More on photosynthesis
Monacia Williams, Contributor

Last week we looked at photosynthesis. We noted that the process utilised two raw materials, carbon dioxide (CO2) and water (H2O), and produced two end products, glucose (C6H12O6) and oxygen (O2) under two conditions - sunlight and chlorophyll. We noted also that it took place in two stages, the light-dependent and the light-independent stages.

This week we will focus on where the process takes place - the leaf - which is the organ of photosynthesis. What makes the leaf suitable for photosynthesis? We already know that it contains the chloroplasts, which has the green pigment chlorophyll - one of the conditions necessary for the process to take place. What other features does it have to make it a suitable and efficient site?

Let us take a look at the leaf as it appears on the stem. You should notice that:

• The surface area is relatively large and positioned on the stem so that it is perpendicular to the light source
• Leaves are thin; light is only absorbed in the first millimetre, or so, of tissue.
• The arrangement of the leaves on the stem is in a mosaic pattern. This minimises overlapping or shading, and maximises the exposure of the leaf to light.
• The plant's shoot system holds the leaf in optimum position for illumination and the uptake of carbon dioxide.

These represent the external adaptations of the leaf, but the internal structure is also adapted to suit its function. We will look at these adaptations. Take time to learn them; you will find this knowledge useful in answering your examination questions.

The above picture, copied from www.biologycorner.com/bihttp, provides us with the internal features of the leaf. Starting from the upper section, we will look at each structure in turn.

• Cuticle - it is transparent, allowing light to pass through. It is also waxy, which helps it to reduce water loss from the leaves.
• Upper epidermis - it is only one cell thick, so it is very thin. It protects the moist inner regions of the leaf from the entry of pathogens.
• Palisade mesophyll - this is the major site of photosynthesis. This area consists of cells that are packed vertically with chloroplasts. The chloroplasts may move by cytoplasmic streaming in order to absorb maximum sunlight. The cells are long, narrow and arranged like a picket fence under the epidermis.
• Bundle sheath - this provides additional support for the veins, preventing the leaves from collapsing. Leaves are structurally weak because of the large amounts of air spaces in them.
• Xylem - this transports water and mineral salts from the roots to the leaves. The heavily lignified cells of the xylem help to maintain the extension of the leaf blade.
• Phloem sieve tubes - these remove the products of photosynthesis, transporting them to other areas of the plant.
• Spongy mesophyll - these are irregular-shaped cells which fit together loosely to leave large air spaces which allow gases to enter and leave the photosynthetic tissue.
• Lower epidermis - this has a similar protective function to the upper; the cuticle is usually thinner.
• Guard cells - these are the only cells in the lower epidermis with chloroplasts. Their walls are unevenly thickened, which allows for the opening and closing of the stomata as the cells change turgidity.
• Stoma - this allows the diffusion of oxygen and carbon dioxide down concentration gradients when they are open.

I hope that, from now on, when you look at a leaf you will consider the many things that are taking place in it. See you next week, when we will look at some experiments.

Dr Walton Small (right), president of the Inter-Secondary Schools' Sports Association (ISSA), and Angella Chaplain, chairperson for netball, ISSA, applaud members of the Holmwood Technical High School team as they celebrate winning the senior all-island netball final against Vauxhall High, at the stadium courts recently. Holmwood won the Under-19 title with a 47-44 extra-time scoreline. - Junior Dowie/Staff Photographer

Monacia Williams teaches at Glenmuir High School.