The Chemistry of Living Systems CHAPTER 18 The Chemistry of Living Systems 18.2 Photosynthesis and Respiration

Glucose Glucose is the most important energy-yielding molecule Photosynthesis Cellular respiration 6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g) C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)

Photosynthesis Glucose is the most important energy-yielding molecule “light” This reaction is an endothermic reaction that is driven by sunlight. It occurs exclusively in plants and algae. Photosynthesis 6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g)

Why do leaves have a green color? Sunlight (all wavelengths) Why do leaves have a green color?

Chlorophyll Sunlight (all wavelengths) Chlorophyll absorbs blue, violet and red wavelengths Plants appear green because of a large molecule called chlorophyll. Chlorophyll emits green wavelengths

Chlorophyll Chlorophyll absorbs sunlight, and transfers the energy to electrons. These high-energy electrons are used in nonspontaneous reactions.

Photosynthesis Plant cells contain structures called chloroplasts. This is where photosynthesis takes place. Chlorophyll molecules are found inside chloroplasts.

6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g) Photosynthesis light 6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g) Step 1: light dependent reactions Step 2: “dark” reactions

Photosynthesis Step 1: light dependent reactions sunlight Chlorophyll absorbs sunlight, and transfers the energy to electrons Step 1: light dependent reactions high-energy electrons help break down water molecules 2H2O(l) → 4H+ + O2(g) + 4e–

Photosynthesis Step 1: light dependent reactions We need special “carrier” molecules to transport these high-energy electrons to the next step 2H2O(l) → 4H+ + O2(g) + 4e–

Photosynthesis NADP+ + 2e– + H+ → NADPH Energy from the light reaction Energy stored to be transported to the next step NADPH: a molecule that carries two high-energy electrons and stores sunlight as chemical energy.

Photosynthesis ATP ADP + Phosphate group Energy is stored in the bonds between phosphate groups ATP ADP + Phosphate group energy ATP: adenosine triphosphate; a molecule that carries chemical energy from spontaneous chemical reactions to nonspontaneous reactions.

Energy is transported to the next step of photosynthesis Step 1: light dependent reactions Step 2: “dark” reactions

Photosynthesis During the Calvin cycle, no light is needed. Energy comes from ATP and NADPH molecules. CO2 is added, glucose is formed 6 CO2 ~ 1 glucose molecule, C6H12O6 Step 2: “dark” reactions for energy (starch) for plant structure (cellulose)

NADP+ and ADP are recycled Photosynthesis NADP+ and ADP are recycled Step 1: light dependent reactions Step 2: “dark” reactions

Photosynthesis Photosynthesis takes place in chloroplasts. Chlorophyll molecules are found inside chloroplasts. Light reactions Dark reactions

Glucose Glucose is the most important energy-yielding molecule Photosynthesis Cellular respiration 6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g) C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)

Cellular respiration Unlike plants, we cannot produce glucose from sunlight, but must obtain it from food. Glucose is broken down for energy through a process called cellular respiration. As the name indicates, this process takes place inside the cell.

Energy stored for transport Like NADPH in photosynthesis, NADH is a molecule that chemically stores energy. NAD+ + 2e– + H+ → NADH Energy Energy stored for transport NADH: nicotinamide adenine dinucleotide; a molecule capable of carrying high-energy electrons and transferring them to another pathway.

Energy carrying molecules NAD+ + 2e– + H+ → NADH Energy Energy stored for transport NADH and ATP help to store and transport energy ATP ADP + Phosphate group energy

Cellular respiration Step 1: Glycolysis Step 2: Krebs cycle C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) Step 1: Glycolysis Step 2: Krebs cycle Step 3: Electron transport chain

The first step occurs in the cytoplasm (cellular fluid). Cellular respiration Glyco-lysis for “glucose” “breakdown” The first step occurs in the cytoplasm (cellular fluid). Step 1: Glycolysis

Glycolysis Some energy in glucose (6 carbons) At the beginning of glycolysis, some energy is needed to drive the breakdown of glucose

1 molecule of glucose yields 2 molecules of pyruvic acid Glycolysis Some energy in More energy released glucose (6 carbons) pyruvic acid (3 carbons) More energy released More energy is obtained at the end of the process 1 molecule of glucose yields 2 molecules of pyruvic acid

Glycolysis This process is also called “anaerobic respiration” because it requires no oxygen glucose (6 carbons) pyruvic acid (3 carbons) Sprint runners can get some energy from glycolysis, but cannot do this for a long time. They also have to breathe heavily afterward to compensate for this oxygen debt.

Glycolysis More energy can be extracted from glucose! to step 2 of respiration glucose (6 carbons) pyruvic acid (3 carbons) to step 3 of respiration

Cellular respiration Step 1: Glycolysis Step 2: Krebs cycle C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) Step 1: Glycolysis Step 2: Krebs cycle Step 3: Electron transport chain

Krebs cycle Pyruvic acid → → → CO2 + ATP + e– many steps Pyruvic acid → → → CO2 + ATP + e– Step 2: Krebs cycle Steps 2 and 3 of cellular respiration take place in the mitochondrion (an organelle in animal cells)

Electron transport chain The last step receives electrons from NADH molecules to produce more ATP Step 3: Electron transport chain

Electron transport chain The last step receives electrons from NADH molecules to produce more ATP Very large and specific proteins, called cytochromes, help to pass electrons around. Oxygen is the final electron acceptor. 4H+ + 4e– + O2(g) → 2H2O(l) Step 3: Electron transport chain

Cellular respiration 100% 90% Energy left that can be used 62% Only about 38% of the energy in glucose is extracted chemically by the body. The rest is lost as heat.

6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g) Photosynthesis light 6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g) Light reactions Dark reactions

Cellular respiration Step 1: Glycolysis Step 2: Krebs cycle C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) Step 1: Glycolysis Step 2: Krebs cycle Step 3: Electron transport chain