5.6- Photosynthesis

oxidation

• atom gains oxygen

• atom loses electron

• atom loses hydrogen

reduction

• atom loses oxygen

• atom gains an electron

• atom gains hydrogen

relationship between photosynthesis and respiration

the products of one reaction are the raw materials for the other

autotroph

an organism that uses an external energy source and inorganic molecules to make complex organic molecules

• photoautotroph- uses light energy to make complex organic molecules

compensation point

the point when photosynthesis and respiration proceed at the same rate- no net gain or loss of carbohydrate

structure of chloroplast

• 2-10μm long, 1μm diameter

• outer membrane is highly permeable

structure of thylakoid membrane

• form discs that contain chlorophyll

• each stack of thylakoids is called granum- can contain many thylakoids

• some thylakoids have tube shape extensions- join with thylakoids in another grana→ intergranal lamellae

• light dependant stage takes place in thylakoids

stroma

• fluid filled matrix around grana

• light independent stage takes place here

adaptations of chloroplasts to their function (6)

• granal membranes provide large SA for chlorophyll electron carriers and enzymes to attach

• network of proteins in grana hold chlorophyll in specific way to allow max. light absorption

• fluid in stroma contains enzymes needed for Light independent stage of photosynthesis

• chloroplast is membrane bound→ optimal pH for enzyme function

• granal membranes have ATPase channels→ catalyse ATP production

• stroma surrounds grana→ products of LDR can easily diffuse into stroma LIDR

ATP

• cells cannot get energy directly from glucose→ energy released from glucose during respiration used to synthesis ATP

• ATP acts as energy carrier→ transports energy around cell

• ATP synthesis from ADP and inorganic phosphate→ catalysed ATP synthase

products of light dependent reaction

• For Calvin Cycle:

  • ATP

  • NADPH

• Oxygen

• occurs in thylakoids

photosynthetic pigments

• molecules which absorb light energy→ absorbs certain wavelengths and reflects others

• arranged in photosystems in thylakoid membranes

types of chlorophyll

• chlorophyll a

• chlorophyll b

chlorophyll a

• primary pigment

• two types:

  • P₆₈₀

  • P₇₀₀

• both found at centre of photosystems→ primary reaction centre

• P₆₈₀ found in photosystem II→ peak absorption is light at wavelength 680nm

• P₇₀₀ found in photosystem I→ peak absorption is light at wavelength 700nm

• chlorophyll a also absorbs blue light wavelength 450nm

structure of chlorophyll

• long hydrocarbon chain and a porphyrin head (ring structure) containing magnesium ion in the centre

• when light hits magnesium 2 electrons become excited and leave chlorophyll

accessory pigments

• absorb wavelength that are not well absorbed by chlorophylls→ pass energy to chlorophyll a at base of the photosystem

• chlorophyll b:

  • absorb light at wavelengths between 500-640nm

  • appears blue-green

• carotenoids:

  • absorb blue light (400-500nm)

  • reflect yellow and orange light

light dependant stage of photosynthesis

1. water is in thylakoid membrane. light hits PSII, breaking water molecule down→ photolysis

   • H₂O→ 2H⁺+2e^-+1/2O₂

   • H+ accumulates, e- used to restabilise chlorophyll a, O2 by product

2. light energy excites 2e^-.

3. Transferred into electron carriers→ take electrons through electron transport chain:

   • each carrier has higher affinity for e^- than the previous.

   • e^- move in series of redox reactions

   • ATP produced→ releases energy

4. energy released along ETC used to actively transport protons into thylakoid space. H+ move through channel coupled to ATP synthase. Movement drives production of ATP from ADP and P_i.

5. arrive at PSI. Light energy excited 2e^-. Electrons can either undergo cyclic phosphorylation OR passed to ferredoxin.

6. 2H+ from photolysis, 2e- and NADP form reduced NADP→ catalysed by NADP reductase

phosphorylation

• making of ATP using light energy

• 2 types:

  • cyclic

  • non-cyclic

non-cyclic phosphorylation

• light energy absorbed by PSII

• light energy excites electrons in chlorophyll

• high energy electrons move along ETC to PSI→ energy released used to synthesise ATP

  1. Light energy also absorbed in PSI→ excites electrons to higher energy level

  2. electrons transferred to NADP along with H+ to form NADPH

cyclic phosphorylation

• only uses PSI

• electrons used aren’t passed to NADP, but passed back to PSI via electron carriers

• electrons recycled and used repeatedly to flow through PSI

• produces small amounts of ATP

• no photolysis of water or generation of NADPH

the light independent stage

• produce organic compounds needed by the plant including glucose

• requires ATP and NADPH

• reactions happen without light but will eventually stop in its absence as NADPH and ATP is not being replaced

The calvin cycle

1. carbon fixation/ carboxylation: RuBP (5 carbon compound) and CO₂ react to make unstable 6C compound→ catalysed by RuBISCO

2. 6C compound breaks down into 2 GP (3 carbon)

3. NADPH from LDR reduces GP into 2 TP molecules (3 carbon)

4. 1 carbon used to produce organic molecule e.g. glucose→ remainder regenerated into RuBP

law of limiting factors

at any given moment, the rate of photosynthesis is limited by the factor that is least favourable

limiting factors of photosynthesis

• light intensity

• carbon dioxide concentration

• temperature

light intensity

• higher LI= higher rate of e- excitation= higher rate of LDS

• after certain point, no change as other factor is limiting

CO2 concentration

• more CO2= more carboxylation=higher rate

• after certain point, no change as another factor becomes limiting

temperature

• higher temp= more collisions= more ESCs made= higher rate

• after optimum temp, enzymes denature= less products made

how light affects levels of GP, TP and RuBP

as light levels fall:

• RuBP levels fall due to less TP

• TP levels fall

• GP levels rise due to accumulation as a result of low TP

how CO₂ levels affect levels of GP, TP and RuBP

as CO2 levels fall:

• RUBP levels increase as they cannot accept CO2

• GP and TP levels fall as GP cannot be made

affect of water on photosynthesis

• cells become plasmolysed as insufficient water taken up by roots

• plant roots produce abscissic acid→ causes stomata to close, reducing gas exchange

• tissue becomes flaccid

• rate of photosynthesis falls