IGCSE Biology | Answers | 25 Questions
State two physical features of most leaves that make them well adapted for photosynthesis.
Name the waxy, transparent layer found on the outer surface of a leaf.
State two functions of the cuticle.
Name the two types of mesophyll found in a dicotyledonous leaf.
Which layer of the leaf is the main site of photosynthesis? Give one reason why.
It contains the greatest number of chloroplasts and is positioned near the top of the leaf, closest to the light source.
What are stomata? State where they are mainly found on a leaf.
State the function of guard cells.
Name the two types of tissue found in a vascular bundle.
State the function of xylem in a leaf.
State the function of phloem in a leaf.
Explain why palisade mesophyll cells are found near the top of the leaf, just below the upper epidermis, rather than near the bottom.
Light enters the leaf from the top. By being positioned just below the upper epidermis, palisade cells receive the greatest light intensity. Since palisade cells are the main site of photosynthesis and contain the most chloroplasts, placing them where light is strongest maximises the rate of photosynthesis.
A student examines a leaf cross-section under a microscope. She sees loosely arranged, irregularly shaped cells with large gaps between them.
(a) Name this tissue. (b) Explain how the gaps between these cells help the leaf carry out photosynthesis.
The gaps (air spaces) allow carbon dioxide to diffuse freely from the stomata through the leaf to the mesophyll cells, where it is needed for photosynthesis. The air spaces also provide a large surface area of cell membrane exposed to gases, making diffusion faster and more efficient.
Epidermal cells contain no chloroplasts. Explain how this feature helps the plant photosynthesise.
Because epidermal cells have no chloroplasts, they are transparent. This means that light passes straight through the epidermis without being absorbed, and reaches the palisade mesophyll cells below. If the epidermis contained chloroplasts, it would absorb some of the light before it reached the main photosynthetic layer, reducing the rate of photosynthesis.
Describe how guard cells control the opening and closing of a stoma.
Opening: When guard cells absorb water, they become turgid (swollen). Their inner walls are thicker than their outer walls, so they curve outward, creating a gap — the stoma opens.
Closing: When guard cells lose water, they become flaccid (limp). They straighten back, and the gap closes — the stoma closes.
A plant is kept in the dark overnight. (a) Predict what will happen to its stomata during the night. (b) Explain what happens to gas exchange when the plant is returned to light the next morning.
In the dark, photosynthesis stops, so CO₂ is no longer needed. Guard cells lose water and become flaccid, closing the stomata to reduce water loss.
When light returns, the guard cells absorb water, swell, and the stomata open. CO₂ can then diffuse into the leaf through the stomata and air spaces, reaching the mesophyll cells. O₂ produced by photosynthesis can diffuse out. Gas exchange resumes, allowing photosynthesis to proceed.
Explain why palisade mesophyll cells contain more chloroplasts than spongy mesophyll cells.
Palisade cells are positioned near the top of the leaf, where light intensity is greatest. More chloroplasts allow more light energy to be absorbed, maximising the rate of photosynthesis at this location.
Spongy mesophyll cells are deeper in the leaf, where less light reaches them. Fewer chloroplasts are needed because the light energy available is lower.
A leaf is large and flat, and is also very thin. Explain how each of these two features is an adaptation for photosynthesis.
Large, flat surface area: A larger surface area exposes more chloroplasts to sunlight at once. More light energy can be absorbed across the leaf, increasing the rate of photosynthesis.
Thin: Carbon dioxide only has to travel a short distance from the stomata to reach the mesophyll cells where it is needed. A shorter diffusion distance means CO₂ arrives at the cells more quickly, keeping photosynthesis efficient.
Explain how the cuticle reduces water loss while still allowing photosynthesis to occur in the cells below.
The cuticle is made of wax, which is waterproof. Water cannot evaporate through the waxy surface, so water loss is reduced.
At the same time, the cuticle is transparent, so light passes through it unobstructed and reaches the palisade mesophyll cells below, where photosynthesis takes place.
A student says: “Sealing all the stomata permanently would help the plant by stopping water loss.” Give two reasons why this would actually harm the plant.
- CO₂ cannot enter the leaf — without carbon dioxide, photosynthesis cannot take place, so the plant cannot make glucose for energy and growth.
- O₂ cannot leave the leaf — oxygen produced by photosynthesis would build up inside the leaf and could not be released.
Phloem carries sugars away from the leaf. Explain why this transport is important for photosynthesis to continue.
If sugars are not removed from the leaf, they build up inside the mesophyll cells. A high concentration of sugars would slow or stop the reactions of photosynthesis. By constantly transporting sugars away through the phloem, the concentration inside the cells stays low, allowing photosynthesis to continue at a steady rate.
A cross-section through a dicotyledonous leaf shows the following layers, from top to bottom:
- Layer 1: A thin, waxy, transparent coating
- Layer 2: A single layer of transparent, flat cells with no chloroplasts
- Layer 3: Tall, tightly packed cells containing many chloroplasts
- Layer 4: Loosely arranged, irregular cells with large air spaces between them
- Layer 5: A single layer of flat cells containing small pores, each controlled by two curved cells
- Layer 1: Cuticle
- Layer 2: Upper epidermis
- Layer 3: Palisade mesophyll
- Layer 4: Spongy mesophyll
- Layer 5: Lower epidermis (with stomata and guard cells)
- Contains a large number of chloroplasts, so it can absorb a large amount of light energy for photosynthesis.
- Positioned near the top of the leaf, closest to the light source, so light intensity is highest here.
- The loose arrangement of cells creates large air spaces, giving CO₂ and O₂ room to diffuse freely through the leaf.
- The irregular cell shape creates a large surface area of cell membrane exposed to the air spaces, allowing gases to diffuse in and out of cells quickly.
Vascular bundles run through the leaf as veins, containing xylem and phloem. (a) State the substance carried by xylem and explain why it is essential for photosynthesis. (b) State the substance carried away by phloem and explain why it must be removed. (c) Explain why having xylem and phloem together in the same bundle is an advantage.
Water is a raw material for photosynthesis — it is needed, along with carbon dioxide, to make glucose. Without a constant water supply, photosynthesis cannot continue.
If sugars are not removed, they accumulate in the mesophyll cells. This build-up raises the sugar concentration inside the cells, which slows and eventually stops photosynthesis. Removing the sugars keeps the concentration low and allows photosynthesis to proceed continuously.
Having both tissues in the same location means that every part of the leaf that receives water (from xylem) also has phloem nearby to remove the sugars produced. This ensures efficient supply and removal throughout the whole leaf, with no mesophyll cells being too far from either tissue.
Palisade mesophyll cells and spongy mesophyll cells both play important roles in the leaf. (a) Describe two structural differences. (b) Explain why palisade cells carry out more photosynthesis. (c) Explain why spongy mesophyll is still important.
- Shape: Palisade cells are tall and column-shaped, tightly packed together. Spongy cells are irregular in shape and loosely arranged.
- Chloroplasts: Palisade cells contain many chloroplasts. Spongy cells contain fewer chloroplasts.
- They contain more chloroplasts, so they can absorb more light energy at one time.
- They are positioned near the top of the leaf where light intensity is greatest, so more light energy is available to drive photosynthesis.
The loosely arranged spongy cells create air spaces that allow CO₂ to diffuse from the stomata to the palisade cells above. Without this gas exchange pathway, the palisade cells would not receive enough CO₂ and their rate of photosynthesis would fall. Spongy cells also carry out some photosynthesis themselves.
Stomata and guard cells control gas exchange and water loss. (a) Describe what happens to guard cells in bright light and how this opens the stoma. (b) Explain why stomata on the lower epidermis is an advantage. (c) State one benefit and one disadvantage of stomata that are permanently open.
In bright light, guard cells absorb water and become turgid (swollen). Because their inner walls (facing the stoma) are thicker than their outer walls, they curve outward when turgid. This bending creates a gap between the two guard cells — the stoma opens.
The lower surface of the leaf is shaded from direct sunlight by the leaf itself. This means it is cooler and less exposed to evaporation. Stomata on the lower surface can stay open for gas exchange while losing significantly less water than if they were on the upper surface, which is in direct sunlight.
Benefit: CO₂ can continuously enter the leaf, so there is always carbon dioxide available for photosynthesis, even in changing conditions.
Disadvantage: The plant continuously loses water vapour through the open stomata. In dry or hot conditions, the plant would lose water faster than it can be replaced by the roots, leading to wilting and eventual damage.
A student says: “Every layer and structure in a leaf works together to make photosynthesis as efficient as possible.”
Using your knowledge of leaf structure, explain how four different structures in a dicotyledonous leaf are adapted to maximise the rate of photosynthesis. For each structure: name it, describe the relevant feature, and explain how that feature helps photosynthesis.
Any four of the following, each with a clear name → feature → explanation chain:
Tall cells packed with many chloroplasts, positioned near the top of the leaf. The high chloroplast count allows maximum absorption of light energy, and the position near the light source means they receive the strongest light intensity. This gives the leaf its highest rate of photosynthesis.
These layers are transparent and contain no chloroplasts. They do not absorb or block the incoming light, allowing it to pass straight through to the palisade cells below. This ensures that the cells doing the most photosynthesis receive as much light as possible.
Stomata on the lower epidermis allow CO₂ to diffuse into the leaf. The air spaces in the spongy mesophyll then distribute CO₂ freely throughout the leaf, so it reaches all the mesophyll cells. Without this supply of CO₂, photosynthesis could not continue regardless of how much light is available.
Xylem delivers water from the roots to the leaf cells. Water is a raw material for photosynthesis — it is needed, along with carbon dioxide, to make glucose. A continuous water supply through the xylem ensures that this raw material is always available in the leaf.
