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Photosynthesis. Photosynthesis. All cells can break down organic molecules and use the energy that is released to make ATP. Some cells can manufacture organic molecules from inorganic substances using energy from light (photosynthesis) or from inorganic chemicals (chemosynthesis).

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photosynthesis1
Photosynthesis
  • All cells can break down organic molecules and use the energy that is released to make ATP.
  • Some cells can manufacture organic molecules from inorganic substances using energy from light (photosynthesis) or from inorganic chemicals (chemosynthesis).
  • Photosynthesis is the ultimate source of almost all organic molecules used by living organisms. It is also the main source of O2 in the atmosphere.
chloroplasts sites of photosynthesis in plants

Leaf cross section

Vein

Mesophyll

Stomata

O2

CO2

Mesophyll cell

Chloroplast

5 µm

Outer

membrane

Thylakoid

Intermembrane

space

Thylakoid

space

Stroma

Granum

Innermembrane

1 µm

Chloroplasts: Sites of Photosynthesis in Plants
  • Leaves are the major locations of photosynthesis
  • Microscopic pores called stomata allow CO2 to enter the leaf and O2 to exit
  • The leaf’s green color is from chlorophyll, the green pigment within chloroplasts
  • Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast
chloroplasts sites of photosynthesis in plants1

Leaf cross section

Vein

Mesophyll

Stomata

O2

CO2

Mesophyll cell

Chloroplast

5 µm

Outer

membrane

Thylakoid

Intermembrane

space

Thylakoid

space

Stroma

Granum

Innermembrane

1 µm

Chloroplasts: Sites of Photosynthesis in Plants
  • Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
  • A typical mesophyll cell has 30-40 chloroplasts
  • The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
  • Chloroplasts also contain stroma, a dense fluid where carbon fixation reactions occur.
overall reaction
Overall Reaction
  • Photosynthesis:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

  • During photosynthesis, H is removed from H2O (leaving O2 as a waste product), energized by light, and then used to reduce CO2 to form glucose.
the splitting of water

Reactants:

12 H2O

6 CO2

6 H2O

6 O2

C6H12O6

Products:

Figure 10.4

The Splitting of Water
  • Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
photosynthesis2

H2O

CO2

Light

NADP+

ADP

+

P

i

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

Chloroplast

[CH2O]

(sugar)

O2

Photosynthesis
  • Photosynthesis occurs in 2 stages:
  • Light dependent stage or energy capturing reactions
  • Light independent “Dark” stage or carbon fixation reactions (also called the Calvin cycle)
the nature of sunlight
The Nature of Sunlight
  • Light is a form of electromagnetic energy, also called electromagnetic radiation
  • Like other electromagnetic energy, light travels in waves
  • Wavelength ( ) = distance between crests of waves
  • Wavelength determines the type of electromagnetic energy
the electromagnetic spectrum

1 m

(109 nm)

10–3 nm

103 nm

106 nm

10–5 nm

103 m

1 nm

Gamma

rays

Micro-

waves

Radio

waves

X-rays

Infrared

UV

Visible light

650

750 nm

500

550

600

700

450

380

Shorter wavelength

Longer wavelength

Higher energy

Lower energy

The Electromagnetic Spectrum
  • The entire range of electromagnetic energy, or radiation
  • Visible light consists of colors we can see, including wavelengths that drive photosynthesis
visible spectrum
Visible Spectrum
  • The portion of the electromagnetic spectrum that we can see
  • White light contains all  of the visible spectrum
  • Colors are the reflection of specific  within the visible spectrum
  •  not reflected are absorbed
  • Composition of pigments affects their absorption spectrum
photosynthetic pigments the light receptors

Light

Reflected

light

Chloroplast

Absorbed

light

Granum

Transmitted

light

Photosynthetic Pigments: The Light Receptors
  • Pigments are substances that absorb visible light
  • Different pigments absorb different wavelengths
  • Wavelengths that are not absorbed are reflected or transmitted
  • Leaves appear green because chlorophyll reflects and transmits green light
the absorption spectra of 3 pigments in chloroplasts

Chlorophyll a

Chlorophyll b

Carotenoids

Absorption of light by

chloroplast pigments

Wavelength of light (nm)

The Absorption Spectra Of 3 Pigments In Chloroplasts

(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

chlorophyll a

CH3

in chlorophyll a

in chlorophyll b

CHO

CH2

CH3

CH

H

C

C

C

Porphyrin ring:

Light-absorbing

“head” of molecule;

note magnesium

atom at center

C

C

CH3

C

H3C

C

CH2

C

C

N

N

H

C

Mg

C

H

N

C

C

N

H3C

C

C

CH3

C

C

C

C

C

H

H

CH2

H

C

C

O

CH2

O

O

C

O

O

CH3

CH2

Hydrocarbon tail:

interacts with hydrophobic

regions of proteins inside

thylakoid membranes of

chloroplasts: H atoms not

shown

Chlorophyll a
  • Chlorophyll a is the main photosynthetic pigment
  • Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
  • Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
excitation of chlorophyll by light

Excited

state

e–

Heat

Energy of election

Photon

(fluorescence)

Ground

state

Photon

Chlorophyll

molecule

Excitation of Chlorophyll by Light
  • When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
  • When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence
  • If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat
photosystems ps

Thylakoid

Photosystem

STROMA

Photon

Light-harvesting

complexes

Reaction

center

Primary electron

acceptor

e–

Thylakoid membrane

Special

chlorophyll a

molecules

Pigment

molecules

Transfer

of energy

THYLAKOID SPACE

(INTERIOR OF THYLAKOID)

Photosystems (PS)
  • A PS is a collection of pigments and proteins found within the thylakoid membrane that harness the energy of an excited electron to do work
  • Captured energy is transferred between PS molecules until it reaches the chlorophyll amolecule at the reaction center
  • At the reaction center are 2 molecules
    • Chlorophyll a
    • Primary electron acceptor
  • The chlorophyll a is oxidized as the electron is passed to primary electron acceptor which is reduced
photosystems
Photosystems

There are two types of photosystems in the thylakoid membrane

Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

Photosystem I is best at absorbing a wavelength of 700 nm

The two photosystems work together to use light energy to generate ATP and NADPH

[CH2O] (sugar)

O2

Primary acceptor

Primary acceptor

4

Fd

e

Pq

e

e

e

H2O

NADP+

+ 2 H+

Cytochrome

complex

2 H+

NADP+

reductase

+

O2

NADPH

1⁄2

Pc

e

+ H+

P700

5

e

Light

P680

Light

6

6

ATP

Photosystem I (PS I)

Photosystem II (PS II)

photosystems electron flow

e–

ATP

e–

e–

NADPH

e–

e–

e–

Mill

makes

ATP

Photon

e–

Photon

Photosystem II

Photosystem I

Photosystems – Electron Flow
electron flow in photosystems
Electron Flow in Photosystems
  • There two routes for the path of electrons stored in the primary electron acceptors depending on the photosynthetic organism
    • Noncyclic electron flow - Plants, algae, cyanobacteria
    • Cyclic electron flow - Bacteria other than cyanobacteria
  • Both pathways begin with the capturing of photon energy and utilize an electron transport chain with cytochromes for chemiosmosis
noncyclic electron flow

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

O2

[CH2O] (sugar)

Primary

acceptor

e–

H2O

2 H+

+

O2

1/2

e–

e–

Energy of electrons

Light

P680

Photosystem II

(PS II)

Noncyclic Electron Flow
  • Uses both Photosystem II and I
  • Electrons from Photosystem II are removed and replaced by electrons donated from water
  • Synthesizes ATP and NADPH
  • Electron donation converts water into O2 and 2H+
  • Light excites electrons
  • The electrons energize the reaction center as they are passed to the primary acceptor
  • H2O split via enzyme catalysed reaction forming 2H+, 2e-, and O2. Electrons move to fill orbital vacated by removed electron
noncyclic electron flow1

O2

[CH2O] (sugar)

Electron

Transport

chain

Primary

acceptor

Primary

acceptor

Electron transport chain

Fd

e–

Pq

e–

e–

e–

NADP+

H2O

Cytochrome

complex

2 H+

+ 2 H+

NADP+

reductase

+

O2

NADPH

1/2

Pc

e–

+ H+

P700

Energy of electrons

e–

Light

P680

Light

ATP

Photosystem I

(PS I)

Photosystem II

(PS II)

Noncyclic Electron Flow
  • Each excited electron is passed along to an electron transport chain
  • ETC produces ATP through chemiosmotic phosphorylation
  • The electron is now lower in energy and enters photosystem I where it is re-energized utilizing sunlight
  • This e- is then passed to a different electron transport system that includes ferridoxin. The enzyme NADP+ reductase assists in the oxidation of ferridoxin and subsequent reduction of NADP+ to NADPH
cyclic electron flow

Primary

acceptor

Primary

acceptor

Fd

Fd

NADP+

Pq

NADP+

reductase

Cytochrome

complex

NADPH

Pc

Photosystem I

ATP

Photosystem II

Cyclic Electron Flow
  • Uses Photosystem I only
  • Electrons from Photosystem I are recycled
  • Synthesizes ATP only
    • Electron in Photosystem I is excited and transferred to ferredoxin that shuttles the electron to the cytochrome complex.
    • The electron then travels down the electron chain and re-enters photosystem I
photosystems electron flow1

e–

ATP

e–

e–

NADPH

e–

e–

e–

Mill

makes

ATP

Photon

e–

Photon

Photosystem II

Photosystem I

Photosystems – Electron Flow
comparison of chemiosmosis in chloroplasts mitochondria

Mitochondrion

Chloroplast

CHLOROPLAST

STRUCTURE

MITOCHONDRION

STRUCTURE

Diffusion

H+

Thylakoid

space

Intermembrane

space

Electron

transport

chain

Membrane

ATP

synthase

Key

Stroma

Matrix

Higher [H+]

Lower [H+]

ADP +

P

i

ATP

H+

Comparison of Chemiosmosis in Chloroplasts & Mitochondria
  • Both the Mitochondria and Chloroplast generate ATP via a proton motive force resulting from an electrochemical inbalance across a membrane
  • Both utilize an electron transport chain primarily composed of cytochromes to pump H+ across a membrane.
  • Both use a similar ATP synthase complex
  • Source of “fuel” for the process differs
  • Location of the H+ “reservoir” differs
overview
Overview
  • Water is split by photosystem II on the side of the membrane facing the thylakoid space
  • The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase
  • ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

O2

[CH2O] (sugar)

STROMA

(Low H+ concentration)

Cytochrome

complex

Photosystem I

Photosystem II

NADP+

reductase

Light

Light

2 H+

NADP+ + 2H+

Fd

NADPH

+ H+

Pq

Pc

H2O

O2

1/2

THYLAKOID SPACE

(High H+ concentration)

2 H+

+2 H+

To

Calvin

cycle

Thylakoid

membrane

ATP

synthase

STROMA

(Low H+ concentration)

ADP

+

ATP

P

i

H+

the calvin cycle
The Calvin Cycle
  • The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
  • During the carbon fixation reactions (Calvin cycle) energy from ATP and hydrogen from NADPH are used to reduce CO2 and form glucose.
  • Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
the calvin cycle1

H2O

CO2

Input

Light

(Entering one

at a time)

3

NADP+

CO2

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

Phase 1: Carbon fixation

NADPH

Rubisco

O2

[CH2O] (sugar)

3

P

P

Short-lived

intermediate

P

6

3

P

P

3-Phosphoglycerate

Ribulose bisphosphate

(RuBP)

6

ATP

6 ADP

3 ADP

CALVIN

CYCLE

6

P

P

3

ATP

1,3-Bisphosphoglycerate

6

NADPH

Phase 3:

Regeneration of

the CO2 acceptor

(RuBP)

6 NADP+

6

P

i

P

5

G3P

P

6

Glyceraldehyde-3-phosphate

(G3P)

Phase 2:

Reduction

1

P

G3P

(a sugar)

Glucose and

other organic

compounds

Output

The Calvin Cycle
  • The Calvin cycle has three phases:
    • Carbon Fixation – attached to 5C sugar
    • Reduction of CO2, NADPH oxidation
    • Regeneration of the CO2 acceptor (RuBP)
carbon fixation
Carbon Fixation
  • Starts with CO2
  • A molecule of CO2 is converted from its inorganic form to an organic molecule (fixation) through the attachment to a 5C sugar (ribulose bisphosphate or RuBP).
  • Catalysed by the enzyme RuBP carboxylase (Rubisco).
reduction
Reduction
  • The formed 6C sugar immediately cleaves into 3-phosphoglycerate
  • Each 3-phosphoglycerate molecule receives an additional phosphate group forming 1,3-Bisphosphoglycerate (ATP phosphorylation)
  • NADPH is oxidized and the electrons transferred to 1,3-Bisphosphoglycerate cleaving the molecule as it is reduced forming Glyceraldehyde 3-phosphate
regeneration
Regeneration
  • The final phase of the cycle is to regenerate RuBP
  • Glyceraldehyde 3-phosphate is converted to RuBP through a series of reactions that involve the phosphorylation of the molecule by ATP
  • For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2
slide30

Summary

Light reactions

Calvin cycle

H2O

CO2

Light

NADP+

ADP

+

P

i

RuBP

3-Phosphoglycerate

Photosystem II

Electron transport

chain

Photosystem I

ATP

G3P

Starch

(storage)

NADPH

Amino acids

Fatty acids

Chloroplast

O2

Sucrose (export)

the importance of photosynthesis a review
The Importance of Photosynthesis: A Review
  • The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
  • Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
  • In addition to food production, photosynthesis produces the oxygen in our atmosphere
slide32

H2O

CO2

Light

NADP+

ADP

CALVIN CYCLE

LIGHT REACTIONS

ATP

NADPH

Electron

Transport

chain

[CH2O] (sugar)

O2

Primary acceptor

7

Primary acceptor

4

Electron transport chain

Fd

e

Pq

e

8

e

2

e

H2O

NADP+

+ 2 H+

Cytochrome

complex

2 H+

NADP+

reductase

+

O2

NADPH

3

1⁄2

Pc

e

+ H+

5

P700

e

Energy of electrons

Light

P680

Light

1

1

6

6

ATP

Photosystem I (PS I)

Photosystem II (PS II)