11. PHOTOSYNTHESIS Flashcards
Write a full and balanced photosynthesis symbol equation
6CO2 + 6H2O (+ light) → C6H12O6 + 602
Where does the energy for photosynthesis originate from?
Sunlight
Name five ways the glucose produced from photosynthesis is used
Respiration, stored as starch, converted into amino acids, cellulose and lipids
Name three parts of a plant that contains photosynthesising cells
Leaves, stems and branches
Describe the two adaptations of leaves that maximises light absorption
They have a large surface area and are arranged to minimise overlapping
Why are leaves thin?
To provide a short diffusion distance, and to ensure light can be absorbed in the first few μm (micrometer)
Why are the leaf cuticle and epidermis transparent?
To allow light to pass through to the photosynthetic palisade mesophyll tissue underneath
Why is the waxy cuticle waterproof?
To reduce the rate of transpiration from the top of the leaf
How is the palisade mesophyll tissue adapted for a maximum rate of photosynthesis?
The cells are tightly packed together, and the cells contain more chloroplasts
What is the function of stomata?
To allow gas exchange between the atmopshere and the inside of the leaf
What do the stomata open and close in response to?
Changing light intensity and changing temperature
How is the spongy mesophyll tissue adapted to its function?
It has air spaces between cells to allow gas exchange between the atmopshere and the palisade mesophyll tissue
Name the three photosynthetic pigments found in the chloroplast
Chlorophyll a, chlorophyll b and carotenoids (accessory pigments)
Which colours are absorbed by the photosynthetic pigments?
Violet, blue and red are absorbed
Which colours are reflected by the photosynthetic pigments?
Green, yellow and orange are reflected
Why do plants have more than one photosynthetic pigment?
To absorb a wider range of wavelengths of light
Name the two reactions that make up the process of photosynthesis
Light Dependent Reaction and Light Independent Reaction
Where does the LDR occur?
In the thylakoid membranes of the chloroplast (or grana)
What is the first stage in the LDR?
Photoionisation
Describe photoionisation
Chlorophyll absorbs light energy, which excites electrons. Two electrons leave the chlorophyll
What is the purpose of photolysis?
To replace the electrons lost from the chlorophyll in photoionisation
Write a full and balanced symbol equation for photolysis
H2O → 2H+ + 2e- + 1/2 O2
What happens to the O2 formed from photolysis?
It is used in respiration / diffuses out of the leaf as waste
What happens to the H+ formed from photolysis?
They bond with NADP+ and 2e- to make NADPH
Describe what happens to the excited electrons from photoionisation
They travel down the electron transport chain releasing energy
How is the energy used that is released from the excited electrons as they travel down the ETC?
It is used to pump H+ from the stroma into the thylakoid lumen
What does the pumping of the H+ from the stroma to the lumen create?
An electrochemical gradient over the thylakoid membrane
Describe chemiosmosis
H+ flow through ATP Synthase down an electrochemical gradient
Describe how chemiosmosis facilitates the production of ATP
The flow of H+ through ATP Synthase causes it to bond ADP and Pi to make ATP
What is phosphorylation?
The bonding of ADP and Pi to create ATP
Write an equation for the production of ATP by phosphorylation
ADP + Pi → ATP
Where is ATP Synthase found?
It is embedded in the thylakoid membranes at the end of the ETC
What is the final electron acceptor in the ETC?
NADP+
Describe how NADPH is produced in the LDR
Two electrons from the ETC bond with NADP+ and H+ to form NADPH
What is a coenzyme?
A molecule that transports particles from one reaction to another
Name the coenzyme used in photosynthesis
NADPH
What happens to the NADPH and ATP produced from the LDR?
They are transported to the stroma where they are used in the LIR
What are the reduced and oxidised versions of the NADP+/NADPH ?
Reduced version = NADPH
Oxidised version = NADP+
Where does the LIR occur?
In the stroma
Name the stages of the LIR
Carbon fixation, reduction and regeneration
Apart from ATP and NADPH, name all the molecules involved in the LIR, and how many carbon atoms they contain
RuBP (5C)
CO2 (1C)
GP (3C)
Triose Phosphate (3C)
Glucose (6C)
Describe what happens in the carbon fixation stage of the LIR
1 molecule of RUBP bonds with 1 molecule of CO2 (catalysed by rubisco) to form 2 molecules of GP
Describe what happens in the reduction stage of the LIR
2 molecules of GP are reduced to 2 molecules of TP by 2 molecules of NADPH using energy released from 2 molecule of ATP
How many carbon atoms from 2 molecules of Triose Phosphate are used to make glucose?
One
Describe what happens in the regeneration stage of the LIR
Triose Phosphate is regenerated into 1 molecule of RuBP using the energy from 1 molecule of ATP
How many turns of the calvin cycle does it take to produce one moelcule of glucose?
Six
How many molecules of ATP and NADPH are used in one turn of the calvin cycle?
3 x ATP
2 x NADPH
How many molecules of ATP and NADPH are used to form one molecule of glucose?
18 x ATP
12 x NADPH
Name the three limiting factors of photosynthesis
Light intensity, temperature and carbon dioxide concentration
Explain why increasing light intensity increases the rate of the light dependent reaction of photosynthesis?
It increases the light energy absorbed by the chlorophyll, which increases the number of electrons travelling down the ETC, which increases the NADPH & ATP formed.
Explain why increasing light intensity increases the rate of the light independent reaction of photosynthesis?
It increases the NADPH & ATP formed in the LDR, which increases the glucose formed in the LIR
Describe the relationship between light intensity and rate of photosynthesis when light is a limiting factor
It is directly proportional
Why does the rate of photosynthesis fluctuate through a normal day?
Because the light intensity changes
Describe how the rate of photosynthesis fluctuates through a normal day
The rate of photosynthesis will be zero at night, then increase as the sun rises in the morning. It will peak at the warmest and lightest part of the day (early afternoon), and reduce again to zero as the sun sets
What is a plant’s compensation point?
The point at which the rate of photosynthesis and rate of respiration are the same
When a plant is operating above its compensation point, which gas is there a net production of?
Oxygen
When a plant is operating below its compensation point, which gas is there a net production of?
Carbon Dioxide
Why is a plant not able to grow when it is operating below its compensation point?
Because there is no excess glucose for the plant to convert into amino acids for growth
Describe three adaptations of shade-adapted plants
Lower respiratory losses (lower rate of respiration)
More chloroplasts per cell
More thylakoids per chloroplast
Why does increasing temperature above a certain point cause the rate of photosynthesis to reduce to zero?
Because rubisco will become denatured, reducing the rate of carbon fixation to zero
Describe the relationship between temperature and rate of photosynthesis when temperature is a limiting factor
It is directly proportional
Why does increasing temperature up to the optimum increase the rate of photosynthesis?
Because increasing temperature increases the rate of rubisco action
Describe the stress response in plants
Stomata close to reduce the rate of transpiration. This can also reduce the rate of photosynthesis.
What is photorespiration?
When oxygen binds to rubisco instead of carbon dioxide
Name the two conditions under which photorespiration will occur
High temperatures
A low ratio of carbon dioxide to oxygen
What are the effects of photorespiration?
Less GP, Triose Phosphate and sugars are made.
RuBP also takes longer to regenerate.
Name two ways farmers and food producers maintain optimum conditions to maximise the rate of photosynthesis in their crops
They will maintain the optimum temperature for rubisco action
They will increase the ratio of carbon dioxide to oxygen
Describe the relationship between carbon dioxide concentration and rate of photosynthesis when carbon dioxide concentration is a limiting factor
It is directly proportional
Why is carbon dioxide concentration most likely to be the limiting factor in nature?
The optimum carbon dioxide concentration for photosynthesis is 1%, however the carbon dioxide concentration in the atmosphere is ~0.04%
Why would increasing carbon dioxide concentration increase the rate of photosynthesis?
The rate of carbon fixation would increase, which would increase the rate of sugars made in the LIR
When carbon dioxide concentration is not limiting, why are the levels of GP and RuBP constant?
GP and RuBP are being formed at the same rate as they are being used
When carbon dioxide concentration is not limiting, why is there twice as much GP as RuBP?
Because one molecule of RuBP binds with CO2 to make two molecules of GP
When carbon dioxide becomes a limiting factor, why does the amount of RuBP increase?
There is less CO2 to bind with RuBP in carbon fixation, therefore less RuBP is converted into GP
When carbon dioxide becomes a limiting factor, why does the amount of GP decrease?
There is less CO2 to bind with RuBP in carbon fixation, therefore less RuBP is converted into GP
Name the two forms of DCPIP, and their colours
Oxidised form = blue
Reduced form = colourless
What is DCPIP used to indicate in plants?
The presence of the LDR (by indicating the presence of electrons)
Where are the electrons coming from that are accepted by DCPIP?
The electron transport chain
How would altering the volume of DCPIP used alter the time it took to decolourise?
The higher the volume, the longer the time it would take to decolourise
Why were the leaves blended before being used in the DCPIP experiment?
To release the chloroplasts from the cells
Why is it important not to overblend the leaves in the DCPIP experiment?
So as not to damage the chloroplasts
Why does the isolating medium need to be ice cold in the DCPIP experiment?
To reduce enzyme activity as much as possible before the experiment begins to reduce the amount of photosynthesis prior to the experimental period
What are the two experimental proxies we use to measure the rate of photosynthesis?
Oxygen produced and carbon dioxide uptake
Which experimental proxy for photosynthesis is most useful when measuring the rate of the LDR?
Oxygen produced
Which experimental proxy for photosynthesis is most useful when measuring the rate of the LIR?
Carbon dioxide uptake
