Photosynthesis (Outline)

1. Overview of photosynthesis 2. Producers, consumers, and decomposers of the ecosystem (source

of carbon and energy) 3. Plant structures: organ, tissue, cells, sub-cellular organelle, and

molecules. 4. Visible Light and its wavelengths 5. Overview of the two pathways of photosynthesis Light reaction Substrates, products, cellular components and their location Calvin cycle Substrates, products, cellular components and their location 6. Specifics of the Light reaction and the Calvin Cycle - Light Reaction: The light receptors (Pigments), Photosystems,

Location within chloroplast, Photophosphorylation, Flow of energy 7. Comparison of chemiosmosis in respiration and photosynthesis - The Calvin cycle: Energy molecules, Carbon source, RuBp, G3P 8. Photorespiration Role of Rubisco Challenge to plants on hot dry days Adaptations of C4 and CAM plants

Photosynthesis

Life on the surface of Earth is powered by solar energy that is converted into chemical energy in organic molecules, for use in cellular respiration

Light

energy 6 CO2 6 + H2O

Carbon dioxide Water

C6H12O6 6 + O2

Glucose Oxygen gas

Producers of the ecosystem Autotrophs Produce their own food and sustain themselves without eating other organisms Bacteria Algae Plants

Based on source of energy: - Photoautotrophs solar energy - Chemoautotrophs chemical energy of inorganic

substances Bacteria can uniquely oxidize sulfur and ammonia

Heterotrophs

– consumers and decomposers of the biosphere.

– completely dependent on autotrophs

Leaf Cross Section

Leaf

Mesophyll Cell

Mesophyll

Vein Stoma CO2 O2

Chloroplast

Chloroplast

Grana Stroma

TEM

9,7

50 ×

Stroma

Granum Thylakoid Thylakoid space

Outer membrane Inner membrane Intermembrane space

LM 2

,600

×

In plants, photosynthesis takes place in green leaves Mesophyll Stoma

Necessary components for Photosynthesis 1.Light 2.Chloroplast Thylakoids (stacked as grana) The light Reaction Stroma The Calvin Cycle Chloroplast

Grana Stroma

TEM

9,7

50 ×

Stroma

Granum Thylakoid Thylakoid space

Outer membrane

Inner membrane

Intermembrane space

Light as a source of energy

- Solar radiation received by Earth. - Visible light consists of photons of different

wavelengths making up the colors of the rainbow.

Increasing energy

10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m

Gamma rays

X-rays UV Infrared Micro- waves

Radio waves

Visible light

400 500 600 700 750

650 nm

Wavelength (nm) 380

Light

CO2 H2O Chloroplast

LIGHT REACTIONS

(in thylakoids) CALVIN CYCLE

(in stroma)

NADP+

ADP +

Pi

ATP

NADPH

O2 Sugar

An Overview of Photosynthesis

The Light reactions - Converts light energy to chemical energy (ATP & NADPH) - Produces O2 from breakdown of water

The Calvin cycle - uses ATP & NADPH from light reaction to assemble sugar

molecules from CO2

Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules during the light reaction

Specific details of the Light reactions: The Light Receptors

• Photosynthetic Pigments • Different pigments absorb different

wavelengths • Wavelengths that are not absorbed are

reflected or transmitted • Pigments are present within photosystems

Chloroplast

Light

Reflected light

Absorbed light

Granum

Light-capturing Pigments: Chlorophyll

In chlorophyll an electron from magnesium in the

porphyrin ring that is excited.

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center

Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown

I. Chlorophyll a participates directly in the light

reactions II. Accessory photosynthetic pigments.

Photosynthetic Pigments

1. Chlorophyll b different absorption spectrum funnels the energy from these wavelengths to

chlorophyll a. 2. Carotenoids funnel the energy from other wavelengths to

chlorophyll a participate in photoprotection against excessive light. Chemistry of autumn leaf color http://chemistry.about.com/library/weekly/aa082602a.htm

II. Accessory Pigments

H2O

LIGHT REACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADP P + i CALVIN

CYCLE

[CH2O] (sugar)

Leaf Cross Section

Leaf

Mesophyll

Vein Stoma CO2 O2 Chloroplast

Chloroplast Components • Light reactions: - Structures: Thylakoid membrane

- Photosystems II & I - Photosynthetic pigments - Electron acceptor

- Electron transport chain - ATP Synthase - Electron transport chain - NADP + Reductase

- Energy and chemical factors: light, Water, ADP +Pi, NADP+

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html#

Light-capturing Pigments: Chlorophyll

In chlorophyll an electron from magnesium in the

porphyrin ring that is excited.

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center

Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown

Thylakoid

Photon

Light-harvesting complexes

Photosystem

Reaction center

STROMA

Primary electron acceptor

e–

Transfer of energy

Special chlorophyll a molecules

Pigment molecules

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Thyl

akoi

d m

embr

ane

Photosystem consists of a

reaction center surrounded by light-harvesting

complexes

Two types of photosystems that work together to use light energy to generate ATP and NADPH.

• Photosystem II • Photosystem I

Splitting of water

e–

ATP

Mill makes

ATP

Photosystem II Photosystem I

NADPH e–

e– e–

e–

e–

e–

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html#

STROMA (Low H+ concentration)

Light

Photosystem II Cytochrome

complex

2 H+

Light

Photosystem I NADP+

reductase Fd

Pc Pq

H2O O2 +2 H+

1/2 2 H+

NADP+ + 2H+

+ H+ NADPH

To Calvin cycle

THYLAKOID SPACE (High H+ concentration)

STROMA (Low H+ concentration)

Thylakoid membrane

ATP synthase

ATP

ADP + P H+

i

[CH2O] (sugar) O2

NADPH

ATP

ADP

NADP+

CO2 H2O

LIGHT REACTIONS

CALVIN CYCLE

Light

Light P680

e–

Photosystem II (PS II)

Primary acceptor

Ener

gy o

f ele

ctro

ns

[CH2O] (sugar)

NADPH

ATP

ADP CALVIN CYCLE

LIGHT REACTIONS

NADP+

Light

H2O CO2

O2

e–

e–

+

2 H+

H2O

O2 1/2

Pq

Cytochrome complex

Pc

ATP

P700

e–

Primary acceptor

Photosystem I (PS I)

e–

e–

NADP+ reductase

Fd

NADP+

NADPH + H+

+ 2 H+

Light

Electron Flow during the light reaction

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria generate ATP by chemiosmosis, using different sources of energy

• Mitochondria transfer chemical energy from

food to ATP; chloroplasts transform light energy into the chemical energy of ATP

MITOCHONDRION STRUCTURE

Intermembrane space

Membrane

Electron transport

chain

Mitochondrion Chloroplast

CHLOROPLAST STRUCTURE

Thylakoid space

Stroma

ATP

Matrix

ATP synthase

Key

H+ Diffusion

ADP + P

H+

i

Higher [H+]

Lower [H+]

O2

The Calvin Cycle

• Uses ATP and NADPH • CO2 enters the cycle and leaves as a 3C

sugar, glyceraldehyde-3-phosphate (G3P).

• Regenerates RuBP

The Calvin Cycle

Light

CO2 H2O

Light reactions Calvin cycle

NADP+

RuBP

G3P ATP

Photosystem II Electron transport

chain Photosystem I

O2

Chloroplast

NADPH

ADP

+ P i

3-Phosphoglycerate

Starch (storage)

Amino acids Fatty acids

Sucrose (export)

Leaf Cross Section

Leaf

Mesophyll

Vein Stoma

CO2 O2

Chloroplast

• Terrestrial plants face dehydration. • The stomata close to limit loss of water. • This limits amount of CO2 while O2 production continues • A wasteful process called photorespiration • On a hot dry day photorespiration drains 50% of the carbon

that could be fixed by the Calvin Cycle.

Challenge to plants in hot and dry area

Photorespiration

Rubisco can use both O2 and CO2 as substrates and can catalyze a wasteful reaction known as photorespiration when O2 is the substrate.

O2 and organic fuel are consumed without

producing sugar Photorespiration is an evolutionary relic

• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound

• In C3 plants, a drop in CO2 and rise in O2 when stomata close on hot dry days divert the Calvin cycle to photorespiration

• C3 leaf anatomy

Leaf Cross Section

Leaf

Mesophyll

Vein Stoma

CO2 O2

Chloroplast

Some plants have special adaptations to save water and limit “Photorespiration” by separating CO2 fixation from sugar formation

1. C4 plants (spatial separation) corn, sugar cane 2. CAM plants (temporal separation) succulents (cacti and pineapple)

Photosynthetic cells of C4 plant leaf

Mesophyll cell

Bundle- sheath cell

Vein (vascular tissue)

C4 leaf anatomy

Stoma Bundle- sheath cell

Pyruvate (3 C) CO2

Sugar

Vascular tissue

CALVIN CYCLE

PEP (3 C)

ATP ADP

Malate (4 C)

Oxaloacetate (4 C)

CO2 PEP carboxylase Mesophyll cell

C4 leaf anatomy and the C4 pathway

• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

• These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 for use in the Calvin cycle

C4 Plants

Sugarcane

C4 plant

CALVIN CYCLE

3-C sugar

CO2

4-C compound

CO2 Mesophyll cell

Bundle-sheath cell

• CAM plants open their stomata at night, incorporating CO2 into organic acids.

• Stomata close during the day, and CO2 is released from organic acids are used in the Calvin cycle

CAM Plants

CAM plant

Day

CALVIN CYCLE

3-C sugar

CO2

4-C compound

Night

Pineapple

CO2

Comparing C4 and CAM photosynthesis

Bundle- sheath cell

Mesophyll cell Organic acid

C4 CO2

CO2

CALVIN CYCLE

Sugarcane Pineapple

Organic acids release CO2 to Calvin cycle

CO2 incorporated into four-carbon organic acids (carbon fixation)

Organic acid

CAM CO2

CO2

CALVIN CYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night