Photosynthesis Flashcards
Chloroplast structure ?
- 2-10 μm diameter
- Surrounded by a double membrane envelope - each envelope membrane is phospholipid bilayer
- outer membrane is permeable to ions/small molecules
- inner membrane has transport proteins - allow certain molecules to enter/leave chloroplast
What is stroma?
Fluid filled matrix in chloroplasts (CO2,sugars,enzymes/other molecules dissolved in stroma)
What system of membranes are found in stroma?
Consists of a series of flattened fluid-filled sacs - THYLAKOIDS
(contain PHOTOSYSTEMS that contain pigments,enzymes,electron carriers)
- thylakoids stack up to form GRANA —> CREATE LARGE SA
GRANA connected by stroma lamellae
What is a photosystem?
System of photosynthetic pigments found in thylakoids of chloroplasts
- different pigment molecules arranged in funnel-like structures in thylakoid membrane
- large no. Pigment molecules in an arrangement —> MAX LIGHT ABSORBED
What does the stroma also contain?
70S ribosomes- proteins coded for by the loop of DNA produced at 70S ribosome
Loop of DNA- codes for some chloroplast proteins
Starch grains - sugars formed during photosynthesis stored as starch in starch grains
Where does each stage of photosynthesis take place?
Light dependent - thylakoids membranes of the GRANA
Light independent - in stroma
Adaptations of chloroplasts to photosynthesis?
STROMA : gel-like fluid contains ENZYMES that catalyse reactions of light independent stage
- rapid transport of products from light dependent stage as surrounds GRANA/thylakoid membranes
GRANA: Granal stacks create LARGE SA-> more photosystems can be present —> max light absorption
- more space for electron carriers/ATP synthase enzymes
DNA: chloroplast DNA (cpDNA) has genes that code for some proteins/enzymes used in photosynthesis
Ribosomes: allows translation of proteins coded by cpDNA
Inner membrane of chloroplast envelope: selective transport proteins control flow of molecules between stroma/cytoplasm of plant cell (cytosol)
2 types of pigment?
Chlorophylls (a/b)- absorb blue-violet/red regions of light spectrum
- reflect green
ACCESSORY PIGMENT: Carotenoids - absorb wavelengths in blue-violet region
- reflect yellow/orange
Why are accessory pigments useful?
They surround primary pigment + Absorb both similar/different wavelength of light to chlorophyll
- expands the wavelength range that can be absorbed for use in photosynthesis
What are the different photosystems?
Photosystem 1 (PSI) - P700
chlorophyll a which has a max absorption of light at 700nm
Photosystem 2 (PSII)- P680
Chlorophyll a has max absorption of light at 680nm
Stages of light dependent stage of photosynthesis?
- Light harvesting at photosystems
- Photolysis of water - light energy used to breakdown water —>
- ATP produced in photophosphorylation
- Reduced NADP (NADPH) produced when H+ ions in the stroma + electrons in ETC combine with carrier molecule, NADP
What is photophosphorylation?
Using light energy/electron transport chain to phosphorylate ADP to ATP
What is photolysis?
Splitting of water into H+ ions, electrons, oxygen in thylakoid lumen
2H20 —> 4H+ + 4e- + O2
Non cyclic photophosphorylation? 1-3
- Light absorbed by PSII
- 2 electrons in chlorophyll are excited to higher energy level + emitted from chlorophyll molecule —> PHOTOIONISATION
- Electron passes down electron transport chain to PSI (chain of electron carriers (proteins with Fe at centre)
Non cyclic photophosphorylation 4-6?
- As excited electrons leave PSII to PSI, they are replaced by electrons from photolysis
5.Excited electrons lose energy as they pass along ETC - this energy used to transport protons into thylakoid via proton pumps
Thylakoid has higher conc of protons than stroma- PROTON GRADIENT - Protons move down their conc grad into stroma via enzyme ATP synthase - chemiosmosis
Energy from this movement combines ADP + Pi —> ATP
Non cyclic photophosphorylation 7-8?
- Light energy absorbed by PSI, excites electrons to even HIGHER energy level
- Electrons transferred to NADP with a proton (H+ ions from stroma) to form reduced NADP
2H+ + 2e- + NADP —> NADPH
Cylic photophosphorylation?
INVOLVED PSI ONLY
Light absorbed by PSI
Pair of electrons gain energy/become excited to higher energy level
Electrons pass down electron transport chain and passed back to PSI
As electrons travel along electron carriers, small amount of atp generated (BUT NO PHOTOLYSIS OCCURS SO NO proton,oxygen, NADPH produced)
- energy used to transport H+ from stroma to thylakoid lumen via PROTON PUMP
- build up of proton in thylakoid used to **synthesis ATP from ADP+ Pi **(in chemiosmosis)
How does cylic and non cyclic photophosphorylation differ?
Cyclic only involved PSI , non clyclic involved both PS1/PS11
Cyclic does not produced reduced NADP (non cylic does)
Which products of the light dependent stage are needed for light independent reaction and what does this stage produce?
Energy from ATP and hydrogen from REDUCED NADP used for light independent reaction (Calvin cycle)
Needed to produce carbohydrates and other organic molecules :
Starch - storage
Sucrose - translocation around plant
Cellulose- make cell walls
Why cant light independent stage continue indefinitely in darkness?
Requires inputs of ATP and reduced NADP
Stages of Calvin cycle?
- CO2 combines with RuBP (5C) - catalysed by enzyme RuBisCo
By accepting COO- group RuBP becomes 6C unstable intermediate , which breaks down - Product of this is 2x GP (3C) - CO2 has now been fixed
- hydrogens from NADPH and ATP (from light dependent stage) used to reduce GP —> TP
- 1/6 of TP molecules used to produce useful organic products needed by the plant (e.g glucose)
- 5/6 of TP molecules used to regenerate RuBP —> NEED ATP FOR THIS
TURNS 6 TIMES TO MAKE ONE HEXOSE SUGAR(needs to produce 2xTP molecules)
- 18 ATP and 12 NADPH needed from light dependent reaction
Uses of TP?
- TP can condense to become hexose phosphates (6C) —> used to produce starch,sucrose,cellulose
- TP can be converted to glycerol/GP can be converted to fatty acids (MOLECULES JOIN TO FORM LIPIDS FOR CELL MEMBRANES)
- TP used in production of amino acids for protein synthesis
- TP recycles to regenerate RuBP
Limiting factors of photosynthesis? What are limiting factors?
If any limiting factor is below optimum level, rate of photosynthesis reduced
CO2 Supply
Light
Temperature
How does rate of photosynthesis depend on light intensity (given CO2/temp are constant)?
Rate of photosynthesis increases as light intensity increase
- greater light intensity, more energy supplied to the plant , so faster light dependent stage
Produces more ATP/reduced NADP for Calvin cycle , so light independent can also happen at greater rate
What happens if light intensity continues to increase ?
Rate of photosynthesis will plateau
- no longer a limiting factor of photosynthesis
How does CO2 conc affect photosynthesis (if light/temp are constant)?
The rate of photosynthesis increases as carbon dioxide concentration increases
- needed for light independent stage of photosynthesis (carbon fixation)
- so the more CO2 , the faster this step is in the Calvin cycle —> faster rate of photosynthesis
- Trend continues unless another factor is in limit/prevent rate from increasing
How does temperature affect photosynthesis (if light/CO2 are constant)?
As temperature increases the rate of photosynthesis increases, as the reaction is controlled by enzymes
- trend only continues to certain temp as *enzymes will denature/rate of reaction will decrease
TEMP affects CALVIN CYCLE as enzyme controlled (RuBisCO)
Increasing temp can also cause:
- stomata to close to reduce water loss —> CO2 cannot enter leaves —> photosynthesis slows down
- Membrane permeability - dissipation of proton gradients in thylakoid (for light dependent) —> SLOWER
Why does temp not have much effect on light dependent reactions?
As thye are driven by energy from light instead of KE of reacting molecules
What is the effect of light intensity on GP,TP and RuBP levels?
LIGHT : decrease in light intensity causes DECREASE IN TP/RuBP conc + slight INCREASE IN GP
-Less light so light dependent stage STOPS/ doesn’t form ATP and NADPH for LI STAGE
-So GP builds up as not converted to TP
-Lack of TP means RuBP wont form
-Fixation of CO2 will stop/conc of GP will plateau
What is the effect of CO2 on GP,TP and RuBP levels?
Low CO2 CONC - decreases conc of GP + TP, increase RuBP conc
RuBP accepts CO2 so when lack of CO2, it remains unfixed/builds up*
Lack of carbon fixation prevents GP/TP from forming
How are limiting factors balanced in agriculture ?
Glasshouses - sensors monitor light intensity , humidity, CO2 conc
So they grow through night /all year round
Computer manages all factors/adjustable to maximise photosynthesis rate
Pesticides/natural pests used to MAX YIELD
Must find balance between cost/environment /yields
Differences between photosynthesis and respiration?
- ATP produced by photophosphorylation in photosynthesis but produced by substrate level and oxidative phosphorylation in respiration
- Occurs in chloroplasts , aerobic in mitochondria, anaerobic in cytosol/cytoplasm
- NADP vs NAD used
- ETC involved in photosynthesis/aerobic , not in anaerobic
How many TP molecules are made in relation to CO2 molecules in Calvin cycle?
2 molecules of TP made for every CO2 molecule used
