Types of Carbon Concentrating Mechanism (CCM) - Overview - PowerPoint Presentation

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Types of Carbon Concentrating Mechanism (CCM) - Overview

Undergraduate level notes

Biochemical

Mechanisms in Plants

Variations

on C3 photosynthesis in which the drawing down of CO

2

is not directly performed by

RuBisCO

Carbon

is initially “fixed” as a C4 acid by

phosphoenolpyruvate

carboxylase (PEPC), which is delivered to

RuBisCO

and converted back to CO

2

, before being fixed as C3 compounds via the CBB cycle.

Two

main variants: C4 and CAM (some variants are also found in aquatic plants).

C4 and CAM - overview

C4

photosynthesis is a spatial separation of the drawing down of CO

2

and its actual fixation by

RuBisCO

.

This

spatial separation is facilitated by

Kranz

anatomy.

CAM

is a temporal separation of the two processes, facilitated in higher plants by a specific, phased pattern of

stomatal

opening and closing.

Biophysical

Mechanisms

Variants

in algae (eukaryotic) and in cyanobacteria (prokaryotic).

Less

well characterised than biochemical CCMs in higher plants.

In

algae, the main CCM component is the

pyrenoid

and in cyanobacteria, it is the

carboxysome

.

The

Pyrenoid

and Carboxysome

Pyrenoid

in algae is where most of the

RuBisCO

in the chloroplast is found, but is not membrane-bound.

Pyrenoid

is part of a CCM comprising a series of bicarbonate (

HCO

3

-

)

pumps and carbonic anhydrases (to interconvert

HCO

3

-

and CO

2

).

Carboxysome

is a more clearly defined

microcompartment

of

cyanobacterial

cells and has a protein shell; bicarbonate ions are also delivered by active transport.

Summary

Biochemical

vs

biophysical mechanisms

Biochemical

: C4 and CAM (higher plants and some algae); Biophysical:

pyrenoid

(algae) and

carboxysome

(cyanobacteria).

Biochemical

CCMs separate CO

2

drawdown and fixation; biophysical CCMs actively concentrate CO

2

around

RuBisCO

by transporting

HCO

3

-

.