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Chapter 08 Lecture and Animation Outline To run the animations you must be in Slideshow View. Use the buttons on the animation to play, pause, and turn audio/text on or off. Please Note: Once you have used any of the animation functions (such as Play or Pause), you must first click on the slide’s background before you can advance to the next slide.

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8.1 Overview of Photosynthesis • Photosynthesis converts solar energy into chemical energy of carbohydrates • Organisms that carry on photosynthesis are called autotrophs – Plants, algae, and cyanobacteria are organisms capable of photosynthesis

• Heterotrophs are organisms that feed on other organisms

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8.1 Overview of Photosynthesis • Autotrophs and heterotrophs use organic molecules produced by photosynthesis • Pigments allow photosynthetic organisms to capture solar energy • Most photosynthetic organisms contain the pigment chlorophyll • Another common pigment group are carotenoids 3

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Flowering Plants as Photosynthesizers • Photosynthesis occurs in the green parts of plants – Particularly leaves, contain chlorophyll and other pigments

• Leaves contain mesophyll tissue specialized for photosynthesis • Raw materials are water and CO2

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8.1 Overview of Photosynthesis – Water is taken up by roots and transported to leaves by veins – Carbon dioxide enters through openings in the leaves called stomata – Light energy is absorbed by chlorophyll and other pigments in thylakoids of chloroplasts

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8.1 Overview of Photosynthesis • Chloroplast structure – The chloroplast and its fluid-filled interior called stroma are surrounded by a double membrane – Thylakoids are a different membrane system within the stroma that form flattened sacs – Thylakoids are stacked together to from grana – Thylakoid space is formed by a continuous connection between individual thylakoids

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cuticle Leaf cross section upper epidermis

mesophyll

lower epidermis

CO2 O2

Leaf vein

stomata

Figure 8.2

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inner membrane outer membrane stroma

stroma granum

Chloroplast, micrograph

Chloroplast

37,000x

thylakoid space thylakoid membrane

Grana channel between thylakoids © Dr. George Chapman/Visuals Unlimited

Figure 8.2

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cuticle Leaf cross section upper epidermis

mesophyll

lower epidermis

CO2 O2

leaf vein

stomata inner membrane outer membrane

stroma

stroma granum

Chloroplast

Chloroplast, micrograph

37,000x

thylakoid space thylakoid membrane

Figure 8.2

Grana channel between thylakoids © Dr. George Chapman/Visuals Unlimited

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Photosynthetic Reaction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

solar energy CO2 + 6 H2O

pigments

C6H12O6 + 6 O2

• Glucose and oxygen are the products of photosynthesis • The oxygen given off comes from water • CO2 gains hydrogen atoms and becomes a carbohydrate 10

Two Sets of Reactions • Photosynthesis consists of two sets of reactions – Photo refers to capturing solar energy – Synthesis refers to producing a carbohydrate

• The two sets of reactions are called the: – Light Reactions (light-dependent) – Calvin Cycle Reactions (light-independent) • Nicotinamide adenine dinucleotide phosphate (NADP+) links these reactions 11

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H2O

CO2

solar energy

ADP + P NADP+

Light reactions

Calvin cycle reactions NADPH ATP

stroma

thylakoid membrane O2

Figure 8.3

CH2O

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8.2 Plants as Solar Energy Converters • During the light reactions, different pigments within the thylakoid membranes absorb energy • Solar energy can be described in terms of its wavelength and energy content

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8.2 Plants as Solar Energy Converters • The electromagnetic spectrum extends from very short gamma rays to very long radio waves

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Increasing wavelength

Increasing energy

• White or visible light is only a small portion of the spectrum

Gamma X rays rays

UV

Infrared

Microwaves

Radio waves

visible light

• Visible light is further divided into wavelengths between 380 and 750 nm

380

500

600

750

Wavelengths (nm)

Figure 8.4

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Visible Light • Visible light contains various wavelengths • The colors of visible light range from: – Violet light • Shortest wavelength but high energy

– Red light • Longest wavelength but lowest energy – Only about 42% of solar radiation that hits Earth’s atmosphere reaches the surface of Earth – most is in the visible-light range – Higher wavelengths are screened by the ozone layer 15

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Visible Light

• Can absorb specific various portions of visible light • The absorption spectrum shown in figure on the right

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Chlorophyll a Chlorophyll b carotenoids Relative Absorption

• Most photosynthetic pigments in cells are chlorophylls a and b and the carotenoids

380

500

600

750

Wavelengths (nm)

Figure 8.5

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Visible Light • Green light is reflected and only minimally absorbed – Leaves appear green

• Other plant pigments become noticeable in the fall when chlorophyll breaks down and the other pigments are uncovered

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Light Reactions • Light Reactions – Take place in thylakoid membrane – Light reactions consist of two pathways: • Noncyclic electron pathway • Cyclic electron pathway

– Both pathways transform solar energy to chemical energy – Both pathways produce ATP – Only the noncyclic pathway produces NADPH 18

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Noncyclic Electron Pathway • Noncyclic electron pathway, named because electron flow is traced from water to NADP+ – Uses two photosystems (Photosystems I and II) – A photosystem consists of a pigment complex and electron acceptors within the thylakoid membrane – The pigment complex can be described as a “antenna” for gathering solar energy

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Noncyclic Electron Pathway • Noncyclic Electron Pathway begins with photosystem II (PSII) – Pigment complex absorbs solar energy – Energy passes from one pigment to another until it is concentrated in reaction center • Chlorophyll a molecule

– Electrons in the reaction center chlorophyll become so energized • Escape from the reaction center and move to a nearby electron acceptor 20

Noncyclic Electron Pathway • Photosystem II would disintegrate without replacement electrons – Electrons provided by splitting water – Releases oxygen (O2) to atmosphere which benefits all organisms that use O2 – Hydrogen ions (H+) stay in the thylakoid space • Contribute to formation of hydrogen ion gradient

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Noncyclic Electron Pathway • In PSII, an electron acceptor receives energized electrons from the reaction center • It sends those electrons down an electron transport chain, (series of carriers that pass electrons from one to the other) • Energy is released to pump hydrogen ions (H+) into thylakoid space forming gradient • When hydrogen ions flow through ATP synthase it makes ATP 22

Noncyclic Electron Pathway • PSI comes next in noncyclic electron pathway – When the photosystem I complex absorbs solar energy, energized electrons leave reaction center and are captured by a different electron acceptor • Low energy PSII electrons used to replace those lost by PSI

– Electron acceptor in photosystem I passes its electrons to NADP+ and it becomes NADPH 23

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sun

sun

energy level

electron acceptor

electron acceptor

e– e–

e–

e–

NADP+ H+

reaction center pigment complex

pigment complex

Photosystem I e–

Photosystem II CO2

H2O

Figure 8.6

NADPH

e–

e– reaction center

2H+

1 – 2 O2

CH2O

Calvin cycle reactions

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CO2

H2O solar energy

ADP +

sun

P

sun

NADP +

Light reactions

Calv in cycle cycle reactions NADPH ATP

energy level

electron acceptor

electron acceptor

thylakoid membrane O2

CH2O

e– e–

e–

e–

NADP+ H+ NADPH

e–

e– reaction center

reaction center pigment complex

pigment complex

Photosystem I e–

Photosystem II CO2

H2O

Figure 8.6

2H+

CH2O

Calvin cycle reactions

1 – 2 O2

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Cyclic Electron Pathway • Uses only photosystem I (PSI) and begins when PSI complex absorbs solar energy – Energized electrons escape from the reaction center and travel down electron transport chain – Released energy is stored in the form of a H+ gradient, which causes ATP production by ATP synthase

• Spent electrons return to PSI (cyclic) • Pathway only results in ATP production 26

• ATP from cyclic electron transport used in Calvin cycle to make carbohydrates Figure 8.7

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sun

energy level

• Energized electrons leave the photsytem I reaction center and return to photosystem by an electron transport chain

electron acceptor ATP e–

CO2

e–

CH2O

Calvin cycle reactions and other enzymatic reactions

reaction center

Pigment complex Photosystem I

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The Organization of the Thylakoid Membrane • The following molecular complexes are present in the thylakoid Membrane: – PS II • Pigment complex and electron acceptors • Water is split to replace energized electrons • Oxygen (O2) is released

– Electron transport chain • Carries electrons from PS II to PS I • Uses energy to pump H+ from the stroma into thylakoid space 28

The Organization of the Thylakoid Membrane – PS I • Pigment complex and electron acceptors • Adjacent to enzyme that reduces NADP+ to NADPH

– ATP synthase complex • Has a channel for H+ flow • Flow drives ATP synthase to join ADP and P

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photosystem II H+

electron transport chain photosystem I

H+

NADP reductase

Pq e–

e– e–

e–

NADP+

NADPH

e– H+ H+

H2O

2

H+ +

1 2

O2

thylakoid space

ATPsynthase complex

H+

H+

ATP H+ H+ chemiosmosis P + ADP Stroma

Figure 8.8 30

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thylakoid membrane thylakoid thylakoid space granum photosystem II H+

electron transport chain

stroma photosystem I

H+

NADP reductase

Pq e–

– e– e

e–

NADP+

NADPH

e– H+ H+

H2O

2

H+ +

1 2

O2

ATPsynthase complex

H+

H+

ATP

thylakoid space

H+ H+ chemiosmosis P + ADP Stroma

Figure 8.8 31

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H2 O

CO2

solar energy

ADP + P NADP+ Light reactions

Calvin cycle reactions

NADPH ATP

thylakoid membrane thylakoid thylakoid space

thylakoid membrane O2

CH2O

granum photosystem II H+

electron transport chain

stroma photosystem I

H+

NADP reductase

Pq e–

e– e–

e–

NADP+

NADPH

e– H+ H+

H2O

2

H+ +

1 2

O2

ATPsynthase complex

H+

H+

ATP

thylakoid space

H+ H+ chemiosmosis P + ADP Stroma

Figure 8.8 32

ATP Production • ATP Production – Thylakoid space acts as a reservoir for hydrogen ions (H+) • H+ from water being split within thylakoid space • Pumped in by electron transport chain

– More H+ in thylakoid space than stroma • Electrochemical gradient

– H+ can only flow through ATP synthase – Energy powers making ATP by chemiosmosis 33

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8.3 Plants as Carbon Dioxide Fixers • The Calvin Cycle (named after Melvin Calvin) – Series of reactions that use CO2 from the atmosphere to produce carbohydrate – Humans other most other organisms take in O2 and release CO2 – Includes • Carbon dioxide fixation • Carbon dioxide reduction • Ribulose-1,5-bisphosphate (RuBP) regeneration 35

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2

H2 O solar energy

ADP+ P NADP+ Light reactions

Calvin cycle

Metabolites of the Calvin Cycle

NADPH ATP

RuBP stroma

O2

3 CO2

CH2O

intermediate

ribulose-1,5-bisphosphate

3PG

3-phosphoglycerate

BPG

1,3-bisphosphoglycerate

G3P

glyceraldehyde-3-phosphate

3 C6

3 RuBP C5

3ADP + 3

6 3PG C3

CO2 fixation

Calvin cycle

P

CO2 reduction

regeneration of RuBP These ATP molecules were produced by the light reactions.

3 ATP

5 G3P C3

6 ATP

6ADP + 6 P

These ATP and NADPH molecules were produced by the light reactions.

6 BPG C3 6 NADPH 6 G3P C3

net gain of one G3P

Other organic molecules

6 NADP+

x2 Glucose

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Fixation of Carbon Dioxide • Carbon dioxide fixation is the 1st step of the Calvin cycle – CO2 is attached to 5-carbon RuBP molecule • This reaction occurs three times • The result is a 6-carbon molecule that splits into two 3-carbon molecules 3-phoshoglycerate (3PG)

– RuBP Carboxylase is the enzyme that makes this happen • Comparatively slow enzyme so there is a lot of it 37

Reduction of Carbon Dioxide • Reduction of Carbon Dioxide – Each 3PG molecules undergoes reduction to G3P in two steps – Energy and electrons needed for this reaction are supplied by ATP and NADPH (from light reaction)

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Reduction of Carbon Dioxide Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

ATP

ADP +

3PG

BPG

NADPH

P

G3P

NADP+

As 3PG becomes G3P ATP becomes ADP +

P , and NADPH becomes NADP+. 39

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Regeneration of RuBP • Regeneration of RuBP – It takes three turns of the Calvin cycle to allow one G3P to exit – For every three turns of Calvin Cycle, five G3P (3-carbon molecule) used – This re-forms three RuBP (5-carbon molecule) – 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

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Regeneration of RuBP Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5 G3P

3 ATP

3 RuBP

3 ADP + P

As five molecules of G3P become three molecules of RuBP, three molecules of ATP become three molecules of ADP + P .

5 × 3 (carbons in G3P) = 3 × 5 (carbons in RuBP) 41

Importance of the Calvin Cycle • G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules – These molecules meet the plant needs

• The hydrocarbon skeleton of G3P can form: – Fatty acids and glycerol to make plant oil – Glucose phosphate (simple sugar) – Fructose (+ glucose = sucrose) – Starch and cellulose – Amino acids 42

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8.4 Alternate Pathways for Photosynthesis • C3 Photosynthesis – The leaves of C3 plants have a different structure and means of fixing CO2 than C4 plants – C3 plants such as wheat, rice, oats have mesophyll cells of leaves in parallel layers – Bundle sheath cells around the plant veins do not contain chloroplasts – As a result, cells using Calvin cycle exposed to CO2 43

C3 Photosynthesis • RuBP carboxylase binds O2 as well as CO2 – When bound to O2, the enzyme undergoes photorespiration – Wasteful reaction because it uses O2 and releases CO2, decreasing output of Calvin cycle – O2 concentration in leaf rises when weather is hot and dry, because plant keeps stomata closed to conserve water

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CO2

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mesophyll cells

RuBP Calvin cycle 3PG (C3)

bundle sheath cell

vein stoma

G3P mesophyll cell

a. C3 Plant a. CO2 fixation in a C3 plant, tuplip © The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer

Figure 8.11

Figure 8.10

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C4 Photosynthesis • C4 plants, such as sugarcane and corn, the mesophyll cells are arranged in concentric rings around the bundle sheath cells – They also contain chloroplasts – In the mesophyll cells, CO2 is initially fixed into a 4-carbon molecule – The 4-carbon molecule is later broken down into a 3-carbon molecule and CO2 – CO2 enters the Calvin cycle 46

C4 Photosynthesis • C4 Pathway – C4 plants tend to be found in hot, dry climates – In these climates, stomata tend to close to conserve water – Oxygen then builds-up in the leaves – But, RuBP carboxylase is not exposed to this O2 in C4 plants and photorespiration does not occur – Instead, in C4 plants, the CO2 is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces 47

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CO2 mesophyll C4 cell bundle sheath cell

mesophyll cells

CO2

Calvin cycle

bundle sheath cell b. C4 Plant

Figure 8.11

vein stoma

G3P b. CO2 fixation in a C4 plant, corn © Corbis RF

Figure 8.10

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C4 Photosynthesis • When the weather is moderate, C3 plants ordinarily have the advantage. • When the weather is hot and dry, C4 plants have the advantage, and can be expected to predominate. • In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over. 49

CAM Photosynthesis • CAM Pathway – This pathway is prevalent among most succulent plants that grow in deserts, including the cacti. – CAM plants partition carbon fixation according to time. • During the night, CAM plants fix CO2, forming C4 molecules. • The C4 molecules are stored in large vacuoles. • During daylight, C4 molecules release CO2 to the Calvin cycle. 50

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night

CO2

C4

day

CO2

Calvin cycle

G3P c. CO2 fixation in a CAM plant, pineapple Figure 8.10

© S. Alden/PhotoLink/Getty RF

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8.5 Photosynthesis Versus Cellular Respiration • Both plant and animal cells carry out cellular respiration. – Occurs in mitochondria – Breaks glucose down – Utilizes O2 and gives off CO2

• Plant cells photosynthesize, but animal cells do not. – Occurs in chloroplasts – Builds glucose – Utilizes CO2 and gives off O2

• Both processes utilize an electron transport chain and chemiosmosis for ATP production.

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ADP

Thylakoid membrane

ATP

H2O

O2

H2O

CO2

solar energy

ADP + P NADP+

Light reactions

Calvin cycle reactions

NADPH ATP

stroma thylakoid membrane

O2

CH2O

NADP+ CH2O

NADPH CO2

Stroma

Photosynthesis

Figure 8.12

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ADP

Cristae

ATP

O2

NADH+H+

H2O e– NADH+H+

e–

e–

e– e–

NADH+H+ and FADH2

e– Glycolysis glucose pyruvate

Citric acid cycle

Preparatory reaction

Electron transport chain

2 ATP 2 ADP 4 ADP 4 ATP total 2

ATP

2 ADP 2

net gain

Matrix

NAD+ CH2O

ATP

32 ADP 32 ATP or 34 or 34

NADH CO2

Cellular Respiration

Figure 8.12

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ADP

Thylakoid membrane

ADP

ATP

O2

H2O H2O

ATP

O2

Cristae

H2O

CO2

solar energy

NADH+H+

e– NADH+H+

e–

e–

e– e– ADP + P NADP+ Light reactions

NADH + H+ and FADH2

e–

Glycolysis glucose pyruvate

Calvin cycle reactions

Citric acid cycle

Preparatory reaction

Electron transport chain

NADPH

ATP

2 ATP 2 ADP

stroma thylakoid membrane

Stroma

4 ADP 4 ATP total

O2

CH2O

NADPH CO2 Photosynthesis

Figure 8.12

2

NADP+ CH2O

ATP

net gain

Matrix

2 ADP

2

CH2O

32 ADP 32 ATP or 34 or 34

ATP

NAD+

NADH CO2

Cellular Respiration

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