5.6 photosynthesis

Question 1

define autotrophic nutrition

Answer 1

converting light energy into chemical energy to synthesis large organic molecules

Question 2

What is the general equation for photosynthesis

Answer 2

6CO2 + 6H2O —> C6H12O6 + 6O2

Question 3

How do photosynthesis and respiration interrelate

Answer 3

the products of one process are the raw materials for the other

Question 4

What is the compensation point

Answer 4

when photosynthesis and respiration occur at the same rate

Question 5

Define the compensation period

Answer 5

time taken for a plant to reach its compensation point

Question 6

Describe the structure of a chloroplast

Answer 6

2-10micrometres long surrounded by a double membrane
intermembrane space
fluid filled stroma
thylakoids = granum is thylakoid stacks
intergranal lamellae

Question 7

How are chloroplasts adapted

Answer 7

stroma contains enzymes needed to catalyse reactions, starch, oils and ribosomes and DNA
outer membrane is permeable to ions
inner membrane less permeable to ions but has transport proteins embedded
granum provide a large surface area for electron carriers and ATP synthase and photosystems

Question 8

How are photosystems held in place

Answer 8

proteins embedded in thylakoid membranes hold photosystems in pace

Question 9

Describe photosystems

Answer 9

within thylakoid membranes are funnel shaped structures called photosystems which contain photosynthetic pigment which absorbs light at a particular wavelength

Question 10

Describe how light travels in a photosystem

Answer 10

The energy of the wavelengths of light captures is funnelled down to the primary pigment centre consisting of a type of chlorophyll at the base

Question 11

What is the difference between chlorophyll a and b

Answer 11

A has absorption peaks of P700 and P680 and appears blue-green
B absorbs 400-500nm and around 640nm and appears yellow-green

Question 12

Describe accessory pigments

Answer 12

carotenoids absorb blue light wavelengths s400-500nm and reflect orange yellow light
xanthophyll’s absorb blue and green light 375-550nm and reflect yellow light

Question 13

where does the light-dependant stage of photosynthesis occur

Answer 13

the granum / thylakoid membranes

Question 14

what stages does the light dependant stage of photosynthesis consist of

Answer 14

light harvesting at the photosystems
photolysis of water
photophosphorylation
formation of reduced NADP

Question 15

Describe photolysis

Answer 15

there is an enzyme in PS2 that in the presence of light splits water molecules into protons, electrons and oxygen

Question 16

Give the equation for photolysis

Answer 16

2H20 –> 4H+ + 4e- + O2

Question 17

What is the role of water in photosynthesis

Answer 17

source of protons used in photophosphorylation
donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
source of by-product oxygen
keeps plant cells turgid allowing them to function

Question 18

Define photophosphorylation

Answer 18

the generation of ATP from ADP and Pi in the presence of light

Question 19

What are the two types of photophosphorylation

Answer 19

cyclic and non-cyclic

Question 20

What is the difference between cyclic and non-cyclic photophosphorylation

Answer 20

non-cyclic involves PS1 and PS2 it produces ATP NADPH and oxygen
whereas cyclic involves only PS1 and produces only ATP but in smaller quantities

Question 21

What are the similarities between cyclic and non-cyclic photophosphorylation

Answer 21

Both produce ATP and both involve iron-containing proteins embedded in the thylakoid membranes

Question 22

Describe non-cyclic photophosphorylation

Answer 22

When a photon of light strikes PS2 its energy is channelled to the primary pigment which excites a pair of electrons in the chlorophyll A and escape from the molecule which are then captured by an electron carrier - these electrons are replaced by those derived from photolysis

Electrons are passed along the electron transport chain and this generates some energy which is used to pump the protons into the thylakoid membrane space across the thylakoid membrane

Eventually electrons are captured by another molecule in PS1 which replace the electrons lost due to excitation by light

A protein-iron-sulphur complex called FERREDOXIN accepts the electrons from PS1 and passes it to NADP in the stroma

As protons accumulate in the thylakoid membrane space a proton gradient forms and protons diffuse down their concentration gradient through special channels in the membrane associated with ATP synthase and the flow of protons causes ADP+Pi –>ATP

Protons are accepted by NADP and this reduces the NADP (catalysed by NADP reductase)

Question 23

Describe cyclic photophosphorylation

Answer 23

This uses only PS1
As light strikes it it excites a pair of electrons in the primary pigment Chlorophyll A p700

The electrons escape and pass to an electron carrier and then pass back to PS1 and during the passage of electrons along the electron carriers some ATP is generated

Question 24

Where does only cyclic photophosphorylation occur

Answer 24

In the stomata because it only contains PS1

Question 25

Describe the Calvin cycle

Answer 25

6CO2 combines with a CO2 acceptor called RuBP which is catalysed by the enzyme RuBisCO and by accepting the carboxyl group the ribulose biphosphate becomes carbonylated forming an unstable intermediate 6 carbon compound that immediately breaks down into a 3 carbon molecule GP and the CO2 has now been fixed

12GP is then reduced using hydrogens from the NADPH into 12TP and energy is used at a rate of 2 molecules of ATP/CO2

In 10 of every 12 TP is rearranged to regenerate 6 of the 5carbon RuBP

The remaining 2 TP are the product

Question 26

What is GP

Answer 26

Glycerate-3-phosphate

Question 27

What is TP

Answer 27

Triose phosphate

Question 28

What is used to reduce the 12 GP to 12 TP

Answer 28

12 ATP to 12 ADP + 12Pi
12 NAPH to 12 NADP

Question 29

What is needed to turn 10 TP to 6 RuBP

Answer 29

6 ATP to 6 ADP and 6Pi

Question 30

Describe why there is a low concentration of RuBP

Answer 30

Because it is continuously being recycled

Question 31

Why does the calvin cycle only run in the daylight

Answer 31

1 - Products from light-dependant stage is required for the Calvin cycle to run
2-Hydrogen ions are pumped into the thylakoid space from the stroma decreasing its pH to 8 which is optimum for RuBisCO
3- In daylight Magnesium ion concentration increases which acts as a cofactor for RuBisCO
4- Ferredoxin that is reduced by electrons from PS1 activates enzymes involved in reactions for the Calvin cycle

Question 32

What are the uses of triose phosphate

Answer 32

Used to synthesis organic molecules e.g.
glucose converted to sucrose, starch or cellulose
used to synthesise amino acids, fatty acids and glycerol

Question 33

What are the products of the light - independent stage

Answer 33

2TP -> 6C (glucose )

Question 34

Name the limiting factors

Answer 34

CO2 concentration
light intensity
temperature

Question 35

What are the effects of light intensity on the Calvin cycle

Answer 35

GP cannot be reduced to TP due to no ATP or NADPH produced from the light-dependant stage so TP levels fall and GP accumulated and RuBP cannot be regenerated

Question 36

What are the effects of CO2 concentration on the Calvin cycle

Answer 36

RuBP has nothing to accept so GP and TP cant be made and RuBP accumulates

Question 37

What is photorespiration

Answer 37

When RuBisCO accepts O2 instead of CO2 so less GP produced and although initially creating an accumulation of RuBP it can no longer be regenerated due to lack of TP

Question 38

efine water stress

Answer 38

When not enough water is available to the plant

Question 39

How do plants respond to water stress and how does this affect their ability to photosynthesise

Answer 39

Cells lose water and become plasmolysed, plant roots produce abscisic acid that when translocated to leaves causes stomata to close reducing gaseous exchange so tissues become flaccid and leaves wilt so photosynthesis greatly reduces