PhotosynthesisChapter 6

Table of Contents

Section 1 The Light Reactions

Section 2 The Calvin Cycle

Section 1 The Light ReactionsChapter 6

Objectives

• Explain why almost all organisms depend on photosynthesis.

• Describe the role of chlorophylls and other pigments in photosynthesis.

• Summarize the main events of the light reactions.

• Explain how ATP is made during the light reactions.

Section 1 The Light ReactionsChapter 6

Obtaining Energy

ATP used as energy source for cellular functions- energy storing molecule (from Ch 3)

- ATP becomes ADP when phosphate group is released (energy released)

• Animals (us) break down food to make ATP • Ex: carbs, fats, proteins

Section 1 The Light ReactionsChapter 6

• Autotrophs (producers) use energy from sunlight to create organic compounds (energy, food)

Ex: plants

• some use chemosythesis: organic compounds (energy, food from inorganic sources)– No sunlight necessar

Ex: archaea bacteria living in harsh environments at the bottom of the ocean

• Heterotrophs (consumers) – must get energy from food, cannot get energy directly from sun or inorganic sources

Ex: humans, dogs, giraffes

Section 1 The Light ReactionsChapter 6

Overview of Photosynthesis

Photosynthesis uses light energy to convert CO2 and water into organic compounds

Equation:

6CO2 + 6H2O sunlight C6H12O6 + 6O2

* Pg 289 photosynthesis and respiration *

Section 1 The Light ReactionsChapter 6

Capturing Light Energy

• The light reactions begin with the absorption of light in chloroplasts

Chloroplast Diagram

**Fig 6-3. pg 114**

Chloroplast Diagram

• Chloroplasts contain pigments• Absorb light from the sun

**Why are leaves green?**

Pigments

Chlorophyll A (red light)

Chlorophyll B (blue light)

Carotenoids (accessory pigments)

*located in thylakoid membrane

Pigments and proteins found in thylakoid membrane are collectively called a photosystem.

Section 1 The Light ReactionsChapter 6

Overview of Photosynthesis• Photosynthesis begins with light dependent reactions

and proceeds to Calvin Cycle (light independent reactions).

Section 1 The Light ReactionsChapter 6

Converting Light Energy To Chemical Energy

• Pigments and proteins found in thylakoid membrane are collectively called a photosystem.

1) Photosystem II absorbs sunlight first and energizes electrons which leave chlorophyll A (oxidized).

2) Electrons then accepted by primary electron acceptor (reduced).

3) Electrons are moved down the electron transport chain which drives the movement of protons (H+) into the thylakoid. (Where do these protons come from?? Pg 117, Fig 6-7)

Section 1 The Light ReactionsChapter 6

Converting Light Energy To Chemical Energy, continued4) Light absorbed in Photosystem 1 (at same time as PS II) and

pass energized electrons down the ETC.

- these electrons replaced by PS II

5) Electrons and protons combine with NADP+ which is reduced to form NADPH (Nicotinamide adenine dinucleotide phosphate)

6) ATP is synthesized through chemiosmosis (DON’T CONFUSE WITH CHEMOSYNTHESIS)

- Proton concentration gradient drives ATP synthase (protein) to combine ADP with phosphate

The Light ReactionsChapter 6

Chemiosmosis

Proton concentration gradient drives synthesis of ATP

The Light ReactionsChapter 6

The Light ReactionsChapter 6

Section 1 The Light ReactionsChapter 6

Summary of Processes in Light Reactions

Section 2 The Calvin CycleChapter 6

Objectives

• Summarize the main events of the Calvin cycle.• Describe what happens to the compounds that are

made in the Calvin cycle.• Distinguish between C3, C4, and CAM plants.

• Summarize how the light reactions and the Calvin cycle work together to create the continuous cycle of photosynthesis.

• Explain how environmental factors influence photosynthesis.

Chapter 6

Carbon Fixation

• The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle.

• In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.

Section 2 The Calvin Cycle

Chapter 6

Carbon Fixation, continued

• The Calvin cycle, which occurs in the stroma of the chloroplast, is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar.

• Most of the three-carbon sugars (G3P) generated in the Calvin cycle are converted to a five-carbon sugar (RuBP) to keep the Calvin cycle operating. But some of the three-carbon sugars leave the Calvin cycle and are used to make organic compounds, in which energy is stored for later use.

Section 2 The Calvin Cycle

Chapter 6 Section 2 The Calvin Cycle

The Calvin Cycle

Chapter 6

Alternative Pathways

• The C4 Pathway

– Some plants that evolved in hot, dry climates fix carbon through the C4 pathway. These plants have their stomata partially closed during the hottest part of the day.

– Certain cells in these plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. These compounds are then transported to other cells, where the Calvin cycle ensues.

Section 2 The Calvin Cycle

Chapter 6

Alternative Pathways, continued

• The CAM Pathway– Some other plants that evolved in hot, dry climates

fix carbon through the CAM pathway. These plants carry out carbon fixation at night and the Calvin cycle during the day to minimize water loss.

Section 2 The Calvin Cycle

Chapter 6 Section 2 The Calvin Cycle

Ongoing Cycle of Photosynthesis