Questions that were answered incorrectly

Name and describe five ways substances can move across the cell surface membrane into a cell (5)

1. Simple diffusion of small/non-polar molecules down a concentration gradient

2. Facilitated diffusion down a concentration gradient via protein carrier/channel

3. Osmosis of water down a water potential gradient

4. Active transport against a concentration gradient via protein carrier using ATP

5. Co-transport of 2 different substances using a carrier protein

The movement of substances across cell membranes is affected by membrane structure. Describe how. (5)

1. Phospholipid bilayer allows movement/diffusion of non-polar/lipid soluble substances

2. Phospholipid bilayer prevents movement/diffusion of polar substances (or) proteins allow polar-charged substances to cross the membrane

3. Carrier proteins allow active transport

4. Channel/carrier proteins allow facilitated diffusion/co-transport

5. Shape/charge of channel/carrier determines which substances move

6. Number of channels/carriers determines how much movement

7. Membrane surface area determines how much diffusion

8. Cholesterol affects fluidity/rigidity/permeability

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells. (6)

• TEM uses electrons and optical uses light

• TEM allows greater resolution

• (TEM) smaller organelles can be observed/observed in greater detail

• TEM can only view dead specimens and optical can only view live

• TEM does not show colour and optical can

• TEM requires thinner specimens

• TEM requires a more complex preparation

• TEM uses magnets and optical uses lenses/glasses

Describe and explain the processes that occur during meiosis that increase genetic variation (5)

• Homologous chromosomes pair up

• Independent segregation of homologous chromosomes in meiosis I, and of chromatids in meiosis II

• Random allocation of chromosomes into each daughter cell (maternal and paternal chromosomes are re-shuffled in any combination)

• Crossing over leads to exchange of parts of non-sister chromatids/alleles between homologous chromosomes

• (Both) create new combinations of alleles

Describe how vaccination can lead to protection against a disease (7)

• Vaccine contains antigens from pathogen which are injected into bloodstream

• Antigen-presenting cell/B-cell presents antigens on its surface

• T cell with complementary receptor protein binds to antigen

• Activates/stimulates B cell to divide by mitosis/produce clones

• B cells develop into plasma cells/memory cells

• Plasma cells release antibodies

• Antibodies cause pathogens to agglutinate which are killed/engulfed by phagocytes

• On second exposure memory cells rapidly produce antibodies/produces more antibodies

Describe the process involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels (5)

• Micelles contain bile salts and fatty acids/monoglycerides

• Make fatty acids/monoglycerides more soluble in water
  (or) bring/release fatty acids/monoglycerides to cell lining of the ileum

• Fatty acids/monoglycerides absorbed by simple diffusion

• Triglycerides reformed in cells and cholesterol and lipoproteins added to form chylomicrons

• Chylomicrons released by exocytosis

Describe how a polypeptide if formed by translation of mRNA (6)

• mRNA attaches to ribosome

• tRNA anticodons bind to complementary mRNA codons

• tRNA brings a specific amino acid

• Amino acids join by peptide bonds

• Amino acids join together with the use of ATP

• tRNA released (after amino acid joined to polypeptide)

• The ribosome moves along the mRNA to form the polypeptide

Describe how mRNA is produced in a plant cell (6)

• Hydrogen bonds between DNA bases break

• Only one DNA strand acts as a template

• Free RNA nucleotides align by complementary base pairing

• Uracil base pairs with adenine/used in the place of thymine

• RNA polymerase joins (adjacent) RNA nucleotides by making phosphodiester bonds

• Pre-mRNA is spliced (or introns are removed)

Describe how the structure of glycogen is related to its function (4)

• Helix/coiled/branched so compact

• Polymer of glucose so easily hydrolysed

• Branched so more ends for faster hydrolysis

• Glucose (polymer) so provides respiratory substrate for energy

• Insoluble so does not affect water potential/osmosis

Explain why viruses are described as acellular and non-living? (2)

• No cell surface membrane
  (or) not made of cells
  (or) do not have organelles/cell structures

• Have no metabolic reactions
  (or) cannot independently replicate/respire

One reason why antibiotics are not effective against viruses (1)

• Do not have bacterial structures/enzymes
  (or) do not have metabolic processes
  (or) do not have a cell wall/murein

Explain the importance of one adaptation of the gas exchange surface in the tracheal system of an insect (2)

• Tracheole wall thin/one cell thick

• So rapid diffusion into cells/short diffusion pathway

(or)

• Tracheoles are highly branched

• So short diffusion pathway
  (or) So large surface area for diffusion

Explain the importance of the xylem being kept open as a continuous tube (3)

• Allows unbroken water column/no barrier to water movement

• Cohesion from H bonds between water molecules

• Transpiration/evaporation creates tension
  (or) water moves from xylem into cells to create tension
  (or) pull up water creates tension in xylem

What is meant by genetic diversity? (1)

• The number of different alleles of each gene
  (or) number of different base sequences found of each gene

Suggest two reasons why populations might show very low levels of genetic diversity (2)

• Population might have been very small/genetic bottleneck

• Population might have started with a small number of individuals/founder effect (one pregnant female)

• Inbreeding

Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst (3)

• Substrate binds to the active site
  (or) Enzyme-substrate complex forms

• Active site changes shape so that it is complementary to the substrate
  (or) Active site changes shape, distorting/breaking bonds in the substrate

• Reduces activation energy

Describe the complete digestion of starch by a mammal. (4)

1. Hydrolysis

2. of glycosidic bonds

3. Starch to maltose by amylase

4. maltose to glucose by maltase/disaccharidase

5. disaccharidase/maltase is membrane-bound

Define ‘gene mutation’ and explain how a gene mutation can have:

• no effect on an individual

• a positive effect on an individual. (4)

• Gene mutation is a change in the base/nucleotide sequence of DNA

• Results in the formation of new allele
  

(Has no effect because)

• Genetic code is degenerate (so amino acid sequence may not change)
  (or)
  Mutation is in an intron

• Does change amino acid but no effect on tertiary structure

• (New allele) is recessive so does not influence phenotype

(Positive effect because)

• Results in change in polypeptide that positively changes the properties of the protein

• May result in increased reproductive success or increased survival chances

Describe how viruses are replicated (4)

1. Virus attachment proteins attach to cell receptors

2. Viral nucleic acid/genetic information enters cell

3. Viral nucleic acid is replicated (or) reverse transcriptase produces DNA from RNA

4. Viral protein is produced

5. Viral components assembled and released from cell

Describe and explain how the structure of DNA results in accurate replication (4)

• 2 strands therefore semi-conservative replication (if possible)

• base pairing/hydrogen bonds hold strands together

• hydrogen bonds weak and easily broken, allows strands to separate

• base sequence exposed so acts as a template

• A with T, C with G, complementary copy

• DNA one parent and one new strand

Describe the behaviour of chromosomes during mitosis and explain how this results in the production of two genetically identical cells. (7)

1. chromosomes shorten/thicken/supercoiling

2. each chromosome has 2 identical chromatids (due to replication)

3. chromosomes/chromatids move to equator

4. attach to individual spindle fibres

5. spindle fibres contract/centromeres divide

6. sister chromatids separate and move to opposite poles

7. each pole receives identical copies of each chromosome

8. nuclear envelope forms around each group of chromosomes at each pole

Describe the role of haemoglobin in the loading, transport and unloading of oxygen (5)

1. Hb loads/associates oxygen in the lungs

2. At high partial pressure of oxygen

3. Binding of an oxygen molecule to Hb makes binding of subsequent oxygen easier (positive co-operativity)

4. Oxygen transported as oxyhaemoglobin in red blood cells

5. Hb unloads/dissociates in the respiring cells/tissues

6. At a low partial pressure of oxygen

Describe the cohesion-tension theory of water transport in the xylem (5)

• Water is lost from leaf due to transpiration/evaporation of water from stomata

• Lowers water potential of mesophyll/leaf cells

• Water pulled up xylem (creating tension)

• Water molecules cohere/stick together by hydrogen bonding

• Forming a continuous water column

• Adhesion of water molecules to the walls of xylem

Describe and explain how the structure of the mammalian breathing system enables efficient uptake of oxygen into the blood (6)

1. alveoli provide a large surface area

2. walls of alveoli thin to provide short diffusion pathway

3. walls of capillary thin to provide short diffusion pathway

4. cell membranes permeable to gases

5. many blood capillaries provides a large surface area

6. intercostal/diaphragm muscles/ventilate lungs/maintain diffusion gradient

7. wide trachea/branching of bronchi and bronchioles for efficient flow of air

8. cartilage rings keep airways open

Describe how the structure of the insect gas exchange system
- provides cells with sufficient oxygen
- limits water loss (5)

1. spiracles allow diffusion (of oxygen) / oxygen diffusion through trachea or tracheoles

2. Tracheoles are highly branched so large surface area

3. Tracheole (walls) thin so short diffusion distance to cells
   (or) Highly branched so short diffusion distance

4. Tracheole walls are permeable to oxygen/air

5. Exoskeleton (impermeable) so reduces water loss

6. Spiracles can close so no/less water loss

7. Hairs around spiracles reduce water loss

Describe how oxygen in the air reaches capillaries surrounding alveoli in the lungs. Details of breathing are not required (4)

• Trachea and bronchi and bronchioles

• down pressure gradient

• down diffusion gradient

• across alveolar epithelium

• across capillary endothelium

Describe the structure and function of the nucleus [4]

• Nuclear envelope/double membrane and pores

• Chromosomes/chromatin (or) DNA with histones

• Nucleolus

• Stores genetic information/material for polypeptide production (or) is the code for polypeptides

• DNA replication occurs

• Production of mRNA/tRNA (or) transcription occurs

• Production of rRNA/ribosomes

Explain how the use of antibiotics has led to antibiotic-resistant strains of bacteria becoming a common cause of infection acquired when in hospital (3)

1. Some bacteria have resistant alleles

2. Exposure to antibiotics acts as a selection pressure
   (or) Resistant bacteria survive/reproduce
   (or) Non-resistant bacteria die

3. More antibiotics used in hospital (compared with elsewhere)
   (or) Patients already have weakened immune systems
   (or) High frequency of resistance allele within bacterial population

NHE3 actively transports one sodium ion into the cell in exchange for one hydrogen ion out of the cell
Use your knowledge of transport across cell membranes to suggest how NHE3 does this (3)

• Co-transport

• Uses hydrolysis of ATP

• Hydrogen ion and sodium ion bind to carrier protein

• Protein changes shape (to move sodium/hydrogen across the membrane)

Describe how bacteria divide (2)

• Binary fission

• Replication of circular DNA

• Cytoplasm divides to produce 2 daughter cells

• Each with single copy of circular DNA

Suggest one advantage to a bacterium of secreting an extracellular protease in its natural environment (2)

• To digest protein

• So they can absorb amino acids for growth/reproduction/protein synthesis of organelles
  (or) To destroy a toxic substance/protein

In DNA replication, why can new nucleotides only be added in a 5’ to 3’ direction? (4)

1. Reference to DNA polymerase

2. Which is specific

3. Only complementary with/binds to 5’ end (of strand)

4. Shapes of 5’ end and 3’ end are different/description of how different

Describe the mass flow hypothesis for the mechanism of translocation in plants (4)

1. In source/leaf sugars actively transported into phloem (co-transport with hydrogen ions)

2. By companion cells

3. Lowers water potential of sieve tube element and water enters by osmosis

4. Increase in pressure causes mass movement of sucrose towards sink

5. Sugars used/converted in root for respiration or storage (starch)

Describe the structure of proteins (5)

1. Polymer of amino acids

2. Joined by peptide bonds

3. Formed by condensation reaction

4. Primary structure is the order of amino acids

5. Secondary structure is folding of polypeptide chain due to hydrogen bonding (alpha helix and beta pleated sheet)

6. Tertiary structure is 3D folding due to hydrogen bonding, ionic bonding and disulphide bridges

7. Quaternary structure is two or more polypeptide chains

Describe how proteins are digested in the human gut (4)

1. Hydrolysis of peptide bonds

2. Endopeptidases break polypeptides into smaller polypeptide chains

3. Exopeptidases remove terminal amino acids

4. Dipeptidases hydrolyse/break down dipeptides into amino acids

[Cognito]
Suggest one advantage and one disadvantage of replanting hedges (2)

(Advantage)

• Increased biodiversity may increase predation of pests

• Increased predation of pests would mean fewer pesticides required

• Increase in pollinators leading to increased yield/outcome

(Disadvantages)

• Reduced land area for growth of crops

• Greater biodiversity may increase number of pests

• Increase in pest population may reduce yield/income

• Seeds/plants from hedges may be blown onto fields and compete with crops

There is currently no effective vaccine available for HIV. Suggest one reason why (1)

Because HIV RNA mutates often, this means a higher genetic diversity (vaccines may not be effective against all strains of the virus)