Chapter 17- Energy for Biological Process

Question 1

What is autotrophic nutrition?

Answer 1

• organisms that can use chemical energy to synthesise large organic molecules, which form the building blocks of living cells, from simple inorganic molecules like H20 + C02

Question 2

What are photoautrophs?

Answer 2

• organisms that photosynthesise
  they use light as the energy source for autotrophic nutrition

Question 3

What are photoautrophs also known as + why?

Answer 3

• producers bc they are at the first trophic level of a food chain + provide energy and organic molecules to other, non-photosynthetic organisms.

Question 4

What are the 2 stages of photosynthesis?

Answer 4

• Light-dependent stage (energy from sunlight is absorbed + used to form ATP)
  -> H from H20 is used to reduce coenzyme NADP to reduced NADP
• Light-independent stage (H from reduced NADP + CO2 is used to build organic molecules like glucose)
  -> ATP supplies the required energy

Question 5

What are the 2 types of photophosphorylation?

Answer 5

• non-cyclic photophosphorylation (involves PSI + PSII) -> produces ATP, O2 and reduced NADP
• cyclic photophosphorylation (involves PSI) -> produces ATP but in smaller quantities than are made by NCP

Question 6

What do both non-cyclic and cyclic photophosphorylation involve?

Answer 6

• iron-containing proteins embedded in the thylakoid membranes that accept + donate electrons and form an ETS

Question 7

What are the first few steps in non-cyclic photophosphorylation?

Answer 7

• when a photon of light strikes PSII, its energy is channelled to the primary pigment reaction centre
• the light energy excites a pair of electrons inside the chlorophyll molecule
• energised electrons escape from the chlorophyll molecule + are captured by an electron carrier
• these electrons are replaced by electrons derived from photolysis
• when this fe+ combines with an electron, it becomes reduced. it can then donate the electron, becoming reoxidised, to the next EC in the chain
• as electrons are passed along a chain of EC embedded in the thylakoid membrane at each step some energy associated with the electrons is released

Question 8

What are the last few steps in non-cyclic photophosphorylation?

Answer 8

• this energy is used to pump protons across the thylakoid membrane into the thylakoid space
• eventually the electrons are captured by another molecule of chlorophyll a in PSI. These electrons replace those lost from PSI due to excitation by light energy
• a protein-iron-sulfur complex called ferredoxin accepts the electrons from PSI + passes them to NADP in the stroma
• as protons accumulate in the thylakoid space, a proton gradient forms across the membrane
• protons diffuse down their conc gradient through special channels in the membrane associated with ATP synthase enzymes + the flow of protons causes ADP and inorganic phosphate to join, forming ATP
• as protons pass through the channel they are accepted, along with electrons, by NADP which becomes reduced. The reduction of NADP is catalysed by NADP reductase

Question 9

What happens in cyclic photophosphorylation?

Answer 9

• as light strikes PSI, a pair of electrons in the chlorophyll molecule at the reaction centre gain energy + become excited
• they escape from the chlorophyll + pass to an ECS + pass back to PSI
• during passage of electrons along the ECs, a small amount of ATP is generated
• no photolysis of H20 occurs, so no protons or O2 are produced + no reduced NADP is generated

Question 10

What do chloroplasts in guard cells contain + produce?

Answer 10

• contain only PSI (P700)
• produce only ATP which actively brings K+ into the cells, lowering the H20 potential so H20 follows by osmosis
• causes guard cells to swell + opens the stoma

Question 11

Where does the light-independent stage of photosynthesis take place?

Answer 11

• in the stroma of chloroplasts

Question 12

What does the light-independent stage use?
(Calvin bestie)

Answer 12

• ATP and reduced NADP from the light-dependent stage to produce glucose

Question 13

What is the calvin cycle?

Answer 13

• series of reactions whereby CO2 is converted to organic molecules is called the Calvin cycle

Question 14

How does CO2 travel into the stroma?

Answer 14

• enters the leaf through the stomata + diffuses through the spongy mesophyll layer to the palisade layer, into the palisade cells, through their thin cellulose cell walls, and then through the chloroplast envelope into the stroma

Question 15

What are the 5 stages of the Calvin Cycle?

Answer 15

1. CO2 combines with CO2 acceptor (5C RuBP)
   - reaction is catalysed by RuBisCo
2. RuBP becomes carboxylated by accepting the carboxyl group which forms an unstable intermediate 6C compound that immediately breaks down
3. product of this reaction is 2 molecules of a 3C compound, GP (glycerate-3-phosphate)
   - C02 is now fixed
4. GP is then reduced, using Hs from the reduced NADP made during the light-dependent stage, to TP (triose phosphate)
   - energy from ATP, also made during LD, is used at this stage at the rate of 2 molecules of ATP for every molecule of C02 fixed during stage 3
5. in 10 of every 12 TP molecules, atoms are rearranged to regenerate 6 molecules of RuBP.
   - process requires phosphate groups
   - chloroplasts contain low levels of RuBP, as it is converted to GP and also continually regenerated
   -remaining 2 molecules of TP = product

Question 16

What happens during the light- dependent stage?

Answer 16

• H+ are pumped from the stroma into the thylakoid spaces, so the conc of free protons in the stroma falls, raising the pH to around 8, which is the optimum for RuBisCO, which is activated by extra ATP in the stroma

Question 17

What are the 3 uses of triose phosphate?

Answer 17

• some glucose is converted to sucrose, some to starch and some to cellulose
• some TP is used to synthesise amino acids, fatty acids + glycerol
• the rest of the TP is recycled to regenerate supply of RuBP (5 molecules of 3C compound TP interact to form 2 molecules of the 5C compound RuBP)

Question 18

What are the 4 limiting factors of photosynthesis?

Answer 18

• carbon dioxide concentration
• light intensity
• temperature
• water stress (stomatal closure)

Question 19

What happens when light intensity is low?

Answer 19

• rate of photosynthesis of low

Question 20

What happens when light intensity is high?

Answer 20

• rate of photosynthesis is high

Question 21

What happens when there is little or no light on the Calvin cycle?

Answer 21

• GP cannot be reduced to TP
• TP levels fall + GP accumulates
• If TP levels fall, RuBP cannot be regenerated

Question 22

What is the effect of low C02 on the Calvin Cycle?

Answer 22

• RuBP cannot accept it and accumulates
• GP cannot be made
• so TP cannot be made

Question 23

What are the effects of having low temperatures on Calvin cycle?

Answer 23

• if plants have enough H20 + C02 + light intensity, rate of photosynthesis increases as temp increases

Question 24

What are the effects of having temperatures above 30C on Calvin cycle?

Answer 24

• growth rate may reduce due to photorespiration: 02 competes with C02 for RuBisCO’s active site
• reduces C02 being accepted by RuBP and reduces quantity of GP + so TP produced, whilst initially causing accumulation of RuBP
• lack of TP = RuBP cannot be regenarated

Question 25

What are the effects of having temperatures above 45C on Calvin cycle?

Answer 25

• enzymes involved in photosynthesis may be denatured
• reduces conc of GP and TP, and eventually RuBP as lack of TP so can’t regenerate

Question 26

What 5 things happens if there is not enough water available to the plant (water stress)?

Answer 26

• the roots are unable to take up enough water to replace that lost via transpiration
• cell lose water + become plasmolysed
• plant roots produce abscisic acid that, when translocated to leaves, causes stomata to close, reducing gaseous exchange
• tissues become flaccid + leaves wilt
• rate of photosynthesis reduces

Question 27

How is the rate of photosynthesis usually found?

Answer 27

• measuring the vol. of 02 produced per min by an aquatic plant

Question 28

What are the limitation of measuring vol of 02?

Answer 28

• some of the 02 produced by the plant will be used for its respiration
• may be some dissolved nitrogen in the gas collected

Question 29

How is a photosynthometer set up?

Answer 29

• air tight
• no air bubbles in the capillary tubing
• gas given off by the plant, over a known period of time, collects in the flared end of the capillary tube
• as the syringe is manipulated, the gas bubble can be moved into the part of the capillary tube against the scale + its length measured

Question 30

What are the 4 stages the light-dependent stage consists of?

Answer 30

• light harvesting at photosystem 2 (only photosystem with enzyme for photolysis)
• photolysis of water
• photophosphorylation - production of ATP in the presence of light
• the formation of reduced NADP