AQA A-Level Biology Paper 1, November 2021: Question 5
10 marks · Medium difficulty · Short Answer
Describe how to isolate chloroplasts from leaves, identify features of chloroplasts that synthesize proteins, calculate ratios of protein to chlorophyll, and explain the effects of iron deficiency on plant growth based on electron micrographs.
Practise this questionQuestion
Question text
05.1 Describe how a sample of chloroplasts could be isolated from leaves.
[4 marks]
05.2 Scientists grew two groups of plants:
• control plants with all the inorganic ions needed
• iron-deficient plants with all the inorganic ions needed but without iron ions.
After 1 week, the scientists measured the mass of protein and the mass of chlorophyll
in the chloroplasts isolated from samples of leaves of these two groups of plants.
Table 4 shows the scientists’ results.
Table 4
Mass of protein / percentage of Mass of chlorophyll / percentage of
control control
40 10
Some proteins found inside the chloroplast are synthesised inside the chloroplast.
Give one feature of the chloroplast that allows protein to be synthesised inside the
chloroplast and describe one difference between this feature in the chloroplast and
similar features in the rest of the cell.
[2 marks]
Feature
*14* Structural difference
05.3 The ratio of protein to chlorophyll in control plants is 9:1
Use the information in Table 4 to calculate the ratio of protein to chlorophyll in
iron-deficient plants.
[1 mark]
Ratio
05.4 The scientists also observed the chloroplasts from the samples of leaves using an
electron microscope.
Figure 6 shows a chloroplast from a control plant (image A) and a chloroplast from an
iron-deficient plant (image B).
*15* Figure 6
This source has been removed due to third-party copyright restrictions.
Use Figure 6 to suggest why iron-deficient plants have a reduced growth rate.
[3 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
05.1 1. Break open cells/tissue and filter
1. Accept
OR homogenise and
filter
Grind/blend cells/tissue/leaves and filter;
2. In cold, same water potential/concentration, pH
controlled solution;
2. Accept for ‘same
3. Centrifuge/spin and remove nuclei/cell debris; water potential/
4. (Centrifuge/spin) at high(er) speed, chloroplasts concentration’,
settle out; isotonic
2. Accept for ‘pH
controlled’, buffered
05.2 Mark in pairs, 1 and 2 OR 3 and 4
1. DNA;
2. Is not associated with protein/histones but
nuclear DNA is
OR
Is circular but nuclear DNA is linear 2
OR 4. Accept: 70S
ribosomes in
Is shorter than nuclear DNA; chloroplast, but 80S
ribosomes in
3. Ribosomes; cytoplasm
– A-LEVEL BIOLOGY – –
4. Are smaller than cytoplasmic ribosomes;
05.3 Correct answer for 1 mark, 36:1; 1
05.4 1. Less (thylakoid) membrane
OR 4. Accept reduced
Fewer/smaller grana; rate of any named
biochemical process
2. Smaller surface area (of membrane in 3 max in photosynthesis;
chloroplast)/less chlorophyll; 11
eg. reduced
3. (Less chlorophyll so) reduced light absorption; production of
ATP/reduced NADP
4. (So) slower rate of photosynthesis;
How to answer it
Cell Ultrastructure, Chloroplast Isolation & Photosynthesis
What this question tests
This exam question evaluates your practical skills in cell fractionation (specifically isolating organelles via differential centrifugation), knowledge of chloroplast self-sufficiency (organelle DNA and ribosomes compared to nuclear/cytoplasmic counterparts), data interpretation involving proportions, and linking ultrastructure changes to metabolic consequences like photosynthesis and plant growth.
Isolating Chloroplasts from Leaves
✅ Correct Answer / Mark Scheme
- Step 1: Homogenise / break open cells and filter.
- Step 2: Solution must be cold, buffered (controlled pH), and isotonic (same water potential).
- Step 3: Centrifuge / spin to remove cell debris and nuclei.
- Step 4: Centrifuge at a higher speed so that chloroplasts settle out.
💡 Key Knowledge
- Cold: Reduces enzyme activity so that digestive enzymes (lysozymes) do not break down the chloroplasts.
- Isotonic: Prevents osmosis, stopping chloroplasts from bursting or shrivelling due to water movement.
- Buffered: Maintains a constant pH so that membrane proteins and enzymes are not denatured.
🧠 Exam Technique
This is a classic practical method recall question. Examiners strictly look for the condition trifecta: cold, isotonic/same water potential, and buffered/pH controlled. Do not lump them together vaguely; state each condition and the specific reason why it is necessary.
❌ Common Errors
- Saying "same concentration" without specifying "water potential".
- Stating "kill bacteria" instead of "slow down enzyme activity" for why the solution must be cold.
- Failing to mention filtration before centrifugation to remove large plant tissue debris.
Chloroplast Autonomy & Organelle Features
✅ Correct Answer / Mark Scheme
Must mark in pairs (Feature 1 + Difference 1 OR Feature 2 + Difference 2):
- Pair A: Feature: DNA. Difference: Chloroplast DNA is not associated with proteins/histones (or is circular / shorter) whereas nuclear DNA is associated with proteins/histones (or is linear / longer).
- Pair B: Feature: Ribosomes. Difference: Chloroplast ribosomes are smaller (70S) than cytoplasmic ribosomes (80S).
💡 Key Knowledge
- Chloroplasts (and mitochondria) contain their own 70S ribosomes and circular DNA because of the Endosymbiotic Theory. This allows them to independently synthesise specific proteins encoded in their own genome.
🧠 Exam Technique
Pay close attention to instructions requiring "marked in pairs". If you state a feature (e.g., DNA) but give a difference belonging to ribosomes or general cytoplasm without proper contrast, you will lose marks. Always contrast the chloroplast feature directly against the rest of the cell.
❌ Common Errors
- Writing "ribosomes" as the feature, but stating the difference is "they make proteins" (both do this, so it is not a structural difference).
- Stating chloroplast DNA is "naked" without clarifying the absence of histone proteins relative to nuclear DNA.
Calculation: Protein to Chlorophyll Ratio
✅ Correct Answer
36 : 1
📐 Step-by-Step Calculation
- Understand the control baseline: Control plant ratio = 9 : 1 (Protein : Chlorophyll).
- Apply Table 4 percentages: Iron-deficient plants have protein mass at 40% of control ( 9 × 0.40 = 3.6 ) and chlorophyll mass at 10% of control ( 1 × 0.10 = 0.1 ).
- Calculate new ratio: 3.6 : 0.1
- Simplify: Multiply both sides by 10 to get 36 : 1 . Alternatively: (9 × 40) / (1 × 10) = 360 / 10 = 36 .
Linking Ultrastructure to Plant Growth
✅ Correct Answer / Mark Scheme
- Point 1: Less thylakoid membrane / fewer or smaller grana.
- Point 2: Smaller surface area of membrane in chloroplast / less chlorophyll present.
- Point 3: Less chlorophyll results in reduced light absorption.
- Point 4: Leads to a slower rate of photosynthesis (accept reduced production of ATP / reduced NADP).
💡 Key Knowledge
Iron is an essential mineral cofactor required for chlorophyll synthesis and electron transport chains (e.g., in cytochromes/ferredoxin). Without iron, chloroplast development is stunted, directly limiting the light-dependent reaction.
🧠 Exam Technique
Build a clear logical chain: Structural change → Biochemical consequence → Overall physiological impact. Examiners look for cause-and-effect sequences: less chlorophyll → less light absorbed → less ATP/reduced NADP made → slower photosynthesis → reduced growth rate.
❌ Common Errors
- Vaguely stating "the plant dies" or "photosynthesis stops" instead of explaining a slower rate of photosynthesis.
- Confusing chlorophyll with chloroplasts—remember that iron deficiency disproportionately affects pigment and internal membrane proliferation.
Topics
Biology · Practical skills · 3.2 Cells · 3.5 Energy transfers in and between organisms (A-level only) · Data analysis · Experimental design
Question and mark scheme from the AQA A-Level Biology examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.