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Photosynthesis. Unit 1 Communication, Homeostasis and Energy. Think about it!!! 10 minutes. Which process evolved first on Earth – aerobic respiration or photosynthesis? Give reasons for your answer You are going to present your answer with your reasons to the rest of the class!!. Answer.

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Unit 1

Communication, Homeostasis and Energy

Think about it!!! 10 minutes

Which process evolved first on Earth – aerobic respiration or photosynthesis?

Give reasons for your answer

You are going to present your answer with your reasons to the rest of the class!!


Plants and animals rely on aerobic respiration

Requires oxygen

Oxygen is a by-product of photosynthesis

Until photosynthesis evolved there was no free oxygen in the atmosphere

Photosynthesis evolved first!!

Test yourself

Where in plants does photosynthesis take place?

What are the raw materials needed for photosynthesis?

What is the energy source for photosynthesis?

Draw a flow diagram showing how energy from sunlight is used to produce muscle contractions in your arm.

Learning outcomes

  • Define the terms autotroph and heterotroph.

  • State that light energy is used during photosynthesis to produce complex organic molecules.

  • Explain how respiration in plants and animals depends upon the products of photosynthesis.

  • State that, in plants, photosynthesis is a two-stage process taking place in chloroplasts.

The importance of photosynthesis

Photosynthesis transfers light energy into the chemical potential energy of organic molecules.

Photosynthesis releases oxygen from water, so all aerobes depend on photosynthesis for their respiration.

Heterotroph and Autotroph


an organism that uses an external energy source and inorganic molecules to make complex organic molecules.




Organism that needs to take in complex organic molecules which act as a source of energy and as usable carbon compounds.

Energy in Living Organisms

  • In order to maintain life, organisms need a source of energy.

  • In most organisms this is provided by the oxidation of organic molecules.

  • Autotrophic nutrition

    • Synthesise organic materials from inorganic sources e.g. photosynthesis

  • Heterotrophic nutrition

    • Obtained in organic form

Photosynthesis: an outline

Photosynthesis (p/s) is the fixation of carbon dioxide and its reduction to carbohydrate, using hydrogen from water

Photosynthesis Equations

Word equation for photosynthesis

Light energy

Carbon dioxide + watercarbohydrate + oxygen


Photosynthesis Equations

  • Overall Chemical Equation

Light energy

nCO2 + nH20  (CH2O)n + nO2


  • Balanced Equation for hexose sugars

Light energy

6CO2 + 6H2O  C6H12O6 + 6O2


Photosynthesis experiments

Testing a leaf for starch

What are the requirements for photosynthesis



carbon dioxide

Factors affecting the rate of photosynthesis

Factors limiting photosynthesis

chlorophyll (enzymes)

carbon dioxide





Photosynthesis is a 2 stage process

Light dependent reactions

thylakoid membranes

Light independent reactions


Learning outcome

Explain, with the aid of diagrams and electron micrographs, how the structure of chloroplasts enables them to carry out their functions.


Chloroplast – electro micrograph




Chloroplast Structure

3 – 10μm diameter

Envelope of 2 phospholipid membranes

Stroma = fluid interior

Thylakoids are series of flattened sacs, which form stacks (grana) in places

Chloroplast function


Provides a LSA to hold pigments, electron carriers, and enzymes for light dependent reactions.

Photosystems arranged in funnel like structure in thylakoid

Membrane of grana holds ATPsynthase (chemiosmosis)

Chloroplast Function


Site of light independent reactions (carbon fixation)

Contains sugars, organic acids and enzymes for Calvin cycle

Store starch grains

Loop DNA – codes for chloroplast proteins

Chloroplast Function


Do not contain chlorophyll

Form a network between the grana

Learning Outcomes

Define the term photosynthetic pigment.

Explain the importance of photosynthetic pigments in photosynthesis.

State that the light-dependent stage takes place in thylakoid membranes and that the light-independent stage takes place in the stroma.

Trapping Light energy

The fate of light which strikes the leaf

Trapping the Light Energy

The fate of light which strikes the leaf 

Light shining on leaf (100%)

12% light reflected

83% light absorbed, but only 4% of this is used in photosynthesis

5% of light transmitted

These values will be affected by

the amount of chloroplasts in the leaf

how shiny the leaf is

how thick the leaf is

Features of light that make it important

spectral quality (colour)

intensity (brightness)

duration (time)

Visible light has a wavelength between 400nm and 700nm

Absorption of Light

Leaves contain a variety of photosynthetic pigments, of which chlorophyll is the most obvious.

It is these pigments which absorb light energy.

There are two different groups of pigments

chlorophylls – chlorophyll a, chlorophyll b

Carotenoids – xanthophyll, carotene

Different photosynthetic pigments absorb different wavelengths.

Absorption and Action Spectra

Absorption Spectrum

A graph of absorbance of different wavelengths of light by a pigment

Action Spectrum

A graph of the rate of photosynthesis at different wavelengths of light.

Chlorophylls absorb red and blue violet regions of light, and reflect green

Carotenoids absorb the blue-violet region of the spectrum.

Absorption Spectrum

Action Spectrum

Absorption and Action Spectra

The Chemistry of photosynthesis

Learning Outcomes

Outline how light energy is converted to chemical energy (ATP and reduced NADP) in the light-dependent stage.

Explain the role of water in the light-dependent stage.

Photosynthesis is a two-stage process

Evidence for this comes from experiments with isotopes of oxygen.

Plants provided with C18O2 combine the atoms into carbohydrates

Plants provided with H218O release the 18O atoms as oxygen gas

All the oxygen released by photosynthesis comes from water.

Harvesting Light

In p/s the light energy absorbed by the p/s pigments is converted to chemical energy.

The absorbed light energy excites electrons in pigment molecules.

In functioning photosystems this is the energy which drives the process of photosynthesis.

There are two categories of p/s pigment

Primary pigments

chlorophyll a

Accessory pigments

chlorophyll a, chlorophyll b and carotenoids

Light-dependent reactions

Water is split in a reaction called photolysis,

These reactions provide the energy to:

Synthesis ATP from ADP and Pi (photophosphorylation)

Transfer H+ and e- to NADP to form reduced NADP


Photophosphorylation can be cyclic or non cyclic depending on the pattern of electron flow in one or both photo systems

Cyclic photophosphorylation


Non cyclic photophosphorylation


Harvesting LightPhotosystems

Pigments are arranged into light harvesting clusters called photosystems

light energy absorbed by pigments is passed to the primary pigment, which acts as a reaction centre.


Photosystem I

Arranged around chlorophyll a molecule with an absorption peak at 700 nm.

Reaction centre P700

Photosystem II

Chlorophyll a molecule with absorption peak at 680nm

Reaction centre P680

Light harvesting system

Cyclic Photophosphorylation

ADP + Pi




Electron carriers


Light energy absorbed by Chlorophyll a


Cyclic Photophosphorylation

involves only photosystem I, which has a chlorophyll a with a reaction centre P 700.

An electron from the molecule is excited to a higher energy level.

It is captured by an electron acceptor, and then is passed back to one of the chlorophyll a P700 molecules.

This happens due to a chain of electron carriers .

Cyclic photophosphorylation

The whole process releases energy to make ATP from ADP and inorganic phosphate.

This ATP will then be used in the light – independent reaction.

Non cyclic photophosphorylation

ADP + Pi





Acceptor B


Acceptor A


Electron carrier

Electron carrier




Light energy

Light energy

H2O ½O2 + 2e- + 2H+

Non-cyclic photophosphorylation

involves both photosystems.

Both absorb light and the electrons which are excited leave the reaction centres of P680 and P700 of the chlorophyll a molecules.

Electron acceptors pass the electrons along chains of electron carriers.

The P700 of the photosystem I absorbs electrons from photosystem II.

Replacement electrons from the photolysis of water go to photosystem II.

Non-cyclic photophosphorylation

The electrons lose energy passing along the electron chain and this goes towards synthesising ATP.

The photolysis of water releases two protons/H+s

H+ combine with electrons from photosystem I and NADP to give reduced NADP (NADPH + H+)

The Photolysis of water

H2O 2H+ + 2e- + ½O2

Oxygen is released as a waste product

The H+ and e- are transferred to NADP to give reduced NADP

2H+ + 2e- + NADP  reduced NADP

The reduced NADP then passes onto the light independent reactions

Pupil Activity

Complete the diagram of Photophosphorylation

Prep Question – 10 marks

Describe the structure of a chloroplast and then give an account of the role played by chlorophyll in photosynthesis. Refer to action and absorption spectra in your answer.

Write in bullet points and include a diagram.

Jan 03 Question 1


blue and red light used in photosynthesis;

(light of) wavelength 420 – 450 nm, gives high rate / AW;

(light of) wavelength 650 – 690 nm, gives high rate / AW;

(light of) wavelength of 500 – 650 nm / green light, less effective / reflected;

sharp / AW, drop after 680 – 690 nm;



chlorophyll a;

chlorophyll b;

carotenoids / carotene;




absorb/ trap/ capture / harvest, light / transfer energy / transfer electrons;


granum/ thylakoid (membrane) / lamella / quantasome;

Light independent reactions

The light independent reaction involves the fixation of carbon dioxide, and it takes place in the stroma of the chloroplast.

Learning Outcomes

Outline how the products of the light-dependent stage are used in the light-independent stage (Calvin cycle) to produce triose phosphate (TP).

Explain the role of carbon dioxide in the light-independent stage (Calvin cycle).

State that TP can be used to make carbohydrates, lipids and amino acids.

State that most TP is recycled to RuBP.

a.k.a. Calvin cycle

Calvin CycleLight independent reactions

The stages are:

Carbon dioxide is linked with a molecule of ribulose bisphosphate (RuBP), which is a 5 carbon sugar, using the enzyme ribulose bisphosphate carboxylase.

A highly unstable 6C structure is formed which immediately splits into 2 molecules of the 3 carbon compound glycerate-3-phosphate (GP).

GP is converted into triose phosphate (3C) with the addition of hydrogen from reduced NADP and energy from ATP

The fate of triose phosphate

Triose phosphate has two purposes within the cell

Synthesis of molecules

Synthesis of hexose sugars, starch and cellulose

Synthesis of amino acids

5/6 are used in the conversion to RuBP so that more CO2 can be taken up

The fate of the products of photosynthesis

Specialist carbohydrates






Amino acids and proteins

Nucleic acids

Growth factors, vitamins, hormones, pigments

Factors limiting photosynthesis

Factors Limiting Photosynthesis

How can the rate of photosynthesis be measured?

Which Environmental factors could limit the rate of photosynthesis?

Learning outcomes

Discuss limiting factors in photosynthesis, with reference to carbon dioxide concentration, light intensity and temperature.

Describe how to investigate experimentally the factors that affect the rate of photosynthesis.

Factors affecting photosynthesis

Raw materials


Carbon dioxide

Energy in the form of sunlight

Light independent stage requires a relatively high temperature

The light-dependent reactions are not directly affected by temperature, why is this?

Quick revision

In the light-dependent stage what is water a source of?

Hydrogen ions used in chemiosmosis

Hydrogen ions accepted by NADP

Electrons to replace lost by oxidised chlorophyll

Limiting factors

If any of these factors are in short supply, it can limit the rate at which photosynthesis takes place

The factor in the shortest supply is known as the limiting factor.

Light Intensity

Light Intensity

Light drives the light-dependent reactions

More light, more photosynthesis

At a point where increasing light intensity has no effect on the rate of photosynthesis, light is no longer the limiting factor

Carbon Dioxide Concentration

Carbon dioxide in air is about 0.04%

Carbon dioxide is needed for the Calvin cycle

If a plant is given extra CO2 they will photosynthesis faster

Carbon Dioxide Concentration

Over which part of this curve is carbon dioxide the limiting factor for photosynthesis?

Suggest why the curve flattens out at high levels of CO2.

Carbon dioxide concentration


Temperature affects the kinetic energy of molecules

Higher the temperature, the faster the molecules move

More collisions

Rate of reaction increases

At temperatures that are too high, enzyme molecules denature and the rate of reaction slows down.

Learning outcomes

Describe the effect on the rate of photosynthesis, and on levels of GP, RuBP and TP, of changing carbon dioxide concentration, light intensity and temperature.

Effect of light on the Calvin Cycle

The Calvin cycle depends on the products from the light-dependent reactions.

Effect of light on the Calvin Cycle

Explain why the Calvin cycle stops running when there is no light and the TP is used up.

Effect of light on the Calvin Cycle

Make a copy of this diagram and add another line to show what you would expect to happen to the levels of RuBP during this 8 minute period.

Effect of temperature on the Calvin cycle

What effect would you expect a rise or a fall in temperature to have on the relative levels of GP, TP and RuBP?

When answering this assume that the temperature does not go high enough to denature the enzymes.

Explain your reasoning.

Effect of carbon dioxide concentration on the Calvin cycle

If CO2 is in short supply

Less for RuBP to react with

Less GP

Less TP

Initial accumulation of RuBP

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