adjust the amount of light passing through to get the clearest view - Repeat with the stained sample (11,12) Exam Questions 2014 – HL – Section B – Question 8 (a) Answer the following questions with reference to the microscope. (i) State the function of the part labelled A in the diagram. To bring the image into focus using the coarse focus wheel (ii) Lens E is marked 10× and lens O is marked 40×. A cell is viewed through lenses E and O. The image of the cell is 0.8 mm in diameter. What is the actual diameter of the cell? 0.8 ÷ 400 = 0.002 mm (b) Answer the following questions in relation to the procedures that you followed when preparing animal cells for examination with a light microscope. (i) Describe how you obtained a sample of cells. The inside of the mouth was swabbed using a cotton wool bud (ii) What stain did you use on the sample? Methylene blue (iii) Outline how you used the coverslip. The coverslip was lowered from a 45o angle using a mounted needle (iv) Explain why a coverslip is used. To protect the lens (v) Describe how you examined the cells using the microscope. - Microscope was set up with the lowest power objective lens in position - Image of the cells on the slide was focused using the coarse focus wheel and the low power lens - The high power lens was carefully moved into position - The image was focused using the fine focus wheel and the high power lens (vi) Draw a labelled diagram of the cells as seen at high magnification. 2005 – HL – Section B – Question 7 (a) State a function of each of the following components of a cell. (i) Ribosome – Protein Synthesis (ii) Cell membrane – Controls entry and exit of molecules (b) Answer the following questions in relation to the preparation, staining and microscopic observation of a slide of an animal cell. (i) What type of animal cell did you use? Human Cheek Cell How did you obtain the cell? By swabbing the inside of the mouth with a cotton wool bud (ii) Name the stain that you used – Methylene Blue Describe how you applied the stain – Use a dropper to add the methylene blue to the slide. Place a drop of clean water to one side of the coverslip and using a piece of tissue on the other side of the coverslip, draw the stain through to remove any excess stain (iii) After staining, a cover slip is placed on the slide. Give a reason for this – To protect the lens (iv) How did you apply the cover slip – At a 45o angle to the slide using a mounted needle Why did you apply it in this way – To prevent air bubbles (v) Describe the difference in colour or depth of colour, if any, between the nucleus and cytoplasm when the stained cell was viewed under the microscope - The colour of the nucleus is deeper as the cytoplasm is clear References Thoughtco.com Shmoop.com Pinterest.com Wfjclass.net Sciencenotes.org Istockphoto.com BYJUS.com Dreamstime.com BYJUS.com Toppr.com Wikipedia.com Microscopy-uk.org.uk