biology - 5/6markers

describe the structure of DNA (5)

long double helix structure held together by hydrogen bond between the complementary nitrogenous bases. each dna polynucleotide strands made up of alternating deoxyribose sugar, phosphate group to form the sugar-phosphate backbone

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

• By osmosis:
  - from high to low water potential/down water potential gradient
  - through aquaporins/water channels
  By facilitated diffusion:
  - channel/carrier proteins
  - down concentration gradient
  By active transport:
  - carrier protein/protein pump
  - against concentration gradient
  - using energy from ATP
  By endocytosis/phagocytosis:
  - correct description of process
  By exocytosis/secretion:
  - correct explanation including fusion of Golgi vesicles with cell surface membrane

contrast the structure of the animal cells and prokaryotic cell

describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels

-micelles contain bile salts and fatty acids
-bring fatty acids to the lining of the ileum
- fatty acids absorbed by diffusion
-triglycerides reformed in cells
-vesicles move to cell membrane

describe how the structure of a protein depends on the amino acids it contains

1. strucutre determined by position of amino acid/R group
2. primary structure is the sequence if amino acid
3. secondary strucutre due H-bonds between amino acids formung alpha helices and beta pleated sheets
4. tertiary strucutre formed by hydrogen bonds, ionic bonds, disulphide bonds interactions
5. Creates specific proteins e.g. enzyme active site, antibodies
6. quaternary structure contains more than 1 polypeptide chain

describe how mRNA is formed by transcription in eukaryotes

mRNA is formed in the nucleus of eukaryotic cells, where DNA helicase breaks the hydrogen bonds between the two DNA strands and parallel nucleotides, causing the two DNA strands to unwind and the double helix to break. RNA polymerase then attaches to one DNA strand that acts as the template strand and joins free flowing nucleotides to the exposed nucleotides on the template strand by complementary base pairing and creating phosphodiester bonds between adjacent nucleotides to form the pre-mRNA strand. Base A joins to T, base U to A, and bases C and G to G and C respectively. The RNA polymerase then stops this production of the pre-mRNA strand once it reaches a stop codon on the template strand. The pre-mRNA then undergoes splicing as the double helix of the DNA reforms. The introns are removed and mRNA is formed, where it can leave the nucleus through the nuclear pores.

define gene mutation and explain how a gene mutation can have:

• no effect on an individual

• a positive effect on individuals

Gene mutations are changes in the base sequence of DNA, resulting in the formation of a new allele.

It has no effect because the genetic code is degenerate so the amino acid sequence may not change. It could also be possible that the mutation is located in an intron so the amino acid sequence would also not be changed. If the mutation did change the amino acid sequence, it could still not have an effect on the protein's tertiary structure or the new allele is recessive and so does not influence the individual's phenotype.

A gene mutation could have a positive effect by resulting in a change in a polypeptide that positively changes its properties.

describe the roles of iron ions, sodium ions and phosphate ions in cells

IRON
->As a component of haemoglobin and myoglobin, iron is needed for the transport of oxygen and carbon dioxide in our bodies.
->It is an essential component of cytochromes that are a component of the electron transport chain.
SODUM
->Sodium ions are needed for the maintenance of osmotic pressure and fluid balance, nerve impulse conduction, and automatic rhythmicity of the heart.
PHOPHATE
->Phosphate ions are required for the synthesis of high energy compounds like ATP, GTP, etc. The bonds between the phosphate ions in ATP release a high amount of energy when they are broken.
->Phosphate ions are one of the three components of nucleotides. Thus, they are involved in the synthesis of nucleic acids like DNA and RNA.

the movement of substances across the cell membrane is affected by membrane structure. describe how.

-phospholipid bilayer allows diffusion of non polar/lipid soluble substances
- Phospholipid bilayer prevents diffusion of polar/charged/lipid soluble substances
- carrier proteins allow active transport
- channel/carrier proteins allow facilitated diffusion/co-transport
- shape/charge of carrier protein determines which substances are transported/moved
-number of channel/carrier proteins determine how much movement
- membrane surface area determines how much movement
- cholesterol affects fluidity/permeability

explain five properties that make water important for organisms

1: a metabolite inn condensation/hydrolysis/photosynthesis/respiration

2. A solvent so (metabolic) reactions can occur

3. High heat capacity so buffers changes in temperature

4. Large latent heat of vaporisation so provides a cooling effect (through evaporation)

5. Cohesion (between water molecules) so supports columns of water (in plants)

6. Cohesion (between water molecules) so produces surface tensions supporting (small) organisms

describe the biochemical tests you would use to confirm the presence of lipids, non-reducing sugar, and amylase in a sample

Lipid

1. Add ethanol/alcohol then add water and shake/mix

OR

Add ethanol/alcohol and shake/mix then pour into/add water;

2. White/milky emulsion

OR

emulsion test turns white/milky;

Non-reducing sugar

3. Do Benedict's test and stays blue/negative;

4. Boil with acid then neutralise with alkali;

5. Heat with Benedict's and becomes red/orange (precipitate);

Amylase

6. Add biuret (reagent) and becomes purple/violet/mauve/lilac;

7. Add starch, (leave for a time), test for reducing sugar/absence of starch;

describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers. give two name examples of polymers and their associated monomer to illustrate your answer

1. condensation reaction joins monomers together AND forms a chemical bond AND releases water
2. hydrolysis reaction breaks a chemical bond between monomers AND uses water
3. A suitable example of polymers and the monomers from which they are made
4. A second suitable example of polymers and monomers from which they are made
5. reference to a correct bond within a named polymer

e.g. amino acid, polypeptide, peptide bond, protein

compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells

1. DNA in Eukaryotic cells are linear, DNA in prokaryotic cells are circular
2.DNA is associated with histone proteins in Eukaryotic cells , DNA not associated with histone proteins in prokaryotic cells
3. The DNA nucleotides in both are the same , and have phosphodiester bonds
4. Eukaryotic DNA contain introns, prokaryotic DNA does not.

describe the gross structure of human gas exchange system and how we breathe in and out

Trachea, bronchi, bronchioles, alveoli
External intercostal muscles contract, diaphragm contracts opening chest cavity
Causes volume increase + pressure decrease in the thoracic cavity
Breathing out = diaphragm relaxes + internal intercostal muscles contract
Causes volume decrease + pressure increase

mucus produced by epithelial cells in the human gas exchange systems contains triglycerides and phospholipids. compare and contrast the structure and properties of triglycerides and phospholipid

1. Both contain ester bonds (between glycerol and fatty acid);
2. Both contain glycerol
3. Fatty acids on both may be saturated/unsaturated
4. Both are insoluble in water;
5. Both contain C, H and O but phospholipids also contain P;
6. Triglyceride has three fatty acids and phospholipid has two fatty acids plus phosphate group;
7. Triglycerides are hydrophobic/non-polar and phospholipids have hydrophilic and hydrophobic region;
8. Phospholipids form monolayer (on surface)/micelle/bilayer (in water) but triglycerides don't

describe the cohesion tension theory of water transport in the xylem

- Water evaporates from leaves (transpiration)
- This creates tension (suction), pulling more water up the lead
- Water molecules are cohesive (stick tougher due to H bonds) so when some are pulled up, others follow. SO whole column of water moves upwards.
- Water enters the stem through roots.

describe how mRNA is produced in a plant cell

1. The DNA strands separate by breaking the H bonds;
OR
H bonds broken between (complementary) (DNA) bases;
2. (Only) one of the strands/template strand is used (to make mRNA/is transcribed);
3. (Complementary) base pairing so A→U, T→A, C→G, G→C;
4. (RNA) nucleotides joined by RNA polymerase;
5. pre-mRNA formed;
6. Splicing / introns removed to form mRNA;

contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells

-TEM use electrons and optical use light.

-TEM allows a greater resolution.

-TEM doesn't show colour and optical can.

-TEM requires thinner specimens.

-TEM focuses using magnets and optical uses lenses.

-TEM requires a more complex preparation.

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell. (6)

1. Helicase;
2. Breaks hydrogen bonds;
3. Only one DNA strand acts as template;
4. RNA nucleotides attracted to exposed bases;
5. (Attraction) according to base pairing rule; 6. RNA polymerase joins (RNA) nucleotides together;
7. Pre-mRNA spliced to remove introns.

Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.
Do NOT include descriptions of transcription and splicing in your answer. (5)

1. mRNA associates with a ribosome / ribosome attaches to mRNA;
2. Ribosome moves to / finds the start codon / AUG;
3. tRNA brings / carries (appropriate / specific) amino acid;
4. Anticodon (on tRNA complementary) to codon (on mRNA);
5. Ribosome moves along to next codon;
OR
Ribosome 'fits' around two codons / can fit two tRNAs;
6. (Process repeated and) amino acids join by peptide bonds / condensation reaction (to form polypeptide);
OR
(Process repeated and) amino acids joined using (energy from) ATP (to form polypeptide);

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. (5)

Comparisons
1. Nucleotide structure is identical; 2. Nucleotides joined by phosphodiester bond;
OR
Deoxyribose joined to phosphate (in sugar, phosphate backbone);
3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes;

Contrasts
4. Eukaryotic DNA is longer;
5. Eukaryotic DNA contain introns, prokaryotic
DNA does not;
6. Eukaryotic DNA is linear, prokaryotic DNA is circular;
7. Eukaryotic DNA is associated with / bound to protein / histones, prokaryotic DNA is not;

Transcription and translation are stages in protein synthesis. Describe what happens during transcription and translation. (6)

Transcription
1. DNA strands separate (by DNA helicase)
2. (Free) nucleotides bind to (one) strand by specific base pairing(
3. U to A (and C to G) / uracil instead of thymine;
4. RNA polymerase joins nucleotides to form mRNA;

Translation
1. mRNA translated at/moves to ribosome;
2. Codon on mRNA;
3. Codon binds with (complementary) anticodon on tRNA;
4. Each tRNA specific to one amino acid;
5. Formation of peptide bonds;

Describe the structure of a cellulose molecule and explain how cellulose is adapted for its functionin cells (6 marks)

1. made from β-glucose;

2. joined by condensation to form glycosidic bond;

3. 1 : 4 link described;

4. "flipping over" of alternate molecules;

5. hydrogen bonds linking long straight chains;

6. cellulose makes cell walls strong;

7. can resist turgor pressure/osmotic pressure;

8. bond difficult to break;

9. resists action of enzymes

Describe how the structures of starch and cellulose molecules are related to their functions. (5 marks)

Starch (max 3)
1.Helical/spiral shape so compact;

2.Molecule is insoluble so osmotically inactive (does not affect water potential)

3.Branched so glucose is easily accessible by enzymes to break down for respiration

4.Large molecule so cannot leave cell/cross cell-surface membrane

Cellulose (max 3)
5.Long, straight & unbranched chains of β glucose

6.Joined by hydrogen bonding, to form (micro/macro) fibrils

8.These provide rigidity/strength;

How are monosaccharide and amino acids absorbed from the lumen of the small intestine into the blood stream? ( 6 marks)

- sodium potassium pump
- active transport of Na+ out of epithelial cells into the blood/capillaries
- reduced concentration of Na+ in epithelial cells
- Na+ enters epithelial cells by facilitated diffusion
-Cotransporters carry glucose/amino acids into epithelial cells at the same time as Na+
- glucose/amino acids enter capillaries/ blood by facilitated diffusion/diffusion

How are lipids digested in the human digestive system? (5 marks)

- hydrolysis of ester bonds
- emulsification of lipid by bile salts/bile
-explanation of emulsification (large droplets to smaller droplets)
- Lipase from the pancreas /in pancreatic juice
- digestion of lipids in the ileum
- digestion to glycerol and fatty acids

Describe the role of enzymes in the digestion of proteins in a mammal (4 marks)

-hydrolysis of peptide bonds
- endopeptidases act in middle of proteins/polypeptides producing shorter polypeptides with more free ends
- exopeptidases act at ends of polypeptides producing dipeptides and amino acids
- dipeptides hydrolyse peptide bonds between 2 amino acids/in dipeptides producing single amino acids

How are proteins digested in the human digestive system? (7 marks)

- hydrolysis of peptide bonds by peptidases
- endopeptidases in stomach break bonds near the middle/within of polypeptides
- produce smaller peptides/polypeptides
- endopeptidases and exopeptidases in pancreatic juice
- exopeptidases in ileum break bonds near the ends of polypeptides
- produce dipeptides
- dipeptidases in plasma membranes/surface membranes of cells lining/epithelial cells ileum/small intestine
- produce amino acids

How is starch digested in the human digestive system? (6 marks)

- hydrolysis of glycosidic bonds
- amylase in saliva/salivary amylase
- Digestion of amylose to maltose in the mouth
- pancreatic amylase
- digestion of amylase to maltose in the ileum
- maltase in membranes of epithelial cells of ileum
- digests maltose to glucose

Bacterial meningitis is a potentially fatal disease affecting the membranes around the brain.
N meningitidis (MN) is a leading cause of meningitis
In the UK children are vaccinated against this disease. Describe how vaccination can lead to protection against bacterial meningitis (6 marks)

- The antigens (from the vaccine) bind to complementary antibody/receptor on a B lymphocyte
- Antigen engulfed by endocytosis and presented in membrane of B lymphocyte
- T lymphocytes/helper cells stimulate mitosis of B lymphocytes
- B lymphocyte undergoes mitosis/produces a clone/clonal selection
- B lymphocytes differentiate to produce plasma cells which release antibodies
- And B memory cells which produce antibodies faster (on future exposure to the same antigen)
- Herd immunity - disease less likely to spread if majority of population vaccinated

Explain how the destruction of cancer cells can be brought about by T lymphocytes (5 marks)

- T lymphocyte with complementary receptors on surface binds to non self antigen/abnormal proteins on cancer cell (antigen presenting)
- T lymphocyte undergoes mitosis and differentiation/clone of T lymphocytes produced
- Cytotoxic T cells
- Bind to non self antigen/abnormal proteins on cancer cell
- Cytotoxic T cells produce proteins which punch holes in cell membrane
- Cancer cell lyses/bursts and dies

Explain how exposure of a child to the chicken pox virus sets of an immune response leading to the production of antibodies (5 marks)

- B lymphocyte with complementary antibodies on surface binds to antigen on virus/antigen presenting phagocyte
- Antigen is engulfed by endocytosis and presented in the membrane of the B lymphocyte
- T helper cells with complementary receptors to the antigen bind to antigen on B cell
- B cell undergoes mitosis
- clonal selection
- Plasma cells produce antibodies

When a pathogen enters the body, it may be destroyed by phagocytosis. Describe how. (5 marks)

- Phagocyte is attracted to chemical attractants released by pathogen/recognise antigens on bacteria as foreign
- Pathogen engulfed/ingested
-Pathogen enclosed/inside phagosome/vacuole/vesicle
-Lysosomes fuse with phagosome/vacuole/vesicle and empty enzymes into lysosome
- Pathogen digested/hydrolysed

In humans, the enzyme maltase breaks down maltose to glucose. This takes place at normal body temperature.
Explain why maltase:
-only breaks down maltose
-allows this reaction to take place at normal body temperature. (5)

1. Tertiary structure / 3D shape of enzyme (means);
2. Active site complementary to maltose / substrate / maltose fits into active site / active site and substrate fit like a lock and key;
3. Description of induced fit;
4. Enzyme is a catalyst;
5. Lowers activation energy / energy required for reaction;
6. By forming enzyme-substrate complex;

scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase. Describe competitive and non-competitive inhibition of an enzyme.

1. Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;

(Competitive inhibition),

2. Inhibitor similar shape (idea) to substrate;

3. (binds) in to active site (of enzyme);

4. (Inhibition) can be overcome by more substrate;

(Non-competitive inhibition),

5. Inhibitor binds to site on enzyme other than active site; (allosteric site)

6. Prevents formation of active site / changes (shape of) active site;

7. Cannot be overcome by adding more substrate;

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells

1. Cell homogenisation to break open cells;
2. Filter to remove (large) debris/whole cells;
3. Use isotonic solution to prevent damage to mitochondria/organelles;
4. Keep cold to prevent/reduce damage by enzymes / use buffer to prevent protein/enzyme denaturation;
5. Centrifuge (at lower speed/1000 g) to separate nuclei/cell fragments/ heavy organelles;
6. Re-spin (supernatant / after nuclei/pellet removed) at higher speed to get mitochondria in pellet/at bottom;

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure. (5)

Principles:
1. Electrons pass through/enter (thin) specimen;
2. Denser parts absorb more electrons;
3. (So) denser parts appear darker;
4. Electrons have short wavelength so give high resolution;
Limitations:
5. Cannot look at living material / Must be in a vacuum;
6. Specimen must be (very) thin;
7. Artefacts present;
8. Complex staining method / complex/long preparation time;
9. Image not in 3D / only 2D images produced;

Explain how the transport of sodium ions is involved in absorption of glucose

1. sodium ions leave epithelial cells and enter the blood
2. by active transport using ATP
3. so sodium ion concentration lower in cell than lumen
4. sodium ions enter the epithelial by facilitated diffusion down concentration gradient
5. glucose co-transported/absorbed with sodium ions against it's concentration gradient

Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein.

Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. (5 marks)

1. chloride ions water soluble/charged/polar
2. cannot cross lipid bilayer of membrane
3. chloride ions transported by facilitated diffusion or diffusion involving carrier/channel proteins
4. oxygen not charged or polar
5. oxygen can diffuse across the lipid bilayer

Describe the processes involved in the transport of sugars in plant stems

1. at source, sucrose actively transported into the phloem sieve tube elements

2. by companion cells

3. lowers the water potential in phloem sieve tube elements so water enters by osmosis

4. produces high hydrostatic pressure

5. mass flow towards the sink/roots/storage tissue

6. at the sink root sugars are removed/unloaded

Describe the structure of proteins

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation;
4. Primary structure is sequence of amino acids;
5. Secondary structure is folding of polypeptide chain due to hydrogen bonding;
Accept alpha helix / pleated sheet
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
7. Quaternary structure is two or more polypeptide chains;

Describe how proteins are disgusted in the human gut

1. hydrolysis of peptide bonds
2. endopeptidases break polypeptides into smaller peptide chains
3. exopeptidases remove terminal amino acids
4. dipeptidases hydrolyse dipeptides into amino acids

Bacterial meningitis is a potentially fatal disease affecting the me,brands around the brain.

Neisseria meningitidis (Nm) is a leading cause of bacterial meningitis

Penicillin…….

Describe how the population of Neisseria meningitidis can become resistant to these antibiotics

1. mutation
2. results in Nm cells with allele resistance to one antibiotic
3. this cell survives and passes are the allele for resistance to offspring
4. process repeated with different genes conferring resistance to each other antibiotics

Contrast the structure of a bacterial cell and the structure of a human cell.

1. Bacterial cell is much smaller than a human cell;
2. Bacterial cell has a cell wall but human cell does not;
3. Bacterial cell lacks a nucleus but human cell has a nucleus;
4. Bacterial cell lacks membrane-bound organelles but human cell has membrane-bound organelles;
5. Bacterial ribosomes smaller than human ribosomes
6. Bacterial DNA is circular but human DNA is linear;
7. Bacterial DNA is 'naked' whereas human DNA is bound to histones / proteins

Describe the appearance and behaviour of chromosomes during mitosis

(During prophase)

1. Chromosomes, coil / condense / shorten / thicken / become visible;

2. (Chromosomes) appear as (two sister) chromatids joined at the centromere;

(During metaphase)

3. Chromosomes line up on the equator / centre of the cell;

4. (Chromosomes) attached to spindle fibres;

5. By their centromere;

(During anaphase)

6. The centromere splits / divides;

7. (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell / separate;

(During telophase)

8. Chromatids / chromosomes

uncoil / unwind / become longer / thinner.

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

1. TEM use electrons and optical use light;
2. TEM allows a greater resolution;
3. So with TEM smaller organelles can be seen and in greater detail
4. TEM view only dead specimens and optical can view live specimens;
5. TEM does not show colour and optical can;
6. TEM requires thinner specimens;
7. TEM requires a more complex and time consuming preparation;
8. TEM focuses using magnets and optical uses (glass) lenses;

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree.
He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a population of trees in the countryside.

Describe how he should collect and process data from these seeds to investigate whether there is a difference in seed size between these two populations of trees

1. Use random sample of seeds (from each population);
2. Use (large enough) sample to be representative of whole population;
3. Indication of what size was measured eg mass;
4. Calculate a mean and standard deviation (for each population);
5. Use the (Student's) t-test;
6. Analyse whether there is a significant difference between (the means of) the two populations;

Haemoglobin are chemically similar molecules found in different species.

Differences in the primary structures of the haemoglobin molecules can provide evidence of phylogenic relationships between species.

Explain how.

1. mutations change base/nucleotide sequence
2. so change the amino acid sequence
3. mutations build up overtime
4. more mutations more differences in amino acids base sequence in closely related species
5. Closely related species have recent common ancestors

Describe how DNA is replicated

1. Strands separate / H-bonds break;
2. DNA helicase (involved);
3. Both strands/each strand act(s) as (a) template(s);
4. (Free) nucleotides attach;
5. Complementary/specific base pairing / AT and GC;
6. DNA polymerase joins nucleotides (on new strand);
7. H-bonds reform;
8. Semi-conservative replication / new DNA molecules contain one old strand and one new strand;

Describe and explain the processes that occur during meiosis that increase genetic variation.

1. Homologous chromosomes pair up;
2. Independent segregation;
3. Maternal and paternal chromosomes are re-shuffled in any combination;
4. Crossing over leads to exchange of parts of (non-sister) chromatids / alleles between homologous chromosomes;
5. (Both) create new combinations of alleles;

Compare and contrast the structure and properties of triglycerides and phospholipids

1. Both contain ester bonds between glycerol and fatty acid;
2. Both contain glycerol;
3. Fatty acids on both may be saturated or unsaturated;
4. Both are insoluble in water;
5. Both contain C, H and O but phospholipids also contain P;
6. Triglyceride has three fatty acids and phospholipid has two fatty acids plus phosphate group;
7. Triglycerides are hydrophobic/non-polar and phospholipids have hydrophilic and hydrophobic region;
8. Phospholipids form monolayer/micelle/bilayer in water but triglycerides don't;