HOW CELLS RELEASE ENERGY The last chapter was concerned with how certain cells cyanobacteria, algae, plants) capture energy (photosynthesis). This chapter is concerned with how cells release that energy. Chapter 7

All cells (prokaryotic & eukaryotic) require energy to: combat entropy carry out day-to-day functions repair/replace worn out organelles reproduce What form of energy do cells use? ATP

Most cells break down nutrients to make ATP by: How do cells obtain ATP? All cells must make their own ATP from nutrients they have either synthesized (autotrophs) or consumed (heterotrophs). Most cells break down nutrients to make ATP by: Cellular respiration (aerobic process) Fermentation (anaerobic process) Aerobic - requiring oxygen (O2) Anaerobic - lacking or not requiring oxygen (O2)

A. Cellular Respiration (aka. Aerobic Respiration) Biochemical pathways that extract energy from nutrients, in the presence of oxygen. Occurs in cells of most eukaryotes & some prokaryotes. General equation for cellular respiration of glucose: C6H12O6 + 6O2  6CO2 + 6H2O + 30 ATP Eukaryotes - protists, fungi, plants & animals Prokaryotes - bacteria & archaea

Electron Transport Chain Cellular respiration occurs in 3 stages: Eukaryotic cells Prokaryotic cells Cytoplasm Glycolysis Krebs Cycle Electron Transport Chain Cytoplasm Mitochondria Glycolysis, Krebs cycle & electron transport chain occur in different locations within cells. Cell membrane

1. Glycolysis (“glucose-splitting”) Glucose (6C) is split into two pyruvate (3C) molecules. does not require oxygen energy harvested/glucose: 2 ATP (via substrate-level phosphorylation) 2 NADH (actively transported into mitochondria of eukaryotic cells) Pyruvate = pyruvic acid Eukaryotic cells have to use 2 ATP molecules to shuttle these 2 NADHs into mitochondria. Energy stored in the bonds of NADH is used by electron transport chain to produce ATP.

First half of glycolysis activates glucose. Note: two ATP molecules have to be used to get the reaction going.

Second half of glycolysis extracts energy. Total energy yield during the 2nd half of glycolysis is 4 ATPs and 2 NADHs. Since 2 ATPs were consumed in the first half of glycolysis, there is a net gain of only 2 ATPs. Note: ATP is synthesized in glycolysis by substrate-level phosphorylation. This means that an enzyme transfers a phosphate group from an organic molecule (substrate) to ADP, forming ATP.

Pyruvic acid must be converted to Acetyl CoA before it can enter Krebs cycle. This transition step takes place in the mitochondria of eukaryotic cells & cytoplasm of bacterial cells. During the conversion, CO2 is released & NAD+ is reduced to NADH. For every glucose molecule that enters glycolysis, 2 pyruvates are produced & converted into 2 acetyl CoA molecules.

2. Krebs Cycle (aka. citric acid cycle) Acetyl CoA is broken down completely to CO2. cells use carbon skeletons of intermediates to produce other organic molecules (amino acids). energy harvested per acetyl CoA: 1 ATP (via substrate-level phosphorylation) 3 NADH 1 FADH2 Known as citric acid cycle because first molecule formed after acetyl CoA enters is citric acid (citrate).

Thus far, how much useable energy has been produced from the breakdown of 1 glucose molecule? 4 ATPs Need the electron transport chain to harvest potential energy in NADHs & FADH2s.

3. Electron Transport Chain (ETC) Series of proteins & electron carriers embedded in the inner mitochondrial membrane (eukaryotes) or cell membrane (prokaryotes). O2 is the final electron acceptor H2O is the final product energy harvested/NADH: 2.5 ATPs (via chemiosmotic phosphorylation) energy harvested/FADH2: 1.5 ATPs (via chemiosmotic phosphorylation)

Chemiosmotic phosphorylation occurring in mitochondria is essentially the same as that occurring in chloroplasts. NADH & FADH2 molecules pass electrons to the ETC. As electrons move down the chain, they release energy which is used to pump protons (H+) out of the mitochondrial matrix & into the intermembrane space. A proton gradient is established. Gradient drives ATP synthesis (protons pass through ATP synthase channels from the intermembrane space to the matrix; ADP is phosphorylated, forming ATP). NOTE: Some insecticides, like 2,4-dinitrophenol, kill by making the inner mitochondrial membrane permeable to protons (destroys the proton gradient). Insect dies when it runs out of ATP. Cyanide & carbon monoxide kill because they block the transfer of electrons to oxygen. Organism dies when it runs out of ATP.

How many ATPs can 1 glucose yield? Most of the ATP generated from cellular respiration comes from chemiosmotic phosphorylation. NADH molecules yield more ATPs than FADH2 molecules (2.5 vs. 1.5/molecule) because their electrons enter the chain a step earlier than FADH2 electrons. How efficient is cellular respiration? About 32% of the energy in glucose is passed to ATP. Doesn’t seem very efficient until you compare it to the efficiency of an automobile (20-25%). NOTE: 30 ATPs per glucose is an estimate. ATP yield is variable for several reasons: highly active cells (muscle, liver) tend to generate more ATPs. not all NADHs produced in cellular respiration are used to produce ATP. not all protons pumped by ETC flow back through ATP synthase. many of the intermediates of glycolysis & Krebs cycle may be used to form other organic molecules.

Can cells use proteins & lipids to produce energy? Most cells use carbohydrates as the primary source of energy (ATP); however, many cells* can use monomers of proteins & lipids to produce energy. *Neurons cannot utilize energy in proteins & lipids. They must break down carbohydrates to obtain energy. Plant cells use energy derived from lipids to fuel activities such as seed germination.

Proteins are digested to amino acids Proteins are digested to amino acids. To produce ATP from amino acids, cells must convert them into pyruvic acid, acetyl CoA or intermediates of Krebs cycle. Fats are digested to glycerol & fatty acids. Glycerol is converted to pyruvic acid. Fatty acids are converted to acetyl CoA.

1. Alcoholic fermentation B. Fermentation Biochemical pathways that extract energy from nutrients, in the absence of oxygen. 1. Alcoholic fermentation Pyruvic acid is broken down to ethanol and carbon dioxide. Ex. yeast (used in production of baked goods & alcoholic beverages)

Notice that alcoholic fermentation yields only 2 ATPs (from glycolysis).

2. Lactic acid fermentation Pyruvic acid is broken down to lactic acid. Examples: certain bacteria (used in production of cheese & yogurt) human muscle cells in oxygen debt Under “oxygen-debt” conditions (cells are working so strenuously that their production of pyruvic acid exceeds the oxygen supply), human muscle cells revert to lactic acid fermentation to extract energy. If enough lactic acid accumulates, the muscle fatigues & cramps.

Notice that lactic acid fermentation yields only 2 ATPs (from glycolysis).

Photosynthesis, glycolysis & cellular respiration are interrelated. Products of photosynthesis (O2 & glucose) are reactants in cellular respiration. Products of cellular respiration (CO2 & H2O) are reactants in photosynthesis. Glycolysis is probably the most ancient of the energy pathways because it is common to nearly all cells (bacteria, archaea, protists, fungi, plants, animals).