What are Photosynthesis and Respiration?

Photosynthesis and Respiration are the two most important chemical reactions on our planet. Between them they are responsible for all of the oxygen in our atmosphere and all of the biomass on the planet. The net result of the two processes is the conversion of light energy to heat energy.

Photosynthesis and respiration are essentially the reverse of each other. Photosynthesis uses up energy and involves building up complicated molecules - this is an anabolic reaction. Respiration releases energy and involves breaking down complicated molecules - this is a catabolic reaction. Both processes occur in special organelles in the cell:

• Photosynthesis occurs in the chloroplasts
• Respiration occurs in the mitochondria

The most common misconception held by my students regarding plant biology is where plants get their food from. Many pupils believe that plants get their food from the soil; if plants really ate the soil, then there would be no soil left! Plants actually grow by making their own food in a process called 'Photosynthesis.'

The raw materials for photosynthesis are:

• Carbon Dioxide (from the air)
• Water (from the soil)

The energy to drive this reaction comes from the Sun in the form of Light energy. In this chemical reaction, a green molecule called chlorophyll absorbs light energy to change carbon dioxide and water into glucose and oxygen (a waste product). This site of this reaction is the green leaves of the plant, which are packed with chlorophyll.

Photosynthesis is actually a many staged process broken into two sections: the light-dependent reactions and the light-independent reactions.

6CO₂ + 6 H₂O → C₆H₁₂O₆ + 6O₂

The glucose made in Photosynthesis is used for one of several things:

1. Repiration: That's right - plants respire. Some of the glucose is used immediately to release energy needed for life processes.
2. Storage: Some of the glucose is converted into insoluble starch for storage (rather like animals do with fat) These stores of energy can be used later when needed
3. Biosynthesis: Some of the glucose is used to make new chemicals such as proteins, sugars and fats.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6 H₂O (+ energy)

The different life processes all require a steady flow of energy. Animals get this energy from the food they digest. Glucose is a store of chemical energy that is a product of digestion. Glucose can react with oxygen to release large amounts of energy in a process called respiration.

Looking at the word equation for respiration, it is similar to the equation for complete combustion of a fuel. Namely:

Fuel + oxygen → Carbon Dioxide + Water + Energy

Unlike combustion, respiration is a controlled reaction - it would be inconvenient if your cells caught fire every time they tried to release energy.

Respiration comes in two forms: Aerobic (with air) and Anaerobic (without air). Just like photosynthesis, respiration is actually a complicated series of reactions comprising at least 30 separate steps.

Respiration converts glucose into a molecule called Adenosine Triphosphate, or ATP. This is the energy currency of the cell. If all the energy stored in glucose was released in a single step, the cell would be destroyed. ATP is a small, convenient package of energy that the cell can use safely and quickly.