Essay Practice

Question 1

Exemplar Plan for Essay

Answer 1

Question 2

Bad vs good paragraph about photosynthesis

Answer 2

Weaker answers will focus on all of photosynthesis – as a process
involving membranes – perhaps mention membranes in chloroplasts (this
approach would gain a maximum of about 18 out of 25).

Good answers will focus on thylakoid membranes in the chloroplast and
the roles of components of these membranes in holding pigments, components of the electron transfer chain, ATP synthase and the membrane as a barrier allowing maintenance of a proton gradient – and, perhaps, role of membranes in maintaining the special chemical environment inside chloroplasts

Question 3

Tips for the essay

Answer 3

• An example not from the specification has to be at (or above) A-level
  standard – not GCSE, or what anyone who hasn’t studied A-level Biology
  would know
• No introduction or conclusion is required – it wastes time that could be
  used for more content
• Content has to be of A-level standard to score highly – this includes scientific terminology and the explanation of ‘importance’.
• A-level detail is required – though not necessarily all the detail of a
  particular topic, just the relevant detail.

Question 4

Content has to be of A-level standard to score highly – this includes scientific terminology and the explanation of ‘importance’.

Example of bad vs good

Answer 4

Content has to be of A-level standard to score highly – this includes scientific terminology and the explanation of ‘importance’.

Example – importance of gas exchange in humans – many students gave reasonable degrees of detail about gas exchange in the lungs. Most then said this was important ‘to stay alive’, or ‘for respiration’ – not A-level standard.

Strongest responses linked oxygen uptake to oxidative phosphorylation in respiration, as a source of most of the ATP the body requires.

Or to prevent increase in concentration of carbon dioxide in blood – leading to fall in pH and adverse effects on eg enzymes

Question 5

What the essay is not

Answer 5

• ‘Think of every possible thing that relates to the title and write as much as
  you can about it, with no thought of the main theme/idea.’ -This would make it just a memory test (AO1).
• ‘Write at a very high level (above A-level) about one or two topics.’ - This is not a synoptic approach.

Question 6

💥Your key to success is PRAT:

Answer 6

Plan – list 5 relevant topics you will write about

Range – Make sure you are not writing about the same concept repeatedly (e.g. don’t write about chemiosmosis in photosynthesis and then again in respiration)

Apply – Once you have explained the key concepts then APPLY this to the question by explain WHY it is important, this often links to another topic in the A-level.

Terminology – Make sure you have includes LOT of A level standard key terms.

Question 7

Describe the structure of DNA.

Answer 7

1. Polymer of nucleotides;

Accept ‘Polynucleotide’

Accept for ‘phosphate’. phosphoric acid

2. Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base;
3. Phosphodiester bonds (between nucleotides);
4. Double helix/2 strands held by hydrogen bonds;
5. (Hydrogen bonds/pairing) between adenine, thymine and cytosine, guanine

Question 8

Name and describe five ways substances can move across the cell-surface membrane into a cell.

Answer 8

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;

For ‘carrier protein’ accept cotransport protein

Question 9

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

Answer 9

1. Micelles contain bile salts and fatty acids/monoglycerides;
2. Make fatty acids/monoglycerides (more) soluble (in water)

OR

Bring/release/carry fatty acids/monoglycerides to cell/lining (of the ileum)

OR

Maintain high(er) concentration of fatty acids/monoglycerides to cell/lining (of the ileum);

Accept lipid/fat for fatty acid/ monoglyceride

3. Fatty acids/monoglycerides absorbed by diffusion;
4. Triglycerides (re)formed (in cells);

Accept chylomicrons form

5. Vesicles fuse to cell membrane;

Accept exocytosis for ‘vesicles move’

Question 10

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

Answer 10

b) 1. Structure is determined by (relative) position of amino acid/R group/interactions;

Accept for ‘interactions’, hydrogen bonds / disulfide bridges / ionic bonds / hydrophobic-hydrophilic interactions

2. Primary structure is sequence/order of amino acids;
3. Secondary structure formed by hydrogen bonding (between amino acids);

Accept alpha helix/β-pleated sheet for ‘secondary structure’

4. Tertiary structure formed by interactions (between R groups);

Accept for ‘interactions’, hydrogen bonds / disulfide bridges / ionic bonds / hydrophobichydrophilic interactions

5. Creates active site in enzymes

OR

Creates complementary/specific shapes in antibodies/carrier proteins/receptor (molecules);

6. Quaternary structure contains more than 1 polypeptide chain

OR

Quaternary structure formed by interactions/bonds between polypeptides;

Accept for ‘intereactions’, hydrogen bonds/ disulfide bridges/ionic bonds/hydrophobichydrophilic interactions
Accept prosthetic (group)

Question 11

Describe how mRNA is formed by transcription in eukaryotes.

Answer 11

1. Hydrogen bonds (between DNA bases) break via RNA Polymerase;

Ignore DNA helicase.
Reject hydrolysing hydrogen bonds.

2. (Only) one DNA strand acts as a template;
3. (Free) RNA nucleotides align by complementary base pairing;
4. (In RNA) Uracil base pairs with adenine (on DNA)

OR
(In RNA) Uracil is used in place of thymine;

5. RNA polymerase joins (adjacent RNA) nucleotides;

Reject suggestions that RNA polymerase forms hydrogen bonds or joins complementary bases.

6. (By) phosphodiester bonds (between adjacent nucleotides);
7. Pre-mRNA is spliced (to form mRNA)
   OR
   Introns are removed (to form mRNA);

Question 12

Describe how a polypeptide is formed by translation of mRNA.

Answer 12

1. (mRNA attaches) to ribosomes
2. (tRNA) anticodons (bind to) complementary (mRNA) codons;
3. tRNA brings a specific amino acid;
4. Amino acids join by peptide bonds;
5. (Amino acids join together) with the use of ATP;
6. tRNA released (after amino acid joined to polypeptide);
7. The ribosome moves along the mRNA to form the polypeptide;

Question 13

Define ‘gene mutation’ and explain how a gene mutation can have:
- no effect on an individual
- a positive effect on an individual.

Answer 13

For 4 marks at least one mark must be scored in each section of the answer.

1. Change in the base/nucleotide (sequence of chromosomes/DNA);
2. Results in the formation of new allele;

(Has no effect because)

3. Genetic code is degenerate (so amino acid sequence may not change);

OR
Mutation is in an intron (so amino acid sequence may not change);

Accept description of ‘degenerate’, eg some amino acids have more than one triplet/codon.

4. Does change amino acid but no effect on tertiary structure;
5. (New allele) is recessive so does not influence phenotype;

(Has positive effect because)

6. Results in change in polypeptide that positively changes the properties (of the protein)

OR
Results in change in polypeptide that positively changes a named protein;

For ‘polypeptide’ accept ‘amino acid sequence’ or ‘protein’.

7. May result in increased reproductive success

Question 14

Describe the sequence of events involved in transmission across a cholinergic synapse. Do not include details on the breakdown of acetylcholine in your answer.

Answer 14

1. Depolarisation of presynaptic membrane;

Accept action potential for depolarisation.

2. Calcium channels open and calcium ions enter (synaptic knob);
3. (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter;
4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
5. (Acetylcholine attaches) to receptors on the postsynaptic membrane;
6. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Question 15

Explain five properties that make water important for organisms.

Answer 15

1. A metabolite in condensation/hydrolysis/photosynthesis/respiration
2. A solvent so (metabolic) reactions can occur

OR
A solvent so allowing transport of substances;

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 tension supporting (small) organisms;

Question 16

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

Answer 16

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

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 brick red 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;

Question 17

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers

Give two named examples of polymers and their associated monomers to illustrate your answer.

Answer 17

1. A condensation reaction joins monomers together and forms a (chemical) bond and releases water;
2. A 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;

eg. Amino acid and polypeptides
Alpha glucose and glycogen
Nucleotide and polynucleotide

4. A second suitable example of polymers and the monomers from which they are made;
5. Reference to a correct bond within the polymers eg. glycosidic, phosphodiester, peptide

Question 18

Lord Howe Island in the Tasman Sea possesses two species of palm tree which have arisen via sympatric speciation. The two species diverged from each other after the island was formed 6.5 million years ago. The flowering times of the two species are different.

Using this information, suggest how these two species of palm tree arose by sympatric speciation.

Answer 18

1. Occurs in the same habitat / environment / population;
2. Mutation/s cause different flowering times;
3. Reproductive separation / isolation

OR
No gene flow
OR
Gene pools remain separate;

4. Different allele/s passed on / selected

OR
Change in frequency of allele/s

5. Disruptive (natural) selection;
6. Eventually different species cannot (inter)breed to produce fertile offspring;

Question 19

Describe how the structures of starch and cellulose molecules are related to their functions.

Answer 19

Starch (max 3)

1. Helical/ spiral shape so compact;
2. Large (molecule)/insoluble so osmotically inactive;
3. Branched so glucose is (easily) released 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;
7. To form (micro/macro)fibrils;
8. Provides rigidity/strength;

Question 20

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

Answer 20

1. (At source) sucrose is actively (transported) into the phloem/sieve element/tube;

Accept: ‘sugar/s’ for sucrose but reject other named sugars e.g. glucose.

Accept: co-transport (with H+ ions).

2. By companion/transfer cells;
3. Lowers water potential in phloem/sieve element/tube and water enters by osmosis;
4. (Produces) high (hydrostatic) pressure;

Accept: pressure gradient.

5. Mass flow/transport towards sink/roots/storage tissue;

Accept: sieve element/tube.

6. At sink/roots sugars are removed/unloaded;

Question 21

Describe the appearance and behaviour of chromosomes during mitosis.

Answer 21

(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.

Question 22

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

Answer 22

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;

Question 23

Essay Topic - ATP

Answer 23

AO1 Structure:

• Adenine nitrogenous base
• Ribose pentose sugar
• 3x phosphates
• nucleotide derivative
• small, soluble but can’t leave cell

AO1 Function:

• hydrolysis releases small manageable amounts of energy ATP ADP+P (this is reversible)
• Energy stored in bonds between phosphates
• Quick process of hydrolysis and synthesis
• ATP hydrolase / ATP synthase involved
• Phosphorylation-addition of phosphate more reactive

Examples and links to topics:

• phosphocreatine/muscle contraction
• Condensation/hydrolysis
• Co-transport/active transport
• Respiration/photosynthesis

Question 24

Essay Topic - Haemoglobin

Answer 24

AO1 Structure:

• large protein with quaternary structure
• made up of 4 polypeptide chains
• each chain has a haem group which contains an iron ion
• each human haemoglobin carries 4 oxygen molecules

AO1 Function:

• In the lungs, oxygen binds to oxygen to form oxyhaemoglobin
• oxyhaemoglobin is formed from a reversible reaction and near the body oxygen dissociates from oxyhaemoglobin and make just haemoglobin
• It’s the Fe²+ that actually binds to the oxygen in haemoglobin-so it’s a pretty key component. When oxygen is bound, the Fe²+ ion temporarily becomes an Fe3+ ion, until oxygen is released.

Examples and links to topics:

• proteins
• Ions
• shapes

Question 25

Essay Topic - LDR - Chemiosmosis

Answer 25

1. Light energy is absorbed by PSII. The light energy excites electrons in chlorophyll which are released. 2. The electrons move down an electron transport chain and to a higher energy level and go to PSI 3. Light energy is used to split water into protons, electrons and oxygen 4. The energy from the excited electrons is used to actively pump protons from stroma into the thylakoid membrane which causes a concentration gradient to form 5. Protons move down the concentration gradient back into the stroma via ATP synthase in which the movement drives synthesis of ADP + Pi to produce ATP 6. Light energy is absorbed by PSI which causes electrons to raise to a higher energy level and electrons are transferred to make reduced NADP

Question 26

Essay Topic - IPS (5)

Answer 26

- iPS cells are created by scientists in the lab. The process involves ‘reprogramming’ specialised adult body cells so that they become pluripotent. - The adult cells are made to express a series of transcription factors that are normally associated with pluripotent stem cells. - The transcription factors cause the adult body cells to express genes that are associated with pluripotency. - One of the ways that these transcription factors can be introduced to the adult cells is by infecting them with a specially-modified virus. The virus has the genes coding for the transcription factors within its DNA. - When the virus infects the adult cell, these genes are passed into the adult cell’s DNA, meaning that the cell is able to produce the transcription factors.

Question 27

Genetic Fingerprinting

Answer 27

In medical diagnosis, a genetic fingerprint can refer to a unique pattern of several alleles. It can be used to diagnose genetic disorders and cancer. - It’s useful when the specific mutation isn’t known or where several mutations could have caused the disorder, because it identifies a broader, altered genetic pattern.