Photosynthesis - An Overview

There are 3 basic types of photosynthesis:  C3, C4, and CAM.  Each has advantages and disadvantages for plants living in different habitats.

All 3 forms of photosynthesis are based on two pathways.  The first are the light reactions, where energy from the sun is absorbed by the chlorophyll of the plant, and the energy is transferred to hydrogen ions which are split off from water (Oxygen is produced at this point).  The second pathway is the so-called "dark reactions" - better known as carbon fixation or the Calvin Cycle.  Here, carbon dioxide is taken from the air and combined to make sugars.  The energy absorbed in the light reactions is used to drive this process, with the end result being sugar molecules with energy stored in the carbon-carbon bonds of the sugar.  The 3 types of photosynthesis differ in details of how these processes are carried out.

"Normal: photosynthesis is carried out by most plants growing in areas with sufficient water.  In this type of photosynthesis, an enzyme called RuBP carboxylase grabs CO2 in one of the first steps of photosynthesis.  This works fine as long as there is plenty of carbon dioxide and relatively little oxygen.  If there is too much oxygen, RuBP carboxylase will grab that instead of the CO2, and a process called photorespiration will occur.  Photorespiration does not help build up any sugars, so if photorespiration occurs, growth stops.  Normally, oxygen (produced in photosynthesis) exits the plant through the stomata; however, if there isn't enough water available (as would happen under bright, hot, sunny conditions), excess oxygen may build up and trigger photorespiration.  If water is present, however, this process is very efficient because both the light reactions and carbon fixation can occur simultaneously in the same cell, and almost all of the cells in the leaf will be producing sugars.

C4 photosynthesis differs in 2 key ways.  First, instead of RuBP carboxylase, a different enzyme, PEP carboxylase, is used to grab CO2.  The PEP carboxylase is less likely to bind to oxygen, thus photorespiration is less likely to occur, a decided advantage under hot, dry conditions where water may be scarce and the stomata remain closed for long periods, trapping oxygen in the plant.  A second difference is that not all of the photosynthetic reactions take place in the same cells.  Carbon is taken up in the mesophyll cells of the leaf (by PEP carboxylase), added to other carbons to form a 4-carbon chemical (hence the name C4) called oxaloacetate, and the oxaloacetate is pumped into adjacent bundle sheath cells where the rest of the Calvin Cycle takes place.  Because many mesophyll cells are all transferring carbon to a relatively few bundle sheath cells, the ration of carbon dioxide to oxygen remains high in the bundle sheath cells and this also inhibits photorespiration.  All of this pumping of CO2 is inherently inefficient, but if water is in short supply the inefficient C4 route is still better than the C3 route with photorespiration.  Also, since there are fewer cells involved in making sugars, fewer sugars can be made.

The final form of photosynthesis is CAM photosynthesis or Crassulacean-Acid metabolism.  This type of photosynthesis is most common in desert plants where water is at a premium.  In CAM photosynthesis, CO2 is taken up only at night and is  stored in vacuoles.  This causes a  build up of oxaloacetate (acidic) - which we just met in C4 photosynthesis - in those vacuoles.  The effect is to partition the initial uptake of CO2 and the rest of the Calvin Cycle over time (as opposed to the partition of these reactions in space that occurs in C4 photosynthesis).  The stomata are open only at night, when it is relatively cool and humid; they stay closed during the hot, dry day.  In the morning, sunlight driving the light reactions allows the stored carbon to be combined into sugars.  The stored carbon may run out fairly quickly, however, meaning that photosynthesis only proceeds for part of the day, and that means little production of sugars.  Thus desert plants can survive the dry conditions, but at the cost of rapid growth.  Desert plants are often very slow to grow, and this is one of the reasons they invest so much energy in defensive structures (spines) and chemicals - they can't afford to lose such hard-earned biomass.

 Conclusion?  C3  is best under moist conditions, C4 under warm, sunny, dry conditions, CAM under desert conditions  

 

Characteristics of Photosynthesis in C3, C4 and CAM plants

Characteristic

C3 Plant

C4 Plant

CAM Plant

Photorespiration

Yes

Little

none

Lower temp limit for photorespiration

15-25o C

30-40o C

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Rubisco present

Yes

Yes

Yes

PEP Carboxylase present

No

Yes

Yes

Initial CO2 fixation

directly into Calvin Cycle via Rubisco

into OAA via PEP carboxylase, then to malic acid  which moves from mesophyll cell to bundle sheath cell and then releases CO2.

into OAA via PEP carboxylase, then to malic acid  which moves into vacuole (during night). CO2 released from malate during day.

Secondary CO2 fixation

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In bundle sheath cell using Rubisco

In “mesophyll”* cell using Rubisco – in morning

Site of Calvin cycle

mesophyll cells

bundle sheath cells

“mesophyll” cells

Site of Light Reactions

mesophyll cells

mostly in mesophyll cells

“mesophyll” cells

 

* Many CAM plants have photosynthetic cells on the stem; technically these are not mesophyll cells.

 

We see evidence of the differences in photosynthesis in our everyday lives.  Most of our turf grasses have C3 photosynthesis and do fine in the spring and early summer while there is abundant rain.  Late in the summer, however, conditions often dry out and crabgrass may take over the lawn.  Crabgrass is  a C4 plant that thrives in late summer; however its inefficient C4 photosynthesis cannot compete with the C3 grasses earlier in the year when moisture is abundant.  A notable C4 crop is corn, which can be grown under fairly dry conditions; corn originated in dry areas of the western hemisphere. CAM plants, in turn, are often restricted to the desert.