Chapter 6Photosynthesis

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Section 1 Vocabulary Pretest Autotroph Photosynthesis Heterotroph Light Reactions Chloroplasts Thylakoid Stroma

A. Cellular organelles where photosynthesis occurs

B. The first stage of photosynthesisC. An organism that can make its

own foodD. An organism that can not make

its own foodE. The process of converting

energy from the sun into chemical energy of food

F. A membrane system found inside chloroplasts

G. Solution surrounding the thylakoids inside chloroplasts

Granum Pigment Chlorophyll Carotenoid Photosystem Primary

Electron Acceptor

Electron Transport Chain

Chemiosmosis

H. Compounds that absorb lightI. A stack of thylakoidsJ. Yellow, orange and brown

accessory pigmentsK. A cluster of pigments that

harvest light energy for photosynthesis

L. Green pigment in plantsM. Movement of protons down a

gradient to make ATPN. Movement of electrons from

one molecule to another O. Accepts electrons

Answer Key Autotroph C Photosynthesis E Heterotroph D Light Reactions B Chloroplasts A Thylakoid F Stroma G Granum I Pigment H Chlorophyll L Carotenoid J Photosystem K Primary Electron Acceptor O Electron Transport Chain N Chemiosmosis M

Obtaining Energy The sun is the direct or indirect source

of energy for most living things. Autotrophs —organisms that can make

their own food Heterotrophs —organisms that can not

make food. They obtain energy from eating food.

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Photosynthesis Photosynthesis is the

process used by autotrophs to convert light energy from sunlight into chemical energy in the form of organic compounds. Involves a complex series

of chemical reactions known as a biochemical pathway. Product of one reaction is

consumed in the next reaction

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Overview Photosynthesis is often summarized in

the following equation: 6CO2 + 6H2O C6H12O6 + 6O2

The Reactants are carbon dioxide and waterThe Products are glucose and oxygen

Light energy

The Stages of Photosynthesis There are two stages to the

process Light Reactions —light

energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH

Dark Reactions (Calvin Cycle)—organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH http://bioweb.uwlax.edu/bio203/s2009/

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The Light Reactions Require light to happen Take place in the chloroplasts Chloroplasts contain pigments that absorb

sunlight. Pigment —a compound that absorbs light

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The Structure of a Chloroplast Surrounded by an outer and inner membrane Thylakoids —membrane system arranged as

flattened sacs. (from the Greek meaning “pocket”) Grana (pl.) Granum (singular)—stacks of

thylakoid membrane sacs Stroma —solution that surrounds the grana

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Thylakoids contain the pigments known as chlorophylls.

Chlorophylls —absorb colors other than green. Therefore, green is reflected and is visible. Two types: Chlorophyll a and Chlorophyll b Chlorophyll a —directly involved in the light reactions Chlorophyll b —accessory pigment that assists in

photosynthesis Carotenoids —accessory pigments responsible for fall

colors and also assist in photosynthesis

Converting Light Energy to Chemical Energy Chlorophylls and carotenoids are

grouped in clusters embedded in proteins in the thylakoid membrane.

These clusters are called photosystems

Two photosystems exist, each with its own job to do: Photosystem I and Photosystem II Plants have both photosystems.

Prokaryotic autotrophs only have photosystem II. It is only numbered as II because it was the second one discovered. However, it probably evolved 1st.

How does it work? Light is absorbed by accessory pigments in

photosystem II. When the absorbed energy from the light reaches

the chlorophyll a molecules of photosystem II, it “excites” electrons to a higher energy level.

These excited electrons will leave the chlorophyll a molecule (this is an oxidation reaction)

The electrons are accepted by the primary electron acceptor (this is a reduction reaction) and begin to move from molecule to molecule down an “electron transport chain”

The energy they lose as they are transported is used to pump H+ ions from the stroma into the thylakoid space, creating a concentration gradient.

Stroma

Thylakoid Space

II

At the same time that light is absorbed by photosystem II, it is also being absorbed by photosystem I, again, exciting electrons.

These electrons move down a different electron transport chain and are added to NADP+ to form NADPH.

The lost electrons from photosystem I are replaced by the electrons moving down the transport chain from photosystem II.

II

I

Stroma

Thylakoid Space

Photosystem II replaces its electrons by splitting water, using a water-splitting enzyme.

2H2O 4H+ + 4e- + O2

For every two molecules of water that are split, four electrons become available to replace those lost by the chlorophyll molecules in photosystem II.

The hydrogen ions and the oxygen molecules are released into the thylakoid space. This is where the oxygen gas given off by photosynthesis comes from.

II

I

The build up of H+ in the thylakoid space stores potential energy. This energy is harvested by an enzyme called ATP synthase.

As H+ ions diffuse through ATP synthase down their concentration gradient, the enzyme uses the energy of the moving ions to make ATP.

This is done by adding a phosphate group to ADP in a process called chemiosmosis.

ATP will then be used in the second stage of photosynthesis called the Calvin Cycle.

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The Calvin Cycle Named for Melvin Calvin Most common pathway for carbon fixation

Carbon fixation —changing CO2 into organic compounds (carbohydrates)

It is the second set of reactions in photosynthesis and does not require light.

It uses the energy that was stored in ATP and NADPH during the light reactions to produce organic compounds in the form of sugars.

The Calvin Cycle occurs in the stroma of the chloroplasts and requires CO2

The Calvin Cycle Step by Step Step 1: Create 6 molecules of 3-PGA

Three molecules of CO2 diffuse into the stroma An enzyme combines each CO2 molecule with a 5-

carbon molecule called RuBP (ribulose bisphosphate) to make 3 very unstable 6-carbon molecules. Each immediately breaks down into two 3-carbon molecules called 3-PGA (3-phosphoglycerate). This results in 6 molecules of 3-PGA.

3 molecules of CO2

6 molecules of 3-PGA

3 molecules of RuBP

Step 2: Convert 3-PGA to G3P Each of the 6 molecules of 3-PGA

is converted into a molecule of G3P (glyceraldehyde 3-phosphate)

This is a two-step process First: 6 ATP molecules (from the

light reactions) donate a phosphate group to the 3-PGA. (Changing ATP to ADP)

Second: 6 NADPH molecules (from the light reactions) donate a H+ (Changing NADPH to NADP+) and the phosphate group is released.

The result is 6 molecules of G3P. The ADP, NADP+ and phosphates that are released can be used again in the light reactions to make more ATP and NADPH

6 ATP

6ADP

6- 3PGA

6 molecules of G3P

6NADPH

6NADP+ 6 P

Step 3: Make organic compounds One of the G3P molecules leaves the

Calvin cycle and is used to make organic compounds (carbohydrates) in which energy is stored for later use.

1 molecule of G3P

starch

glucose

Step 4: Convert G3P to RuBP The remaining G3P molecules are converted back into

RuBP by adding phosphate groups from ATP molecules. The RuBP is used again in the cycle.

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3 CO2

6- 3PGA 6 ATP

6 ADP

6 NADPH

6NADP+

6 G3P1 G3P

starch

glucose

3 ATP

3 ADP

3 RuBP

5 G3P

6 P

Plant species that fix carbon using the Calvin Cycle only are known as C3 plants because of the three-carbon compound that is initially formed in the process. They include most plants.

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Alternative Pathways Plants living in hot, dry climates

have trouble using the Calvin Cycle to fix carbon. This is because they must partially

close their stomata to conserve water.

This allows less CO2 to enter and an excess of O2 to build up, both of which inhibit the Calvin Cycle

Two alternate pathways have evolved for these plants—both allow the plants to conserve water.

They are the C4 pathway and the CAM pathway

The C4 Pathway C4 plants include: corn,

sugar cane and crab grass

Cells called mesophyll cells in C4 plants use an enzyme to fix CO2 into a four carbon compound

This compound travels to other cells where CO2 can be released and enter the Calvin Cycle

These plants lose about ½ as much water as C3 plants when producing the same amount of carbohydrates.

The CAM Pathway CAM plants include:

cactuses, pineapples, and jade plants.

These plants open their stomata at night and close them during the day (opposite of most plants).

CO2 absorbed at night can enter the Calvin Cycle during the day, allowing the stomata to stay closed and conserve water.

These plants lose less water than any other plants