A student examined the lower surface of a leaf under a microscope to study where gases diffuse in and out. Fig. 17.1 shows part of the surface as seen throu...

The pore in the lower epidermis is a stoma, controlled by two guard cells. This tests measuring, naming and drawing the stoma, and explaining diffusion of gases through it.

(a) Greatest width of pore Q [1] Measuring across the widest part of the pore gives about \(3\ \text{mm}\) as drawn (accept an honest reading). [1]

(b) Length of one curved cell [1] Measuring along one curved cell gives about \(12\ \text{mm}\) as drawn (accept an honest reading). [1]

(c) Name of pore Q [1] A stoma (stomatal pore). [1]

(d) The two curved cells [1] Guard cells. [1]

(e) Gas diffusing in during photosynthesis [1] Carbon dioxide. [1]

(f) Gas diffusing out during photosynthesis [1] Oxygen. [1]

(g) Labelled drawing of the pore and guard cells [4]

Marks: a large clear drawing of the two curved guard cells with the pore between them [1]; single clean lines using more than half the space [1]; a ruled label line to the pore [1]; a ruled label line to a guard cell [1].

(h) Why gases diffuse through the pore [3] There is a concentration difference (gradient) between the inside of the leaf and the outside air [1]; the pore is an opening in the leaf surface [1]; so gas particles move through it from a higher to a lower concentration by diffusion [1].

(i) How the air spaces help [3] The air spaces connect to the pore and let gases move freely inside the leaf [1]; they provide a large surface area for exchange [1]; so gases diffuse easily between the air spaces and the cells [1].

(j) Two other features that suit diffusion [2] Any two of: a thin, flat shape giving a short diffusion distance; a large surface area; moist cell surfaces; many internal air spaces. [2]

(k) Why more pores on the lower surface [2] Fewer pores on the upper surface reduce water loss [1]; the lower surface is cooler and shaded, so less water is lost while gases can still diffuse in and out [1].