What is Photosynthesis ? How Photosynthesis takes place.

  • Photosynthesis is a chemical reaction that takes place inside a plant, producing food for the plant to survive.
  • Carbon dioxide, water and light are all needed for photosynthesis to take place.
  • Photosynthesis happens in the leaves of a plant.


There are following factors involve in photosynthesis and can effect on the rate of photosynthesis

Factor # 1. Temperature:

Photosynthesis can take place over a wide range of temperature. Plants of cold climates photosynthesis at much lower temperatures than do those of warm climates. The photosynthesis takes place in certain evergreen species of cold regions, even at temperatures below than 0°C. It has been reported to occur in some species of conifers at temperatures as low as -35°, and in some kinds of lichens at -20°C.
Tropical plants normally do not photosynthesize below about 5°C. On the other hand algae in the water of hot springs may carry on photosynthesis at a temperature as high as 75°C. Many semi-desert and tropical species can withstand air temperatures of 55°C. Most ordinary temperate climate plants, however, photosynthesize best between temperature of 10° and 35°C.
If there is adequate light intensity and a normal supply of carbon dioxide (i.e., no factor is limiting), the rate of photosynthesis of most ordinary land plants increases with rise in temperature up to a point (usually 25°C) which varies somewhat from one kind of a plant to another; above this range there is rapid decline in the rate of photosynthesis primarily form injurious effects of higher temperatures on the protoplasm.
At these higher temperatures the time of exposure is of importance.
The higher the temperature the sooner the decline in photosynthetic rate. The decline in the rate of photosynthesis with time particularly marked at higher temperatures, is evidence of the increasingly limiting effect of some internal factor generally called the “time-factor”. As regards the nature of this time factor, it is possible that it may represent the composite influence of several internal conditions.
They are as follows:
(i) An inactivation of enzymes at higher temperatures.
(ii) The accumulation of the end products of the reaction which may exert a retarding effect on the rate of photosynthesis.
(iii) Failure of the diffusion of carbon dioxide toward the chloroplasts to keep pace with its use in photosynthesis.
(iv) Similar time factor effects are present in the temperature relations of other plant processes such as respiration and growth

Factor # 2. Carbon Dioxide Concentrations:

Nearly 0.032% by volume of carbon dioxide is present in the atmosphere and at this low level it acts as a limiting factor. Under laboratory conditions when light and temperature are not limiting factors, increase in CO2 concentration in the atmosphere from 0.03% to 0.3-1% raises rate of photosynthesis.
With the further increase in the concentration of CO2 progressively the rate of carbon assimilation increases slightly and then it becomes independent of CO2 concentration.
Thereafter, it remains constant over a wide range of CO2concentrations. Plants vary in their ability to utilize high concentrations of CO2. In tomatoes, high concentration of CO2, above the physiological range, exerts harmful influence causing leaf senescence. During the early period of the earth, the concentration of CO2 in the atmosphere was as high as 20%.

Factor # 3. Light:

It is one of the major factors affecting photosynthesis. Photosynthesis cannot occur in the dark and the source of light for the plants is sunlight. Three attributes of light are important for photosynthesis:
• Intensity: Photosynthesis begins at low intensities of light and increases till it is maximum at the brightest time of the day. The amount of light required varies for different plants. Photosynthesis uses maximum up to 1.5 % light in the process and so light is generally not a limiting factor at high intensity. However, the light becomes a limiting factor in low intensity because no matter how much water or CO2 is present, without light photosynthesis cannot occur. At high intensities, the temperature of the plant increases which leads to increased transpiration in the plant. This leads to the closing of the stomata which leads to a reduced CO2 intake. Thus, leading to a reduction and finally stoppage of photosynthesis. Therefore, excessive light inhibits photosynthesis.
• Quality: Experiments conducted by Engelmann prove that the chlorophyll most effectively absorbs red and blue wavelengths from the entire spectrum of light. Thus, maximum photosynthesis occurs when the plant is exposed to the light of these wavelengths.
• Duration: The longer the plant is exposed to light, the longer the process of photosynthesis will continue. As long as the temperature of the plant remains balanced, photosynthesis will occur.

Factor # 4. Intensity:

When CO2 and temperature are not limiting and light intensities are low, the rate of photosynthesis increases with an increase in its intensity. At a point saturation may be reached, when further increase in light intensity fails to induce increase in photosynthesis.
Optimum or saturation intensities may vary with different plant species e.g., C4 and C3. C3 plants become saturated at levels considerably lower than full sunlight but C4 plants are usually not saturated at full sunlight.
When the intensity of light falling on a photosynthesizing organ is increased beyond a certain point, the cells of that organ become vulnerable to chlorophyll catalyzed photo-oxidations. Consequently, these organs begin to consume O2 instead of CO2 and CO2 is released. Photo-oxidation is maximal when O2 is present or carotenoids are absent or CO2 concentration is low.

Factor # 5. Quality:

The action spectrum for photosynthesis in leaves shows two major peaks, one in the red and the other one in the blue (Fig. 14-1). In these regions, chlorophylls absorb maximal light. Most effective wavelengths differ with different plants.
It is of interest to note that plants show high photosynthesis in the blue and red light while red algae do so in green light and brown algae in blue light. The blue-green algae have action spectrum peak in yellow or orange light.

Factor # 6. Duration:

In general, a plant will accomplish more photosynthesis when exposed to long periods of light. It has also been found that uninterrupted and continuous photosynthesis for relatively long periods of time, may be sustained without any visible damage to the plant. We would also do well to bear in mind that if we remove the source of light, the rate of CO2 fixation falls to zero immediately.
Clearly, no species has evolved and/or has developed a storage battery in its leaves whereby the immediate products of the photochemical reactions can be retained in significant amounts to be utilized for the fixation of CO2 later on.

Factor # 7. Oxygen:

Optimum levels of oxygen are favorable for photosynthesis. Oxygen is needed for photorespiration in C3 plants and the by-product of photorespiration is CO2 which is essential for photosynthesis. Also, the energy generated during the oxygen respiration is needed for the process of photosynthesis as well. However, an increase in the oxygen levels beyond the optimum for the plant leads to inhibition of photosynthesis.
This is because oxygen tends to break down the intermediaries that are formed in photosynthesis. Oxygen also completes with CO2 to combine with RUBISCO which a part of the dark reaction of photosynthesis and photorespiration. Therefore, increased levels of O2 would mean that RUBISCO will combine with O2 to initiate photorespiration and photosynthesis will slow down.
Oxygen has been shown to inhibit photosynthesis in C3 plants while C4 plants show little effect. It is suggested that C4 plants have photorespiration and high O2 stimulates it. The rate of photosynthesis increases by 30-50% when the concentration of oxygen in air is reduced from 20% to 0.5% and CO2, light and temperature are not the limiting factors.
Oxygen is inhibitory to photosynthesis because it would favour a more rapid respiratory rate utilizing common intermediates, thus reducing photosynthesis. Secondly, oxygen may compete with CO2 and hydrogen becomes reduced in place of CO2. Thirdly, O2 destroys the excited (triplet) state of chlorophyll and thus inhibits photosynthesis.
It may be stated that direct effect of O2 on photosynthesis remains to be understood.

Factor # 8. Water:

Water is an essential raw material in carbon assimilation. Less than 1% of the water absorbed by a plant is used in photosynthesis. The decrease in water contents of the soil from field capacity to the permanent wilting point results in the decreased photosynthesis.
The inhibitory effect is primarily attributed to increased dehydration of protoplasm and also stomatal closure. The removal of water from the protoplasm also affects its colloidal state, impairs enzymatic efficiency, inhibits vital processes like respiration, photosynthesis etc. Dehydration may even damage the micromolecular structure of the chloroplasts.
It is also assumed that primary factor of dehydration in retarding photosynthesis is due to stomatal closure which reduces CO2absorption. Water deficiency may cause drying of the cell walls of mesophyll cells, reducing their permeability to CO2. Water deficiency may accumulate sugars and thus increase respiration and decrease photosynthesis.

Factor # 9. Mineral Elements:

As discussed earlier, several minerals are essential for plant growth. These include Mg, Fe, Cu, CI, Mn, P and are closely associated with reactions of photosynthesis.

Factor # 10. Air Pollutants:

Gaseous and metallic pollutants decrease photosynthetic activity. These include ozone, SO2, oxidants, hydrogen fluorides, etc.
Factor # 11. Chemical Compounds:
Compounds like HCN, H2S, etc. when present even in small quantities, depress the rate of photosynthesis by inhibiting enzymes. In addition chloroform, ether etc., also stop photosynthesis and the effect is reversible at low concentrations. However, at high concentrations the cells die.

Factor # 12. Chlorophyll Contents:

The rate of photosynthesis in two varieties of barley having normal green leaves and yellow leaves was studied. CO2, light and temperature were not limiting factors. The rate of assimilation per unit area of leaf surface in the two varieties was the same even though the green-leaved variety contained ten times more chlorophyll than the yellow one. Clearly, the chlorophyll in the green leaves is surplus. Leaves having high chlorophyll content do not photosynthesize rapidly since they lack the enzymes or co-enzymes to use the products of the light reactions to reduce available CO2.

Factor # 13. Protoplasmic Factor:

Besides chlorophyll certain protoplasmic factors also influence the rate of photosynthesis. They affect the dark reactions. It has been shown that these factors are absent in the young stage and develop as the seedling becomes old.
That these protoplasmic factors appear to be enzymatic is indicated by the fact that the capacity for photosynthesis is lost at temperatures above 30°C or at strong light intensities in many plants even though cells are green and living.

Factor # 14. Accumulation of Carbohydrates Intensity:

Accumulation of photosynthate in the plant cells, if not translocate, slows down and finally stops the process. The accumulated products increase the rate of respiration. Sugar is also converted into starch and the accumulation of starch in chloroplasts reduces their effective surfaces and the process slows down.

As discussed earlier, several minerals are essential for plant growth. These include Mg, Fe, Cu, CI, Mn, P and are closely associated with reactions of photosynthesis.