Photosynthesis and ATP Synthesis

Topic 1: Photosynthesis and ATP Synthesis

• Photosynthesis involves autotrophs, which use inorganic compounds to create organic substances, or carbohydrates.
• These carbohydrates store energy for the organism in the form of ATP.
• The usage of ATP is called respiration and the resultant energy is the universal energy currency.
• Photosynthetic organisms house chloroplasts or chlorophyll in membranes.

Photosynthesis equations

• The word equation is: Carbon dioxide + Water -> Glucose + Oxygen.
• The chemical equation is: 6CO2 + 6H2O -> C6H12O6 + 6O2.
• The process is divided into light-dependent and light-independent reactions.

Light-dependent reactions

• Light energy breaks down water molecules during photolysis.
• Oxygen is released as a byproduct of the process.
• Hydrogen molecules pair with carbon dioxide to form glucose.
• These reactions occur in the thylakoids within the chloroplast.

Light-independent reactions

• These reactions take place in the stroma of the chloroplast.

Dicotyledonous Leaf Structure/Function

Upper Epidermis

• Thin and transparent cells, coated with a waterproof waxy cuticle.
• The waxy cuticle limits water loss, protects from insects and microbes, and allows sunlight penetration.

Lower Epidermis

• Thin cell layer with guard cells that form stomata.
• Guard cells curve to open the stomata by absorbing water through osmosis, enabling diffusion and transpiration.

Spongy Mesophyll

• Loosely packed cell layer with numerous air spaces.
• It facilitates material diffusion and gaseous exchange between the palisade mesophyll and stomata.

Palisade Mesophyll

• Cylindrical upright cells with many chloroplasts to maximize light absorption in long, narrow air spaces.
• Contains a large vacuole to keep chloroplasts positioned for maximum light, and is adjacent to vascular bundles that supply water via the xylem.
• Thin cell walls facilitate gaseous diffusion.
• Chloroplasts are mobile due to proteins in the cytoplasm.

Chloroplast Structure

• Double-membraned organelles containing lamellae and thylakoids, which form grana.
• Large surface area efficiently traps light.
• Thylakoid lumens hold H+ ions to aid in ATP production.
• Contains photosynthetic pigments like chlorophyll a and b, plus carotenoids.
• Chlorophyll a absorbs longer light wavelengths.
• Chlorophyll absorbs all wavelengths, except green, with carotenoids absorbing all wavelengths except red and orange.

Photophosphorylation & Photosystems

• Photophosphorylation uses light to attach a phosphate to an organic compound.
• Light energy enables an electron to move through a series of electron carriers forming an electron transport chain in the thylakoid membranes..
• Electron is like a 'ball in a pinball machine' powered by light captured by a chlorophyll-protein photosystem complex.
• The two types of chlorophyll-protein photosystem complexes are PSI and PSII.
• PSI generates ATP or NADPH.
• NADPH is an electron donor and reducing agent essential for photosynthesis.
• Vitamin B3 is a key cofactor for NAD and NADP, easy to remember with “P” (photosynthesis).

Light-Dependent Phosphorylation - Cyclic vs. Non-cyclic

• ATP is the main goal for both.
• Both involve excited electrons from chlorophyll molecules passing through an electron transport chain.

Cyclic

• Defined as ATP synthesis during the light reaction stage.
• Electrons cycle to and from Photosystem I (PSI), returning to the PSI chlorophyll.
• Usually occurs in isolated chloroplasts and photosynthetic bacteria only.

Non-Cyclic

• Defined as ATP synthesis requiring an electron donor while producing oxygen.
• Excited electrons pass from PSII to PSI, replacing any lost electrons in PSI.
• Electrons reduce NADP to NADPH, while PSI produces ATP.
• This process usually occurs in green plants.

Other Differences

• Photosystem involved: cyclic uses PSI only, non-cyclic uses PSI and PSII.
• Reduced NADP made: No in cyclic, Yes in non-cyclic.
• PSI replacement electrons found in cyclic only via the electron that it emitted itself, or in from the photolysis of water in non-cyclic.
• PSII is not involved in cyclic.
• Predominance: Anaerobic conditions for cyclic, aerobic conditions for non-cyclic.
• Evolution of oxygen: no in cyclic, yes in non-cyclic.
• During photolysis water is split and oxygen and hydrogen released.
• H+ ions reduce NADP.
• Electrons return to PSI

Calvin Cycle - Light-Independent Reactions

• ATP and NADPH are key products of photophosphorylation, to excite electrons and occur in the thylakoids.
• The goal is to produce glucose and other carbohydrates occurring in the stroma of the chloroplast.
• RUBISCO helps to combine carbon dioxide with RuBP; glucose needs a carbon to be 6C (hexose) and RuBP is a 5C sugar (pentose).
• Build some 3C sugars (triose), glucose comes from triose phosphate, or glycerate-3-phosphate (G3P, PGA).
• G3P is also used to re-initiate the cycle.

Carbon Fixation

• A CO2 molecule and 5C RuBP combine with rubisco.
• The intermediate (not glucose) splits into 3C triose phosphates.

Reduction

• ATP and NADPH convert those 3C sugars to G3P.
• NADPH donates electrons.
• G3P molecules make glucose, or amino acids if N is added.

Regeneration

• ATP is used to recycled unused G3P molecules back to RuBP to restart cycle.

Key Compounds and Functions

• NADPH: reduced NADP, reducing agent for G3P formation.
• G3P: forms glucose, sucrose, cellulose and other carbs, helps reform RuBP, restarts Calvin.
• RuBP: a 5C sugar that takes in CO2 to become 6C and eventually glucose.
• Rubisco: enzyme plants use to combine RuBP with CO2.
• H+ ions: protons released during photolysis to help form NADPH.

Rate of Photosynthesis Limiting Factors

• Photosynthesis requires: energy, raw materials, reasonable temperture, light-capturing pigments like chlorophyll.
• Low light decreases rate, increasing light intensity past optinum has no effect.
• Increasing or decreasing the optimum temperature 25C and 35C. has less rate of photosyntiesis.
• Low carbon dioxide coniditions decease rate and saturates.
• Greenhouses use artificial light
• Paraffin lamps release carbon dioxide to improve.
• Growth chambers precisely control temperature/CO2.
• Hydroponics optimizes mineral ion delivery.

Factors that can impact rate

• Sunlight provides energy, electrons become excited in chlorophyll centers to eventually trigger reactions.
• Limited chlorophyll molecules cause light to stop and then the level is saturated.
• Carbon dioxide donates carbon atoms to RuBP through rubisco to generate hexose sugars and glucose.
• Limited CO2 slows Calvin Cycle process, rate decreases and saturates, leading to plateaus.
• Enzymes have globular proteins that typically have tertiary structures which bonds break with higher kinetic energy.
• RuBisCO's bonds can break, leading to enzyme structure change to malfunction by combining RuBP with OXYGEN that wastes RuBP, decreasing photosynenthic levels.

Dicotyledonous Leaf Structure/Function

Chloroplast

• Cylindrical cells arranged in an upright manner to provide a lot of chloroplsts for light absorption which creates long and narrow air spaces.
• Contains a very large vacuoloe to keep keep chloroplasts on outside parts of the cells, and adjacent to vasuclar bundles.
• Thin cell walls facilatate afficient diffusiong of gases.
• Chloroplasts are more Mobile due to contain proteins in the cytoplasm.

Green Houses

• Have artificial light can continue beyond daylight and at a high intesity intensity.
• Using paraffin increases photosynthesis due to produing CO2 and heat.

Growth Chamber

• Precisely can regulates temperature and CO2 regulation crops can become sensitive.

Hydroponics

• Plants have growon in liquids systems can give mineral ions to make organic matter production.