LIFE 102: Respiration

Cellular respiration

Catabolic pathway that breaks down organic fuel in an exergonic process by oxidizing it to release energy used to make ATP

Redox reaction

Chemical reaction that transfers electrons between reactants

Oxidation

A substance loses electrons (is oxidized)

Reduction

A substance gains electrons (is reduced)

OIL RIG

Oxidation Is Loss and Reduction Is Gain (of electrons)

Why oxygen releases energy when electrons move to it

Oxygen is very electronegative - electrons move closer to its nucleus into a lower energy state and release energy

What is oxidized and reduced in cellular respiration

Glucose (the fuel) is oxidized and O2 is reduced

NAD+

A coenzyme that first accepts electrons from organic compounds during respiration

NADH

Reduced form of NAD+ - stored energy that is tapped to make ATP

Electron carriers in respiration

NADH and FADH2

Cellular respiration overall equation

C6H12O6 + 6 O2 yields 6 CO2 + 6 H2O + energy

Energy released by glucose oxidation

686 kcal per mole

Why glucose is broken down in little steps

Small enough for the cell to handle and large enough to drive ATP production

Electron transport chain (ETC)

Passes electrons from NADH and FADH2 in a series of steps to O2 instead of one explosive reaction

Role of O2 in the ETC

Pulls electrons down the chain in an energy-yielding tumble and forms H2O

Three stages of cellular respiration

Glycolysis, citric acid cycle, and oxidative phosphorylation (electron transport chain and chemiosmosis)

Percent of ATP made by oxidative phosphorylation

Almost 90%

Substrate-level phosphorylation

Enzyme transfers a phosphate directly to ADP to make ATP - occurs in glycolysis and the citric acid cycle

Maximum ATP per glucose

Up to 32 ATP

Glycolysis location

Cytoplasm

Glycolysis function

Breaks glucose down into two molecules of pyruvate

Does glycolysis need O2?

No - it occurs whether or not O2 is present

Glycolysis ATP accounting

4 ATP made and 2 ATP consumed for a net of 2 ATP

Glycolysis NADH production

Two NAD+ are reduced to NADH

End products of glycolysis

2 pyruvate (starting substrates for the citric acid cycle and fermentation)

Link reaction before the citric acid cycle

Pyruvate is converted to acetyl CoA which links glycolysis to the citric acid cycle

Citric acid cycle location

Mitochondrial matrix

Citric acid cycle function

Completes the breakdown of pyruvate to CO2 by oxidizing organic fuel

Citric acid cycle yield per turn

1 ATP, 3 NADH, and 1 FADH2

Electron transport chain location

Inner membrane (cristae) of the mitochondrion

Does the ETC make ATP directly?

No - it breaks the large free-energy drop from food to O2 into smaller steps and builds an H+ gradient

Oxidative phosphorylation

ATP synthesis powered by the electron transport chain

Chemiosmosis

Use of energy in an H+ gradient to drive cellular work (ATP synthesis)

How the ETC creates the H+ gradient

Electron transfer causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space

ATP synthase

Protein through which H+ flows back across the membrane - uses this exergonic flow to phosphorylate ADP into ATP

Energy flow in respiration

Glucose to NADH to electron transport chain to H+ gradient to ATP

Efficiency of energy transfer from glucose to ATP

About 34% of the energy in glucose is transferred to ATP

ATP yield from NADH and FADH2

10 NADH give about 25 ATP and 2 FADH2 give about 3 ATP

Why net yield is lower than 32 ATP in some accounts

Pyruvate is charged and cannot cross lipid bilayers so 2 ATP are used to actively transport it into the mitochondrion (about 30 ATP per glucose)

Carbohydrate metabolism

Complex carbohydrates are hydrolyzed to monosaccharides which enter glycolysis

Fat metabolism

Fats are broken down into glycerol and fatty acids

What happens to glycerol

It is converted to glyceraldehyde 3-phosphate and enters glycolysis

What happens to fatty acids

Broken down by beta-oxidation into 2-carbon molecules that enter the citric acid cycle as acetyl-CoA

ATP from fats vs glucose

Fats produce 2x as much ATP per gram as glucose

Protein metabolism

Proteins are broken down into amino acids and the amino groups are removed and excreted as ammonia (NH3)

Where amino acids enter respiration

At glycolysis or the citric acid cycle

Anabolic pathways (biosynthesis)

Small molecules from food glycolysis or the citric acid cycle are diverted to build other substances like fats and amino acids

Main control mechanism of catabolism

Feedback inhibition - regulating enzyme activity at strategic points in the pathway

Feedback regulation of respiration by ATP

If ATP drops respiration speeds up and when there is plenty of ATP respiration slows down

Aerobic respiration

Respiration that needs O2 for the electron transport chain

What happens without O2

Glycolysis couples with fermentation or anaerobic respiration to produce ATP

Why pyruvate is a key intersection in catabolism

It is the end product of glycolysis and feeds the citric acid cycle or fermentation

Energy flow through ecosystems

Energy flows in as sunlight and leaves as heat - photosynthesis and respiration cycle the matte