Photosynthesis

Question 1

Define autotrophs

Answer 1

• organisms that synthesis their own organic carbon compounds from light or chemical energy

Question 2

Define photoautotrophs

Answer 2

• organisms that synthesise their own organic carbon compounds using light energy via photosynthesis

Question 3

Define heterotrophs

Answer 3

• organisms that rely on other organisms for organic carbon compounds

Question 4

What are the two stages of photosynthesis and where in the chloroplast do each of them happen?

Answer 4

• light dependent stage.. happens in the thylakoid membranes

- light independent stage… happens in the stroma

Question 5

What organelles and features can you find in a chloroplast?

Answer 5

• double membrane(inner and outer)
• own DNA
• ribosomes
• thylakoid membranes
• stacks of thylakoid membranes called grant
• grana joined by lamellae
• fluid called stroma
• starch grains

Question 6

Why are there starch grains in the chloroplast?

Answer 6

• glucose made in photosynthesis that isn’t used up in respiration gets stored in the form of starch.

Question 7

How are chloroplasts adapted for their function?

Answer 7

• many thylakoid membranes maximise the surface area more photosystems containing pigment molecules which can maximise light absorption
• stroma contains appropriate pH and enzymes for the Calvin cycle to occur

Question 8

How is light harvested in the light dependent stage?

Answer 8

• there are many different photosynthetic pigments on the photo system like chlorophyll, carotenoids and xanthophylls which can absorb different wavelengths of light
• accessory pigments can absorb a greater variety of light wavelengths
• this light energy passes from pigment to pigment on the photo system until it reaches the primary pigment on the reaction centre
• the electrons are now excited

Question 9

What happens in the light dependent stage?

Answer 9

• photosynthetic pigments on PS11 absorb photons and become excited
• these leave PS11 and are accepted by an electron carrier
• this is photoionisation
• photolysis happens at PS11 whirred water is broken down into 2H+, 2e- and an oxygen
• electrons lost by PS11 replaced
• the electrons that left PS11 enter the electron transport chain
• as they travel to each neighbouring electron carrier, they lose energy
• this energy is used to pump hydrogen ions through the protein molecules from the stroma to the thylakoid lumen
• H+ now moves down concentration gradient through ATP synthase enzymes
• this chemiosmosis causes the production of ATP… photophosphorylation
• the electron arrives at PS1 and is excited again by photons
• it enters a second electron transport chain resulting in the production of more ATP
• at the end of the electron transport chain., the electron is accepted with a H+ by oxidised NADP to from reduced NADP

Question 10

What happens in the light independent stage?

Answer 10

• reduced NADP and ATP from the light dependent reactions are used in the light independent reactions
• carbon dioxide which diffuses in through stomata is fixed by ribulose bisphosphate to produce an unstable 6C intermediate
• this is catalysed by the most abundant enzyme, RuBIsco
• as it is very reactive and unstable, it splits to form three glycerate-3 phosphate molecules.
• glycerate-3 phosphate is reduced into triose phosphate molecules using ATP and reduced NADP
• majority of the TP is regenerated into RuBP, by using ATP, so that the Calvin cycle can continue

Question 11

What is the fate of glycerate-3 phosphate and triose phosphate.

Answer 11

• glycerate three phosphate synthesises fatty acids and triose phosphate synthesises glycerol molecules
• triose phosphate produces alpha and beta glucose
• the glucose molecules can be used to synthesis sucrose
• triose phosphate synthesises nuclei acids, amino acids

Question 12

What is the difference between cyclic and non cyclic photophosphorylation

Answer 12

• cyclic only contains one photosystem and the same electron is recycled so there is no formation of reduced NADP

Question 13

What are the limiting factors of photosynthesis?

Answer 13

• light intensity
• temperature
• carbon dioxide concentration

Question 14

What would happen if a plant was placed in a dark room?

Answer 14

• there would be absence of light
• the pigment molecules cannot absorb photons and therefore excite any electrons
• electrons can’t enter the electron transport chain and so chemiosmosis and photophosphorylation can’t happen
• no production of ATP and reduced NADP
• accumulation of RuBP but concentrations of GP and TP would decrease
• over time the concentration of RuBP will decrease because there isn’t enough TP to regenerate the RuBP

Question 15

How will temperature affect the rate of photosynthesis?

Answer 15

• very low temperatures will cause enzymes to be inactive due to the lack of kinetic energy and successful collisions between the enzyme and substrate
• optimums temperature is where the rate of photosynthesis is the highest
• if the temperature continues to increase, there will be increased vibrations within the enzyme causing binds to break, the tertiary structure to change and so the active site to change shape
• enzyme denaturation
• rate of photosynthesis stops

Question 16

How does carbon dioxide concentrations increase?

Answer 16

• If the carbon dioxide concentrations are too low, there will be competitive inhibition as oxygen is also complementary to the active site of rubisco
• rate of photorespiration increases
• if they increase, the rate of photosynthesis can increase but only till a point because then another factor becomes a limiting factor

Question 17

Increasing the temperature decreases the rate of photosynthesis for another reason that isn’t the denaturation of enzymes. What is it?

Answer 17

• increasing the temperature also increases successful collisions between oxygen and rubisco so rate of photorespiration increases.

Question 18

How can water stress affect rate of photosynthesis?

Answer 18

• if there is a lack of water uptake by roots, water will diffuse out of the plant cells causing them to become plasmolysed
• leaves will wilt and this will prevent maximum absorption of light
• guard cells also lose water resulting in the closure of stomata preventing CO2 from diffusing in for photosynthesis

Question 19

How many cycles of the Calvin cycle to produce one molecule of glucose?

Answer 19

6

Question 20

How does light intensity affect the stomata.

Answer 20

• increase in light intensity also causes stomata to open
• more CO2 can diffuse in
• increased CO2 concentrations can cause the rate of photosynthesis to increase up to a point when another factor limits the rate

Question 21

do plants have fat droplets as a store of energy and if not, why would there be fat droplets in chloroplasts?

Answer 21

• no, they prefer storing energy in the form of starch
• fat droplets could be for membrane formation or cholesterol formation or glycolipid formation
• fatty acids can be used for the production of pigment molecules

Question 22

what is the difference between cyclic and non cyclic photophosphorylation

Answer 22

• non cyclic photophosphorylation involves both photosystems
• cyclic photophosphorylation only involves photosystem 1, there is no splitting of water, when the electrons reach the end of the electron transport chain, they return back to the start of the chain to photosystem 1 instead of helping making reduced NADP and there is no oxidised NADP in the first place
• ATP is the only product of cyclic photophosphorylation

Question 23

why does cyclic photophosphorylation occur?

Answer 23

• to save energy and time
• cyclic photophosphorylation is just the process by which organisms such as prokaryotes concert ADP to ATP for immediate energy for the cells
• the chloroplast shifts to this process when the levels of ATP drop and the levels of reduced NADP rises
• often, the amount of ATP required to keep the Calvin cycle going is more than what is produced by non-cyclic photophosphorylation
• Calvin cycle will slow or even stop without sufficient AtP
• CYCLIC PHOTOPHOSPHORYLATION occurs until the ATP levels have been replenished

Question 24

What are some of the proteins that DNA needs to code for that’s involved in photosynthesis?

Answer 24

• ATP synthase
• enzyme involved in photolysis
• protein electron carriers
• enzymes for Calvin cycle like rubisco
• for photosystem synthesis

Question 25

what is the formula to find the volume of a cylinder?

Answer 25

• PIr^2h

Question 26

what happens when dry ice melts?

Answer 26

• it turns into carbon dioxide gas

Question 27

why is it not possible for all the oxygen produced by photosynthesis to not be collected?

Answer 27

• some may be used for oxidative phosphorylation in aerobic respiration
• some may escape the collection apparatus
• some may be trapped in air spaces in the leaf
• some may dissolve in the water

Question 28

What features would plants in the shade have?

Answer 28

• features that would help maximise photosynthesis

Question 29

What features would plants that are exposed to a lot of sun have?

Answer 29

• features that would reduce transpiration

Question 30

what are some precautions you need to take in this experiment investigating photosynthesis and why?

Answer 30

• the plants should be the same size so they have the same surface area allowing for the same number of pigment molecules
• sample should be taken from the same part of the plant so there are the same number of pigment molecules
• the same thickness of the filter so the light intensity is the same
• the same distance of light source or lamp so the light intensity is the same
• there should be only one source of light in the room so only light of desired wavelength occurs
• same volume and concentration of indicator so the colour changes are comparable
• carry out repeats to calculate means and identify anomalies
• shield between light source and tube to prevent temperature changes
• constant temperature as temperature affects enzyme activity

Question 31

how can we control things better in greenhouses?

Answer 31

• the glass can control the light intensity
• burning fuel to control carbon dioxide concentrations
• heater to increase temperature and fan to reduce
• prevents damage to plants by wind, rain, hail