Modes of Nutrition

• There are two possible sources of energy for living organisms: light energy or chemical energy.

MODES OF NUTRITION

ENERGY SOURCE

Light Chemicals

CA

RB

ON

SO

UR

CE Carbon

dioxide

Produce oxygen: • Plants • Algae

• Cyanobacteria Do not produce oxygen: • Some prokaryotes

• Some prokaryotes

(all use inorganic substances, such as H2S)

Organic compounds

• Some prokaryotes

• All animals • All fungi

• Most protists • Most prokaryotes

Metabolic Pathways

• There are two major types of metabolic pathways:

o Pathways that assemble small molecules into larger ones, such as photosynthesis, are called anabolic pathways; they are energy-requiring.

o Pathways that break large molecules down into smaller ones, such as cellular respiration, are called catabolic pathways; they are energy-releasing.

Glucose Catabolism

• To remain alive, most cells depend on the chemical energy stored in organic molecules, primarily carbohydrates, although lipids and proteins can be used when carbohydrates are not available. But glucose is the fuel of choice.

• The cell cannot use thermal energy to overcome the large activation energy required to release energy from glucose by direct burning because it would destroy the cell. Instead, the cell uses two strategies:

o The overall process of releasing energy from glucose is split into several chemical reactions, each of which has a much smaller activation energy. They form a metabolic pathway.

o Each of these chemical reactions has its own enzyme, which lowers the activation energy of each reaction even further.

• The available energy in a glucose molecule is stored in the electrons in the C—H covalent bonds.

• At certain points in glucose breakdown, these bonding electrons are removed from the glucose molecule and are added to carrier molecules.

• These carriers transport the high-energy electrons to an electron transport chain (ETC), where their energy is used to regenerate ATP from ADP. An ETC is a series of molecules embedded in a membrane.

• The low-energy electrons are added to a final electron acceptor.

Electrons and Energy

• Glucose breakdown involves two electron carriers: NADH and FADH2.

o NAD+ and FAD are empty carriers—each is missing two high-energy electrons.

o NADH and FADH2 are full carriers—each is carrying two high-energy electrons.

o Full carriers transfer the high-energy electrons to an ETC. o Carriers that give up electrons to an ETC are then recycled back to

earlier steps where they can be reused.

Glycolysis

• Glucose breakdown begins with glycolysis, a series of 10 chemical reactions that convert one 6-carbon molecule of glucose into two 3-carbon molecules of pyruvate.

o Two ATP molecules and two molecules of NADH are produced. o Glycolysis takes place in the cytoplasm and does not require oxygen. o Glycolysis is a major ATP-producing pathway that occurs in all cells,

eukaryotes and prokaryotes alike. This is evidence that it evolved early in the history of life on Earth.

• If both an electron transport chain (ETC) and a final electron acceptor are available, respiration, the next stage of glucose breakdown, begins.

Respiration

• In respiration, each pyruvate is completely broken down to three CO2 molecules by the transition reactions and the Krebs (or citric acid) cycle.

o Four NADH molecules, one FADH2 molecule, and one molecule of ATP are produced for each pyruvate during this process.

Transition Reactions and the Krebs Cycle

Electron Transport Chain

• Also in respiration, an electron transport chain releases the large amount of potential energy in the electrons carried by NADH and FADH2 to make about 32 or 34 ATP molecules.

o The low-energy electrons at the end of the ETC are passed to a final electron acceptor.

• If the final electron acceptor is molecular oxygen (O2), this type of respiration is called aerobic respiration. The end product is H2O.

o The theoretical maximum yield of ATPs per glucose molecule in prokaryotes is 38; in eukaryotes, it is 36.

Aerobic Respiration

• Most cells do aerobic respiration. The transition reactions and Krebs cycle occurs in the mitochondrial matrix in eukaryotes and in the cytoplasm of prokaryotes. The ETC is in the mitochondrial inner membrane in eukaryotes and in the plasma membrane of prokaryotes.

Anaerobic Respiration

• If the final electron acceptor is an inorganic substance other than molecular oxygen (O2)—such as sulfate or nitrate compounds—this type of respiration is called anaerobic respiration.

o Anaerobic respiration is used mainly by prokaryotes that live in extremely-low-oxygen environments, such as deep in soil, deep underwater, or in the digestive tracts of animals such as humans.

o Not only do they not use oxygen, but many are actually poisoned and/or killed if exposed to it.

o The yield of ATP by anaerobic respiration varies but is always somewhat less than aerobic respiration.

• Fermentation is an alternative to respiration if the cell lacks an electron transport chain, or if there is no final electron acceptor available to operate the chain.

o Pyruvate is not completely broken down to CO2. o Pyruvate is the final electron acceptor. It accepts the high-energy

electrons from NADH to form a fermentation product, regenerating NAD+ molecules.

o A total of only 2 ATPs are obtained by glycolysis and fermentation. o Fermentation occurs in the cytoplasm. o Many textbooks incorrectly refer to fermentation as anaerobic

respiration.

Fermentation

• Various fermentation products are possible, for example:

o Brewers’ or bakers’ yeast produces CO2 and ethanol. (This organism routinely does fermentation even when oxygen is present.)

o In the absence of O2, human muscle cells produce CO2 and lactate.

Aerobic Respiration vs. Fermentation

• The final products of both aerobic respiration and the direct burning of glucose are H2O and CO2, so both processes release the same amount of energy.

o In direct burning, this energy disperses to the surroundings as heat (thermal energy); none is stored.

o But in aerobic respiration, much (but not all!) of the released energy is stored in ATP.

• However, the final products of fermentation include organic compounds that have more chemical energy than H2O and CO2 combined. Therefore, less energy is released for making ATP than in aerobic respiration or the direct burning of glucose.

• Of the available energy in glucose, 2% can be captured as ATP in fermentation but 40% in aerobic respiration (in prokaryotes):

Photosynthesis

• Photosynthesis (or carbon fixation) is the anabolic pathway that makes organic compounds, especially glucose, from carbon dioxide using the energy from sunlight.

o Green plants, algae, and cyanobacteria do photosynthesis that releases oxygen. This is called oxygenic (oxygen-producing) photosynthesis.

o Some types of bacteria do photosynthesis without releasing oxygen. This is called anoxygenic (not oxygen-producing) photosynthesis.

Pigments Capture Light Energy

• Pigments are molecules that absorb specific wavelengths (energies) of light and reflect all others; the color we see is the net effect of all the light reflecting back at us.

o When a photosynthetic pigment absorbs light energy, an electron in its outermost shell is boosted to a higher electron shell.

o The “excited” electron and its potential energy can be transferred to another molecule.

o The major pigment used in oxygenic photosynthesis (green plants) is chlorophyll a.

Absorption spectra for

chlorophyll a and chlorophyll b.

Oxygenic Photosynthesis

• Oxygenic photosynthesis consists of two major stages:

o The light-dependent reactions, which require sunlight;

o The light-independent reactions (or Calvin cycle), which require ATP and NADPH.

The Light-Dependent Reactions

• The light-dependent reactions occur in the thylakoid membrane of the chloroplast:

1. Light energy is absorbed by chlorophyll a, energizing (“exciting”) electrons in its outermost shell which then leave the molecule.

2. Chlorophyll a replenishes its electrons with low-energy electrons obtained by splitting water molecules, which also produces O2 and H+.

3. The energy of excited electrons that have left chlorophyll a is used to make ATP in an electron transport chain.

4. The excited electrons and H+ are picked up by NADP+, which becomes NADPH.

The Light-Independent Reactions

• The light-independent reactions (or Calvin cycle) occur in the stroma of the chloroplast.

o High-energy electrons from NADPH and energy from ATP are used to convert carbon dioxide into glucose.