C4 Pathway : Hatch and Slack Pathway

Hatch and Slack pathway: C4 Pathway

M.D. Hatch and C. R. Slack in 1967, demonstrated an alternate pathway of carbon dioxide fixation, in higher plants found in tropical region. They termed it as the C4 pathway.  They found that in certain plants, the first product of photosynthesis is a 4 carbon acid, i.e., oxaloacetic acid (OAA), instead of 3 carbon compound.

This type of carbon dioxide fixation was first demonstrated in some plants of family poaceae like sorghum, maize, sugarcane (monocots) and in some dicots, for example, Atriplex, Amaranthus, Euphorbia etc. These plants are termed as C4 plants because of the first stable photosynthetic product being a 4 carbon compound. 

Characteristics of C4 plants:

Following are the characteristics of  C4 plants:

The C4 plants appear to be better equipped to withstand drought and are also able to maintain active photosynthesis under condition of water stress.  Water stress would lead to stomatal closure in C3 plants and consequent reduction in carbon dioxide uptake, whereas in C4 plants, carbon dioxide concentration is higher resulting in the suppression of photorespiratory carbon dioxide loss.  C4 plants often live in hot, arid and saline habitats.  Thus, C4 plants can tolerate halophytic conditions.

Kranz anatomy:

Another basic feature of C4 plants is the occurrence of Kranz anatomy in their leaves.  The anatomy of a typical C4 leaf is different from that of C3 leaf.  The photosynthetic parenchyma cells in a typical C3 leaf are organized into two distinct tissues-an upper region of tightly packed palisade cells and the more loosely arranged spongy mesophyll cells bordering large intercellular spaces.

C4 leaves are generally thinner than C3 leaves.  The vascular bundles of C4 leaves are closer and also have smaller air spaces .  Besides, in C4 leaves there are only one type of mesophyll cells which are loosely arranged like those of spongy mesophyll in C3 leaves.

In C4 leaves the chloroplasts present in bundle sheath cells are of abnormal type.  They are large in size, centripetally arranged and lack well-organized grana.  They also contain starch grain.  The chloroplast of mesophyll cells are normal.  Hence, in C4 plants, chloroplasts are dimorphic in nature.

C4 leaves are also characterized by the presence of tightly packed, thick-walled bundle sheath cells all around the vascular bundle.  Because of the wreath-like configuration of these bundle sheath cells, this arrangement is termed as Kranz anatomy .  Bundle sheath cells are well protected from oxygen being released from mesophyll cells.

The bundle sheath cells contain large number of chloroplasts. Thus one can easily recognize C4 plants by their prominent dark green veins.

C4 pathway

In C4 plants, initial fixation of carbondioxide occurs in mesophyll cells . The primary acceptor of carbondioxide is phosphoenolpyruvate.  It combines with carbondioxide in the presence of phosphoenol pyruvate carboxylase to form oxaloacetic acid .

Oxaloacetic acid reduces to malic acid.  Inside the bundle sheath cells malic acid is decarboxylated to form pyruvate and carbondioxide.  Carbondioxide is again fixed inside the bundle sheath cells through calvin cycle.  RuBP is termed as secondary or final acceptor of carbondioxide of C4 plants. Therefore, C4 plants have 2 carboxylation reaction.

Following are the steps involved in Hatch and Slack pathway or C4 pathway :

1. Phosphoenolpyruvic acid accepts carbon dioxide.  As a result it forms oxaloacetic acid inside mesophyll cells in the presence of enzyme, phosphoenolpyruvate carboxylase.
2. Oxaloacetic acid is reduced by NADPH₂.  Thus it forms malic acid in the presence of enzyme, malate dehydrogenase.
3. Oxaloacetic acid also produces aspartic acid by a transamination reaction with the help of enzyme, transaminase.
4. Malic acid is transported to bundle sheath cells, where it is decarboxylated by NADP and specific malic enzyme. As a result pyruvic acid and carbon dioxide is produced. Carbondioxide is again fixed inside the bundle sheath cells through calvin cycle.
5. Pyruvic acid is sent back to mesophyll cell. There it converts to phosphoenolpyruvate with the help of ATP. This results in the formation of AMP (adenosine monophosphate) instead of ADP (adenosine diphosphate).

Hence, regeneration of ATP from AMP requires 2ATP each or 12 ATP for formation of 6 molecules of phosphoenol pyruvate.  Therefore ,C4 pathway requires 12 additional ATP or total 30 ATP(18 ATP in C3 cycle+12 additional ATP).

Difference between C3 Plants and C4 Plants

Following are the differences between C3 and C4 plants:

C3 Plants	C4 Plants
1.  Carbon dioxide fixation occurs at only one site.	1.  Carbon dioxide fixation occurs at two sites-first in mesophyll cells and then in bundle sheath.
2.  Ribulose diphosphate is the first acceptor of CO2.	2.  Phosphoenol pyruvate is the first acceptor of CO2, while ribulose diphosphate is the second acceptor.
3. Phosphoglyceric acid is the first product.	3.  Oxalo acetic acid is the first product.
4.  The plants operate only Calvin cycle.	4.  Plants also operate a dicarboxylic acid cycle in addition to Calvin cycle.
5.  Carbon dioxide compensation point is 25-100 ppm.	5.  Carbon dioxide compensation point is 0-10 ppm.
6.  The rate of carbon assimilation is slow.	6.  The rate of carbon assimilation is rapid.
7.  Optimum temperature for photosynthesis is 10-25 degree centigrade.	7.  Optimum temperature for photosynthesis is 30-45 degree centigrade.
8.  The leaves do not possess Kranz anatomy.	8.  The leaves do possess Kranz anatomy.
9.  Cloroplasts are monomorphic (i.e., they are of only one type).	9.  Cloroplasts are dimorphic (i.e., there are two types of chloroplasts).
10.  Bundle sheath usually do not contain chloroplasts.	10.  Bundle sheath contain prominent chloroplasts.
11. In higher plants operating C3 cycle, the chloroplasts are all granal.	11.  There are two types of chloroplasts, granal in mesophyll cells and agranal in bundle sheath cells.
12. CO2 fixing enzymes i.e., ribulose diphosphate carboxylase is inefficient.	12.  CO2 fixing enzymes i.e., ribulose diphosphate carboxylase is efficient because CO2 concentration is high.
13.  Fixation of one molecule of CO2 uses 3 ATP. It also requires 2NADPH.	13.  Fixation of one molecule of CO2 uses 3 ATP. It also requires 2NADPH.
14.  The plants are unable to perform photosynthesis at very low CO2 concentration.	14.  The plants are able to perform photosynthesis even also at very low CO2 concentration.

Significance:

1. C4 plants are more efficient in picking up CO2 even in low concentration because of the high affinity of PEP.
2. C4 plants consume more energy. However, sufficient energy is available in the tropics where the plants grow.  Further C4 plants have little photorespiration while in C3 plants, more than half of photosynthetic carbon may be lost in photorespiration.  C4 pathway, is therefore, of adaptive advantage.
3. The concentric arrangement of mesophyll cells produces a small area in relation to volume. Thus helpful for better utilization of available water. Thus it reduces the intensity of solar radiations.
4. They can tolerate excess salts because of the presence of organic acids.
5. They can also adapt high temperature and intense radiation of tropics.
6. Normal oxygen concentration is not inhibitory for the growth in contrast to C3 plants.