(a) Fig. 3.1 shows a drawing of a root hair cell and Fig. 3.2 shows a drawing of a palisade cell.
Explain the reasons for the difference in the numbers of mitochondria and chloroplasts between the root hair cell and the palisade cell, shown in Fig. 3.1 and Fig. 3.2.
(b) Fig. 3.3 is a photomicrograph of a cross‑section of part of a xerophyte leaf.
(i) Explain why the part labelled A in Fig. 3.3 is a tissue.
(ii) Describe two ways the structure labelled B in Fig. 3.3 is adapted for its function.
(iii) Describe one way the leaves of xerophytes are adapted to their environment.
(iv) Describe one way the roots of xerophytes are adapted to their environment.
▶️ Answer/Explanation
(a)
Mitochondria: Root hair cells have more mitochondria because they require energy for active transport of minerals/ions against a concentration gradient. Palisade cells have fewer mitochondria as their primary function is photosynthesis.
Chloroplasts: Palisade cells contain chloroplasts for photosynthesis, while root hair cells lack chloroplasts because they are underground and do not receive light.
(b)(i)
The part labelled A is a tissue because it consists of a group of similar cells working together to perform a specific function (e.g., photosynthesis or structural support).
(b)(ii)
Adaptations of structure B (xylem vessel):
- Thick lignified walls for mechanical strength.
- Hollow lumen for efficient water transport.
(b)(iii)
Xerophyte leaf adaptation: Thick waxy cuticle to reduce water loss through transpiration.
(b)(iv)
Xerophyte root adaptation: Deep or widespread roots to maximize water absorption from a large soil area.
Fig. 1.1 is a photomicrograph of a cross-section of part of a leaf.
(a) (i) State the letter in Fig. 1.1 that identifies where:
gas exchange is controlled
most photosynthesis occurs
spongy mesophyll cells are found.
(ii) Draw a circle on Fig. 1.1 to identify a vascular bundle.
(iii) Translocation occurs in the vascular bundle.
State the names of two substances that are only moved by translocation.
(b) Explain how xylem vessels are adapted for their functions.
(c) Describe how water moves through a plant from the soil to the air spaces in a leaf.
(d) Explain why plants need nitrate ions.
▶️ Answer/Explanation
(a)(i) Gas exchange is controlled by L (stomata). Most photosynthesis occurs in N (palisade mesophyll). Spongy mesophyll cells are found in M.
(a)(ii) A vascular bundle contains xylem and phloem and appears as a distinct cluster in the leaf cross-section.
(a)(iii) Two substances moved by translocation are sucrose and amino acids.
(b) Xylem vessels are adapted by having thick lignified walls for support, no cell contents for uninterrupted water flow, and continuous tubes formed by end-to-end cell joining.
(c) Water moves via root hair cells (osmosis) → xylem (cohesion-tension) → mesophyll cells (diffusion) → air spaces (evaporation).
(d) Plants need nitrate ions to synthesize amino acids, which form proteins essential for growth and cellular functions.