LIGHT DEPENDENT REACTIONS

Question 1

What are accessory pigments? (1)

Answer 1

light-absorbing molecules that pass on the energy they absorb to chlorophyll molecules at the start of photosynthesis.

Question 2

what are the common types of photosynthetic pigments? (3)

Answer 2

-chlorophylls
-carotenes
-others

Question 3

what photosynthetic pigments come underneath chlorophylls? (2)

Answer 3

Chlorophyll a - green
chlorophyll b - green

Question 4

what photosynthetic pigments come underneath carotenes? (2)

Answer 4

B - carotene - orange
Xanthophyll - yellow

Question 5

what photosynthetic pigments come underneath others? (2)

Answer 5

phycoerithrin - red
fucoxanthin - brown

Question 6

what is the purpose of photosynthetic pigments? (1)

Answer 6

to absorb light energy and convert it into chemical energy.

Question 7

what is meant by hydrophilic? (1)

Answer 7

having an affinity for water and soluble in water.

Question 8

what is meant by hydrophobic? (1)

Answer 8

substances that repel water, they are mainly insoluble in water but can be soluble in lipids.

Question 9

what are the two parts of a chlorophyll molecule? (2)

Answer 9

-the head of the molecule is a hydrophilic ring structure with a magnesium atom in its centre.
-attached to this is a long hydrophobic hydrocarbon tail

Question 10

what does the arrangement of a chlorophyll molecule mean? (1)

Answer 10

means that chlorophyll molecules are attached to the membranes of the chloroplast by their long tails whilst the heads lie flat on the membrane surface to absorb the maximum amount of light.

Question 11

what is the absorption spectrum? (1)

Answer 11

shining light of different wavelengths through a solution of the pigment and measuring how much is absorbed - the graph of the amount of light absorbed at each wavelength is known as the absorption spectrum.

Question 12

what is the action spectrum? (1)

Answer 12

by plotting a graph of the rate of photosynthesis of a green plant at different wavelengths.

Question 13

why are the action spectrum and absorption spectrum very similar? (1)

Answer 13

this provides good evidence to support the model of light being trapped by these pigments and used in photosynthesis.

Question 14

why do plants need more than one absorption pigment? (1)

Answer 14

a combination of two or more pigments means that a greater range of wavelengths can be absorbed efficiently.

Question 15

What are the main properties of the structure of chloroplast? (6)

Answer 15

• has outer and inner membranes
  -contains 70s ribosomes
  -the liquid is called the stroma
  -has a stack of disks (grana) and individually called a thykloid
  -have DNA
  -contains a lamella (between granium)

Question 16

where is chlorophyll found in a chloroplast? (1)

Answer 16

within the thylakoid

Question 17

what is autotrophic nutrition? (1)

Answer 17

the synthesis of larger organic molecules from simpler inorganic compounds as carried out by plants during photosynthesis

Question 18

what is the word equation for photosynthesis? (1)

Answer 18

Carbon dioxide + water + light energy —-> organic compounds (sugars) + oxygen

Question 19

where do light dependent reactions take place? (1)

Answer 19

occurs in the grona

Question 20

what is a summary of light dependent reaction? (3)

Answer 20

light energy is used directly to split water (photolysis). Hydrogen is then removed and retained by the photosynthetic specific hydrogen acceptor (NADP+). At the same time ATP is generated for ADP and phosphate, dissolving energy from light - this is known as photophosphorylation.

Question 21

do chlorophylls occur haphazardly? (1)

Answer 21

chlorophylls do not occur haphazardly but are grouped together in structure called photosystems held in the thylakoid membranes of the grana.

Question 22

what is photosystem 1? (1)

Answer 22

has a reaction activated at the wavelength 700nm (also known as P700)

Question 23

What is photosystem 2? (1)

Answer 23

has a reaction activated by the light wavelength of 680nm (also known as P680)

Question 24

What occurs in the first stage of photosystem 2? (1)

Answer 24

the excited electrons from photosystem 11 are picked up by, and passed along a chain of electron carriers. As these excited electrons pass, some of the energy causes the pumping of hydrogen ions (protons) from the chloroplasts matrix into the thylakoid spaces, there they accumulate - incidentally causing the pH to drop. The result is a proton gradient that is created across the thylakoid membrane, and which sustains the synthesis of ATP.

Question 25

What occurs in the second stage of photosystem 2? (1)

Answer 25

as a result of these energy transfers, the excitation level of the electrons falls back to ‘ground state’ and they come to fill the vacancies in the reaction of photosystem 1.

Question 26

What occurs in the third stage of photosystem 2? (1)

Answer 26

the holes in the reaction centre of photosystem 2 are filled by electrons (in their ground state) from water molecules positively charged ‘vacancies’ in the photosystem 2 are powerful enough to cause the splitting of water in the presence of a specific enzyme.

Question 27

What occurs in the fourth stage of photosystem 2? (1)

Answer 27

the reaction this enzyme catalyses then triggers the release of hydrogen ions and oxygen atoms as well as ground state electrons.

Question 28

What occurs in the fifth stage of photosystem 2? (1)

Answer 28

the oxygen atoms combine to form molecular oxygen (waste product) the hydrogen is used in the reduction of NADP+

Question 29

What occurs in the sixth stage of photosystem 2? (1)

Answer 29

(in the grana) the synthesis of ATP is coupled to electron transport via the movement of protons (chemiosmosis). The hydrogen ions are trapped within the thylakoid space flow out via ATP synthesise enzyme down their electrochemical gradient, at the same time ATP is synthesised from ADP and phosphate (called photophosphorylation)

Question 30

what is non-cyclic photophosphorylation? (1)

Answer 30

the production of ATP from ADP using light energy in a linear series of reactions.