Overview of Photosynthesis

Photosynthesis (English pronunciation: /foʊtoʊˈsɪnθəsɪs/; from the Greek φώτο- [photo-], "light," and σύνθεσις [synthesis], "putting together", "composition") is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight.^[1] Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is vital for all aerobic life on Earth. In addition to maintaining normal levels of oxygen in the atmosphere, photosynthesis is the source of energy for nearly all life on earth, either directly, through primary production, or indirectly, as the ultimate source of the energy in their food,^[2] the exceptions being chemoautotrophs that live in rocks or around deep sea hydrothermal vents. The rate of energy capture by photosynthesis is immense, approximately 100 terawatts,^[3] which is about six times larger than the power consumption of human civilization.^[4] As well as energy, photosynthesis is also the source of the carbon in all the organic compounds within organisms' bodies.
Although photosynthesis can happen in different ways in different species, some features are always the same. For example, the process always begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls. In plants, these proteins are held inside organelles called chloroplasts, while in bacteria they are embedded in the plasma membrane. Some of the light energy gathered by chlorophylls is stored in the form of adenosine triphosphate (ATP). The rest of the energy is used to remove electrons from a substance such as water. These electrons are then used in the reactions that turn carbon dioxide into organic compounds. In plants, algae and cyanobacteria, this is done by a sequence of reactions called the Calvin cycle, but different sets of reactions are found in some bacteria, such as the reverse Krebs cycle in Chlorobium. Many photosynthetic organisms have adaptations that concentrate or store carbon dioxide. This helps reduce a wasteful process called photorespiration that can consume part of the sugar produced during photosynthesis.

Photosynthetic organisms are photoautotrophs, which means that they are repositories of energy, they are able to synthesize food directly from carbon dioxide, water, and using energy from light. They accrue it as part of their potential energy. However, not all organisms that use light as a source of energy carry out photosynthesis, since photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.^[2] In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae and cyanobacteria, the overall process is quite similar in these organisms. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes carbon dioxide but does not release oxygen.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is a redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert carbon dioxide into a carbohydrate, which is a reduction reaction. In general outline, photosynthesis is the opposite of cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, and release chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.

The general equation for photosynthesis is therefore:

2n CO₂ + 2n DH₂ + photons → 2(CH₂O)_n + 2n DO

Carbon dioxide + electron donor + light energy → carbohydrate + oxidized electron donor

6H₂O + 6CO₂ ----------> C₆H₁₂O₆+ 6O₂

Most of us don't speak chemicalese, so the above chemical equation translates as:

six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen

 


 

OVERVIEW OF PHOTOSYNTHESIS:

The reactions of photosynthesis take place in two main stages:

a). those that capture energy ( Light Reactions )

b). those that use energy to make carbohydrates ( Calvin Cycle )

The process of photosynthesis essential involves removing e- from the oxygen in water and redistributing them around the carbons which come from the CO2. This is a redox reaction. The two half reactions are:

 

2nH2O ->nO2 + 4ne- + 4nH+ (oxidation) which leads to production of ATP + NADP This reaction uses the suns energy to remove electrons from the H2O in Photosystem II of the light reactions. Again this is not something that is easily done. We have seen before that oxygen tends to hold on to electrons tightly. It is the same in this case. at the end of the chain a low energy electron enters Photosystem I. Location: chloroplast membranes

4nH+ + 4ne- + nCO2 -> (CH20)n + nH2O (reduction) This reaction is what essentially happens in the dark reaction. Location: stroma

Diagram :