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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Communication, 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
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.
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.
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.
Photosynthesis (p/s) is the fixation of carbon dioxide and its reduction to carbohydrate, using hydrogen from water
Word equation for photosynthesis
Carbon dioxide + watercarbohydrate + oxygen
nCO2 + nH20 (CH2O)n + nO2
6CO2 + 6H2O C6H12O6 + 6O2
Testing a leaf for starch
What are the requirements for photosynthesis
Factors limiting photosynthesis
Photosynthesis is a 2 stage process
Light dependent reactions
Light independent reactions
Explain, with the aid of diagrams and electron micrographs, how the structure of chloroplasts enables them to carry out their functions.
3 – 10μm diameter
Envelope of 2 phospholipid membranes
Stroma = fluid interior
Thylakoids are series of flattened sacs, which form stacks (grana) in places
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)
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
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)
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.
A graph of absorbance of different wavelengths of light by a pigment
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.
The Chemistry of photosynthesis
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 H218O 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
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
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.
Arranged around chlorophyll a molecule with an absorption peak at 700 nm.
Reaction centre P700
Chlorophyll a molecule with absorption peak at 680nm
Reaction centre P680
ADP + Pi
Light energy absorbed by Chlorophyll a
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.
ADP + Pi
H2O ½O2 + 2e- + 2H+
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.
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;
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.
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
Amino acids and proteins
Growth factors, vitamins, hormones, pigments
Factors limiting photosynthesis
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.
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?
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 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 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
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
Initial accumulation of RuBP