1. Photosynthesis: Using Light to Make Food

2. Introduction: Plant Power
Photosynthesis - a process that converts light energy to chemical energy (sugars – which are carbohydrate!)
Plants use water and carbon dioxide to produce a simple sugar and give off oxygen
Photosynthesis nourishes almost the entire living world directly or indirectly. Almost all plants are autotrophs, meaning that they sustain themselves without eating anything derived from other living beings.
Plants produce oxygen, a by-product of photosynthesis, that is used in respiration.
The solar energy used in photosynthesis traveled 150 million kilometers from the sun to Earth to be converted into chemical energy.
You may want to reintroduce the terms producers and consumers within the context of this chapter.
Copyright © 2009 Pearson Education, Inc.

3. Light energy Glucose Oxygen gas Photosynthesis6
CO2
+ 6
H2O
C6H12O6
+ 6 O2
Carbon dioxide
Water
Glucose
Oxygen gas
Photosynthesis

4. Energy needs of life All life needs a constant input of energy
Heterotrophs (animals, fungi)
get their energy from “eating others”
Eat organic molecules
Autotrophs (Plants)
produce their own energy (sugars)
Use energy of sunlight to build
organic molecules
make energy & synthesize sugars through photosynthesis (use light)
consumers
producers

5. Plant structure Obtaining raw materials sunlight CO2 H2O nutrients
leaves = solar collectors (major sites of chlorophyll & photosynthesis)
CO2
In through stomates (stomata) = pores in leaves for gas exchange
H2O
uptake from roots
nutrients
N, P, K, S, Mg, Fe…

6. Plant structure Chloroplasts Thylakoid membrane contains
double membrane
stroma
fluid-filled interior
Thylakoids – membranous sacs
grana – stacks of
thylakoids
Thylakoid membrane contains
chlorophyll molecules
electron transport chain
ATP synthase
H+ gradient built up within thylakoid sac
outer membrane
inner membrane
granum
stroma
thylakoid
A typical mesophyll cell has chloroplasts, each about 2-4 microns by 4-7 microns long.
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma are membranous sacs, the thylakoids.
These have an internal aqueous space, the thylakoid lumen or thylakoid space.
Thylakoids may be stacked into columns called grana.

7. In photosynthesis, electrons gain energy
Light energy captured by chlorophyll pigments provides the boost for the electrons (electrons carry energy)
As a result, light energy is converted to chemical energy, which is stored in the chemical bonds of sugar molecules
The sugar produced in photosynthesis is stored for later use or as raw material for biosynthesis of new plant material.
Teaching Tips
1. 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.
Copyright © 2009 Pearson Education, Inc.

8. 1) the light reactions (in thylakoids) 2) the Calvin cycle (in stroma)
photosynthesis occurs in two metabolic stages
1) the light reactions (in thylakoids)
2) the Calvin cycle (in stroma)
The two metabolic stages are the light reactions and the Calvin cycle.
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.
Teaching Tips
1. 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.
2. Figure 7.5 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure demonstrates 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.
Copyright © 2009 Pearson Education, Inc.

9. 1) the light reactions (in thylakoids)
light energy is converted to chemical energy  ATP and NADPH
Water is split to provide the electrons and H+ ions [as well as the oxygen gas (O2) that is given off]
NADPH (an electron carrier ) and ATP are generated
The two metabolic stages are the light reactions and the Calvin cycle.
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.
Teaching Tips
1. 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.
2. Figure 7.5 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure demonstrates 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.
Copyright © 2009 Pearson Education, Inc.

10. 2) Calvin cycle (in stroma)
cyclic series of reactions
CO2 is incorporated into organic compounds (sugars)
NADPH from the light reactions provides the electrons
ATP from the light reactions provides chemical energy
Final outcome  builds sugars (glucose)
The two metabolic stages are the light reactions and the Calvin cycle.
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.
Teaching Tips
1. 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.
2. Figure 7.5 is an important visual organizer that notes the key structures and functions of the two stages of photosynthesis. This figure demonstrates 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.
Copyright © 2009 Pearson Education, Inc.

11. H2O CO2 Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN CYCLE
(in stroma)
(in thylakoids)
ATP
Electrons
Figure 7.5 An overview of the two stages of photosynthesis that take place in a chloroplast.
Figure 7.5 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.
NADPH O2
Sugar

12. Photosynthesis gets energy by absorbing wavelengths of light
Chlorophylls & other pigments
Light absorbing molecules embedded in thylakoid membrane inside
Chlorophyll
absorbs best in red & blue wavelengths & least in green
accessory pigments also absorb light of different wavelengths to add more energy
chlorophyll b, carotenoids, xanthophylls
Why are plants green?

13. Photosystems capture solar power
Pigments in chloroplasts are responsible for absorbing light energy causing release of electrons
Student Misconceptions and Concerns
1. Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths.
Teaching Tips
1. The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat.
Copyright © 2009 Pearson Education, Inc.

14. The products of the light reactions
NADPH
ATP O2
Teaching Tips
1. The authors develop a mechanical analogy for the energy levels and movement of electrons in the light reaction. Figure 7.8B equates the height of an electron with its energy state. Thus, electrons captured at high levels carry more energy than electrons in lower positions. Although this figure can be very effective, students might need to be carefully led through the analogy to understand precisely what is represented.
Copyright © 2009 Pearson Education, Inc.

15. PART 2: THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
Copyright © 2009 Pearson Education, Inc.

16. ATP and NADPH power sugar synthesis in the Calvin cycle
The Calvin cycle makes sugar within a chloroplast
Requires atmospheric CO2, and the ATP & NADPH generated in the light reactions
Produces  an energy-rich, three-carbon sugar which the plant uses to make glucose
The important enzyme of the Calvin cycle is rubisco
The Calvin cycle is called a “cycle” because the starting material is regenerated as the process occurs.
Student Misconceptions and Concerns
1. As noted in Module 7.5, the terms light reactions and dark reactions can lead students to conclude that each set of reactions occurs at a different time of the day. However, the Calvin cycle in most plants occurs during daylight, when NADPH and ATP from the light reactions are readily available.
Teaching Tips
1. Glucose is not the direct product of the Calvin cycle, as might be expected from the general equation for photosynthesis. Instead, G3P, as noted in the text, is the main product. Clarify the diverse uses of G3P in the production of many important plant molecules for students.
Copyright © 2009 Pearson Education, Inc.

17. Which is used to make glucose
ATP
NADPH
Input
CALVIN
CYCLE
Figure 7.10A An overview of the Calvin cycle.
Output:
G3P
Which is used to make glucose

18. Photosynthesis summary
Light reactions
produced ATP
produced NADPH
consumed H2O
produced O2 as byproduct
Calvin cycle
consumed CO2
Produced sugar
regenerated ADP
regenerated NADP
ADP
NADP

19. Summary of photosynthesis
6CO2
6H2O
C6H12O6
6O2
light
energy  +
Where did the CO2 come from? Air through stomates
Where did the CO2 go? Converted to sugars
Where did the H2O come from? roots
Where did the H2O go? Split to provide electrons and protons and the Oxygen is given off by the plant (O2)
Where did the energy come from? light
What’s the energy used for? Make sugars
What will the glucose be used for? energy
What else is involved…not listed in this equation? ATP and NADPH – made in light reactions and used in the Calvin cycle