AS 1 2018-style biological molecules question · 6 marks
Describe the structure of a protein and explain how its tertiary structure is maintained.
Marked instantly, no account needed
CCEA AS Biology 2018 past paper and mark scheme
Summer 2018 is part of the current CCEA GCE Biology (2016) specification, so every question style in it is still examinable. The papers and mark schemes themselves are published by CCEA; BioCCEA is an independent study tool and is not endorsed by CCEA.
What you sit in AS 2018
AS 1: Molecules and Cells
AS 1 written paper (1 hour 30 minutes)
Biological molecules, enzymes, cell structure, transport across membranes, cell division, nucleic acids and the immune response.
AS 1 topic list and exam focusAS 2: Organisms and Biodiversity
AS 2 written paper (1 hour 30 minutes)
Gaseous exchange, transport in plants and mammals, adaptation, biodiversity, classification, sampling and human impact.
AS 2 topic list and exam focusAS 3: Practical Skills in AS Biology
AS 3 practical booklet plus a written practical-skills paper (1 hour)
Planning, manipulating apparatus, recording, processing and evaluating experimental data at AS level.
AS 3 topic list and exam focus
How to sit the 2018 paper properly
- 1Print the paper and sit it in one block, to time, with no notes. A paper done in pieces tells you nothing about your exam performance.
- 2Mark it with the official CCEA mark scheme in front of you and award a mark only where the scheme's point is actually made.
- 3Log every dropped mark by unit topic, not by question number, so patterns appear across papers.
- 4Rewrite the two worst answers in full, using mark-scheme wording, then re-mark them.
- 5Come back to the same paper three weeks later and redo only the questions you lost marks on.
Sit the AS 2018 style questions
Answer each one under exam conditions. Your wording is marked against every mark scheme point before the full-mark answer appears.
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These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Worked AS 2018 questions with mark schemes
Question styles that carried the most marks in the Summer 2018 papers, each with the full list of creditworthy mark scheme points, a model answer at full marks, and the marking point candidates most often missed.
AS 1 2018-style biological molecules question · 6 marks
Describe the structure of a protein and explain how its tertiary structure is maintained.
Command word: Describe — how to answer itMark scheme — 6 creditworthy points
- Primary structure: the sequence of amino acids joined by peptide bonds
- Secondary structure: α-helix or β-pleated sheet held by hydrogen bonds between C=O and N–H groups
- Tertiary structure: the overall three-dimensional folding of the polypeptide
- Held by hydrogen bonds, ionic bonds between charged R groups and hydrophobic interactions
- Disulfide bridges form between the R groups of cysteine residues and are the strongest of these bonds
- Quaternary structure: two or more polypeptide chains, sometimes with a prosthetic group such as haem
Full-mark answer
The primary structure of a protein is the sequence of amino acids in the polypeptide, joined by peptide bonds formed in condensation reactions. The secondary structure is the regular folding of that chain into an α-helix or β-pleated sheet, held in place by hydrogen bonds between the C=O and N–H groups of the backbone. The tertiary structure is the overall three-dimensional shape into which the whole polypeptide folds, and it is maintained by hydrogen bonds, ionic bonds between oppositely charged R groups, hydrophobic interactions between non-polar R groups in the interior, and disulfide bridges between the R groups of cysteine residues, which are the strongest of these bonds. Where a protein consists of more than one polypeptide chain, sometimes with a prosthetic group such as the haem group in haemoglobin, it also has a quaternary structure.
Examiner insight: Disulfide bridges must be linked to cysteine specifically. Naming the four levels without naming the bonds that hold each one usually caps at 3.
AS 2 2018-style immunity question · 6 marks
Explain how the specific immune response leads to the destruction of a bacterial pathogen, and explain why a second infection with the same pathogen causes no symptoms.
Command word: Explain — how to answer itMark scheme — 6 creditworthy points
- Antigen-presenting cells (macrophages) display the bacterial antigen on their surface
- A T helper cell with a complementary receptor binds and is activated, releasing cytokines
- This activates the B cell with a complementary antibody - clonal selection
- The B cell divides by mitosis to form clones of plasma cells - clonal expansion
- Plasma cells secrete specific antibodies that agglutinate the bacteria and act as opsonins for phagocytosis
- Memory B cells remain; on second infection they divide rapidly into plasma cells, so antibody is produced faster and in greater quantity, destroying the pathogen before symptoms develop
Full-mark answer
A macrophage engulfs the bacterium and displays its antigen on the cell surface membrane, acting as an antigen-presenting cell. A T helper cell whose receptor is complementary to that antigen binds to it and is activated, releasing cytokines that stimulate the B cell carrying the complementary antibody - this is clonal selection. That B cell divides repeatedly by mitosis to give a clone of plasma cells, which secrete large numbers of specific antibodies. The antibodies bind to antigens on the bacteria, agglutinating them and acting as opsonins so that phagocytes engulf and digest them. Some of the clone remain as memory B cells, so if the same pathogen enters again these divide rapidly into plasma cells and antibody is produced much sooner and in far greater quantity than in the primary response. The pathogen is destroyed before its numbers are high enough to cause symptoms.
Examiner insight: The secondary response mark requires both 'faster' and 'more antibody'. Vague references to 'immunity' without memory cells score nothing.
AS 2 2018-style surface area to volume question · 5 marks
Explain why large multicellular organisms need a transport system, using surface area to volume ratio in your answer.
Command word: Explain — how to answer itMark scheme — 5 creditworthy points
- As size increases, volume increases faster than surface area, so surface area to volume ratio decreases
- Diffusion distances to the centre of the organism become too great, so diffusion alone is too slow
- A high metabolic rate means a greater demand for oxygen and nutrients per unit volume
- A mass transport system moves substances quickly over long distances by bulk flow
- Exchange surfaces (e.g. lungs, gills, small intestine) are specialised to increase surface area and are linked to the transport system
Full-mark answer
As an organism gets larger its volume increases faster than its surface area, so its surface area to volume ratio falls. This means the body surface is too small to supply the whole volume of the organism with oxygen and nutrients. At the same time the diffusion distance from the surface to the cells in the centre becomes very large, and diffusion over such distances is far too slow to meet demand, particularly in an active animal with a high metabolic rate. A mass transport system is therefore needed to carry substances rapidly by bulk flow to and from every cell, and it is linked to specialised exchange surfaces such as the lungs or small intestine, whose folded structures provide the large surface area that the body surface alone cannot.
Examiner insight: The first mark needs the comparison 'volume increases faster than surface area'. Answers that only say 'they are too big for diffusion' cap at 2.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemePractise the AS 2018 paper in the app
These open BioCCEA with AS already selected, so you can sit questions in this paper's style and have them marked against the CCEA mark scheme.
Get your 2018 answers marked
Photograph a written answer from this paper and see it marked line by line in CCEA mark-scheme language, with the exact phrase you needed to earn each missing mark.