Photosynthesis Flashcards
autotrophic nutrition
build up of organic and inorganic molecules (eg. carbon dioxide, water) into more complex molecules eg. glucose, starch
photoautotrophs
light energy is converted into chemical energy which is stored within compounds
endosymbiont theory
photosynthetic bacteria (eg. chloroplasts) were acquired by eukaryotic cells by endocytosis to produce the first plant cell, these are then passed on through generations
chloroplast structure and function - membrane
- double membrane with space between them
- outer membrane permeable to small ions and molecules
- inner membrane has transport proteins to allow specific molecules to enter/leave
chloroplast structure and function - stroma
- gel-like matrix that fills inside of chloroplast
- contains carbon dioxide, enzymes and sugars dissolved
- contains ribosomes, loop of DNA, starch grains
chloroplast structure and function - thylakoids
- flattened, fluid filled sacs
- stacks of these make a granum
- contain photosynthetic pigments, enzymes, electron carriers
chloroplast structure and function - stromal lamellae
- connect grana and ensure they are separated
pigments in chloroplasts
chlorophyll a - blue-green
chlorophyll b - yellow-green
carotene - orange
xanthrophyll - yellow
The colour of the pigment is the one that is not absorbed and is reflected
which colours do chlorphylls mainly absorb and reflect
absorb mainly red, blue, violet
reflect mainly green
which colours do carotenoids mainly absorb and reflect
absorb mainly blue, violet
reflect orange, yellow
what happens to the wavelengths absorbed in the leaves?
they are used in photosynthesis
what is a photosystem and why do we need them?
a light-harvesting cluster of pigment molecules in thylakoid membranes
They have a large surface area maximising the amount of light absorbed for photosynthesis
how do photosystems work?
- different pigment molecules arranged in funnel structure
- each pigment molecule is passed down to the next until it reaches the primary pigment reaction centre
- only chlorophyll A can participate in light reactions in photosynthesis but accessory pigments (chlorophyll b and carotenoids) can absorb wavelengths that chrolophyll A can’t
- accessory pigments pass photons (energy) to chlorophyll A broadening the spectrum that drives photosynthesis
difference between photosystem 1 and 2
- PS1 - mostly absorbs wavelengths 700nm (P700)
- PS2 - mostly absorbs wavelengths 680nm (P680)
light-dependent stage of photosynthesis (non-cyclic photophosphorolation)
- light is harvested from accessory pigments and passed to primary pigment reaction centre
- photolysis of water - water is split into protons, electrons and oxygen
- photoionisation - electrons donated to PS2 and are exited to a higher energy level
- electrons are passed down electron transport chain, losing energy as they go
- H+ ions are actively pumped from the stroma into the thylakoid lumen using some of this energy, creating an electrochemical gradient
- photoionisation occurs again by PS1 using light energy absorbed and electrons are excited to higher energy level
- electrons pass over electron carriers
- ferrodoxin donates electrons and protons to NADP reducing it to make NADPH as a product
- H+ ions travel down electrochemical gradient through ATP synthase which catalyses the production of ATP from ADP and inorganic phosphate
- ATP and NADPH are now in stroma
chemiosmosis
- diffusion of protons down conc grad through partially permeable membrane
- this movement releases energy used in formation of ATP
cyclic photophosphorylation
- only uses photosystem 1
- no reduced NADP produced
- electrons return to PS1 after leaving the electron transport chain instead of being used to form NADPH
light-independent stage of photosynthesis (Calvin cycle)
- carbon fixation - CO2 is absorbed by the plant and locked away, this is catalysed by rubisco
- rubisco catalyses reaction of RuBP + CO2 to produce an unstable intermediate with 6 carbons
- unstable intermediate splits into 2 GP molecules with 3 carbons each
- GP is reduced to TP using energy from ATP and hydrogen from NADPH (products from light-dependent stage)
- 1/6 of TP is removed from the cycle to form glucose, starch, cellulose, lipids, amino acids etc.
- 5/6 of TP used to regenerate RuBP so the cycle can continue
- It takes 2 cycles to produce one glucose molecule
light intensity as a limiting factor for photosynthesis
- no photolysis or photoionisation so no products of light dependent stage (ATP and NADPH)
- graph is r-shaped as light can only increase rate so much until temperature or CO2 is limiting factor
CO2 concentration as limiting factor of photosynthesis
- no carbon fixation so none of stages of the calvin cycle can happen - no products
- graph is r-shaped as CO2 can only increase rate so much until light or temperature is limiting factor
temperature as limiting factor of photosynthesis
- high temps - rubisco denatures so no calvin cycle
- stomata close to reduce water loss - no CO2 let in
- graph is n-shaped as there is optimum temperature between denaturing
effect of light intensity on levels of TP, GP and RuBP
low light intensity
- low levels of RuBP and TP because there will be no products from the light dependent stage therefore no ATP to regenerate RuBP and no NADPH to reduce GP to TP
- high levels of GP because there is no ATP to reduce it so it accumulates
effect of CO2 levels on levels of TP,GP and RuBP
low CO2 levels
- low levels of GP and TP because there is no carbon fixation so there’s nothing for RuBP to react with to form them
- high level of RuBP because there’s nothing for it to react with to form the unstable intermediate
why do we need ATP?
movement
active transport
metabolism
cell division
maintenance of body temp
law of conservation of energy
energy can neither be destroyed or created, only converted from one form to another
how do plants convert light energy into ATP?
- light energy converted to chemical energy during photosynthesis - glucose made
- glucose is broken down into ATP during respiration
- ATP used by cells as energy
