Photosynthesis

Making Glucose

Where does all that glucose come from?

Photosynthesis

Trees From Rocks?

What is Photosynthesis?

CO2

CO2

CO2

CO2

CO2

CO2

radiant energy

Photosynthesisglucose

What is Photosynthesis?In the process of photosynthesis, plants convert radiant energy from the sun into chemical energy in the form of glucose (sugar)

An Overview

Photosynthetic Organisms

cyanobacteria

protists plants

That’s a lot of

photosynthesis

http://www.smhi.se/weather/baws_ext/info/2004/Baltic_algae_2004_en.htm

So is

this!

O2

glucose

CO2 H2OH+H+ EnergyCO2 + H2O + energy glucose + O2

Photosynthesis

glucose + O2 CO2 + H2O + energy CO2 H2O

H+H+ Energy

O2

glucose

Cellular RespirationPhotosynthesis

• Although photosynthesis and cellular respiration appear to simply be reverse processes, they are not!

• They use different mechanisms (enzymes)

Main Similarities

• Use cytochrome complexes to generate a proton gradient in a double membraned organelle

• Use ATP synthase

• Use similar energy carriers eg. NADH vs NADPH

Recall: The Leaf

vascular bundle

palisade mesophyll

spongy mesophyll

epidermis

cuticle

stomate

EpidermisMesophyllVascular Bundles

Where Does Photosynthesis Occur?

O2

CO2H2O

Where Does Photosynthesis Occur?

O2H2OCO2

nucleus

chloroplast

cytosol

cell wall

nucleus

chloroplast

cytosol

cell wall

All photosynthetic reactions occur in the chloroplasts

The Chloroplast

Outer membrane

Inner membrane

Stroma

Thylakoid

Granum

The Chloroplast

Thylakoid (contains chlorophyll)

Stroma

Lamella (connects grana)

Thylakoid Space (Lumen)

Stages of Photosynthesis

• There are two main stages of photosynthesis:

PHOTOSYNTHESIS

Light Reactions Dark Reactions

makesmakes

NADPH ATP

some

glucoseglucose

Stages of Photosynthesis

• There are two main stages of photosynthesis

PHOTOSYNTHESIS

Light Reactions Dark Reactions

makesmakes

NADPH ATP

some

glucoseglucose

DON’T BE FOOLED!!Both light and dark reactions

occur during the day. The “Dark” reactions don’t

REQUIRE light while the “Light” reactions do!

Stages of Photosynthesis

• There are two main stages of photosynthesis:

PHOTOSYNTHESIS

Light Dependent Reactions Light Independent Reactions

makesmakes

NADPH ATP

some

glucoseglucose

Light

• The full range of wavelengths of light emitted from the sun is known as the electromagnetic spectrum

• Visible light is between 400-700nm

Particle And A Wave

• Light travels as an electromagnetic wave of energy

• It is possible to view light as traveling in bundles of energy called photons

• Chlorophyll can absorb photons of particular wavelengths

PhotonPhoton

Photon

Photon

Photon

What Happens to Photons?

Chlorophyll

• Chlorophyll is a pigment– This means it is a chemical that absorbs

certain wavelengths of light

• When chlorophyll absorbs light, its electrons may enter an excited state

Capturing Light

• The most common pigments in plants are chlorophyll a and chlorophyll b

Chlorophyll a

Chlorophyll b

absorbed absorbed

Absorbance:

Capturing Light

• The most common pigments in plants are chlorophyll a and chlorophyll b

Chlorophyll a

Chlorophyll b

GREENWhat do we see?

Reflection:

reflected

Other Pigments

• Though chlorophylls are the most common pigments used in photosynthesis, other pigments are used too in order to absorb at other wavelengths:

egsCarotenoidsPhycobilinsXanthophylls

Other Pigments

Converting Radiant Energy to Chemical Energy

• Chlorophyll in the thylakoid membrane excites its electrons using radiant energy

• These energized electrons can then be transferred to a primary electron acceptor via redox reaction

Light Reactions(Light-Independent Reactions)

• Protein complexes in the thylakoid membrane contain:– Antannae pigments: mostly chlorophyll b– Reaction Center: a molecule of chlorophyll a– A primary electron acceptor

• These complexes are called photosystems

a

PrimaryElectron Acceptor

Light Reactions(Light-Independent Reactions)

• The photosystems are known as– P700 or PSI– P680 or PSII

• The numbers correspond to the wavelength at which their absorption spectrum peaks

Thylakoid

PS I PS IIP700 P680

Light Reactions(Light-dependent Reactions)

1. Photons of light are absorbed by antenna pigments causing them to move from ground state to an excited state.

2. “Excitation” energy is passed along chlorophyll molecules until it reaches the reaction centre.

3. Chlorophyll a in the reaction centre absorbs the energy.

4. The high-energy state of chlorophyll a causes it to emit 2 electrons.

5. The primary electron acceptor takes the electrons from chlorophyll a. this process is called photoexcitation

Thylakoid

PS I PS IIa aa

Electron Accepto

r

Non-Cyclic Electron Pathway

• Photosystem II aquires a supply of electrons by using the sun’s energy to hydrolyze water.

• This is called photolysis.

PS II

High

Low

En

ergy

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H2O

H+H+

O2

H+

H+

H+

H+

Stroma

Thylakoid Lumen

Thylakoid Membrane

Non-Cyclic Electron Pathway

• The reaction center passes electrons through an electron transport system containing a cytochrome complex (cytochrome b6f)

• This complex generates a proton gradient

PS II

High

Low

En

ergy

H+

H+

H+

H+

Thylakoid Membrane

Stroma

Thylakoid Lumen

E T C

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

PS I

O2

ATP

Electrons are re-energized at photosystem I The high energy electrons are involved in a redox

reaction to generate the high energy NADPH molecule

The proton gradient is used to generate ATP through ATP Synthase

Non-Cyclical Electron Pathway

PS II

High

Low

En

ergy

H+

H+

H+

H+

Thylakoid Membrane

Stroma

Thylakoid Lumen

E T C

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+H

+H

+

H+

H+

H+

H+

PS I

ATP

NADP+

NADPH

The End Result

• These end products of the light-dependent reactions can now be used to synthesize glucose

ATPNADPH

Cyclic Electron Pathway

High

Low

En

ergy

H+

H+

H+

H+

Membrane

E T C

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

PS IATP

Simple organisms like bacteria are able to meet their energy demands by using PS I alone to generate ATP

In this way they generate cellular energy without synthesizing glucose.

Cyclic Electron Pathway

High

Low

En

ergy

H+

H+

H+

H+

Membrane

E T C

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

PS IATP

ATP

The Dark Reaction

The Whole Process