The Structure And Function Of Carbohydrates Essay Mark Scheme

Previous IB Exam Essay Questions: Unit 3

Use these model essay question responses to prepare for essay questions on your in class tests, as well as the IB Examination, Paper 2. These questions have appeared on recent IB examinations, exactly as shown below. Following each question is the markscheme answer which was used to evaluate student answers on the examination paper.
The first 15 essay questions are possible candidates for the unit test
1. Outline the thermal, cohesive and solvent properties of water.5 marks
  • 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.

2. Describe the significance of water to living organisms.6 marks

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
  • transparency - allows plants to photosynthesize in water / allows animals to see
  • (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

3. Describe the use of carbohydrates and lipids for energy storage in animals.5 marks

Answers must discuss both carbohydrates and lipids to receive full markscarbohydrates: 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

4. List three functions of lipids.3 marks

  • 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

5. Describe the significance of polar and non-polar amino acids.5 marks

For the maximum mark the response must have polar and non-polar amino acids

polar amino acids: 3 max

  • hydrophilic
  • can make hydrogen bonds
  • found in hydrophilic channels/parts of proteins projecting from membranes
  • found on surface of water-soluble protein

non-polar amino acids: 3 max

  • hydrophobic
  • forms van der Waals/hydrophobic interactions with other hydrophobic amino acids
  • found in protein in interior of membranes
  • found in interior of water soluble proteins

6. Outline the role of condensation and hydrolysis in the relationship between amino acids and dipeptides.4 marks

  • 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

7. Describe the structure of proteins.9 marks

  • (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

8. List four functions of proteins, giving an example of each.4 marks

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

9. Distinguish between fibrous and globular proteins with reference to one example of each protein type.6 marks

  • fibrous proteins are strands/sheets whereas globular proteins are rounded;
  • fibrous proteins (usually) insoluble whereas globular proteins (usually) soluble;
  • globular more sensitive to changes in pH/temperature/salt than fibrous;
  • fibrous proteins have structural roles / other specific role of fibrous protein;
  • globular proteins used for catalysis/transport/other specific role of globular protein;
  • another role of globular protein;
  • named fibrous proteins e.g. keratin/fibrin/collagen/actin/myosin/silk protein;
  • named globular protein e.g. insulin/immunoglobulin/hemoglobin/named enzyme;
Do not accept statements about fibrous proteins having only secondary structure and globular proteins having only tertiary structure.

10. (a) Lactase is widely used in food processing. Explain three reasons for converting lactose to glucose and galactose during food processing. 3 marks

  • 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);

(b) Simple laboratory experiments show that when the enzyme lactase is mixed with lactose, the initial rate of reaction is highest at 48 °C. In food processing, lactase is used at a much lower temperature, often at 5 °C. Suggest reasons for using lactase at relatively low temperatures.2 marks

  • 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;

11. Outline how enzymes catalyse reactions.7 marks

  • they increase rate of (chemical) reaction;
  • remains unused/unchanged at the end of the reaction;
  • lower activation energy;
  • activation energy is energy needed to overcome energy barrier that prevents reaction;
  • annotated graph showing reaction with and without enzyme;
  • 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);
  • induced fit model / change in enzyme conformation (when enzyme-substrate/ES complex forms);
  • making the substrate more reactive;

12. Explain the effect of pH on enzyme activity.3 marks

  • 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

13. Compare the induced fit model of enzyme activity with the lock and key model.4 marks

  • 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

14. Draw graphs to show the effect of enzymes on the activation energy of chemical reactions5 marks

(for the first graph, which may be either exothermic or endothermic, award up to 1 mark for any of the following, up to 4 marks)

  • vertical axis with energy label and horizontal axis with time label
  • labels showing reactant / substrate and product
  • labeled line showing correct shape and curve without enzyme
  • labeled line showing correct shape and curve with enzyme
  • labels for activation energy with and without enzymes

(Award 1 mark for a second graph which shows the correct curves for an endergonic reaction if the first graph was exothermic or vice versa. For the second graph, no marks will be awarded for labels)

15. Explain, using one named example, the effect of a competitive inhibitor on enzyme activity.6 marks

  • 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

These following 4 questions will not be candidates for the unit test
1. Describe the structure of triglycerides.6 marks
  • composed of C, H and O (must be stated)
  • relatively more C and H/less O than carbohydrates
  • composed of fatty acids and glycerol
  • glycerol is CH2.OH.CH.OH.CH2OH/ diagram showing it separately or as part of a triglyceride
  • fatty acids are carboxyl groups with hydrocarbon chain attached/ diagram showing it separately or as part of a triglyceride
  • ester bonds/diagram showing C-O-C=O
  • three fatty acids/hydrocarbon chains linked to each glycerol (must be stated)
  • 12-20 carbon atoms per hydrocarbon tail/diagram showing this number
  • saturated if all the C-C bonds are single/unsaturated if one or more double bonds
  • whole molecule is nonpolar/hydrophobic

2. Discuss the solubility of proteins in water.4 marks

many proteins are soluble in water:

  • solubility depends on what amino acids /R groups are present
  • smaller proteins are more soluble than big ones
  • proteins with many polar / hydrophilic amino acids / R groups are more soluble / soluble
  • proteins with polar / hydrophilic amino acids / R groups on the outside are soluble
  • example of a polar amino acid / group
  • globular proteins are more soluble than fibrous proteins

however, the solubility of proteins can be limited:

  • solubility of proteins may also be affected by conditions (pH, temperature, salinity)
  • denaturation makes proteins insoluble
  • proteins do not form true solutions in water but colloidal solutions

3. Outline enzyme-substrate specificity5 marks

  • active site of enzyme binds to specific substrate
  • shape of the active site and substrate fit/complement each other
  • lock and key model
  • chemical properties of substrate and enzyme attract/opposite charges
  • enzyme/active site is not rigid and substrate can induce slight changes in shape
  • allows substrates of similar structure to bind with same enzyme
  • induced fit
  • causes weakening of bonds in substrate to lower activation energy

4. Explain how proteins act as enzymes, including control by feedback inhibition in allosteric enzymes.9 marks

  • 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
  • How Bacteria affect Human Lives

  • How microorganisms can benefit humans

  • The functions of mineral ions in plants and animals

  • How the structure of different cells is related to their function

  • The effect of temperature on living organisms and the processes which occur in them

  • Relationships between different species of organisms

  • The importance of osmosis in plants and animals

  • The ways in which genes and the environment contribute to variation

  • How different organisms obtain the element nitrogen

  • The functions of cell-surface membranes

  • The ways in which genes and environment effects phenotype of an organism

  • The part played by microorganisms in nutrient cycles

  • The similarities and differences between nervous and hormonal control in animals

  • The passage of water through plants

  • The effect of ecological conditions on the distribution of organisms

  • The functions of proteins

  • Adaptations to living on land

  • Homeostasis in a mammal

  • How the body of a mammal obtains nutrients before and after birth

  • Energy flow through ecosystems

  • How light affects living organisms

  • The importance of carbohydrates in living organisms

  • The functions of blood

  • The advantages and disadvantages of living in water

  • The adaptations of parasites to their way of life

  • The genetic code

  • The relation between the structure of different cells and their functions

  • Support and movement in living organisms

  • Applications and implications of gene technology

  • Roles of pigments in living organisms

  • Control of the internal environment in living organisms

  • The role of enzymes in living organisms

  • Gas exchange in animals and flowering plants

  • Lipids in living organisms

  • Chemical coordination in plants and animals

  • The movement of molecules and ions through membranes

  • The chemical and biological control of insect pests

  • Transport systems in mammals and flowering plants

  • ATP and its roles in living organisms

  • Production and elimination of waste products in animals

  • The role of water in the lives of organisms

  • The factors affecting the growth and size of populations

  • The functions of proteins in plants and animals

  • Natural selection and the effects of environmental change

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