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Photosynthesis and Plant Nutrition: Question 5

Syllabus 6.2

Multiple choice Core 1 mark

A cross-section through a privet leaf shows four layers, in order from the upper surface of the leaf down to the lower surface:

  1. a thin, transparent, waxy layer that lets light through and reduces water loss from the leaf's surface;
  2. a layer of tall, closely packed cells, positioned just beneath layer 1, containing many chloroplasts;
  3. a layer of loosely packed, irregularly shaped cells with large air spaces between them;
  4. a layer containing pairs of guard cells surrounding small pores.

Which named tissue is layer 2, and why is its position, just beneath the upper surface, an advantage for photosynthesis?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify layer 2 from its description

Layer 2 is described as tall, closely packed cells containing many chloroplasts, sitting directly beneath the upper waxy layer. This matches the palisade mesophyll, whose column-shaped cells are packed tightly together, leaving little space between them, and are filled with chloroplasts. This rules out the spongy mesophyll, which is the loosely packed layer with large air spaces described as layer 3, further down.

Step 2: Work out why its position is an advantage

Because layer 2 sits directly beneath the transparent, light-permeable waxy layer (layer 1), it is the first cell layer that light reaches after entering the leaf. Being packed with chloroplasts and positioned closest to the incoming light means the palisade mesophyll can absorb the most light for photosynthesis, before that light is scattered or absorbed by the layers beneath it.

Step 3: Why the other options are wrong

  • A and D wrongly name layer 2 as the spongy mesophyll, but the spongy mesophyll is the loosely packed layer with large air spaces (layer 3), not the tall, closely packed layer described in layer 2.
  • C correctly names layer 2 as the palisade mesophyll, but gives the wrong reason: reducing water loss is the role of the waxy cuticle (layer 1), not the reason the palisade layer sits near the top.

Final answer

Layer 2 is the palisade mesophyll, positioned just beneath the upper surface so that it receives the most light for photosynthesis, option B.