CHAPTER 10
PHOTOSYNTHESIS
1
I. OVERVIEW
A. Photosynthesis
1. Photosynthesis is the process that converts light
energy (sunlight) into chemical energy stored in
sugars and other organic molecules
2. Directly or indirectly, photosynthesis nourishes
almost the entire living world
2
B. Two Types of Nutrition
1. Autotrophic
•Producers
•Synthesize organic molecules from inorganic raw
materials (CO2)
•Require an energy source
•Ex: plants, algae, some protists, and some
prokaryotes
•Photoautotrophs—use light energy source
-Ex: plants
•Chemoautotrophs—use oxidation of inorganic subs
(sulfur or ammonia) as energy source
-rare form of autotrophic nutrition found in some
bacteria
3
Examples
of
Autotrophs
4
2. Heterotrophic
• Consumers
• Acquire organic molecules from compounds
produced by other organisms
• Includes decomposers (fungi and bacteria) and
animals that eat plants or other animals
• Depend on photoautotrophs for food and oxygen
5
II. CONCEPT 10.1: CHLOROPLASTS:
SITES OF PHOTOSYNTHESIS
A. Introduction
1. Chloroplasts are structurally similar to and likely
evolved from photosynthetic bacteria
2. The structural organization of these cells allows for the
chemical reactions of photosynthesis
B. Chloroplasts
1. All green plant parts have chloroplasts, but leaves are
the major organs of PS
2. Their green color is from chlorophyll, the green
pigment within chloroplasts
3. Are surrounded by 2 membranes
6
4. Light energy absorbed by chlorophyll drives the
synthesis of organic molecules in the chloroplast
5. CO2 enters and O2 exits the leaf through microscopic
pores called stomata
6. Chloroplasts are found mainly in cells of the mesophyll,
the interior tissue of the leaf
7. A typical mesophyll cell has 30–40 chloroplasts
8. The chlorophyll is in the membranes of thylakoids
(connected sacs in the chloroplast); thylakoids may be
stacked in columns called grana
9. Chloroplasts also contain stroma, a dense fluid
10. Photosynthetic prokaryotes lack chloroplast but have
chlorophyll built into plasma membrane or into the
membranes of vesicles.
7
Leaf cross section
Vein
Mesophyll
Stomata
Chloroplast
CO2
O2
Mesophyll cell
5 µm
9
10
C. Photosynthesis: An Overview
1. Equation of PS
•Reverse of CR
• 6 CO2 + 12 H2O* + light energy
C6H12O6 + 6 O2* + 6H2O
•Simplest photosynthetic equation:
CO2 + H2O
[CH2O] + O2
2. Splitting of Water
•C. B. van Niel hypothesized that plants split H2O as a
source of H+ and release O2 as a by-product.
3. Photosynthesis is a redox process in which H2O is
oxidized and CO2 is reduced
11
Fate of Atoms During
Photosynthesis
12
D. A Preview of Photosynthesis
1. Photosynthesis consists of the light reactions (the photo
part) and Calvin cycle (the synthesis part)
2. The light reactions (in the thylakoids):
• Split H2O
• Release O2
• Reduce NADP+ to NADPH
• Generate ATP from ADP by photophosphorylation
3. The Calvin cycle (in the stroma) forms sugar from CO2,
using ATP and NADPH
4. The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules
13
14
15
16
17
III. Concept 10.2: The light
reactions
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the chemical
energy of ATP and NADPH
A. The Nature of Sunlight
1. Light is a form of electromagnetic energy, also called
electromagnetic radiation
2. Like other electromagnetic energy, light travels in
rhythmic waves
3. Wavelength is the distance between crests of waves
4. Wavelength determines the type of electromagnetic
energy
18
5. The electromagnetic spectrum is the entire range of
electromagnetic energy, or radiation
6. Visible light consists of wavelengths (including those
that drive photosynthesis) that produce colors we can
see
7. Light also behaves as though it consists of discrete
particles, called photons
B. Photosynthetic Pigments: Light Receptors
1. Pigments are substances that absorb visible light
2. Different pigments absorb different wavelengths
3. Wavelengths that are not absorbed are reflected or
transmitted
4. Leaves appear green because chlorophyll reflects
and transmits green light
19
20
5. A spectrophotometer measures a pigment’s ability to
absorb various wavelengths
6. Chlorophyll a is the main photosynthetic pigment and
absorbs best in the red and blue wavelengths, and
least in the green.
7. Accessory pigments, such as chlorophyll b, broaden
the spectrum used for photosynthesis
8. Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll
21
22
C. Excitation of Chlorophyll by Light
1. When a pigment absorbs light, it goes from a ground
state (lowest-energy state) to an excited state (orbit of
higher potential energy), which is unstable
2. When excited electrons fall back to the ground state,
photons are given off, an afterglow called
fluorescence
3. If illuminated, an isolated solution of chlorophyll will
fluoresce, giving off light and heat
23
24
D. Photosystem
1. A photosystem consists of a reaction-center complex
(a type of protein complex) surrounded by lightharvesting complexes
2. The light-harvesting complexes (pigment molecules
bound to proteins) funnel the energy of photons to the
reaction center
3. A primary electron acceptor in the reaction center
accepts an excited electron from chlorophyll a
4. Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
25
26
5. There are two types of photosystems in the thylakoid
membrane
6. Photosystem II (PS II) functions first (the numbers
reflect order of discovery) and is best at absorbing a
wavelength of 680 nm
7. The reaction-center chlorophyll a of PS II is called
P680
8. Photosystem I (PS I) is best at absorbing a
wavelength of 700 nm
9. The reaction-center chlorophyll a of PS I is called
P700
27
E. Linear Electron Flow
1. During the light reactions, there are two possible
routes for electron flow: cyclic and linear
2. Linear electron flow, the primary pathway, involves
both photosystems and produces ATP and NADPH
using light energy
3. A photon hits a pigment and its energy is passed
among pigment molecules until it excites P680
4. An excited electron from P680 is transferred to the
primary electron acceptor
28
29
5. P680+ (P680 that is missing
an electron) is a very strong
oxidizing agent
6. H2O is split by enzymes, and
the electrons are transferred
from the hydrogen atoms to
P680+, thus reducing it to
P680
7. O2 is released as a byproduct of this reaction
30
8. Each electron “falls”
down an electron
transport chain from the
primary electron acceptor
of PS II to PS I
9. Energy released by the
fall drives the creation of
a proton gradient across
the thylakoid membrane
10.Diffusion of H+ (protons)
across the membrane
drives ATP synthesis
31
11. In PS I (like PS II), transferred light energy excites
P700, which loses an electron to an electron acceptor
12. P700+ (P700 that is missing an electron) accepts an
electron passed down from PS II via the electron
transport chain
32
33
13. Each electron “falls” down an electron transport chain
from the primary electron acceptor of PS I to the
protein ferredoxin (Fd)
14. The electrons are then transferred to NADP+ and
reduce it to NADPH
15. The electrons of NADPH are available for the
reactions of the Calvin cycle
34
Linear Electron Flow
35
36
F. Cyclic Electron Flow
1. Cyclic electron flow uses only photosystem I and
produces ATP, but not NADPH
2. Cyclic electron flow generates surplus ATP, satisfying
the higher demand in the Calvin cycle
3. Cyclic electron flow is thought to have evolved before
linear electron flow and may protect cells from lightinduced damage
37
Cyclic Electron Flow
38
39
G. Comparison of Chemiosmosis in Chloroplast and
Mitochondria
1. Ways Similar:
•ETC located in membranes
•ATP synthase in same membrane as ETC
•ATP synthase complexes and some electron carriers
and cytochromes are very similar
2. Ways Different:
•Chloroplasts and mitochondria generate ATP by
chemiosmosis, but use different sources of energy
•Mitochondria transfer chemical energy from food to
ATP; chloroplasts transform light energy into the
chemical energy of ATP
40
•Spatial organization of chemiosmosis differs between
chloroplasts and mitochondria but also shows similarities
•In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis as they
diffuse back into the mitochondrial matrix
•In chloroplasts, protons are pumped into the thylakoid
space and drive ATP synthesis as they diffuse back into
the stroma
41
42
3. ATP and NADPH are produced on the side facing the
stroma, where the Calvin cycle takes place
4. In summary, light reactions generate ATP and
increase the potential energy of electrons by moving
them from H2O to NADPH
43
H. Summary of the Light Reaction
1. Linear Electron Flow produces:
•NADPH—electron donor used to reduce CO2 to
sugar in the Calvin Cycle
•ATP—energy source for the Calvin Cycle
•O2—released as by-product
2. Cyclic Electron Flow produces:
•ATP
44
IV. Concept 10.3 Calvin Cycle
A. The Calvin cycle, like the citric acid cycle, regenerates
its starting material after molecules enter and leave the
cycle
B. The cycle builds sugar from smaller molecules by using
ATP and the reducing power of electrons carried by
NADPH
C. Requires ATP and NADPH from the light reaction
D. Carbon enters the cycle as CO2 and leaves as a sugar
named glyceraldehyde-3-phospate (G3P)
E. For synthesis of 1 G3P, the cycle must take place three
times, fixing 3 molecules of CO2
45
F. Three Phases of Calvin Cycle for each CO2
1. Carbon Fixation
•Each CO2 molecule is attached to a five-carbon
sugar, ribulose bisphosphate (RuBP)
•This reaction is catalyzed by RuBP carboxylase, or
rubisco which is probably the most abundant
protein on Earth and is the most abundant protein in
chloroplasts
•The six-carbon intermediate is unstable and splits in
half to form two molecules of 3-phosphoglycerate for
each CO2
46
2. Reduction
•Couples ATP hydrolysis with reduction (pair of
electrons donated by NADPH) of 3-phosphoglycerate
to glyceraldehyde 3-phosphate (G3P)
•Requires 6 ATP and 6 NADPH
•1 G3P exits the cycle to be used by the plant cell
3. Regeneration of CO2 Acceptor (RuBP)
•5 G3P are used to regenerate 3 RuBP
•Requires 3 ATP
47
G. Summary of the Calvin Cycle
1. To produce one G3P molecule requires:
•9 ATP
•6 NADPH
•3 CO2
2. G3P is the raw material used to synthesize glucose
and other carbohydrates.
3. Two G3P molecules are needed to produce 1 glucose
molecule.
4. It takes 18 ATP, 12 NADPH and 6 CO2 to produce 1
glucose molecule.
48
49
50
51
52
V. Concept 10.4 Alternative
Mechanisms of Carbon Fixation
Dehydration is a problem for plants, sometimes
requiring trade-offs with other metabolic processes,
especially photosynthesis
On hot, dry days, plants close stomata, which
conserves H2O but also limits photosynthesis
The closing of stomata reduces access to CO2 and
causes O2 to build up
These conditions favor a seemingly wasteful process
called photorespiration
53
A. Photorespiration: An Evolutionary Relic?
1.In most plants (C3 plants), initial fixation of CO2, via
rubisco, forms a three-carbon compound
2.In photorespiration, rubisco adds O2 instead of CO2 in
the Calvin cycle
3.Photorespiration consumes O2 and organic fuel and
releases CO2 without producing ATP or sugar
4.Photorespiration may be an evolutionary relic because
rubisco first evolved at a time when the atmosphere
had far less O2 and more CO2
5.Photorespiration limits damaging products of light
reactions that build up in the absence of the Calvin
cycle
54
6.In many plants, photorespiration is a problem because
on a hot, dry day it can drain as much as 50% of the
carbon fixed by the Calvin cycle
7.The two most important photosynthetic adaptations are
C4 photosynthesis and CAM
B. C4 Plants
1.C4 plants minimize the cost of photorespiration by
incorporating CO2 into four-carbon compounds in
mesophyll cells
2.This step requires the enzyme PEP carboxylase
3.PEP carboxylase has a higher affinity for CO2 than
rubisco does; it can fix CO2 even when CO2
concentrations are low
55
4.These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2 that is
then used in the Calvin cycle
5.Adaptive process which enhances carbon fixation
under conditions that favor photorespiration (hot, arid)
6.Occurs in corn, sugarcane, agricultural grasses
56
C4 Leaf Anatomy
57
C. CAM Plants
1.Some plants, including succulents, use crassulacean
acid metabolism (CAM) to fix carbon
2.CAM plants open their stomata at night, incorporating
CO2 into organic acids
3.Stomata close during the day, and CO2 is released
from organic acids and used in the Calvin cycle
58
59
D. The Importance of Photosynthesis: A Review
1.The energy entering chloroplasts as sunlight gets
stored as chemical energy in organic compounds
2.Sugar made in the chloroplasts supplies chemical
energy and carbon skeletons to synthesize the organic
molecules of cells
3.Plants store excess sugar as starch in structures such
as roots, tubers, seeds, and fruits
4.In addition to food production, photosynthesis
produces the O2 in our atmosphere
5.50% for cellular respiration
6.Sucrose—transport form of carbohydrate
60
Overview of Photosynthesis
61
E. Comparison of CR and PS
Cellular Respiration
Photosynthesis
Exergonic redox process
Endergonic redox process
Energy released from
oxidation of sugar
Electrons from sugar’s H+
lose potential energy in ETC
as transported to O2 forming
water
Produces ATP
Energy required to reduce
CO2
Light is energy source that
boost potential energy of eas they move from water to
sugar
Produces sugar
62
You should now be able to:
1. Describe the structure of a chloroplast
2. Describe the relationship between an action spectrum
3.
4.
5.
5.
6.
7.
and an absorption spectrum
Trace the movement of electrons in linear electron flow
Trace the movement of electrons in cyclic electron flow
Describe the similarities and differences between
oxidative phosphorylation in mitochondria and
photophosphorylation in chloroplasts
Describe the role of ATP and NADPH in the Calvin cycle
Describe the major consequences of photorespiration
Describe two important photosynthetic adaptations that
minimize photorespiration
63