Unit 5 - Energy Transfer

Equation for photosynthesis

Carbon dioxide + water + light energy → Glucose + Oxygen
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Leaf adaptations for photosynthesis

A large surface area to absorb sunlight

Thin leaves to minimise diffusion distance

Transparent cuticle to allow light through

Many chloroplasts

Stomata take in CO₂ for LIR

Xylem and phloem transport reactants/products

Chloroplasts

Found in mesophyll of leaf tissue, site of photosynthesis.

Thylakoids contain chlorophyll and are the site of the light-dependent reaction.

Stroma is fluid around thylakoids and is the site of the light-independent reaction.

Light-dependent reaction

LDR uses water and light energy to produce ATP and NADPH (and oxygen as a by-product).

Light energy is absorbed by PSII (photosystem 2). This excites electrons in chlorophyll, moving the electrons to a higher energy level. These high energy electrons leave the chlorophyll and move down the electron transport chain (ETC) to PSI.

Photolysis of water. These lost electrons from PSII must be replaced, so light energy splits water into H⁺ ions (protons), electrons and oxygen. H₂O → 2H⁺ + ½O₂.

The excited electrons lose energy as they move along ETC, this energy is used to transport H+ ions from stroma to thylakoid, creating a proton gradient - this is chemiosmosis. Protons now move down the gradient from thylakoid to stroma through enzyme ATP Synthase. The energy from this forms ATP.

Light energy is absorbed by PSI which excites the electrons again to a higher energy level. These electrons are transferred to NADP. NADP is reduced (with H⁺ in stroma from ATP Synthase) to form NADPH.

Light-independent reaction

LIR/Calvin Cycle uses products of LDR, NADPH and ATP, to make organic substances for the plant. Takes place in stroma.

CO₂ enters leaf through stomata and diffuses into stroma of chloroplast where it is combined with RuBP and catalysed with enzyme rubisco to create an unstable 6C compound which immediately breaks down into 2× 3C glycerate 3-phosphate (GP).

The hydrolysis of ATP (from LDR)provides energy to reduce GP to triose phosphate (TP). This reduction reaction also requires H⁺ ions which come from oxidising NADPH. 1/6 of triose phosphate is converted into useful organic compounds e.g. glucose. 5/6 is used to regenerate RuBP, with the help of ATP. Therefore the LIR must turn 6 times to produce one hexose sugar (glucose).

Limiting factors of photosynthesis

Light intensity - Light is needed to provide energy to the LDR.

Light wavelength - The chlorophyll in leaves reflects green light so only blue and red light is used for photosynthesis.

Temperature - The enzymes involved (ATP synthase and rubisco) will denature at extreme temperatures. At high temperatures, stomata close to prevent water loss, limiting the amount of CO₂.

CO₂ concentration - 0.4% gives the best rate of photosynthesis.

Water - Plants need a constant supply of water.