3.1 AS Unit: Cells, Exchange and Transport Module 1: Cells 1.1.1 Cell Structure Candidates should be able to: (a) state the resolution and magnification that can be achieved by a light microscope, a transmission electron microscope and a scanning electron microscope; Light Microscope Transmission Electron Microscope Scanning Electron Microscope Resolution 0.2 μ (200nm) 0.2nm 0.2nm Magnification 1500 / 2000 Over 500 000 250 000 (b) explain the difference between magnification and resolution; Resolution Magnification Resolving power the ability of an optical system to distinguish between two adjacent objects increases the apparent size of an object the degree of detail that can be seen with a microscope The resolving power is inversely proportional to the wavelength of the radiation used (i.e. the shorter the wavelength, the greater the resolution). (c) explain the need for staining samples for use in light microscopy and electron microscopy; Stains: - most biological structures are transparent - the stain gives a contrast between different structures - the stain combines with certain chemicals in the structure - Iodine solution: Starch blue-black - Eosin solution: cytoplasm pink - Feulgens agent - Aceto-orcein agent DNA dark red / purple (d) calculate the linear magnification of an image (HSW3); Page 1 of 8
(e) describe and interpret drawings and photographs of eukaryotic cells as seen under an electron microscope and be able to recognise the following structures: nucleus, nucleolus, nuclear envelope, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, ribosomes, mitochondria, lysosomes, chloroplasts, plasma (cell surface) membrane, centrioles, flagella and cilia; Key 1 Plasma Membrane 2 Smooth Endoplasmic Reticulum 3 Vacuole 4 Golgi Body 5 Mitochondrion 6 Ribosomes 7 Centriole 8 Rough Endoplasmic Reticulum Parts of the nucleus 9 Nuclear Envelope 10 Nucleolus 11 DNA / Chromosomes 12 Nuclear Pore 13 Nucleoplasm (f) outline the functions of the structures listed in (e); Organelle Present in plant cell Present in animal cell Function 1. Cell wall Yes No 2. Plasma membrane Yes Yes 3. Nucleus Yes Yes 4. Mitochondria Yes Yes strength, resist pressure created when water enters selectively controls the movement of substances into and out of cells contains DNA which holds the genetic information produces large amounts of A.T.P. by aerobic respiration Page 2 of 8
5. Chloroplast Yes No photosynthesis 6. Rough endoplasmic reticulum 7. Smooth endoplasmic reticulum Yes Yes protein synthesis Yes Yes synthesis of lipids 8. Golgi apparatus Yes Yes modification and packaging of proteins 9. Centriole Yes Yes organises spindle during mitosis 10. Flagella Yes Yes locomotion 11. Cilia Yes Yes chemical sensation, signal transduction (g) outline the interrelationship between the organelles involved in the production and secretion of proteins (no detail of protein synthesis is required); Proteins are created via a process called protein synthesis. This beings in the nucleus and then moves to the Golgi apparatus where it is modified and packaged. This means that certain chemical groups may be added to it. This protein can then be expelled if needed via a process called exocytosis or can be used within the cell. (h) explain the importance of the cytoskeleton in providing mechanical strength to cells, aiding transport within cells and enabling cell movement; The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm. The cytoskeleton is present in all cells; it was once thought this structure was unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion (using structures such as flagella, and cilia), and plays important roles in both intracellular transport (the movement of organelles, for example) and cellular division. Page 3 of 8
(i) compare and contrast, with the aid of diagrams and electron micrographs, the structure of prokaryotic cells and eukaryotic cells; Prokaryotic Cell There may also be lipid or glycogen granules within the cell, so look out for those! (for Eukaryotic cell, see part (e)) (j) compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of plant cells and animal cells See part (f) for the difference between plant and animal cells. Cell Membrane - highly selective region made of phospholipids - regulates the uptake and release of materials from the cell - is approximately 7.5nm in thickness - the outside contains glycocalyx which has unique markings for cell recognition - the advantage of this is that it has an immunological memory so it can learn which cells to kill therefore there is efficient recognition. - the disadvantage of this is that it means that there can be rejection in transplants Cellulose cell wall - Plant cells only - not part of the cell: extra cellular only - provides rigidity - fully permeable: prevents cell from rupturing o prevents the intake of excess water - polysaccharide Page 4 of 8
Cytoplasm - subdivided - the protoplasm is the name for all of the materials inside the cell membrane - the cytoplasm is the name for all the material inside the membrane but not the nucleus - i.e. all the ground materials including organelles are in the cytoplasm - the cytoplasm forms a cytoskeleton o 90% water and inorganic salts and organic molecules o organelles are absent in prokaryotic cells Mitochondria - size: 1μ wide, 2.5 μ long (would not be seen under light microscope) - numbers vary according to the type of cell (normally 1000 per cell) - muscle (and sperm) cells have more - site of ATP production - electron microscope reveals the internal structure - organelle bounded by a double membrane (which is highly folded) - giving rise to CRISTAE, which project into the interior of the organelle - cristae are involved in oxidative phosphorylation and electron transport - many of the enzymes are embedded in the wall of the cristae - interior o consists of organic matrix containing numerous chemical compounds o site of Krebs Cycle - DNA is present in mitochondria so that it can replicate itself - in plant mitochondria, the cristae are plate-like - in animal mitochondria, the cristae are finger-like Cell Wall - consists of many cellulose fibres - cemented together by a mixture of other organic substances - cellulose: polysaccharide (polymer of glucose) o consists of long chains of glucose molecules o bound by adjacent molecules - in the cell wall, there are around 2000 parallel cellulose molecules, which are packed to form microfibrils - there, in turn, are bundled together to form fibrils - the structure is like fibre glass in the sense that the cell wall has great strength due to the many strong fibres and glue that holds them together - All plant cells start by having a primary wall, which is flexible - this grows with the cell - the fibrils in this wall run in all directions - Most plant cells develop a secondary wall Page 5 of 8
- this is thicker than the primary wall - many additional layers are deposited outside the primary wall - in each layer of the secondary wall, the fibrils run mainly in the same direction - in older cells, more layers are laid down, but the fibrils run at different angles - the consequence of this is that the overall structure has great strength and prevents any further increase in size Example: Xylem - lignin is laid down - this further strengthens the secondary wall - this increases the strength of the supporting tissues (e.g. in trees and shrubs) Conclusion - cell wall has several functions: (a) rigidity and strength (i.e. resists expansion when the cells are turgid) (b) allows communication between cells (i.e. cytoplasmic connections plasmodesmata in cell wall) (c) forces cell to grow in a certain way (i.e. shape) (e.g. long tube xylem) Chloroplast - before the electron microscope, this was only seen as a body with a series of layers - with the electron microscope, lamellae are confirmed to be these layers - also: grana: stacks of densely packed membranes o linked by lamellae - stroma (cytoplasm of chloroplast) - double membrane (responsible for fixation of CO 2 ) (i.e. the site of enzymes, which fix CO 2 ) - in eukaryotes only - size: 3 10 μ in diameter - just visible with a light microscope (but is undifferentiated) - photosynthetic pigments are located on internal membranes (grana) (a) Membrane system - site of light reactions (photosynthesis) - chlorpophyll pigments - enzymes - electron carriers - flattened, fluid-filled sacs (called thylakoids which are stacked to form grana) - lamellae between grana (b) Stroma - site of dark reactions in photosynthesis - photosynthetic gel containing enzymes associated with calvin cycle and sugars and organic acids (c) Starch grains - excess sugars Page 6 of 8
Golgi apparatus (a.k.a. Golgi body) - series of complex tubules - flattened cisternae - present in cells which actively produce enzymes - modify and package proteins produced by ribosomes - e.g. carbohydrates are added to proteins in the golgi body to become glycoproteins, which are released into the cytoplasm and are fused with the surface membrane to be released outside. - the golgi body is also involved with the transportation of lipids within the cells and the formation of lysosomes (which contain digestive enzymes) - lysosomes fuse with and digest unwanted substances in the cells or old worn-out organelles released from golgi vesicle cisternae cytasol (between cysternae) starts from packages from the endoplasmic reticulum - protein molecules are transferred between cisterneae - secretory vesicles pass through the cytosol and fuse with adjacent cisterna Endoplasmic Reticulum - membrane-bound flattened sacs or tubules cisternae - form a network continuous with outer membrane of nuclear envelope - same structure as other membranes in cell - increases the surface area within the cell o sites of metabolic activity o channels for movement of substances o separate different activities of the cell proceeding simultaneously 2 types 1. Rough endoplasmic reticulum - the bulk of the endoplasmic reticulum - encrusted with ribosomes - isolates and transports proteins synthesised by the ribosomes - many of these proteins are not required by the cell, but are secreted by the cell (e.g. digestive enzymes, hormones etc.) - rough endoplasmic reticulum acts as a transport system for these products. 2. Smooth endoplasmic reticulum - not encrusted with ribosomes - involved in synthesis and transport of lipids and steroids Page 7 of 8
ribosomal Ribosomes - composed of RNA (rrna) - each consists of 3 sub-units one slightly larger than the other ( cottage loaf shape) - assembles amino acids into proteins - operates in conjunction with mrna - bound to endoplasmic reticulum - often several ribosomes collectively (polysome) - N.B. not all ribosomes are associated with the endoplasmic reticulum. In growing cells or cells that are making proteins for internal use, large numbers of ribosomes are found free in the cytoplasm. Lysosomes - formed by golgi body - they are vesicles containing enzymes - carry out intracellular digestion - 0.2μ 0.5μ wide - enzymes include nuclease, protease, lipase, acid phosphotase (i.e. hydrolases) o Function: to release enzymes outside the cell (exocytosis) to fuse with a particle taken in by exocytosis and digest it internally (residual bits may be expelled by exocytosis) Autophagy: digest old worn-out parts of cell / organelle Centrioles - not seen in plant cells - act as anchors for spindle fibres when they pull chromosomes apart Microtubules - fine tubular organelles making up the cytoskeleton in the cytoplasm (i.e. maintain shape) - straight and hollow of various lengths - made of protein sub-units called tubulin - used to make centrioles and spindles for cell division Page 8 of 8