CHAPTER 10

PHOTOSYNTHESIS

The human brain, so frail, so perishable, so full of inexhaustible

dreams and hungers, burns by the power of the leaf." Loren Eiseley, The Unexpected Universe

Photosynthesis

An anabolic, endergonic, carbon dioxide

(CO2) requiring process that uses light energy

(photons) and water (H2O) to produce organic

macromolecules (glucose).

6CO2 + 6H2O C6H12O6 + 6O2

glucose

SUN

photons

Autotrophs produce their organic

molecules from CO2 and other inorganic

raw materials

Plants and other autotrophs are the

producers of the biosphere

Photoautotroph

Cyanobacteria Protist- Euglena

Chemoautotroph – Purple

Sulphur Bacteria

Photoautotroph

Heterotrophs live on organic compounds

produced by other organisms.

Green organelle = chloroplasts

Half a million chloroplasts mm2 of leaf

Chlorophyll – is the green pigment

inside the chloroplasts.

Chloroplasts are the sites of

photosynthesis in plants

Elodea

Leaf Structure

2 layers of Mesophyll Cells

Stomata

Cuticle

Vein

Air Spaces

Epidermis

6H2O 6CO2 + C6H12O6 + 6O2

Leaf Structure

2 layers of Mesophyll Cells

Contain Chloroplasts

Stomata (CO2 enters, O2 exits)

Cuticle - prevents water loss

Vein (carries water to leaf

in xylem, and glucose away

from leaf in phloem)

Air Spaces

holds gases

Epidermis

(transparent, so light goes

through; makes cuticle)

6H2O 6CO2 + C6H12O6 + 6O2

SPONGY Mesophyll - glucose made here

Palisade Mesophyll - glucose

made here

Ques: Where will the

leaves be thin - top

layer or in the lower

layers of this forest?

Why?

Ques: Where will the

leaves have more

stomata - tropical rain

forest or desert? Why?

Fig. 10.2

Has Chlorophyll in

Membrane to trap light

energy and ETC + ATP

Synthase to make ATP

ATP is used in

the stroma to link

up C, O, and H to

form glucose by

Calvin’s cycle

THYLAKOID- LIGHT DEPENDENT REACTION

STROMA - LIGHT INDEPENDENT REACTION/CALVIN’S CYCLE

Thylakoid

Granum

Thylakoid Membrane has

Chlorophyll + Reaction

Centers that can absorb

light and “energize”

electrons. These electrons

are passed along an ETC to

make ATP

Thylakoid space involved

in creating a Proton

pH Gradient (Chemiosmosis!)

ADP + Pi ATP

H+ H+

H+ H+ H+

Source of Atoms in Photosynthesis

6CO2 + 12H2O + light -> C6H12O6 + 6O2 + 6H2O

Old Hypothesis:

Step 1: CO2 -> C + O2

Step 2: C + H2O -> CH2O

CO2 + H2O + light energy -> CH2O + O2

–CH2O -general formula for a sugar.

Actual:

Step 1: H2O -> 2H+ + 2e- + 1/2O2

Step 2: CO2 + 2H+ + 2e- -> CH2O

Photosynthesis is a redox reaction.

– It reverses the direction of electron flow in respiration.

Water is split and electrons transferred

with H+ (protons) from water to CO2,

reducing it to sugar.

Fig. 10.3

Photosynthesis – photo = light; synthesis

= making of sugar

The light reactions convert

to chemical NRG(ATP)

2 Parts of Photosynthesis are - The

Light reactions and the Calvin cycle

SOLAR ENERGY

CO2 The Calvin cycle incorporates from

the atmosphere

Calvin’s cycle uses energy from the light

reaction to convert the new carbon to

sugar - C6H12O6

Electron Donor

High Energy Electron

Acceptor

Thylakoids (Grana)

Contain Chlorophyll

Stroma

Photophosphorylation

The thylakoid chlorophyll convert light energy

into the chemical energy of ATP and

NADPH.

The light reactions : a closer look

When light meets matter, it may be

reflected, transmitted, or absorbed.

– Different pigments absorb photons of

different wavelengths.

Fig. 10.6

A spectrophotometer measures the

ability of a pigment to absorb various

wavelengths of light.

Absorption spectrum plots a pigment’s

light absorption vs wavelength

Fig. 10.7

– Chlorophyll a, the dominant pigment,

absorbs best in the red and blue

wavelengths, and least in the green.

Fig. 10.8a

Collectively, these photosynthetic pigments determine an

overall action spectrum - plots changes in

photosynthetic rate as light wavelength is changed

Engleman’s Exeriment: You

are the light of my life!

Fig. 10.8c

High O2 High O2

High O2 High O2

Blue and red light = more photosynthesis in algae because? ;

This means more oxygen in that part of the spectum;

This then implies that more bacteria will be supported in

blue/red areas.

When a molecule absorbs a photon, one

of that molecule’s electrons is elevated to

an orbital with more potential energy.

Chlorophyll is in the thylakoid membrane

In chlorophyll a and b, it is an electron from

magnesium in the porphyrin ring that is

excited by light.

Some pigments, including chlorophyll,

release a photon of light, in a process

called fluorescence, as well as heat.

Fig. 10.10

Chlorophyll is organized along with proteins and

smaller organic molecules into photosystems.

A photosystem acts like a light-gathering

“antenna complex” consisting of a few hundred

chlorophyll a, chlorophyll b, and carotenoid.

There are two types of photosystems.

Photosystem I has a reaction center

chlorophyll, the P700 center, that has an

absorption peak at 700nm.

Photosystem II has a reaction center with a

peak at 680nm.

During the light reactions, there are two

possible routes for electron flow: cyclic and

noncyclic.

Noncyclic electron flow, the predominant

route, produces both ATP and NADPH.

CO2

O2

carbohydrate end product (e.g., sucrose, starch, cellulose)

Light-Independent Reactions

glucose P

ADP + Pi ATP

NADPH NADP+

e–

H+

H+

H+ H+

H+

O

H+

H2O

SUNLIGHT

Overview of Photosynthesis

Stroma

PSII PSI ETC

Carbohydrate making phase

(light independent reaction)

ATP and NADPH making phase

(light dependent reaction)

inside a Thylakoid

ATP Synthase

Noncyclic Electron Flow

- Z scheme

P700

Photosystem I P680

Photosystem II

Primary

Electron

Acceptor

Primary

Electron

Acceptor

ETC

Enzyme

Reaction

H2O

1/2O2 + 2H+

ATP

NADPH

Photon

2e-

2e-

2e-

2e-

2e-

SUN

Photon

Noncyclic

photophosphorylation

Photon

Noncyclic Electron Flow

ADP + ATP

NADP+ + H NADPH

Oxygen comes from the splitting of

H2O, not CO2

H2O 1/2 O2 + 2H+

(Reduced)

P (Reduced

)

(Oxidized)

Fig. 10.12

Chemiosmosis

Proton Pump Powers ATP synthesis.

Located in the thylakoid membranes.

Uses Electron Transport Chain and ATP

synthase (enzyme) to make ATP.

Protons are pumped into thylakoid space

from stroma to form a gradient during

Light Reactions. When they flow back

through ATP synthase, ATP is made

Photophosphorylation: addition of

phosphate to ADP to make ATP using the

energy provided by light

Chloroplast

Granum Thylakoid

Stroma

Outer Membrane

Inner Membrane

Chemiosmosis - proton pumping

H+ H+

ATP Synthase

H+ H+ H+ H+

H+ H+ high H+

concentration

H+ ADP + P ATP

PS II PS I E

T C

low H+

concentration

H+

Thylakoid

SUN (Proton Pumping)

Thylakoid

Space

Stroma

Low pH

High pH

Fig. 10.13

The light reactions use the solar power of

photons absorbed by both

photosystem I and

photosystem II to

provide chemical

energy in the form

of ATP and reducing

power in the form

of the electrons

carried by NADPH.

Cyclic Photophosphorylation

Cyclic Electron Flow

P700

Primary

Electron

Acceptor

e-

e-

e-

e-

ATP

produced

by ETC

Photosystem I

Accessory

Pigments

SUN

Photons

Satisfy the higher demand for ATP in Calvin’s cycle

Fig. 10.14

Fig. 10.16

Noncyclic electron flow pushes

electrons from water, where they

are at low potential energy, to

NADPH, where they have high

potential energy.

– This process also produces ATP.

– Oxygen is a byproduct.

Cyclic electron flow converts light

energy to chemical energy in the

form of ATP.

Calvin Cycle

Carbon Fixation (light independent rxn).

C3 plants (80% of plants on earth).

Occurs in the stroma.

Uses ATP and NADPH from light rxn.

Uses CO2. Fixes 1C per turn. In reality

Calvin’s makes a 3C compound (2 of them

join to form the 6C Glucose)

To produce glucose: it takes 6 turns and

uses 18 ATP and 12 NADPH.

Chloroplast

Granum Thylakoid

Stroma

Outer Membrane

Inner Membrane

Calvin Cycle (C3 fixation)

6CO2

6C-C-C-C-C-C

6C-C-C 6C-C-C

6C-C-C-C-C

12PGA

RuBP

12G3P

(unstable)

6NADPH 6NADPH

6ATP 6ATP

6ATP

C-C-C-C-C-C

Glucose

(6C)

(36C)

(36C)

(36C)

(30C)

(30C)

(6C)

6C-C-C 6C-C-C

C3

glucose

RUBISCO

Calvin Cycle

Remember: C3 = Calvin Cycle

Great under normal, cool, moist

conditions

C3

Glucose

Photorespiration

Occurs on hot, dry, bright days.

Stomates close = more O2 and less CO2

Fixation of O2 instead of CO2 because RUBISCO (first enzyme in Calvin’s cycle) acts as an OXYGENASE = acts different when oxygen concentration rises in a leaf cell.

Light reaction and Calvin’s cycle take place, BUT

Produces 2-C molecules (not glucose) instead of 3-C sugar molecules.

Produces no glucose molecules and uses up ATP - what a waste!

Photorespiration

Because of photorespiration: Plants

have special adaptations to limit the effect

of photorespiration.

1. C4 plants

2. CAM plants

C4 Plants

Hot, moist environments - stomata open .

15% of plants (grasses, corn, sugarcane).

Divides photosynthesis spatially to prevent photorespiration.

Light rxn - mesophyll cells (makes ATP and NADPH and oxygen!).

Calvin cycle - bundle sheath cells - cells surrounding xylem and phloem.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

C4 Plants

Mesophyll Cell

CO2

C-C-C

PEP

C-C-C-C

Malate

ATP

Bundle Sheath Cell

C-C-C

Pyruvic Acid

C-C-C-C

CO2

C3

Malate

Transported

glucose

Vascular

Tissue

PEP

Carboxylase RUBISCO

CAM Plants

Crassulacean Acid Metabolism

Hot, dry environments.

5% of plants (cactus and ice plants).

Stomates closed during day.

Stomates open during the night.

Light rxn - occurs during the day.

Calvin Cycle - occurs when CO2 is present.

CAM Plants

Night (Stomates Open

Bring in CO2)

Day (Stomates Closed)

Vacuole

C-C-C-C

Malate

C-C-C-C

Malate Malate

C-C-C-C CO2

CO2

C3

C-C-C Pyruvic acid

ATP C-C-C

PEP glucose

Questions:

The O2 released during photosynthesis comes from (A) CO2 (B) H2O (C) NADPH (D) RuBP (RuDP) (E) C6H12O6

The carbon that makes up organic molecules in plants is derived directly from (A) combustion of fuels (B) carbon fixed in photosynthesis (C) carbon dioxide produced in respiration (D) carbon in the lithosphere (E) coal mines

Carbohydrate-synthesizing reactions of photosynthesis directly require (A) light (B) products of the light reactions (C) darkness (D) O2 and H2O (E) chlorophyll and CO2

If plants are grown for several days in an atmosphere containing 14CO2 in place of 12CO2, one would expect to find (A) very little radioactivity in the growing leaves (B) large amounts of radioactive water released from the stomates (C) a large increase in 14C in the starch stored in the roots (D) a large decrease in the rate of carbon fixation in the guard cells (E) an increase in the activity of RuBP carboxylase (rubisco) in the photosynthetic cells.

Which of the following is an important difference between light-dependent and light-independent reactions of photosynthesis? (A) The light-dependent reactions occur only during the day; the light-independent reactions occur only during the night. (B) The light-dependent reactions occur in the cytoplasm; the light-independent reactions occur in the chloroplasts. (C) The light-dependent reactions utilize CO2 and H2O; the light -independent reactions produce CO2 and H2O. (D) The light-dependent reactions depend on the presents of both photosystems I and II; the light-independent reactons require only photosystem I. (E) The light-dependent reactions produce ATP and NADPH; the light-independent reactions use energy stored in ATP and NADPH.