Photosynthesis

Types of Nutrition Heterotrophic – organisms break down complex organic molecules into simple soluble ones. Animals, fungi, some bacteria. Autotrophic – means ‘self-feeding’. Organisms build up complex organic molecules from simple inorganic ones. Plants are photoautotrophs; some bacteria are chemoautotrophs.

Photosynthesis overview Overall equation: 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Where do the oxygen atoms in O 2 come from? Experiments with radioactive isotopes of 18 O showed that they are derived from water.

Stages of Photosynthesis Stage 1 Light-dependent reaction Chlorophyll pigments capture light Light energy converted to chemical energy. Production of ATP and NADPH Stage 2 Light-independent reaction Reduction of CO 2 to glucose Thylakoid membranes Stroma

Light-dependent reaction Chlorophyll molecules are arranged on the thylakoid membranes in groups called PHOTOSYSTEMS. There are two types of photosystems PSI and PSII.

Thylakoid membranes Electron carrier molecules and ATP synthase are also located on the thylakoid membranes.

The energy captured in the light-dependent reaction is used for: 1.Direct synthesis of ATP – a process called photophosphorylation. 2.Splitting water into H + ions and OH - ions – a process called photolysis.

PSI e- Electrons are passed along a series of carriers. Energy from the electrons is used to synthesise ATP. ADP + Pi ATP The electron returns to the chlorophyll a molecule e- Photons of light hit chlorophyll molecules in photosystem I Electrons are excited out of the chlorophyll a molecules in the centre of the photosystem. Cyclic Photophosphorylation

Non-cyclic photophosphorylation PSII e-e- PSI e- ADP + Pi ATP 2H 2 O → + 4e - + 4H + + O 2 Light energy is used to split water molecules e- NADP NADPH Electron excited out of PSII

The Electron Transport Chain As electrons are transferred from one carrier to the next, they lose energy. This energy is used to pump H + ions from the stroma into the lumen between thylakoid membranes. Stroma pH = 8; thylakoid lumen = 4. A proton gradient is established. H + flow out of the lumen down their concentration gradient through an enzyme complex called ATP synthase. As the H + flow through, this provides the energy required to phosphorylate ADP: ADP + Pi →ATP Electron Transport Chain animation

Light energy is used to split water molecules: 2H 2 O → 4H + + 4e - + O 2 The electrons replace the electrons excited out of the chlorophyll molecules in PSII. The H + ions reduce NADP NADP NADPH Oxygen gas is given off – either used in plant’s respiration or released into atmosphere. Light Dependant Reactions of Photosynthesis Animation Light Dependant Reactions of Photosynthesis Animation Photolysis

The Light-dependent reaction Summary: Light energy is used to synthesise ATP (photophosphorylation). Reduced NADP is produced. O 2 is given off. The ATP and NADPH produced in the light dependent reaction are used in the second stage of photosynthesis to synthesis glucose from carbon dioxide.

The Light-independent reaction Calvin Cycle Takes place in the stroma of the chloroplast. CO 2 enters the plant’s leaves via stomata and dissolves in water around the palisade cells. It then diffuses through cell membrane, cytoplasm and chloroplast membranes into the stroma.

Sequence of events in Calvin Cycle CO 2 is fixed by combining it with ribulose bisphosphate, RuBP a 5C compound using the enzyme ribulose bisphosphate carboxylase (rubisco). The resulting 6C compound is unstable and immediately splits into two 3C molecules of 3- phosphoglycerate (PGA). The ATP from the light dependent reaction provides the energy needed to convert the GP into 1,3 diphosphoglycerate which is then reduced by the coenzyme to produce glyceraldehyde 3-phospate (G3P) (You have seen this in glycolysis). H + are also needed to reduce the PGA – these are provided by the NADPH produced in the light dependent stage.

Summary of Calvin Cycle CO 2 Ribulose bisphosphate 5 carbon 2 Glycerate 3 phosphate 3 carbon 2 G3P 3 carbon Glucose ATP and NADPH ADP + Pi and NADP ATP ADP + Pi Animation

Fate of G3P Most G3P is used to reform ribulose bisphosphate, using ATP from the light dependent reaction. Some G3P is converted to glucose. The Calvin Cycle has to turn six times to make one molecule of glucose. Glycerol can be synthesised from G3P.