UNIT A: Cell Biology

Chapter 2: The Molecules of Cells

Chapter 3: Cell Structure and Function

Chapter 4: DNA Structure and Gene Expression

Chapter 5: Metabolism: Energy and Enzymes

Chapter 6: Cellular Respiration

Chapter 7: Photosynthesis: Section 7.3

In this chapter you will learn how certain pigments, like the ones that give leaves their particular colours, trap energy from the Sun and use it for photosynthesis.

UNIT A Chapter 7: Photosynthesis

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Chapter 7: Photosynthesis

Which pigments provide the maximum efficiency for a plant as it conducts photosynthesis?

Why do leaves appear green in the spring and summer and then turn to red or yellow in the fall?

7.3 Plants as Carbon Dioxide FixersThe Calvin cycle reactions use atmospheric CO2

to produce carbohydrate. The cycle is divided into three sections:1.Fixation of carbon dioxide: carbon dioxide fixation involves reaction between CO2 and RuBP (a 5-carbon molecule)

• Three CO2 and three RuBP react to produce three 6-carbon molecules; these split forming six 3-carbon molecules (3-phosphoglycerate (3PG))

• ATP is used• RuBP carboxylase catalyzes the reaction; it

is highly abundant in the cell

UNIT A Chapter 7: Photosynthesis Section 7.3

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Calvin Cycle Reactions2. Reduction of carbon dioxide: 3PG is reduced

to G3P and NADPH is oxidized to NADP+

• Each 3PG undergoes reduction to G3P in two steps

• ATP and NADPH are used

UNIT A Chapter 7: Photosynthesis Section 7.3

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3. Regeneration of RuBP: the starting material of the Calvin cycle (RuBP) is replaced.• ATP is used• It takes three turns of the Calvin cycle

for one G3P to exit. For every three turns, five G3P are used to re-form three RuBP and the cycle continues

UNIT A Chapter 7: Photosynthesis Section 7.3

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Calvin Cycle Reactions

UNIT A Chapter 7: Photosynthesis Section 7.3

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Figure 7.9 The Calvin cycle reactions.

The Calvin Cycle

The Importance of the Calvin Cycle

The product of the Calvin cycle, glyceraldehyde-3-phosphate (G3P) can be converted to many other molecules.•Glucose phosphate is made, which is the molecule most often metabolized for energy •Glucose phosphate combines with fructose to produce sucrose, which is used to transport carbohydrates within plants•Glucose phosphate is used for synthesis of starch (storage form of glucose) and cellulose (in cell walls)•G3P is also used for synthesizing fatty acids, glycerol, and amino acids

UNIT A Chapter 7: Photosynthesis Section 7.3

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UNIT A Section 7.3

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Chapter 7: Photosynthesis

Check Your Progress

1. Describe how carbon dioxide is fixed and then reduced to a carbohydrate.

2. Illustrate why it takes three turns of the Calvin cycle to produce one glucose molecule.

3. Explain why G3P is an important molecule in plant metabolism.

UNIT A Section 7.3

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Chapter 7: Photosynthesis