Topic 2

Question 1

Explain how the structure and properties of phospholipids help to maintain the structure of cell membranes. 9 marks

Answer 1

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

Question 2

Explain the role of vesicles in transportation of materials within cells. 8 marks

Answer 2

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

Question 3

Describe the process of active transport. 4 marks

Answer 3

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

Question 4

Outline the ways in which substances move passively across membranes. 5 marks

Answer 4

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

Question 5

Distinguish between active and passive movements of materials across plasma membranes, using named examples. 4 marks

Answer 5

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.

Question 6

Outline, with an example, the process of exocytosis. 5 marks

Answer 6

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.

Question 7

Explain the reasons for cell division in living organisms. 8 marks

Answer 7

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

Question 8

Outline the processes that occur in a cell during interphase, including those needed to prepare for mitosis. 4 marks

Answer 8

DNA replication

DNA transcription

enzyme/ protein synthesis

biochemical reactions/ example of a biochemical reaction

cell respiration

growth

organelles replicated

Question 9

Outline the thermal, cohesive and solvent properties of water. 5 marks

Answer 9

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.

Question 10

Describe the significance of water to living organisms. 5 marks

Answer 10

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

Question 11

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

Answer 11

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

Question 12

List three functions of lipids. 3 marks

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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);

Question 16

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

Answer 16

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;

Question 17

Outline how enzymes catalyse reactions. 4 marks

Answer 17

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;

Question 18

Explain the effect of pH on enzyme activity. 3 marks

Answer 18

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

Question 19

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

Answer 19

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

Question 20

Describe the structure of proteins. 9 marks

Answer 20

(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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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