IGCSE Biology | Answers & Worked Solutions
Define transpiration.
Transpiration is the loss of water vapour from a plant. Water evaporates from the surfaces of the mesophyll cells into the air spaces inside the leaf, then diffuses out through the stomata.
State two structural features of a root hair cell that make it well adapted for absorbing water from the soil.
Any two of:
- Long, thin hair-like extension — greatly increases the surface area in contact with soil water, so more water can be absorbed at once.
- Large permanent vacuole — contains dissolved salts which lower the water potential inside the cell, so water enters by osmosis.
- Thin cell wall — water has only a short distance to travel to enter the cell.
Name the process by which water enters a root hair cell from the soil.
State two functions of xylem in a flowering plant.
- Transport of water and dissolved mineral ions from the roots to the leaves.
- Structural support — the thick, rigid walls of xylem vessels help to support the plant.
Name the tissue in plants that transports sucrose and amino acids.
State three structural features of xylem vessels.
- Thick walls (strengthened and rigid).
- No cell contents — the vessels are hollow (no cytoplasm, nucleus, or organelles).
- Cells joined end-to-end with no cross walls, forming one long continuous tube.
List, in order, the tissues that water passes through on its journey from the soil to a leaf mesophyll cell.
Define translocation.
Translocation is the movement of sucrose and amino acids through the phloem, from parts of the plant that produce or store them, to parts that use or store them.
State two environmental factors that increase the rate of transpiration.
Any two of:
- Increased wind speed
- Increased temperature
- Increased light intensity
- Decreased humidity
Name the two types of substance transported in the phloem.
A student increases the wind speed around a plant. Explain why this causes the rate of transpiration to increase.
Wind carries water vapour away from the surface of the leaf. This keeps the concentration of water vapour outside the leaf low. The concentration gradient between the inside of the leaf (high water vapour) and the outside air (low water vapour) stays steep. Water vapour therefore diffuses out through the stomata faster, so the rate of transpiration increases.
A plant is placed in a very humid atmosphere. Explain why its rate of transpiration will be lower than in dry air.
Humid air already contains a high concentration of water vapour. This reduces the concentration difference between the inside of the leaf and the air outside. The concentration gradient is less steep, so water vapour diffuses out through the stomata more slowly. The rate of transpiration therefore decreases.
Explain why a plant wilts if it loses water faster than its roots can absorb it.
When the plant loses water faster than the roots absorb it, the cells lose water and can no longer remain turgid (firm and full of water). Without enough water pressure inside the cells, the plant cannot support its own weight and droops — this is wilting.
A student examines a transverse section of a non-woody dicotyledonous stem under a microscope. Describe where she would find the vascular bundles, and where xylem and phloem are positioned within each bundle.
The vascular bundles are arranged in a ring near the outside of the stem. Within each bundle:
- Xylem is on the inner side (facing towards the centre of the stem).
- Phloem is on the outer side (facing towards the outside of the stem).
Explain how two structural features of a root hair cell make it efficient at absorbing water by osmosis.
Feature 1 — Long, thin hair-like extension: This greatly increases the surface area of the cell that is in contact with soil water. A larger surface area means more water can be absorbed at the same time, so absorption is faster.
Feature 2 — Large permanent vacuole: The vacuole contains dissolved salts (cell sap), which lower the water potential inside the cell. The soil water has a higher water potential than the cell sap. Water therefore moves into the cell by osmosis down the water potential gradient.
A student covers all the stomata on a plant’s leaves with petroleum jelly. Explain the effect this would have on:
(a) the rate of transpiration. (b) the movement of water up the xylem.
The stomata are blocked, so water vapour cannot diffuse out of the leaf. The rate of transpiration will fall greatly or stop altogether.
Because transpiration has stopped, there is no longer a pulling force drawing water out of the xylem at the leaf. The transpiration pull is lost, so water movement up the xylem slows greatly or stops.
Explain how two structural features of xylem vessels make them well suited to transporting water from root to leaf.
Feature 1 — No cell contents (hollow): There are no organelles, cytoplasm, or nucleus blocking the inside of the tube. Water can therefore flow freely and quickly through the vessel without any obstacles.
Feature 2 — Cells joined end-to-end with no cross walls: Individual xylem cells are connected with the end walls broken down, forming one long, unbroken continuous tube from root to leaf. Water can flow the full length without any barriers.
A plant is moved from a dim room into bright sunlight. Predict and explain the effect on its rate of transpiration.
Prediction: The rate of transpiration will increase.
Explanation: In bright light, the stomata open wider to allow carbon dioxide in for photosynthesis. Wider stomata allow more water vapour to escape from the leaf, so transpiration increases.
Mineral ions are found in higher concentrations inside root cells than in the surrounding soil. Explain how these ions are absorbed into the root hair cells.
Mineral ions are already in higher concentration inside the root cells than in the soil, so they cannot enter by diffusion (diffusion moves substances from high to low concentration). Instead, the ions are absorbed by active transport. The root hair cells use energy from respiration to pump ions in against the concentration gradient.
A plant is growing in hot, dry, and windy conditions. Give two reasons why this plant is at high risk of wilting.
Reason 1 — High temperature: Warm conditions increase the rate of evaporation from mesophyll cells and the rate of diffusion through stomata, so water is lost from the leaf very quickly.
Reason 2 — High wind speed: Wind removes water vapour from around the leaf surface, keeping the concentration gradient between inside and outside the leaf steep. Water vapour diffuses out faster, so water is lost rapidly.
Both factors together cause the plant to lose water much faster than its roots can absorb it, leading to wilting.
This question is about the mechanism that moves water up the xylem.
Water evaporates from the surfaces of mesophyll cells into the air spaces inside the leaf. Water vapour then diffuses out through the stomata (transpiration). As water leaves the mesophyll cells, they draw water out of the xylem in the leaf veins. This creates a tension — a pulling force — that acts on the water column in the xylem, pulling it upward.
Cohesion is the ability of water molecules to stick together. Because of cohesion, the water in the xylem forms a continuous, unbroken column. When the transpiration pull acts at the top, the whole column moves upward together — the molecules do not separate, so the pull is transmitted all the way down to the roots.
The upward pull (tension) created by transpiration acts on the walls of the xylem vessel. If the walls were thin, this tension would cause the vessel to collapse inward, breaking the water column and stopping the flow of water. Thick walls are rigid and resist this tension, keeping the tube open so water can continue to flow.
This question is about the position and function of vascular tissues.
Vascular bundles are arranged in a ring near the outside of the stem. Within each bundle, xylem is on the inner side (towards the centre of the stem) and phloem is on the outer side (towards the surface of the stem).
In a root, the vascular tissue is found in the centre of the root. The xylem forms a star shape in the middle, and phloem is found in the gaps between the arms of the xylem star. There is no ring arrangement as seen in the stem.
- Function 1 — Transport of water and minerals: Xylem vessels have no cell contents and no cross walls, forming a long unbroken hollow tube. This allows water to flow freely from root to leaf without obstruction.
- Function 2 — Structural support: Xylem vessels have thick walls which are rigid and strong. This helps to support the stem and resist the tension created by transpiration pull.
This question is about the movement of water from soil to air.
The soil water has a higher water potential than the cell sap inside the root hair cell. The vacuole of the root hair cell contains dissolved salts, which lower the water potential inside. Water therefore moves by osmosis from the region of higher water potential (soil) into the region of lower water potential (root hair cell).
Transpiration removes water from the mesophyll cells at the top of the xylem. This lowers the water potential in the mesophyll cells, so they draw water out of the xylem. This creates a lower water potential in the xylem compared to the root cortex, pulling water across the cortex and from the soil. Without transpiration continuing, this chain of water potential differences would not be maintained and water would stop moving up.
A student investigates the effect of different conditions on the rate of transpiration by measuring how fast a plant takes up water.
Wind carries water vapour away from the surface of the leaf. This prevents water vapour from building up around the stomata, keeping the concentration of water vapour outside the leaf low. The concentration gradient between the inside of the leaf and the outside air remains steep, so water vapour diffuses out faster and transpiration rate increases.
Higher temperature increases the rate at which water evaporates from the surfaces of mesophyll cells. It also gives water vapour molecules more kinetic energy, so they diffuse faster through the stomata. Both effects increase how quickly water vapour leaves the leaf, so transpiration rate increases.
Not all the water absorbed at the roots is lost through transpiration — some is used in photosynthesis or stays inside the plant (for cell growth and other processes). Water uptake is therefore slightly greater than actual transpiration, so this method slightly overestimates the true rate of transpiration.
This question compares transport in xylem and phloem.
The phloem transports sucrose and amino acids. Sucrose is produced in the leaves during photosynthesis. Both sucrose and amino acids move from parts of the plant that produce or store them (e.g. leaves, storage roots) to parts that use or need them (e.g. growing shoot tips, fruits, roots). This movement can go in both directions — upward or downward through the plant.
- Substances transported: Xylem carries water and dissolved mineral ions; phloem carries sucrose and amino acids.
- Direction of flow: Xylem transport is in one direction only — upward from root to leaf; phloem transport moves in both directions (up and down the plant).
- Harm 1: Sucrose made in the leaves during photosynthesis cannot be transported to the rest of the plant. Roots and other organs are cut off from the sugar supply they need for energy and growth. The plant will be weakened and eventually the deprived parts will die.
- Harm 2: Amino acids cannot be distributed to growing parts of the plant. Without amino acids, the plant cannot make proteins needed for growth and repair. Growth will stop and new tissue cannot be produced.
