17.5

17.0 Photosynthesis

1) 17.1 Overview of photosynthesis 2) 17.2 Absorption spectrum of

photosynthesis pigments 3) 17.3 Light Dependent Reaction 4) 17.4 Light Independent Reaction/Calvin

Cycle 5) 17.5 Alternative mechanisms of Carbon

Fixation : Hatch Slack (C4) and Crassulacean Acid Metabolism (CAM) pathways

Learning outcomes :

17.5 Alternative mechanism of carbon

fixation : Hatch-Slack (C4) and

Crassulacean Acid Metabolism

(CAM)pathways

Describe carbon fixation in C4 and CAM plants

HATCH-SLACK

PATHWAY

IN C4 PLANTS

Learning Outcomes : 17.5 : Describe carbon fixation in C4 and CAM plants


C4 carbon fixation is one of three biochemical mechanisms, along with C3 and CAM photosynthesis,

used in carbon fixation

It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in

contrast to the 3-carbon molecule products in C3 plants

INTRODUCTION to C4 plants

A group of plants (C4 plant) has evolved by using an alternative method of fixing carbon

dioxide.

This C4 plant have Krantz anatomy leaf

Eg : corn, sorghum & sugarcane

It consists of mesophyll cells which is arranged concentrically around bundle

sheath cells which also has chloroplast

Mechanism in C4 plants


CO2 is fixed twice by 2 different pathways :

1st carbon dioxide fixation is known as Hatch-Slack pathway

2nd carbon dioxide fixation is known as Calvin Cycle

Mechanism in C4 plants


DIFFERENCES BETWEEN C3 AND C4 PLANTS


1st carbon dioxide fixation :

Occurs in mesophyll cell CO2 will bind to

phosphoenolpyruvate (PEP) to

form oxaloacetate (OAA)

This process catalysed by PEP carboxylase

PEP carboxylase has higher affinity to CO2


1st carbon dioxide fixation :

- PEP carboxylase can fix

CO2 even when

concentration of CO2 is

very low

- OAA reduced to malate

- Then malate is

transported to bundle

sheath cell


2nd carbon dioxide fixation : Occurs in bundle sheath cell Malate undergoes

decarboxylation to produce

pyruvate and CO2 CO2 will combine with RuBP and

undergo Calvin cycle

Pyruvate returns to mesophyll cell Pyruvate phosphorylates to PEP

by using energy from ATP


- The ability of PEP

carboxylase to bind with

CO2 at low concentration

can increase the CO2 in

bundle sheath cell

- Thus, enable RuBisCO to

fix CO2 instead of O2 with

RuBP and enters Calvin

cycle

- Tendency of

photorespiration can be

reduced


C3 C4 CO2 fixation Occurs once

In mesophyll cells

Occurs twice,

first in mesophyll cells, then in bundle sheath cells

CO2 acceptor RuBP, a 5C compound Mesophyll cells ;

PEP

Bundle sheath cells; RuBP

CO2 fixing enzyme RuBP carboxylase PEP carboxylase which is very efficient

RuBP carboxylase

Bundle sheath cells Non photosynthetic/ no chloroplast

Photosynthetic/ possess chloroplast

First product of photosynthesis

3-phosphoglycerate Oxaloacetate (OAA)

Photorespiration High/ more Low/ less/ none

Efficiency of photosynthesis

Less / low/ not occur More / high/ occur

Comparison between C3 and C4 plants

C3 C4 Leaf anatomy Krantz anatomy absent.

Bundle sheath is surrounded by loosely packed spongy mesophyll

Krantz anatomy present.

Bundle sheath is surrounded by a ring of closely packed palisade mesophyll

Chloroplast Only one type/ form of chloroplast

Two types/ form of chloroplast

CO2 pathway Calvin cycle only Hatch-slack pathway and Calvin cycle

CRASSULACEAN

ACID METABOLISM

(CAM)


- An adaptations in hot and dry condition also

evolved in succulent plant

- These plants open their stomata during the night

- And close stomata during the day to conserves

water and prevent entry of CO2

- but open at night to collect CO2

CRASSULACEAN ACID METABOLISM


This mode of carbon fixation is called

crassulacean acid metabolism (CAM)

Occur in mesophyll cell

Eg : - cactus

- pineapples


During the night :

CAM plants open the stomata

CO2 enter and fixed to PEP to form OAA

Catalysed by PEP carboxylase

OAA then reduced to malate

The malate are stored in vacuoles until morning


During the day:

CAM plants close the stomata

Malate released from vacuole and decarboxylated

Form CO2 and 3C compound

CO2 is released to the Calvin cycle

The light dependent reaction supply ATP and NADPH to Calvin cycle


Comparison between CAM and C4

CAM C4 CO2 fixation Occurs twice, both in

mesophyll cells but

occurs at different

times : night and day

Occurs twice,

first in mesophyll cells, then in

bundle sheath cells

CO2 acceptor Mesophyll cells ;

PEP

Mesophyll cells;

PEP

Bundle sheath

cells; RuBP

CO2 fixing enzyme During night:PEP

carboxylase

During day:

RuBisCO

Mesophyll cells;

PEP carboxylase

which is very

efficient

Bundle sheath

cells ; RuBisCO

CAM C4 CO2 fixation Occurs twice, both in

mesophyll cells but occurs at different times : night and day

Occurs twice,

first in mesophyll cells, then in bundle sheath cells

CO2 acceptor Mesophyll cells ;

PEP

Mesophyll cells; PEP

Bundle sheath cells; RuBP

CO2 fixing enzyme During night:PEP carboxylase

During day: RuBisCO

Mesophyll cells; PEP carboxylase which is very efficient

Bundle sheath cells ; RuBisCO

17.5

Documents

cam plants c4 carbon

c4 plants

product of carbon fixation

cam plantsc3 c4 co2

co2 co2

co2 learning outcomes

hatch slack c4

hatchslack c4