Topic 2

phospholipid structure hydrophobic tail/hydrophilic head head made from glycerol and phosphate tail made from two fatty acids saturated/ unsaturated fatty acid (in tail) arrangement in membrane phospholipids form a bilayer heads face outside the membrane/ tails face inside the membrane/ hydrophic interior/ hydrophilic exterior of membrane A suitable annotated diagram may incorporate all or many of the above points. Award 5 marks maximum for a suitable diagram that is labelled correctly. phospholipids held together by hydrophobic interactions phospholipid layers are stabilized by interaction of hydrophilic heads and surrounding water phospholipids allow for membrane fluidity/ flexibility fluidity/ flexibility helps membranes to be (functionally) stable phospholipids with short fatty acids/ unsaturated fatty acids are more fluid fluidity is important in breaking and remaking membranes (e.g. endocytosis/ exocytosis) phospholipids can move about/ move horizontally/ “flip flop” to increase fluidity hydrophilic/ hydrophobic layers restrict entry/ exit of substances

vesicles are membrane bound packages/droplets formed by pinching off/budding off a piece from a membrane can carry proteins rough ER synthesizes proteins proteins enter/accumulate inside the ER transported to Golgi apparatus for processing targeted to/transported to specific cellular organelles fuse with membrane of organelle so contents of vesicle join the organelle transported to the plasma membrane fuses with plsma membrane releases/secretes contents exocytosis

uses/ requires energy/ ATP goes against concentration gradient/ lower to higher concentration requires a protein in the cell membrane/ pump/ carrier protein (reject channel) hydrolysis of ATP/ ATP –> ADP + phosphate involves a conformational change in the pump/ protein/ diagram to show this

diffusion (is a method of passive transport across the membrane) pore/ channel proteins for facilitated diffusion/ to allow hydrophilic particles across movement from high to low concentration/ down the concentration gradient membrane must be permeable to the substance diffusing oxygen/ other named example of a substance than can diffuse through membranes from a region of lower to a region of higher solute concentration/ higher to lower water potential membranes are (nearly) always freely permeable to water

passive: diffusion / osmosis / facilitated diffusion, whereas, active transport: ion pumps / exocytosis / pinocytosis / phagocytosis a second passive method (from above), whereas, active transport: a second active method; (from above) passive: does not require energy, whereas, active transport: requires energy/ATP; passive: down concentration gradient, whereas, active transport: against concentration gradient; passive: no pumps needed, whereas, active transport: requires protein pumps; passive: oxygen across alveoli / other example, whereas, active transport: glucose absorption in ileum / other example; Both the passive and active movements must be contrasted to receive a mark. Award 3 max if no examples are given. Responses do not need to be shown in a table format.

vesicles carry material to plasma membrane; vesicle fuses with membrane; (by joining of) phospholipid bilayers; aided by the fluidity of the membrane; material released/expelled from the cell; membrane flattens; name of example e.g. exocytosis of neurotransmitter / exocrine secretion/endocrine secretion / hormone secretion / release of cortical granules; outline of example: (in the presence of calcium), neurotransmitter vesicles release their contents into the synapse / hormones released from one cell have an effect on another cell etc.; Accept these points if clearly made in an annotated diagram. 4 max if no example given.

to increase the number of cells in an organism to allow differentiation/ cell specialization for greater efficiency to replace damaged/ lost cells example binary fission asexual reproduction of unicellular organisms gamete/ spore formation cells only arise from pre-existing cells refer to Virchow cells cannot grow beyond a certain size surface area to volume ratio becomes too small transport across the membrane too slow example nucleus cannot control the cell control of cell division sometimes lost tumor formation

DNA replication DNA transcription enzyme/ protein synthesis biochemical reactions/ example of a biochemical reaction cell respiration growth organelles replicated

water has a high specific heat capacity; a large amount of heat causes a small increase in temperature; water has a high latent heat of vaporization; a large amount of heat energy is needed to vaporize/evaporate water; hydrogen bonds between water molecules make them cohesive/stick together; this gives water a high surface tension / explains how water rises up xylem; water molecules are polar; this makes water a good solvent; Award 4 max if thermal, cohesive and solvent properties are not all mentioned.

Each feature or property must be related to living organisms in order to receive a mark. Features may include: surface tension - allows some organisms (e.g. insects) to move on water’s surface polarity / capillarity / adhesion - helps plants transport water (excellent) solvent - capable of dissolving substances for transport in organisms (excellent) thermal properties (high heat of vaporization) - excellent coolant ice floats - lakes / oceans do not freeze, allowing life under the ice buoyancy - supports organisms structure - turgor in plant cells / hydrostatic pressure habitat - place for aquatic organisms to live

Answers must discuss both carbohydrates and lipids to receive full marks carbohydrates: 3 max stored as glycogen (in liver) short-term energy storage more easily digested than lipids so energy can be released more quickly more soluble in water for easier transport lipids: 3 max stored as fat in animals long-term energy storage more energy per gram than carbohydrates lipids are insoluble in water so less osmotic effect

energy storage / source of energy / respiration substrate (heat) insulation protection (of internal organs) water proofing / cuticle buoyancy (structural) component of cell membranes electrical insulation by myelin sheath (steroid) hormones glycolipids acting as receptors

diagram of peptide bond drawn condensation / dehydration synthesis: water produced (when two amino acids joined) hydrolysis: water needed to break bond dipeptide –> amino acids - hydrolysis occurs amino acids –> dipeptide - condensation occurs

name of function and named protein must both be correct for the mark storage - zeatin (in corn seeds)/casein (in milk) transport - hemoglobin/lipoproteins (in blood) hormones - insulin/growth hormone/TSH/FSH/LH receptors - hormone receptor/neurotransmitter receptor/receptor in chemoreceptor cell movement - actin/myosin defense -antibodies/immunoglobin enzymes - catalase/RuBP carboxylase structure - collagen/keratin/tubulin/fibroin electron carriers - cytochromes pigments - opsin active transport - sodium potassium pumps/calcium pumps facilitated diffusion - sodium channels/aquaporins mark first four functions only, but allow other named examples

it allows people who are lactose intolerant/have difficulty digesting lactose to consume milk (products); galactose and glucose taste sweeter than lactose reducing need for additional sweetener (in flavoured milk products); galactose and glucose are more soluble than lactose / gives smoother texture / reduces crystalization in ice cream; (bacteria) ferment glucose and galactose more rapidly (than lactose) shortening production time (of yoghurt/cottage cheese);

less denaturation / enzymes last longer at lower temperatures; lower energy costs / less energy to achieve 5 °C compared to 48 °C; reduces bacterial growth / reduces (milk) spoilage; to form products more slowly / to control the rate of reaction;

they increase rate of (chemical) reaction; remains unused/unchanged at the end of the reaction; substrate joins with enzyme at active site; to form enzyme-substrate complex; active site/enzyme (usually) specific for a particular substrate; enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron transfer); making the substrate more reactive;

enzymes have an optimal pH lower activity above and below optimum pH / graph showing this too acidic / base pH can denature enzyme change shape of active site / tertiary structure altered substrate cannot bind to active site / enzyme- substrate complex cannot form hydrogen / ionic bonds in the enzyme / active site are broken / altered

in both models substrate binds to active site substrate fits active site exactly in lock and key, whereas fit is not exact in induced fit substrate / active site changes shape in induced fit, whereas active site does not change shape in lock and key in both models an enzyme - substrate complex is formed in lock and key binding reduces activation energy, whereas in the induced fit change to substrate reduces activation energy lock and key model explains narrow specificity, whereas induced fit allows broader specificity induced fit explains competitive inhibition, whereas lock and key does not

(primary structure is a) chain of amino acids/sequence of amino acids (each position is occupied by one of) 20 different amino acids linked by peptide bonds secondary structure formed by interaction between amino and carboxyl/-NH and -C=O groups (weak) hydrogen bonds are formed (α-) helix formed / polypeptide coils up or (ß-) pleated sheet formed tertiary structure is the folding up of the polypeptide stabilized by disulfide bridges / hydrogen / ionic / hydrophobic bond quaternary structure is where several polypeptide subunits join conjugated proteins are proteins which combine with other non-protein molecules for example metals / nucleic acids / carbohydrates / lipids

competitive inhibitor has similar shape/structure to the substrate therefore it fits to the active site no reaction is catalyzed so the inhibitor remains bound substrate cannot bind as long as the inhibitor remains bound only one active site per enzyme molecule substrate and inhibitor compete for the active site therefore high substrate concentrations can overcome the inhibition as substrate is used up ratio of inhibitor to substrate rises named example of inhibitor plus inhibited enzyme / process / substrate

enzymes are globular proteins there is an active site substrate(s) binds to active site shape of substrate (and active site) changed / induced fit bonds in substrate weakened activation energy reduced sketch of energy levels in a reaction to show reduced activation energy in feedback inhibition a (end) product binds to the enzyme end-product is a substance produced in last / later stage of a pathway modulator / inhibitor / effector / product binds at the allosteric site / site away from the active site binding causes the enzyme / active site to change shape substrate no longer fits the active site the higher the concentration of end-product the lower the enzyme activity enzyme catalyzes the first / early reaction in pathway so whole pathway is inhibited prevents build-up of intermediates allosteric inhibition is non-competitive