(a) Name the processes by which water moves from [1.5]
(i) a to b
(ii) c to d
(iii) e to f
(b) Give the name of the plant parts in which water moves [1.5]
(i) from a to b
(ii) at c
(iii) through e to f
(c) Explain why the leaves often show temporary wilting at noon in hot summer. [2]
(d) Explain why seedlings often wilt when transplanted. [2]
(a)(i) a to b – Transpiration pull, capillary action [1/2]
(ii) c to d – Osmosis [1/2]
(iii) e to f – evaporation and diffusion [1/2]
(b)(i) from a to b – xylem vessels in stem [1/2]
(ii) xylem vessel in vein [1/2]
(iii) intercellular air space through stoma [1/2]
(c) Because the transpiration rate is higher than the rate of water absorption during noon, so there is a net loss of water;
The cells in the leaves become lose their turgidity and become flaccid, and the leaves start to droop during wilting;
(d) This is because some roots are damaged during the transplanting process (OR roots do not take up as much water as they are not spread out in the soil), resulting in lower rate of water absorption than the rate of water loss through transpiration, which results in a net loss of water and wilting;
2. Compare and contrast how the structure of the leaves in plants and the lungs in mammals allow them to be effective gas exchange surfaces.
Similarities between lungs and leaves as gas exchange surfaces:
1. Large surface area to volume ratio for rapid diffusion of gases.
2. Layer of moisture lining both the inside of the alveolar wall and the outside of mesophyll cells where gases can dissolve.
3. Both are thin structures: The leaf is only made up of a few layer of cells, while the alveolar walls are one-cell thick, reducing diffusion distance and hence increase rate of diffusion of gases into the cells of the leaf and blood.
4. Presence of air spaces in both lungs (alveoli) and leaves (intercellular air spaces) where air moves in and out of these spaces and then into the cells or blood.
Differences between lungs and leaves:
1. Lungs have capillaries which transport oxygen in blood away, and bring in more carbon dioxide, maintaining a diffusion gradient, while leaves do not have capillaries.
2. Air enters the alveoli via branching airways while air enters the leaf via openings known as stomata.
3. Movement of air into and out of the alveoli is due to breathing movements brought about by the action of the intercostal muscles, ribcage and diaphragm, where air flows in and out due to a pressure gradient, while air movement in the leaves is due to diffusion of gases through the stomatal openings down a concentration gradient.
3. Glasswort, shown in the figure above is a plant that grows in salt marshes. The plants are covered by seawater at each high tide. The roots grow in mud which contains a high concentration of salt. The drawing shows a shoot of the plant.
(a) Explain how two features of the structure of the shoot help the plant to survive in conditions where it is difficult to take up water. [2]
(b) Explain how the structure of a leaf is adapted to obtain the raw materials needed for photosynthesis to occur. [6]
(c) Explain why seedlings have a better chance of survival when transplanted in the evening. [2]
(a) Absence of leaves: Reduced surface area for water loss / evaporation /transpiration;
Shiny surface: Indicates waxy cuticle / covering to reduce water loss;
Swollen stems: Store water;
(any two)
(b) Carbon dioxide [1/2]
• Carbon dioxide from the atmosphere diffuses into the leaf through the stomata which are controlled by pairs of guard cells;
• Carbon dioxide dissolves in the film of water around the cell walls before diffusing into the mesophyll cells;
• Numerous air spaces between mesophyll cells decrease diffusion distance of carbon dioxide into the leaf;
Water [1/2]
• The veins branch repeatedly within the leaf so that every mesophyll cell is close to a xylem vessel which carries water, decreasing diffusion distance for water to travel;
• The guard cells control the opening of the stomata to regulate transpiration rate and closes to reduce loss of water when the plant wilts;
(c) During the process of transplanting, many small roots of the plants may be slightly damaged thus reducing the rate of water uptake/absorption;
When transplanting is done in the evening, the plants have a greater chance of surviving as the stomatal openings are smaller and the rate of transpiration is reduced significantly, due to cooler temperatures;
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