Leaf Structure and Function, worked answers

The palisade mesophyll cells are packed more densely with chloroplasts than the spongy mesophyll cells, and the cells are arranged more closely together with fewer air spaces between them. Because chlorophyll is concentrated in the chloroplasts, a tissue with more chloroplasts per unit area transmits less light and appears as a darker green under the microscope. The spongy mesophyll, in contrast, has large air spaces and fewer chloroplasts per cell, so it appears paler.

chloroplastsdensely packedchlorophyllair spacespalisade mesophyll

Guard cells have a cell wall that is thicker on the side facing the stomatal pore than on the outer side. When water enters the guard cells by osmosis, because the water potential inside the cells is lower than outside, the cells become turgid and turgor pressure builds up against the wall. Because of this uneven thickening, the thinner outer wall stretches more than the thicker inner wall, causing the pair of guard cells to bow apart from each other and open the pore between them. When the guard cells lose water and become flaccid, turgor pressure falls, the cells straighten, and the pore closes.

guard cellsturgidunevenly thickened wallosmosisbow apartstomatal pore

The rate of water uptake is expected to be higher in air moving from the fan than in still air. This is because air movement removes the humid boundary layer that forms near the stomata in still air, maintaining a steeper water vapour concentration gradient between the air spaces inside the leaf and the outside air. This steeper gradient speeds up the rate of diffusion of water vapour out through the stomata, which in turn speeds up the rate at which the shoot takes up water to replace it, raising the potometer reading.

potometerair movementboundary layerwater vapour concentration gradientrate of transpiration

The compensation point is the light intensity at which a plant's rate of photosynthesis exactly equals its rate of respiration. When light intensity is below the compensation point, the rate of photosynthesis is lower than the rate of respiration, so the plant cannot produce enough oxygen to meet its own respiratory needs. As a result, the plant becomes a net absorber of oxygen from the surrounding air and a net releaser of carbon dioxide into it.

compensation pointrate of photosynthesisrate of respirationnet absorber of oxygennet releaser of carbon dioxide

A broad, thin leaf has a large surface area relative to its volume, allowing more light and carbon dioxide to be absorbed at once than a thick, narrow leaf could manage. This thin shape also shortens the diffusion distance between the leaf surface and the mesophyll cells inside, allowing carbon dioxide to reach the chloroplasts more quickly and reducing the time taken for gases and light to reach every cell carrying out photosynthesis.

large surface areathin laminashort diffusion distancelight absorptioncarbon dioxide

The stomata that allow carbon dioxide to diffuse in for photosynthesis unavoidably also allow water vapour to diffuse out along its own concentration gradient. Transpiration therefore occurs as an unavoidable side effect of gaseous exchange rather than something the plant can fully control, so the amount of water lost far exceeds the amount actually used in the photosynthetic reaction itself. The waxy cuticle on the upper epidermis is one structural feature that limits water loss, because it reduces direct evaporation from the leaf surface and forces most water loss to occur through the stomata alone.

stomatagaseous exchangeside effectcuticlewaterproof layer

The midrib contains the leaf's main vascular bundle, the xylem and phloem, which carries water and minerals into the leaf and sugars out of it. Besides transport, the midrib also gives mechanical support to the thin, flexible lamina, keeping it rigid and angled toward the light. Without this support, the lamina would collapse and overlap with neighbouring leaves, reducing the amount of light each leaf could intercept for photosynthesis.

vascular bundlemechanical supportxylem and phloemlight interception