1. Photosynthesis: Using Light to Make Food
Chapter 7
Photosynthesis: Using Light to Make Food

3. Biology and Society: Green Energy
Wood has historically been the main fuel used to:
Cook
Warm homes
Provide light at night
Industrialized societies replaced wood with fossil fuels including coal, gas, and oil.
To limit the damaging effects of fossil fuels, researchers are investigating the use of biomass (living material) as efficient and renewable energy sources.
© 2010 Pearson Education, Inc.

4. Figure 7.0
Figure 7.0 Capturing solar energy 4

5. Biology and Society: Green Energy
Fast-growing trees, such as willows:
Can be cut every three years
Do not need to be replanted
Are a renewable energy source
Produce fewer sulfur compounds
Reduce erosion
Provide habitat for wildlife
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6. Biology and Society: Biofuels
There are several types of biofuels.
Bioethanol is a type of alcohol produced by the fermentation of glucose made from starches in crops such as grains, sugar beets, and sugar cane.
Bioethanol may be used
directly as a fuel source in specially designed vehicles or
as a gasoline additive.
Cellulosic ethanol is a type of bioethanol made from cellulose in nonedible plant material such as wood or grass.
Biodiesel is made from plant oils or recycled frying oil
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7. THE BASICS OF PHOTOSYNTHESIS
Is used by plants, some protists, and some bacteria
Transforms light energy into chemical energy
Uses carbon dioxide and water as starting materials
The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules.
Student Misconceptions and Concerns
1. Students may understand the overall chemical relationships between photosynthesis and cellular respiration, but many struggle to understand the use of carbon dioxide in the Calvin cycle. Photosynthesis is much more than gas exchange.
2. Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants.
Teaching Tips
1. When introducing the diverse ways that plants impact our lives, consider challenging your students to come up with a list of products made from plants that they encounter regularly. Perhaps you might only list those encountered in a single day of college life. The list can be surprising and help to build up your “catalog of examples.”
2. Within the living world, there are many examples of adaptations to increase surface area. Some examples are the many folds of the inner mitochondrial membrane, the highly branched surfaces of fish gills and human lungs, and the highly branched system of capillaries in the tissues of our bodies. Consider relating this broad principle seen elsewhere to the extensive folding of the thylakoid membranes.
3. In our world, energy is frequently converted to a usable form in one place and used in another. For example, electricity is generated by power plants, transferred to our homes, and used to run computers, create light, and help us prepare foods. Consider relating this common energy transfer to the two-stage process of photosynthesis.
4. You might wish to discuss the evolution of chloroplasts from photosynthetic prokaryotes if you will not address this subject elsewhere in your course.
5. Students who have not read all of chapter 7 may not realize that glucose is not the direct product of photosynthesis. Although glucose is shown as a product of photosynthesis, a three-carbon sugar is directly produced (G3P). A plant can use G3P to make many types of organic molecules, including glucose. (The authors address the production of G3P under the section “The Calvin Cycle” later in this chapter.)
6. Figure 7.3 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure reminds students where water and sunlight are used in the thylakoid membranes to generate oxygen, ATP, and NADPH. The second step, in the stroma, reveals the use of carbon dioxide, ATP, and NADPH to generate carbohydrates.

8. Photosynthetic Protists Photosynthetic Bacteria
Photosynthetic autotrophs: Producer for most ecosystem
Organisms that use photosynthesis are:
Photosynthetic autotrophs
The producers for most ecosystems
Plants
(mostly on land)
Photosynthetic Protists
(aquatic)
PHOTOSYNTHETIC AUTOTROPHS
Photosynthetic Bacteria
Micrograph of cyanobacteria
Kelp, a large alga
Forest plants LM
Figure 7.1 A diversity of photosynthetic autotrophs

9. Chloroplasts: Sites of Photosynthesis
Chloroplasts are:
The site of photosynthesis
Are chemical factories powered by the sun
Convert solar energy into chemical energy
Found mostly in the interior cells of leaves
Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits.
Photosynthetic
cells
Vein
CO2 O2
Stomata
Leaf cross section
Figure 7.2-1
Figure 7.2 Journey into a leaf (step 1) 9

10. Journey into the chloroplast
Inside chloroplasts are membranous sacs called thylakoids, which are suspended in a thick fluid, called stroma.
Thylakoids are concentrated in stacks called grana.
The green color of chloroplasts is from chlorophyll, a light- absorbing pigment that plays a central role in converting solar energy to chemical energy.
Interior cell LM
Stroma
Granum
Thylakoid
space
Chloroplast
Inner and Outer
membrane
TEM
Figure 7.2 Journey into a leaf (Step 1)

11. The Overall Equation for Photosynthesis
In the overall equation for photosynthesis, notice that:
The reactants of photosynthesis are the waste products of cellular respiration.
Carbon
dioxide
6 O2
6 CO2
6 H2O
C6H12O6
Water
Glucose
Photo-
synthesis
Oxygen gas
Light energy
This simple chemical equation highlights the relationship between photosynthesis and cellular respiration.
The reactants of photosynthesis are the waste products of cellular respiration – PS takes the exhaust of CR and rearrange its atoms to produce food and oxygen.
During respiration, a fall of e- from food molecules to O2 to form water release the energy that mitochondria can use it to make ATP. The opposite occurs in PS. E- are boosted uphill and added to CO2 to produce sugar. This process requires lot of energy which is supplied by the light energy and e- from hydrogen atom of water molecule

12. The Overall Equation for Photosynthesis
In photosynthesis:
Sunlight provides the energy
Electrons are boosted “uphill” and added to carbon dioxide
Sugar is produced
During photosynthesis, water is split into:
Hydrogen
Oxygen
Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar.
Oxygen escapes through stomata into the atmosphere.

13. THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
Photosynthesis occurs in two stages:
The light reactions convert solar energy to chemical energy
The Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide
Light
H2O O2
Chloroplast
reactions
NADPH
ATP
Calvin
cycle
CO2
NADP+
ADP P
Sugar
(C6H12O6)
© 2010 Pearson Education, Inc.
Figure 7.3 A road map for photosynthesis (step 2)
In LR
chlorophyll in thylakoid membrane absorbs solar energy, convert them to chemical energy of ATP and NADPH (e- carrier)
Water is split, provides of source of e- and releases O2 gas
In DR
Uses the products of LR to power the production of sugar from CO2
Enzymes in stroma are used
NADPH provides the high energy e- for the reduction of CO2 to glucose

14. A Photosynthesis Road Map
The initial incorporation of carbon from the atmosphere into organic compounds is called carbon fixation.
This lowers the amount of carbon in the air.
Deforestation reduces the ability of the biosphere to absorb carbon by reducing the amount of photosynthetic plant life.
BioFlix Animation: Photosynthesis
© 2013 Pearson Education, Inc. 14

15. THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
Chloroplasts
- are chemical factories powered by the sun and
- convert sunlight into chemical energy.
Chlorophyll in thylakoid membrane absorbs solar energy, convert them to chemical energy of ATP and NADPH (electron carrier)
Water is split, provides of source of electrons and releases O2 gas
Figure 7.6 Why are leaves green?

16. The Nature of Sunlight
Sunlight is a type of energy called radiation, or electromagnetic energy.
The full range of radiation is called the electromagnetic spectrum.
Visible light
Wavelength (nm)
Radio
waves
500
600
750
700
Wavelength =
580 nm
Micro-
Gamma
rays
Infrared UV
X-rays
10–5 nm
10–3 nm
103 nm
1 nm
106 nm
1 m
103 m
Increasing wavelength
The distance between the crests of two adjacent waves is called a wavelength.
The full range of radiation is called the electromagnetic spectrum.
Electromagnetic energy travels as rhythmic waves, oscillates between electric and magnetic field.
The distance between the crests of 2 adjacent waves is called a wavelength.
The full range of radiation of very short wavelength of gamma rays to very long wavelength of radiowaves is called the electromagnetic spectrum

17. The Process of Science: What Colors of Light Drive Photosynthesis?
Observation: In 1883, German biologist Theodor Engelmann saw that certain bacteria living in water tend to cluster in areas with higher oxygen concentrations.
Question: Could this information determine which wavelengths of light work best for photosynthesis?
Hypothesis: Oxygen-seeking bacteria will congregate near regions of algae performing the most photosynthesis.
© 2010 Pearson Education, Inc.

18. The Process of Science: What Colors of Light Drive Photosynthesis?
Experiment: Engelmann:
Laid a string of freshwater algal cells in a drop of water on a microscope slide
Added oxygen-sensitive bacteria to the drop
Used a prism to create a spectrum of light shining on the slide
Figure 7.5 Investigating how light wavelength affects photosynthesis

19. The Process of Science: What Colors of Light Drive Photosynthesis?
Results: Bacteria:
Mostly congregated around algae exposed to red-orange and blue-violet light
Rarely moved to areas of green light
Conclusion: Chloroplasts absorb light mainly in the blue- violet and red-orange part of the spectrum.
Animation: Light and Pigments
Figure 7.6 Why are leaves green?

20. 20 Light Prism Microscope slide Bacteria Bacteria Number of bacteria
Algal cells
400
500
600
700
Wavelength of light (nm)
Figure 7.6
Figure 7.6 Investigating how light wavelength affects photosynthesis 20

21. Animation: Light and Pigments
Why are leaves green?
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light (detected
by your eye)
Animation: Light and Pigments
Figure 7.5
Figure 7.5 Why are leaves green? 21

22. Chloroplast Pigments Chloroplasts contain several pigments:
Chlorophyll a:
Absorbs mostly blue-violet and red light
Participates directly in the light reactions
Chlorophyll b:
Absorbs mostly blue and orange light
Participates indirectly in the light reactions

23. Chloroplast Pigments Carotenoids:
Absorb mainly blue-green light
Participate indirectly in the light reactions
Absorb and dissipate excessive light energy that might damage chlorophyll
The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids.
Figure 7.7 Photosynthetic pigments
Carotenoids may help protect our eyes from the bright light.
The falling autumn temperature causes a decrease in the chlorophyll level

24. How Photosystems Harvest Light Energy
Light behaves as photons, discrete packets of energy.
Chlorophyll molecules absorb photons.
Electrons in the pigment gain energy.
As the electrons fall back to their ground state, energy is released as heat or light.
Electrons in the pigment gain energy and become excited – has raised from ground state to excited state – looses its excess energy and falls back to ground state by releasing the heat energy.
Some pigments emits light as well as heat energy

25. Excited electrons in pigments
Excited state
The electron
falls to its
ground state.
Absorption of a
photon excites
an electron. e–
Heat
Light
Light
(fluorescence)
Photon
Ground
state
Chlorophyll
molecule
(a) Absorption of a photon
(b) Fluorescence of a glow stick
Figure 7.8
Figure 7.8 Excited electrons in pigments 25

26. Breaking a glass vial starts a chemical reactions that excites electrons of a fluorescent dye
(b) Fluorescence of a glow stick
Figure 7.8 Excited electrons in pigments
The fluorescent light in a glow stick is due to chemical reaction that excites electrons of a fluorescent dye. The excited e- falls back to ground state after releasing the energy in the form of fluorescent light

27. How Photosystems Harvest Light Energy
In the thylakoid membrane, chlorophyll molecules are organized with other molecules into photosystems.
A photosystem is a cluster of a few hundred pigment molecules that function as a light-gathering antenna.
The reaction center of the photosystem consists of chlorophyll a molecules that sit next to another molecule called a primary electron acceptor, which traps the light-excited electron from chlorophyll a.
Another team of molecules built into the thylakoid membrane then uses that trapped energy to make
ATP and
NADPH. 27

28. A photosystem
A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna.
Chloroplast
Cluster of pigment
molecules
Photon
Primary
electron
acceptor
Reaction
center
Electron
transfer e–
Reaction-
center
chlorophyll a
Pigment
molecules
Transfer
of energy
Thylakoid membrane
Photosystem
Figure 7.9
Figure 7.9 A photosystem: light-gathering molecules that focus light energy onto a reaction center 28

29. How the Light Reactions Generate ATP and NADPH
The light reactions are located in the thylakoid membrane.
Two types of photosystems cooperate in the light reactions:
The water-splitting photosystem
The NADPH-producing photosystem
An electron transport chain:
Connects the two photosystems
Releases energy that the chloroplast uses to make ATP

30. Light reactions (Step 1)
Primary
electron
acceptor
Water-splitting
photosystem
Light
H2O
2 H
Reaction-
center
chlorophyll 2e – O2 +
Figure 7.10 Light reactions (Step 1)

31. Light reactions (Step 2)
Energy
to make
ATP
Primary
electron
acceptor 2e –
Electron transport chain
Light
Reaction-
center
chlorophyll
H2O 2e –
Water-splitting
photosystem
2 H + + O2
Figure 7.10 Light reactions (Step 2)
2. Energised e- from PS II pass down the ETC to the NADPH-producing PS.
The chloroplast uses the energy released by this e- fall to make ATP

32. Light reactions (Step 3)
Primary
electron
acceptor
NADP 2e –
Energy
to make
ATP
Primary
electron
acceptor 2e –
NADPH 2e –
Light
Electron transport chain
Light
Reaction-
center
chlorophyll
Reaction-
center
chlorophyll
NADPH-producing
photosystem
H2O 2e –
Water-splitting
photosystem
2 H + + O2
Figure 7.10 Light reactions (Step 3)
3. The NADPH-producing PS transfers its light-excited e- to NADP+ to, reducing it to NADPH

33. How the thylakoid membrane converts light energy to the chemical energy of NADPH and ATP– O2
ATP
NADP
Stroma
Inside thylakoid
Photosystem
Electron transport chain
NADPH
ADP  P H+
synthase
To Calvin cycle H
Electron flow
This figure shows the location of light reaction in the thylakoid membrane

34. In summary, light reactions take place in the thylakoid membranes
Products are
NADPH
ATP
Oxygen.
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35. THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE
Functions like a sugar factory within the stroma of a chloroplast
Regenerates the starting material with each turn
Inputs are CO2 , energy from ATP and high energy electrons from NADPH.
Output G3P as raw material for making glucose and other organic compounds and the starting material to continue the cycle
© 2010 Pearson Education, Inc.
It is called cycle because the starting material is regenerated with each turn of the cycle.
Inputs are CO2 , energy from ATP and high energy e- from NADPH.
Output G3P as raw material for making glucose and other organic compounds and the starting material to continue the cycle

36. Calvin cycle (Step 1) Calvin cycle CO2 (from air) RuBP sugar
Three-carbon molecule
RuBP sugar
CO2 (from air)
An enzyme adds each Co2 to a 5-C sugar called RuBP and the resulting molecule breaks into two 3-C molecules

37. Calvin cycle (Step 2) Calvin cycle CO2 (from air) RuBP sugar
Three-carbon molecule
ATP P P
ADP  P
Calvin
cycle
NADPH
NADP
G3P sugar P
The Calvin cycle (Step 2)
Using energy from ATP and NADPH produced by the LR, enzymes convert each 3-C molecule to the 3-C sugar (G3P)

38. Calvin cycle (Step 3) Calvin cycle CO2 (from air) RuBP sugar
Three-carbon molecule
ATP P P
ADP  P
Calvin
cycle
NADPH
NADP
G3P sugar
G3P sugar P P
G3P sugar
Glucose (and
other organic
compounds) P
Figure
The Calvin cycle (Step 3)
For every 3 molecules of CO2 that enters the cycle, the net output is one G2P sugar. The other G3P sugars continue in the cycle

39. Calvin cycle (Step 4) Calvin cycle CO2 (from air) RuBP sugar
Three-carbon molecule
ATP P P
ADP  P
ADP  P
Calvin
cycle
NADPH
ATP
NADP
G3P sugar
G3P sugar P P
G3P sugar
Glucose (and
other organic
compounds) P
Figure
The Calvin cycle (Step 4)
Using energy from ATP, enzymes rearrange the remaining G3P sugars to regenerate RuBP

40. Summary: Calvin cycle Calvin cycle NADPH ADP P NADP ATP G3P CO2
Glucose and
other compounds
Figure 7.UN7 Summary: Calvin cycle

41. Evolution Connection: Solar-Driven Evolution
Other alternative mode of Photosynthesis
C3 plants:
Use CO2 directly from the air
Are very common and widely distributed
C4 plants:
Close their stomata to save water during hot and dry weather
Can still carry out photosynthesis
CAM plants:
Are adapted to very dry climates
Open their stomata only at night to conserve water
© 2010 Pearson Education, Inc.
So far we learned that
Plants convert solar energy to chemical energy via PS
Plants use certain wavelength of light to drive these reactions
How do plants synthesize food under hot, dry weather? Alternative mode of incorporating carbon from CO2 have evolved in some plants that allows them to save water without shutting down PS

42. Incorporate C from CO2 into a 4-C compounds
C4 plants
Incorporate C from CO2 into a 4-C compounds
2 types of cells are involved – mesophyll cell and bundle sheath cell
Sugar
C4 Pathway
(example: sugarcane)
C4 plant
CAM plant
Calvin
cycle
Day
Cell
type 1
Four-carbon
compound
Night
type 2
CAM Pathway
(example: pineapple)
ALTERNATIVE PHOTOSYNTHETIC PATHWAYS
CO2
CAM plants
Incorporate C from CO2 into a 4-C compounds at night
Release the 4-C compounds to Calvin cycle during day
Figure 7.14 C4 and CAM photosynthesis
C4 plants incorporate C from CO2 into a 4-C compounds even when the leaf CO2 level is low.
2 types of cells – mesophyll cell and bundle sheath cell: mesophyll incorporates C in 4-C compound and send it to BSC

43. Review: Photosynthesis equation
Light
energy
6 CO2
6 H2O
C6H12O6
6 O2
Photo-
synthesis
Carbon
dioxide
Water
Glucose
Oxygen gas
Figure 7.UN1 Photosynthesis equation

44. Review: light reaction
Figure 7.UN2 Orientation diagram: light reactions

45. Review: light reaction
reactions
CO2 O2
H2O
(C6H12O6)
NADPH
Sugar
ATP
ADP P
NADP
Calvin
cycle
Figure 7.UN2 Orientation diagram: light reactions

46. Summary: light reaction
NADPH-producing
photosystem
Water-splitting
photosystem
Figure 7.UN6 Summary: light reactions

47. Summary: light reaction
NADP+ e – 2e –
ADP
ATP
acceptor e – 2e –
acceptor 2e
NADPH –
Photon
Electron transport chain
Photon
Chlorophyll
H2O
Chlorophyll
NADPH-producing
photosystem 2e –
Water-splitting
photosystem
2 H 2 1 + O2 +
Figure 7.UN6 Summary: light reactions

48. Review: Calvin cycle
Figure 7.UN3 Orientation diagram: Calvin cycle

49. Review: Calvin cycle CO2 H2O NADP ADP Calvin Light cycle reactions
(C6H12O6)
NADPH
Sugar
ATP
ADP P
NADP
Calvin
cycle
Figure 7.UN3 Orientation diagram: Calvin cycle