SeCTion 4.2 Overview of Photosynthesis - Weebly

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4.2 Overview of Photosynthesis

Teacher Notes and Answers

SECTION 2

Instant Replay 1. It absorbs energy from visible light. 2. Energy from sunlight is absorbed. Water molecules are broken down and oxygen is released. Energy-carrying molecules, including ATP, transfer energy. Carbon dioxide molecules are used to build sugars. Six-carbon simple sugars are produced. The sugars are often used to build starches and cellulose.

Vocabulary Check 1. chlorophyll and thylakoid 2. photosynthesis 3. light-dependent reactions 4. light-independent reactions

The Big Picture 5. in the chloroplast 6. chlorophyll 7. Because they produce their own source of chemical energy and energy that is used by other organisms that eat producers. 8. Light-dependent reactions: thylakoid membrane, O2 and ATP. Light-independent reactions: stroma, ATP and CO2

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section

4.2

Overview of

Photosynthesis

Key Concept The overall process of photosynthesis produces sugars that

store chemical energy.

Photosynthetic organisms are producers.

Some organisms are called producers because they produce their own sources of chemical energy. Other organisms also use the chemical energy that producers make. Plants are producers. Plants capture energy from sunlight, and store it as chemical energy in the form of sugars. These sugars are made through photosynthesis. Remember from Section 3.2 that chloroplasts are organelles in plant cells where photosynthesis takes place.

Chlorophyll (KLAWR-uh-fihl) is a molecule in chloroplasts that absorbs some of the energy in visible* light. Plants use energy in visible light for photosynthesis. Visible light looks white, but it is made up of different wavelengths, or colors, of light.

Plants have two kinds of chlorophyll, called chlorophyll a and chlorophyll b. These two types of chlorophyll absorb red and blue wavelengths of light. Neither type of chlorophyll absorbs much green light. Plants are green because green wavelengths of light are reflected by chlorophyll.

What is the role of chlorophyll in photosynthesis?

4B, 9B chloroplast

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Photosynthesis in plants occurs in chloroplasts.

Chloroplasts are plant cell organelles. Two main parts of chloroplasts are needed for photosynthesis: ? The grana (singular, granum) are stacks of compartments called

thylakoids (THY-luh-koydz). Thylakoids are shaped like coins, flat and circular. The thylakoid compartments are enclosed by membranes. These membranes contain chlorophyll. ? The stroma is fluid that is all around the grana inside the chloroplast. There are two stages of photosynthesis, which are called the lightdependent reactions and the light-independent reactions. The light-dependent* reactions capture energy from sunlight. These reactions happen in the thylakoids and their membranes. Chlorophyll

* Academic Vocabulary

dependent needs something to function visible able to be seen

Chloroplasts in plant cells contain a lightabsorbing molecule called chlorophyll, not shown here.

Interactive Reader 59

absorbs energy from sunlight. The energy moves along the thylakoid membrane and is transferred to molecules that carry energy, such as ATP. During this process, water (H2O) molecules are broken down, and oxygen (O2) molecules are released.

The light-independent* reactions use the energy from the lightdependent reactions to make sugars. These reactions happen in the stroma. During this process, carbon dioxide (CO2) and energy from the light-dependent reactions are used to build sugars, usually glucose (C6H12O6).

The equation for the whole process of photosynthesis is shown below. You can see that there are many arrows between the reactants--6CO2 and 6H2O--and the products--C6H12O6 and 6O2. Although there is only one arrow, there are many steps in the process.

6CO2 1 6H2O

C6H12O6 1 6O2

Carbon Dioxide Water light, enzymes a sugar oxygen

Overview of Photosynthesis

STAGE 1: Light-Dependent Reactions

1 Energy from sunlight is absorbed. Water molecules are broken down and oxygen is released.

6H2O

chloroplast sunlight

granum (stack of thylakoids)

chloroplast (t he plant cell organelle where photosynthesis takes place)

6O2

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2 thylakoid

Energy-carrying

STAGE 2: Light-Independent Reactions

energy

molecules, including ATP, transfer energy.

6CO2

stroma (fluid outside the thylakoids)

1 six-carbon sugar

3 Carbon dioxide molecules are used to build sugars.

C6H12O6

4 Six-carbon simple sugars are produced. The sugars are often used to build starches and cellulose.

Circle the four main steps of photosynthesis shown on the diagram.

* Academic Vocabulary independent can function without something

60 Holt McDougal Biology

Simple sugars, such as glucose, are not the only carbohydrates that come from photosynthesis. Plants use simple sugars to build starch and cellulose molecules. Another important product of photosynthesis is oxygen. You will read more about that in Sections 4.4 and 4.5.

4.2 Vocabulary Check

photosynthesis chlorophyll thylakoid

light-dependent reactions light-independent reactions

Mark It Up

Go back and highlight each sentence that has a vocabulary word in bold.

1. Which two terms are things that are found in the chloroplast?

2. Which term is a process that is made up of two other terms?

3. Which term is the part of photosynthesis when light energy is captured and transferred to a molecule, such as ATP?

4. Which term is the part of photosynthesis when energy is used to make sugars like glucose?

4.2 The Big Picture

5. Where in the plant cell does photosynthesis take place? 6. What molecule absorbs sunlight for photosynthesis? 7. Why are plants called producers?

8. Complete the chart below.

Reaction

Light-dependent reactions Light-independent reactions

Where does it happen?

What is needed in the reaction?

water (H2O) and sunlight

What is produced by the reaction?

sugar (C6H12O6)

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Interactive Reader 61

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