Photosynthesis: Light Reactions Ms. Bush, Ms. Cohen Plants and Photosynthesis Unit

What exactly do we mean when we say that photosynthesis is powered by light energy from the sun? Sunlight is a type of energy called radiation, or electromagnetic energy, which travels as rhythmic waves. Light absorbing molecules called pigments, built into the thylakoid membranes, absorb some wavelengths of light and reflect other wavelengths.

Visible radiation drives the light reactions Different pigments absorb light of different wavelengths. Chloroplasts contain several kinds of pigments (chlorophyll a and b as well as accessory pigments such as carotenoids). LAB: Chromatography Lab separating pigments in a spinach leaf.

Light & Pigments Chlorophyll a absorbs mostly blue-violet and red light. Chlorophyll b absorbs mostly blue and orange light. NOTE: Chlorophyll absorbs light very well in the blue and red regions, but not the green. This is why plants are green. Colors that you see are wavelengths of light that are reflected back to your eye. Accessory Pigments such as the carotenoids absorb mainly blue-green light; they reflect yellow-orange light. The colors of the leaves in autumn show the colors of the carotenoid pigments.

Photosystems capture solar power. When a pigment molecule absorbs a photon, one of the pigment’s electrons jumps to a higher energy level called an excited state. The excited state e- is very unstable so it drops back down to its ground state releasing excess energy as heat and light. NOTE: This is what makes a black car so hot on a sunny day. DEMONSTRATIONS: CHEM. - The Flame Test - different solutions of dissolved salts heated up in the bunsen burner turn the flame different colors. BIO. - Some isolated pigments, including chlorophyll, emit light as well as heat after absorbing photons. Heat up spinach leaves in alcohol, take the chlorophyll solution produced and illuminate. The chlorophyll emits photons that produce a reddish afterglow called fluorescence. Of course chlorophyll in the thylakoid membrane behaves a bit differently. The dashed box in the figure represent neighboring molecules that capture the excited electron before it drops back to the ground state.

Photosystems capture excited electrons In the thylakoid membrane, pigments and proteins are organized into clusters called photosystems. From the photosystem, the captured electron is passed to an electron transport chain.

Two types of photosystems have been identified in the thylakoid membrane. Photosystem I Photosystem II Photosystem II (P680) - absorbs 680 nm of light best Photosystem I (P700) -absorbs 700 nm of light best Named after their order of discovery (although II functions first in the sequence of steps that make up the light reaction).

Light Reactions: Pigment absorbs photons of light. Energy is passed to other pigment molecules and finally to the reaction center of Photosystem II, where an electron is excited. Excited electron is captured. Water is split, and electrons replenish ones that were lost in PII. Oxygen atoms combine to form O2 as a waste product, and hydrogen ions build up in the thylakoid space. Electrons are passed along a transport chain that provides the energy for the synthesis of ATP. Light energy excites an electron of pigments in PI, which are quickly replaced by an incoming electron from the ETC. The excited electron of PI is passed through a shorter electron transport chain to NADP+, reducing it to NADPH.

Chemiosmosis powers ATP synthesis in the light reactions. The process of chemiosmosis drives ATP synthesis using the potential energy of a concentration gradient of hydrogen ions across a membrane. The gradient is created when an electron transport chain pumps hydrogen ions across the membrane from the stroma into the thylakoid space as it passes electrons down the chain. The energy of the concentration gradient drives H+ back across the membrane through ATP synthase. The enzyme, ATP synthase, couples the flow of H+ to the phosphorylation of ADP. In photosynthesis, this chemiosmosis production of ATP is called photophosphorylation because the initial energy input is light energy.

A mechanical analogy of the light reactions: