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Chapter 6

Photosynthesis

Section 1 Vocabulary Pretest



Autotroph



Photosynthesis



Heterotroph



Light Reactions



Chloroplasts



Thylakoid



Stroma

A.

B.

C.

D.

E.

F.

G.

Cellular organelles where photosynthesis occurs The first stage of photosynthesis An organism that can make its own food An organism that can not make its own food The process of converting energy from the sun into chemical energy of food A membrane system found inside chloroplasts Solution surrounding the thylakoids inside chloroplasts



Granum



Pigment



Chlorophyll



Carotenoid



Photosystem



Primary Electron Acceptor



Electron Transport Chain



Chemiosmosis

H.

I.

J.

K.

L.

M.

N.

O.

Compounds that absorb light A stack of thylakoids Yellow, orange and brown accessory pigments A cluster of pigments that harvest light energy for photosynthesis Green pigment in plants Movement of protons down a gradient to make ATP Movement of electrons from one molecule to another Accepts electrons

Answer Key



Autotroph



Photosynthesis

            

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

make food. They obtain energy from eating food.

—organisms that can not http://image.wistatutor.com/content/environment/food-chain-system.jpeg



Photosynthesis

Photosynthesis

process used by autotrophs to is the

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 http://www.vtaide.com/png/images/photosyn.jpg

Overview

 Photosynthesis is often summarized in the following equation:

6CO 2 Light energy + 6H 2 O C 6 H 12 O 6 + 6O 2

The

Reactants

The

Products

are carbon dioxide and water are glucose and oxygen

The Stages of Photosynthesis

 There are two stages to the process 

Light Reactions ATP

molecule —light energy is converted to chemical energy, which is temporarily stored in and the energy carrier

NADPH



Dark Reactions (Calvin Cycle)

— organic compounds are formed using

CO 2

and the chemical energy stored in

ATP

and

NADPH

http://bioweb.uwlax.edu/bio203/s2009/schroeer_ paul/images/484px Simple_photosynthesis_overview_svg.png

The Light Reactions

  Require

light

to happen Take place in the

chloroplasts

 Chloroplasts contain

pigments

sunlight.



Pigment

that absorb —a compound that absorbs light http://www.quranandscience.com/images/stories/chloroplasts2.jpg

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

membrane sacs (singular)—stacks of thylakoid

Stroma

—solution that surrounds the grana http://www.s cool.co.uk/assets/learn_its/alevel/biolog y/cells-and organelles/organelles/chloroplast-b.gif

  Thylakoids contain the pigments known as chlorophylls.

Chlorophylls

—absorb colors other than green. Therefore, visible. Two types:    

green is reflected

and is

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 1 st .



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

II

Thylakoid Space

 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.

Stroma I I I Thylakoid Space

 Photosystem II replaces its electrons by

splitting water

, using a water-splitting enzyme.

2H 2 O 4H + + 4e + O 2

  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

.

http://www.biojourney.org/modchemiosmosis.jpg

The Calvin Cycle

 Named for

Melvin Calvin

 Most common pathway for

carbon fixation

   

Carbon fixation

—changing

CO 2

compounds

(carbohydrates)

into organic 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

chloroplasts and requires

CO 2

of the

The Calvin Cycle Step by Step

 Step 1:

Create 6 molecules of 3-PGA

 

Three molecules of CO 2 diffuse into the stroma An enzyme combines each CO 2 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 CO 2 3 molecules of RuBP 6 molecules of 3-PGA

 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 +

NADPH and

phosphates

that are released can be used again in the light reactions to make more ATP and

6 molecules of G3P 6- 3PGA 6NADP+ 6NADPH 6 P 6 ATP 6ADP

 Step 3:

Make organic compounds

 One of the

G3P molecules

compounds leaves the Calvin cycle and is used to make organic

(carbohydrates)

in which energy is stored for later use.

glucose 1 molecule of G3P starch

 Step 4:

Convert G3P to RuBP

 The remaining

RuBP G3P

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

3 ADP 3 CO 2 3 ATP 3 RuBP 6- 3PGA 6 ATP glucose 5 G3P 1 G3P 6 G3P 6 NADPH starch 6NADP+ 6 P

http://bioap.wikispaces.com/file/view/Carbon_Fixation.gif/120055293/Carbon_Fixation.gif

6 ADP

 Plant species that

fix carbon

using the

Calvin Cycle only

are known as

C 3 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 trouble using the Calvin Cycle to fix carbon.

in hot, dry climates

have  This is because they must partially close their

stomata

to conserve water.

 This allows less CO excess of O 2 2 to enter and an 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 C

4

Pathway

C 4 plants

include:

corn, sugar cane

and

crab grass

 Cells called mesophyll cells in C 4 plants use an enzyme to fix CO 2 into a

four carbon compound

 This compound travels to other cells where Calvin Cycle

CO 2

can be released and enter the  These plants lose about

½ as much water

as C 3 plants when producing the same amount of carbohydrates.

The CAM Pathway

   

CAM plants

cactuses, pineapples, and jade plants.

include: These plants

open

stomata at

night

their and

close

them during the

day

(opposite of most plants).

CO 2 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