Photosynthesis

Photosynthesis is the fundamental process used by plants, algae, and some bacteria to convert light energy into chemical energy. These organisms, called autotrophs or producers, create their own food. They take simple inorganic molecules from the environment—carbon dioxide from the air and water from the soil—and use sunlight as the energy source to convert them into glucose, a type of sugar that provides energy and building materials for the plant. Oxygen is released as a by-product of this reaction. This process is not only vital for the plant but also for almost all life on Earth, as it forms the base of most food chains and produces the oxygen we breathe.

Photosynthesis takes place inside specialised organelles within plant cells called chloroplasts. These are mainly found in the cells of the leaves, particularly in the palisade mesophyll layer. Each chloroplast contains a green pigment called chlorophyll, which is brilliant at absorbing light energy from the sun. The chloroplast's structure is highly adapted for its role: it has a large surface area of internal membranes which hold the chlorophyll. This allows the maximum amount of light to be absorbed. The raw materials get to the chloroplasts as water is transported from the roots via the xylem, and carbon dioxide diffuses in from the atmosphere through tiny pores in the leaf called stomata.

A plant's leaf is brilliantly engineered for photosynthesis. Its structure is a classic example of adaptation for function. Most leaves are broad and flat, creating a large surface area to absorb as much sunlight as possible. They are also thin, which allows carbon dioxide to diffuse quickly to all cells. A waxy cuticle on the surface prevents water loss. The upper layer of cells, the palisade mesophyll, is packed with chloroplasts and arranged vertically to maximise light absorption. Below this, the spongy mesophyll has large air spaces, allowing gases like carbon dioxide and oxygen to move easily to and from the cells. Tiny pores called stomata, mainly on the underside of the leaf, allow CO₂ to enter and O₂ to exit. These pores are opened and closed by guard cells. Veins containing xylem and phloem transport water to the leaf and take glucose away.

The rate of photosynthesis can be affected by several factors in the environment. A 'limiting factor' is a factor that is in shortest supply and is therefore restricting the rate of the process. Even if other factors are plentiful, the rate will be limited by this one scarce resource. The three main limiting factors for photosynthesis are light intensity, carbon dioxide concentration, and temperature. For example, on a sunny day, CO₂ might be the limiting factor. At night, light is always the limiting factor. Temperature affects the enzymes that control the reactions. If it's too low, the enzymes work slowly; if it's too high, they can be damaged (denatured), which stops photosynthesis altogether. Understanding these factors is crucial for commercial growers who manipulate them in greenhouses to maximise crop yield.

It's a common mistake to think plants only photosynthesise. In fact, plant cells respire all the time, just like animal cells, to release energy from glucose. Photosynthesis only happens in the light. This leads to a daily cycle of gas exchange. During the day, the rate of photosynthesis is much higher than the rate of respiration. This means there is a net intake of CO₂ and a net release of O₂. At night, with no light, photosynthesis stops completely. However, respiration continues, so the plant takes in O₂ and releases CO₂. There is a point, usually at dawn and dusk, when the light intensity is such that the rate of photosynthesis exactly equals the rate of respiration. This is called the light compensation point. At this point, there is no net exchange of gases between the plant and the environment.

The glucose produced during photosynthesis is a vital molecule with multiple fates. It is the plant's primary source of energy and its basic building block. The main uses are: 1. Respiration: A large portion of glucose is immediately broken down by respiration to release energy (ATP). This energy powers all other metabolic processes, such as growth and active transport. 2. Storage as Starch: Glucose is soluble and can affect the water potential of cells. To store it, plants convert glucose into starch, which is a large, insoluble molecule. Starch can be stored in chloroplasts or in special storage organs like tubers (e.g., potatoes) and can be converted back to glucose when needed. 3. Making Cellulose: Glucose is used to build cellulose, a strong structural carbohydrate that makes up the plant's cell walls, providing support. 4. Making Proteins and other molecules: Glucose can be combined with mineral ions, such as nitrates from the soil, to synthesise amino acids. These amino acids are then joined together to form proteins. Glucose can also be converted into lipids (fats and oils) for energy storage, often in seeds.