1. Photosynthesis: Acquiring energy from the sun
Chapter 6

2. An Overview of Photosynthesis
Most of the energy used by almost all living cells ultimately comes from the sun.
Plants, algae, and some bacteria capture the sunlight energy by a process called photosynthesis.
Only about 1% of the available energy in sunlight is captured.
Cross-section of leaf
Cuticle
Epidermis
Mesophyll
Vascular
bundle
Bundle
sheath
Stoma
Vacuole
Nucleus
Cell wall
Chloroplasts
Inner membrane
Outer membrane
Granum
Stroma
Chloroplast
Thylakoid
Mesophyll cell

3. An Overview of Photosynthesis
The leaf cells of plants contain chloroplasts.
The chloroplast contains internal membranes called thylakoids.
The thylakoids are stacked together in columns called grana.
The stroma is a semi liquid substance that surrounds the thylakoids.

4. An Overview of Photosynthesis
The photosystem is the starting point of photosynthesis.
It is a network of pigments in the membrane of the thylakoid.
The primary pigment of a photosystem is chlorophyll.
The pigments act as an antenna to capture energy from sunlight.
Individual chlorophyll pigments pass the captured energy between them.

5. An Overview of Photosynthesis
Photosynthesis takes place in three stages:
Capturing energy from sunlight.
Using the captured energy to produce ATP and NADPH.
Using the ATP and NADPH to make carbohydrates from CO2 in the atmosphere.

6. An Overview of Photosynthesis
The process of photosynthesis is divided into two types of reactions.
Light-dependent reactions take place only in the presence of light and produce ATP and NADPH.
Light-independent reactions do not need light to occur and result in the formation of organic molecules.
More commonly known as the Calvin cycle.

7. An Overview of Photosynthesis
The overall reaction for photosynthesis may be summarized by this simple equation:
6 CO2
carbon
dioxide +
6 H2O
water
Light
energy
C6H12O6
glucose
6 O2
oxygen

8. How Plants Capture Energy from Sunlight
Light is comprised of packets of energy called photons.
Sunlight has photons of varying energy levels.
The possible range of energy levels is represented by an electromagnetic spectrum.
Human eyes only perceive photons of intermediate energy levels.
This range of the spectrum is known as visible light.

9. Increasing energy of photon
Increasing wave length
0.001 nm
1 nm
1,000 nm
10 nm
0.01 cm
1 cm
1 m
100 m
Radioactive
elements
X-ray
machines
Light
bulbs
People
Radar
Microwave
ovens
FM radio
AM radio
Gamma rays X
rays UV
light
Infrared
Microwaves
Radio waves
Visible light
400 nm
430 nm
500 nm
560 nm
600 nm
650 nm
740 nm

10. How Plants Capture Energy from Sunlight
Pigments are molecules that absorb light energy.
The main pigment in plants is chlorophyll.
Chlorophyll absorbs light at the ends of the visible spectrum, mainly blue and red light.

11. How Plants Capture Energy from Sunlight
Plants also contain other pigments, called accessory pigments, that absorb light levels that chlorophyll does not.
These pigments give color to flowers, fruits, and vegetables.
They are present in leaves too, but are masked by chlorophyll until the fall when the chlorophyll is broken down.

12. Why are plants green? 1. All wave lengths except
500- to600-nanometer wave lengths
(green) absorbed by leaf
4. Brain perceives
“green”
2. Green reflected
by leaf
3. Green absorbed
by eye pigment

13. Absorption spectra of chlorophylls and carotenoids
Chlorophyll b
Chlorophyll a
Relative light absorption
400
450
500
Wavelength (nm)
550
600
650
700

14. Organizing Pigments into Photosystems
In plants, the light-dependent reactions occur within a complex of proteins and pigments called a photosystem.

15. Organizing Pigments into Photosystems
Photon
Photosystem e–
Reaction
center
chlorophyll
Electron
donor
Chlorophyll
molecules
acceptor
Light energy is first captured by any one of the chlorophyll pigments.
The energy is passed along to other pigments until it reaches the reaction center chlorophyll molecule.
The reaction center then releases an excited electron, which is then transferred to an electron acceptor.
The excited electron that is lost is then replaced by an electron donor.

16. Organizing Pigments into Photosystems
The light-dependent reactions in plants and algae use two photosystems.
Photosystem II
Captures a photon of light and releases an excited electron to the electron transport system (ETS).
The ETS then produces ATP.

17. Organizing Pigments into Photosystems
Photosystem I
Absorbs another photon of light and releases an excited electron to another ETS.
The ETS produces NADPH.
The electron from photosystem II replaces the electron from the reaction center.
Energy of electrons
Electron transport system
Photon
Photosystem II
NADP+ + H+
Photosystem I
2H2O
4H+ + O2 H+
P700
P680 e– 1 2 3 4 5
Excited
reaction
center
Reaction
Proton gradient
formed for ATP
synthesis
Water-splitting
enzyme
ATP
NADPH

18. How Photosystems Convert Light to Chemical Energy
Plants produce both ATP and NADPH by noncyclic photophosphorylation.
The excited electrons flow through both photosystems and end up in NADPH.
High energy electrons generated by photosystem II are used to make ATP and are then passed along to photosystem I to drive the production of NADPH.

19. The photosynthetic electron transport system
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Stroma
Calvin
cycle
Photon
Photon
Antenna
complex
H+ + NADP+
NADP
Thylakoid
membrane H+
ATP
NADP e– e– e–
Light-dependent
reactions e–
2H2O
Proton
gradient T h y l a k o i d s p a c e H+
Water-splitting
enzyme H+ H+ O2
4 H+ H+
Thylakoid
space H+
Photosystem II
Electron transport system
Photosystem I
Electron transport system

20. How Photosystems Convert Light to Chemical Energy
As a result of the proton pump of the ETS, a large concentration of protons builds up in the thylakoid space.
The thylakoid membrane is impermeable to protons.
Protons can only re-enter the stroma by traveling through a protein channel called ATP synthase.
As the protons follow their concentration gradient and pass through ATP synthase channels, ADP is phosphorylated into ATP.
This process is called chemiosmosis.

21. Chemiosmosis in a chloroplast
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Stroma H+
Calvin
cycle
Photon
ADP
ATP H+
ATP
NADPH
Light-dependent
reactions e– e–
Thylakoid space
2H2O H+ H+ H+ H+
Thylakoid
space O2
4 H+ H+
Photosystem II
Electron transport system
ATP synthase

22. Building New Molecules
Cells use the products of the light-dependent reactions to build organic molecules.
ATP is needed to drive endergonic reactions.
NADPH is needed to provide reducing power in the form of hydrogens.

23. Building New Molecules
The synthesis of new molecules employs the light-independent, or Calvin cycle, reactions.
These reactions are also known as C3 photosynthesis.

24. Building New Molecules
The Calvin cycle reactions occur in 3 stages
1. Carbon fixation
Carbon from CO2 in the air is attached to an organic molecule, RuBP.
2. Making sugars
The carbons are shuffled about through a series of reactions to make sugars.
3. Reforming RuBP
The remaining molecules are used to reform RuBP.

25. 2 P The Calvin cycle begins when a carbon
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P
The Calvin cycle begins when a carbon
atom from a CO2 molecule is added to
a five-carbon molecule (the starting
material). The resulting six-carbon
molecule is unstable and immediately
splits into three-carbon molecules.
(Three “turns” of the cycle are indicated
here with three molecules of CO2
entering the cycle.)
Then, through a series of reactions,
energy from ATP and hydrogens from
NADPH (the products of the light- dependent reactions) are added to the
three-carbon molecules. The now- reduced three-carbon molecules either
combine to make glucose or are used to
make other molecules.
Most of the reduced three-carbon
molecules are used to regenerate the
five-carbon starting material, thus
completing the cycle. 6
3-phosphoglycerate
Glucose 1 5 3
CO2 2
RuBP
(Starting material)
ATP
Glyceraldehyde
3-phosphate
NADPH

26. Building New Molecules
The Calvin cycle must “turn” 6 times in order to form a new glucose molecule.
Only one carbon is added from CO2 per turn.
The Calvin cycle also recycles reactants needed for the light-dependent reactions.
It returns ADP so that it is available for chemiosmosis in photosystem II.
It returns NADP+ back to the ETS of photosystem I.

27. Reactions of the Calvin cycle3
Rubisco
Carbon fixation
3-phosphoglycerate 6 1 5
CO2
6 ADP
1,3-bisphosphoglycerate
6 NADP
Glyceraldehyde 3-phosphate
Glyceraldehyde
3-phosphate
3 ADP
RuBP
Stroma of chloroplast
2 Pi
Making sugars P i
Glucose and
other sugars
3 ATP
6 ATP
Reforming
Thylakoid space
Light-dependent
reactions
ATP
Calvin
cycle
NADPH
6 NADPH
Rubisco is thought to be the most abundant protein on earth

28. Photorespiration: Putting the Brakes on Photosynthesis
Many plants have trouble carrying out C3 photosynthesis when it is hot.
Plants close openings in their leaves, called stomata (singular, stoma), in order to prevent water loss.
The closed stoma also prevent gas exchange.
O2 levels build up inside the leaves while the concentrations of CO2 fall.
The enzyme rubisco fixes oxygen instead of carbon.
This process is called photorespiration and short-circuits the Calvin cycle and photosynthesis.

29. Plant response in hot, arid weather
Leaf
epidermis
Heat
Under hot, arid
conditions, leaves
lose water by
evaporation through
openings in the
leaves called stomata.
H2O
H2O
Stoma
The stomata close
to conserve water
but as a result, O2
builds up inside the
leaves, and CO2
cannot enter the
leaves. This leads
to photorespiration. O2 O2
CO2
CO2

30. Carbon fixation in C4 plants
CO2
Phosphoenol-
pyruvate (PEP)
Oxalo-
acetate
Mesophyll
cell
Pyruvate
Malate
ATP
PPi +
AMP
Pi +
Bundle-
sheath
Calvin
cycle
Glucose
Some plants have adapted to hot climates by performing C4 photosynthesis.
These C4 plants include sugarcane, corn, and many grasses.
They fix carbon using different types of cells and reactions than C3 plants and do not run out of CO2 even in hot weather.
CO2 becomes trapped in cells called bundle-sheath cells.

31. Photorespiration: Putting the Brakes on Photosynthesis
Another strategy to avoid a reduction in photosynthesis in hot weather occurs in many succulent (water-storing) plants, such as cacti and pineapples.
These plants undergo crassulacean acid metabolism (CAM).
Photosynthesis occurs via the C4 pathway at night and the C3 pathway during the day.

32. Comparing carbon fixation in C4 and CAM plants
C4 plants
Glucose
CO2
CAM plants
Night
Day C4
pathway
Calvin
cycle
Mesophyll
cell
Bundle-
sheath