The C4

Pathwa

y

Plants that are adapted to dry tropical regions have the C4 pathway as said before.

Despite having oxaloacetic acid as the first CO2 fixation product they use the C3 pathway or the Calvin cycle as the main biosynthetic pathway.

How are C4 plants different from C3

plants?C4 plants are special :-

They have a special type of leaf anatomy

They tolerate higher temperatures They show a response to high light

intensities They lack a process called

photorespiration They have greater productivity of

biomass.

Anatomy of C4 plant leaf The large cells present around the

vascular bundles of the C4 pathway plants are called BUNDLE SHEATH CELLS.

The leaves which have such anatomy are said to have ‘KRANZ’ anatomy.

The bundle sheath cells may form several layers around the vascular bundles ,they are characterized by the following :-

• a large number of chloroplasts. • thick walls unaffected by gaseous

exchange.• no intercellular spaces.

HATCH AND SLACK PATHWAY

What happens in Hatch and Slack

Pathway? In this pathway, Phospho-enol-pyruvate (PEP) combines with atmospheric CO2 in the presence of catalyst enzyme Phospho-enol-pyruvate carboxylase (PEP CASE) to form Oxaloacetic acid (OOA).

This reaction occurs in the mesophyll cell of the plants.

OOA is then converted into another 4 carbon compound Malic acid which is transported into bundle sheath cells from mesophyll cells.

In the bundle sheath cells, the malic acid gets decarboxylated or breaks down to release CO2 and a 3 carbon molecule (Pyruvic Acid).

The CO2 liberated is used for Calvin cycle in the bundle sheath cells and the 3 carbon molecule (Pyruvic Acid) is transported back to the mesophyll cell, where it is used in the regeneration of PEP. Thus completing the cycle.

Photorespiration

In C3 plants some O2 does bind to RuBisCO and hence CO2 fixation is decreased.

So the RuBP instead of being converted to 2 molecules of PGA binds with O2 to form 1molecule of phosphoglycerate and phosphoglycolate in a pathway called Photorespiration.

In the photorespiratory pathway, there is neither synthesis of sugar nor of ATP but results in the release of CO2 with the utilization of ATP.

Therefore, photorespiration is regarded as a wasteful process.

In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site.

This results in increasing the intracellular concentration of CO2, which ensures that the RuBisCO functions as carboxylase and minimize the oxygenase activity.

Factors Affecting

Photosynthesis

Photosynthesis occur under the influence of many factors both internal(plant) and external factors.

The plant/ internal factors include :-The number, size, age and orientation of leavesMesophyll cells and chloroplastsInternal CO2 concentration & the amount of

chlorophyll

These factors depend on the genetic predisposition and the growth of the plant.

The external factors would include the availability of the following :-SunlightCO2 concentrationTemperatureWater

All these factors simultaneously affects the rate of photosynthesis. Since all these factors interact and affect photosynthesis or CO2 fixation, one of these factors are the major cause or is the one that limits the rate of photosynthesis. So at any point the rate will be determined by the factors available at sub-optimal levels

When several factors affect any bio-chemical process, Blackman's LAW OF LIMITING FACTORS comes into effect .

The law states that :-

If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed .

Frederick Frost Blackman

For example :-Despite the presence of a green leaf and optimal light and CO2 conditions, the plant may not photosynthesise if the temperature is low . If this leaf gets the optimal temperature it will start photosynthesizing.

AFFECTSOF

LIGHT

There is a linear relationship between incident light and CO2 fixation rates at low light intensities. At higher light intensities gradually the rate does not show further increase as other factors become limiting.

The light saturation occurs at 10% of full sunlight. So except for plants in shades or in dense forests, light is rarely a limiting factor in nature.

Also increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

Graph of light intensity on the rate of photosynthesis

AFFECTSOF

CARBON DIOXIDE

CONCENTRATION

Carbon dioxide is the major limiting factor for photosynthesis. The concentration of CO2 is very low in atmosphere (0.03% to 0.04%). Increase in concentration upto 0.05% can cause an increase CO2 fixation rates. Beyond this, the levels can become damaging over longer periods.

The C3 and C4 plants respond differently to CO3 concentrations :-At high light conditions none of the group responds

to high CO2 conditionsAt low light conditions both groups show increase in

the rates of photosynthesis.The C4 plants show saturation at about 360µ1L-1

while C3 responds to increase CO concentration and saturation for them is seen only at 450 µ1L-1 and thus current availability of CO2 levels is limiting to the C3 plants.

AFFECTSOF

TEMPERATURE

The dark reactions being enzymatic are temperature controlled. Though light reactions are also temperature sensitive they are affected by temperature to a much lesser extent.

The C4 plants responds to higher temperature and shows higher rate of photosynthesis

C3 plants have a much lesser temperature optimum.

The temperature optimum for photosynthesis of different plants depends upon the habitant they are adapted to.

For example :- Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

Affects of Water

Though water is one of the reactants in the light reaction, the effect of water as a factor is more on the plant, rather than directly on photosynthesis.

Water stress causes the stomata to close reducing the CO2 availability.

Water stress also makes leaves wilt, thus reducing the surface area of the leaves and their metabolic activity as well