Comparison between photosynthesis and respiration

Photosynthesis and respiration are both processes that are necessary for the survival of organisms. However, in many ways these two processes are very different as they are responsible for contrasting necessities. Therefore they have both similarities and differences.

One major difference between the two is that photosynthesis occurs wholly in plants while respiration occurs in both plants and animals. This is because plants are autotrophic so have to produce their own food while animals are heterotrophic as they obtain food by consuming it in the form of other animals or plants. Plants produce their own food via photosynthesis and as animals do not need to produce their own food they have no need for photosynthesis. Both plants and animals require respiration because it is the process in which food is converted into energy in the form of ATP, which is used for many processes within all organisms.

Photosynthesis begins when a pigment molecule within a photosystem absorbs light energy which is then passed from one pigment molecule to another until a high energy electron is emitted which is used to reduce NADP. Therefore it appears that the process of photosynthesis begins with the presence of energy, in this case light energy. For respiration to begin ATP must be present as it is used to phosphorylate glucose to set off glycolysis. Two ATP molecules are needed for each glucose molecule as they both provide a phosphate group forming fructose bisphosphate after two phosphorylations. Therefore both photosynthesis and respiration require energy in order to begin the whole process. However, the form of energy is different as for respiration the energy comes in the shape of ATP rather than the light energy that is used in photosynthesis. ATP is actually used during photosynthesis but in a later stage. It is used during the Calvin cycle to produce triose phosphate and to recycle triose phosphate to form ribulose bisphosphate.

Respiration and photosynthesis need coenzymes that can be reduced to carry hydrogen. In photosynthesis the coenzyme present is NADP which becomes reduced NADP when it accepts a hydrogen. This is used to oxidise glycerate 3-phosphate into triose phosphate which then proceeds to become glucose and then starch. In respiration two such enzymes are used; NAD and FAD. All of these coenzymes are used to transport hydrogen to where it is needed in the process.

A basic difference between the two is the actual reactants and products formed. In respiration the reactants are oxygen and glucose. The oxygen is ...

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A basic difference between the two is the actual reactants and products formed. In respiration the reactants are oxygen and glucose. The oxygen is the final acceptor of the hydrogen atoms in a reaction catalysed by cytochrome oxidase forming water. Oxygen is essential for the whole aerobic process – everything after glycolysis – and if not present will cause a back up through the electron transport chain up to glycolysis. The glucose is used to produce pyruvate which commences the Kreb’s cycle. Water and carbon dioxide are given off; the water formed from the reduction of oxygen and the carbon dioxide formed from during the Kreb’s cycle. Photosynthesis, on the other hand, is quite the reverse. Carbon dioxide and water are actually the reactants rather than the products. The carbon dioxide is used to form ribulose bisphosphate during the Calvin Cycle while the water undergoes photolysis producing two electrons and two hydrogen ions used in the synthesis of ATP and reduced NADP. Oxygen is produced during the photolysis of light while glucose is produced during the Calvin Cycle from the oxidation of ribulose bisphosphate.

Both of these processes feature an electron transport chain which transports electrons and hydrogen atoms down a decreasing energy level. In photosynthesis this is used to synthesis ATP and to reduce NADP. In respiration it is also used to produce ATP and to allow the hydrogen atoms to eventually combine with oxygen and form water. Both respiration and photosynthesis seem to come in two parts. For respiration there is the anaerobic process that features glycolysis and produces very little ATP and the aerobic process which produces most of the ATP. In photosynthesis there is the light dependent reaction in the thylakoid and the light independent reaction in the stroma. Another similarity in respect to different parts of each process is the actual location of each part. Anaerobic respiration occurs in the cytoplasm and aerobic respiration occurs in the mitochondria. The difference in location mirrors that of photosynthesis although within photosynthesis the whole process takes place within the chloroplasts; anaerobic respiration takes place outside the mitochondria.

In conclusion, although respiration and photosynthesis are fundamentally very different processes with regards to purpose and the products and reactants involved they are still strikingly similar in many respects. This is due to the fact that both are essentially organic processes that involve very similar carbon based molecules as well as a number of other molecules such as ATP and the coenzymes. Both processes are ultimately concerned with provided the organism with energy and a source of energy to enable it to carry out other necessary processes.