Photosynthesis Flashcards
Chloroplast envelope
- double membrane
- function: keep reactants for photosynthesis close to reaction site
Thylakoids (fluid filled sacs)
- stacked up in chloroplast; grana
- contain chlorphyll
- site of light-dependent reaction
- high conc. ATP
- large SA
Name chloroplast organelles
- outer membrane
- inner membrane
- stroma
- thylakoids
- granum (stack of thylakoids)
- lamella
- lumen (inside of thylakoid)
Photosynthetic pigments
- found in thylakoid membrane
- attached to proteins
- protein + pigment = photosystem
- absorb diff wavelengths
- capture light and provide high energy electrons used to drive chemical reactions; result in production of carbohydrates
- if action spectrum and absorption spectrum are plotted on the same graph, it can be seen they closely resemble each other; proof pigments involved in absorption of light
Stroma
- contains enzymes, sugars, organic acids for light-independent reaction
chlorophyll a
- bluish green pigment
- red light = 680-700 nm
- blue light = 450nm
chlorophyll b
- yellowish green pigment
- red light = 640 nm
- blue light = 450 nm
why leaves are green
- little light between 500-700nm is reflected
- pigment absorb little green light but instead reflected
action spectrum
- wavelength of light absorbed by each pigment
1. pondweed photosynthesise for set time in light at each wavelength
2. measure vol. oxygen
3. plot graph; rate of photosynthesis against wavelength of light
absorption spectrum
- overall rate of photosynthesis at each wavelength of light
1. measure how much light is absorbed using colorimeter for each pigment
why deep water seaweeds appear red at surface
- absorb other wavelengths other than red
- red light does not penetrate deep water so it is reflected
photosynthesis processes
- light-dependent reaction
2. light-independent reaction
light-dependent reaction
- only occur in light
- thylakoid
- light energy is absorbed by pigments and converted into chemical energy
- electrons excited to a higher energy level by the energy trapped by chlorophyll molecules in the thylakoid membranes
- electrons then passed down the electron transport chain from one electron carrier to the next
- > this process generates ATP from ADP and Pi in a process called photophosphorylation - phosphorylation can be cyclic or non-cyclic
- Reduced NADP is also generated in the light-dependent stage, as the electrons are transferred to NADP along with a proton
- Both ATP and reduced NADP are used in the light-independent stage
light-independent reaction
- can occur in dark too
- stroma
- involves reduction of CO2
1. CO2 combined with H makes TP
2. TP makes glucose - cycle to regenerate RuBP
- also known as calvin cycle
- final stage of photosynthesis
- uses ATP (source of energy) and reducing NADP (reducing power to produce glucose
Note:
RuBP = ribulose bisphosphate
GP = glycerate 3-phosphate
PS1/PS2 = photosystem 1/2
Photosynthesis equation
CO2 + water -> Glucose + oxygen
Coenzyme
- aid function of enzyme
- e.g NADP
- NADP transfers H from one molecule to another; can reduce (give oxygen) or oxidise (take hydrogen from) a molecule
Light-dependent stage (Non-cyclic photophosphorylation)
- photon hits chlorophyll in PS2
- electrons are excited
- electrons are taken up by an electron acceptor, passed along an electron transport chain to PS1 chlorophyll. Energy is released, ATP is synthesised.
- photon hits chlorophyll in PS1
- photolysis: water dissociates into hydrogen and hydroxide ions. Replaces lost electrons in PS2 chlorophyll
- electrons are excited
- electrons are taken up by an electron acceptor, passed along an electron transport chain to NADP
- NADP takes up an H+ ion from dissociated water and forms reduced NADP
- hydroxide ions react together to form water and oxygen
How does chemiosmosis produce ATP in the light independent stage?
- H+ ions (protons) move down their conc. gradient from thylakoid space into the stroma via the transmembrane channel protein ATP synthase
- ATP synthase catalyses ADP + Pi -> ATP
light-dependent stage (cyclic photophosphorylation)
- a photon of light hits a chlorophyll molecule
- electrons are excited
- electrons taken up by an electron acceptor
- electrons passed along an electron transport chain. Energy is released, ATP is synthesised
- electron returns to PS1 chlorophyll
difference between non-cyclic and cyclic photophosphorylation
non-cyclic photophosphorylation
= produces ATP and reduced NADP for Calvin cycle to produce biological compounds
- electrons don’t come back to the same molecule
- first electron donor is water
- involves both PS1 and PS2
- last electron acceptor is NADP
cyclic photophosphorylation
= produces additional ATP to meet surplus energy demands of cell
- electrons come back to the same molecule
- first electron donor is PS1
- involves PS1 only
- last electron acceptor is PS1
- ATP only product
light-independent stage - Calvin Cycle
- carbon fixation
- reduction
- regneration
CARBON FIXATION
1. RuBP is combined with CO2 in a reaction called carbon fixation, catalysed by RUBISCO
2. RuBP is converted into 2 GP molecules
REDUCTION
3. reduced NADP + ATP used to convert GP -> GALP
REGNERATION
4. Some GALP molecules are used to make glucose, which is then converted to essential organic compounds e.g polysaccharides, lipids, amino acids and nucleic acids
5. remaining GALP molecules are used to reform RuBP with the help of ATP
photolysis
- light energy splits molecules of water
products:
- H+ ions: move out of thylakoid space via ATP synthase and are used to reduce the coenzyme NADP
- electrons: replace electrons lost from chlorophyll
- oxygen: used for respiration or diffuses out of leaf as waste gas
reduced NADP
How:
- NADP + 2H + 2e -> reduced NADP
- catalysed by dehydrogenase enzymes
Where
- stroma of chloroplasts
roles of ATP and reduced NADP in the light-independent reaction
ATP: reduction of GP to GALP
Reduced NADP: coenzyme transports electrons needed for reduction of GP to TP
Limiting factor
= factor that determines maximum rate of a reaction, even if other factors change to become more favourable
- light intensity (light-dependent stage)
- CO2 levels (light-independent stage)
- temperature (enzyme controlled)
Compensation point
- during daylight, plants both photosynthesise and respire
- during night, plants only respire
- as light intensity increases from dawn, there comes a point at which rate of respiration is equal to rate of photosynthesis
Sun plants
= when exposed to high light intensities, can absorb much of light available to cells
- thick cuticle: reduce transpiration losses in more intense sunlight
- more than one palisade layer with smaller chloroplasts
- thicker leaf; greater distance between upper and lower epidermis
- higher levels of respiration and compensation points
Shade plants
= can absorb light available at lower light intensities. If exposed to high light, most would pass through
- thin cuticle
- single palisade layer with larger chloroplasts to maximise light absorption
- thinner leaf; shorter distance between upper and lower epidermis
- lower rates of respiration and compensation points
