Photosynthesis

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Introduction Almost all the energy transferred to all the ATP

molecules in living organisms originally comes from the energy in sunlight

Introduction Green plants, some protoctista and some bacteria are

able to transfer sunlight energy into energy trapped in the molecular structure of carbohydrates.

Introduction This is the process called photosynthesis. Once carbohydrates such as glucose have been made,

plants can convert some of them to other organic substances such as oils, nucleic acids and proteins

Introduction Animals cannot make organic molecules from inorganic

one and so rely entirely on plants for their supply of organic molecules.

Photosynthesis – a summary

Photosynthesis can be summarised by the equation:

nCO2 + nH2O (CH2O)n + nO2

This shows that photoautotrophs synthesise carbohydrate using carbon dioxide, water and light energy.

light

carbohydrate

Q. Is photosynthesis a reduction or an oxidation?A. CO2 is reduced; water is oxidised.

This simple summary hides the fact that photosynthesisis a series of reactions controlled by specific enzymes.

Stages of photosynthesis

The light dependent stage (LDS).

In these reactions, ATP and a reduced coenzyme (NADPH) are made.

Oxygen is a waste product of this

stage.

The light independent stage (LIS).

In these reactions, the products of the light dependent reactions are used to reduce carbon dioxide to carbohydrate.

The reactions of photosynthesis can be divided into two distinct stages.

An overview

water (H2O)light energyADPPioxidised NADP

light-dependent stage

light-independent stage

oxygen (O2) carbon dioxide (CO2)

carbohydratesADPinorganic phosphateoxidised NADP

ATPreduced NADP

Note: the light-independent stage is also known as the Calvin cycle.

The chloroplast

outer membraneinner membranechloroplast envelope

Stroma. The site of the Calvin cycle

Small circular DNA coding for some

chloroplast proteins

Lamella. A pair of membranes containing chlorophyll

Thylakoid. A membranous sac

Granum. A stack of

thylakoid membranes

Starch grain. Produced from sugars made in photosynthesis

Lipid droplet. Made from the sugars made in photosynthesis

Thylakoid space. Space between lamellae.

Ribosomes. Smaller than cytoplasmic ribosomes.

Structure to function: chloroplast

Internal compartmentalisation. The two stages of photosynthesis are effectively separated, thus allowing rate-determining factors such as pH and enzyme concentrations to be optimized

DNA and ribosomes mean chloroplast can code for and produce its own proteins such as RuBPC

Double membrane provides control of substances entering/leaving the organelle

Thylakoid membranes provide a large surface area for light absorption

Trapping light energy Light energy is trapped by photosynthetic pigments. Different pigments absorb different wavelengths of

light. The photosynthetic pigments of higher plants form

two groups: the chlorophylls and the carotenoids.

Chlorophylls absorb mainly in the red and blue-violet regions of the light spectrum. They reflect green light which is why plants look green.

The carotenoids absorb mainly in the blue-violet region of the spectrum.

Pigment Colour

Chlorophylls: chlorophyll a Yellow-green

chlorophyll b Blue-green

Carotenoids: ß carotene Orange

xanthophyll Yellow

carotenoid

chlorophyll a

Thylakoid membranesthe possible arrangement of chlorophyll and associated molecules within the thylakoid membranes based on studies of isolated grana

electron carriers

chlorophyll combined with protein

stalked particles containing the enzymes for catalysing the synthesis of ATP

Trapping light energy The photosynthetic pigments fall into two categories:

primary pigments and accessory pigments The primary pigments are two forms of chlorophyll a with

slightly different absorption peaks. The accessory pigments include other forms of

chlorophyll a, chlorophyll b and the carotenoids. The pigments are arranged in light-harvesting clusters called photosystems.

In a photosystem, several hundred accessory pigment molecules surround a primary pigment molecule and the energy of the light absorbed by the different pigments is passed to the primary pigment.

The primary pigments are said to act as reaction centres.

Absorption spectra: An absorption spectrum is a graph of

the absorbance of different wavelengths of light by a pigment.

Action spectra: An action spectrum is a graph of the

rate of photosynthesis at different wavelengths of light.

Absorption spectra:

400 450 500 550 600 650 700Wavelength of light (nm)

ab

sorb

an

ce

chlorophyll a chlorophyll b

carotenoids

Action spectrum:


Rate

of

ph

oto

syn

thesi

s

Photosystems Photosystem I

This is arranged around a molecule of chlorophyll a with a peak absorption at 700nm

The reaction centre of photosystem I is therefore known as P700

Photosystem II This is arranged

around a molecule of chlorophyll a with a peak absorption at 680nm

The reaction centre of photosystem II is therefore known as P680


ab

sorb

an

ce

chlorophyll a

Found in PS1Found in PS2

A photosystem:

thylakoid membrane

photosystem

light

accessory pigments

primary pigment reaction centreP700 or P680

An overview






ATPreduced NADP


The light-dependent reaction Occurs in the thylakoids

Results in photophosphorylation

This can be either cyclic photophosphorylation (CPP) or non-cyclic photophosphorylation (NCP)

CPP produces – ATP– Hydrogen ions

NCP produces– Oxygen– Reduced NADP– ATP

Once the light energy is passed to the reaction centre, electrons are energised to a level where they are emitted from the chlorophyll.

These are used in the light dependent stage to:– Produce reduced NADP [NADPH]– Transfer light energy to ATP by the process

of photophosphorylation

Depending on the route taken by the released electrons this can be either – non-cyclic photophosphorylation or – cyclic photophosphorylation.

Non cyclic photophosphorylation produces oxygen, NADPH and ATP

While cyclic photophosphorylation produces just ATP and hydrogen ions

At the same time water molecules are split to produce electrons

These replace the electrons ejected from the chlorophyll and hydrogen ions.

Oxygen is given off as a waste product

Cyclic photophosphorylation

PSI

2e-

electron carrierelectron carrier

electron carrier

electron carrier

ADP + Pi

ATP

light

Electrons are cycled (PSI carriers PSI carriers etc.)

PSI

2e-

electron carrierelectron carrier

electron carrier

electron carrier

ADP + Pi

ATP

light

PSII

light

2e-

H2O O2 + 2e- + 2H+21

NADP NADPH

2e-

waste product

to LIS

NON-CYCLIC PHOTOPHOSPHORYLATION

The production of ATP in non-cyclic photophosphorylation

When light energy is passed to the reaction centre in PSII, electrons are energised to a level where they are emitted from the chlorophyll


As the result of the flow of electrons from PSI to PSII and the breakdown of water there is a build up of hydrogen ions.


These accumulate within the thylakoid space and create a concentration gradient.


The consequent passage of H+ across the thylakoid membranes provides the energy for the production of ATP in the presence of ATPase. (chemiosmosis).

When light is absorbed by the chlorophyll in photosystem I (PSI), an electron is ejected and taken up by an electron acceptor (ferredoxin).

The production of NADPH in non-cyclic photophosphorylation

This in turn passes the electron to a molecule of NADP, which is thus reduced to NADPH.

The production of NADPH in non-cyclic photophosphorylation

This process would eventually stop if the released electrons were not replaced in PSI. This happens as a result of light energy displacing electrons from PSII.

Non-cyclic photophosphorylation

Electrons from PSII are passed along a series of electron carriers (cytochromes) andeventually replace the lost electrons in PSI

Non-cyclic photophosphorylation

If there is sufficient NADPH then the plant will

automatically switch to CYCLIC

PHOTOPHOSPHORYLATION

In this, the electrons follow a different route; PSI is both the donator and acceptor of the electrons; i.e. they follow a cyclical route.

As light enters PSI electrons are lost from the chlorophyll and pass along a chain of electron carrier molecules before re-entering PSI.

The accumulation of H+ still occurs in the thylakoid space, with the consequent synthesis of ATP, but no NADPH is formed

The Hill Reaction The photolysis of water was first shown by Robert Hill in 1939 Working on isolated chloroplast he showed that they had

‘reducing power’ In the presence of an oxidising agent, oxygen was liberated

from water He used a substance that changed colour on reduction This can be demonstrated with a variety of substances but is

usually shown using the blue dye DCPIP (dichlorophenolindophenol)

This is substituting for the plant’s NADP. DCPIP becomes colourless when reduced

oxidised DCPIP reduced DCPIP

H2O O2 21

The Hill ReactionC

olo

rim

ete

r re

ad

ing

s

colourless

blue

time

Chloroplasts in the light

Chloroplasts in the dark for 5 minutes and then in the light

An overview






ATPreduced NADP


The light-independent reaction

Occurs in the stroma of the chloroplast

The product is sugar which can be converted into fats, amino acids etc

The light-independent reaction Also known as the Calvin cycle. This is the stage that fixes

carbon dioxide from the atmosphere

Carbon dioxide combines with a 5-carbon sugar called ribulose bisphosphate (RuBP). The reaction is catalysed by the enzyme RuBPC. [ribulose bisphosphate carboxylase]

The 6-carbon compound formed is unstable and immediately splits to give two molecules of a 3-carbon compound called glycerate-3-phosphate (GP).

GP is reduced to 3-carbon triose phosphate (TP) in the presence of ATP and reduced NADP [from the light-dependent stage]

The triose phosphate is then either used to regenerate RuBP or to make carbohydrates and other metabolites for the plant

unstable intermediate(6C)

2 x glycerate 3-phosphate

(3C)

2 x triosephosphate (3C)

ATP

ADP + Pi

reduced NADP

NADP

CO2

(1C)

RuBP(5C)

ATP

ADP

The Calvin cycle

Glucose (6C), amino acids and lipids

The Calvin cycle

Fate of triose phosphate

Some is used to regenerate RuBP

Some molecules condense to form hexose phosphates, sucrose, starch and cellulose

Some are converted to acetyl coenzyme A to make amino acids and lipids

The light-independent reaction

This cycle of events was worked out by Calvin, Benson and Bassham between 1946 and 1953

The cycle is usually called the Calvin cycle

The enzyme ribulose bisphosphate carboxylase (ribisco or RuBPC for short), which catalyses the combination of carbon dioxide and RuBPC is the most common enzyme in the world!

Photosynthesis – a summary

6O2

24 electrons

6CO2

+

6CO2

12H2O

chlorophyll

12H2O

Light energy

ADP + Pi

ATP

24H+ +

6H2O + C6H12O6

24H+

1

2

3

4

5

6

Light-dependent stage (thylakoid membranes) Light-independent stage (stroma)

Structure to function:the leaf Thin, therefore rapid light

penetration Waxy cuticle reduces water

loss Upper epidermis

transparent to light Palisade mesophyll

arranged at 90o to surface thus minimising amount of light absorbed by cell walls

Chloroplasts in mesophyll can be moved to maximise absorption Spongy mesophyll has many air sps spaces for rapid gas diffusion

Light and shadeA variety of environmental factors can affect the size and thickness of leaves. In many species, leaves grown under high light intensity (sun leaves) are smaller and thicker than those grown under low light intensity (shade leaves). Increased thickness of sun leaves is due to greater development of palisade parenchyma.

Acer : mapleShade leaf Sun leaf

Photosynthesis

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