Download Cell Membrane Structure and Function and more Exams Advanced Education in PDF only on Docsity!
Paper 1 AQA A level Biology 2022
Explain why plasma membranes are said to have a fluid mosaic structure. - 1. Fluid because the phospholipids are constantly moving. 2. Mosaic because the proteins are scattered throughout the membrane-like tiles in a mosaic. What effect does cholesterol have on the cell membrane? - 1.Cholesterol molecules fit between the phospholipids and bind to the hydrophobic tails of the phospholipids. 2.Causes them to pack together more closely, restricting the movement of other molecules in the membrane. 3.Cholesterol makes the membrane less fluid and more rigid. 4.Cholesterol helps to maintain the shape of animal cells, which is important because animal cells don't have cell walls (especially important for cells which are not supported by other cells e.g. red blood cells). State 3 functions of glycolipids in the cell membrane. - 1. Act as recognition sites. 2. Maintain the stability of the membrane. 3. Help cells attach to one another and so form tissues. State 3 functions of glycoproteins in the cell membrane. - 1.Act as recognition sites.2.Help cells to attach to one another so form tissues.3.Allow cells to recognise one another e.g. lymphocytes can recognise an organisms's own cells. Describe the functions of membranes within cells. - 1.Control the entry and exit of materials in membrane-bound organelles such as mitochondria and chloroplasts.2.Separate organelles from cytoplasm so that specific metabolic reactions can take place.3.Provide an internal transport system e.g. endoplasmic reticulum.4.Isolate enzymes which could damage the cell e.g. lysosomes.5.Provide surfaces on which reactions can occur e.g. protein synthesis on ribosomes on rough ER. Suggest reasons why most molecules do not diffuse freely through the cell membrane. - 1.They're not lipid soluble.2.Too large to pass through protein channels.3.Have the same charge as the charge on the protein channels, so they are repelled even if they are small enough to pass through the channel. State two factors which affect the permeability of cell membranes - .1.Temperature2.Solvent concentration How can substances move across a cell surface membrane? - 1.Simple diffusion (passive)2.Facilitated diffusion (passive)3.Osmosis (passive)4.Active transport (active)5.Co-transport (active)
Describe how substances can move across a cell surface membrane. - 1.Small/non-polar/lipid-soluble substances can cross by simple diffusion down a concentration gradient.2.Large/polar molecules - pass through channel proteins/carrier proteins, by facilitated diffusion, down a concentration gradient.3.Water - moves by osmosis from an area of high water potential to an area of lower water potential.4.Active transport - moves substances against a concentration gradient, involves protein carriers and requires ATP.5.Glucose/amino acids can be co-transported across a cell membrane alongside sodium ions.* DON'T EVER SAY THAT A MOLECULE MOVES 'ACROSS' OR 'ALONG' A CONCENTRATION GRADIENT - MAKE SURE YOU SAY 'UP' OR 'DOWN' OR 'AGAINST' Define diffusion. - The net movement of molecules or ions from an area of higher concentration to an area of lower concentration until evenly distributed.Define osmosis.The movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane. Define active transport. - The movement of molecules from an area of lower concentration to an area of higher concentration using ATP and carrier proteins. State factors which affect the rate of diffusion. - 1.Concentration gradient2.Area over which diffusion takes place3.Thickness of exchange surface4.Temperature5.Size of molecules (smaller the molecule, the faster the rate of diffusion). What is meant by the term simple diffusion - .The diffusion of molecules directly through a cell membrane.(most molecules don't easily pass across the cell surface membrane - those that do are small, non-polar molecules). What is Ficks Law? - The rate of diffusion is proportional to : (Surface Area x Concentration) Thickness1.The larger the SA, the faster the rate of diffusion.2.The higher the concentration gradient, the faster the rate of diffusion.3.The thinner the exchange surface, the faster the rate of diffusion. What is meant by the term passive process. - No energy is needed for it to happen. Why are oxygen and carbon dioxide able to diffuse across cell membranes? - 1.They're small, so they can pass through the spaces between the phospholipids.2.They're non-polar, so they are lipid-soluble and so can dissolve in the phospholipid bilayer.
opposite site of the membrane.3.No ATP is required, the molecules move from an area of higher concentration to an area of lower concentration, using the kinetic energy of the molecules themselves. Discuss two factors which affect the rate of facilitated diffusion. - 1.The concentration gradient: the higher the concentration gradient, the faster the rate of diffusion, up to a point which is determined by how many protein channels and carriers are present in the membrane.2.Once all the proteins in a membrane are in use, the rate of facilitated diffusion can't increase any further. State the 3 factors which affect the rate of osmosis. - 1.Water potential gradient.2.Thickness of exchange surface.3.Surface area of exchange surface. What is meant by a selectively permeable membrane? - A membrane that is permeable to water molecules (and a few other small molecules), but not to larger molecules. What is the water potential of pure water under standard conditions of pressure and temperature? - Zero If two solutions are isotonic, what does this mean?They have the same water potential .Explain why the water potential of distilled water is higher than the water potential of cytoplasm. - 1.Distilled water contains no solute and so its water potential is zero.2.Presence of solutes in cytoplasm, reduce the water potential of the cytoplasm. Describe the property of the cell surface membrane which allows osmosis to take place. - 1.Partially permeable so cell surface membranes allow water molecules to pass through, but not solute molecules. What happens if you surround an animal cell with pure water? - It swells and bursts. What happens if you surround a plant cell with pure water? - 1.It swells, but doesn't burst. 2.The cell wall prevents it from bursting because it is made of cellulose, which is a strong material.3.The cell is turgid. What happens if an animal cell is surrounded by a concentrated salt/sugar solution? - 1.It shrinks because water leaves the cell by osmosis.2.Water moves from area of higher water potential in the cell to area of lower water potential in the solution
.What happens if a plant cell is surrounded by a concentrated salt/sugar solution? - 1.Water leaves the cell by osmosis2.but the cell wall prevents the cell from shrinking by keeping the cell rigid. Describe two ways in which osmosis and diffusion are similar. - 1.Both are passive and so don't require ATP.2.Both involve movement down a concentration gradient. Give two differences between osmosis and facilitated diffusion. - 1.Osmosis only involves the movement of water.2.Facilitated diffusion involves carrier proteins. Why can glucose molecules not pass easily through the phospholipid bilayer? - Only lipid-soluble substances diffuse across the phospholipid bilayer easily, Water-soluble substances like glucose, diffuse very slowly. Suggest two changes to the cell surface membrane that would increase the rate at which glucose could be transported in the cell. - 1.More proteins with pores which span the phospholipid bilayer.2.Increase the surface area of the cell surface membrane with microvilli .State two properties that a drug should have if it is to enter a cell rapidly. - 1.Lipid-soluble2.Small What is a co-transporter? - A type of carrier protein that can bind two molecules at a time (the concentration of one of the molecules is used to move the other molecule AGAINST its own concentration gradient). Describe how glucose is absorbed by co-transport in the mammalian ileum (final part of the small intestine). - 1.Sodium ions are actively transported out of the ileum epithelial cells by the sodium- potassium pump.2.Creates a concentration gradient, causing sodium ions to diffuse into the epithelial cells down their concentration gradient via sodium-glucose co-transporter proteins.3.The co-transporter carries glucose into the cell with the sodium.4.The concentration of glucose inside the cell increases.5.Glucose diffuses out the cell into the blood, down its concentration gradient through protein channels, via facilitated diffusion. Why is it necessary for glucose to be absorbed from the lumen of the ileum by co-transport? - The concentration of glucose in the ileum is too low for glucose to diffuse out through the epithelial cell surface membrane into the blood.
•Explain why the percentage increase in mass of a small cube would be greater than the percentage increase in mass for a large cube. - Due to the larger surface area:volume ratio of the small cubes for osmosis. •Explain why the potato cylinder in distilled water gained mass. - Water moved into the potato cells by osmosis, from a less negative water potential outside the cells to a more negative water potential inside the cells. Explain what is meant by the term eukaryotic cell. - •Cell that has a membrane-bound nucleus and chromosomes.•Has other membrane-bound organelles, such as mitochondria and endoplasmic reticulum. Give examples of eukaryotic cells. - Animal cells / Plant cells / Fungi / Algae Give an example of a prokaryotic cell. - Bacteria Describe the structure and function of the cell-surface membrane. - Structure - membrane found on surface of animal cells and just inside the surface of other cells / made mainly of lipids and proteins.Function:•Regulates the movement of substances into and out of the cell.•Has receptor molecules on it (allows it to respond to e.g. hormones). Describe the structure and function of the nucleus. - Structure:•Surrounded by nuclear envelope (double membrane with many pores).•Contains chromosomes (consist of linear DNA + histone proteins).•Contains one or more nucleoli (singular = nucleolus)Function: •Contains genetic material of the cell in the form of DNA - therefore controls the cell's activities.•Pores allow movement of substances e.g. mRNA between the nucleus and the cytoplasm.•Nucleolus manufactures ribosomes. Describe the structure and function of mitochondria. - Structure:•Oval-shaped with a double membrane,•Inner membrane folded to form cristae.•Matrix contains enzymes involved in respiration.Function:•Site of aerobic respiration and therefore, where ATP is produced.•Found in large numbers in cells which are very active and therefore need lots of energy. Describe the structure and function of chloroplasts. - Structure:•Small, flattened structure found in plants and algae.•Double membrane.•Contains thylakoid membranes - stacked to form grana.•Grana
are linked together by lamellae.•Thylakoids are surrounded by fluid called stroma.Function:•Site of photosynthesis - thylakoid membranes are the site of the light-dependent reaction and light- independent reaction takes place in the stroma. Describe the structure and function of the Golgi apparatus. - Structure - fluid-filled, membrane-bound sacs / vesicles often seen at the edges of the sacs.Function:•Processes and packages new lipids and proteins.•Makes lysosomes. Give the structure and function of the lysosome. - Structure - round organelle surrounded by a membrane / type of Golgi vesicle.Function:•Contains digestive enzymes called lysozymes - breakdown worn out components of the cell or digest invading cells. Give the structure and function of the ribosome. - Structure:•Very small round organelle, with no surrounding membrane.•Can be free floating in the cytoplasm or attached to rough ER.•Made of proteins and ribosomal RNA.Function:•Site of protein synthesis. Give the two types of molecules from which a ribosome is made. - •protein•(ribosomal) RNA Give the structure and function of the rough endoplasmic reticulum (RER).Structure - system of membranes enclosing a fluid-filled space / surface is covered with ribosomes.Function:•Folds and processes proteins that have been made at the ribosomes. Give the structure and the function of the smooth endoplasmic reticulum (SER).,Structure -system of membranes enclosing a fluid-filled space / no ribosomes.Function:•Synthesises and processes lipids. Give the structure and function of the cell wall. - Structure:•Rigid structure that surrounds plants, algae and fungi.•In plants and algae, it's made mainly of cellulose.•In fungi, it's made of chitin.Function:•Supports cells and prevents them changing shape. Give the structure and function of the cell vacuole. - Structure:•A membrane-bound organelle found in the cytoplasm of plant cells.•Contains cell sap.•Vacuolar membrane = tonoplast.Function:•Helps to maintain pressure inside the cell - keeps the cell rigid - stops plants wilting.•Involved in isolation of unwanted chemicals inside the cell. Name 3 organelles found in plant cells but not in animal cells. - 1.Cell wall2.Cell vacuole3.Chloroplasts
Suggest how neutrophils are adapted to their function of ingesting and destroying microbes. - 1.Large amounts of lysosomes, which contain enzymes to destroy ingested microbes.2.Have a lobed nucleus, which allows for greater flexibility. Suggest how a root hair cell is adapted to its function of absorbing water and minerals from the soil. - 1.Large surface area due to hair-like projections, which increases the uptake of minerals and water.2.Have a large amount of mitochondria for active transport. What is meant by the term prokaryotic? - 1.A cell of an organism belonging to the kingdom Prokaryote.2.Do not have a nucleus OR membrane-bound organelles. What is a plasmid? - 1.A small, circular piece of DNA found in bacterial cells, that is separate from the main circular DNA molecule.2.Plasmids can be passed between prokaryotic cells.3.Not all prokaryotes contain plasmids, some contain more than one plasmid. What is a flagellum? - A long hair-like structure that rotates to make the prokaryotic cell move. How do prokaryotic cells differ from eukaryotic cells? - 1.Prokaryotic cells don't contain a nucleus - DNA floats free in the cytoplasm.2.Prokaryotic DNA is circular DNA.3.Prokaryotic DNA is not attached to histone proteins.4.Prokaryotes are much smaller than eukaryotic cells.5.Prokaryotes have no membrane-bound organelles.6.Prokaryotes have smaller ribosomes.7.Prokaryotes have a cell wall composed of murein (a glycoprotein).Name 3 features of a prokaryotic cell, which would not be found in a eukaryotic cell.1.One or more plasmids.2.A capsule surrounding the cell.3.One or more flagella. Describe the structure and function of the prokaryotic cell wall.Made of murein - the cell wall supports the cell and prevents it from changing shape. - What is the function of the capsule in prokaryotic cells? - To protect bacteria from attack by cells of the immune system. Give an example of a gene which is carried on a plasmid? - Genes for antibiotic resistance. What is a virus? - 1.Acellular and non-living.2.Consist of nucleic acids surrounded by a capsid (protein coat).3.Attachment proteins stick out from the capsid - allow the virus to attach to host cells.4.No
plasma membrane or cytoplasm or ribosomes.5.Viruses can't reproduce independently - they invade and reproduce inside host cells. Method by which prokaryotic cells replicate? - Binary fission Describe how a prokaryotic cell divides. - 1.The circular DNA and plasmid(s) replicate.2.The main DNA loop is replicated once, plasmids can be replicated multiple times.3.The cells gets bigger and the DNA loops move to opposite poles of the cell.4.The cytoplasm begins to divide and new cell walls begin to form.5.The cytoplasm divides and two daughter cells are produced.6.Each daughter cell has one copy of the circular DNA, but can have a variable number of copies of the plasmid(s). How to calculate the number of bacteria at the end of a growth period? - bacteria at beginning of growth period x 2number of divisionse.g. if 1 bacteria divides 25 times, there will be 1 x 225 bacteria Why can some viruses only infect one cell type? - 1.Different viruses have different attachment proteins.2.Therefore they require different receptor proteins on host cells.3.Therefore, some viruses can only bind to and infect certain cell types. Define magnification. - The number of times bigger the image/drawing is compared to the real size of the specimen. Define resolution. - The minimum distance needed to differentiate between two adjacent objects. Distinguish between magnification and resolution. - 1.Magnification is how many times bigger the image is compared to the real object.2.Resolution is the minimum distance apart that two objects can be in order for them to be distinguished as two separate objects. Why will increasing the magnification not necessarily increase the resolution? - 1.Microscopes have a limit of resolution.2.Beyond the limit of resolution, increasing the magnification will result in larger, but more blurred images. State the formula for calculating magnification. - magnification = size of image/size of object* make sure you convert the numbers into the same units before doing the calculation.
Why do the samples for an SEM not need to be as thin as for a TEM? - Because the electrons do not penetrate the specimen Explain why specimens have to be kept in a near-vacuum in order to be viewed effectively using an electron microscope. - 1. Electrons are absorbed by molecules in the air.2. So, if air was present, this would prevent the electrons reaching the specimen. Explain why the theoretical resolving power of an electron microscope cannot always be achieved. - 1. The preparation of the specimens may not be good enough.2. A higher energy electron beam is required, which may destroy the specimen. Explain what is meant by the term artefact in microscopy. - A structural detail that results from processing of the specimens and is not a part of the natural specimen e.g. fingerprint, air bubble or inaccuracies caused staining the sample etc. Why are artefacts more common when using electron microscopes? - 1. Specimens need a lot of preparation before they can be viewed under an electron microscope.2. Therefore, there are more stages at which an artefact could be introduced. How did the first scientists to use electron microscopes distinguish between artefacts and organelles? -
- They had to repeatedly prepare samples in different ways.2. If an object could be seen with one preparation technique, but not another, the object was more likely tobe an artefact than an organelle. When using a light microscope - which 2 things do you need in order to calculate the size of the cells you are looking at?,1. Eyepiece graticule (has numbers but not units - therefore needs calibrating)2. Stage micrometer (has a scale with units) Explain how you would calibrate the eyepiece graticule. - 1. Line up the eye piece graticule and the stage micrometer.2. Calculate the length of each division on the stage micrometer by dividing the total length by the number of divisions.3. Work out how many eyepiece divisions are the same as one micrometer division.4. Divide the length of one division on the stage micrometer by the number of divisions on the eye piecegraticule that you worked out in step 3. What is meant by cell fractionation? - Cells are broken up and their organelles are separated out.
What is meant by homogenisation? - Cells are broken up by a homogeniser and organelles are released from the cell. Why must the homogenate be filtered? - To remove any large cell/tissue debris and any whole cells. Explain what is meant by ultracentrifugation. - Organelles in the filtered homogenate are separated using a centrifuge. Describe and explain how centrifugation allows scientists to obtain a cell-free liquid. - 1. Centrifugation causes dense cells2. to form a pellet at the bottom of the tube.3. The cell-free supernatant can then be removed. Describe the solution that is used for cell fractionation. - 1. Ice-cold - to reduce the activity of enzymes that can break down organelles.2. Buffered - to maintain the pH.3. Isotonic - to prevent damage to the organelles* through osmosis*DON'T SAY 'TO PREVENT CELLS FROM BURSTING ' - THE CELLS HAVE BEEN BROKEN UP USING A HOMOGENISER TO RELEASE THE ORGANELLES. Describe briefly how you might obtain a sample rich in chloroplasts. - 1. Store the plant cells in an ice- cold, isotonic and buffered solution.2. Homogenate the cells to break them open and release the organelles.3. Filter the homogenate to remove large debris and intact cells.4. Centrifuge at low speed and remove the supernatant.5. First pellet obtained will contain the nuclei (the heaviest organelle).6. Spin the supernatant at a higher speed, which will cause the next heaviest organelle, the chloroplasts,toform a pellet. List the organelles in size order starting with the heaviest. - Nucleus, chloroplast, mitochondria, endoplasmic reticulum/golgi/lysosomes,ribosomes.
