Photosynthesis

Photosynthesis is the fundamental process by which plants, algae, and some bacteria convert light energy into chemical energy. They use carbon dioxide from the air and water from the soil to synthesise glucose, a sugar that serves as their food for energy and growth. This process takes place inside chloroplasts, which contain the green pigment chlorophyll that absorbs sunlight. Oxygen is released as a by-product, which is essential for most life on Earth. In essence, photosynthesis is how plants make their own food.

A plant's leaf is perfectly adapted to be a highly efficient solar-powered food factory. Its broad, flat shape provides a large surface area to absorb sunlight. A waxy cuticle on the surface prevents water loss. The upper epidermis is transparent to let light through to the palisade mesophyll layer below. Palisade cells are packed with chloroplasts and arranged vertically to maximise light absorption. Below this, the spongy mesophyll layer has air spaces to allow gases like carbon dioxide to diffuse to the photosynthesising cells. Stomata, small pores usually on the underside of the leaf, open to allow CO₂ in and O₂ out, and are controlled by guard cells. Xylem vessels in the veins bring water to the leaf, while phloem tubes transport the glucose made during photosynthesis away to other parts of the plant.

While glucose provides energy and carbon building blocks, plants need more than just CO₂, water, and light. They also require mineral ions, which they absorb from the soil through their roots via active transport. These minerals are essential for creating key biological molecules. Two crucial minerals are nitrates and magnesium. Nitrate ions (NO₃⁻) are vital because they provide the nitrogen needed to synthesise amino acids. Amino acids are the building blocks of all proteins, which are required for growth, repair, and making enzymes. Magnesium ions (Mg²⁺) are a central component of the chlorophyll molecule itself. Without magnesium, the plant cannot make chlorophyll, cannot absorb sunlight, and therefore cannot photosynthesise. A lack of these minerals leads to deficiency symptoms, such as stunted growth (nitrate deficiency) or yellowing leaves (magnesium deficiency).

The rate of photosynthesis depends on several external factors. At any given moment, the rate is determined by the factor that is in the shortest supply. This is called the 'limiting factor'. The main limiting factors are light intensity, carbon dioxide concentration, and temperature. For example, on a sunny day, light is plentiful, but if CO₂ levels are low, the rate of photosynthesis will be limited by the lack of CO₂. If you increase the CO₂ concentration, the rate will increase until another factor, like light intensity (perhaps as evening approaches), becomes limiting. Temperature affects the enzymes that control the reactions of photosynthesis. As temperature increases, the rate increases up to an optimum point (usually 25-35°C). Beyond this, the enzymes begin to denature, and the rate rapidly decreases. Commercial greenhouse growers manipulate these factors, using artificial lighting, CO₂ generators, and heaters to create optimal conditions and maximise crop yield.

Scientists can perform several experiments to investigate photosynthesis. To test if photosynthesis has occurred, a leaf is tested for starch, which is how plants store the glucose they produce. The procedure involves destarching the plant first (leaving it in the dark for 48 hours), then performing the starch test: boil the leaf in water to kill it and break down cell walls, boil in ethanol (in a water bath for safety) to remove the chlorophyll, rinse in hot water to soften it, and finally add iodine solution. If starch is present, the iodine will turn from brownish-orange to blue-black. To prove light, chlorophyll, and CO₂ are necessary, you can selectively remove each factor. For example, cover part of a leaf with black card to exclude light, use a variegated leaf (with white and green parts) to test for chlorophyll, or place a plant in a sealed jar with a chemical that absorbs CO₂ (like soda lime). In each case, only the parts of the leaf where all factors were present will test positive for starch. The rate of photosynthesis can be measured by collecting the oxygen produced by an aquatic plant, like Elodea (pondweed), over a set time.

It's a common misconception that plants only photosynthesise. In reality, plant cells respire 24 hours a day, just like animal cells, to release energy from glucose for life processes. Photosynthesis, however, only occurs when there is light. This leads to a net change in gas exchange over a 24-hour period. During the day, the rate of photosynthesis is much higher than the rate of respiration. Therefore, the plant has a net intake of CO₂ and a net output of O₂. During the night, with no light, photosynthesis stops completely. Respiration continues, so the plant has a net intake of O₂ and a net output of CO₂. There is a point, at dawn and dusk, where the light intensity is such that the rate of photosynthesis exactly equals the rate of respiration. At this point, called the compensation point, there is no net exchange of gases between the plant and the atmosphere.