Photosynthesis

Photosynthesis in Overview Process by which plants and other autotrophs convert the energy of sunlight into sugars. Requires sunlight, water, and carbon dioxide. Overall equation: 6 CO2 + 6 H20  C6H12O6 + 6 O2 Occurs in the leaves of plants in organelles called chloroplasts.

Leaf Structure Most photosynthesis occurs in the palisade layer. Gas exchange of CO2 and O2 occurs at openings called stomata surrounded by guard cells on the lower leaf surface. H2O is brought up from roots via tubes called xylem. Palisade Spongy

Chloroplast Structure Thylakoid – membrane bound sacs Chlorophyll is embedded in the membrane (found as part of Photosystem I and Photosystem II) Electron Transport Chain and ATP synthase is here Light reactions occur here Thylakoid lumen – liquid inside thylakoid membranes Granum – a stack of thylakoids LAMELLA – unstacked thylakoids that connect grana Stroma – liquid region outside of the thylakoid membranes (Calvin Cycle or dark reactions occur here

Pigments Chlorophyll A is the most important photosynthetic pigment. It is blue-green in colour and is the only pigment that can participate directly in photosynthesis. Other pigments called antenna or accessory pigments are also present in the leaf. They broaden the spectrum that can be absorbed and transfer the energy over to chlorophyll a Chlorophyll B (yellow / green) Carotenoids (orange / red) Xanthophylls (yellow / brown) These pigments are embedded in the membranes of the chloroplast in groups called photosystems.

Figure 10.9 Location and structure of chlorophyll molecules in plants The pigment molecules have a large head section (porphyrin ring with Mg at center) that is exposed to light in the surface of the membrane; the hydrocarbon tail anchors the pigment molecules into the lipid bilayer.

Figure 10.11 How a photosystem harvests light Chlorophyll a

Photosynthesis: The Chemical Process Occurs in two main phases. Light reactions Light reactions are the “photo” part of photosynthesis. Light is absorbed by pigments AND used to make ATP and coenzymes NADPH that will drive the dark reactions. Dark reactions (aka – the Calvin Cycle) Dark reactions are the “synthesis” part of photosynthesis. ATP and NADPH generated in light reactions are used to make glucose from CO2.

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 3)

Light Reactions Light-dependent reactions occur on the thylakoid membranes. Light and water are required for this process. Energy storage molecules are formed. (ATP and NADPH) Oxygen gas is made as a waste product.

Dark Reactions Dark reactions (light-independent) occur in the stroma. Carbon dioxide is “fixed” into the sugar glucose. ATP and NADPH molecules created during the light reactions power the production of this glucose in a cyclic process

Figure 10.x1 Melvin Calvin