BIO 10 BIO 10 Lecture 7Lecture 7BIO 10 BIO 10 Lecture 7Lecture 7

THE VITAL FORCE: THE VITAL FORCE:

RESPIRATIONRESPIRATION

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 CO2 and H2O to produce energy usable for life processes

(C6H10O5)n

oxidation CO2 + H2O + 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 CO2 + H2O 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:– C6H12O6 +6O2 + ADP 6CO2 + 6H2O + 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 fastOccurs in the cytoplasmNo 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 CO2 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 CO2

• 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 FADH2 (2 total per glucose)

– 1 ATP (2 total per glucose)

• NADH and FADH2 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 O2 + 2 electrons + 2 H+ = H2O

• 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?