Understanding Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are reciprocal processes. In other words, the products of one process are the raw materials (reactants) of the other process.

Do not confuse cellular respiration with breathing. Breathing is a physical action that helps you get the oxygen you need to carryout respiration which is a chemical process.

The main event of photosynthesis is food production not oxygen production. Oxygen is a byproduct of photosynthesis.

The main event of cellular respiration is the release of energy from food and storing that energy in a smaller more usable form (ATP). Carbon dioxide is a byproduct. Oxygen must be present for aerobic respiration to take place.

All organisms perform the process of cellular respiration. Only photosynthetic organisms (green plants, algae, etc.) carryout the process of photosynthesis.

During photosynthesis, the energy from the sun is stored in the bonds of food the product. During cellular respiration, the energy is released from the bonds of the food and stored in the bonds of a smaller molecule (ATP) for more accessible use.

The is the equation for photosynthesis: CO 2 + H 2 O + energy (sunlight) → C 6 H 12 O 6 + O 2 This is the equation for cellular respiration: C 6 H 12 O 6 + O 2 → CO 2 + H 2 O + energy

Photosynthesis takes place in the chloroplasts. Cellular Respiration takes place in the mitochondria.

Photosynthesis takes as two distinct set of reactions: Reactions that must take place in the presence of light – light reactions Reactions that may take place with or without the presence of light – Calvin cycle also referred to as dark reactions

Cellular respiration always begins with a process called glycolysis – the breakdown of glucose (food). Glycolysis is an anaerobic process that does not require oxygen. Glycolysis takes place in the cytoplasm of the cell. Glycolysis produces a net 2 ATP molecules. (More molecules are produced but some must be used to supply energy to the process).

After glycolysis takes place the molecules produced (pyruvate or pyruvic acid) can enter the Krebs Cycle (citric acid cycle) which takes place in the mitochondrion. Electrons that are produced may be transported to the electron transport chain in the mitochondria by carrier molecules of NADH.

The Krebs Cycle and the electron transport chain are part of aerobic respiration and require oxygen to occur. Aerobic respiration produces a net 36 ATP molecules.

The pyruvic acid produced during glycolysis may also be used in fermentation. Fermentation is the process that takes place when oxygen is not available or when the body is using oxygen faster than it can be supplied. Fermentation is an anarerobic process.

There are two types of fermentation. Lactic acid fermentation – which takes place in muscle cells and some prokaryotic cells. Alcoholic Fermentation – which takes place in yeast and a few other microorganisms resulting in the production of ethyl alcohol.

Anaerobic respiration only produces a net 2 ATP molecules. Aerobic respiration is 18 times more efficient at releasing energy from food molecules and storing it in ATP.

Organisms that are able to carryout photosynthesis producing their own food are known as autotrophs or producers. Organisms that must obtain food for energy by eating other organisms are known as heterotrophs or consumers.