5.6-photosynthesis Flashcards

Question

What is a light harvesting system?

Answer 1

-chlorophyll b, xanthophylls and carotenoids are all embedded within the thylakoid membrane and form a light harvesting system -The light harvesting system is where light energy of different wavelengths in absorbed, and this energy is then transferred to the reaction centre. • The reaction centre contains chlorophyll a and is where light dependent reactions occur.

Answer 2

Light-dependent stage (Occurs on the thylakoid membrane (grana)) It involves 4 stages: 1-Non-cyclic phosphorylation 2-Cyclic phosphorylation 3-photolysis 4-chemiosmosis

Answer 3

-Photosystem II (PSII) is the first to be used, absorbs light with a wavelength of 700nm -Photosystem I (PSI) the second to be used, absorbs light with a wavelength of 680nm -both are involved in non-cyclic phosphorylation

Answer 4

-the light energy that is absorbed in PSII causes electrons within the reaction centres to become excited and released -the electrons released from PSII and PSI move along the electron transport chain -this results in ATP production by chemiosmosis -the electrons lost from PSII are replaced by electrons from photolysis and the electrons lost from PSI are replaced by electrons at the end of the electron transport chain from PSII -at the end of the electron transport chain from PSI, the electrons are accepted by the coenzyme NADP. NADP accepts the electrons, along with H+ (from photolysis) to form reduced NADP -the reduced NADP and ATP made in this process are used in the light-independent reaction

Answer 5

-some of the electrons that are released from PSI are not picked up by NADP and instead are recycled back into PSI -the transport of electrons still results in ATP production through chemiosomosis -however no photolysis occurs so no reduced NADP is produced

Answer 6

-in PSII there is an enzyme that in the presence of light splits water molecules into protons, electrons and oxygen (photolysis) -H2O —> 1/2 O2 + 2e^- + 2H^+ -the H+ is used to form NADPH -the electrons are passed along a chain of electron carrier proteins -the oxygen is either used for respiration or diffuses out of the leaf through the stomata

Answer 7

-the electrons that gained energy and left the chlorophyll move along a series of proteins embedded within the thylakoid membrane -as they move along they release energy and some of the energy from the electrons is used to pump the protons across the chloroplast membranes (from stroma into thylakoid lumen) -an electrochemical gradient is formed -the protons pass through the enzyme ATP synthase which results in the production of ATP (as it causes an inorganic phosphate to join ADP) -coenzyme NADP is reduced forming reduced NADP

Answer 8

-molecules that can accept one or more electrons and then donate those electrons to another carrier -proteins embedded in thylakoid membranes are electron carriers and form an electron transport chain -NAD and NADP are also electron carriers

Answer 9

1-Carbon dioxide reacts with a five carbon compound called ribulose biphosphate (RuBP) This reaction is catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase) 2-the product of this reaction is two molecules of a three-carbon compound GP (glycerate-3-phosphate) The carbon dioxide has now been fixed 3-GP is reduced to TP (triode phosphate) using hydrogens from the reduced NADP made during the light-dependent stage. ATP that was made during the light-dependent stage is also used at the rate of 2 molecules of ATP for every molecule of carbon dioxide that has been fixed during stage 2 4-some of the carbon from TP leaves the cycle each turn to be converted into useful organic substances (e.g. glucose) 5/6 of the molecule is used to regenerate RuBP with the energy from ATP

Answer 10

Cyclic photophosphorylation only involves photosystem I (whereas non-cyclic photophosphorylation involves photosystems I and II) Cyclic photophosphorylation does not produce reduced NADP (whereas non-cyclic photophosphorylation does)

Answer 11

-used as a starting material for the synthesis of carbohydrates, lipids and amino acids -TP is recycled to regenerate the supply of RuBP

Answer 12

The rate of photosynthesis increases as light intensity increases: The greater the light intensity, the more energy supplied to the plant and therefore the faster the light-dependent stage of photosynthesis can occur This produces more ATP and reduced NADP for the Calvin cycle (light-independent stage), which can then also occur at a greater rate During this stage, light intensity is said to be a limiting factor of photosynthesis At some point, if light intensity continues to increase, the relationship above will no longer apply and the rate of photosynthesis will reach a plateau At this point, light intensity is no longer a limiting factor of photosynthesis – another factor is limiting the rate of photosynthesis

Answer 13

A decrease in light intensity causes a decrease in TP and RuBP concentrations but a slight increase in GP concentration When there is less light available the light-dependent stage stops and does not form any more products needed for the light-independent stage (ATP and NADPH) As a consequence, GP builds up as it is not converted to TP A lack of TP means that RuBP will not form Over time the fixation of carbon dioxide will stop and the concentration of GP will plateau

Answer 14

The rate of photosynthesis increases as carbon dioxide concentration increases, more carbon dioxide that is present, the faster carbon fixation during the Calvin cycle can occur and the faster the overall rate of photosynthesis This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is in short supply

Answer 15

Very low concentrations of carbon dioxide (less than 0.01%) causes a decrease in the concentration of GP and TP but an increase in RuBP concentration RuBP accepts carbon dioxide so when there is a lack of carbon dioxide molecules it remains unfixed and builds up The lack of carbon dioxide fixation prevents GP and TP molecules from forming

Answer 16

The effects on changing temperature on the Calvin cycle are: -from low temperatures to temperatures of 25-30 °C if plants have enough water and carbon dioxide and a sufficient light intensity the rate of photosynthesis increases as temperature increases -at temperatures above 30 °C for most plants, growth rates may reduce due to photorespiration- oxygen competes with carbon dioxide for the enzyme RuBisCO’s active site. This reduces the amount of carbon dioxide being accepted by RuBP and therefore reduces the quantity of GP and therefore TP being produced, whilst initially causing a accumulation of RuBP however, due to the lack of TP, RuBP cannot be regenerated -at temperatures above 45 °C enzymes involved in photosynthesis may be denatured, this would reduce concentrations of GP and TP and eventually of RuBP as it could not be regenerated due to the lack of TP

Answer 17

If not enough water is available to the plant (water stress): -the roots are unable to take up enough water to replace that lost via transpiration -cells lose water and become plasmolysed -plant roots produce abscisic acid that when translocated to leaves, causes stomata to close, reducing gaseous exchange -tissues become flaccid and leaves wilt -the rate of photosynthesis is greatly reduced

Answer 18

-membraneous channels that connect thylakoids