6.2 Leaf structure & 6.3 Mineral nutrition

Table of Contents

6.2 Leaf structure

broad, flat shape to obtain maximum sunlight
thin so that carbon dioxide only needs to diffuse across short distances to reach the inner cells of the leaves

Stomata are minute openings in the lower epidermis enclosed by two guard cells. Stomata regulate the amount of carbon dioxide into the leaf during photosynthesis and prevent too much loss of water vapour when photosynthesis stops—the opening and closing of stomata is controlled by guard cells in response to environmental conditions.
Guard cells are bean-shaped and are the only epidermal cells that contain chloroplasts, so they can manufacture food by photosynthesis. In sunlight, the concentration of potassium ions increaseand along with the sugars formed, lowers the water potential of the guard cells. This causes the water from other cells to enter the guard cells. This flow of water into the guard cells increases the pressure inside the guard cells. Because the guard cells have a thicker cellulose cell wall around the stomatal pore, the swollen guard cells curve and pull the stomata open.

The mesophyll layer lies between the upper and the lower epidermis. The mesophyll layer is made up of two layers - upper palisade mesophyll and lower spongy mesophyll layer
The Palisade layer is the main site for photosynthesis and contains numerous chloroplasts to absorb the maximum amount of sunlight for photosynthesis. They are long and cylindrical cells closely packed together with fewer intercellular spaces.
Spongy Mesophyll cells are irregular in shape and loosely arranged so that there are numerous large intercellular air spaces. The surface of mesophyll cells is always covered with a thin film of water so that the carbon dioxide can dissolve in it. They contain chloroplasts but fewer than the palisade mesophyll cells. The chloroplasts in the spongy mesophyll cells absorb sunlight and use it for joining carbon dioxide and water molecules

The large air spaces between the mesophyll cells make it easy for carbon dioxide and oxygen to diffuse in and out. They also play a vital role in transpiration.

The mesophyll cells contain xylem and phloem. The xylem brings water and dissolved mineral salts to the leaf from the soil, absorbed by the root hair cells. Once out of the veins, these raw materials diffuse from cell to cell through the mesophyll of the leaf.
Once the leaves receive all the raw materials, they produce glucose, which is then transported throughout the plant by phloem vessels

Upperepidermis - single layer of closely packed cells covered by the cuticle. Cuticle protects the leaf, prevents excessive evaporation, focuses light on the mesophyll layer
Lower epidermis - contains guard cells and stomata.

Nitrates are a source of nitrogen needed to make amino acids(to build proteins). Its deficiency causes stunted growth and yellowing of leaves

Magnesium is needed to make chlorophyll, and its deficiency causes yellowing between the veins of leaves (chlorosis)

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School

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