Topic 5.2 Photosynthesis

Question 1

Light

Answer 1

Only 2% of the suns radiation landing on an area is turned into sugar by photosynthesis
-Some is transmitted
-Some is reflected
-Some misses
-Some goes through
-Some is ‘wrong’ wavelength

Question 2

Pigments: Absorption Spectrum

Answer 2

The absorption of light compared to wavelength.

(Graph that shows the percentage of each wavelength of light that a pigment absorbs)

Question 3

Pigments: Action Spectrum

Answer 3

The rate of photosyntehsis against wavelength

(Graph that shows the overall rate of photosynthesis at each wavelength of light. Strongly corresponds to absorption spectrum of chlorophyll a)

Question 4

What is photophosphorylation?

Answer 4

A process that drives the production of ATP.

Question 5

Photosystem absorption

Answer 5

-Each system contains as different combination of chlorophyll pigments and so absorbs light in a slightly different area of the spectrum.
-PSI: wavelength of 700nm
-PSII: wavelength of 680nm

Question 6

The Light Dependent Reaction
(Summary)

Answer 6

Occurs on the thylakoids:
1) Photon of light hits chlorophyll molecules.
2) The pigment becomes ‘excited’ and emits two high energy electrons.
3) These excited electrons pass down an electron transfer chain on the thylakoid membrane.
4)Chemiosmosis results in the phosphorylation of ADP into ATP.

Question 7

Cyclic photophosphorylation

Answer 7

1) Light hits photosystem I
2) Electrons are excited to a higher energy level and leave the photosystem
3) The excited electrons pass along the ETC, releasing energy as they do so
4) The energy released provides energy to drive chemiosmosis
5) At the end of the electron transport chain the electrons rejoin photosystem I in a complete circle
6) The ATP produced enters the light-independent reaction

Question 8

Non-Cyclic photophosphorylation: Part 1

Answer 8

1) Light energy hits photosystem II in the thylakoid membrane
2) Two electrons gain energy and are excited to a higher energy level
3) The excited electrons leave the photosystem and pass to the first protein in the electron transport chain (and generate ATP)
(As the excited electrons leave the photosystem II they are replaced by electrons from the photolysis of water)
4) The electrons pass down the ETC and release energy which enables chemiosmosis

Question 9

Chemiosmosis simplified

Answer 9

H+ ions move through the membrane along a concentration gradient

Question 10

Non-Cyclic photophosphorylation: Part 2

Answer 10

6) At the end of the ETC the electrons from photosystem II are passed to photosystem I
7) Light energy also hits photosystem I, exciting another pair of electrons which leave the photosystem
8) The excited electrons form photosystem I also pass along an ETC
9) These electrons combine with hydrogen ions from the photolysis of water and the coenzyme NADP to NADPH (H+ + 2e- + NADP —> NADPH)
10) The reduced NADP and the ATP pass to the light-independent reactions

Question 11

Light Dependent Reaction summary

Answer 11

-Thylakoids of chloroplast which contain chlorophyll
-Photons hit chlorophyll which becomes excited
-Two high energy electrons are emitted
-Electrons pass down ETC on thylakoid membrane
-ATP is synthesised by photophosphorylation
-Energy is used for photolysis (splitting of water) to produce two electrons (to replace those lost)
-Electrons are used to reduce NADP to NADPH
-NADPH and ATP are used in the Light Independent reaction
-Oxygen is released into the atmosphere

Question 12

Light independent reaction

Answer 12

-Calvin benson cycle
-ATP and reducted NADP used
-CO2 fixed
-CO2 converted to glucose
-Occurs in stroma of chloroplasts
(Discovered by Melvin Calvin using his ‘lollipop’ apparatus).

Question 13

The Calvin cycle

Answer 13

Reduction of carbon dioxide by a series of reactions known as the Calvin-Benson cycle.
3 stages:
1) Carbon fixation
2) Reduction
3) Regeneration

Question 14

Where does the independent reaction occur?

Answer 14

In the chloroplasts

Question 15

Limiting factors: External

Answer 15

-Light intensity (affects the amount of chlorophyll excited)
-Light wavelength
-Wind speed
-Water availability
-Temperature (enzyme activity)
-Carbon dioxide concentration (Calvin cycle)

Question 16

Limiting factors: Internal

Answer 16

-Type and concentration of photosynthetic pigments
-Enzyme and water content
-Leaf structure

Question 17

Compensation point

Answer 17

When the carbon dioxide produced by respiration is equal to the carbon dioxide taken up by photosynthesis.

Question 18

Suggest how changing wavelength of light affects rate of photosynthesis

Answer 18

-Highest rate in violet range ~ 450
-Chlorophylls reflect green light so rate slows 490-570
-Rate increases in organs range 590-620
-Slowest rate above 650

Question 19

The 2 main groups of photosynthetic pigment

Answer 19

-Chlorophyll (made of chlorophyll a & chlorophyll b)
-Caroteniods (carotene & xanthophyll)

Question 20

Where are photosynthetic pigments found?

Answer 20

Embedded in thylakoid membrane within chloroplasts.

Question 21

Explain the role of chlorophyll a

Answer 21

Primary synthetic pigment.
Mainly absorbs wavelengths in violet-blue (430nm) & orange-red (662nm) parts of the spectrum.
Emits electrons after absorbing photons of light.

Question 21

Explain the role of chlorophyll b

Answer 21

Accessory pigment with absorption peaks 453nm & 642nm.

Question 22

Explain the role of carotenoids

Answer 22

Act as antioxidants to prevent damage to other pigments via non-photochemical quenching of excess photons.
Mainly absorb wavelengths in the blue green part of the spectrum.

Question 23

Why do many plants have a variety of photosynthetic pigments?

Answer 23

To widen the range of wavelengths of light that can absorb to ensure maximum rate of photosynthesis. Particularly important for plants in shaded conditions.

Question 24

Method of extracting photosynthetic pigments

Answer 24

Use a pestle and motar to grind a leaf with an extraction solvent, eg. propanone.

Question 25

Structure of a chloroplast

Answer 25

-Usually disc-shaped
-Double membrane (envelope)
-Thylakoids: flattened discs stack to form grana
-Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
-Stroma: fluid-filled matrix with high enzyme and substrate concentration

Question 26

What are the purposes of cyclic and non-cyclic photophosphorylation

Answer 26

-Cyclic: produces additional ATP to meet surplus energy demands of cell
-Non-cyclic: produces ATP and reduced NADP for Calvin cycle to produce biological compounds.

Question 27

What happens in photolysis of water?

Answer 27

Light energy splits molecules of water
2H2O —> 4H+ + 4e- + O2

Question 28

What happens to the products of the photolysis of water?

Answer 28

-H+ ions: move out of the thylakoid space via ATP synthase & are used to reduce the coenzyme NADP
-e-: replace electrons lost from chlorophyll
-O2: used for respiration or diffuses out of leaf as waste gas.

Question 29

The Calvin cycle: What happens during carbon fixation?

Answer 29

1) Reaction between CO2 & ribulose bisphosphate (RuBP) catalysed by ribulose bisphosphate carboxylase (RUBISCO).
2) Forms unstable 6C intermediate that breaks down into 2 x glycerate 3-phosphate (GP).

Question 30

The Calvin cycle: What happens during reduction?

Answer 30

3) 2 x GP are reduced to 2 x glyeraldehyde-3-phosphate (GALP).
4) Requires 2 x reduced NADP & 2 x ATP.
5) Forms 2 x NADP & 2 x ADP.

Question 31

The Calvin cycle: What happens during regeneration?

Answer 31

-After 1C leaves the cycle, the 5C compound RuP forms
-RuBP is regenerated from RuP using 1 x ATP
-Forms 1 x ADP