Photosynthesis

1
Photosynthesis

• 6CO2 12H20 C6H12O6 6O2 6 H2O

2
Overview

• Light-dependent reactions of photosynthesis
• Light-independent reactions of photosynthesis
• C3 versus C4 metabolism

• Write an essay on the light-dependent (light)
  reactions of photosynthesis
• Write an essay on the light-independent (Dark)
  reactions of photosynthesis
• Compare and contrast C3 and C4 metabolism

3
Photosynthesis
CO2
Calvin/Benson Cycle (Dark Reactions)
Light reactions
H2O O2
Sucrose
4
Photosynthesis occurs only in the chloroplasts
Vacuole
Stroma
Grana
Chloroplast
Palisade cell
Thylakoids
Thylakoid space
PSI
PSII
5
The light reactions of photosynthesis
6
Plants absorb Light
Chlorophyll a
Chlorophyll b
Abs
Carotenoids
400
Green
Red
500
600
700
Blue
Wavelength of light (nm)
7
Absorption of light leads to excited electrons

• When light energy is absorbed by chlorophyll, an
  electron is promoted to a higher (more
  energetic) orbital. This energy can be lost by
• from one orbital to the next with loss of heat
• back to the ground state with loss of light
  (fluorescence) and heat
• transfer to another chlorophyll.

8
Chlorophylls are organised into photosystems to
trap light energy
Special chlorophyll molecule - primary electron
acceptor
Light
Transfer of light energy from one chlorophyll to
another
Chlorophylls are organised into photosystems
9
Cyclic photosphorylation generates ATP
Stromal side
Fd
PQ
PQ
P700
PSII
PSI
Thylakoid space side
10
Non-Cyclic photosphorylation generates ATP and
NADPH
NADP
Stromal side
NADPH
Q
Fd
PQ
PQ
P700
P680
PSII
PSI
H20 2H 2e- 1/2 O2
Thylakoid space side
11
Generation of ATP
H
Stroma

• H gradient generated by
• PQ shuttle (H)
• Splitting of water (2H)
• NADPH production (2H removed from stroma)
• ATP generated by an ATP synthase as H ions flow
  through it

ATP
H
ADP Pi
H
H
H
H
H
12
Light-independent reactions
13
The Calvin/Benson cycle
Ribulose biphosphate
Carbon
CO2
Phosphorus
3-phosphoglycerate
ATP
ADP
ADP
ATP
1,3 diphospho- glycerate
NADPH
NADP
Sugars
G-3-P
Glyeraldehyde-3-phosphate
14
C4 metabolism
15
C3 and C4 metabolism

• Inefficient as RUBISCO can act as both a
  carboxylase and as an oxygenase.
• Oxygenase activity leads to loss of carbon that
  has already been fixed.
• Some plants have an alternative pathway where CO2
  is first fixed into C4 organic acids (C4 pathway)
  and then liberated later to undergo the
  Calvin/Benson cycle (C3 metabolism).

16
C4 metabolism

• 2 types
• C3 and C4 metabolism are separated in space
  (different cells) e.g. sugar cane Characteristic
  aggregation of cells around the vascular bundles
  - Krantz morphology
• C3 and C4 metabolism are separated in time (same
  cell) e.g. pineapple.

17
Comparison of C3 C4 leaves

• C3 - note the lack of chloroplasts in the bundle
  sheath

• C4 - note the extensive chloroplasts in the
  bundle sheath

18
Separation in space
CO2
PEP (3C)
Mesophyll cell surrounding bundle sheath
OAA (4C)
ADP
ATP
Pyruvate (C3)
CO2
C3
Bundle sheath cell
sugar
Vascular tissue