Introduction to Photosynthesis

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Importance of Photosynthesis

Photosynthesis is essential to all life on earth. Both plants and animals depend on the ability of this process to capture energy originating in outer space and convert it into chemical compounds.

Photoautotrophs

Plants, algae and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis.
- These organisms manufacture their own food using light energy and are therefore called photoautotrophs (self-feeders using light).
- Other organisms, like animals, fungi and most other bacteria are called heterotrophs (other feeders), as they rely on sugars produced by photosynthetic organisms for their energy.

Storing energy

Photosynthesis is important not only for capturing sunlight's energy, but also for storing it in the carbon-carbon bonds of carbohydrate molecules.
- These carbohydrates are the energy source that heterotrophs use for synthesising ATP by respiration.
- Through a chain of energy transfers, photosynthesis powers 99% of Earth's ecosystems.

Equation for Photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy) and water as substrates. It produces simple carbohydrate molecules and oxygen as a byproduct.

Chemical equation

$Carbon\ dioxide + Water \rightarrow Glucose + Oxygen$
- This basic equation is deceptively simple, as the process takes place in many steps involving intermediate reactants and products.

Photosynthesis and respiration

$Carbon\ dioxide + Water \rightarrow Glucose + Oxygen$
- You may also have noticed that the photosynthesis reaction is the reverse of the reaction for respiration.
  - This makes sense, as respiration is about releasing energy, whereas photosynthesis is the process by which energy is captured and stored.

Structure of the Leaf

In plants, photosynthesis generally takes place in the leaves, which consist of several layers of cells. Photosynthesis occurs in a middle layer called the mesophyll.

Stomata

The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings in the underside of the leaf, called stomata.
- Stomata are also involved in water balance, with guard cells regulating the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

Chloroplasts

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast.
- Chloroplasts have a double membrane envelope (comprising an outer and inner membrane) and contain chloroplast DNA.

Thylakoids

Within a chloroplast are stacked, disc-shaped structures called thylakoids.
- Embedded in the thylakoid membrane are:
  - Chlorophyll, a pigment (light-absorbing molecule) responsible for the initial interaction between light and plant material.
  - Proteins which make up the electron transport chain.
A stack of thylakoids is called a granum and the liquid-filled space surrounding the granum is the stroma.