Lesson Two

1
Lesson Two

• Photosynthesis

2
Photosynthesis

• Photosynthesis occurs only in the
  chlorophyll-containing cells of green plants
  , algae ,
• and certain protists and
  bacteria.

3
Photosynthesis

• Overall, it is a process that converts light
  energy into chemical energy that is stored in the
  form of molecular bonds.

Carbondioxide
Water
Glucose
Oxygengas
PHOTOSYNTHESIS
4
Photosynthesis

• From the point of view of chemistry and
  energetics, it is the opposite of cellular
  respiration.

5
Photosynthesis

• Whereas cellular respiration is highly exergonic
  and releases energy,

reactants
Amounts of energy output
Potential energy of molecules
products
6
Photosynthesis

• .photosynthesis requires energy and is highly
  endergonic.

products
Amount of energy input
Potential energy of molecules
reactants
7
Photosynthesis

• Photosynthesis starts with CO2 and H2O as raw
  materials and proceeds through two sets of
  partial reactions. In the first set, called the
  light-dependent reactions, water molecules are
  split (oxidized), O2 is released, and ATP and
  NADPH are formed.

8
(No Transcript)
9
Photosynthesis

• In the second set, called light-independent
  reactions, CO2 is reduced (via the addition of H
  atoms) to carbohydrates. These chemical events
  rely on the electron carrier NADPH and ATP
  generated by the first set of reactions.

10
3
CO2
1
3
P
P
P
6
RuBP
3-PGA
6
ATP
3 ADP
Light-independent reactions (CALVIN CYCLE)
6 ADP
P
3
ATP
6
NADPH
4
6 NADP
6
P
5
P
G3P
G3P
3
Glucoseand other compounds
OUTPUT
1
P
G3P
11
Photosynthesis

• Both sets of reactions take place in
  chloroplasts.

12
Photosynthesis

• Most of the enzymes and pigments for the
  light-dependent reactions are embedded in the
  thylakoid membrane of chloroplasts.

13
Photosynthesis

• The dark reactions take place in the stroma.

stroma
14
How Light Energy Reaches Photosynthetic Cells

• The energy in light photons in the visible part
  of the spectrum can be captured by biological
  molecules to do constructive work.

15
How Light Energy Reaches Photosynthetic Cells

• The pigment chlorophyll in plant cells absorbs
  photons within a particular absorption spectrum
  a statement of the amount of light absorbed by
  chlorophyll at different wavelengths.

16
How Light Energy Reaches Photosynthetic Cells

• When light is absorbed it alters the arrangement
  of electrons in the absorbing molecule. The added
  energy of the photon boosts the energy condition
  of the molecule from a stable state to a
  less-stable excited state.

17
How Light Energy Reaches Photosynthetic Cells

• During the light-dependent reactions of
  photosynthesis, as the absorbing molecule returns
  to the ground state, the excess excitation
  energy is transmitted to other molecules and
  stored as chemical energy.

18
How Light Energy Reaches Photosynthetic Cells

• All photosynthetic organisms contain various
  classes of chlorophylls and one or more
  carotenoid (accessory) pigments that also
  contribute to photosynthesis.

19
How Light Energy Reaches Photosynthetic Cells

• Groups of pigment molecules called antenna
  complexes are present on thylakoids.

20
How Light Energy Reaches Photosynthetic Cells

• Light striking any one of the pigment molecules
  is funneled to a special chlorophyll a molecule,
  termed a reaction-center chlorophyll, which
  directly participates in photosynthesis.

21
How Light Energy Reaches Photosynthetic Cells

• Most photosynthetic organisms possess two types
  of reaction-center chlorophylls, P680 and P700,
  each associated with an electron acceptor
  molecule and an electron donor. These
  aggregations are known respectively as
  photosystem I (P700) and photosystem II (P680).

22
How Light Energy Reaches Photosynthetic Cells
23
The Light-Dependent Reaction Converting Solar
Energy into Chemical-Bond Energy
24
The Light-Dependent Reaction

• The photosystems of the light-dependent reactions
  are responsible for the packaging of light energy
  in the chemical compounds ATP and NADPH. The
  packaging takes place through a series of
  oxidation-reduction reactions set in motion when
  light strikes the P680 reaction center in
  photosystem II.

25
The Light-Dependent Reaction

• In this initial event water molecules are
  cleaved, oxygen is released, and electrons are
  donated. These electrons are accepted first by
  plastoquinone and then by a series of carriers as
  they descend an electron transport chain.

26
The Light-Dependent Reaction

• For each four electrons that pass down the chain,
  two ATPs are formed. The last acceptor in the
  chain is the P700 reaction center of photosystem
  I.

27
The Light-Dependent Reaction

• At this point incoming photons boost the energy
  of the electrons, and they are accepted by
  ferredoxin.

28
The Light-Dependent Reaction

• Ferredoxin is then reoxidized, and the coenzyme
  NADP is reduced to the NADPH. The ATP generated
  previously and the NADPH then take part in the
  light-independent reactions.

29
The Light Dependent Reaction

• The production of ATP from the transport of
  electrons excited by light energy down an
  electron transport chain is termed
  photophosphorylation. The one-way flow of
  electrons through photosystems II and I is called
  noncyclic photophosphorylation

30
The Light-Dependent Reaction

• plants also derive additional ATP through cyclic
  photophosphorylation, in which some electrons are
  shunted back through the electron transport chain
  between photosystems II and I.

31
The Light-Independent Reactions Building
Carbohydrates

• In the light-independent reactions of
  photosynthesis, which are driven by ATP and
  NADPH, CO2 is converted to carbohydrate. The
  reactions are also known as the Calvin-Benson
  cycle.

32
The Light-Independent Reactions

• Atmospheric CO2 is fixed as it reacts with
  ribulose 1,5-bisphosphate (RuBP), a reaction that
  is catalyzed by the enzyme ribulose
  1,5-bisphosphate carboxylase (rubisco).

33
The Light-Independent Reactions

• The reduction of CO2 to carbohydrate (fructose
  1,6-bisphosphate) is completed via several more
  steps of the cycle.

Glyceraldehyde 3-phosphate
34
The Light-Independent Reactions

• Finally, RuBP is regenerated so that the cycle
  may continue.

35
Oxygen An Inhibitor of Photosynthesis

• High levels of oxygen in plant cells can disrupt
  photosynthesis and can also cause
  photorespiration an inefficient form of the
  dark reactions in which O2 is fixed rather than
  CO2 and no carbohydrate is produced.

rubisco
36
Reprieve from Photorespiration The C4 pathway

• Most plants are C3 plants they experience
  decreased carbohydrate production under hot, dry
  conditions as a result of the effects of
  photorespiration.

37
The C4 pathway

• Among C4 plants, however, special leaf anatomy
  and a unique biochemical pathway enable the plant
  to thrive in arid conditions.

38
The C4 pathway

• Thus C4 plants lessen photorespiration by
  carrying out photosynthesis only in cells that
  are insulated from high levels of CO2. They also
  possess a novel mechanism for carbon fixation.

39
Glossary

• Absorption spectrum
• Calvin-Benson cycle
• Carotenoid
• Chlorophyll
• Chloroplast
• Cyclic photophosphorylation
• C3 plant
• C4 plant
• Light-dependent reactions

• Light-independent reactions
• Noncyclic photophosphorylation
• Photon
• Photophosphorylation
• Photosynthesis
• Photosystem
• Ribulose 1,5-bisphosphate
• thylakoid

40
Absorption spectrum

• A graphic representation of the amount of light
  energy absorbed by a substance plotted against
  the wavelength of the light (From Taiz and
  Zeiger, Plant Physiology, 3e).

41
Calvin-Benson Cycle (Calvin Cycle)

• The biochemical pathway for the reduction of CO2
  to carbohydrate. The cycle involves three phases.

42
Carotenoid

• Linear polyenes arranged as a planar zigzag
  chain, with the repeating conjugated double-bond
  system CHCH-CCH3CH-. These orange pigments
  serve both as antenna pigments and
  photoprotective agents.

43
Chlorophyll

• A group of light absorbing green pigments active
  in photosynthesis.

44
Chloroplast

• The organelle that is the site of photosynthesis
  in eukaryotic photosynthetic organisms.

45
Cyclic photophosphorylation (cyclic electron flow)

• In photosystem I, flow of electrons from the
  electron acceptors through the cytochrome b6f
  complex and back to P700, coupled to proton
  pumping into the lumen. This electron flow
  energizes ATP synthesis but does not oxidize
  water or reduce NADP.

46
C3 plant

• Plants in which the first stable product of
  photosynthetic CO2 fixation is a three-carbon
  compound (i.e. 3-phosphoglycerate).

47
C4 plant

• Plants in which the first stable product of CO2
  assimilation in mesophyll cells is a four-carbon
  compound that is immediately transported to
  bundle sheath cells and decarboxylated. The CO2
  released enters the Calvin cycle.

48
Light-dependent reactions

• photosynthetic electron transport
• Electrons flow from light-excited chlorophyll and
  the oxidation of water, through PSII and PSI, to
  the final electron acceptor NADP.

49
Light-independent reactions

• Dark reactions
• Calvin-Benson cycle
• Calvin cycle

50
Noncyclic photophosphorylation

• Noncyclic electron flow
• Electrons flow from light-excited chlorophyll and
  the oxidation of water, through PSII and PSI, to
  the final electron acceptor NADP.

51
Photon

• A discrete physical unit of radiant energy.
• A particle that has zero mass or charge and unit
  spin, the quantum of the electromagnetic field
  and carrier of the electromagnetic force

52
Photophosphorylation

• The formation of ATP from ADP and inorganic
  phosphate (Pi) using light energy stored in the
  proton gradient across the thylakoid membrane.

53
Photorespiration

• Uptake of atmospheric O2 with a concomitant
  release of CO2 by illuminated leaves. Molecular
  oxygen serves as substrate for rubisco and the
  formed 2-phosphoglycolate enters the
  photorespiratory carbon oxidation cycle. The
  activity of the cycle recovers some of the carbon
  found in 2-phosphoglycolate, but some is lost to
  the atmosphere.

54
Photosynthesis

• The conversion of light energy to chemical energy
  by photosynthetic pigments using water and CO2,
  and producing carbohydrates.

55
Photosystem

• A functional unit in the chloroplast that
  harvests light energy to power electron transfer
  and to generate a proton motive force used to
  synthesize ATP.

56
Ribulose 1,5-bisphosphate (RuBP)

• A compound with a backbone of five carbon atoms
  that is required for carbon fixation in the
  Calvin-Benson cycle of photosysthesis.

57
Thylakoid

• The specialized, internal, chlorophyll containing
  membranes of the chloroplast where light
  absorption and the chemical reactions of
  photosynthesis take place.