2024 AQA A LEVEL BIOLOGY 3.2 CELLS EXAM WITH CORRECT ANSWERS, Exams of Advanced Education

Download 2024 AQA A LEVEL BIOLOGY 3.2 CELLS EXAM WITH CORRECT ANSWERS and more Exams Advanced Education in PDF only on Docsity! 2024 AQA A LEVEL BIOLOGY 3.2 CELLS EXAM WITH CORRECT ANSWERS    Structure of Nucleus - CORRECT-ANSWERS1. Nuclear envelope: double membrane surrounding nucleus, outer membrane continuous with the (R)ER of the cell. 2. Nuclear pores: allow the passage of larger molecules, such as mRNA, out of the nucleus. 3. Nucleoplasm: granular, jelly-like material making up the bulk of the nucleus. 4. Chromosomes: protein-bound, linear DNA. 5. Nucleolus: small spherical region(s) in nucleoplasm. Manufactures ribosomal RNA and assembles ribosomes. Function of the Nucleus - CORRECT-ANSWERS1. Controls cell's activities - produces mRNA and tRNA - protein synthesis. Controls entry and exit of materials, and contains nuclear reactions. 2. Retains genetic material in the form of DNA and chromosomes. 3. Manufactures ribosomal RNA and ribosomes. Define Monoclonal Antibodies - CORRECT-ANSWERSAntibodies produced from a single group of genetically identical B-cells. Monoclonal antibodies are identical in structure. List uses of monoclonal antibodies - CORRECT-ANSWERS1. Targeting cancer drugs. 2. Pregnancy testing. 3. Diagnosing HIV etc. Outline how monoclonal antibodies are used in targeting cancer drugs. - CORRECT-ANSWERS1. Monoclonal antibodies are made that will bind to tumour markers (cancer cell antigens), with anti-cancer drugs attached to antibody. 2. Antibodies bind to the tumour markers - drug will only accumulate in areas of body where there are cancer cells. 3. Therefore, side-effects of an antibody-based drug are lower than other drugs because they accumulate near specific cells. Outline how monoclonal antibodies are used in pregnancy testing (Medical Diagnosis). - CORRECT-ANSWERSApplication area contains antibodies for hCG, bound to a blue-coloured bead. 2. When hCG present in urine, hCG will bind to the antibody on the beads, forming an antigen-antibody complex. 3. Urine moves up the stick to test strip, carrying beads. 4. Test strip contains immobilised hCG antibodies. 5. Antibodies bind to hCG if present --> hCG-antibody complex with the blue beads attached becomes more concentrated --> turns blue. 6. If no hCG present, beads will pass through the test area --> no visible colour change. Outline how monoclonal antibodies are used in the ELISA test. - CORRECT- ANSWERSEnzyme-linked immunosorbent assay: => allows us to see if a patient has antibodies for a specific antigen, usually HIV. Indirect ELISA - used to see if a patient has HIV antibodies. 1. HIV antigen bound to bottom of a well in a well plate. 2. Patient's blood plasma sample (containing antibodies) added to the well. Any HIV-specific antibodies will bind to the antigen in well. 3. Well washed out to remove any unbound antibodies. 4. Secondary antibody attached to a specific enzyme which can attach to primary antibody added. Well washed out again to remove unbound secondary antibody. 5. Cell's enzyme used to make mRNA, from viral DNA, passing out through nuclear pores to ribosomes for translation, producing viral RNA and proteins ---> new HIV particles. 6. New HIV particles break away from Th-cell, using a part of the T-cell- surface membrane to form their lipid envelopes. How does HIV cause the symptoms of AIDS? - CORRECT-ANSWERS1. HIV particles kill/interfere with Th-cell, drastically reducing their numbers in the blood. 2. Th-cells cannot stimulate B-cells to produce antibodies or the Tc-cells that kill infected cells. => body is unable to produce an adequate immune response and becomes susceptible to other infections and cancers. Why are antibiotics innefective against viruses? - CORRECT- ANSWERSAntibiotics inhibit the enzymes required for the synthesis and assembly of the peptide cross-linkages in bacterial cell walls. This weakens the walls, making them unable to withstand pressure ---> water enters bacterium by osmosis naturally and cell dies. BUT Viruses lack their own metabolic pathways and cell structure and rely on those of their host cell: - antibiotics are ineffective because there are no metabolic mechanisms or structures for them to disrupt. Viruses also have a protein coat, not a murein cell wall ---> do not have sites where antibiotics can work. => once viruses are within an organism's own cells, antibiotics cannot reach them. Structure of Mitochondria - CORRECT-ANSWERS1. Double membrane surrounding organelle - controls entry and exit of material. 2. Cristae - extensions of the inner membrane, providing a large surface area for the attachment of enzymes and other proteins during respiration. 3. Matrix - makes up the remainder - contains proteins, lipids, ribosomes and DNA (allows mitochondria to produce own proteins) and some respiratory enzymes. Function of the Mitochondria - CORRECT-ANSWERS1. Sites of Krebs Cycle and oxidative phosphorylation pathway in aerobic respiration - responsible for ATP production. Structure of Chloroplasts - CORRECT-ANSWERSFound in plants and algae. 1. Chloroplast envelope - double plasma membrane, highly selective, surrounds the organelle. 2. Grana - stacks of disc-shaped thylakoid membrane. 3. Thylakoids - contain chlorophyll used in photosynthesis, can be linked by lamellae to other grana. 4. Stroma - fluid-filled matrix where Calvin Cycle takes place. Also contains starch grains. Functions of Chloroplasts - CORRECT-ANSWERSSite of Photosynthesis: LDR in thylakoid membranes. LIR in stroma. 1. Granal membranes provide a large SA for LDR - photosystems, e- carriers and enzymes etc. 2. Chloroplasts contain DNA and ribosomes - can quickly and easily manufacture some of the proteins needed for photosynthesis. Structure of Endoplasmic Reticulum - CORRECT-ANSWERS1. 3D system of sheet-like membranes - continuous with the outer membrane of the nuclear double membrane. 2. Membrane contains a network of tubules and flattened sacs called cisternae. 3. RER - ribosomes on the outer surface of the membranes. 4. SER - lacks ribosomes on its surface and is often more tubular in its appearance. Function of Endoplasmic Reticulum - CORRECT-ANSWERSRER 1. Large SA for protein/glycoprotein synthesis. 2. Provides a pathway for material transport throughout the cell, especially for proteins. SER 1. Synthesises, stores and transports lipids and carbohydrates. Structure of Golgi Apparatus - CORRECT-ANSWERS1. Compact system of flattened sacs and stacked membranes (cisternae). 2. Vesicles - modified proteins and lipids transported to cell membrane where they fuse with it, and then egest contents to the outside. Function of Golgi Apparatus - CORRECT-ANSWERS1. Form glycoproteins by adding carbs to proteins. 2. Produce secretory enzymes, such as those secreted by the pancreas - apparatus is developed in secretory cells, especially those in the small intestine. 3. Secrete carbs, such as cellulose for plant cell walls. 4. Transports, modifies and stores lipids. 5. Forms lysosomes. Structure of Lysosomes - CORRECT-ANSWERSGolgi vesicles with proteases, lipase and lysozymes. Functions of Lysosomes - CORRECT-ANSWERS1. Hydrolyse foreign material ingested by phagocytes. 2. Exocytosis of enzymes to destroy extra-cellular material. 3. Apoptosis - programmed cell death. Autolysis - breaking down cells after death. 4. Digest worn out organelles - can recycle chemicals. Structure of Ribosomes - CORRECT-ANSWERS1. Small cytoplasmic granules found in all cells, free-floating or associated with RER. 2. 80S - found in eukaryotic cells, slightly larger. 3. 70S - in prokaryotic cells, slightly smaller. Principles of Optical Microscopes - CORRECT-ANSWERSSimple convex glass lenses used in pairs in a compound light microscope to focus an object at a short distance by 1st lens, then magnified by 2nd lens. Can you describe the pros and cons of optical microscopes? - CORRECT- ANSWERSPros: - cheap - images in colour - no training required - live specimens Cons: - low magnification x1500 - low resolution - 2D images Principles of Transmission Electron Microscopes - CORRECT-ANSWERS1. Electron gun produces e- beam, focused onto the specimen by a condenser electromagnet. 2. 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. Can you describe the pros and cons of transmission electron microscopes? - CORRECT-ANSWERSPros: - high resolution images - high magnification - visible internal structures Cons: - expensive - training is required - no colour images - 2D images - only thin specimens Principles of Scanning Electron Microscopes - CORRECT-ANSWERS1. Beam of e- directed onto surface of specimen - passed back and forth across specimen. 2. e- scattered by specimen - scattering pattern analysis allows us to get a 3D image. Can you describe the pros and cons of scanning electron microscopes? - CORRECT-ANSWERSPros: - 3D images - high magnification - high resolution - thick specimens Cons: - expensive - training is required - no colour images How do you prepare a slide for an optical microscope? - CORRECT- ANSWERS1. Pipette a drop of water onto the slide 2. Use tweezers to place a thin section of your specimen on top of the droplet 3. Add a drop of a stain 4. Add a cover slip - remove all air bubbles What is the difference between magnification and resolution? - CORRECT- ANSWERSMAGNIFICATION - 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. What is the formula to calculate Magnification? - CORRECT-ANSWERS Can you describe the principles of cell fractionation and ultracentrifugation in separating cell components? - CORRECT-ANSWERS1. Homogenisation - tissue is broken up in a cold, isotonic buffer solution to release the organelles into a solution 2. Filtration - the homogenised cell solution is filtered through a gauze - this separates any large cell debris 3. Ultracentrifugation - the cell fragments are poured into a test tube and placed in a centrifuge and spun at a low speed - a thick sediment - the pellet - is at the bottom of the tube and the fluid above is the supernatant - the supernatant is drained into a new tube and spun again at a higher speed - a new pellet forms and again, the supernatant is drained off and spun again at an even higher speed - this process is repeated at higher speeds each time until all the organelles are separated out Why is a cold, isotonic buffer needed? - CORRECT-ANSWERSCOLD - to reduce enzyme activity that could break down organelles. ISOTONIC - same water potential as tissue sample - to prevent water moving in or out of the cells by osmosis, causing lysis. BUFFERED - to prevent changes in pH which could affect/denature enzymes. How are the organelles separated out during centrifugation? - CORRECT- ANSWERSThey are separated in order of mass and the order is usually: - nuclei - mitochondria - lysosomes - endoplasmic reticulum - ribosomes Define Virus - CORRECT-ANSWERSAcellular, non-living particles. Smaller than bacteria. Can you explain that viruses are acellular and non-living? - CORRECT- ANSWERSViruses are nucleic acids surrounded by protein so they are not living. Can you describe the structure of virus particules? - CORRECT-ANSWERS- no plasma membrane, cytoplasm, ribosomes - a core of genetic material : DNA or RNA - capsid surrounding core - attachment proteins around edge of capsid Outline the differences between mitosis and meiosis. - CORRECT- ANSWERSMitosis results in 2 genetically identical diploid daughter cells. Meiosis results in 4 genetically different haploid daughter cells. List the stages of Mitosis - CORRECT-ANSWERSInterphase Prophase Metaphase Anaphase Telophase Outline Interphase - CORRECT-ANSWERSPrecedes Mitosis 1. Cell is not dividing. 3. Protein channels - water filled tubes allowing water-soluble ions to diffuse across the membrane. 4. Protein carriers - bind to ions or molecules like glucose/amino acids - then change shape to move these molecules across the membrane. 5. Help cells adhere together. 6. Form cell-surface receptors for identifying cells. Outline the role of cholesterol in cell-surface membrane structure. - CORRECT-ANSWERSWithin the phospholipid bilayer of cell-surface membranes: 1. Reduce lateral movement of other molecules (including phospholipids) - pulls together the fatty acid tails, limiting movement without making the membrane too rigid. 2. Make the membrane less fluid at high temperatures. 3. Prevent leakage of water and dissolved ions from the cell, as cholesterol molecules are very hydrophobic. Outline role of glycolipids in cell-surface membrane structure. - CORRECT- ANSWERSCarbohydrate covalently bonded to a lipid in the membrane: 1. Carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell - there acts as a cell-surface receptor for specific chemicals (ABO blood system) 2. Help maintain the stability of the membrane. 3. Help cells to attach to one another and so form tissues. Outline the role of glycoproteins in cell-surface membrane structure - CORRECT-ANSWERSCarbohydrate chains attached to many proteins on the outer surface of the cell-surface membrane. 1. Cell-surface receptors for hormones and neurotransmitters. 2. Help cells to attach to one another and so form tissues. 3. Allow cells to recognise one another - lymphocytes can recognise an organism's own cells. Comment on the permeability of the cell-surface membrane. - CORRECT- ANSWERSControls the movement of substances into/out of the cell. Most molecules don't freely diffuse across it because many are: 1. Not lipid-soluble 2. Too large to pass through 3. Same charge as protein channel charges - repelled even if small 4. 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. - CORRECT-ANSWERS= 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 - CORRECT-ANSWERSThe net movement of molecules/ions/particles from a region of higher concentration to a region of lower concentration. Define Facilitated Diffusion - CORRECT-ANSWERSThe movement of larger/charged/polar species made easier by protein channels/carriers that span the membrane. It is a passive process, only difference is that fd occurs at specific points on the plasma membrane where there are protein molecules. Comment on the roles of the proteins involved in facilitated diffusion. - CORRECT-ANSWERSCHANNEL 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 - CORRECT-ANSWERSThe 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 the water potential of a cell - CORRECT-ANSWERSThe water potential is the pressure created by water molecules. The addition of water will lower the wp. wp = 0 when pure water at 298K, standard conditions. Water will move towards the area of lowest wp by osmosis, until an equilibrium is established. Comment on osmosis in plant cells when water moves in/out. - CORRECT- ANSWERSProtoplast (cell parts within cell wall) pushes on cell wall when swells - turgid. If water leaves by osmosis, a point is reached where protoplast no longer presses on cellulose cell wall - incipient plasmolysis. Further loss of water by osmosis -> cell contents shrink further and protoplast pulls away from cell wall - cell is said to be plasmolysed. Define Active Transport - CORRECT-ANSWERSThe movement of particles into/out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins. Outline how active transport takes place - CORRECT-ANSWERS1. Molecules bind to a carrier protein and ATP attaches to the membrane protein on the inside of cell/organelle. 2. Binding of phosphate ion to protein changes shape, allowing access for molecules to inside of the membrane but closed to the outside. Outline the process of Co-transport in the ileum - CORRECT-ANSWERS1. Na+ actively transported out of epithelial cells into blood via Na+/K+ pumps (carrier molecules in epithelial cell-surface membrane). 2.Increased Na+ conc maintained in lumen of intestine than in epithelial cells. 3. Na+ diffuse into epithelial cells down Na+ gradient through a co-transport protein ---> carrying glucose (/amino acids) with them. 4. Glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier, against their conc gradient. => Indirect active transport - as [Na+] gradient provides energy for process, not ATP - but ATP needed for 1. Outline the process of Cell-Mediated Immunity - CORRECT-ANSWERS1. Pathogens invade body cells/phagocytes. 2. Phagocyte places pathogen's antigens on its own cell-surface membrane. 3. Receptors on specific T-Cell complementary to this antigen. 4. Attachment activates rapid T-cell division by mitosis, to form clones of genetically identical cells. 5. Cloned T-Cells: a) stimulate phagocytes to engulf pathogens by phagocytosis. b) activate Tc-Cells which produce perforin protein which make pathogen membrane freely permeable. c) develop into memory cells, enabling a rapid secondary immune response to future infections by the same pathogen. d) stimulate clonal selection of B-Cells and stimulate them to secrete their antibody. What are plasma cells? - CORRECT-ANSWERSCells which secrete antibodies (usually) into blood plasma. They only survive for a few days - lead to destruction of pathogen. What are memory cells? - CORRECT-ANSWERSCells which circulate in blood and tissue fluid - divide rapidly into plasma cells and more memory cells -> more antibodies at a faster rate (faster response). Define humoral immunity - CORRECT-ANSWERSThis involves B-cells and antibodies, soluble in blood and tissue fluids of body. The B-cells produce specific antibodies, specific to one antigen. Outline the process of humoral immunity - CORRECT-ANSWERS1. Surface antigens of an invading pathogen are taken up by a B-cell. 2. B-cell processes antigens and presents them on its surface. 3. Th-cells (activated earlier) attach to the processed antigens on the B-cell and activate it. 4. B-cell is now activated to divide by mitosis to give a clone of plasma cells. 5. Cloned plasma cells produce monoclonal antibody and secrete it; antibody exactly complementary to antigen on pathogen's surface. 6. Antibody attaches to antigens on the pathogen + destroys them by marking them for phagocytosis. 7. Some B-cells then develop into memory cells, circulating in blood. What is the primary immune response? - CORRECT-ANSWERSProduction of antibodies and memory cells from new B-cells, after antigen enters body for the first time. What is the secondary immune response? - CORRECT-ANSWERSQuicker, stronger immune response due to memory cells -> clonal selection happens faster, B-memory cells activated and divide into plasma cells, producing necessary antibody. Memory T-cells activated and divide into correct T-cells to kill cells presenting the antigen. Define antibody - CORRECT-ANSWERSA protein produced by lymphocytes in response to the presence of the appropriate antigen The structure of an antibody - CORRECT-ANSWERS- 4 polypeptide chains - antigen-binding sites. - constant and variable regions (variable region dependant on antibody). - receptor binding site. Outline how antibodies help to destroy pathogens - CORRECT-ANSWERSTheir 2 binding sites mean that antibodies can cause agglutination of the bacterial cells. This makes it easier for phagocytes to locate them, as not spread out throughout the body. They then serve as markers, stimulating phagocytes to engulf and destroy the bacterial cells to which the antibodies are attached.