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Biology A Level > Photosynthesis > Flashcards

Photosynthesis Flashcards

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1
Q

Where does the light dependent stage of photosynthesis occur?

A

In the grana (stacks of thylakoids) of chloroplasts

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2
Q

What does the light dependent stage of photosynthesis consist of?

A
  • Light harvesting at he photosystems
  • Photolysis of water
  • Photophosphorylation (production of ATP in presence of light)
  • Formation of NADPH (reduced NADP)
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3
Q

What are the two different types of photosystems?

A
  • Photosystem 1 (PS1), PS1 contains chlorophyll a pigment at the primary reaction with a peak absorption of light of wavelength 700nm
  • Photosystem 2 (PS2), PS2 contains chlorophyll a pigment at the primary reaction with a peak absorption of light of wavelength 780nm
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4
Q

Where does photolysis occur?

A

PS2

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5
Q

How does photolysis occur?

A
  • An enzyme in PS2 splits water into protons, electrons and oxygen
  • 2H2O → 4H+ + 4e- + O2
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6
Q

What is the oxygen produced in photolysis used for?

A

Some of the oxygen produced in photolysis is used for aerobic respiration in the plant cells, however in periods of high light intensity, the rate of photosynthesis is greater than the rate of respiration so oxygen diffuses out of the leaves through the stomata into the surrounding atmosphere

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7
Q

What are the two types of photophosphorylation?

A
  • Non-cyclic photophosphorylation occurs in PS1 and PS2, it produces ATP, oxygen and reduced NADP (NADPH)
  • Cyclic photophosphorylation involves only PS1, produces ATP but in smaller quantities than in non-cyclic phosphorylation
  • Both types of photophosphorylation involve iron based electron carriers embedded in the thylakoid membrane
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8
Q

What is the process of non-cyclic photophosphorylation?

A
  • A photon of light strikes PS2 (P680), its energy is channelled to the primary pigment reaction centre
  • Light energy excites a pair of electrons inside the chlorophyll molecule
  • Energised electrons escape from the chlorophyll molecule and are captured by an electron carrier (a protein with iron at its centre) embedded in the thylakoid membrane
  • The electrons that have been excited and escaped the chlorophyll molecule are replaced by electrons derived from photolysis
  • When an electron combines with the Fe3+ ion in the electron carrier, the Fe3+ becomes reduced to an Fe2+, the Fe2+ can then donate the electron to the next electron carrier in the chain to become reoxidised to Fe3+ again
  • As electrons are passed along the chain of electron carriers embedded in the thylakoid membrane, energy is released each time an electron is passed on
  • This energy is used to pump protons across the thylakoid membrane and into the thylakoid space
  • Eventually the electrons moving along the electron transport chain are captured by another molecule of chlorophyll a in PS1, this replaces the electrons lost from PS1 due to excitation by light energy (this happens in cyclic photophosphorylation)
  • The electrons from PS1 are accpeted by an iron sulfur complex called ferredoxin and they are passed to NADP in the stroma
  • As protons accumulate in the thylakoid space, a proton gradient is formed across the membrane
  • Protons diffuse down their concentration gradient through ATP synthase channel proteins, the flow of protons provides energy for the synthesis of ATP from ADP and an inorganic phosphate molecule
  • As the protons pass through the channel, they’re accepted along with electrons by NADP to form NADPH, the reduction of NADP is catalysed by NADP reductase
  • ATP and NADP are now in the stroma ready for the light independent stage of photosynthesis
  • Oxygen is also produced from the photolysis of water that happens at the beginning of the light dependent stage
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9
Q

What is the process of cyclic photophosphorylation?

A
  • As a photon of light strikes PS1, a pair of electrons in the chlorophyll molecule (P700) become excited and escape the chlorophyll molecule
  • These electrons pass through an electron carrier system and pass back to PS1
  • During the passage of electrons along the electron carriers, a small amount of ATP is produced but not as much as in non-cyclic photophosphorylation
  • Cyclic photophosphorylation only produces ATP, not NADPH or oxygen because photolysis of water doesn’t occur
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10
Q

How do guard cells open stomata?

A
  • Chloroplasts in guard cells only contain PS1 meaning they only produce ATP
  • This ATP is used to to actively bring potassium ions (K+) into the cell, this lowers the water potential of the guard cells which causes water to enter the cells by osmosis
  • This causes the guard cells to swell and the stomata to open
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11
Q

How does CO2 enter a leaf?

A

CO2 from the air enters the leaf through the stomata and diffuses through the spongey mesophyll layer and into the palisade layer, into the palisade cells through their thin cellulose cell walls through the chloroplast envelope and into the stroma, here it is fixated into organic molecules which helps to maintain a concentration gradient (higher concentration in air than in the leaf) that aids the diffusion of CO2 into the stomata

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12
Q

Describe the process of the Calvin cycle

A
  • CO2 combines with a carbon dioxide acceptor called RuBP (5C), this is catalysed by RuBisCO
  • RuBP accepts the carboxyl group (COO-) and forms an unstable intermediate (6C) that immediately breaks down into 2GP (3C), the carbon dioxide has now been fixed
  • 2GP are then reduced, using hydrogens from the NADPH made in the light dependent stage, to triose phosphate (3C)
  • 2 molecules of ATP are hydrolysed for each molecule of CO2 that is fixed, one molecule of NADPH is needed to reduce each molecule of GP
  • In 10 out of every 12 TP molecules the atoms are rearranged to regenerate six molecules of RuBP, this process requires phosphate groups
  • The remaining 2 TP molecules are the product
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13
Q

How many molecules of TP are required to form 1 molecule of glucose?

A

2

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14
Q

How many TP molecules out of 12 are reconverted into RuBP

A

10, 5 molecules of RuBP are formed

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15
Q

How many turns of the Krebs cycle are required to form 2 molecules of TP?

A

6

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16
Q

Why can the Calvin cycle only take place in daylight?

A
  • It requires products formed in the LDR (ATP and NADPH)
  • Also, in the LDR, protons are pumped from the stroma and into the thylakoid space which raises the pH to around 8 which is optimum for the enzyme RuBisCO
  • RuBisCO is also activated by the increased amount of ATP in the stroma
  • In daylight the concentration of Mg2+ ions increases in the stroma, these ions attach to the active site of RuBisCO and act as cofactors to activate it
  • The ferredoxin that is reduced by electrons in PS1 activates enzymes involved in the Calvin cycle
17
Q

How can TP molecules be used to synthesise organic compounds?

A
  • Some glucose, formed from two molecules of TP, is converted to sucrose, some to starch and some to cellulose
  • Some TP is used to synthesise amino acids, fatty acids and glycerol which can be undergo a condensation reaction to form lipids
18
Q

What are the limiting factors of photosynthesis?

A
  • CO2, water and light intensity
  • Availability of chlorophyll, electron carriers and relevant enzymes
  • Temperature and turgidity of the cell is also importatn
19
Q

What is the role of light in photosynthesis?

A
  • Light provides energy to excite the electrons in photosystems 1 and 2
  • It also is essential for the photolysis of water, photolysis is essential for producing ATP and NADPH which are necessary for the next stage of photosynthesis (the light independent reaction) to take place
  • Light also causes the stomata to open, when stomata are open, transpiration occurs which leads to uptake of water and it’s delivery to leaves
  • The stomata being open also means CO2 can diffuse into the leaves, CO2 is the starting molecule in the Calvin cycle
20
Q

What is the effect of decreasing light intensity on the Calvin cycle?

A
  • GP can’t be reduced to TP as ATP isn’t present from the light dependent reaction
  • TP levels fall and GP accumulates
  • TP falls meaning RuBP can’t be regenerated so the cycle stops
  • NADPH won’t be present, this means that the GP won’t be able to accept hydrogens to form TP
21
Q

What is the effect of decreasing CO2 concentration on the Calvin cycle?

A
  • RuBP accumulates as CO2 is not present for RuBP to combine with to form GP
  • This means that TP can’t be formed either as GP isn’t formed in the first place
22
Q

What is the effect of changing temperature on the Calvin cycle?

A
  • From low temperatures to temperatures of 25-30º, if light intensity is high enough, there is a sufficient amount of water and a high enough CO2 concentration, rate of photosynthesis increases as temperature increases
  • At temperatures of 30º and above, the rate of photosynthesis decreases as oxygen is able to compete with CO2 for RuBisCO’s active site, this reduces the rate at which RuBP combines with CO2, the 2TP that is produced as a product for every 6 turns of the Calvin cycle will still be converted into glucose or other organic molecules at the same rate meaning that even if less than 12TP molecules are available, 2 glucose molecules will be produced as a product. This means that less RuBP will be regenerated for each RuBP molecules that combines with CO2
  • At temperatures of above 45º enzymes involved in photosynthesis may be denatured, this would mean that no RuBP is regenerated as the other enzymes involved in the Calvin cycle would not be able to function properly, so concentration of RuBP will decrease as the TP will be converted into glucose and other organic molecules at the same rate while the Calvin cycle is continuing at a rate of almost 0, meaning that RuBP concentration will decrease to 0. Concentration of GP and TP will decrease as they will be converted into glucose at a faster rate than the Calvin cycle is taking place
23
Q

What are the effects of water stress (not enough water being present)?

A
  • Roots are unable to uptake water to replace that lost in transpiration
  • Cells lose water and become plasmolysed
  • Plant roots produce abscisic acid that when translocated to the leaves causes the stomata to close, reducing gaseous exchange
  • Tissues become flaccid and leaves wilt
  • The rate of photosynthesis greatly decreases

Biology A Level (21 decks)

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