GCSE revision

• Click the hyperlink to see the movie

• Photosynthesis.wmv

• You should take notes of the important concepts

• If you do not know all that you see in the movie already, you should go back to the text books of GCSE and revise.

Energy Transfer

But where does this energy come from?

1. Autotrophic Nutrition

2. Heterotrophic Nutrition

Organisms need energy to undertake the various forms of biological work including:

• Movement

• Active transport

• Synthesis of organic compounds

• Cell division

Heterotrophic Nutrition

Heterotrophs are unable to make their own food like autotrophs and rely on the compounds produced by autotrophs

Heterotrophs obtain organic food molecules by consuming other organisms or their by-products

Autotrophic NutritionAutotrophs are ‘Producers’ and are able to synthesise organic molecules from inorganic raw materials obtained from the environment

In particular PHOTOAUTOTROPHS use light as a source of energy to synthesise carbohydrates.

This process is PHOTOSYNTHESIS

Photosynthesis nourishes almost all the living world, either directly or indirectly

ATPAdenosine

Triphosphate Adenine

Ribose

Adenosine

Phosphate groups

ATP acts like a rechargeable

battery

ATP as an energy source

ATP is formed by adding a 3rd phosphate ion (Pi ) to ADP

ATP + H2O ADP + Pi + 30kJ

•This involves the hydrolysis of ATP and releases a large amount of energy

•This energy can be used for other energy-requiring reactions

ATP breaks down to form ADP and a phosphate ion

•This reaction uses a large amount of energy

•This energy comes from chemical reactions like those that occur in photosynthesis and respiration

Why is ATP better than glucose as an energy source?

The breakdown of ATP to ADP is a single step reaction whereas the breakdown of glucose to CO2 and H20 is a complex multi-step process:

Energy is instantly available with ATP

The breakdown of ATP releases small amounts of energy, ideal for fuelling the energy requirements of the body but glucose produces more energy than is required

ATP is a more efficient energy source

PhotosynthesisDefinition:

The conversion of light energy into chemical energy that is stored in glucose or other organic compounds

It involves:

•Trapping of light energy to form ATP

•Fixation of CO2 (ie converting it into organic compounds)

The products can be used directly by the plants or can be transferred to other organisms which feed on the plants or break down dead plant material

Photosynthesis

Light-dependent reactions

Light energy

Water Oxygen

ADP + Pi ATP NADP reduced NADP

Light-independent reactionsCarbon

Dioxide Carbohydrate

Photosynthesis

6CO2 + 6H2O C6H12O6 + 6O2

Used in the light independent reaction

Used in the light dependent reaction

Product of the light dependent reaction

Product of the light independent reaction

Light energy

Leaf organization 3

ChloroplastThe site of photosynthesis

The photograph shown below details chloroplast structureas viewed with a transmission electron microscope

Courtesy of Dr. JulianThorpe – EM & FACS Lab,

Biological Sciences University Of Sussex

A single granumChloroplast molecules areLocated here for the light dependent stage of photosynthesis

Chloroplast envelope visible as two membranes

StromaEnzymes for the light independent stage of

photosynthesis located here

Lipiddroplets

Chloroplast

Photosystems

•The light harvesting units of photosynthesis, Photosystem I and Photosystem II

•They are made of clusters of chlorophyll and other light harvesting pigments

•There are 2 in the chloroplast located in the thylakoid membrane

•Consists of an antenna complex and reaction centre

Light absorption

Diagram of the Antenna system

Eventually one pigment molecule takes on all photon-absorbed energyThis molecule is the reaction center

The absorbed energy can Be passed onto other pigmentmolecules This is the antenna systemThe organization of pigments in a photosynthetic organism Then it releases the energy by

Giving up an electron which can in turn reduce a suitable molecule This is what starts the

“light” reactions

Chlorophyll

• Other pigments contribute to light absorption:– Carotenes (absorb in the blue: appear orange-yellow)– Xanthophylls (absorb in the blue-green: appear reddish)

• These pigments pass their energy to chlorophyll

Light-Dependent Reactions

-makes NADPH + H+ and ATP -involves 2 separate reaction centers or photosystems:

photosystem II and photosystem I

1. Electron transport

2. Photosystem II

2 H2O 4H+ + O2 + 4 electrons

This “splitting” reaction provides constant source of electrons to the antenna system in the presence of light

The electrons are passed from the reaction center to an electron transport chain which sets up a proton gradient and this drives ATP synthesis

3. Photosystem I – receives an electron from the transport chain

Here the electron is passed to an electron carrier and eventually to ferredoxin which gives the electrons to NADP+ reductase to make NADPH + H+

The chlorophyll receives electrons from water

PS II

PS I

Electron carriers

Electron carriers

electrons

electrons

water Oxygen

Hydrogen ions

PhytolysisNADP

reduced NADPATP

ADP + Pi

Light

Light

1

2electrons

34

5

6

7

8

9

Light-dependent reactions

Light-Independent (Dark) Reactions

This is the Calvin Cycle

•Occurs in the chloroplast stroma

Ribulose-1,5- bisphosphate (RuBP) has a new carbon (C from CO2) covalently attached by the enzyme “rubisco”

Phase 2:Reduction phase

-Requires ATP consumption and electrons from NADPH-Converts 3-

phosphoglycerate to glyceraldehyde-3-phosphate

-Some of this G3P goes into making glucose

• Requires ATP consumption and allows the cycle to continue by resupplying the CO2 acceptor

Phase 3: regeneration of RuBP

•Uses the ATP and NADPH + H+ from the light reactions to make carbohydrates

Phase 1: Fixation of CO2

Light-Independent (Dark) Reactions

1

2

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2 x

2 x

x 2

x 2

x 2

x 2