11 Photosynthesis

Question 1

Adaptations for photosynthesis

Answer 1

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

Question 2

Leaf structure

Answer 2

see textbook page 269

Question 3

Summary of photosynthesis

Answer 3

See textbook pg 276

Question 4

Three main stages of photosynthesis

Answer 4

1. 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

Question 5

Structure of chloroplasts

Answer 5

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

Question 6

In LD the capture of light is for 2 purposes:

Answer 6

To add Pi to ADP to make ATP To split H20 into H+ and OH- (photolysis)

Question 7

What happens when chlorophyll molecule absorbs light energy

Answer 7

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

Question 8

What happens after e- leave chlorophyll

Answer 8

Taken up by molecule called an e- carrier Chlorophyll has been oxidised, e- carrier had been reduced

Question 9

The chemiosmotic theory

Answer 9

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

Question 10

Equation for photolysis of water

Answer 10

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

Question 11

What happens to protons produced by photolysis

Answer 11

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

Question 12

Why is reduced NADP important

Answer 12

Further potential source of Chemical energy

Question 13

How is O2 from photolysis used

Answer 13

Respiration or diffuses out of leaf as waste product

Question 14

How ATP and reduced NADP are produced during LDS

Answer 14

See textbook page 273

Question 15

How are chloroplasts adapted to carrying out LDS

Answer 15

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

Question 16

The light independent reaction

Answer 16

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

Question 17

The Calvin cycle summary

Answer 17

See textbook pg 275

Question 18

Calvin cycle steps

Answer 18

1. 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

Question 19

How is chloroplast adapted for LIS

Answer 19

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

Question 20

Law of limiting factors

Answer 20

At any given moment the rate of a physiological process is limited by the factor that is at its least favourable value

Question 21

Compensation point

Answer 21

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

Question 22

Optimum conc of CO2 and importance

Answer 22

0.1 % In atmosphere present at around 0.04% CO2 affects enz activity particularly the combo of RuBP and CO2 in LIS

Question 23

How temp affects rate of photosynthesis

Answer 23

Increases in direct proportion to temp Between 0 and 25 degrees rate of photos is approx doubled for each 10 degrees rise in temp

Question 24

How to measure photosynthesis

Answer 24

Measure vol of O2 produced by photosythometer

SEE textbook pg 278