CIE iGCSE Co-Ordinated Science B8.4 Translocation Exam Style Questions Paper 4
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic B8.3 — Transpiration (Part (a)(i), (a)(ii))
• Topic B8.1 — Xylem and phloem (Part (b)(i))
• Topic B8.4 — Translocation (Part (b)(ii))
• Topic B9.1 — Circulatory systems (Part (c)(i), (c)(ii))
▶️ Answer/Explanation
(a)(i) High wind speed increases the rate of transpiration because:
- Wind removes water vapour from around the leaf.
- This maintains a steep concentration gradient of water vapour between the leaf and the outside air.
- Increased diffusion of water vapour from the stomata occurs.
Transpiration is the loss of water vapour from the leaves. Wind increases the transpiration rate by removing the layer of humid air that builds up around the leaf surface. This maintains a steep concentration gradient between the water vapour inside the leaf (high concentration) and the air outside (low concentration), which increases the rate of diffusion of water vapour out through the stomata.
(a)(ii) Effect on distance: Increases (for 25°C compared to 15°C).
Explanation (any two from):
- Increase in the rate of transpiration.
- Increased rate of water evaporating from the mesophyll cell surfaces.
- Increased rate of diffusion of water vapour out of the leaf through the stomata.
Higher temperature increases the kinetic energy of water molecules, causing them to evaporate more rapidly from the surfaces of mesophyll cells inside the leaf. This increases the concentration of water vapour in the air spaces, leading to a greater diffusion gradient and a faster rate of transpiration, so the air bubble moves a greater distance.
(b)(i) Mineral ions.
Xylem vessels transport water and dissolved mineral ions (such as nitrates, phosphates, and potassium ions) absorbed from the soil by root hair cells, up the plant to the leaves.
(b)(ii) Translocation is the movement of sucrose and amino acids from sources to sinks through phloem vessels.
Translocation is the transport of sucrose (produced during photosynthesis) and amino acids (produced from nitrate ions) in the phloem. Sucrose and amino acids are transported from sources (where they are made or stored, e.g., leaves) to sinks (where they are used or stored, e.g., roots, fruits, growing tips).
(c)(i) In a single circulatory system, blood only goes through the heart once during each complete circuit of the body.
Path: Heart → Gills (for oxygenation) → Body (for delivery of oxygen and nutrients) → Heart.
In fish, blood is pumped from the heart to the gills where it is oxygenated. It then flows directly to the rest of the body where oxygen is delivered and carbon dioxide is picked up, before returning to the heart. This means blood passes through the heart only once per complete circuit.
(c)(ii) Any one from:
- Allows faster metabolism because blood can be pumped at higher pressure to the body.
- Allows different blood pressures in the pulmonary circulation (low pressure to lungs) and systemic circulation (high pressure to body).
- Oxygenated blood and deoxygenated blood are kept separate, making oxygen delivery more efficient.
The double circulation in mammals has two separate circuits: pulmonary circulation (heart to lungs and back) and systemic circulation (heart to body and back). This separation allows blood pressure to be lower in the lungs, preventing damage to delicate capillaries, while maintaining higher pressure for efficient delivery of oxygen and nutrients to the rest of the body, which supports a faster metabolic rate.
Question
• sink.
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic B8.4 — Translocation (Part (a))
• Topic B5.1 — Enzymes (Parts (b) & (c))
▶️ Answer/Explanation
(a) Source label to the cotyledons; Sink label to the roots.
The cotyledons act as the source because they store sucrose and amino acids which are transported away. The growing roots act as the sink because they receive and use these substances for growth and respiration.
(b) Three reasons why seeds need amylase:
• Starch in seeds acts as an energy store for the growing embryo.
• Starch is a large, insoluble molecule and cannot be transported out of the cotyledons.
• Amylase breaks down starch into simple reducing sugars (such as maltose/glucose).
• These sugars are used in respiration to release energy needed for growth of the seedling.
• The soluble sugars can be transported to the growing regions.
(c)(i) Two comparative differences:
• α-amylase has a narrow, single optimum pH (around pH 7), whereas β-amylase is active across a broader pH range (approximately pH 4–10).
• α-amylase has higher activity at its optimum pH compared to β-amylase at its optimum.
• α-amylase activity drops sharply away from its optimum pH, while β-amylase shows more gradual changes in activity.
(c)(ii) At pH 9, α-amylase shows very low activity or no activity because the enzyme is denatured.
The pH is outside the enzyme’s optimum range, causing the active site to change shape. The active site is no longer complementary in shape to the starch substrate, so enzyme-substrate complexes cannot form effectively.
(c)(iii) β-amylase needs to be active over a wider pH range because it works outside bacterial cells.
In the human body (where α-amylase works), the pH is maintained relatively constant (e.g., in the small intestine). However, outside bacterial cells, the pH of the environment can vary considerably, so β-amylase must function effectively across a wider range of pH values to digest food in different conditions.