4.1-Capturing Solar Energy: Light Dependent Reactions

4.1-Capturing Solar Energy:Light Dependent Reactions

SBI4U1

Photosynthesis

Overall Chemical Equation:6CO2 (g) + 6H2O (l) + energy C6H12O6 (s) + 6 O2 (g)

Simply stated…

Carbon dioxide + water with addition of energy from the sun yields glucose + oxygen.

Photosynthesis…

-Photosynthesis transforms radiant energy of sunlight into chemical energy-Photosynthesizing agents/organisms use approx. 2% of the sun’s energy.• Photosynthesis allows plants to:– Make glucose • Converted to cellulose in cell walls• Starch for energy storage

Two sets of reactions that make up photosynthesis…

“Photo” = light“Synthesis” = rxns that synthesize carbohydrate

Rx. 1- Light Dependent Reaction:Light energy is trapped and used to generate ATP and NADPH( similar to NADH in Cell. Resp)Rx. 2- Light Independent ReactionEnergy from ATP and reducing power of NADPH are used to make glucose

Light Dependent Rxn: rxn that uses solar energy to generate ATP and NADPH (similar to NADH)

Light Independent Rxn: rxn that uses the energy of ATP and reducing power of NADPH to make a high energy organic molecule

*Note: cyanobacteria, algae and plants all carry out photosynthesis, but we are just going to focus on plants for simplicity

Structure of the Chloroplasts

• Photosynthesis factory • 3-8µm in length and 2-3µm in diameter• Outer and inner membranes enclose a space

filled w/ protein rich semi-liquid material called stroma– Thylakoids are within the stroma• Flattened discs

– Stacked thylakoids are called grana, unstacked thylakoids are called lamellae

Levels of Organization in a Plant LeafCarbon dioxide and water that are used to synthesize glucose through photosynthesis are taken upby the leaf and then enter into plant cells and chloroplasts. Water enters the leaf through veins, andcarbon dioxide enters via openings called stomata.

Grana

• Inside the thylakoid sac is the thylakoid lumen– Water filled

• Chlorophyll and ETC proteins are embedded in thylakoid membrane– Chlorophyll is a green coloured pigment that

absorbs light– Common Forms of chlorophyll:chlorophyll a and chlorophyll b

Why Chlorophyll Appears GreenA) Leaves appear green because chlorophyll molecules in leaf cells reflect green and yellow

wavelengths of light and absorb other wavelengths (red and blue).B) This absorbance spectrum for three photosynthetic pigments shows that each pigment

absorbs a different combination of colours of light.

Absorption of Light

• Light is absorbed in packets of energy called photons

• Wavelengths (colour) of light are related to energy– Shorter wavelength more

energy– Longer wavelength less

energy– Electrons can absorb a photon

only if it carries exactly enough energy to allow the electron to move up to another allowed energy level.

Pigment: compound that absorbs visible light– E.g. Chlorophyll a and chlorophyll b

Chlorophyll a absorbs at 400-450 nm and 650-700 nmChlorophyll b absorbs at 450-500 nm and 600-650 nm– Both types of chlorophyll reflect green

Photosystems• Protein based complexes composed of clusters of

pigments that absorb light energy in the thylakoid membrane

• When a pigment molecule absorbs a photon, the molecule passes the energy to the chlorphyll a molecules.

• Light reactions of photosynthesis occur in the thylakoid membrane

• Divided into 3 parts1. Photoexcitation: e- gets excited2. Electron transport: e- transferred to e- carriers,

and protons pump into lumen3. Chemiosmosis: formation of ATP

Photosystems are made up of two parts:1. Antenna complex• Made up of chlorophyll molecules and other

pigments• Absorbs a photon and transfers energy from pigment

to pigment until it reaches chlorophyll a in reaction centre

2. Reaction Centre:• Transmembrane protein complex • Contains chlorophyll a • e- absorb energy and passes e- to an electron acceptor

The Reaction Centre

• The antenna complex is also sometimes referred to as the light-harvesting complex because it gathers (harvests) energy from light so that the energy can be directedto the P680 molecule in the reaction centre.

There are 2 Photosystems:1. Photosystem One - PS I:– Primary pigment is chlorophyll a. – Absorption peak at 700nm– Called P700

2. Photosystem Two - PS II:– Primary pigment is chlorophyll b – Absorption peak at 680nm– Called P680

Arrangement of Photosystem Iand Photosystem II

In the light-dependent reactions, photosystem II passes electrons to photosystem I via an electron transport system, which contains the b6-f complex. This complex acts as a proton pump to produce a proton gradient across the thylakoid membrane. The electrons lost from the reaction centre of photosystem II are replenished by the oxidation of water. Photosystem I uses the electrons to reduce NADP+ to NADPH.

•PSII passes electrons to PSI via an electron transport system (w/ b6-f complex proton pump)•e- lost from PSII are replenished by oxidation of H2O•PSI uses e- to reduce NADP+ to NADPH

PSI and PSII work together to produce ATP and NADPH

Animation: http://www.biology4all.com/resources_library/source/61a.swf

• ATP synthesis in light-dependent rxns is the same as in aerobic respiration

• It is called photophosphorylation– Using photons to drive phosphorylation of ADP to

produce ATP via chemiosmosis

• PSI and PSII, an electron transport system, and ATP synthase enzyme are in the thylakoid membrane

• They works together to make ATP

Animation: http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf

Making ATP by Chemiosmosis

Photosystem I, photosystem II, an electron transport system, and the ATP synthase enzyme are embedded in the thylakoid membrane of chloroplasts. ATP synthesis by chemiosmosis in chloroplasts occurs in a way that is very similar to the way it occurs in mitochondria.

Cyclic Photophosphorylation

Cyclic Phosphorylation

In cyclic photophosphorylation, an electron in P700 is excited by a photon and begins taking the same path that it took in noncyclic photophosphorylation. However, the electron is not used to reduce NADP+ but instead is passed back to the b6-f complex, where the energy is used to generate the proton gradient.

• Chloroplast produce more ATP through cyclic photophosphorylation– Only PSI– Photon excited an e- from P700 in PSI• it follows the same path as in noncyclic• e- is not used to reduce NADP+

• It is passed back to b6-f complex• Creatinfga proton gradient to generate ATP, but not

NADPH– Believed to be used by early bacteria

Learning Expectations...

• Reactants and products of photosynthesis• Structure and function of chloroplasts• Chlorophyll a vs. chlorophyll b• Photosystems (PSI vs. PSII)• ATP production and chemiosmosis in plants• Cyclic vs. noncyclic photophosphorylation

** there are a lot of details here, re-watching the animations may be beneficial**