AQA A Level Biology Topic 2 Cells PDF

AQA A Level Biology Topic 2 Cells Model answer notes by @biologywitholivia Topic Sub-topic Understand Memorise Practise 2.1 Cell 2.1.1 Structure of eukaryotic cell...

AQA A Level Biology Topic 2 Cells Model answer notes by @biologywitholivia Topic Sub-topic Understand Memorise Practise 2.1 Cell 2.1.1 Structure of eukaryotic cells structure 2.1.2 Structure of prokaryotic cells and of viruses 2.1.3 Methods of studying cells 2.2 2.2 All cells arise from other cells All cells arise from other cells Required practical 2 2.3 2.3 Transport across cell membranes Transport across cell Required practical 3 membranes Required practical 4 2.4 2.4 Cell recognition and the immune system Cell recognition & the immune system AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia 2.1 Cell structure 2.1.1 Structure of eukaryotic cells What are the distinguishing features of eukaryotic cells? Cytoplasm containing membrane-bound organelles So DNA enclosed in a nucleus Describe the general structure of eukaryotic cells Algal and fungal cells are similar to plant cells. See differences above. Describe the structure of the cell-surface membrane More detail covered in 2.3 Transport across cell membranes. Describe the function of the cell-surface membrane Selectively permeable → enables control of passage of substances in / out of cell Molecules / receptors / antigens on surface → allow cell recognition / signalling 2 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe the structure of the nucleus Describe the function of the nucleus Holds / stores genetic information which codes for polypeptides (proteins) Site of DNA replication Site of transcription (part of protein synthesis), producing mRNA Nucleolus makes ribosomes / rRNA Describe the structure of a ribosome Made of ribosomal RNA and protein (two subunits) Not a membrane-bound organelle Describe the function of a ribosome Site of protein synthesis (translation) Describe the structure of rough (rER) & smooth endoplasmic reticulum (sER) Describe the function of rER and sER rER Ribosomes on surface synthesise proteins Proteins processed / folded / transported inside rER Proteins packaged into vesicles for transport eg. to Golgi apparatus sER Synthesises and processes lipids Eg. cholesterol and steroid hormones 3 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe the structure of Golgi apparatus and Golgi vesicles Describe the function of Golgi apparatus and Golgi vesicles Golgi Modifies protein, eg. adds carbohydrates to produce glycoproteins apparatus Modifies lipids, eg. adds carbohydrates to make glycolipids Packages proteins / lipids into Golgi vesicles Produces lysosomes (a type of Golgi vesicle) Golgi Transports proteins / lipids to their required destination vesicles Eg. moves to and fuses with cell-surface membrane Describe the structure of lysosomes Describe the function of lysosomes Release hydrolytic enzymes (lysozymes) To break down / hydrolyse pathogens or worn-out cell components Describe the structure of mitochondria Describe the function of mitochondria Site of aerobic respiration To produce ATP for energy release Eg. for protein synthesis / vesicle movement / active transport 4 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe the structure of chloroplasts in plants and algae Describe the function of chloroplasts in plants and algae Absorbs light energy for photosynthesis To produce organic substances eg. carbohydrates / lipids Describe the structure of the cell wall in plants, algae and fungi Composed mainly of cellulose (a polysaccharide) in plants / algae Composed of chitin (a nitrogen-containing polysaccharide) in fungi Describe the function of the cell wall in plants, algae and fungi Provides mechanical strength to cell So prevents cell changing shape or bursting under pressure due to osmosis Describe the structure of the cell vacuole in plants Describe the function of the cell vacuole in plants Maintains turgor pressure in cell (stopping plant wilting) Contains cell sap → stores sugars, amino acids, pigments and any waste chemicals 5 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe how eukaryotic cells are organised in complex multicellular organisms In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Tissue Group of specialised cells with a similar structure working together to perform a specific function, often with the same origin. Organ Aggregations of tissues performing specific functions. Organ system Group of organs working together to perform specific functions. Describe how you can apply your knowledge of cell features / organelles to explain adaptations of eukaryotic cells General answer format: [Named cell] has many [named organelle, eg. ribosomes] To [link organelle function to cell function eg. increase rate of protein synthesis, making many antibodies] Exam insight: common mistakes ❌ Mistake Explanation “Mitochondria make energy in Energy can’t be made, only released. You also need to specify that ATP respiration.” (the ‘energy currency’ of cells) is made, which releases energy. “The nucleus controls cell This is low-level GCSE standard. Instead, refer to processes related to activities.” the DNA it stores, such as DNA replication or transcription. “A cell is adapted for protein Almost all cells have ribosomes, so this is not an adaptation. If a cell has synthesis by having ribosomes.” a high rate of protein synthesis, it will have more ribosomes. “[Named cells] have many vesicles This is often too vague. Use the information in the question to identify for transport.” where the vesicles are transporting molecules to. “The rER…” The full name of the rough endoplasmic reticulum is given in the specification with no abbreviation offered as an alternative, so ‘rER’ alone is normally not enough to get a mark. *Not being able to label an electron Students find this difficult. Practise labelling chloroplast and micrograph of an organelle.* mitochondria electron micrographs, as these are common. 6 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia 2.1.2 Structure of prokaryotic cells and of viruses What are the distinguishing features of prokaryotic cells? Cytoplasm lacking membrane-bound organelles Examples of prokaryotic organisms: So genetic material not enclosed in a nucleus bacteria and archaea (always unicellular) Describe the general structure of prokaryotic cells Compare and contrast the structure of eukaryotic and prokaryotic cells Eukaryotic cell Prokaryotic cell Has membrane-bound organelles No membrane-bound organelles eg. mitochondria, endoplasmic reticulum eg. no mitochondria, endoplasmic reticulum Has a nucleus No nucleus Containing DNA DNA is is free in cytoplasm DNA is long & linear DNA is short & circular & associated with histone proteins & not associated with proteins Larger (80S) ribosomes (in cytoplasm) Smaller (70S) ribosomes Cell wall only in plants, algae and fungi Cell wall in all prokaryotic cells Containing cellulose or chitin Containing murein, a glycoprotein Plasmids / capsule never present Plasmids, flagella and a capsule (sometimes flagella) sometimes present Larger overall size Much smaller overall size 7 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Explain why viruses are described as acellular and non-living Acellular - not made of cells, no cell membrane / cytoplasm / organelles Non-living - have no metabolism, cannot independently move / respire / replicate / excrete Describe the general structure of a virus particle 1. Nucleic acids surrounded by a capsid (protein coat) 2. Attachment proteins allow attachment to specific host cells 3. No cytoplasm, ribosomes, cell wall, cell-surface membrane etc. 4. Some also surrounded by a lipid envelope eg. HIV Exam insight: common mistakes ❌ Mistake Explanation “All prokaryotes have plasmids, a These aren’t present in all prokaryotic cells, so be careful when using capsule and flagella.” them in comparison questions. “Bacteria have a capsid and Bacterial (‘slime’) capsules provide protection and help with adhesion. viruses have a capsule.” Viral capsids (‘protein coat’) protect genetic material. “Prokaryotic cells have Prokaryotic cells have no membrane-bound organelles, so no mitochondria to make ATP.” mitochondria. They still perform respiration to make ATP. *Not making direct comparisons Statements in compare / contrast questions need to reference both between cell types.* sides, eg. ‘A prokaryotic cell has… whereas a eukaryotic cell has…’ "Prokaryotic cells have small / 70S Small/70S ribosomes are found in both prokaryotic cells and the ribosomes; eukaryotic cells don't." mitochondria and chloroplasts of eukaryotic cells. “Viruses are acellular because they This relates to viruses being non-living. Viruses are acellular as they are cannot reproduce on their own.” not made of cells and don’t have cell organelles. “All viruses have a lipid envelope.” HIV has a lipid envelope, but not all viruses do. 8 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia 2.1.3 Methods of studying cells Describe the difference between magnification and resolution Magnification = number of times greater image is than size of the real (actual) object ○ Magnification = size of image / size of real object Resolution = minimum distance apart 2 objects can be to be distinguished as separate objects Compare the principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes Optical microscope Transmission Scanning electron microscope (TEM) electron microscope (SEM) Light focused using Electrons focused using Electrons focused using glass lenses electromagnets electromagnets Light passes through specimen, Electrons pass through specimen, Electrons deflected / bounce different structures absorb denser parts absorb more and off specimen surface different amounts & wavelengths appear darker Generates a 2D image of a Generates a 2D image of a Generates a 3D image cross-section cross-section of surface Low resolution due to long Very high resolution due to short High resolution due to short wavelength of light wavelength of electrons wavelength of electrons Can’t see internal structure of Can see internal structures of Can’t see internal structures organelles or ribosomes organelles and ribosomes Specimen = thin Specimen = very thin Specimen does not need to be thin Low magnification (x 1500) High magnification (x 1,000,000) High magnification (x 1,000,000) Can view living organisms Can only view dead / dehydrated Can only view dead / dehydrated specimens as uses a vacuum specimens as uses a vacuum Simple preparation Complex preparation so Complex preparation so artefacts often present artefacts often present Can show colour Does not show colour Does not show colour Students should be able to appreciate that there was a considerable period of time during which the scientific community distinguished between artefacts (eg. dust, air bubbles occurring during preparation) and cell organelles. To overcome this, scientists prepared specimens in different ways. If an object was seen with one technique but not another, it was more likely to be an artefact than an organelle. 9 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia List the steps in calculations involving magnification, real size & image size 1 Note formula / rearrange if necessary (I = AM) 2 Convert units if necessary - image and actual size must be in same unit 3 Calculate answer and check units required or if standard form etc. is required Note - a scale bar may also be used. Describe how to convert between different units Unit Equivalent number of metres Centimetre (cm) 1/100 m 0.01 m 10-2 m Millimetre (mm) 1/1000 m 0.001 m 10-3 m Micrometre (µm) 1/1000000 m 0.000001 m 10-6 m Nanometre (nm) 1/1000000000 m 0.000000001 m 10-9 m Describe how the size of an object viewed with an optical microscope can be measured 1. Line up (scale of) eyepiece graticule with (scale of) stage micrometre 2. Calibrate eyepiece graticule - use stage micrometre to calculate size of divisions on eyepiece graticule 3. Take micrometre away and use graticule to measure how many divisions make up the object 4. Calculate size of object by multiplying number of divisions by size of division 5. Recalibrate eyepiece graticule at different magnifications Eg. using the stage micrometre to calculate the size of divisions on the eyepiece graticule. 4 eyepiece graticule divisions = 10 stage micrometre divisions In this stage micrometre, 1 subdivision = 10 µm So 4 eyepiece graticule divisions = 10 µm x 10 = 100 µm So 1 eyepiece graticule division = 100 µm/4 = 25 µm 10 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe and explain the principles of cell fractionation and ultracentrifugation as used to separate cell components 1. Homogenise tissue / use a blender Disrupts cell membrane, breaking open cells and releasing contents / organelles 2. Place in a cold, isotonic, buffered solution Cold to reduce enzyme activity → so organelles not broken down / damaged Isotonic so water doesn’t move in or out of organelles by osmosis → so they don’t burst Buffered to keep pH constant → so enzymes don’t denature 3. Filter homogenate Remove large, unwanted debris eg. whole cells, connective tissue 4. Ultracentrifugation - separates organelles in order of density / mass Centrifuge homogenate in a tube at a high speed Remove pellet of heaviest organelle and respin supernatant at a higher speed Repeat at increasing speeds until separated out, each time pellet made of lighter organelles (nuclei → chloroplasts / mitochondria → lysosomes → ER → ribosomes) Exam insight: common mistakes ❌ Mistake Explanation “Cost is a limitation of electron microscopes.” This is too basic and won’t achieve a mark. “Electron microscopes produce clearer images.” This is too vague. They have a higher resolution. *Not relating resolution to wavelength.* Explaining resolution in terms of long / short wavelength of light / electrons almost always achieves a mark. “Light microscopes have a long wavelength.” Be careful with your wording. It’s the light with the long wavelength, not the microscope. “A black & white image proves a scanning All electron microscope images are B&W. However, only a electron microscope was used.” SEM produces 3D images. “[Named organelle eg. nucleus] is not visible in Not all features may be present in an image as it is only a the TEM / optical microscope image because it section / slice; the organelle could be in another part of the is not present in the cell.” cell. It may also have not been stained. *Mixing up the role of an eyepiece graticule and The eyepiece graticule spans the full field of view. With no the role of a stage micrometer.* fixed units, it requires calibration at different magnifications using a micrometre (has units) on the stage. 11 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia 2.2 All cells arise from other cells Describe the stages of the cell cycle in eukaryotic cells 1. Interphase (S phase) DNA replicates semi-conservatively ○ Leading to 2 chromatids (identical copies) joined at a centromere (G1/G2) number of organelles & volume of cytoplasm increases, protein synthesis 2. Mitosis Nucleus divides To produce 2 nuclei with identical copies of DNA produced by parent cell 3. Cytokinesis Cytoplasm and cell membrane (normally) divide To form 2 new genetically identical daughter cells Describe the behaviour of chromosomes & role of spindle fibres in mitosis Stage 1 Chromosomes condense, becoming shorter / thicker (so visible) Students should be Prophase ○ Appear as 2 sister chromatids joined by a centromere able to recognise Nuclear envelope breaks down the stages of the Centrioles move to opposite poles forming spindle network cell cycle and explain the Stage 2 Spindle fibres attach to chromosomes by their centromeres appearance of cells Metaphase Chromosomes align along equator in each stage of mitosis. Stage 3 Spindle fibres shorten / contract Anaphase Centromere divides This is covered Pulling chromatids (from each pair) to opposite poles of cell under required practical 2. Stage 4 Chromosomes uncoil, becoming longer / thinner Telophase Nuclear envelopes reform = 2 nuclei Spindle fibres / centrioles break down 12 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Why do some eukaryotic cells not undergo the cell cycle? Within multicellular organisms, not all cells retain the ability to divide (eg. neurons) Only cells that do retain this ability go through a cell cycle Explain the importance of mitosis in the life of an organism Parent cell divides to produce 2 genetically identical daughter cells for… Growth of multicellular organisms by increasing cell number Replacing cells to repair damaged tissues Asexual reproduction Describe how tumours and cancers form Mitosis is a controlled process. Mutations in DNA / genes controlling mitosis can lead to uncontrolled cell division Tumour formed if this results in mass of abnormal cells ○ Malignant tumour = cancerous, can spread (metastasis) ○ Benign tumour = non-cancerous Suggest how cancer treatments control rate of cell division Some disrupt spindle fibre activity / formation More effective against ○ So chromosomes can’t attach to spindle by their centromere cancer cells due to ○ So chromatids can’t be separated to opposite poles (no anaphase) uncontrolled cell division ○ So prevents / slows mitosis but also disrupts cell Some prevent DNA replication during interphase cycle of rapidly dividing ○ So can’t make 2 copies of each chromosome (chromatids) healthy cells. ○ So prevents / slows mitosis Describe how prokaryotic cells replicate Binary fission: 1. Replication of circular DNA 2. Replication of plasmids 3. Division of cytoplasm to produce 2 daughter cells Single copy of circular DNA Variable number of copies of plasmids 13 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe how viruses replicate Being non-living, viruses do not undergo cell division. 1. Attachment proteins attach to complementary receptors on host cell 2. Inject viral nucleic acid (DNA/RNA) into host cell 3. Infected host cell replicates virus particles: a. Nucleic acid replicated b. Cell produces viral protein / capsid / enzymes c. Virus assembled then released Exam insight: common mistakes ❌ Mistake Explanation “Mitosis repairs cells.” Mitosis creates new cells to replace damaged or dead ones, therefore repairing the tissue but not the cells themselves. “DNA replication happens in DNA replication happens in interphase, which is before mitosis. mitosis.” “Chromosomes and chromatids A chromatid is one of the two identical halves of a chromosome that are the same thing.” has been replicated. These are held together by a centromere. *Mixing up centromeres and Centromeres join sister chromatids, while centrioles are organelles centrioles.* involved in spindle formation. *Forgetting to mention that This mark is commonly missed. When sister chromatids are pulled centromeres divide in anaphase.* apart, the centromere holding them together divides. “Cytokinesis always happens.” Some cells (eg. muscle cells) undergo mitosis (nuclear division) without cytokinesis (cytoplasmic division), so have multiple nuclei. “Tumours and cancers form due to Many other cells divide rapidly, but cell division has to be uncontrolled rapid cell division.” for cancers and tumours to form. “In binary fission in prokaryotic cells, Chromosomes consist of linear DNA associated with histones and are bacterial chromosomes replicate.” only found in eukaryotic cells. Bacteria have circular DNA (no histones). 14 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Required practical 2 Preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index. Students should measure the apparent size of cells in the root tip and calculate their actual size using the formula: actual size = size of image / magnification Describe how to prepare squashes of cells from plant root tips 1. Cut a thin slice of root tip (5mm from end) using scalpel and mount onto a slide 2. Soak root tip in hydrochloric acid then rinse 3. Stain for DNA eg. with toluidine blue 4. Lower coverslip using a mounted needle at 45o without trapping air bubbles 5. Squash by firmly pressing down on glass slip but do not push sideways Common questions 1. Why are root tips used? Where dividing cells are found / mitosis occurs 2. Why is a stain used? To distinguish chromosomes Chromosomes not visible without stain 3. Why squash / press down on cover (Spreads out cells) to create a single layer of cells slip? So light passes through to make chromosomes visible 4. Why not push cover slip sideways? Avoid rolling cells together / breaking chromosomes 5. Why soak roots in acid? Separate cells / cell walls To allow stain to diffuse into cells To allow cells to be more easily squashed To stop mitosis Describe how to set-up and use an optical microscope 1 Clip slide onto stage and turn on light 2 Select lowest power objective lens (usually x 4) 3 a. Use coarse focusing dial to move stage close to lens b. Turn coarse focusing dial to move stage away from lens until image comes into focus 4 Adjust fine focusing dial to get clear image 5 Swap to higher power objective lens, then refocus 15 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia What are the rules of scientific drawing? ✓ Look similar to specimen / image ✓ No sketching / shading - only clear, continuous lines ✓ Include a magnification scale (eg. x 400) ✓ Label with straight, uncrossed lines Explain how the stages of mitosis can be identified In interphase (not mitosis), chromosomes aren’t visible but nuclei are. In mitosis, chromosomes are visible. Stage Appearance Explanation Prophase Chromosomes visible / distinct → because condensing But randomly arranged → because no spindle activity / not attached to spindle fibre Metaphase Chromosomes lined up on equator → because attaching to spindle Anaphase Chromatids (in two groups) at poles of spindle Chromatids V shaped → because being pulled apart at their centromeres by spindle fibres Telophase Chromosomes in two sets, one at each pole What is a mitotic index? Proportion of cells undergoing mitosis (with visible chromosomes) Mitotic index = number of cells undergoing mitosis / total number of cells in sample Explain how to determine a reliable MI from observed squashes Count cells in mitosis in field of view Count only whole cells / only cells on top and right edges → standardise counting Divide this by total number of cells in field of view Repeat with many / at least 5 fields of view selected randomly → representative sample Calculate a reliable mean Suggest how to calculate the time cells are in a certain phase of mitosis 1. Identify proportion of cells in named phase at any one time Number of cells in that phase / total number of cells observed 2. Multiply by length of cell cycle 16 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia 2.3 Transport across cell membranes Describe the fluid-mosaic model of membrane structure Molecules free to move laterally in phospholipid bilayer The basic structure of all cell membranes Many components - phospholipids, proteins, (cell-surface membranes & membranes glycoproteins and glycolipids around eukaryotic organelles) is the same. Describe the arrangement of the components of a cell membrane Phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads face outwards Proteins ○ Intrinsic / integral proteins span bilayer eg. channel and carrier proteins ○ Extrinsic / peripheral proteins on surface of membrane Glycolipids (lipids with polysaccharide chains attached) found on exterior surface Glycoproteins (proteins with polysaccharide chains attached) found on exterior surface Cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails Explain the arrangement of phospholipids in a cell membrane Bilayer, with water present on either side Hydrophobic fatty acid tails repelled from water so point away from water / to interior Hydrophilic phosphate heads attracted to water so point to water Explain the role of cholesterol (sometimes present) in cell membranes Restricts movement of other molecules making up membrane So decreases fluidity (and permeability) / increases rigidity Suggest how cell membranes are adapted for other functions Phospholipid bilayer is fluid → membrane can bend for vesicle formation / phagocytosis Glycoproteins / glycolipids act as receptors / antigens → involved in cell signalling / recognition 17 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe how movement across membranes occurs by simple diffusion Lipid-soluble (non-polar) or very small substances eg. O2, steroid hormones Move from an area of higher conc. to an area of lower conc. down a conc. gradient Across phospholipid bilayer Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances) Explain the limitations imposed by the nature of the phospholipid bilayer Restricts movement of water soluble (polar) & larger substances eg. Na+ / glucose Due to hydrophobic fatty acid tails in interior of bilayer Describe how movement across membranes occurs by facilitated diffusion Water-soluble (polar) / slightly larger substances Move down a concentration gradient Through specific channel / carrier proteins Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances) Explain the role of carrier and channel proteins in facilitated diffusion Shape / charge of protein determines which substances move Channel proteins facilitate diffusion of water-soluble substances ○ Hydrophilic pore filled with water ○ May be gated - can open / close Carrier proteins facilitate diffusion of (slightly larger) substances ○ Complementary substance attaches to binding site ○ Protein changes shape to transport substance Describe how movement across membranes occurs by osmosis Water diffuses / moves From an area of high to low water potential (ψ) / down a water potential gradient Through a partially permeable membrane Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances) Water potential is a measure of how likely water molecules are to move out of a solution. Pure (distilled) water has the maximum possible ψ (0 kPA), increasing solute concentration decreases ψ. 18 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe how movement across membranes occurs by active transport Substances move from area of lower to higher concentration / against a concentration gradient Requiring hydrolysis of ATP and specific carrier proteins Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport 1. Complementary substance binds to specific carrier protein 2. ATP binds, hydrolysed into ADP + Pi, releasing energy 3. Carrier protein changes shape, releasing substance on side of higher concentration 4. Pi released → protein returns to original shape Describe how movement across membranes occurs by co-transport Two different substances bind to and move simultaneously via a co-transporter protein (type of carrier protein) Movement of one substance against its concentration gradient is often coupled with the movement of another down its concentration gradient Describe an example that illustrates co-transport Absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum: 1 Na+ actively transported from epithelial cells to blood (by Na+/K+ pump) Establishing a conc. gradient of Na+ (higher in lumen than epithelial cell) 2 Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient Via a co-transporter protein 3 Glucose moves down a conc. gradient into blood via facilitated diffusion The movement of sodium can be considered indirect / secondary active transport, as it is reliant on a concentration gradient established by active transport. 19 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes Increasing surface area of membrane increases rate of movement Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport Increasing concentration gradient increases rate of simple / facilitated diffusion and osmosis Increasing concentration gradient increases rate of facilitated diffusion ○ Until number of channel / carrier proteins becomes a limiting factor as all in use / saturated Increasing water potential gradient increases rate of osmosis Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes Membrane folded eg. microvilli in ileum → increase in surface area More protein channels / carriers → for facilitated diffusion (or active transport - carrier proteins only) Large number of mitochondria → make more ATP by aerobic respiration for active transport Exam insight: common mistakes ❌ Mistake Explanation “Thin cell membranes increase the rate of Cell membranes are less than 5nm wide and vary very diffusion.” little in thickness. A thin layer of cells increases the rate. “Only small molecules move by simple diffusion.” The key marking point here is that these molecules are lipid-soluble / non-polar. “Active transport uses carrier or channel proteins.” Active transport only uses carrier proteins. “Cholesterol increases membrane strength.” Cholesterol decreases fluidity / increases rigidity, but this is not the same as increasing strength. “Osmosis is movement of water from a dilute to a This is a GCSE answer. Osmosis at A Level should be more concentrated solution.” described in terms of water potential. “Diffusion is movement of substances from a high Diffusion is movement from high concentration to low gradient to low gradient.” concentration, not gradient to gradient. “Having carrier / channel proteins is an adaptation For a membrane to be adapted for rapid transport, it for fast facilitated diffusion.” must have more of these proteins, rather than them just being present. 20 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Required practical 3 Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue. Describe how to calculate dilutions Use the formula: C1 x V1 = C2 x V2 C1 = concentration of stock solution C2 = concentration of solution you are making V1 = volume of stock solution used to make new V2 = volume of new solution you are making concentration V2 = V1 + volume of distilled water to dilute with Worked example: describe how you would use a 0.5 mol dm-3 solution of sucrose (stock solution) to produce 30 cm3 of a 0.15 mol dm-3 sucrose solution. 1. Volume of stock solution required, V1 = (C2/C1) x V2 (0.15 ÷ 0.5) x 30 = 9 cm3 2. Volume of distilled water to top up with = V2 - V1 30 - 9 = 21 cm3 distilled water Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato) Part 1: collecting data Step Control variables 1. Create a series of dilutions using a 1 mol Volume of solution, eg. 20 cm3 -3 dm sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm-3 ) 2. Use scalpel / cork borer to cut potato into Size, shape and surface area of plant tissue identical cylinders Source of plant tissue ie variety or age 3. Blot dry with a paper towel and measure / Blot dry to remove excess water before weighing record initial mass of each piece 4. Immerse one chip in each solution and Length of time in solution leave for a set time (20-30 mins) in a Temperature o water bath at 30 C Regularly stir / shake to ensure all surfaces exposed 5. Blot dry with a paper towel and measure / Blot dry to remove excess water before weighing record final mass of each piece Repeat (3 or more times) at each concentration. 21 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Part 2: processing data 6. Calculate % change in mass = (final - initial mass)/ initial mass 7. Plot a graph with concentration on x axis and percentage change in mass on y axis (calibration curve) Must show positive and negative regions 8. Identify concentration where line of best fit intercepts x axis (0% change) Water potential of sucrose solution = water potential of potato cells 9. Use a table in a textbook to find the water potential of that solution Common questions Why calculate % Enables comparison / shows proportional change change in mass? As plant tissue samples had different initial masses Why blot dry Solution on surface will add to mass (only want to measure water taken up or lost) before weighing? Amount of solution on cube varies (so ensure same amount of solution on outside) Explain the changes in plant tissue mass when placed in different concentrations of solute Increase Water moved into cells by osmosis in mass As water potential of solution higher than inside cells Decrease Water moved out of cells by osmosis in mass As water potential of solution lower than inside cells No No net gain/loss of water by osmosis change As water potential of solution = water potential of cells 22 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Required practical 4 Investigation into the effect of a named variable on the permeability of cell-surface membranes. Describe a method to investigate the effect of a named variable (eg. temperature) on the permeability of cell-surface membranes 1. Cut equal sized / identical cubes of plant tissue (eg. beetroot) of same age / type using a scalpel 2. Rinse to remove pigment released during cutting or blot on paper towel 3. Add same number of cubes to 5 different test tubes containing same volume of water (eg. 5cm3) 4. Place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50 oC) 5. Leave for same amount of time (eg. 20 mins) 6. Remove beetroot and measure intensity of colour of surrounding solution: Semi-quantitatively ○ Use a known conc. of extract & distilled water to prepare a dilution series (colour standards) ○ Compare results with colour standards to estimate conc. Quantitatively ○ Measure absorbance (of light) of known concentrations using a colorimeter ○ Draw a calibration curve → plot a graph of absorbance (y) against conc. of extract (x) and draw a line / curve of best fit ○ Absorbance value for sample read off calibration curve to find associated extract conc. Common questions What are the issues with comparing to a Matching to colour standards is subjective colour standard? Colour obtained may not match any of colour standards Why wash the beetroot before placing it Wash off any pigment on surface in water? To show that release is only due to [named variable] Why regularly shake each test tube To ensure all surfaces of cubes remain in contact with liquid containing cubes of plant tissue? To maintain a concentration gradient for diffusion Why control the volume of water? Too much water would dilute the pigment so solution will appear lighter / more light passes through in colorimeter than expected So results are comparable How could you ensure beetroot cylinders Take readings in intervals throughout experiment of were kept at the same temperature temperature in tube using a digital thermometer / throughout the experiment? temperature sensor Use corrective measure if temperature has fluctuated 23 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia What does a high absorbance suggest about the cell-membrane? More permeable / damaged As more pigment leaks out making surrounding solution more concentrated (darker) Explain how temperature affects permeability of cell-surface membranes As temperature increases, permeability increases ○ Phospholipids gain kinetic energy and fluidity increases ○ Transport proteins denature at high temperatures as H bonds break, changing tertiary structure At very low temperatures, permeability increases ○ Ice crystals can form which pierce the cell membrane and increase permeability Explain how pH affects permeability of cell-surface membranes High or low pH increases permeability ○ Transport proteins denature as H / ionic bonds break, changing tertiary structure Explain how lipid-soluble solvents eg. alcohol affect permeability of cell-surface membranes As concentration increases, permeability increases Ethanol (a lipid-soluble solvent) may dissolve phospholipid bilayer (gaps form) 24 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia 2.4 Cell recognition and the immune system What is an antigen? Foreign molecule / protein / glycoprotein / glycolipid That stimulates an immune response leading to production of antibody How are cells identified by the immune system? Each type of cell has specific molecules on its surface (cell-surface membrane / cell wall) that identify it Often proteins → have a specific tertiary structure (or glycoproteins / glycolipids) What types of cells and molecules can the immune system identify? 1. Pathogens (disease causing microorganisms) eg. viruses, fungi, bacteria 2. Cells from other organisms of the same species (eg. organ transplants) 3. Abnormal body cells eg. tumour cells or virus-infected cells 4. Toxins (poisons) released by some bacteria Describe phagocytosis of pathogens (non-specific immune response) 1 Phagocyte attracted by chemicals / recognises (foreign) antigens on pathogen 2 Phagocyte engulfs pathogen by surrounding it with its cell membrane 3 Pathogen contained in vesicle / phagosome in cytoplasm of phagocyte 4 Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes) 5 Lysozymes hydrolyse / digest pathogen Phagocytosis leads to presentation of antigens where antigens are displayed on the phagocyte cell-surface membrane, stimulating the specific immune response (cellular and humoral response). 25 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe the response of T lymphocytes to a foreign antigen (the cellular response) T lymphocytes recognise (antigens on surface of) antigen presenting cells eg. infected cells, phagocytes presenting antigens, transplanted cells, tumour cells etc. Specific helper T cells with complementary receptors (on cell surface) bind to antigen on antigen-presenting cell → activated and divide by mitosis to form clones which stimulate: Cytotoxic T cells → kill infected cells / tumour cells (by producing perforin) Specific B cells (humoral response - see below) Phagocytes → engulf pathogens by phagocytosis Describe the response of B lymphocytes to a foreign antigen (the humoral response) B lymphocytes can recognise free antigens eg. in blood or tissues, not just antigen presenting cells. 1. Clonal selection: Specific B lymphocyte with complementary receptor (antibody on cell surface) binds to antigen This is then stimulated by helper T cells (which releases cytokines) So divides (rapidly) by mitosis to form clones 2. Some differentiate into B plasma cells → secrete large amounts of (monoclonal) antibody 3. Some differentiate into B memory cells → remain in blood for secondary immune response 26 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia What are antibodies? Quaternary structure proteins (4 polypeptide chains) Secreted by B lymphocytes eg. plasma cells in response to specific antigens Bind specifically to antigens forming antigen-antibody complexes Describe the structure of an antibody Explain how antibodies lead to the destruction of pathogens Antibodies bind to antigens on pathogens forming an antigen-antibody complex ○ Specific tertiary structure so binding site / variable region binds to complementary antigen Each antibody binds to 2 pathogens at a time causing agglutination (clumping) of pathogens Antibodies attract phagocytes Phagocytes bind to the antibodies and phagocytose many pathogens at once Explain the differences between the primary & secondary immune response Primary - first exposure to antigen ○ Antibodies produced slowly & at a lower conc. ○ Takes time for specific B plasma cells to be stimulated to produce specific antibodies ○ Memory cells produced Secondary - second exposure to antigen ○ Antibodies produced faster & at a higher conc. ○ B memory cells rapidly undergo mitosis to produce many plasma cells which produce specific antibodies 27 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia What is a vaccine? Injection of antigens from attenuated (dead or weakened) pathogens Stimulating formation of memory cells Explain how vaccines provide protection to individuals against disease 1. Specific B lymphocyte with complementary receptor binds to antigen 2. Specific T helper cell binds to antigen-presenting cell and stimulates B cell 3. B lymphocyte divides by mitosis to form clones 4. Some differentiate into B plasma cells which release antibodies 5. Some differentiate into B memory cells 6. On secondary exposure to antigen, B memory cells rapidly divide by mitosis to produce B plasma cells 7. These release antibodies faster and at a higher concentration Explain how vaccines provide protections for populations against disease Herd immunity - large proportion of population vaccinated, reducing spread of pathogen ○ Large proportion of population immune so do not become ill from infection ○ Fewer infected people to pass pathogen on / unvaccinated people less likely to come in contact with someone with disease Describe the differences between active and passive immunity Active immunity Passive immunity Initial exposure to antigen eg. No exposure to antigen vaccine or primary infection Memory cells involved No memory cells involved Antibody produced and secreted Antibody introduced from another organism eg. by B plasma cells breast milk / across placenta from mother Slow; takes longer to develop Faster acting Long term immunity as antibody can be produced in Short term immunity as response to a specific antigen again antibody hydrolysed (endo/exo/dipeptidases) Explain the effect of antigen variability on disease and disease prevention Antigens on pathogens change shape / tertiary structure due to gene mutations (creating new strains) So no longer immune (from vaccine or prior infection) ○ B memory cell receptors cannot bind to / recognise changed antigen on secondary exposure ○ Specific antibodies not complementary / cannot bind to changed antigen Examples: yearly new flu vaccines, no vaccine for HIV, catch a cold many times 28 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Describe the structure of a HIV particle Describe the replication of HIV in helper T cells 1. HIV attachment proteins attach to receptors on helper T cell 2. Lipid envelope fuses with cell-surface membrane, releasing capsid into cell 3. Capsid uncoats, releasing RNA and reverse transcriptase 4. Reverse transcriptase converts viral RNA to DNA 5. Viral DNA inserted / incorporated into helper T cell DNA (may remain latent) 6. Viral protein / capsid / enzymes are produced a. DNA transcribed into HIV mRNA b. HIV mRNA translated into new HIV proteins 7. Virus particles assembled and released from cell (via budding) Explain how HIV causes the symptoms of acquired immune deficiency syndrome (AIDS) HIV infects and kills helper T cells (host cell) as it multiplies rapidly ○ So T helper cells can’t stimulate cytotoxic T cells, B cells and phagocytes ○ So B plasma cells can’t release as many antibodies for agglutination & destruction of pathogens Immune system deteriorates → more susceptible to (opportunistic) infections Pathogens reproduce, release toxins and damage cells Explain why antibiotics are ineffective against viruses Viruses do not have structures / processes that antibiotics inhibit: Viruses do not have metabolic processes (eg. do not make protein) / ribosomes Viruses do not have bacterial enzymes / murein cell wall What is a monoclonal antibody? Antibody produced from genetically identical / cloned B lymphocytes / plasma cells So have same tertiary structure 29 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Explain how monoclonal antibodies can be used in medical treatments Monoclonal antibody has a specific tertiary structure / binding site / variable region Complementary to receptor / protein / antigen found only on a specific cell type (eg. cancer cell) Therapeutic drug attached to antibody Antibody binds to specific cell, forming antigen-antibody complex, delivering drug Some monoclonal antibodies are also designed to block antigens / receptors on cells. Explain how monoclonal antibodies can be used in medical diagnosis Monoclonal antibody has a specific tertiary structure / binding site / variable region Complementary to specific receptor / protein / antigen associated with diagnosis Dye / stain / fluorescent marker attached to antibody Antibody binds to receptor / protein / antigen, forming antigen-antibody complex Examples vary, eg. pregnancy tests. You’ll need to interpret information in the question on how these work. Explain the use of antibodies in the ELISA (enzyme-linked immunosorbent assay) test to detect antigens Direct ELISA 1. Attach sample with potential antigens to well 2. Add complementary monoclonal antibodies with enzymes attached → bind to antigens if present 3. Wash well → remove unbound antibodies (to prevent false positive) 4. Add substrate → enzymes create products that cause a colour change (positive result) OR sandwich ELISA 1. Attach specific monoclonal antibodies to well 2. Add sample with potential antigens, then wash well 3. Add complementary monoclonal antibodies with enzymes attached → bind to antigens if present 4. Wash well → remove unbound antibodies (to prevent false positive) 5. Add substrate → enzymes create products that cause a colour change (positive result) 30 AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Explain the use of antibodies in the ELISA test to detect antibodies Indirect ELISA 1. Attach specific antigens to well 2. Add sample with potential antibodies, wash well 3. Add complementary monoclonal antibodies with enzymes attached → bind to antibodies if present 4. Wash well → remove unbound antibodies 5. Add substrate → enzymes create products that cause a colour change (positive result) Suggest the purpose of a control well in the ELISA test Compare to test to show only enzyme causes colour change Compare to test to show all unbound antibodies have been washed away Discuss some general ethical issues associated with the use of vaccines and monoclonal antibodies Pre-clinical testing on / use of animals - potential stress / harm / mistreatment ○ But animals not killed & helps produce new drugs to reduce human suffering Clinical trials on humans - potential harm / side-effects Vaccines - may continue high risk activities and still develop / pass on pathogen Use of drug - potentially dangerous side effects Suggest some points to consider when evaluating methodology relating to the use of vaccines and monoclonal antibodies Was the sample size large enough to be representative? Were participants diverse in terms of age, sex, ethnicity and health status? Were placebo / control groups used for comparison? Was the duration of the study long enough to show long-term effects? Was the trial double-blind (neither doctor / patient knew who was given drug or placebo) to reduce bias? Suggest some points to consider when evaluating evidence and data relating to the use of vaccines and monoclonal antibodies What side effects were observed, and how frequently did they occur? Was a statistical test used to see if there was a significant difference between start & final results? Was the standard deviation of final results large, showing some people did not benefit? Did standard deviations of start & final results overlap, showing there may not be a significant difference? What dosage was optimum? Does increasing dose increase effectiveness enough to justify extra cost? Was the cost of production & distribution low enough? 31  ❌ AQA A Level Biology Topic 2 Cells stan.store/biologywitholivia Exam insight: common mistakes Mistake Explanation *Confusing lysosomes & lysozymes* Lysozymes are enzymes found in lysosomes. “Pathogens are contained in a vacuole Sometimes mark schemes only allow the terms vesicle or during phagocytosis.” phagosome, so use these to be safe. “Lysosomes fuse with pathogens.” Lysosomes fuse with the phagosome that contains the pathogen. *Not mentioning the idea of B and T lymphocytes are highly specific due to the wide range of specificity in B / T lymphocytes.* receptors found on their cell-surface membranes. The cellular and humoral response are classed as the specific immune response. “An antibody has an active site.” An enzyme has an active site. An antibody has a binding site. *Describing the full immune response Pay close attention to the wording of the question, as they normally for every immune system question.* tend to focus on a specific aspect of the immune response. “Antibiotics are ineffective against This is not wrong, but was considered to be GCSE level. Instead, viruses as viruses live inside host mention the structures and processes that antibiotics inhibit, that human cells and so are inaccessible.” viruses don’t have. “HIV replicates using the T helper cells’ This is too vague. Refer to specific processes, such as transcription machinery.” and translation. “A monoclonal antibody is a clone of an A monoclonal antibody by definition is produced by a clone of a antibody.” plasma cell. The antibody itself can't be considered a clone. 32