Photosynthesis Chapter 4

Source of almost all energy utilized by living things is the sun Source of almost all energy utilized by living things is the sun. Plants harvest solar energy and convert it into chemical energy (carbohydrates) in the process of photosynthesis. This stored energy is either utilized directly by the plant or passed on through the food chain to animals that feed upon those plants. How is the energy stored in food molecules recaptured and converted into the energy necessary to do work?

Goals for lecture Photosynthesis Respiration Understand how plants use sunlight to convert carbon dioxide and water into sugar Respiration Understand how most creatures (plants, animals, fungi) convert sugar into carbon dioxide and water, thereby releasing energy for use by their cells

Photosynthesis What is it? Who does it? How does it work? Why should we care?

Photosynthesis 6CO2 + 6H2O  C6H12O6 + 6O2

Photosynthesis What is it? 6CO2 + 6H2O  C6H12O6 + 6O2 Carbon dioxide + water + sunlight ---> sugar and oxygen

Photosynthesis Who does it? algae

Photosynthesis Who does it? algae mosses, ferns, higher plants no fungi rarely animals (corals, sea slugs)

Photosynthesis Why should we care?

Photosynthesis How does it work? Light capture (energy storage) Conversion of CO2 and H2O to sugar

Photosynthesis How does it work? Light capture (energy storage) Light reactions Conversion of CO2 and H2O to sugar Calvin Cycle

The light reactions take place in the thylakoids The Calvin cycle take place in the stroma

The “energy currency” of the cell: Sunlight is an energy source that is hard to spend in the cell. It’s kind of like having a really big bill in your pocket: you’re rich, but you can’t use it!

The “energy currency” of the cell: By contrast, the molecules ATP and NADPH are easily spent, like pocket change: chemical bond energy stored in these molecules is easily released, making these molecules quick sources of energy for cellular reactions.

Four things happen in the light reactions of photosynthesis: 1. Chlorophyll absorbs light energy 2. Some of this energy is transferred to chemical bond energy in the manufacture of ATP 3. Some of the energy is used to split water (H2O) into H and O. The O is released from the plant as molecular oxygen. 4. The H from the water combines with NADP+ to form NADPH.

About Chlorophyll …

Figure 10.10 Excitation of isolated chlorophyll by light In fluorescent molecules, energy excites electrons in the molecules, boosting them to a higher energy state. As the electrons return to their normal, or ground, state, they give off some of that energy as light. When light impinges on the chlorophyll molecule, electrons are similarly boosted to a higher state. However, the electron does not return directly to its ground state, but rather leaves the atom and is transferred to another molecule that accepts it, an electron acceptor molecule.

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5) In the chloroplast, chlorophyll and associated electron acceptor molecules are arranged into units called photosystems. There are two kinds, Photosystem One and Photosystem Two, each of which uses a slightly different form of chlorophyll. The photosystems are located in the thylakoid membrane system inside the chloroplast.

Figure 10.13 A mechanical analogy for the light reactions Light energy impinges on Photosystem I, exciting an electron and boosting it to a higher energy state. The electron is then passed to an electron acceptor molecule, which then passes it to another (like an electron bucket-brigade), until they are accepted by the compound NADP+. The NADP+ and the electrons (e-) and protons present in the chloroplast unite to form NADPH.

Figure 10.13 A mechanical analogy for the light reactions Photosystem II is similarly activated by light energy. Again, when this happens, an electron lost from the chlorophyll molecule is boosted up and is accepted by an electron acceptor molecule. It is then passed from one electron acceptor molecule to another through an electron transport chain. A little bit of energy is lost at each step along the way, energy that is captured and used to make ATP.

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5) But what about the electron lost from Photosystem II? There’s a sort of ‘electron hole’ left by its departure: electrons lost from Photosystem II are replaced with electrons pulled away from the hydrogen atoms in water (H2O). The protons (H+) remaining are the ones that get used to from the NADPH at the end of Photosystem I. The oxygen is liberated and released as a gas. This is why plants give off oxygen!

Summary of the Light Reactions Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 2) Summary of the Light Reactions In summary, in the light reactions, energy from the sun is used to generate ATP and NADPH. In the process, water is consumed and oxygen is given off. The ATP and NADPH made during the light reactions are used to power the Calvin cycle.

The Calvin Cycle In the Calvin Cycle, solar energy that was captured and stored (as ATP and NADPH) in the light reactions is used to convert carbon dioxide to sugar. How does this happen?

That is, how do you make sugar (C6H12O6) from carbon dioxide (CO2)?

The light reactions take place in the thylakoids The carbon dioxide used in photosynthesis enters the leaf at the stomates. Once inside the spongy mesophyll of the leaf, the CO2 diffuses across the cell membrane and into the stroma of the chloroplast. The Calvin cycle take place in the stroma

= carbon atom + + CO2 CO2 RuBP RuBP PGA PGA Glucose In the first reaction of the cycle, a molecule of carbon dioxide is added to a five carbon sugar ribulose 1-5 biphosphate (RuBP) to form 2 (3 carbon) molecules of PGA. These are rearranged through a series of energy-requiring reactions, using up ATP and NADPH to generate 2 molecules of PGAL. (If this were done six (6) times we now would have 12 molecules of PGAL. Two (2) of the PGALs are removed to make one glucose while the remaining 10 go back into the cycle to regenerate six (6) of the five (5) carbon sugars ribulose 1-5 biphosphate, the 5-carbon sugar we began with in step one. Thus the process is a cycle, named the Calvin cycle for Milton Calvin, the physiologist who first elucidated the process. PGAL P P P P P P PGAL

Calvin Cycle Arithmetic Six turns of the Calvin cycle yield one molecule of sugar and restore the six RuBP molecules you started with. One turn of the cycle: 1 CO2 + 1 RuBP = 2 PGAL (1 carbon) (5 carbons) (6 carbons, 3 per PGAL) Six turns of the cycle: 6 CO2 + 6 RuBP = 12 PGAL (6 carbons) (30 carbons) (36 carbons, 3 per PGAL) Using 2 PGAL (total = 6 carbons) to make a molecule of sugar leaves 10 PGAL (30 carbons), enough to make 6 molecules of RuBP, which is what you started with. So six turns of the cycle do indeed yield a molecule of sugar AND restore the 6 RuBPs you started with.

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 3) Basically Photosynthetic plants trap solar energy in the form of ATP and NADPH, which they then use as an energy source(s) to make sugars from carbon dioxide. They simultaneously release oxygen into the atmosphere.

Figure 10.0 Sunbeams

Respiration Next we will focus on the process of cellular respiration, in which energy that enters the biosphere as solar energy and is captured as chemical (food) energy during photosynthesis is burned to release that energy for doing work.

All living cells in the plant body carry out cellular respiration!

Respiration What is it? Who does it? How does it work? Why should we care? C6H12O6 + 6O2  6CO2 + 6H2O Plants, animals, fungi, bacteria

Respiration How does it work? Why should we care? C6H12O6 + 6O2  6CO2 + 6H2O Produces chemical energy, which powers cells: Without it, you’re dead!

Respiration How does it work? C6H12O6 + 6O2  6CO2 + 6H2O Energy released by this reaction is used to make ATP. energy stored in sugar energy stored in ATP

Respiration How does it work? Cytoplasm Glycolysis Mitochondria Krebs cycle Electron transport

Citric acid cycle CO2

The last electron acceptor molecule passes its electrons to oxygen to produce a molecule of water. It is because oxygen is required as the final electron acceptor in the electron transport chain that oxygen is required for aerobic respiration.

Clicker Question What becomes of the water your body produces during cellular respiration? You breathe it out as water vapor through your nostrils You sweat it out through your sweat glands It accumulates in your body to prevent dehydration It is excreted via your kidneys You breathe it out as water vapor through your nostrils You sweat it out through your sweat glands It accumulates in your body to prevent dehydration It is excreted via your kidneys