Photosynthesis biology 1

Photosynthesis plays a key role in photo-autotrophic existence Photosynthesis is a redox process, occurring in chloroplasts, and involves two reactions –Light dependent reaction –Light independent reaction (Calvin Cycle) Pigments in chloroplasts are keyed to react to specific wavelengths of light There are different strategies to photosynthesis, including the C 3, C 4 and CAM pathways

The importance of photosynthesis Life on Earth is balanced between autotrophs (“self”-feeders) and heterotrophs (“other”-feeders) Autotrophs synthesize complex organic molecules (e.g., sugar), utilizing energy from –Light (photo-autotrophs) –Oxidation of inorganics (chemo-autotrophs Autotrophs responsible for ‘producing’ organic molecules that enter ecosystem

Photosynthesis as a redox process A general equation for photosynthesis is: 6CO H 2 O C 6 H 12 O 6 + 6O 2 + 6H 2 O Indicating the net consumption of water simplifies the equation to: 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 The simplest form of equation is: CO 2 + H 2 O CH 2 O + O 2 Thus 6 repetitions of the equation produce a molecule of glucose light

Where does the oxygen come from? Van Niel demonstrated in chemo- autotrophs that: CO 2 + 2H 2 S CH 2 O + H 2 O + 2S Therefore, a general form for the synthesis equation: CO 2 + 2H 2 X CH 2 O + H 2 O + 2X Van Niel summarized that oxygen comes from water (later confirmed with radio-isotopes) light

Photosynthesis as a redox process Hydrogen is extracted from water and incorporated into sugar Electrons have higher potential energy in the sugar molecule Light is the energy source that boosts the potential energy of electrons as they are moved from water to sugar When water is split, electrons are transferred form the water to carbon dioxide, reducing it to sugar

The chloroplast Photosynthesis occurs in chloroplasts Chlorophyll (and other pigments), stored in thylakoid membranes captures light energy - this is where light dependent reactions occur The stroma (matrix inside chloroplast) contains light-independent reactions, reducing CO 2 to CH2O Thylakoids and photosynthetic cells are organized in grana and the mesophyll respectively to maximize absorption of light

The light-dependent reaction Light energy is converted to chemical bond energy found in ATP and NADPH, occurring in thylakoid membranes –NADP + (nicotinamide adenine dinucleotide phosphate) is reduced to NADPH, temporarily storing energized electrons transferred from water, and an H + –O 2 is a by-product of splitting water –ATP is produced via photophosphorylation ATP and electrons are used in next stage to fix carbon

The light-independent reaction Also known as the Calvin cycle Main purpose is to fix carbon (process of incorporating carbon into organic molecules NADPH provides the reducing power ATP provides the chemical energy

How the light reaction works The nature of light –Acts as both a particle and a waveform –As a wave, is a type of electromagnetic energy: visible light consists of a spectrum of wavelengths from 380 nm to 750 nm –As a particle, light behaves as discrete particles called photons, each photon having a fixed amount of energy –In photosynthesis, the most used (absorbed) wavelengths are blue and red. Green is transmitted (hence the green color of chlorophyll)

A substance that absorbs light is called a pigment Each pigment has a characteristic absorption spectrum In the light reaction the most important pigment is chlorophyll a However, the action spectrum for chlorophyll a does not match that for photosynthesis - therefore, other pigments are involved –Carotenoids, e.g., xanthophyll –Chlorophyll b These accessory pigments do not participate directly in the light reaction, but work with chlorophyll a to do so

How pigments aid the light- dependent reaction Photo-excitation Absorbed wavelength photons boost one of the pigment molecule’s electrons in its lowest energy state (ground state) to an orbital of higher energy (excited state) The difference in energy is directly equal to the energy of the photon, and therefore specific to a specific wavelength Conclusion: certain pigments are designed to absorb particular wavelengths

Photosystems Photosystems are organizations of photosynthetic pigments, consisting of –An antenna complex (responsible for inductive resonance, the absorption of energy associated with photons, and passing that energy between themselves –A reaction centre chlorophyll. One molecule of chlorophyll a per photosystem can take that energy use it to push out an electron, passing it to… –…the primary electron acceptor (the first step of the light reaction Two types of photosystem: P700 (photosystem I) and P680 (photosystem II)

Non-cyclic electron flow Excited electrons are transferred form P700 to the primary electron acceptor for photosystem I Primary electron acceptor passes excited electrons to NADP + (goes to NADPH) via ferredoxin Oxidized P700 chlorophyll becomes an oxidizing agent (needs electrons to be replaced). These electrons come indirectly from photosystem II

Electrons ejected from P680 are trapped by the PS II primary electron acceptor These electrons are transferred to an electron transport chain (making ATP), eventually ending in P700 (PS I) Electron space vacated in P680 are filled splitting of water H 2 O O + 2H + + 2e - Light + enzyme to P680 chemiosmosis

Production of ATP via the electron transport chain is termed photo- phosphorylation (light energy used to make ADP into ATP) In this case, ATP production is specifically termed noncyclic photo- phosphorylation

Cyclic electron flow Involves only PS I, P700 In this cyclic system, ejected electrons are fed to the ETC to generate ATP (cyclic photo-phosphorylation) P700 acts as the ultimate acceptor of the shunted electrons No NADPH is produced, or O 2 Aim is to produce extra ATP needed for Calvin cycle

The Calvin cycle Powered by ATP and NADPH from light-dependent reaction Carbon enters cycle as CO 2 and leaves as triose sugar, glyceraldehyde 3-phosphate (G3P) Three phases: –Carbon fixation - a molecule of CO 2 is attached to a CO 2 -acceptor, ribulose biphosphate (RBP) Unstable 6-carbon intermediate degrades into 3-carbon molecule –Reduction - endergonic reaction uses ATP (energy) and NADPH (reducing agent) to convert 3- phosphoglycerate to glyceraldehyde 6-phosphate –Regeneration of RBP (requires ATP) Calvin cycle needs 18 ATP and 12 NADPH to produce 1 molecule of glucose

Alternative mechanisms Photorespiration - a metabolic pathway that consumes O 2, produces CO 2, produces no ATP and decreases photosynthetic output Occurs because Rubisco (enzyme in Calvin cycle), can accept O 2 instead of CO 2 When O 2 conc. higher than CO 2 conc. In leaf, rubisco takes O 2 (e.g., when hot, stomata close, CO 2 drops, O 2 increases)

Rubisco transfers O 2 to RuBP Resulting 5-C molecule splits into Three-C molecule Stays in Calvin cycle Two-C molecule (glycolate) goes to mitochondrion via peroxisome Glycolate broken down to CO 2

Strategies to prevent photorespiration C3 plants produce 3 phosphoglycerate, the first stable intermediate in the Calvin cycle (eg, rice, soy, wheat) C4 plants (eg, corn, sugar, grasses) have different morphology. CO 2 fixation occurs in different location In mesophyll cells, CO 2 is added to phosphoenolpyruvate (PEP) to make oxaloacetate (4-carbons). Enzyme is PEP carboxylase, which has a far higher affinity for CO 2 than O 2 Oxaloacetate is converted to malate (4C), and then moved to bundle sheath cells, where remaining Calvin cycle occurs

Another strategy... In succulents adapted to dry, arid environments, stomata closed during day At night, stomata open and CO 2 is incorporated into a number of organic acids (termed crassulacean acid metabolism: CAM) During day, light reactions produce ATP and NADPH which release CO 2 from organic acids In summary, –C4 plants spatially separate carbon fixation from the Calvin cycle –CAM plants temporally separate carbon fixation from the Calvin cycle