IGCSE Biology Paper-3: Specimen Questions with Answers 334 - 335 of 358

Passage

Fig. shows a diagram of a cross-section of a dicotyledonous leaf, as seen using a light microscope.

Dicotyledonous Leaf

Question 334 (3 of 4 Based on Passage)

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Plants use carbon dioxide for photosynthesis.

(i) Describe where and how carbon dioxide enters a leaf.

(ii) State the two products of photosynthesis in green plants.

Explanation

  • On the surface of the leaves of the plants there are many tiny pores known as stomata or stoma. For photosynthesis green plants take carbon dioxide from the air. The carbon dioxide enters the leaves of the plant through the stomata present on their surface. Each stomatal pore is surrounded by a pair of guard cells. The opening and closing of the pores of stomata is controlled by the guard cells only. When water flows into the guard cells, they swell, become curved and cause the pore to open. On the other hand, the guard cells lose water; they shrink, become straight and close the stomatal pore.
  • Photosynthesis is the name given to the set of chemical reactions performed by plants to convert energy from the sun into chemical energy in the form of sugar. Specifically, plants use energy from sunlight to react carbon dioxide and water to produce sugar (glucose) and oxygen.

Question 335 (4 of 4 Based on Passage)

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Name tissue A and cell B.

Explanation

A — Palisade mesophyll

B — Stomata

Dorsi-ventral leaves are found in dicots. These are generally horizontal and sunlight falls on their upper surface (ventral surface or adaxial surface) .

The important anatomical features of dorsiventral leaves are:

  • Upper epidermis: It is single-layered. It is covered with a thick cuticle. It has more stomata than the lower epidermis.
  • Mesophyll: It is differentiated into an upper palisade parenchyma and spongy parenchyma. The cells of the palisade parenchyma are cylindrical in shape and are compactly arranged. They have chloroplasts and, therefore, photosynthesize. They also check transpiration. The spongy parenchyma is loosely arranged with large intercellular spaces. They also contain chloroplasts and carry out photosynthesis.
  • Vascular bundle: There is one prominent vascular bundle and several small ones. The size of the vascular bundles is dependent on the size of the veins. The veins vary in thickness in the reticulate venation of the dicot leaves. It has phloem towards the lower (abaxial) side and xylem towards the upper (adaxial) side. The vascular bundle is enclosed by thick walled bundle sheath.
  • Lower epidermis: It is single layered and has less stomata or may be absent.
T. S. Dicot Leaf

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