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O-Level Nutrition and Transport in Flowering Plants
What the O-Level syllabus expects for Nutrition and Transport in Flowering Plants, and how to practise it.
What the syllabus expects
- Recognise the cell and tissue structure of a dicotyledonous leaf in transverse section under the light microscope, and set out why these features matter for their functions, such as the arrangement of chloroplasts for photosynthesis, the stomata and mesophyll cells for gaseous exchange, and the vascular bundles for transport.
- Locate, and account for the jobs of, the xylem vessels and the phloem (its sieve tube elements and companion cells) within a herbaceous dicotyledonous stem and leaf sectioned and viewed under the light microscope.
- Account for how a root hair cell is built to take up water and ions.
- Note that chlorophyll takes in light energy and turns it into chemical energy used to make carbohydrates and whatever follows from them.
- Give a brief account of why nearly all life depends entirely on photosynthesis.
- Give the equation for photosynthesis in both words and symbols.
Scope: The stages that depend on light and those independent of it need not be detailed. - Set out how carbon dioxide gets to the mesophyll cells inside a leaf.
- Investigate and talk through how changing light intensity, carbon dioxide concentration and temperature affect the rate of photosynthesis, for example in a submerged aquatic plant.
- Talk through how temperature, the concentration of carbon dioxide and the intensity of light can each act as a limiting factor for the rate of photosynthesis.
- Define transpiration and account for it being a by-product of gaseous exchange in plants.
- Account for how water moves between plant cells, and between those cells and their surroundings, in terms of water potential.
Scope: Calculations involving water potential are not required. - Outline the route by which water travels into the roots and up through the xylem vessels to the leaves, driven by transpiration pull.
- Investigate and account for how changes in air movement, temperature, humidity and light intensity affect transpiration rate, and how wilting comes about.
- Define translocation as moving food, chiefly sucrose, through the phloem tissue, and illustrate the process using translocation studies.
How it's examined
Questions on this topic most often ask you to explain, sketch. About 16% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Explain the way in which carbon dioxide travels to the mesophyll cells inside a leaf.
Show the worked answer
Carbon dioxide moves entirely by diffusion. It enters the leaf through the stomata (mainly in the lower epidermis). It then diffuses through the intercellular air spaces of the spongy mesophyll. Because the mesophyll cells are using up CO2 in photosynthesis, the CO2 concentration inside the leaf is lower than outside, so CO2 diffuses down this concentration gradient. At the cell it dissolves in the film of moisture on the cell wall and diffuses into the mesophyll cell.
Example 2 (3 marks)
Tiny pores called stomata are found in the lower epidermis of a leaf. The opening and closing of each pore is controlled by the pair of guard cells that surround it. In the space below, draw a fully labelled diagram of two guard cells enclosing a single stoma.
Show the worked answer
Draw two elongated, sausage/kidney-shaped guard cells lying side by side, curved so that they enclose a central gap (the stoma/pore) between them. The cells are thicker on the inner wall (the wall next to the pore) and thinner on the outer wall. Label the following: guard cell (x2), stoma / stomatal pore (the central opening), nucleus (one in each guard cell), chloroplasts (present inside the guard cells - note guard cells contain chloroplasts unlike other epidermal cells), and the thickened inner cell wall / thinner outer wall. The two guard cells should be shown joined at each end so they form the pore between them.
Example 3 (2 marks)
The tea shrub, Camellia sinensis, is grown mainly in tropical regions because it thrives in a damp location. Explain how such a moist environment affects the plant's rate of transpiration.
Show the worked answer
Transpiration is the loss of water vapour from the leaves through the stomata by evaporation and diffusion. A damp/moist environment means the air around the leaf has a high humidity, so it already holds a lot of water vapour. This reduces the water vapour concentration (water potential) gradient between the inside of the leaf (air spaces, which are saturated) and the outside air. Because the difference in water vapour concentration is smaller, water vapour diffuses out of the leaf more slowly. Therefore a moist environment lowers/decreases the rate of transpiration.
More worked questions on this topic
- An ordinary garden plant, plant R, is placed inside a newly sealed terrarium and set under a st (2 marks)
- Using the data in Table 6.1 (temperatures 10–45 °C and the matching oxygen volumes 9, 15, 23, 3 (4 marks)
More O-Level Pure Biology topics
Cell Structure and Organisation · Movement of Substances · Biological Molecules · Nutrition in Humans · Transport in Humans · Respiration in Humans · all of O-Level Pure Biology