Topic 2 Flashcards
Explain how the structure and properties of phospholipids help to maintain the structure of cell membranes. 9 marks
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
Explain the role of vesicles in transportation of materials within cells. 8 marks
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
Describe the process of active transport. 4 marks
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
Outline the ways in which substances move passively across membranes. 5 marks
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
Distinguish between active and passive movements of materials across plasma membranes, using named examples. 4 marks
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.
Outline, with an example, the process of exocytosis. 5 marks
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.
Explain the reasons for cell division in living organisms. 8 marks
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
Outline the processes that occur in a cell during interphase, including those needed to prepare for mitosis. 4 marks
DNA replication
DNA transcription
enzyme/ protein synthesis
biochemical reactions/ example of a biochemical reaction
cell respiration
growth
organelles replicated
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.
Describe the significance of water to living organisms. 5 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
(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
Describe the use of carbohydrates and lipids for energy storage in animals. 5 marks
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
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
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
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
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);
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;
Outline how enzymes catalyse reactions. 4 marks
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;
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
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
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
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
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
