Photosynthesis occurs inside chloroplasts. Chloroplasts contain chlorophyll, a green pigment found inside the thylakoid membranes. These chlorophyll molecules are arranged in groups called photosystems. There are two types of photosystems, Photosystem II and Photosystem I. When a chlorophyll molecule absorbs light, the energy from this light raises an electron within the chlorophyll molecule to a higher energy state. The chlorophyll molecule is then said to be photoactivated. Excited electron anywhere within the photosystem are then passed on from one chlorophyll molecule to the next until they reach a special chlorophyll molecule at the reaction centre of the photosystem. This special chlorophyll molecule then passes on the excited electron to a chain of electron carriers.
Green plants and other photosynthetic organisms trap light energy and synthesize carbohydrate molecules via metabolic processes collectively called photosynthesis.
The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place.
Summary Stage I: Light-Dependent Reactions.
The light-dependent reactions transform light energy into chemical energy which is trapped and carried by ATP and NADPH to the Calvin Cycle.
The light-dependent reactions require chlorophyll and occur in the thylakoid membranes of the grana of the chloroplast.
Light energy is also used to split water (Photolysis of water) into:
H2O -----> 2H+ + 2e- + 1/2 O2 This reaction produces oxygen and provides electrons and Hydrogen for the reduction of NADP to NADPH (NADP gains H+ and electrons; the water is oxidized because it loses the H+and e-)
The light reactions remove electrons from excited chlorophyll molecules in both Photosystem I and Photosystem II and pass the higher energy electrons along an electron transport chain, releasing energy to make ATP (from ADP and P), or transferring the electrons to NADP.
The light reactions must occur several times to produce enough ATP and NADPH to "run" the Calvin cycle Stage II: Calvin Cycle or C-3 Photosynthesis
(Sometimes called the Dark Reactions)
Six molecules of Carbon dioxide each combine with a 6 molecules of a 5-carbon sugar (Ribulose bisphosphate) and undergo a reduction to form 3-carbon molecules (Glyceraldehyde 3 Phosphate or G3P).
Ten of the 12 molecules of G3P are used to regenerate more ribulose bisphosphate to keep the cycle going.
Two of the 12 G3P are converted to the carbohydrate, glucose.
These photosynthetic reactions do not use light energy for the energy source.
However, although light-independent reactions are, by convention, also called dark reactions, they are not independent of the need of light, for they are driven by ATP and NADPH, products of light.
ATP is a key ingredient in the final phase of photosynthesis, which occurs independent of light.
Light Independent Stage The light independent stage is sometimes called the dark stage, assuming that it takes place in the dark.
So how can these factors have an effect on the rate of photosynthesis? Lets start off with the light intensity. When the light intensity is poor, there is a shortage of ATP and NADPH, as these are products from the light dependent reactions. Without these products the light independent reactions can't occur as glycerate 3-phosphate cannot be reduced. Therefore a shortage of these products will limit the rate of photosynthesis. When the carbon dioxide concentration is low, the amount of glycerate 3-phosphate produced is limited as carbon dioxide is needed for its production and therefore the rate of photosynthesis is affected. Finally, many enzymes are involved during the process of photosynthesis. At low temperatures these enzymes work slower. At high temperatures the enzymes no longer work effectively. This affects the rate of the reactions in the Calvin cycle and therefore the rate of photosynthesis will be affected.
most photosynthesis takes places in leaves, mesophyll cells and organelles known as chloroplast The first step in photosynthesis is the absorption of light energy to be used in the chloroplast.
A limiting factor is a factor that controls a process. Light intensity, temperature and carbon dioxide concentration are all factors which can control the rate of photosynthesis. Usually, only one of these factors will be the limiting factor in a plant at a certain time. This is the factor which is the furthest from its optimum level at a particular point in time. If we change the limiting factor the rate of photosynthesis will change but changes to the other factors will have no effect on the rate. If the levels of the limiting factor increase so that this factor is no longer the furthest from its optimum level, the limiting factor will change to the factor which is at that point in time, the furthest from its optimum level. For example, at night the limiting factor is likely to be the light intensity as this will be the furthest from its optimum level. During the day, the limiting factor is likely to switch to the temperature or the carbon dioxide concentration as the light intensity increases.
As we can see, there is a close relationship between the action spectrum and absorption spectrum of photosynthesis. There are many different types of photosynthetic pigments which will absorb light best at different wavelengths. However the most abundant photosynthetic pigment in plants is chlorophyll and therefore the rate of photosynthesis will be the greatest at wavelengths of light best absorbed by chlorophyll (400nm-525nm corresponding to violet-blue light). Very little light is absorbed by chlorophyll at wavelengths of light between 525nm and 625 (green-yellow light) so the rate of photosynthesis will be the least within this range. However, there are other pigments that are able to absorb green-yellow light such as carotene. Even though these are present in small amounts they allow a low rate of photosynthesis to occur at wavelengths of light that chlorophyll cannot absorb.
The action spectrum of photosynthesis is a graph showing the rate of photosynthesis for each wavelength of light. The rate of photosynthesis will not be the same for every wavelength of light. The rate of photosynthesis is the least with green-yellow light (525 nm-625 nm). Red-orange light (625nm-700nm) shows a good rate of photosynthesis however the best rate of photosynthesis is seen with violet-blue light (400nm-525nm).
The regeneration of RuBP is essential for carbon fixation to continue. Five triose phosphate molecules will undergo a series of reactions requiring energy from ATP, to form three molecules of RuBP. RuBP is therefore consumed and produced during the light-independent reactions and therefore these reactions form a cycle which is named the Calvin cycle.