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AQA A-Level Biology. 3.2 - Cells. May
2024 Exam Review
Structure of Nucleus. - Answer>> 1. Nuclear envelope: double membrane surrounding nucleus, outer membrane continuous with the (R)ER of the cell.
- Nuclear pores: allow the passage of larger molecules, such as mRNA, out of the nucleus.
- Nucleoplasm: granular, jelly-like material making up the bulk of the nucleus.
- Chromosomes: protein-bound, linear DNA.
- Nucleolus: small spherical region(s) in nucleoplasm. Manufactures ribosomal RNA and assembles ribosomes. Function of Nucleus. - Answer>> 1. Controls cell's activities - produces mRNA and tRNA - protein synthesis. Controls entry and exit of materials, and contains nuclear reactions.
- Retains genetic material in the form of DNA and chromosomes.
- Manufactures ribosomal RNA and ribosomes. Structure of Mitochondria. - Answer>> 1. Double membrane surrounding organelle - controls entry and exit of material.
- Cristae - extensions of the inner membrane, providing a large surface area for the attachment of enzymes and other proteins during respiration.
- Matrix - makes up the remainder - contains proteins, lipids, ribosomes and DNA (allows mitochondria to produce own proteins) and some respiratory enzymes. Functions of Mitochondria. - Answer>> 1. Sites of Krebs Cycle and oxidative phosphorylation pathway in aerobic respiration - responsible for ATP production. NB = found in high numbers in metabolically active cells which require much ATP. Structure of Chloroplasts. - Answer>> Found in plants and algae.
- Chloroplast envelope - double plasma membrane, highly selective, surrounds the organelle.
- Grana - stacks of disc-shaped thylakoid membrane.
- Thylakoids - contain chlorophyll used in photosynthesis, can be linked by lamellae to other grana.
- Stroma - fluid-filled matrix where Calvin Cycle takes place. Also contains starch grains. Functions of Chloroplasts. - Answer>> Site of Photosynthesis: LDR in thylakoid membranes. LIR in stroma.
- Granal membranes provide a large SA for LDR - photosystems, e- carriers and enzymes etc.
- Vesicles - modified proteins and lipids transported to cell membrane where they fuse with it, and then egest contents to the outside. Functions of Golgi Apparatus. - Answer>> 1. Form glycoproteins by adding carbs to proteins.
- Produce secretory enzymes, such as those secreted by the pancreas - apparatus is developed in secretory cells, especially those in the small intestine.
- Secrete carbs, such as cellulose for plant cell walls.
- Transports, modifies and stores lipids.
- Forms lysosomes. NB = Golgi Vesicles are 'pinched off' from golgi cisternae. Structure of Lysosomes. - Answer>> Golgi vesicles with proteases, lipase and lysozymes. Functions of Lysosomes. - Answer>> 1. Hydrolyse foreign material ingested by phagocytes.
- Exocytosis of enzymes to destroy extra-cellular material.
- Apoptosis - programmed cell death. Autolysis - breaking down cells after death.
- Digest worn out organelles - can recycle chemicals. NB = very abundant in secretory cells and phagocytes.
Structure of Ribosomes. - Answer>> 1. Small cytoplasmic granules found in all cells, free-floating or associated with RER.
- 80S - found in eukaryotic cells, slightly larger.
- 70S - in prokaryotic cells, slightly smaller.
- 2 Subunits - large and small - contain ribosomal RNA and proteins. Functions of Ribosomes. - Answer>> Carry out translation stage of protein synthesis to produce polypeptides. Structure of Cell Wall. - Answer>> Found in plants, algae and fungi.
- Cellulose microfibrils embedded in a matrix - contribute to overall cell wall strength are considerably strong. and other polysaccharides.
- Middle lamella - marks the boundary between adjacent cell walls and cements adjacent cells together. NB= made of nitrogen-containing chitin in fungi, and made of the glycoprotein murein in bacteria. Functions of Cell Wall. - Answer>> 1. (Cellulose) - to provide mechanical strength to prevent cell wall bursting under pressure created by osmotic entry of water.
- To provide mechanical strength to the cell as a whole.
- Allows water to pass along it - contributes to the movement of water through the plant.
Difference between magnification and resolution? - Answer>> Magnification = increasing the size of an image. Up until the limit of resolution, an increase in magnification = an increase in detail. Resolution = minimum distance apart that two objects can be for them to appear as separate items. Need to appreciate that... - Answer>> ...there was a considerable period of time during which the scientific community distinguished between organelles and artefacts. artefacts = (something in a scientific experiment present due to how expt. was prepared or investigated). Why is cell fractionation needed? - Answer>> Needed to study the structure and function of the various organelles that make up cells. We need a large number of isolated organelles - can get them via cell fractionation. Define cell fractionation. - Answer>> The process in which cells are broken up and the different organelles they contain are separated out. Describe/Outline the process of Homogenisation. Why a cold, isotonic, buffered solution? - Answer>> 1. Tissues placed in a cold, isotonic (relative to tissue), buffered solution.
- Cells then broken up by a homogeniser/blender - releases organelles from cell - resultant fluid = homogenate - then filtered to remove any complete cells or large pieces of debris.
Cold - to reduce enzyme activity, such as lysozymes, that could break down organelles. Isotonic - same water potential as tissue sample - to preven water moving in or out of the cells by osmosis, causing lysis. Buffered - to prevent changes in pH which could affect/denature enzymes. Describe/Outline the process of Ultracentrifugation. - Answer>> Principles of Optical Microscopes. - Answer>> Simple convex glass lenses used in pairs in a compound light microscope - focuses object at a short distance by 1st lens, then magnified by 2nd lens. Limitations of Optical Microscopes. - Answer>> Light has a relatively long wavelength - low resolution. Can only distinguish between objects 0.2 micrometres apart. Principles of Transmission Electron Microscopes. - Answer>> 1. Electron gun produces e- beam, focused onto specimen by a condenser electromagnet.
- Beam passes through a thin section of the specimen from below. Parts absorb e- and appear dark; others let e- pass through and appear bright - produces image on screen - photomicrograph. Limitations of Transmission Electron Microscopes. - Answer>>
- Can't work on living specimens - needs to be in a vacuum.
- Complex staining process.
- Image not in colour.
Ensure method avoids trapping air bubbles! Graticule? Why needed? - Answer>> Glass disc with an etched scale placed in the eyepiece of a microscope. Needed to measure size of objects under objective lens, need to calibrate the eyepiece graticule - each objective lens will magnify to a different degree. Describe how to calibrate the eyepiece graticule. - Answer>> 1. Use a stage micrometer = special microscope slide with an etched scale - line up scales on graticule and micrometer.
- Once lined up, can calculate length of divisions on eyepiece graticule. x40 mag gives 25microm per graticule unit therefore, x400 mag gives 25/10microm per graticule unit. NB = Only need to calibrate once, providing the same objective lens is used. How are cells specialised in complex multicellular organisms? - Answer>> Cells initially all identical in an embryo but as they develop, the switching on/off of certain genes takes place, leading to changes in the organelle numbers and shapes of cells. Define tissue. Give an example. - Answer>> Collection of similar cells that are aggregated together and work together to perform a specific function.
Example = epithelial tissue - consists of sheets of cells, lining the surfaces of organs, often having a protective or secretory function. Define organ. Comment on the difference between capillaries, veins and arteries, relative to the term organ. - Answer>> Combination of aggregated tissues that are co-ordinated together to perform a variety of functions, one of which is the predominant major function. While capillaries, veins and arteries all have the same major function, i.e.e carrying blood, capillaries are not organs, unlike veins and arteries as they are made up of only one tissue - epithelium. Define organ system. Give examples. - Answer>> Organs working together as a single unit - systems may be grouped together to perform particular functions more efficiently. Digestive, Respiratory, Circulatory systems etc. Define eukaryotic cell. - Answer>> Larger cells with a true nucleus bounded by nuclear membrane/nuclear envelope. Define prokaryotic cell. - Answer>> Smaller, have no true nucleus or nuclear envelope. List features of prokaryotic/bacterial cells. - Answer>> 1. No true nucleus, only an area where DNA is found.
- DNA not associated with proteins.
List features of viruses. - Answer>> 1. Contain nucleic acids (DNA/RNA).
- Need to hijack host cell's machinery to reproduce and multiply.
- Enclosed in a protein coat, called a capsid.
- Some viruses, like HIV, also have a further lipid envelope.
- Capsid (if lipid envelope not present) has attachment proteins which are essential to allow the viruses to identify and attach to a host cell. Outline the differences between mitosis and meiosis. - Answer>> Mitosis results in 2 genetically identical diploid daughter cells. Meiosis results in 4 genetically different haploid daughter cells. List stages of mitosis. - Answer>> Interphase Prophase Metaphase Anaphase Telophase Outline Interphase. - Answer>> Precedes Mitosis
- Cell is not dividing.
- Considerable cellular activity - replication of DNA, two copies on centromere. Outline Prophase. - Answer>> 1. Chromosomes become more visible, thicken.
- Centrioles move to opposite ends of the cell (poles).
- Spindle fibres develop from each of the centrioles (spindle apparatus).
NB = plants lack centrioles, but are not essential as they still have the spindle apparatus.
- Nucleolus disappears and the nuclear envelope breaks down, leaving the chromosomes free in cell cytoplasm. Outline Metaphase. - Answer>> 1. Chromosomes seen to be made up of two chromatids.
- Microtubules attach to centromere - chromosomes pulled to the cell equator where they line up. Outline Anaphase. - Answer>> 1. Centromeres divide, separating each pair of sister chromatids.
- Chromatids pulled to their respective poles as spindles contract, centromeres first - v-shaped. NB = Mitochondria provide energy; gather around spindle. Mitosis can be prevented by introducing spindle-destroying chemicals. Outline Telophase. - Answer>> 1. Chromosomes reach their respective poles and then uncoil, become long and thin again - chromosomes again.
- Spindle fibres disintegrate; nuclear envelope and nucleolus reform.
- Cytoplasm divides in cytokinesis. Outline the process of Binary Fission. - Answer>> How prokaryotic cells divide:
- Circular DNA molecule replicates, and both copies attach to the cell membrane.
- Plasmids also replicate.
G2 - cell keeps growing and proteins needed for cell division are made. Mitosis. Cytokinesis. How does cancer arise? Difference between benign and malignant? - Answer>> Result of gene damage controlling mitosis and cell cycle. Mutant cells are structurally and functionally different - most mutant cells die but surviving mutant cells become tumours. Malignant - grow rapidly, less compact, more likely to be life- threatening. Benign - grow more slowly, more compact, less likely to be life- threatening. How can cancer be treated? - Answer>> Involves killing dividing cells by blocking a part of the cell cycle - cell division and therefore cancer growth ceases. Chemo disrupts cell cycle by preventing DNA replication or by inhibiting the metaphase stage of mitosis by interfering with spindle formation. Suggest why we don't use larger or more frequent doses to kill more cancer cells. - Answer>> Not given more frequently because healthy cells would not be able to increase their numbers to near normal again between treatments - numbers would decline more rapidly and possibly kill the patient. Not in higher doses because even more healthy cells killed each time...(same as above).
Outline role of phospholipids in cell-surface membrane structure. - Answer>> Their hydrophilic/hydrophobic interactions lead to the formation of a phospholipid bilayer.
- Allow lipid-soluble substances to enter/exit cell.
- Prevent water-soluble substances entering and leaving cell.
- Make the membrane flexible and self-sealing. Outline role of proteins in cell-surface membrane structure. - Answer>> Some in surface of bilayer:
- Act to provide mechanical support to membrane.
- Along with glycolipids, act as cell receptors for molecules such as hormones. Some span the entire membrane:
- Protein channels - water filled tubes allowing water-soluble ions to diffuse across the membrane.
- Protein carriers - bind to ions or molecules like glucose/amino acids - then change shape to move these molecules across the membrane.
- Help cells adhere together.
- Form cell-surface receptors for identifying cells.
Comment on the permeability of the cell-surface membrane. - Answer>> Controls the movement of substances into/out of the cell. Most molecules don't freely diffuse across it because many are:
- Not lipid-soluble
- Too large to pass through
- Same charge as protein channel charges - repelled even if small
- Charged/polar - can't pass through the non-polar hydrophobic tails in the phospholipid bilayer. Explain the "fluid-mosaic" model of the cell-surface membrane structure. - Answer>> = arrangement of all the various molecules combined into the structure. Fluid = membrane is flexible and can constantly change in shape as individual phospholipid molecules can move relative to one another. Mosaic = Proteins embedded in the phospholipid bilayer vary in shape, size + pattern like tiles in a mosaic. Define diffusion. Give the equation for Fick's Law. - Answer>> = The net movement of molecules/ions/particles from a region of higher concentration to a region of lower concentration. Fick's Law: diffusion rate = (SA x gradient x permeability) / (membrane thickness).
Define facilitated diffusion. Comment on the roles of the proteins involved in facilitated diffusion. - Answer>> Movement of larger/charged/polar species made easier by protein channels/carriers that span the membrane. NB=Passive process, only difference is that fd occurs at specific points on the plasma membrane where there are protein molecules. Channel Proteins = Water-filled hydrophilic channels allowing specific water-soluble ions to pass through - selective channels - only open when specific ion binds to protein causing it to change shape -> open and closed on different sides of the membrane. Carrier Proteins = Molecule specific to the protein binds, changes shape of protein -> molecule released to the inside of the membrane. Define osmosis. - Answer>> The net passage of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane. Comment on water potential. Define haemolysis. Define plasmolysis. - Answer>> = Pressure created by water molecules. Addition of water will lower wp. wp = 0 when pure water at 298K, standard conditions.
