Unit 1Communication, Homeostasis and Energy

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!!

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.

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.

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.

Autotroph an organism that uses an external energy

source and inorganic molecules to make complex organic molecules.▪ Chemoautotroph▪ Photoautotroph

Heterotroph Organism that needs to take in complex

organic molecules which act as a source of energy and as usable carbon compounds.

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. photosynthesisHeterotrophic nutrition

Obtained in organic form

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

Word equation for photosynthesis

Light energyCarbon dioxide + watercarbohydrate +

oxygenchlorophyll

Light energynCO2 + nH20 (CH2O)n + nO2

chlorophyll

Light energy6CO2 + 6H2O C6H12O6 + 6O2

chlorophyll

Overall Chemical Equation

Balanced Equation for hexose sugars

Testing a leaf for starch

What are the requirements for photosynthesis Light Chlorophyll carbon dioxide

Factors limiting photosynthesis chlorophyll (enzymes) carbon dioxide Light Water Temperature

Photosynthesis is a 2 stage process Light dependent reactions▪ thylakoid membranes

Light independent reactions▪ Stroma

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

x

y

z

3 – 10μm diameterEnvelope of 2 phospholipid

membranesStroma = fluid interiorThylakoids are series of flattened

sacs, which form stacks (grana) in places

Grana 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)

Stroma 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

Lamellae Do not contain chlorophyll Form a network between the grana

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.

The fate of light which strikes the leaf

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

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 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.

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.

Evidence for this comes from experiments with isotopes of oxygen. Plants provided with C18O2 combine the

atoms into carbohydrates Plants provided with H2

18O release the 18O atoms as oxygen gas

All the oxygen released by photosynthesis comes from water.

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

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 PSI

Non cyclic photophosphorylation PSII & PSI

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

PSI

Light energy absorbed by Chlorophyll a

Electronacceptor

ADP + Pi

ATP

2e-

Electron carriers

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 .

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

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

PSII

PSI

ElectronAcceptor A

2e-

ElectronAcceptor B

2e-

Light energy

Light energy

AD

P +

Pi

ATP

NADP

NADPH+ H+

H2O ½O2 + 2e- + 2H+

Electron carrier

Electron carrier

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.

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+)

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

Complete the diagram of Photophosphorylation

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.

(a) 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;

(b) (i)

chlorophyll a; chlorophyll b; carotenoids / carotene; xanthophylls; phaeophytin;

(ii) absorb/ trap/ capture / harvest, light / transfer energy /

transfer electrons; (iii)

granum/ thylakoid (membrane) / lamella / quantasome;

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

the stroma of the chloroplast.

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.

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

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

Specialist carbohydrates glucose fructose Sucrose Cellulose

LipidsAmino acids and proteinsNucleic acidsGrowth factors, vitamins, hormones,

pigments

How can the rate of photosynthesis be measured?

Which Environmental factors could limit the rate of photosynthesis?

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.

Raw materials Water Carbon dioxide

Energy in the form of sunlightLight independent stage requires a

relatively high temperature The light-dependent reactions are not

directly affected by temperature, why is this?

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

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 drives the light-dependent reactions

More light, more photosynthesisAt a point where increasing light

intensity has no effect on the rate of photosynthesis, light is no longer the limiting factor

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

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.

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.

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.

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

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

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.

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.

If CO2 is in short supply Less for RuBP to react with Less GP Less TP Initial accumulation of RuBP