M5, C17 Energy for Biological Processes

Question 1

what is the word and balanced symbol equation for photosynthesis

Answer 1

carbon dioxide + water –light energy–> oxygen + glucose

6CO2 + 6H2O -> 6O2 + C6H12O6

Question 2

what does it mean when you say plants are autotrophic

Answer 2

they make their own organic molecules from inorganic molecules/CO2

e.g. plants make organic molecules from photosynthesis

Question 3

explain the importance of photosynthesis to life on earth

Answer 3

Produces oxygen which other organisms use for respiration.
Absorbs carbon dioxide which limits carbon dioxide levels in the atmosphere which could lead to global warming and climate change.
Plants are autotrophs which means they make their own food which heterotrophs can then eat.

Question 4

define heterotrophs

Answer 4

obtains organic molecules from other organisms

feeds on and digests another organism

Question 5

explain why the equation for photosynthesis is an oversimplification

Answer 5

It implies carbon dioxide and water combine and react which lead to the formation of oxygen and glucose. This is not true.
There are separate reactions- one which is light-dependent and one which is light-independent.
There are also lots of steps which happen in each of those 2 reactions.
The equation also doesn’t mention the pigment, chlorophyll, required to absorb the light energy for the light-dependent reaction.

Question 6

name all the parts of a leaf

Answer 6

from top to bottom:
waxy cuticle
upper epidermis
palisade mesophyll
spongy mesophyll with air spaces in between
vascular bundle
lower epidermis
guard cells and stomata
waxy cuticle

Question 7

how is the palisade mesophyll adapted for its function in the leaf

Answer 7

packed full of chloroplasts

absorbs the light energy required for photosynthesis

Question 8

why are their air spaces between the spongy mesophyll in the leaf

Answer 8

makes it easier for diffusion

Question 9

why is the waxy cuticle usually thicker on top of the leaf rather than on the bottom

Answer 9

because there is direct sunlight on the top of the leaf which increases the rate of evaporation
therefore the waxy cuticle has to be thicker to reduce water loss

Question 10

why is the upper epidermis thin

Answer 10

allows maximum sunlight to get through the leaf to the palisade cells so the chloroplasts can absorb maximum light energy for photosynthesis

Question 11

what are photosynthetic pigments

Answer 11

substances that absorb light at certain wavelengths

other wavelengths are reflected

Question 12

what are the 4 photosynthetic pigments found in chloroplasts

Answer 12

chlorophyll a
chlorophyll b
carotene
xanthophyll

(carotene and xanthophyll are also known as carotenoids)

Question 13

Why don’t plant rely on just one photosynthetic pigment

Answer 13

because different pigments absorb different wavelengths of light
so multiple are needed to absorb as much light energy as possible
also in different environments they may be different light sources so ensures at least one of the pigments can absorb the light energy

Question 14

name the features of the chloroplast

Answer 14

stroma
thylakoid - multiple are called granum
ribosome
chloroplast DNA
starch granule
drop of lipids
lamellae
inner and outer membrane

Question 15

where are the pigments found in the chloroplasts

Answer 15

in the thylakoid membranes

arranged in photosystems

Question 16

describe the similarities and differences between the 2 photosystems in the thylakoid membranes

Answer 16

Similarities:
Both photosystems are funnel shaped.
At the thinner end, there is a chlorophyll a molecule, called the primary pigment.
In the rest of the funnel are the accessory pigments which involves carotene, chlorophyll b and xanthophyll.
Light energy is transferred through the accessory pigments to the primary pigment.

Differences:
Light of wavelength 680nm is absorbed by the photosystem II (PSII).
Light of wavelength 700nm is absorbed by the photosystem I (PSI).

Question 17

give a step-by-step process of photophosphorylation for light-dependent photosynthesis reaction

Answer 17

1) photolysis - releases 2e-, 2h+ and 1/2o2 molecules
2) h+ions released into thylakoid and thus increases conc of h+ ions present. eventually via facilitated diffusion via a protein channel down a electrochemical gradeient to stroma (chemiosmosis) this drives formation of ATP) then picked up by nadp
3) psii absorbs 680nm and the energy from the photons excite the electrons released by photolysis of water (as psii was deficient)
4) excited e- leaves psii reaction centre and moved by electron carriers to electron transport chain protein in thylakoid memebrane. movement of electrons along this causes atp production by CHEMIOSMOSIS
(Hydrogen ions actively pumped from stroma into thylakoid)
5)electrons pass reaction centre of psI and excited there by 700nm light
6)excited electrons carried to e- transport chain (causes atp prouduction again) by chemiosmosis
7) electrons leaving e- transport chain after psI - accepted with a H+ ion by coenzyme NADP to form NADPH
8)Electrons lost by PSii - released by another
from photolysis
CYCLIC - REPLACED BY ONE FROM PSII OR ITSELF - ONLY MAKES ATP (NO NADPH)

Question 18

give a step-by-step process of cyclic photophosphorylation for light-dependent photosynthesis reaction

Answer 18

1) Light strikes PSI and a pair of electrons is excited from the chlorophyll molecule.
2) They pass along the chain of electron carriers back to the chlorophyll.
3) As they pass down the carriers, they produce energy.
4) The energy is used to actively pump hydrogen ions into the thylakoid.
5) The hydrogen ions diffuse down a concentration gradient back into the stroma through a channel protein.
6) The ATP synthase is activated to catalyse the reaction to produce ATP.

Question 19

Compare cyclic and non-cyclic photophosphorylation for the light dependent stage of photosynthesis.

a) photosystems involved
b) is photolysis involved
c) fate of electrons released from chlorophyll
d) products made

Answer 19

a) Only PSI is involved in cyclic but both are involved in non-cyclic
b) Photolysis is only involved non-cyclic
c) The electrons in cyclic return back to PSI. For non-cyclic, the electrons made by PSII go to PSI and the electrons from PSI are used by NADP to produce reduced NADP (NADPH)
d) Products of cyclic is just ATP. Products of non-cyclic are ATP and reduced NADP (NADPH)

Question 20

The light-independent stage of photosynthesis is known as the Calvin cycle. Give a step-by-step process of the Calvin cycle.

Answer 20

1) Carbon dioxide from the air diffuses into the leaf though open stomata. It diffuses through the air spaces in the spongy mesophyll and reaches the palisade mesophyll layer. It diffuses into the stroma.
2) Here, the carbon dioxide combines with a 5-carbon compound, ribulose bisphosphate (RuBP). Forms unstable 6C intermediate and breaks down immediately) The reaction of co2 w RuBP is catalysed by rubisco (INEFFICIENT ENZYME - inhibited by o2)
3) The product is 2 molecules of a 3-carbon compound, glycerate 3-phosphate (GP).
4) GP is reduced and phosphorylated to another 3-carbon compound, triose phosphate (TP). ATP and reduced NADP from the light dependent reaction are used in this process. (hydrogen atom used)
5) The majority of TP is recycled back to RuBP by phosphorylation using ATP.
6) Food products are made from TP.

X6 turns to make 1 glucose

Question 21

define limiting factor

Answer 21

a factor present in the lowest amount that reduces the rate of photosynthesis

Question 22

what are the 3 limiting factors of plants

Answer 22

light intensity
carbon dioxide concentration
temperature

Question 23

explain why the levels of GP and TP change when a plant changes from light to dark

Answer 23

when it’s dark, GP can still be made because carbon dioxide is unaffected
TP can only be made with the presence of ATP and NADPH but if there’s no light then the light-dependent reaction can’t happen so these things can’t be produced so TP levels drops
GP is usually converted to TP but because this can’t be made, GP levels increase

Question 24

one experiment to investigate the rate of photosynthesis is to count to the number of bubbles a pondweed produces when placed in a test tube of water
how can this experiment be made more accurate

Answer 24

Counting bubbles produces human errors and the bubbles are all different sizes. So a gas syringe should be attached to the top of the tube in order to fin dthe volume of gas the plant has produced. (note: this doesn’t stop the fact that some bubbles could be carbon dioxide produced during respiration)

Question 25

One experiment used to investigate the rate of photosynthesis is placing leaf discs in a dilute solution of sodium hydrogen carbonate and extracting the air from them using a syringe and seeing how long the leaves floated back to the top.
Why is sodium hydrogen carbonate used?
What does it mean if the leaf discs rise quicker?

Answer 25

Sodium hydrogen carbonate is used in order the provide the carbon dioxide for photosynthesis.

The quicker the leaves rise, the more photosynthesis they are doing because they are producing oxygen during photosynthesis and this refills the air sacs meaning they are more buoyant so will rise quicker.

Question 26

Regeneration of RuBP

Answer 26

For one glucose molecule to form - 6 carbon dioxides are needed to eneter calvin cycle resulting in 6 full turns of the cycle. this results in production of 12 TP molecules( two of whcih removed to make glucose molecule wheras other 10 used to regenerate 6 RuBPs

Question 27

Describe chemiosmosis

Answer 27

ATP is synthesised by chemiosmosis. The reaction involves the diffusion of protons. movement of protons as they move down their ec gradient, releases energy used in the attachment of adp w pi forming ATP.

Chemiosmosis depeneds on the creation of proton conc gradient. Energy to do this comes from excited electrons.

The electrons pass into electron transporter chain and used to generate a proton gradient.

The elctron transporter chain is made up of series of elcetron carriers each w a progressively lower energy level. As electrons move from high energy electron carriers to low, energy is released and used to pump protons across a membrane which creates the proton gradient.

Question 28

Photolysis describe

Answer 28

• water molecules split into h+, e- and o2 using energy from the sun
• Oxygen-evolving complex forms part of psii and catalyses breakdown of water (H20 = H+ + 2E- + 1/2O2
• protons are released into thylakoid lumen increasing conc across membrane. as they move back through membrane down gradient, they drive formation of ATP.. Once back to stroma combine w NADP and e- (from psI) to form NADPH.

removal of H+ from stroma maintains proton gradient across thylakoid membrane.

Question 29

Light intensity as a limiting factor

• Why does it affect rate of photosynthesis
• Why factor necessary
• Effect if factor limited

Answer 29

1) Needed for photolysis. Exciting electrons within photosystem during light dependent phase

2) ATP and NADPH are needed for conversion of GP to TP in calvin cycle
- If AU increases, ATP and NADPH are produced at higher rate

3) Light dpendent reaction will decrease thus less ATP and NADPH made thus less GP and TP made in calvin cycle. Thus regeneration of RuBP decreases so RuBP decreases.

Question 30

CO2 as a limiting factor

• Why does it affect rate of photosynthesis
• Why factor necessary
• Effect if factor limited

Answer 30

1) CO2 source of carbon thus if CO2 increases, the rate of carbon fixation will increase in the calvin cycle and thus increases production of TP
2) Rate of Tp production increases thus there is a greater yield of organic substances
3) GP production will decrease as less carbon fixation. RuBP increases

Question 31

Temp as a limiting factor

• Why does it affect rate of photosynthesis
• Why factor necessary
• Effect if factor limited

Answer 31

1) Increase temp, increases rate of enzyme activity up to a point where the enzyme denatures
2) Increase in temp will increase enzyme controlled reactions such as carbon fixation.

HOWEVER the rate of photorespiration increases after 25 degrees cel thus increasing rates of photosynthesis may not be seen at higher temps when enzyme denatured as photorespiration increasing and thus competitively inhibits RuBisCo

3)KE will decrease thus decreased rate of successful collisions between substrate and enzymes