5A

Photosynthesis is a reaction in which light energy is used to split apart the strong bonds in water molecules (photolysis), to combine hydrogen with carbon dioxide to produce a fuel in the form of glucose. Oxygen is a biproduct.

Rate of photosynthesis: carbon dioxide concentration, light intensity, and temperature

ATP

  • Made of a ribose sugar, three phosphate groups and adenine

  • When energy is needed, third phosphate bond can be broken through a hydrolysis reaction which is catalysed by the enzyme ATPase. The product of this is ADP (free inorganic phosphate group - Pi and energy)

  • Reversible reaction: ATP can be synthesised from ADP and a phosphate group through a condensation reaction, catalysed by the enzyme ATPase

Light Dependent Stage (occurs in the thylakoid membrane)

  1. Light energy excites electrons in the chlorophyll in the thylakoid membrane, causing them to leave the chlorophyll and pass to an electron acceptor at the start of the electron transport chain (photoionisation).

Excites: causes a pair of electrons in chlorophyll a to move to a higher energy level

  1. Electrons pass down the chain from one electron carrier to the next through a series of redox reactions. The energy released as the electrons pass down generates ATP from ADP and a phosphate group in a process called photophosphorylation. (cyclic)

  2. Light splits water into protons (H+ ions), electrons and oxygen, known as photolysis of water. Electrons replace those lost in the first stage, protons are pumped across the membrane using the ATP produced through a process known as chemiosmosis, creating a potential gradient. (non-cyclic)

  3. Reduced NADP is generated as the electrons in the electron transport chain are transferred to NADP along with hydrogen.

  4. Protons pass back through the membrane through an ATP synthase enzyme. ATP and Reduced NADP are used for the light independent stage.

Photosystems (PSI and PSII)

  • Photosystems are involved in absorbing light required for photosynthesis and converting it into chemical energy

Chlorophyll:

Chlorophyll a = (absorb red and blue light, green light is reflected)

  • Chlorophyll a = primary pigment

  • Other = chlorophyll b, carotenoids, xanthophyll, phaeophytin

Primary pigment + other proteins = reaction center

Accessory pigments+ other proteins = light harvesting system 

Together the light harvesting systems and are known as photosystems

Different accessory pigments absorb different wavelengths of light so as much light energy is harvested as possible

Thylakoid membranes contain different photosystems (I or II)

Chemiosmosis

  • Energy is used to pump protons from the stroma into the thylakoid interior (thylakoid interior is impermeable to protons so cannot diffuse back) - concentration of protons is greater in the interior than the stroma (electrochemical potential gradient)

  • Chemiosmosis activates ATP synthase to phosphorylate ADP to form ATP

  • Cyclic photophosphorylation occurs when chloroplasts require an increased amount of ATP

Light Independent Stage (Calvin Cycle) - occurs in stroma

  1. RuBP is combined with carbon dioxide in a reaction called carbon fixation catalysed by the enzyme RUBISCO

  2. RuBP is converted into two glycerate 3-phosphate molecules (GP)

  3. Reduced NADP and ATP are used to reduce GP to form glyceraldehyde 3-phospate (GALP/ TP). Reduced NADP becomes oxidised

  4. 1/6 GALP molecules are used to make simple sugars such as glucose, fructose and galactose, which are then converted into polysaccharides, amino acids, proteins, lipids and nucleic acids

  5. The remaining 5/6 molecules are used to regenerate RuBP with the help of ATP

RuBP - ribulose bisphosphate (5 carbons)

GP - glycerate 3-phosphate (3 carbons)

GALP - glyceraldehyde 3-phosphate (3 carbons)

RUBISCO - ribulose bisphosphate carboxylase/ oxygenase

Carbohydrates (simple sugars) are made by joining two GALP molecules, polysaccharides are made by joining hexose sugars in different ways.

Lipids are made using glycerol (synthesised by GALP) and fatty acids (synthesised by GP)

Amino acids are made from GP

Nucleic acids are made using GALP

Chloroplasts are the site of photosynthesis:

  • Contains stacks of thylakoid membrane called grana which contain photosynthetic pigments

  • Contain stroma (fluid surrounding the grana) which contains all the enzymes required for the light independent stage of photosynthesis

  • Grana are connected by lamella which maximises efficiency of chloroplasts

Absorption spectrum: graph showing the amount of light absorbed by a pigment against the wavelength of light

Action spectrum: a graph demonstrating the rate of photosynthesis against the wavelength of light

  • Reflected light = colour presented

  • Absorbed light = used for photosynthesis

Chromatography can be used to separate chloroplast pigments

  • Extract pigments from a plant by grinding up leaves with propanone and then filtering it

  • The pigment travels up at different speeds separating the pigments as they have different solubilities in the solvent

  • Calculate Rf value = distance travelled by solute/ distance travelled by solvent