ATP

• ATP (adenosine triphosphate) provides an immediate source of energy to cell - energy contained between phosphate groups

• ATP is synthesised in a condensation reaction between ADP and Pi and catalysed by ATP synthase using energy (adding a phosphate = phosphorylation)

• ATP diffuses into cells and used then hydrolysed back into ADP and Pi catalysed by ATP hydrolase which releases energy

Properties of ATP:

• small amount of energy released = energy is not wasted as heat

• small and soluble = easily diffuse into cells

• easily hydrolysed = energy released instantly

• quickly resynthesised = constant supply of energy

• can phosphorylate other molecules = makes other molecules more active

• cannot pass through cell membranes = cells have immediate energy supply

The Structure of a Leaf

• Large surface area = absorbing as much sunlight as possible

• Thin layers = short diffusion pathway for gases

• Transparent cuticle & epidermis = let light through mesophyll cells

• Long, narrow upper mesophyll cells = lots of chloroplasts to collect sunlight

• Lots of Stomata = gas exchange

• Stomata that can open and close = based on light intensity to minimise product/reactant loss

• Many air spaces in lower mesophyll = allow for rapid diffusion

• Network of xylem = bring water to cells and phloem to remove sugars from photosynthesis

The Structure of a Chloroplast

• Double Membrane

• Grana = stacks of thylakoids ( where light-dependent stage of photosynthesis takes place)

• Photosynthetic pigments = absorb light energy needed for photosynthesis (e.g. chlorophyll a, chlorophyll b and carotene)

• Pigments are found in thylakoid membranes attached to proteins = photosystems

• Two Photosystems are PS1 - absorbs light best at wavelength 700nm and PS11 - absorbs light best at wavelength 680nm

• Stroma is a fluid-filled matrix where the light-dependent stage takes place containing various other structures such as starch grains

Process of Photosynthesis

6Co2 + 6H20 → C6H12O6 + 6O2 = Carbon Dioxide + Water → Glucose + Oxygen

• Complete metabolic pathway - involves many immediate reactions = energy transfers where light energy is used to form chemical bonds

• Photosynthesis takes place in chloroplasts of cells in the leaves mostly which involves the absorption of light by the pigment chlorophyll

1) The Light-Dependent Reaction

Involves the absorption of light by chlorophyll in the thylakoids where energy is used to:

• Form ATP from ADP and Pi - photophosphorylation

• Split Water into H+ ions(protons), electrons and oxygen - photolysis

1. Photoionisation

• Chlorophyll in PSII absorbs light energy and electrons become excited and gain energy

• Electrons then leave the chlorophyll so the chlorophyll becomes oxidised

• Electrons which leave the chlorophyll are taken by an electron carrier which becomes reduced (gained an electron)

2. Making ATP

• Electrons are passed along several electron carriers in the thylakoid membrane through oxidation-reduction reactions = electron transport chain

• Electrons lose energy through each stage → PSI from PSII

• The energy lost as they move through the transport chain is used to transport H+ ions into the thylakoid

• Increased H+ ion concentration than in the stroma so protons diffuse back into the stroma through the enzyme ATP synthase channel

• Energy from the movement of protons diffusing back is used to form ATP from ADP and Pi

• Using light = photophosphorylation

Chemiosmotic theory/ chemiosmosis = the process of electrons flowing down the electron transport chain and creating a proton gradient across the membrane of the thylakoid to drive ATP synthase

3. Making NADPH

• Light energy is absorbed by PSI which excites the electron again

• Electrons are transferred to coenzyme NADP with a H+ ion from the stroma which forms reduced NADP = NADPH - used in light-dependent reaction

4. Photolysis of Water

• Loss of electrons when light hits a chlorophyll molecules in photoionisation the chlorophyll becomes short of electrons

• The replacement electrons are formed through splitting water molecules into electrons, H+ ions and oxygen molecules

• The oxygen formed from this is used up in respiration or diffuses out of the cell as a waste product

This cycle is known as the cyclic photophosphorylation - products formed are ATP, reduced NADP(NADPH) and oxygen(waste product)

• Cyclic photophosphorylation only occurs in PSI photosystem and only produces ATP as a product

• Cyclic refers to the fact that the electrons are not being passed on to NADP - pass back through electron carriers so no reduced NADP or oxygen is formed

Chloroplast Adaptations

• Thylakoid membranes have a large surface area = attachment of chlorophyll, electron carriers and enzymes

• Chlorophyll is very precise = maximum absorption of light

• Grana membrane have ATP synthase channels = catalyse production of ATP & selectively permeable to form a proton gradient

• Chloroplasts contain both DNA & Ribosomes so they can quickly produce some proteins involved in the light-dependent reaction

2) The Light-Independent Reaction(Calvin Cycle)

1. CO2 combines with the 5-carbon compound ribulose biphosphate which is catalysed by rubisco

2. The reaction between CO3 and RuBP produces 2 molecules of 3-carbon glycerate 3-phosphate(GP)

3. ATP produced in the LDR provides energy needed to reduce GP to Triose Phosphate(TP)

4. Reduced NADP from the LDR is used to reduce GP → TP - provides Hydrogen

5. Some TP 1/6 is used to make organic substances - glucose, sucrose, amino acids ,etc

6. Most TP 5/6 - used to regenerate RuBP using ATP from the LDR → provides phosphate

Adaptation - fluid of the stroma contains all enzymes needed for the Light-Independent reaction

Fluid surrounds the grana so products from LDR can readily diffuse into the stroma

Contains Both DNA & Ribosomes so it can quickly carry out protein synthesis needs for the LIDR

Calvin Cycle needs to take place 6 times = 1 molecule of glucose

ONLY ATP IS PRODUCED IN THE LIGHT-INDEPENDENT REACTION

Factors during Photosynthesis

Compensation point = the point at which the vol of O2 produced and CO2 absorbed during photosynthesis is exactly balanced to the O2 used and the CO2 given out during cellular respiration - no net exchange of gases

High Light Intensity = more energy for LDR and more C6H12O6 is produced - too high = chlorophyll becomes damaged

Optimum wavelength - Photosynthetic pigments of chlorophyll can only absorb red and blue light from sunlight - rate of photosynthesis increases

Temperature = optimum around 25 Celsius for enzymes to function at the fastest rate - below = enzymes becomes inactive and temperature rises = enzymes denature

CO2 = high Co2 causes stomata to close so less photosynthesis can take place

Water = Constant supply is needed = too much = waterlogging and lack of oxygen to the roots and mineral ions cannot be absorbed = less ATP for active transport of mineral ions needed for chlorophyll

Limiting Factors = Light intensity, Temperature and CO2

Commercial Greenhouses

Factors to increase rate of growth to achieve optimum yield artificially :

• Artificial Lighting - specific wavelengths

• Pump Co2 into glasshouse - propane/ paraffin burners

• Ventilation

• Glass panels - stop heat from escaping and allows light in

• Thermostat / thermometers

• Humidifiers