cellular respiration and energy

• cells use the energy stored in food molecules to make ATP

• cellular respiration is breaking down food (carbohydrates, fats, proteins)

• converts stored energy into ATP, which is usable energy for cells

energy for life work

• cells require energy to perform their many tasks

  • assembling polymers

  • active transport

  • movement

  • reproduction

• ATP- adenosine triphosphate

  • energy molecule for cellular work

cellular respiration is a catabolic pathway

• metabolic pathways that release stored energy by breaking down complex molecules

• cellular respiration

  • aerobic respiration

  • uses O2 to break down glucose

  • yields CO2+H2O+ATP

  • exergonic process

energy transfers

• law of conservation energy (1st law of thermodynamics)

  • energy is not created or destroyed, it is just transformed

• how is energy transferred or transformed?

  • change chemical bonds (break bonds/form bonds)

• the relocation of electrons releases the energy stored in organic molecules and the energy is then used to make ATP

oxidation-reduction reactions

• redox reactions- partial or complete transfer of one or more electrons from one reactant to another (OIL RIG)

  • oxidation is loss of e-

    • reducing agent (electron donor)

  • reduction is gain of e-

    • oxidizing agent (electron acceptor)

• may not be a complete transfer of e-

  • may be a change in the degree of electron sharing in covalent bonds

oxidation of organic fuels

• in cell respiration glucose is oxidized and oxygen is reduced

  • as electrons shift toward a more electronegative atom, they give up potential energy and chemical energy is released

• organic molecules with an abundance of hydrogen are rich in “hilltop” electrons, which release their potential energy when they “fall” closer to oxygen

role of electron carriers and enzymes in cellular respiration

• energy is not released in one single explosive shot

  • organic fuels are broken down slowly

  • series of steps- each catalyzed by an enzyme

• the electron carrier NAD+ assists in the slow release of energy

  • electrons generally travel with a proton (H+)

  • picked up by NAD+ + 2e- +2H+ + NADH + H+

  • passed from NADH down an electron transport chain to O2 (electron acceptor)

overview of cell respiration

• C6H12O6 + 6O2 —> 6CO2 + 6H2O + 38ATP

• 3 stages and 5 chemical reactions of cellular respiration

  • glycolysis —> pyruvate oxidation —> citric acid cycle —> electron transport chain and chemiosmosis

• what you need to know

  • you need the know the inputs and outputs for each of the 5 reactions of cell respiration

glycolysis- “splitting sugar”

• 10 step process occurring in the cytosol

• occurs whether oxygen is available or not

• 2 phases: energy-investment and energy-payoff

  • 2 ATP are needed to start glycolysis (investment)

  • 4 ATP are made by substrate level phosphorylation

    • net gain of 2 ATP+2 NADH (payoff)

• phase one: energy investment

  • cells spend ATP to start process

  • 2 ATP to split glucose molecule

  • glucose (6C) —> 2 three carbon sugars (G3P)

• phase two: energy payoff

  • 2G3P rearranged into 2 pyruvate (3C)

  • 4 ATP produced by substrate level phosphorylation

    • net of 2 ATP per glucose molecule

      • used to do cell work

  • plus 2 molecules of NAD+ —> 2NADH

    • passed to ETC

substrate level phosphorylation

• the ATP made during glycolysis is made by a process called substrate level phosphorylation

• substrate level phosphorylation- the production of ATO through an enzyme that directly transfers a phosphate group from substrate to ADP

the mitochondria

• glycolysis releases less than a quarter of the chemical energy stored in glucose

  • most energy remains in the 2 pyruvate molecules

• if oxygen is present the pyruvate made during glycolysis enters the mitochondria by active transport and is used in aerobic respiration

pyruvate oxidation —> Acetyl CoA

• pyruvate (made during glycolysis) enters the mitochondria by active transport- Pyruvate oxidation occurs in the matrix

• pyruvate converted into acetyl CoA and NADH

  • 2 pyruvate —> 2 acetyl CoA + 2 NADH

  • 2 CO2 given off as a byproduct- 1st release of CO2

  • 3 step reaction

citric acid cycle

• acetyl group of acetyl CoA added to oxaloacetate

  • forms citrate (cycle name)

• citrate is slowly decomposed back to oxaloacetate

  • recycling of oxaloacetate is what makes it a cycle

• also called Krebs cycle for Hans Krebs

  • scientist who worked out the pathway

• each acetyl CoA enters the Krebs cycle

  • 2 acetyl CoA are made per 1 glucose

  • 2 turns of the cycle per molecule of glucose

• outputs produced per glucose (after 2 turns)

  • 2 ATP

  • 6 NADH

  • 2 FADH2

  • 4 CO2- final release of CO2

ATP production so far

• only 4 ATP molecules produced

  • 2 net ATP from glycolysis

  • 2 ATP from citric acid cycle

  • all by substrate level phosphorylation

• NADH and FADH2 hold most of the energy extracted from the glucose

  • these electron carriers enter the electron transport chain for ATP synthesis by oxidative phosphorylation

    • 10 NADH and 2 FADH2

the electron transport chain (ETC)

• a series of proteins that transport electrons, which releases energy used to make ATP

  • embedded in the inner membrane of the mitochondria

  • molecules in the ETC alternate between reduced and oxidized states as they accept and donate electrons

  • the electron transport chain does NOT make any ATP directly- creates a protein gradient that drives chemiosmosis

steps of the ETC

• NADH and FADH2 get oxidized and electrons get transferred to the electron transport chain

• electrons are passed down the ETC until they reach the final electron acceptor, which is O2

  • ½ O2 picks up 2H+ atoms to become H2O

• as electrons move down the chain protons (H+) cross into the inner membrane space creating a proton gradient

• ATP syntheses uses the energy of this proton gradient to make ATP

steps to chemiosmosis

• the energy released during the electron transport chain is used to pump H+ ions from the matrix into the inter membrane space

  • this results in a proton gradient

• H+ (protons) diffuse back across inner mitochondria membrane through ATP synthase (with gradient)

• ATP synthase is the enzyme that makes ATP from ADP + P

ATP bookkeeping

• the ETC is organized into four complexes

  • NADH enters at complex I

  • FADH2 enters at complex II

    • complex II is at a lower energy level

    • provides less energy for ATP synthesis

• for each molecule of NADH that enters the ETC about 3 ATP are made

• for each molecule of FADH2 that enters the ETC about 2 ATP are made

where does each step occur?

• glycolysis- takes place outside the mitochondria in the ctyosol

• pyruvate (made during glycolysis) enters the mitochondria and forms acetyl CoA and NADH

  • acetyl CoA accumulates in the mitochondrial matrix

• Krebs cycle- occurs in the mitochondrial matrix

• electron transport chain- embedded in the inner membrane of the mitochondria

• chemiosomosis- protons are pumped from the matrix to the inter membrane space. they diffuse back across into the matrix through ATP synthase which makes ATP

fermentation

• an extension of glycolysis that makes a small amount of ATP from pyruvate in the absence of oxygen

  • regenerates NAD+ so glycolysis can occur again

  • anaerobic process (without oxygen)

  • takes place in the cytosol of the cell

• glycolysis is a universal biological process

  • evolutionary history- probably evolved in ancient prokaryotes

2 common types of fermentation

• lactic acid fermentation

  • converts the pyretic acid (obtained from glycolysis) into lactic acid

  • NAD+ is regenerated so glycolysis can continue

  • occurs in muscle cells during strenuous activity

  • used in production of cheese and yogurt

• alcohol fermentation

  • converts the pyretic acid (obtained from glycolysis) into ethanol

  • NAD+ is regenerated so glycolysis can continue

  • yeast use alcoholic fermentation, which is then used in the production of wine, beer, and bread

  • many bacterial use alcoholic fermentation

regulation of cell respiration

• feedback inhibition- end product of an anabolic pathways inhibits an enzyme early in the pathway

  • prevents a cell from producing an excess of a particular substance

• supply of ATP in the cell regulates ATP respiration

  • phosphofructokinase- allosteric enzyme in 3rd step of glycolysis is inhibited by ATP and citrate

    • one example- other enzymes in the process play regulatory roles

catabolism of other large biomolecules

• in this lecture, glucose has even used as an example fuel molecule for cellular respiration

  • humans and other animals obtain most of their calories in the form of fats, proteins, sucrose, and other disaccharides, and starch a polysaccharide

  • these various molecules can all be used as fuel with various modifications to the process