Photosynthesis Flashcards
What is the function of chloroplasts?
- site of photosynthesis in plant cells which takes place in the form of light dependent + light independent reactions
What is the structure of chloroplasts?
- surrounded by double phospholipid membrane
- contains stroma (fluid): site of light independent stage
- contains ribosomes, circular chloroplast DNA + starch grains
- thylakoid membranes (fluid filled sacs that stack to form grana): site of light dependent stage + contains pigments, enzymes + e- carriers
- grana are connected by lamellae (membranes): creates large SA to inc. NO° of light dependent reactions
- membranes contain many pigment molecules arranged in photosystems = inc light absorption
What is the equation for photosynthesis?
- carbon dioxide + water —-> oxygen + glucose
- 6CO2 + 6H2O —-> C6H12O6 + 6O2
How are (dicotyledonous) plant leaves adapted to carry out photosynthesis?
- waxy cuticle: prevents water loss
- transparent upper epidermis: so light can reach palisade cells
- palisade mesophyll layer: tightly packed, elongated cells w many chloroplast to absorb light
- air spaces in spongy mesophyll layer: creates larger SA to inc rate of gas exchange
- stomata on bottom of leaf: guard cells control diffusion of gases into + out of leaf
- thin leaves: inc light absorption + has a shorter diffusion distance so inc rate of gas exchange
What are the 2 stages of photosynthesis?
1st stage: light dependent reaction
2nd stage: light independent reaction (Calvin cycle)
Where does the light dependent stage of photosynthesis occur?
- thylakoid membranes + thylakoid spaces which contain pigments, enzymes + e- carriers
What are the diff stages of the light dependent reaction (LDR)?
- photolysis of water
- photoionisation of chlorophyll
- chemiosmosis + photophosphorylation
Describe the process of photolysis of water in the light dependent reaction.
- light energy, absorbed by chlorophyll, splits water in the thylakoid lumen, into oxygen, H+ and e- = H+ gradient bc of a high conc of H+
- H+ moves down conc gradient into stroma
- e- pass along ETC within thylakoid membrane
- oxygen (waste product) is used for respiration or diffuses out of leaf via stomata
Describe the process of photoionisation of chlorophyll in the light dependent reaction.
- light energy, absorbed by chlorophyll in thylakoid membrane, causes e- to become excited to a higher energy lvl + are released from chlorophyll (ionised)
- they then pass along ETC within the thylakoid membrane + are replaced by e-s from photolysis of water
Describe the process of chemiosmosis + photophosphorylation in the light dependent reaction.
- as excited e- pass along ETC, it produces energy to move H+ ions from stroma to thylakoid lumen, creating a conc gradient
- H+ move down conc gradient, by facilitated diffusion, through ATP synthase back into stroma
- produces energy for phosphorylation of ADP into ATP by ATP synthesis
- NADP (coenzyme) combines w e- at end of ETC + H+ passed through ATP synthase to produce reduced NADP/NADPH (used in LIR)
What products are formed from the light dependent reaction?
- ATP
- NADPH
What do pigments do?
- absorb diff. wavelengths of light
What are the 2 groups of pigments called?
- chlorophylls (primary pigments): absorb wavelengths in blue-violet + red regions of light spectrum = reflect green light, so appears green
- carotenoids (accessory pigments): absorbs wavelength in blue-violet region of spectrum
Describe the method for investigating photosynthetic pigments in leafs by chromatography.
- draw a pencil line (origin line) approx. 1cm above bottom of chromatography paper
- extract pigment from leaf by crushing leaf in a solvent (e.g. acetone) using a pestle + mortar
- add drop of pigment to centre of origin line using a capillary tube
- allow to dry + repeat until pigment dot is 3-5mm in diameter
- suspend paper vertically straight in solvent below the pencil line
- remove paper using forceps + draw a pencil line to mark where solvent moved up to
- calculate Rf values to identify pigments present in leaf
What is the equation for the Rf value?
Rf value = distance moved by pigment / distance moved by solvent
What does the Rf value indicate?
- how far a dissolved pigment travels
- larger Rf value = pigment more soluble
What are the approx. Rf values of the diff. pigments present in chloroplasts?
- chlorophyll A: 0.65
- chlorophyll B: 0.45
- carotene: 0.95
- xanthophyll: 0.71
Why must the origin line be drawn in pencil + not pen?
- bc the ink would dissolve in solvent + run
Why should you draw a line where the solvent reaches immediately?
- bc the solvent evaporates rapidly
Why should you measure the Rf value from the middle of the pigment mark?
- bc it standardises the measurements to allow for comparisons
Why should the chromatography paper be vertical + straight?
- so pigments move straight up paper + not runoff the side
Where does the light independent stage (Calvin cycle) of photosynthesis occur?
- stroma
What are the 3 main steps in the light independent stage (Calvin cycle)?
- carbon fixation
- reduction of GP
- regeneration of RuBP
Describe carbon fixation in the light independent stage (Calvin cycle).
- CO2 reacts w RuBP (5C) [ribulose bisphosphate] to form 2 GP (3C) [glycerate 3-phosphate] molecules which is catalysed by the enzyme rubisco (found in stroma)
