Transcript Photosynthesis 1
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: light
n
CO
2
+
n
H
2
O (CH
2
O)
n
+
n
O
2
This shows that photoautotrophs synthesise carbohydrate using carbon dioxide, water and light energy.
Q. Is photosynthesis a reduction or an oxidation?
A. CO 2 is reduced; water is oxidised.
This simple summary hides the fact that photosynthesis is a series of reactions controlled by specific enzymes.
Stages of photosynthesis
The reactions of photosynthesis can be divided into two distinct stages .
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.
An overview
oxygen (O
2
) carbon dioxide (CO
2
) water (H ADP Pi
2
O) light energy oxidised NADP light-dependent stage ATP reduced NADP light-independent stage Note: the light-independent stage is also known as the Calvin cycle.
carbohydrates ADP inorganic phosphate oxidised NADP
The chloroplast
Starch grain. Produced from sugars made in photosynthesis outer membrane Ribosomes. Smaller than cytoplasmic ribosomes.
the Calvin cycle Made from the sugars made in photosynthesis Small circular DNA coding for some chloroplast proteins Thylakoid space. Space between lamellae.
Lamella. A pair of membranes containing chlorophyll
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 .
Pigment Chlorophylls: Carotenoids: chlorophyll a chlorophyll b ß carotene xanthophyll Colour
Yellow-green Blue-green Orange Yellow 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.
carotenoid chlorophyll a
Thylakoid membranes
the possible arrangement of chlorophyll and associated molecules within the thylakoid membranes based on studies of isolated grana chlorophyll combined with protein electron carriers 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:
chlorophyll a chlorophyll b carotenoids 400 450 500 550 600 650 700 Wavelength of light (nm)
Action spectrum:
400 450 500 550 600 650 700 Wavelength of light (nm)
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
chlorophyll a Found in PS2 Found in PS1 400 450 500 550 600 650 700 Wavelength of light (nm)
A photosystem:
light thylakoid membrane photosystem accessory pigments primary pigment reaction centre P700 or P680
An overview
oxygen (O
2
) carbon dioxide (CO
2
) water (H ADP Pi
2
O) light energy oxidised NADP light-dependent stage ATP reduced NADP light-independent stage Note: the light-independent stage is also known as the Calvin cycle.
carbohydrates ADP inorganic phosphate oxidised 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
Electrons are cycled (PSI carriers PSI carriers etc.) electron carrier ADP + P
i
electron carrier electron carrier ATP electron carrier 2e
-
PSI light
2e
-
ADP + P
i
electron carrier electron carrier ATP electron carrier to LIS electron carrier 2e
-
PSI light NADP NADPH 2e
-
PSII light H
2
O
1 2
O
2
+ 2e
-
+ 2H
+
waste product
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
The production of ATP in non-cyclic photophosphorylation 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.
The production of ATP in non-cyclic photophosphorylation These accumulate within the thylakoid space and create a concentration gradient.
The production of ATP in non-cyclic photophosphorylation The consequent passage of H + across the thylakoid membranes provides the energy for the production of ATP in the presence of ATPase. (chemiosmosis).
The production of NADPH in non-cyclic photophosphorylation 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.
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) and eventually replace the lost electrons in PSI
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 formed + still occurs in the thylakoid space, with the consequent synthesis of ATP, but no NADPH is
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 H
2
O
1 2
O
2
blue
The Hill Reaction
Chloroplasts in the light Chloroplasts in the dark for 5 minutes and then in the light colourless time
An overview
oxygen (O
2
) carbon dioxide (CO
2
) water (H ADP Pi
2
O) light energy oxidised NADP light-dependent stage ATP reduced NADP light-independent stage Note: the light-independent stage is also known as the Calvin cycle.
carbohydrates ADP inorganic phosphate oxidised 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
ADP ATP RuBP (5C) CO
2
(1C)
The Calvin cycle
unstable intermediate (6C) reduced NADP 2 x glycerate 3-phosphate (3C) ATP NADP 2 x triose phosphate (3C) Glucose (6C), amino acids and lipids ADP + P
i
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
Light-dependent stage (thylakoid membranes) Light-independent stage (stroma) Light energy 12H 2 O 6CO 2
4
ADP + P i
1
chlorophyll
3
12H 2 O
2
24 electrons + 24H + 6O 2 ATP
5
24H + + 6CO 2 6H 2 O
6
+ C 6 H 12 O 6
Structure to function:the leaf
Thin, therefore rapid light penetration Waxy cuticle reduces water loss Upper epidermis transparent to light Palisade mesophyll arranged at 90
o
to surface thus minimising amount of light absorbed by cell walls sps Chloroplasts in mesophyll can be moved to maximise absorption Spongy mesophyll has many air spaces for rapid gas diffusion
Light and shade
A 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 : maple Shade leaf Sun leaf