1. Photosynthesis and ATP synthesis 2 Flashcards
Explain cyclic photophosphorylation
Light is absorbed by PSI
light energy is passed on to electrons and excites them in the chlorophyll a molecules at the reaction centre.
In each chlorophyll a molecule, one of the electrons becomes so energetic that it moves to a higher energy level and leaves the chlorophyll molecules completely.
The electron is then passed along the chain of electron carriers (e transport system)
The energy from the electron is used to make ATP.
The electron, now having lost its extra energy, eventually returns to chlorophyll a in PSI.
The electron transport system consists of a series of carrier molecules and, as electrons pass from one carrier molecule to the next, energy is released.
This energy release causes
H+ to be actively pumped into the thylakoid from the stroma across the thylakoid membrane.
The concentration of H+ inside the thylakoid space is therefore higher than in the stroma outside.
Thus, a H+ gradient is created across the thylakoid membrane.
The H+ can diffuse back across the thylakoid membrane and into the stroma only via specific protein channels called chemiosmotic channels.
As H+ flow from inside the thylakoid membrane to the outside of the thylakoid down their electrochemical gradient (proton gradient), energy is generated to drive the enzymatic conversion of ADP and P (inorganic phosphate found in the cytosol) into ATP.
The reactions of the Calvin cycle that occur in the stroma require a larger amount of ATP than NADPH and this is where the ATP is used up.
Chemiosmotic channels are formed by
enzyme complexes called ATP synthase
Non-cyclic photophosphorylation involves what type of photosystem
both kinds of photosystem.
PSI and PSII
Non-cyclic photophosphorylation results in
the production of ATP and NADP
what are the three stages of Non-cyclic photophosphorylation
1 ) Photolysis of water produces protons (H+ ions), electrons and 0 2
2) Energy from the excited electrons makes ATP…
3) reduced NADP is generated
explain the first stage of Non-cyclic photophosphorylation where Photolysis of water produces protons (H+ ions), electrons and O2 takes place
The chlorophyll a molecules of PSII absorb light energy
the excited, high energy electron from chlorophyll leaves the reaction centre chlorophyll a molecule of PSII (and moves down the electron chain-photophosphorylation)
As soon as the high-energy electrons leave the reaction centre, PSII immediately takes up replacement electrons from a water molecule
the water molecule splits into two electrons, two H+ (protons) and an oxygen atom. (two oxygen atoms from two water molecules combine to form a molecule of oxygen)
The H+ stay inside the thylakoid space and contribute to the formation of a proton gradient which is used to generate ATP.
the reaction for this is
2H2O → 4H+ + 4e− + O2
The light-dependent splitting of water molecules is called
photolysis
Photolysis is
chemical process by which molecules are broken down into smaller units through the absorption of light
the light-dependent splitting of water molecules
Process by which light energy breaks down a molecule
explain the second stage of Non-cyclic photophosphorylation where energy from the excited electrons makes ATP…
High-energy electrons from PSII are taken up by an electron acceptor, which transfers them to an electron transport system.
The electron transport system consists of a series of molecules that act as electron carriers and as the electrons are transported along,
energy is released and used to transport H+ into the thylakoid and stored in the form of a proton gradient across the thylakoid membrane
The production of ATP occurs when the H+ inside the thylakoid flow down their electrochemical gradient to the outside of the thylakoid (the stroma) via the ATP synthase (which is embedded in the thylakoid membrane)
the energy of this movement combines ADP an inorganic phosphate (Pi) to form ATP
(Chemiosmosis-production of ATP using an electrochemical gradient)
Chemiosmosis is
the process by which a Hydrogen pump pumps protons into the thylakoid membrane.
The production of ATP using an electrochemical gradient
H+ passively flows through the ATP synthase which leads to the creation of ATP.
explain the third stage of Non-cyclic photophosphorylation where reduced NADP is generated
After the high-energy electrons have travelled along the electron transport system they have lost energy and become low-energy electrons.
The low-energy electrons enter PSI.
The PSI antenna complex absorbs solar energy and the high-energy excited electrons leave the reaction centre chlorophyll a
then the electrons are taken up by an electron acceptor, which passes them to NADP. The electrons, NADP and H+ bind and become NADPH
Photosystem I uses light energy to
reduce NADP to NADPH
Photosystem II uses light energy to
oxidize water molecules to form oxygen (O2 ), H+ and electrons
the non-cyclic electron pathway starts with
PSII
the only mechanism by which ATP is generated during photosynthesis is by
the proton gradient across the thylakoid membrane
Cyclic photophosphorylation only uses PSI. It’s called ‘cyclic’ because
the electrons from the chlorophyll molecule aren’t passed onto NADP, but are passed back to PSI via electron carriers.
This means the electrons are recycled and can repeatedly flow through PSI.
This process doesn’t produce any reduced NADP or O , — it only produces small amounts of ATP
Both cyclic and non-cyclic photophosphorylation stop
when
there is no light because ATP synthesis and NADP
reduction (i.e. production of NADPH) require light
The rates of cyclic and non-cyclic photophosphorylation depend on
each other because they are linked by the exchange of ATP and ADP and NADP and NADPH with the Calvin cycle and photosystems
The Z-scheme is
simply a way of summarising what happens to electrons during the light dependent reactions.
It is a kind of graph, with the y-axis indicating the ‘energy level’ of the electron
explain the Z-scheme, summarising non-cyclic photophosphorylation
1) light hits photosystem II. The red vertical line going up shows the increase in the energy level of electrons as they are emitted from this photosystem.
2) If you keep following the vertical line showing the increasing energy in the electrons, you arrive at a point where it starts a steep dive downwards. This shows the electrons losing their energy as they pass along the electron carrier chain.
3) Eventually they arrive at photosystem I.
4) You can then track the movement of the electrons to a higher energy level when PSI is hit by light, before they fall back downwards as they lose energy and become part of a reduced NADP molecule.
the electrons in the z scheme come from
the splitting of water molecules
The thylakoid membrane contains several protein complexes that perform different functions associated with photosynthesis.
The protein complexes are:
PSI is a protein complex in association with the antenna complex of chlorophyll a molecules and the enzyme that reduces NADP to NADPH
PSII is a protein complex in association with the antenna complex of chlorophyll a molecules.
PSII splits water to give oxygen and H+ that contribute to electrochemical gradient across the thylakoid membrane.
ATP synthase complex has an ion channel that allows H+ to flow down their electrochemical gradient from the inside of the thylakoid space to outside in the stroma thereby generating ATP from ADP.
A cytochrome is
a protein that contains a haem group that serves as electron carriers in respiration and photosynthesis
A cytochrome complex transports electrons from PSII and PSI.
A cytochrome complex transports
electrons from PSII and PSI.
haem is an
ion-containing prosthetic group such as that in haemoglobin
The Calvin cycle takes place in
the stroma of the chloroplasts
Products of Calvin Cycle
triose phosphate from CO2 (used to make glucose and other useful organic substances)
ribulose bisphosphate (a 5-carbon compound)
Calvin cycle needs what to keep it going?
ATP and H+ ions
The Calvin cycle is also known as carbon dioxide fixation because
carbon from CO2 is ‘fixed’ into an organic molecule.
Carbon enters the calvin cycle as ___ and leaves as ___
Carbon enters the Calvin cycle as carbon dioxide and leaves as sugar
The fixation of carbon dioxide is the
conversion of carbon dioxide to an organic compound
The Calvin cycle (Light independent stage) can be divided into three basic steps
1) Carbon Fixation
2) Reduction of Glycerate-3-Phosphate
3) Regeneration of RuBP
the Calvin cycle is where
the now ATP and reduced NADP that have been formed in the light-dependent stage are used to help to produce carbohydrates from carbon dioxide.
explain what happens in the first stage of the Calvin cycle where carbon fixation takes place
CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast.
Here, it’s combined with ribulose bisphosphate (RuBP), a 5-carbon compound. This reaction is catalysed by the enzyme rubisco. •
This gives an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate(GP)/ 3-phosphoglycerate(PGA)
What is the first product of the Calvin cycle
3-phosphate(GP)/ 3-phosphoglycerate(PGA)
glycerate 3-phosphate(GP) is also called
3-phosphoglycerate(PGA)
The chloroplast stroma contains an enzyme called
rubisco
Rubisco full name is
Ribulose-1,5-bisphosphate carboxylase
what is thought to be the most abundant enzyme in the world
Rubisco
explain what happens in the second stage of the Calvin cycle where the Reduction of Glycerate-3-Phosphate takes place
The hydrolysis of ATP (from the light dependent reaction) provides energy to turn the 3-carbon compound, GP, into a different 3-carbon compound called triose phosphate (TP).
this reaction also requires ATP to supply the energy and H+ ions, which come from reduced NADP (also from the light-dependent reaction).
Reduced NADP is recycled to NADP.
Some triose phosphate is then converted into useful organic compounds (e.g. glucose) and some continues in the Calvin cycle to regenerate RuBP
what is the first carbohydrate that is made in photosynthesis
triose phosphate (TP)
RuBP molecules each contain
five atoms of carbon
explain what happens in the third stage of the Calvin cycle where the Regeneration of RuBP takes place
Five out of every six molecules of TP produced in the cycle aren’t used to make hexose sugars, but to regenerate RuBP.
energy for this reaction is supplied by ATP
Regenerating RuBP uses the rest of the ATP produced by the light-dependent reaction.
diagram showing the Calvin cycle
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Function of Rubisco
to catalyse the reaction in which carbon dioxide combines with a substance called RuBP(ribulose bisphosphate).
Rubisco is
a plant enzyme involved in the light independent stage of photosynthesis which catalyzes the fixing of atmospheric carbon dioxide during photosynthesis by catalyzing the reaction between carbon dioxide and RuBP
the enzyme that fixes the carbon dioxide is
Ribulose bisphosphate carboxylase (RuBP carboxylase
RuBP carboxylase has a relatively low enzymatic rate because it
catalyses the fixation of a few molecules of its substrate (carbon dioxide) per second
In a cycle, the starting molecule must be
regenerated in order for the cycle to continue
For every three turns of the Calvin cycle what happens?
five molecules of PGAL are used to regenerate three molecules of RuBP
a molecule of carbon dioxide is also entering the Calvin cycle at every turn and the carbon dioxide is reduced
the net gain is one PGAL molecule. This means that one PGAL molecule is available to the plant to be used to generate other organic compounds that it needs.
why is the Calvin cycle needed?
The products of the light-dependent stage cannot be stored as a long-term supply of energy for a plant.
Neither can they be used to build up the large molecules that plants need for structural support, information storage and metabolic functions such as catalysing reactions.
This means a further set of reactions is necessary to produce molecules to fix carbon and therefore make it possible to produce organic molecules
The recycling of RuBP is important because
otherwise it would have to be produced from something else.
For example, it could be produced from stored starch which would be a waste of the energy used to make starch in the first place
Rubisco is active when
active in the light, but less active or inactive in the dark.
Oxygen acts as a competitive inhibitor
Of 12 molecules of triose phosphate produced, ten of these are used to
produce six molecules of RuBP and two may be used to produce hexose or glycerol
what is the starting molecule for the synthesis of starch and cellulose
Glucose phosphate
In times of rapid photosynthesis, starch grains may be
stored temporarily in the chloroplast. Later, the starch is reconverted to glucose and transported to other parts of the plant.
The Calvin cycle is the starting point for
making all the organic substances a plant needs.
Triose phosphate (TP) and glycerate 3-phosphate (GP) molecules are used to make carbohydrates, lipids and amino acids
what are the fates of the triose phosphate produced during reduction of Glycerate-3-Phosphate in the Light Independent/Calvin cycle stage of photosynthesis?
1) Five-sixths of it are used to regenerate RuBP.
2) The remainder can be converted into other carbohydrates.
For example, two triose phosphates can combine to produce a hexose phosphate molecule.
From this, glucose, fructose, sucrose, starch and cellulose can be formed.
3) can also be used to make lipids like glycerol and fatty acids which gives plant oil
eg-coconut, corn and castor oil
4) and to make amino acids.
for amino acid production, plants need TP + nitrogen containing ions from the soil in the form of nitrate ions or ammonium ions or root nodules
glycerol is synthesized from
triose phosphate
fatty acids is synthesized from
glycerate 3-phosphate
what is the form in which plants usually transport sugars through the phloem
sucrose
examples of storage organs of starch
yams, sweet potatoes and cassava
Root nodules are
are enlargements or swellings on the roots of legumes that are inhabited by symbiotic nitrogen-fixing bacteria
The Calvin Cycle Needs to Turn how many times to make One Hexose Sugar?
The Calvin Cycle Needs to Turn Six Times to Make One Hexose Sugar
The Calvin Cycle Needs to Turn Six Times to Make One Hexose Sugar because
Three turns of the cycle produces six molecules of triose phosphate (TP), because two molecules of TP are made for every one CO2 molecule used.
Five out of six of these TP molecules are used to regenerate ribulose bisphosphate (RuBP).
This means that for three turns of the cycle only one TP is produced that’s used to make a hexose sugar.
A hexose sugar has six carbons though, so two TP molecules are needed to form one hexose sugar.
This means the cycle must turn six times to produce two molecules of TP that can be used to make one hexose sugar.
Six turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependent reaction.
Photorespiration is a
process that occurs when the enzyme RuBP carboxylase catalyses a reaction with oxygen instead of carbon dioxide
The conditions under which photorespiration occur are related to
environmental factors.
In hot dry weather, stomata on leaves are closed to prevent water from being lost from inside the leaf
how is phosphoglycolate formed?
When the stomata are closed, photosynthesis can still occur and one of the by-products of photosynthesis is oxygen which builds up in the intercellular spaces.
This abundant oxygen then combines with RuBP carboxylase to form one molecule of PGA and one molecule of phosphoglycolate
phosphoglycolate is
a molecule that contains two carbon atoms.
Phosphoglycolate is not a useful metabolite in plants and energy must be used up to convert it into products that are useful.
No carbon is fixed during photorespiration.
what inputs does Photosynthesis require?
1) raw materials in the form of carbon dioxide and water
2) energy in the form of sunlight
3) The light-independent stage also requires a reasonably high temperature
why does The light-independent stage of photosynthesis require a reasonably high temperature
because the rates of reactions are affected by the kinetic energy of the molecules involved
what happens if any of these requirements of photosynthesis is in short supply
it can limit the rate at which the reactions of photosynthesis are able to take place
what are the factors affecting the rate of photosynthesis
1) Light intensity
2) availability of carbon dioxide
3) Temperature
4) water supply
what does the Light provide in photosynthesis?
Light provides the energy that drives the light dependent reactions, so it is obvious that when there is no light, there is no photosynthesis.
what happens if we provide a plant with more light
it will photosynthesise faster
However, this can only happen up to a point (a point of light intensity)
what happens when we reach a point of light intensity?
if you we give the plant more light, its rate of photosynthesis does not change.
We can say that ‘light saturation’ has occurred.
Some other factor, such as the availability of carbon dioxide or the quantity of chlorophyll in the plant’s leaves, is preventing the rate of photosynthesis from continuing to increase
explain the diagram of the effect of light intensity on the rate of photosynthesis
Over the first part of the curve rate of photosynthesis increase as light intensity increases.
For these light intensities, light is a limiting factor.
The light intensity is limiting the rate of photosynthesis.
If we give the plant more light, then it will photosynthesise faster.
But, from point X onwards, increasing the light intensity has no effect on the rate of photosynthesis.
Along this part of the curve, light is no longer a limiting factor. Something else is.
It is most likely to be the carbon dioxide concentration.
Optimum Conditions for Photosynthesis are
High light intensity of a certain wavelength
Temperature around 25 °C
Carbon dioxide at 0.4%
a constant supply of water
Limiting factors occur when
a chemical process is affected by more than one factor.
The rate of the chemical process is limited by whichever factor is nearest its minimum value.
The limiting factor is
that factor which directly affects a process if its quantity is changed
What this means is that whichever factor is available in the least amount limits the rate of the chemical reaction.
The photosynthesis reactions cannot go faster when the limiting factor is holding it back
what is the concentration of carbon dioxide in the air
very low, only about 0.04%
what is the substance needed for the formation of every organic molecule inside every living thing on Earth
carbon dioxide
why is the concentration of carbon dioxide inside the leaf even lower than in the air outside during daylight
During daylight, carbon dioxide is used in the Calvin cycle in the chloroplasts, so the concentration of carbon dioxide inside the leaf is even lower than in the air outside, providing the diffusion gradient that keeps it moving into the leaf.
Carbon dioxide concentration is often a limiting factor for photosynthesis.
If we give plants extra carbon dioxide, they can photosynthesise faster.
diagram showing the relationship between carbon dioxide concentration and rate of photosynthesis.
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diagram showing the effect of carbon dioxide at different light intensities.
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Increasing the carbon dioxide concentration from 0.04 to 0.4% in photosynthesis does what?
gives a higher rate of photosynthesis, but any higher and the stomata start to close
what is usually the limiting factor on a warm, sunny, windless day,
usually CO2
what is usually the limiting factor during the night
sunlight
if the carbon dioxide concentration is low, then the
rate of carboxylation catalysed by rubisco will be slower than if the concentration was higher
what temperature affects?
Temperature affects the kinetic energy of molecules
Photosynthesis requires the action of enzymes to catalyse reactions and temperature is a factor that influences the rate of enzymatic reactions
what happens at the higher temperature of molecules
The higher the temperature, the faster molecules move, and the more frequently they collide with one another.
They also collide with more energy.
The greater frequency and energy of collisions of molecules means that
the reaction rate increases
The greater frequency and energy of collisions of molecules means that the reaction rate increases.
In photosynthesis, though, this effect is only seen in
the light-independent reactions
the rate of the light-dependent reactions is not directly affected by temperature, because
the energy that drives them comes from light, not the kinetic energy of molecules
why plant enzymes often have lower optimum temperatures than enzymes found in mammals
because they have evolved to work in the environmental temperatures in which the plant normally lives
In living organisms, most reactions are catalysed by enzymes, so we also need to consider the effect of temperature on them.
Just like any molecules, their kinetic energy increases as temperature increases.
However beyond a certain temperature (different for different enzymes) they begin to lose their shape, and therefore their catalytic properties.
plant enzymes often have lower optimum temperatures than enzymes found in mammals because they have evolved to work in the environmental temperatures in which the plant normally lives but what causes things to be complicated?
a peculiar property of the enzyme rubisco.
Rubisco has an unfortunate tendency to stop doing what it is supposed to do - catalyse the combination of carbon dioxide with RuBP - and start doing something else when temperature rises.
It switches to catalysing a reaction in which oxygen is combined with RuBP.
This is very wasteful, as it wastes RuBP.
It is called photorespiration
what is the effect of photorespiration
it can seriously reduce the rate of photosynthesis in many plant species, when temperature and light intensity are high.
Photorespiration is a misleading name because
it is not really respiration at all
the Calvin cycle does not require what?
it does not require energy input from light
It does, however, need energy input from the light-dependent stage, in the form of ATP and reduced NADP
explain the diagram of the effect of light on the Calvin cycle
The effect of light and dark on the relative levels of TP and GP in a chloroplast.
Imagine that light is shining on a chloroplast.
The light-dependent stage is generating ATP and reduced NADP, and the reactions of the Calvin cycle are working continuously.
Now the light is switched off.
The light-dependent stage stops, so the supply of ATP and NADP to the Calvin cycle also stops.
These substances are needed to fuel the conversion of GP to TP. So now GP can no longer be converted into TP, and the GP just builds up.
The rest of the cycle keeps running, until most of the TP is used up.
Then it grinds to a halt this diagram shows what happens to the relative amounts of GP and TP when the light is switched off.
the levels of TP plummet, while the levels of GP rise.
If the light is switched on again, they go back to their ‘normal’ relative levels
diagram of the effect of light and dark on the relative levels of TP and GP in a chloroplast.
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why increasing the supply of any other factor when one factor is already limited has no effect on crop production
If one factor is rate limiting, increasing the supply of any other factor has no effect on the rate of photosynthesis and therefore no effect on crop production but increasing the supply of the limiting factor will have a dramatic impact on crop production
what will have a dramatic impact on crop production
increasing the supply of a limited factor factor that is already limited
factors that affect crop production are
1) increasing the supply of a limited factor factor is already limited
2) light or carbon dioxide could be rate limiting at temperatures within the range tolerated by the crop
3) Water and each mineral nutrient required by a plant
a farmer has limited capacity to affect light or carbon dioxide levels in order to increase the rate of photosynthesis, unless
a crop is being grown in a specialised glasshouse
example of how Water and each mineral nutrient required by a plant can affect crop production
If the supply of potassium is limited in the soil, potassium may be rate limiting for crop growth.
So, increasing the light intensity will have no or little effect on crop production in these circumstances
what can reveal which factor is going to be rate limiting in a range of circumstances and therefore which factor could be increased to make maximum impact on crop production at any one time
Analysis of the climate, soil and atmospheric environment of a crop
Analysis of the climate, soil and atmospheric environment of a crop can reveal what?
which factor is going to be rate limiting in a range of circumstances and therefore which factor could be increased to make maximum impact on crop production at any one time
The ability of plants to transfer light energy into chemical energy means that
they can be used to provide fuels for us to use
for example, for generating electricity or in vehicle engines
As stocks of fossil fuels run down, and as carbon dioxide levels in the atmosphere continue to increase, there has been a sharp increase in the use of
crop plants to produce fuels rather than food.
For example, rape seed is being used to produce biodiesel, rather than food for animals or humans
why is using crop plants to produce fuels good?
Using plants to provide fuels is theoretically ‘carbon-neutral’.
The carbon dioxide that is given out when the fuels are burnt is matched by the carbon dioxide that the plants take in as they photosynthesise and grow
why is using crop plants to produce fuels bad?
the energy that is used in harvesting the plants, converting the biomass to a useful form of fuel and transporting that fuel to points of sale, then there is still a net emission of carbon dioxide to the atmosphere
the increasing quantity of crops to produce biofuels is having an effect on the availability and price of food
effects on ecosystems.
Producing large quantities of biofuels will take up large areas of land
example of how using crop plants to produce fuels is having an effect on the availability and price of food
as huge areas of land in the USA are taken over to grow corn (maize) for fuel, there is less maize on sale for cattle feed or human food.
Prices have increased, in some cases so much so that poorer people, especially in neighbouring countries like Mexico, are finding it much more difficult to buy enough food for their needs
example of how using crop plants to produce fuels is having an effect areas of land
Producing large quantities of biofuels will take up large areas of land
There is a danger that some countries will cut down forests to provide extra land for this purpose, damaging habitats and endangering species that live there
Photosynthesis uses light energy to cause
carbon dioxide and water to react to produce carbohydrates and oxygen
The light energy is transformed to chemical energy in the carbohydrates
The chloroplasts are arranged in the cytoplasm how?
around the outer edge of the cell, and can be moved to maximise light absorption
Inside the chloroplast, membranes form what?
fluid-filled sacs called thylakoids, which in turn form stacks called grana
The chlorophyll molecules are found where?
within photosystems I and II
The factor that is in the shortest supply at any one time during photosynthesis is known as
the limiting factor
If crops are grown in glasshouses, these limiting factors of photosynthesis can be
adjusted to increase the rate of photosynthesis and therefore the productivity of the crop
What is the function of the chlorophyll found in plant cells?
Absorbs light for photosynthesis.
Is photosynthesis an exothermic or endothermic reaction?
Endothermic
Energy needs to absorbed for the reaction to occur
Hydroponics is
the growth of plants in aqueous solutions containing essential minerals (e.g. nitrogen, potassium, phosphorus, magnesium, calcium and sulfur).
hydroponics makes to so that plants can
produce fruits and vegetables without the presence of soil and the readily available water supply aids growth.
In field conditions, plants have to take up water from the soil solution via their root hairs but in hydroponics the roots are suspended in water.
Greenhouse technology is used for
sustainable crop production in places with adverse climatic conditions.
