Photosynthesis and Energy Supply

Photosynthesis

Photosynthesis is the process by which green plants, some bacteria and protists, synthesise organic substances from inorganic substances. This process involves the absorption of light by pigments, the conversion of light energy into chemical energy and the use of chemical energy for the synthesis of organic substances.
Photosynthesis is a two-stage process involving: the light-dependent reactions (light reactions) and the light-independent reactions (Calvin cycle).

Light-dependent Reactions

The light-dependent reactions occur in the thylakoid membranes of the chloroplasts.
They involve the excitation of chlorophyll by light, leading to photolysis of water, release of oxygen, the synthesis of ATP (photophosphorylation) and the generation of reduced NADP.
There are two pathways of photophosphorylation: cyclic and non-cyclic. Cyclic photophosphorylation involves only Photosystem I and generates ATP. In non-cyclic photophosphorylation, both Photosystems I and II are involved. It results in the synthesis of ATP, and the production of reduced NADP and oxygen.

The Calvin Cycle (Light-independent Reactions)

The Calvin Cycle (light-independent reactions) occurs in the stroma of the chloroplasts.
This stage of photosynthesis involves the fixation of carbon dioxide by reaction with a 5-carbon sugar, ribulose 1,5-bisphosphate, to form two 3-carbon sugars (glycerate 3-phosphate).
Some of the ATP and all the reduced NADP produced in the light-dependent reactions provide the energy and hydrogen to reduce the glycerate 3-phosphate to triose phosphate. This sugar can be used to create stored carbohydrates such as glucose and fructose or used in respiration.
Some triose phosphate molecules are also used to regenerate ribulose 1,5-bisphosphate to continue the cycle.

Factors Affecting Photosynthesis

Photosynthesis is a rate process, influenced by several factors, including light intensity, carbon dioxide concentration, and temperature.
Light intensity influences the rate of photosynthesis, with low light intensities limiting the rate. At higher light intensities, other factors may limit the rate.
Low carbon dioxide concentrations limit the rate of photosynthesis, but as with light intensity, raising the concentration has a plateau effect once another limiting factor comes into play.
Photosynthesis is also affected by temperature, functioning best at optimal temperatures (generally around 25-35°C). Higher or lower temperatures may limit enzyme activity, reducing the photosynthetic rate.

Ecosystems and Energy Flow

Energy flow in ecosystems is unidirectional. The primary source of energy for all ecosystems is the sun. This energy is captured by primary producers (plants) during photosynthesis and converted into chemical energy.
The chemical energy stored in plants is passed on to primary consumers (herbivores) when they eat the plants. From the primary consumers, energy is transferred to secondary consumers (carnivores that eat herbivores) and from secondary to tertiary consumers (carnivores that eat other carnivores).
At each transfer level, only around 10% of the energy is transferred to the next level. The majority of the energy is lost to the environment as heat or is used by the organism for processes such as respiration and movement.
Decomposers in the ecosystem, such as bacteria and fungi, break down dead organic matter and release nutrients back into the soil. In this process, they too use the chemical energy for their metabolic needs and release the remaining energy as heat.
Productivity in ecosystems can refer to primary or secondary productivity. Primary productivity is the rate at which solar energy is converted into organic substances by photosynthetic organisms. This can be subdivided into gross primary productivity (total rate of photosynthesis) and net primary productivity (the rate of energy storage in plant tissues available to herbivores). Secondary productivity is the rate at which consumers convert the chemical energy in food into new biomass.