Photosynthesis

Question 1

What is the chemical equation for photosynthesis?

Answer 1

6CO2 + 6H2O ➡️ C6H12O6 + 6O2

Question 2

What are two types reactions involved in photosynthesis?

Answer 2

1. Light dependent reaction
   *Trapping of light energy by photosynthetic pigments in chloroplast
   *Use light energy to excite electrons in chlorophyll (photoactivation) and split water (photolysis)
   *Energy produced as ATP and NADPH (reduced NADP)
   *Energy transferred to light-independent reaction

2.Light-independent reaction
*AKA the Calvin cycle
*energy from light-dependent reactions used for…
*Fixation of carbon dioxide/ carbon fixation to produce of complex organic molecules (such as carbs, amino acids and lipids)

Question 3

What are photosynthetic pigments?

Answer 3

*Trap light energy
*Found on thylakoid membrane of chloroplasts
*pigments are arranged in light-harvesting clusters =photosystems

*Diff pigments absorb different wavelengths of light

Two groups of photosynthetic pigments:
1.Chlorophylls
e.g. chlorophyll a,chlorophyll b

2. Carotenoids
   e.g. Beta-carotene,xanthophyll

Chlorophyll a is a primary pigment
*The rest are accessory pigments

Question 4

How are photosynthetic pigments arranged?

Answer 4

• pigments are arranged in light harvesting clusters = photosystems/antenna complex
  *Primary pigments absorb light and act as reaction centres in the light-dependent reactions of photosynthesis
  *Accessory pigments surround a primary pigment
  ➡️Accessory pigments may absorb diff wavelengths of light
  ➡️Light energy are then passed to primary pigments.

Two types of photosystems:
*Photosystem II (PSII)
*Photosystem I (PSI)

Question 5

What is the colour of the photosynthetic pigments?

Answer 5

Chlorophyll:
Absorbs mainly red and blue-violet lights
Reflects green light

Carotenoids:
Absorb mainly blue-violet lights
Reflects red light

Chlorophylls:
Chlorophyll a=yellow-green
Chlorophyll b=blue-green

Carotenoids:
Beta-carotene=orange
Xanthophyll=yellow

Question 6

What is the light absorption spectrum?

Answer 6

*Graph of light absorbance by pigments at different wavelengths of light.

Question 7

What is photosynthetic action spectrum?

Answer 7

*Graph of rate of photosynthesis at different wavelengths of light.
*Related to absorption spectrum (but different)
*Also dependent of wavelength of light
(Shorter wavelength, more energy!)

Question 8

What is chromatography used for?

Answer 8

Used to separate and identify different pigments

Question 9

What are the steps in chromatography?

Answer 9

1) grind/blend leaves
* form suspension of plant extract with mixture of different pigments

2) place extract on a marked line of the chromatography paper using a small dropper. Dry and repeat.

3) place paper into solvent so that solvent is just below the extract

4) allow solvent to run (rise up paper) for a fixed time duration
*Solvent carry different pigments at different speed, so diff distance from loading line
*Depends on pigments mass
*Pigments separated➡️ chromatogram

5) Measure distance travelled by solvent and pigment
6) Calculate Rf value
7)Compare Rf values against published values to identify pigments

Question 10

How do we calculate Rf value?

Answer 10

Rf= distance travelled by pigment/distance travelled by solvent

Question 11

What are the two pathways of light-dependent reaction?

Answer 11

Two pathways:
1. Non-cyclic photophosphorylation
2. Cyclic photophosphorylation

Question 12

What are the steps in light dependent reaction: Non-cyclic photophosphorylation

Answer 12

Step 1: Photoactivation
Step 2: Electron transport chain and ATP synthesis
Step 3: photolysis and reduction of NADP

Question 13

Describe the first step in non cyclic photophosphorylation: photoactivation

Answer 13

Step 1: Photoactivation
*Light energy absorbed by both photosystems (PSII-p680 and PSI-p700)
*Passed to primary pigment at reaction centre
*Electrons excited to a higher energy level
*Electrons emitted from reaction centres
*Electrons captured by electron acceptors

Question 14

Describe the second step of non-cyclic photophosphorylation: electron transport chain and ATP synthesis.

Answer 14

Step 2: Electron transport chain and ATP synthesis
*Electrons passed along the electron carriers of ETC
*Electrons release energy to produce ATP using chemiosmosis

*Energy used to pump H+ across membrane into the thylakoid membrane
*Proton gradient formed across the thylakoid membrane
*H+ move down gradient into stroma
*VIA ATP synthase
➡️To synthesise ATP (from ADP and Pi)

*ATP made is passed to light-independent reaction
*Electrons passed to PS I

Question 15

Describe what happens in the third step of non-cyclic photophosphorylation: photolysis and reduction of NADP?

Answer 15

Step 3: Photolysis and reduction of NADP
*Occurs at PS II only
*Requires enzyme

*Splitting of water to H+ and OH-
*Electrons are removed from OH-

H20 ➡️ 2H+ + 1/2O2 + 2e-

*Electrons-replace electrons lost from PS II
*Oxygen-waste gas, released
*H+ ion-combine with de-energized electrons from PS I to reduce NADP

NADP + 2H+ +2e- ➡️ NADPH

Question 16

How are electrons lost replaced in non cyclic photophosphorylation?

Answer 16

Electrons lost from:
1) PS II
➡️ Replaced by electrons from photolysis of water
H2O➡️2H+ + 1/2O2 +2e-

2) PS I
➡️ Replenished by electrons from PS II, after passing through the ETC

*Electron donor=H2O
*Final electron acceptor=NADP
*NADP reduced to NADPH

Question 17

What can be seen in the “Z scheme” diagram of non-cyclic photophosphorylation?

Answer 17

Z-scheme
*Involves both photosystems
*Electrons flow from PS II ➡️ PS I
*Involves the photolysis of water
*Carried along electron carriers
➡️ATP is synthesized
*Final electron acceptor=NADP

Question 18

Describe cyclic photophosphorylation of the light dependent reaction.

Answer 18

• involves only PS I
• reaction centre of PS I photoactivated
  *Electrons excited and emitted from chlorophyll
• captured by an electron acceptor
• passed along ETC
• energy released by electrons
  ➡️ATP synthesis by chemiosmosis
  *Electrons returned to original Photosystem, PS I

*No photolysis of water involved
*No reduced NADP formed

*Fins electron acceptor= PS I

Question 19

What is the hill reaction used for?

Answer 19

Used to determine the effect of light intensity/ wavelength of light on chloroplast activity
Based on photolysis of water and reduction of NADP in chloroplast

Question 20

How does the Hill reaction work?

Answer 20

1)Chloroplasts are isolated
2)Presence of light and water
3) Redox agents (e.g. DCPIP)
*DCPIP=dichlorophenolindophenol
*Substitute for NADP
*Acts as an electron acceptor/oxidising agent

When DCPIP reduced:
Blue➡️colourless
O2 is produced

Rate of loss of=measure of chloroplast activity Based

Question 21

What are the steps in the light independent reaction

Answer 21

*Occurs in the stroma of chloroplasts
*Does NOT require light

Steps:
1) Fixation of carbon dioxide
2) Reduction
3) Regeneration

Question 22

What is the first step of light-independent reaction?

AKA Calvin Cycle

Answer 22

Step 1: Fixation of carbon dioxide
*Carbon dioxide (1C) is combined with ribulose biphosphate (RuBP) (5C)
*To produce 2 x glycerate-3-phosphate (GP or PGA) (3C)
*Catalysed by enzyme, ribulose biphosphate carboxylase (rubisco)

Question 23

What is the second step of light-independent reaction?

AKA The calvin cycle

Answer 23

Step 2: Reduction
*The reduction of 2 x GP (Glycerate-3-phosphate) (3C)
*Uses ATP and reduced NADP
*Some NADP regenerated
*Produced 2 x triose phosphate (TP) (3C)

1/6 molecules of TP is converted to other molecules (e.g. carbs, amino acids, lipids, hexose, starch) for uses in the plant cell.

GP used to make amino acids/fatty acids/lipids

Question 24

What is the third step of the light independent reaction?

Answer 24

Step 3: Regeneration of RuBP
*5/6 molecules of TP is used to regenerate RuBP(Ribulose biphosphate)
*Uses ATP

Question 25

How does the structure of chloroplast relate to its function?

Answer 25

*Main photosynthetic organelle *Can be seen with a light microscope *Second largest organelle in the cell (after nucleus) *Appear as biconvex discs *2 phospholipid membranes *Stroma inside *Stack of thylakoids=grana (singular granum)

Question 26

How does the structure of grana relate its function in chloroplast?

Answer 26

1. Grana *Membrane has large surface area *Holds pigments, enzymes, electron carriers needed for light-dep.reactions *Holds ATP synthase needed for ATP synthesis by chemiosmosis *Large no. of pigment molecules arranged to absorb light *Arranged in light-harvesting clusters (photosystems) *For efficient light absorption *Diff pigments arranged in funnel-like structures *Each pigments passes energy to next pigment *Till it reaches the reaction centre

Question 27

How does the structure of stroma relate to its function in chloroplast?

Answer 27

*Site of light-indep. reactions of the calvin cycle *Contains enzymes of the calvin cycle,sugars and organic acids *Bathes grana membranes so received products of light-dep. reactions. *Also has 70S ribosomes, loops of DNA, lipid droplets and starch grains *Ribosomes: produce chloroplast proteins (via translation)

Question 28

How does the structure of a palisade mesophyll cell relate to its function?

Answer 28

1. Overall cell arrangement and shape *Cells are closely packed to absorb maximum light *Light surface area for diffusion of gases *Verticle to surface of leaf to reduce number of cross walls *Cell walls thin for maximum light penetration/diffusion of gases *Moist cell surfaces for diffusion of gases *Near air spaces to circulate gases/provide a reservoir of CO2 2. Chloroplasts *Large number of chloroplasts to absorb maximum light *Large vacoule pushes chloroplasts to edge of cell →Short diffusion path for carbon dioxide →Can absorb maximum light *Chloroplasts can move →Towards light →Away from high light intensity to avoid damage

Question 29

How does the structure of dicotyledonous leaf relates to its function?

Answer 29

*Thin/flat to give large surface area to volume ratio *Held at right angles to sun to allow max.light absorption. *Has cuticle on upper surface to prevent water loss via cuticular transpiration *Has closely packed palisade mesophyll arranged vertical to the surface of leaf *Has spongy mesophyll that provides large surface area for CO2 uptake/gas exchange *Mesophyll cells have moist surfaces for diffusion of gases *Has stomata/guard cell for entry of CO2 *Has xylem to supply water/mineral ions and act as support *Has phloem for translocation of products of photosynthesis

Question 30

What is a limiting factor?

Answer 30

A factor that restricts a process's rate

Question 31

What are the limiting factors affecting the rate of photosynthesis?

Answer 31

1. Light intensity 2. Carbon dioxide concentration 3. Temperature

Question 32

Describe how light intensity acts a limiting factor affecting the rate of photosynthesis?

Answer 32

Rate of photosynthesis increases with light intensity But levels off due to other limiting facrs such as temperature and [CO2]

Question 33

Describe how carbon dioxide acts a limiting factor affecting the rate of photosynthesis?

Answer 33

Rate of photosythesis increases with [CO2] But levels off due to limiting factors: temp and light intensity

Question 34

Describe how temperature acts a limiting factor affecting the rate of photosynthesis?

Answer 34

*Rate of photosynthesis increases with temperature *But decreases after the optimum temp because: *Rubisco has higher tendency to catalyze reaction with O2 with RuBP, instead of CO2 (photorespiration) at high temp and high light intensity *Enzymes also start to denature

Question 35

How can we measure the rate of photosynthesis?

Answer 35

*Use aquatics plants such as Elodea and Cabomba *Use gas syringe/microburette/photosynthometer 1. Cut shoot of aquatic plant *Must cut step cleanly,under water *Plant must be well illuminated b4 use 2. Place shoot in tube of hydrogen carbonate solution (to provide CO2) and water bath to mantain temp *Water needs to be aerated beforehand by bubbling air through it, to prevent other gases from dissolving in water 3. Place in different conditions to investigate its effect on the rate of photosynthesis *Light intensity-Place light source 5 min different distances, d from plant *Wavelength of light-Use min 5 different colour filters *Concentration of cafrbon dioxide-Add min 5 diff concentrations of hydrogen carbonate solution to water surrounding plant *Temperature-use water bath of min 5 diff temperatures 4. Alllow aclimatization 5. Count no. of bubbles of oxygen gas produced per unit time OR collect oxygen gas and measure length/volume produced per unit time 6. Repeat at least three times+mean