Chapter 13: How Cells Obtain Energy from Food

What are the pros of a stepwise sugar oxidation process as opposed to a one step process?

-one-step processes are too much for our cells to handle

-one-step processes would release heat, which we SHOULD NOT have in a cell (remember cells need order and heat is very entropic)

-stepwise processes allow for energy to be captured in small reactions in the form of activated carriers, which can be used in later processes

What are the three stages of metabolism

1) digestion: food is broken down and nutrients are absorbed

2) glycolysis (cytosol): glucose molecules are split into pyruvate

3) Krebs Cycle and Electron transport chain (mitochondria): oxidative phosphorylation breaks down molecules further, producing ATP and consuming O2

Cellular Respiration Process

1) glycolysis

2) acetyl CoA formation

3) Krebs cycle

4) electron transport chain and chemiosmosis

overall: food + O2 -> ATP, NADH, CO2, H2O

Glycolysis

-glucose split into two pyruvate molecules

(1 glucose + 2 ATP -> 2 pyruvate +2 ATP + 2 NADH +2 H2O) net amount

-happens in cytosol

-ATP produced without oxygen

Glycolysis step 1

What happens: glucose phosphorylated

Reactants: glucose + ATP

Products: glucose 6-phosphate + ADP + H+

Enzyme used: hexokinase

overall ATP used: 1

overall ATP produced: 0

Glycolysis step 2

what happens: isomer of glucose 6-phosphate formed

reactants: glucose 6-phosphate

products: fructose 6-phosphate

enzyme used: phosphoglucose isomerase

overall ATP used: 1

overall ATP produced: 0

glycolysis step 3

what happens: fructose 6-phosphate is phosphorylated

reactants: fructose 6-phosphate + ATP

products: fructose 1,6-bisphosphate

enzyme: phosphofructokinase

overall ATP used: 2

overall ATP produced: 0

glycolysis step 4

what happens: Fructose 1,6 bisphosphate is cleaved

reactants: fructose 1,6-bisphosphate

products: dihydroxyacetone phosphate + glyceraldehyde 3-phosphate

enzyme: aldolase

overall ATP used: 2

overall ATP produced: 0

glycolysis step 5

what happens: dihydroxyacetone phosphate is rearranged/isomerized

reactants: dihydroxyacetone phosphate

products: glyceraldehyde 3-phosphate (notice how this is the same as the other product formed in step 4)

enzyme: triose phosphate isomerase

*note: since there are now two molecules of glyceraldehyde 3-phosphate, each step (6-10) is now doubled.

overall ATP used: 2

overall ATP produced: 0

glycolysis step 6

what happens: glyceraldehyde 3-phosphate molecules are oxidized

reactants: glyceraldehyde 3-phosphate + NAD+ + phosphate

products: 1, 3-bisphosphoglycerate + NADH + H+

enzyme: glyceraldehyde 3-phosphate dehydrogenase

overall ATP used: 2

overall ATP produced: 0

overall NADH produced: 2

glycolysis step 7

what happens: phosphate from previous step transferred to ADP

reactants: 1,3-bisphosphoglycerate + ADP

products: 3-phosphoglycerate + ATP

enzyme: phosphoglycerate kinase

overall ATP used: 2

overall ATP produced: 2

overall NADH produced: 2

glycolysis step 8

what happens: ester linkage is rearranged

reactants: 3-phosphoglycerate

products: 2-phosphoglycerate

enzyme: phosphoglycerate mutase

overall ATP used: 2

overall ATP produced: 2

overall NADH produced: 2

glycolysis step 9

what happens: water molecule removed from 2-phosphoglycerate; high energy enol linkage created

reactants: 2-phosphoglycerate

products: phosphoenolpyruvate, H2O

enzyme: enolase

overall ATP used: 2

overall ATP produced: 2

overall NADH produced: 2

glycolysis step 10

what happens: high energy enol-phosphate linkage transferred to ADP

reactants: phosphoenolpyruvate + ADP + H+

products: pyruvate + ATP

enzyme: pyruvate kinase

overall ATP used: 2

overall ATP produced: 4

overall NADH produced: 2

what is the net amount of ATP molecules produced during glycolysis

2

kinase

an enzyme that catalyzes the transfer of a phosphate group to a specified molecule.

-in steps 1 + 3 of glycolysis kinases transfer phosphates from ATP to a substrate

-in steps 7 and 10 of glycolysis kinases transfer phosphates to ADP to make ATP

isomerase

enzyme that catalyzes the rearrangement of bonds in the molecule

-in steps 2 and 5 of glycolysis

dehydrogenase

enzyme that catalyzes oxidation by removing a hydride (H-) ion

-step 6

Mutase

enzymes that catalyze the shifting of a chemical group from one location to another

-step 8

fermentation

Process by which cells break down sugars and release energy in the absence of oxygen

acetyl CoA formation

-pyruvate formed in glycolysis is transported to the mitochondria

-pyruvate dehydrogenase catalyzes the formation of acetyl CoA

-overall: pyruvate -> acetyl CoA + NADH + CO2

what is always required for the Krebs Cycle

Acetyl CoA

what is the net result of the Krebs cycle

2CO2, GTP, 3NADH, FADH2

GTP

-an activated carrier that is similar to ATP

-not used in electron transport chain

FADH2

-electron carrier that is similar to NADH, as it gives up electrons

-participates in the electron transport chain

NADH

electron carrier that stores energy used to make ATP

true or false: oxygen directly enters the Krebs cycle

false.

true or false: the Krebs cycle can function in an anaerobic environment

false, the Krebs cycle requires oxygen, even though oxygen does not enter the cycle directly

oxidative phosphorylation

the chemical energy is captured by activated carriers used to generate ATP

Fatty acid oxidation

-fatty acids can be converted into Acetyl CoA in the mitochondrial matrix by

-removes 2 carbons from the molecule per cycle

-REQUIRES OXYGEN

Which is preferred by our bodies for energy production: glycolysis or fatty acid oxidation

glycolysis, since it is anaerobic

gluconeogenesis

-"reverses" glycolysis to restore glucose supply by building it from pyruvate

-typically occurs during a fasting state

glycogen

-branched glucose polymer that provides energy when we fast

-when more ATP is needed than gained from food, glycogen is broken down via phosphorylation to provide additional ATP

true or false: ATP synthase can work in both directions

true. if the proton gradient is disrupted, ATP synthase can work in the opposite direction to return protons to the matrix. This USES ATP.

Roughly how many ATP molecules are produced from one glucose molecule

about 28-36