11 Photosynthesis Flashcards
(24 cards)
Adaptations for photosynthesis
Large SA Arrangement of leaves to minimise overlap Thin so diffusion distance short Transparent cuticle and epidermis Long narrow upper mesophyll cells packed with chloroplasts to collect sun Numerous stomata for gas exch so all mesophyll cells close by Stomata open and close in response to light intensity Many air spaces for rapid diffusion in gas phase Network of xylem, brings water and phloem, carries away sugars
Leaf structure
see textbook page 269
Summary of photosynthesis
See textbook pg 276
Three main stages of photosynthesis
- capturing of light energy 2. The light dependent reaction, some light energy absorbed conserved in chemical bonds, electron flow created by effect of light on chlorophyll, causing photolysis (H2O splits into H+ e- and O2). Products are reduced NADP, ATP, and O2 3. The light independent reaction H+ used to produce sugars + more
Structure of chloroplasts
Disk shaped 1-10 micrometers Double membrane The grana, stacks of disk like structures called thylakoids where LDS takes place, and contain chlorophyll. Some thyl have tubular extensions that join up with thylakoids in adjacent grana, called intergranal lamellae The stromatolites, fluid filled matrix where LIS takes place, contains starch grains
In LD the capture of light is for 2 purposes:
To add Pi to ADP to make ATP To split H20 into H+ and OH- (photolysis)
What happens when chlorophyll molecule absorbs light energy
Boost energy of a pair of e- within the molecule raising them to a higher energy level E- are in excited state They leave the chlorophyll As a result the chlorophyll becomes ionised, photoionisation
What happens after e- leave chlorophyll
Taken up by molecule called an e- carrier Chlorophyll has been oxidised, e- carrier had been reduced
The chemiosmotic theory
Each thylakoids is an enclosed chamber into with H+ are pumped from the stroma using protein carriers in thylakoids membrane: proton pumps The energy to drive this comes from e- released from photolysis Photolysis also produces H+ which further increases their conc in thylakoid space This creates and maintains a conc gradient of H+ across thylakoids membrane with a high conc inside thylakoid space and low conc in stroma H+ can on,g cross thylakoid memb thru ATP synthase channel proteins, the rest of memb is impermeable to H+, the channels form small granules on memb surface and so are known as stalked granules As H+ pass thru ATP synthase channels, cause changes to structure of enz which catalyses ADP + Pi to form ATP
Equation for photolysis of water
Loss of e- when light strikes a chlorophyll molecule, leaves it short of e- If chlorophyll is to continue absorbing light energy e- must be replaced Replaced e come from water molecules 2H2O—> 4 H+ + 4e- + O2
What happens to protons produced by photolysis
H+ pass out of thylakoid space thru ATP synthase channels and are taken up by e carrier called NADP NADP becomes reduces Reduced NADP is main product of LDS and it enters LIS, taking with it e from chlorophyll
Why is reduced NADP important
Further potential source of Chemical energy
How is O2 from photolysis used
Respiration or diffuses out of leaf as waste product
How ATP and reduced NADP are produced during LDS
See textbook page 273
How are chloroplasts adapted to carrying out LDS
Thylakoid membranes provide a large SA for attachment of chlorophyll , e carriers, and enz that carry out LDS A network of proteins in the grana hold the chlorophyll in very precise manner that allows max absorption of light The grana, membranes have ATP synthase channels within them which catalyse the production of ATP , they are also selectively permeable which allows establishment of H+ gradient Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of proteins involved in LDS
The light independent reaction
Products of LDS, ATP and reduced NADP are used to reduce glycerine 3 phosphate in 2nd stage LIS This stage does not require light but requires products of LDS Takes place in stroma Also called Calvin cycle
The Calvin cycle summary
See textbook pg 275
Calvin cycle steps
- CO2 from atmos diffuses into leaf thru stomata and dissolves in H2O around walls of mesophyll cells. Then diffuses thru cell surf membrane, cytopl and chloroplast membrane into stroma 2. In stroma the CO2 reacts with 5C RuBP (ribulose biphosphate), catalysed by ribulose biphosphate carboxylase (rubisco) 3. Reaction between CO2 + RuBP = 2 molecules of 3C GP (glycerate 3 phosphate) 4. Reduced NADP from LDS used to reduce GP to TP (triose phosphate) using energy from ATP 5. NADP is reformed and goes back to LDS to be reduced again by accepting H+ 6. Some TP converted to organic substances e.g. starch, cellulose, lipids, gluc, aa, nucleotides 7. Most TP used to regenerate RuBP using ATP from LDS
How is chloroplast adapted for LIS
Fluid of stroma contains all enz for LIS, stromal fluid is memb bound in chloroplast meaning chemical environ which has high conc of enz and substrates can be maintained in it Stroma fluid surrounds grana and so products of LDS in grana can readily diffuse into stroma It contains DNA and ribosomes so can quickly manufacture some of proteins involved in LIS
Law of limiting factors
At any given moment the rate of a physiological process is limited by the factor that is at its least favourable value
Compensation point
When light is limiting factor rate of photosynthesis directly proportional to light intensity As Light intensity increases vol of O2 produced and CO2 absorbed by photosyn = O2 absorbed and CO2 produced by respiration. At this point no net exchange of gases. This is light compensation pt
Optimum conc of CO2 and importance
0.1 % In atmosphere present at around 0.04% CO2 affects enz activity particularly the combo of RuBP and CO2 in LIS
How temp affects rate of photosynthesis
Increases in direct proportion to temp Between 0 and 25 degrees rate of photos is approx doubled for each 10 degrees rise in temp
How to measure photosynthesis
Measure vol of O2 produced by photosythometer
SEE textbook pg 278
