BIO 10 Lecture 7 THE VITAL FORCE: RESPIRATION

Respiration = the process by which living organisms harvest the energy in highly ordered, high energy molecules such as carbohydrates and fats by oxidizing them into CO 2 and H 2 O to produce energy usable for life processes (C 6 H 10 O 5 ) n oxidation CO 2 + H 2 O + USABLE ENERGY

Molecules like carbohydrates and fats are energy-rich because they are highly ordered That order can be converted to energy when the molecule loses an electron When a molecule loses an electron during a chemical reaction, it is said to be oxidized Stores less energy than it did before The loss of the electron releases energy –When a molecule gains an electron during a chemical reaction, it is said to be reduced Stores more energy than it did before The gain of the electron requires the input of energy

During photosynthesis, plants take in CO 2 + H 2 O from the environment and, using the energy from the sun, reduce these molecules (add electrons to them) The result is starch, a highly reduced form of carbon, hydrogen, and oxygen Long chains of simple sugars Highly ordered so very energy-rich Used to temporarily store the energy from sunlight until it can be harvested by the cell into a usable form via respiration May also be eaten by animals, who then harvest the stored energy in an identical manner

Types of Respiration: –Anaerobic Does not require oxygen Evolved first Very inefficient Takes place in the cytoplasm Can only support small life forms –Prokaryotes –Some small eukaryotes –Can occur in large eukaryotes for brief periods –Aerobic Requires oxygen Evolved later Highly efficient Takes place in the mitochondria –Eukaryotes only

Usable Energy: ATP Cells cannot use the energy released during the oxidation of carbohydrates and lipids directly –Must convert the energy stored in food into a useable form or “currency” –Much like we store the potential energy we earn by working at our jobs as dollars in the bank, the potential energy from food breakdown is stored as ATP This bond carries small units of usable energy around the cell

Like dollars, ATP molecules carry energy in a form that can be released at any time ATP also happens to carry just about the right amount of energy to drive most cellular reactions During respiration, the energy stored in carbohydrates and lipids is converted to the usable storage molecule ATP

Photosynthesis: Plants use the photons from sunlight to reduce carbon dioxide and water into carbohydrates Respiration: Carbohydrates are oxidized to drive the production of ATP Carbohydrates are stored as potential energy in the plant cell

General reaction: –C 6 H 12 O 6 +6O 2 + ADP  6CO 2 + 6H 2 O + ATP (high energy) (low energy products) Two stages: Glycolysis –All respiring organisms –2 ATP molecules per glucose broken down Krebs Cycle and the Electron transport chain –Eukaryotes only –34 additional ATP molecules per glucose How is Respiration Accomplished? Usable energy

Glycolysis: Glucose enters the cytoplasm of the cell An enzyme immediately breaks apart one ATP molecule into ADP and P (produces energy) –The phosphate (P) group in then attached to the glucose, creating glucose-6-phosphate At this point, the glucose-6-phosphate is rearranged into fructose-6-phosphate and another phosphate is added This requires one more ATP molecule The result is fructose-1,6 phosphate Note that, so far, no ATP have been produced – in fact 2 ATP have been consumed! –Ledger: -2 ATP

Fructose-1,6 diphosphate is now cleaved into two molecules of glyceraldehyde-3-phosphate –Each of these molecules will now continue on through glycolysis in an identical manner A molecule called NAD+ now strips an electron from (oxidizes) each of the glyceraldehyde-3- phosphate molecules –The energy from this reaction is used to add phosphate groups to each of the glyceraldehyde-3- phosphates They are now 1,3-diphosphoglyceric acids

Electron carrier molecules like NAD+ serve to help carry the electrons down their energy gradients during respiration NAD+ strips an electron off one molecule and (as NADH) carries it to another molecule which accepts the electron in a lower energy state than it was when NAD+ got it At each step, therefore, energy can be harvested NAD+ has been reduced to NADH; NADH now carries an extra electron NADH has been oxidized to NAD+; NAD+ can now accept another electron

Note that as NAD+ accepts the electron, it also accepts a hydrogen atom NAD+ thus carries both electrons and H atoms in the cell

In the final steps of glycolysis –Both phosphates are stripped off the two 1,3- diphosphoglyceric acid molecules Are added to ADP to create ATP In all, 4 ATP molecules are produced LEDGER: + 2 ATP

Plus side— Reactions are very fast Occurs in the cytoplasm No oxygen required Minus side— Inefficient! Only 2 ATP molecules produced from each glucose molecule There is much more energy available in the pyruvic acids – how to get it out? In Some Organisms, Respiration Ends with Glycolysis …

A Better Way: The Krebs Cycle and the Electron Transport Chain Pyruvic acid (the final breakdown product of glycolysis) is shuttled into the mitochondria for further breakdown and the production of more ATP Evolved later, but generates MUCH larger quantities of energy (36 total ATP per glucose molecule when combined with glycolysis) Occurs only in mitochondria (only in eukaryotes) and requires oxygen.

The mitochondrion has a double- membrane 4 parts: -Outer membrane -Intramembrane space -Inner membrane -Inner compartment

The Kreb’s Cycle takes place in the inner compartment: –Each of the two pyruvic acids travels into the mitochondrion, where it is converted into acetyl coenzyme-A This reaction strips an electron off (oxidizes) the pyruvic acid and NAD+ picks up the electron One molecule of CO 2 is also produced –Diffuses through the cell membrane into the bloodstream –Picked up by hemoglobin and transported to the lungs for exhalation –The acetyl coenzyme-A then undergoes a series of reactions that oxidizes it completely to CO 2 At each step, NAD+ (or a similar molecule called FAD) pick up the stripped off electrons

For each molecule of pyruvic acid that goes through this process the outcome is: –3 NADH (6 total per glucose) –1 FADH 2 (2 total per glucose) –1 ATP (2 total per glucose) NADH and FADH 2 then continue on to the next stage, taking their precious electrons and hydrogens to the electron transport chain …

The Electron Transport Chain (ETC) is a series of electron carrier molecules embedded in the mitochondrial inner membrane –When a molecule of NADH arrives, it dumps its electron to the first carrier, which accepts the electron in a lower energy state than it was when NAD+ picked it up –This carrier then passes the electron along to the next carrier in the chain, which accepts it in a yet lower energy state –The movement of electrons at each transfer releases energy –The energy is used to power the movement of H + ions (from the oxidation of NADH) across the inner mitochondrial membrane from the inner compartment into the outer compartment

Note that at each step, the downhill run of the electrons provides the electron carrier molecules with the energy to pump H+ ions against their concentration gradient across the inner mitochondrial membrane

Hydrogen ions are then allowed to flow downhill (with their concentration gradient) through an enzyme in the membrane called ATP synthase –Facilitated transport Like a water wheel spinning, as the ions pass, energy is used to transfer phosphate onto ADP to make ATP.

The Greatest amount of ATP is made in this stage (32 ATP per glucose). At the end of the ETC, which carrier accepts the electron? OXYGEN! 1/2 O electrons + 2 H + = H 2 O Which is why organisms that use the Kreb Cycle and the ETC to produce ATP have to breathe in oxygen It’s also why we can sustain a multi-cellular body with an enormous requirement for ATP The miracle of the mitochondrion is that it evolved a way to harvest the energy of pyruvic acid in small chunks to produce ATP

Fats, proteins, carbohydrates, and sugars other than glucose can also enter the pathway to be converted to energy Food eaten in excess of caloric demands can also be converted from amino acids, fatty acids, and sugars into proteins, fats, and carbohydrates for structure or storage –98 percent of energy reserves of animals are fats

Short Review of Lecture 7 What is the difference between anaerobic and aerobic respiration? What do the two processes have in common? What is the difference between oxidation and reduction reactions? Which yield energy? Which require energy? Where does glycolysis take place? Where does the Kreb Cycle take place? Where does the electron transport chain take place? Why are all multicellular eukaryotes obligate anerobes?