Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, in the form of glucose. This process uses carbon dioxide and water as raw materials and releases oxygen as a by-product. It is the primary method by which energy enters most food chains.

Chlorophyll is the primary photosynthetic pigment found in chloroplasts, giving plants their green colour. It absorbs light energy, particularly in the blue-violet and red regions of the visible spectrum, reflecting green light. There are different types, primarily chlorophyll a (the main photosynthetic pigment) and chlorophyll b (an accessory pigment that broadens the range of light absorbed). The absorbed light energy excites electrons in the chlorophyll molecule, initiating the light-dependent reactions. An absorption spectrum shows the wavelengths of light absorbed by a pigment, while an action spectrum shows the rate of photosynthesis at different wavelengths.

This stage occurs in the thylakoid membranes of the chloroplasts. It requires light energy. Water molecules are split (photolysis) by light energy, releasing electrons, protons (H⁺), and oxygen gas. The electrons are passed along an electron transport chain, releasing energy that is used to generate ATP (adenosine triphosphate) from ADP and inorganic phosphate (photophosphorylation). The protons combine with NADP⁺ (nicotinamide adenine dinucleotide phosphate) to form NADPH, a reducing agent. Oxygen is released as a by-product. The main products, ATP and NADPH, carry chemical energy and reducing power to the light-independent stage.

This stage, also known as the Calvin Cycle, occurs in the stroma of the chloroplasts and does not directly require light, but depends on the ATP and NADPH produced during the light-dependent stage. It involves the fixation of carbon dioxide (CO₂) into organic molecules. The cycle can be divided into three main phases: 1. **Carbon Fixation**: CO₂ combines with a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), catalysed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), forming an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). 2. **Reduction**: The 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P) using energy from ATP and reducing power from NADPH. G3P is a three-carbon sugar. 3. **Regeneration**: Most of the G3P molecules are used to regenerate RuBP, a process that requires more ATP, allowing the cycle to continue. For every six molecules of G3P produced, one molecule is used to synthesise glucose or other organic compounds, while the other five are used to regenerate RuBP.

The rate of photosynthesis is influenced by several environmental factors. When one factor is in short supply, it limits the rate of the entire process, even if other factors are optimal. This is known as a limiting factor. 1. **Light Intensity**: As light intensity increases, the rate of photosynthesis increases up to a certain point, after which it plateaus as another factor becomes limiting. 2. **Carbon Dioxide Concentration**: Increasing CO₂ concentration generally increases the rate of photosynthesis, as CO₂ is a raw material. Beyond a certain concentration, the rate will plateau. 3. **Temperature**: Photosynthesis is enzyme-controlled, so its rate increases with temperature up to an optimum. Beyond the optimum, enzymes begin to denature, and the rate rapidly decreases. 4. **Water Availability**: Water is a raw material for the light-dependent stage. Severe water shortage can reduce the rate of photosynthesis, primarily by causing stomata to close, limiting CO₂ uptake. 5. **Chlorophyll Concentration**: The amount of chlorophyll present directly affects the amount of light that can be absorbed. A deficiency in nutrients like magnesium (a component of chlorophyll) can lead to reduced chlorophyll production and thus a lower rate of photosynthesis.