Photosynthesis

Question 1

What is oxidation?

Answer 1

• loss of electrons or hydrogen

- gain of oxygen

Question 2

What is reduction?

Answer 2

• gain of electrons or hydrogen

- loss of oxygen

Question 3

Where does photosynthesis take place?

Answer 3

Chloroplasts within leaves

Question 4

Why are leaves adapted?

Answer 4

to bring together 3 raw materials of photosynthesis

- H2O, CO2 & light & remove O2 and C6H12O6

Question 5

Adaptations of leaves for photosynthesis

Answer 5

• large SA to absorb lots of light
• arrangement of leaves on plant to minimize overlapping (avoids shadowing of 1 leaf by another)
• Thin so DD for gases kept short
• numerous stomata for gaseous exchange so all mesophyll cells only short DD from one
• transparent cuticle & epidermis to let light through to photosynthetic mesophyll cells below
• long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight
• stomata that open & close in response to changes in light intensity
• network of xylem that bring water to leaf cells & phloem that carries sugars away produced during photosynthesis
• many air spaces in lower mesophyll level to allow rapid diffusion in the gas phase of O2 & CO2

Question 6

Photosynthesis equation

Answer 6

6CO2 + 6H2O –>(light on arrow) C6H12O6 + 6O2

Question 7

What is photosynthesis?

Answer 7

• metabolic pathway with many intermediate reactions:
  (process of energy transferral where some of energy is conserved in form of chemical bonds
  1 capturing of light energy
  2 light-dependent reaction
  3 light-independent reaction

Question 8

Shape of chloroplast (where photosynthesis takes place)

Answer 8

• disc-shaped & 2-10 micrometers long & 1 micrometer in diameter

Question 9

Structure of Chloroplast?

Answer 9

• surrounded by double membrane

- inside are 2 distinct structures: grana & stroma

Question 10

What is the grana?

Answer 10

stacks of thylakoids (disc-like structure) (where light dependent stage happens)
- within thylakoids is chlorophyll

Question 11

What is intergranal lamellae ?

Answer 11

tubular extensions that join up with thylakoids in adjacent grana in some thylakoids

Question 12

What is the stroma?

Answer 12

fluid filled matrix (where light-independent stage happens)

-within stoma are other structures e.g. starch grains

Question 13

Uses of glucose

Answer 13

• storage molecule (starch) stores chemical energy
• used in respiration
• cellulose
• used to make lipids (glycerol)
• used to make AAs

Question 14

How does water enter plants?

Answer 14

Roots in transpiration

Question 15

How does carbon dioxide come into plants?

Answer 15

Diffuses into leaves

Question 16

The light-dependent reaction of photosynthesis involves what?

Answer 16

• capture of light whose energy is used for 2 reasons:
• add Pi molecule to ADP making ATP
• split water into H+ ions (protons) & OH- ions, splitting caused by light (PHOTOLYSIS)

Question 17

3 Equations involved in light-dependent reaction of photosynthesis

Answer 17

ADP + Pi -> ATP
NADP + H+ + 2e- –> NADPH (reduced NADP)
H20 –> 2H+ + 2e- + 1/2 O2

Question 18

Model Answer: Describe the events of the light-dependent reaction of photosynthesis

Answer 18

1 photon of light hits chlorophyll in photosystem 2 (PS2) causing photoexcitation - raises 2e- up an energy level, they leave PS2 (photoionisation) in an Electron transport chain (ETC)
2 they travel through electron carriers in a series of redox reactions releasing energy as they go
3 energy released from electron carriers in ETC is used as energy to move H+ ions against their conc gradient from stroma into thylakoid lumen, H+ ions move through ATP synthase down their conc gradient which catalyses ADP and Pi to form ATP (this is chemiosmosis)
ADP + Pi –> ATP
4 another photon of light hits chlorophyll in PS1 & re-excites electrons and they leave PS1 and ETC and join with NADP
H+ ions that moved down their conc gradient combine with NADP also to form reduced NADP in stroma
NADP + H+ + 2e- –> NADPH
5 Photolysis of water provides e- to reduce the chlorophyll in PS2 and replace the H+ used in the reduction of NADP.
H20 –> 2H+ + 2e- + 1/2O2

Question 19

What function is chloroplast adapted to?

Answer 19

capturing sunlight and carrying out light dependent reaction of photosynthesis

Question 20

Site of the light-dependent reaction

Answer 20

thylakoids of chloroplasts

- disc-like structures stacked together in groups (called grana)

Question 21

State 4 ways Chloroplast’s structure is adapted to its function

Answer 21

1 thylakoid membranes provide large SA for attachment of chlorophyll, electron carriers and enzymes that carry out LD reaction
2 chlorophyll arranged in photosystems to maximise absorption of light
3 granal membranes have ATP synthase channels within them (catalyses production of ATP), also selectively permeable which allows establishment of proton gradient
4 chloroplasts contain both DNA & ribosomes so they can quickly & easily manufacture some of the proteins involved in the light-dependent reaction

Question 22

Where does the Light-independent reaction of photosynthesis take place?

Answer 22

stroma of chloroplasts

Question 23

Another term for the light independent reaction?

Answer 23

The Calvin Cycle

Question 24

Model answer: Describe the process of the Calvin Cycle

Answer 24

1 RuBP (Ribulose Biphosphate) combines with CO2 to form a 6-carbon intermediate
2 catalysed by rubisco
3 6 carbon molecule breaks down into 2 x GP (glycerate-3-phosphate)
4 GP reduced to TP (triose phosphate) using reduced NADP - using energy from ATP & (both from light-dependent reaction)
5 some TP is used to manafacture organic substances/hexose sugars, some used to regenerate RuBP

note: organic substances produced from TP could be lipids, glucose, cellulose, amino acids, starch, nucleotides i.e. what the plant requires

Question 25

Why does when RuBP combines with CO2 it breaks down into 2 3-carbon molecules?

Answer 25

it’s an unstable intermediate molecule

Question 26

How is Glycerate-3-phospahte reduced to Triose phosphate?

Answer 26

reduced NADP from LD reaction is used to reduce GP to TP - using energy supplied by ATP

Question 27

State 2 uses of the triose phospahte produced

Answer 27

1 some of the triose phosphate is used to regenerate RuBP in the Calvin cycle
2 some of the triose phosphate is converted to useful organic substances

Question 28

How does the NADP go back into the LD reaction after its use in the calvin cycle?

Answer 28

NADP re-formed and goes back into light-dependent reaction to be reduced again by accepting more protons

Question 29

What are some of the useful organic substances TP (triose phosphate) is converted into?

Answer 29

• starch
• cellulose
• lipids
• AAs
• nucleotides

Question 30

How is the chloroplast adapted to carrying out the light-independent reaction of photosynthesis?

Answer 30

1 fluid of stroma contains all enzymes needed for this reaction. Stromal fluid is membrane-bound in chloroplast so a chemical environment which has a high conc of enzymes and substrates can be maintained there - distinct from environment of cytoplasm
2 stromal fluid surrounds grana and so products of light-dependent reaction in the grana can readily diffuse into stroma
3 contains both DNA and ribosomes so it can quickly and easily manafacture some of the proteins involved in the light-independent reaction

Question 31

Identify environmental factors that limit the rate of photosynthesis

Answer 31

Light intensity
Temperature
CO2 concentration

Question 32

State the importance of sufficient water in photosynthesis

Answer 32

plants need a constant supply of water
too little - photosynthesis stops
too much - soil becomes waterlogged (reducing uptake of minerals)

Question 33

State all optimum conditions for photosynthesis

Answer 33

• high light intensity of a certain wavelength
  (higher light intensity = more energy provided)
• temp around 25 C
  (too low would make enzymes inactive, too high would denature them e.g. rubisco and ATP synthase)
• Carbon dioxide at 0.4%
  (gives a higher rate of photosynthesis but higher and stomata start to close)

Question 34

How agricultural growers (farmers) counteract limiting factors that limit plant growth?

Answer 34

they create optimum conditions in glasshouses:
CO2 conc - CO2 added to air (by burning small amount of propane in CO2 generator)
Light - lights can get in through glass, lamps provide light at night
temp - glasshouses trap heat energy from sunlight, which warms air - heaters & cooling systems used to maintain optimum temp, air circulation makes sure temp is even throughout glasshouse

Question 35

What is the function of the light-dependent reaction?

Answer 35

• to convert solar energy into chemical energy in the form of NADPH and ATP, which are used in calvin cycle and fuel assembly of sugar molecules

Question 36

What is the function of the calvin cycle?

Answer 36

• making simple sugars from carbon dioxide and water

Question 37

state products of the light-dependent reaction

Answer 37

reduced NADP, ATP, Oxygen

Question 38

state products of the light-independent reaction

Answer 38

sugars and other organic molecules

Question 39

suggest why measuring the volume of gas produced by a plant (while investigating photosynthesis) may not be an accurate representation of rate of photosynthesis

Answer 39

• vol of O2 produced will be less than that produced by photosynthesis as some of the O2 will be used in cellular respiration

Question 40

Do plants respire or just photosynthesize?

Answer 40

• plants respire in the normal way i.e. using glycolysis, Krebs cycle, oxidative phosphorylation etc.
• but respiration is masked by the fact that photosynthesis produces oxygen faster than respiration takes it up and photosynthesis uses carbon dioxide faster than respiration produces it

Question 41

The concentrations of Carbon dioxide in the air at different heights above the ground in a forest changes over a period of 24 hours. Use your knowledge of photosynthesis to describe these changes and explain why they occur [5 MARKS]

Answer 41

1. High concentration of carbon dioxide linked with night/darkness;
2. No photosynthesis in dark/night / light
   required for photosynthesis/light dependent reaction;
3. (In dark) plants (and other organisms)
   respire;
4. In light net uptake of carbon dioxide by
   plants/plants use more carbon dioxide than they produce/ rate of photosynthesis greater than
   rate of respiration;
5. Decrease in carbon dioxide concentration with height;
6. At ground level fewer leaves/less
   photosynthesising tissue/more animals/less light;

Question 42

microorganisms make the carbon in polymers in a dead worm available to cells in a leaf. Describe how [5 MARKS]

Answer 42

1. Microorganisms are saprobionts
2. Secrete enzymes (onto dead tissue) /
   extracellular digestion;
3. Absorb products of digestion/smaller
   molecules
4. Respiration (by microorganisms) produces carbon dioxide;
5. Carbon dioxide taken into leaves;
6. Through stomata;