Photosynthesis Flashcards
What are autotrophs?
Organisms that use light energy or chemical energy and inorganic molecules to synthesise complex organic molecules
What is photosynthesis?
The process whereby light energy from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic substances
What are heterotrophs?
Organisms that ingest and digest complex organic molecules, releasing the chemical potential energy in them. These include animals, fungi and some bacteria
What are photoautotrophs?
Organisms that can photosynthesise. These include all plants, some bacteria and some protoctists
Why do autotrophs and heterotrophs depend on photosynthesis?
Both autotrophs and heterotrophs can release the chemical potential energy in complex organic molecules. This is respiration. They rely on photosynthesis to produce the complex organic molecules.
What is the equation to summarise photosynthesis?
6CO2+6H2O+light energy->C6H12O6+ 6O2
Where does photosynthesis take place?
In chloroplasts inside the cells of photoautotrophs
What is the structure of a chloroplast?
They are disc-shaped and are surrounded by a double membrane, with an intermembrane space between membranes.
The other membrane is permeable to many small ions, whilst the inner membrane is less permeable, with transport proteins embedded in it. It is folded into lamellae, which are stacked up like piles of pennies. Each stack is called a granum. Between the grana are intergranal lamellae
There are two distinct regions within a chloroplast - the stroma and the grana
What is the stroma?
A fluid filled matrix within the chloroplast. The reactions of the light independent stage of photosynthesis occur in the stroma, and it contains the necessary enzymes for this. Within the stroma are starch grains and oil droplets, as well as DNA and prokaryote-type ribosomes
What are the grana?
Stacks of flattened membrane compartments called thylakoids. These are the sites of light absorption and ATP synthesis during the light dependent stage of photosynthesis. The thylakoids can only be seen using an electron microscope.
What is the purpose of the inner membrane of the chloroplast?
It can control entry and exit of substances between the cytoplasm and the stroma inside the chloroplasts, using it’s transport proteins.
How are chloroplasts adapted for photosynthesis?
- the many grana, with stacks of up to 100 thylakoid membranes provide a large surface area for photosynthetic pigments, electron carriers and ATP synthase enzymes, all involved in the light dependant reaction.
- the photosynthetic pigments are arranged in photosystems
- proteins in the grana hold the photo systems in place.
- the stroma contains all of the enzymes to catalyse the light independent stage
- the grana are surrounded by the stroma so the products of the light dependent reaction can pass on to the light independent reaction easily.
- chloroplasts can make some of the proteins they need for photosynthesis using chloroplast DNA and ribosomes
What are photosynthetic pigments?
Substances that absorb certain wavelengths of light and reflect others. They appear to us as the colour of the light wavelengths they’re reflecting.
Where are photosynthetic pigments found in the chloroplast?
They are found in the thylakoid membranes, arranged in funnel shaped structures called photosystems, held in place by proteins
What is the structure of a photosystem?
Photosystems are funnel shaped, with the primary pigment reaction centre at the base and accessory pigments leading out from it in a funnel shape. Light energy absorbed by the accessory pigments is passed down the funnel to the primary pigment reaction centre.
What is a primary pigment reaction centre?
The primary pigment reaction centre of a photosystem is found at the centre of the photosystem and can be one of two types of chlorophyll: P680 or P700.
What is the difference between P680 and P700?
P680 is found in photosystem II and it’s peak absorption is light at a wavelength of 680nm
P700 is found in photosystem I and it’s peak absorption is light at a wavelength of 700nm
Where does the light dependent stage of photosynthesis take place?
On the thylakoid membranes of the chloroplasts
What happens in the light dependent stage of photosynthesis?
Light energy is absorbed by photosynthetic pigments in the Photosystems and converted to chemical energy. This energy is used to add a phosphate group to ADP to make ATP and to reduce NADP. During the process, water is oxidised to oxygen.
What products of the light dependent reaction are used in the light independent reaction?
ATP is used to transfer energy and reduced NADP transfers hydrogen to the light independent reaction.
Where does the light independent stage of photosynthesis take place?
In the stroma of the chloroplast
What happens in the light independent stage of photosynthesis?
Glucose is made from CO2, with energy and hydrogen from the ATP and reduced NADP produced in the light dependent reaction
What are the three main things the light energy absorbed by Photosystems is used for in the light dependent reaction?
- making ATP from ADP and inorganic phosphate
- making reduced NADP from NADP
- splitting water into protons (H+ ions), electrons and oxygen.
What is photolysis?
The splitting of water into hydrogen ions, oxygen and electrons, using light energy
What is photophosphorylation?
The process of adding phosphate to a molecule using light; in the case of the light dependent reaction phosphate is added to ADP to make ATP
What are the two types of photo phosphorylation in the light dependent reaction?
Cyclic photophosphorylation and non-cyclic photophosphorylation
What are the products of non-cyclic photophosphorylation?
ATP, reduced NADP and oxygen
Photosystems in the thylakoid membranes are linked by …?
Electron carriers
What are electron carriers?
Proteins that transfer electrons
What happens during non-cyclic photophosphorylation?
- light energy excites electrons in chlorophyll in PSII
- these electrons move down the electron transport chain to PSI
- photolysis of water produces H+ ions, electrons and oxygen
- the electrons from the water replace those lost from PSII
- energy from the excited electrons is lost as they move along the electron transport chain, and is used to make ATP
- the electrons are accepted by PSI
- light energy absorbed by PSI excites the electrons again
- the electrons are transferred to NADP along with a proton, to form reduced NADP
What effect does light energy have on electrons in chlorophyll?
It excites them, causing them to move to a higher energy level
What happens to the products of photolysed water in the light dependent reaction?
The electrons are used to replace those lost from photosystem II.
The hydrogen ions take part in chemiosmosis to make ATP from ADP and Pi
The oxygen is released into the atmosphere
What is chemiosmosis?
The flow of protons down their concentration gradient across a membrane, through an enzyme called ATP synthase. The force of this flow generates ATP from ADP and Pi
What process, other than photosynthesis, is chemiosmosis a part of?
Respiration
What happens to the energy lost from the electrons as they move down the electron transport chain in non-cyclic photophosphorylation?
The energy is used to transport hydrogen ions from the stroma into the thylakoid, so that the thylakoid has a higher concentration of protons, and there is a concentration gradient. This allows protons to move down their concentration gradient into the stroma via ATP synthase, so that ATP can be produced
What does cyclic photophosphorylation produce?
ATP
What photosystems are used in non cyclic and cyclic photophosphorylation?
In cyclic photophosphorylation only photosystem I is used.
In non cyclic photophosphorylation both photosystems I and II are used.
What happens during cyclic photophosphorylation?
Light hits PSI and electrons in chlorophyll are excited. These electrons flow through an electron transport chain, and ATP is produced from the energy they lose. They are then passed back to PSI and are recycled.
What is another name for the light dependent cycle?
The Calvin cycle
Where does the Calvin cycle take place?
In the stroma of chloroplasts
What does the Calvin cycle produce?
Triose phosphate (TP) and ribulose bisphosphate, a 5 carbon compound
What can triose phosphate be used for?
It can be used to make glucose and other useful organic substances, such as carbohydrates, lipids and amino acids.
What substances are used in the Calvin cycle?
CO2, energy from ATP, H+ ions from reduced NADP
What happens in the Calvin cycle?
- CO2 is combined with ribulose bisphosphate (RuBP), catalysed by rubisco
- this produces two molecules of glycerate 3-phosphate (GP), which each have 3 carbons
- GP is reduced and phosphorylated to triose phosphate, using energy and a phosphate group from ATP and H+ ions from reduced NADP
- five out of every six molecules of triose phosphate are recycled to RuBP, using energy from ATP
- the remaining molecules of triose phosphate are converted to useful organic compounds
What proportion of triose phosphate molecules produced in the Calvin cycle are used to regenerate ribulose bisphosphate?
5/6
What is a hexose sugar?
A monosaccharide that has six carbon atoms, e.g. Glucose.
How are hexose sugars made?
By joining two molecules of triose phosphate (3C) together
How many times must the Calvin cycle turn to produce one hexose sugar?
Six times. Each turn of the cycle produces two triose phosphate molecules, but as 5 out of every 6 molecules produced are used to make RuBP, three turns of the cycle are needed to produce one molecule of triose phosphate that can be used to make a hexose sugar.
How is glycerate 3-phosphate produced in the Calvin cycle?
Carbon dioxide is combined with RuBP, catalysed by rubisco, producing two molecules of glycerate 3-phosphate
How is triose phosphate produced in the Calvin cycle?
Glycerate 3-phosphate is reduced and phosphorylated to make TP, using ATP and reduced NADP from the light dependent stage
What is rubisco?
An enzyme that catalyses the combination of carbon dioxide and ribulose bisphosphate in the Calvin cycle
How is RuBP produced in the Calvin cycle?
It is produced from two molecules of triose phosphate, using energy from ATP produced in the light dependent reaction. As there are 6 carbons in 2 molecules of TP, and 5 carbons in 1 molecule of RuBP, one carbon is left over. This is used to make useful organic compounds
How are carbohydrates made?
Hexose sugars such as glucose and fructose are made from two triose phosphate molecules. Larger carbohydrates, such as sucrose, starch and cellulose, are made by joining hexose sugars together in different ways
How are lipids made?
Using glycerol, which is synthesised from triose phosphate, and fatty acids, which are synthesised from glycerate 3-phosphate
Which product of the Calvin cycle can be used to make amino acids?
Glycerate 3-phosphate
Where does the carbon dioxide used in the Calvin cycle come from?
It diffuses into the plant through the stomata, then diffuses into cells and into the stroma of the chloroplasts
What are the optimum conditions for photosynthesis?
- high light intensity of a certain wavelength
- temperature around 25°C
- carbon dioxide concentration at 0.4%
- a good supply of water
Why is high light intensity of a certain wavelength needed for photosynthesis?
Light is needed to provide the energy for the light dependent reaction - the higher the intensity the more energy it provides. However only certain wavelengths of light are used for photosynthesis. The photosynthetic pigments in chlorophyll a, chlorophyll b and carotene only absorb the red and blue light in sunlight
What colours of sunlight are absorbed by the photosynthetic pigments in photosystems?
Red and blue
Why is a temperature of around 25°C optimum for photosynthesis?
Photosynthesis involves enzymes (ATP synthase, rubisco). These are more active at higher temperatures, but if the temperature is more than 45°C they start to denature. Also at high temperatures stomata close to avoid too much water loss, causing photosynthesis to slow down because less carbon dioxide enters the leaf
Why is a CO2 concentration of 0.4% optimum for photosynthesis?
Carbon dioxide makes up 0.04% of the gasses in the atmosphere. Increasing this to 0.4% gives a higher rate of photosynthesis, but any higher and the stomata start to close.
Why is water necessary for photosynthesis?
If a plant has too little water, photosynthesis has to stop as water needs to be photolysed in the light dependent reaction so that the protons from water can replace those lost from PSII
What factors can limit photosynthesis?
- light intensity
- temperature
- CO2 concentration
What will happen to photosynthesis if one of the limiting factors is too low or too high?
It will limit photosynthesis, slowing it down. Even if the other two factors are at the perfect level it will make no difference to the rate of photosynthesis as long as that factor is at the wrong level
What is the saturation point of a limiting factor on photosynthesis?
The point at which the factor is no longer limiting the reaction, and something else has become the limiting factor
How can the rate of photosynthesis, and the effect of light intensity, temperature and CO2 concentration on the rate of photosynthesis, be measured?
Using pondweed. The rate at which oxygen is produced by pondweed can be easily measured, and this corresponds to the rate of photosynthesis
How can the rate at which oxygen is produced by pondweed be measured?
Pondweed should be placed in a test tube filled with water, with a capillary tube, also filled with water, in place above the pond weed. As gas is given off by the plant, an air bubble will be formed in the capillary tube. The length of this can be measured and, using this and the radius of the capillary tube, the volume of oxygen produced can be measured.
How do commercial growers prevent carbon dioxide concentration from limiting the growth of their plants?
In greenhouses, with plants in close proximity to one another, the CO2 concentration in the air can get very low. Farmers can increase the concentration by burning methane
How do commercial growers prevent light intensity from limiting the growth of plants?
In greenhouses light can get in through the glass. Farmers can also add lamps to increase light intensity, and provide light at night time
How do commercial growers prevent temperature from limiting the growth of plants?
Greenhouses trap the heat energy from sunlight, which warms the air. Heaters and cooling systems can also be used to keep a constant optimum temperature.
What effect does low light intensity have on the Calvin cycle?
The products of the light dependent stage will be in short supply, so the phosphorylation and reduction of GP to TP will be slow. This means that the levels of GP will rise, as it is still being made but not used, and the levels of TP and RuBP will fall as they’re being used up but not remade as quickly.
What effect does temperature have on the Calvin cycle?
All the reactions in the Calvin cycle are catalysed by enzymes. At low temperatures all of the reactions will happen more slowly, so levels of GP, TP and RuBP will all fall. GP, TP and RuBP are affected in the same way at high temperatures, when enzymes start to denature.
What effect does carbon dioxide concentration have on the Calvin cycle?
At low CO2 concentrations, conversion of RuBP to GP will happen very slowly, as there’s less CO2 to combine with RuBP to make GP. This means that levels of RuBP will rise as it’s being made but not used up, and levels of GP and TP will fall, as they’re being used to make RuBP but not remade.
What is a coenzyme?
A molecule that aids the function of an enzyme. They work by transferring a chemical group from one molecule to another.
What is a coenzyme used in photosynthesis?
NADP. It transfers hydrogen from one molecule to another, meaning it can reduce or oxidise a molecule
