The Basics of Cellular Respiration (H. Biology)

Cell metabolism involves all chemical reactions

Cell Respiration: Overall Equation

The cell cannot use the amount of energy in a glucose molecule all at once.

The cell uses energy in the smaller “packets” contained in ATP molecules.

• Cell respiration also provides “carbon skeletons” used in biosynthesis of other organic compounds.

• It takes place in both plant and animal cells.

There are 3 major stages of Cell respiration

• Glycolysis (sometimes listed separately from cell respiration because it happens outside the mitochondrion.)

• Krebs cycle (Citric Acid Cycle)

• Electron Transport System (chain) & Oxidative phosphorylation

(E.T.O.P.)

mitochondrion

I) Glycolysis

• It occurs in the cytosol of the cytoplasm.

• It is anaerobic so it requires no oxygen.

cytosol

Glycolysis In Depth

• 1) Glucose is converted to glucose-6-phosphate. (This takes 1 ATP to supply the phosphate)

• 2) Glucose-6-phosphate splits into 2 3C phosphate compounds: PGALs (This takes another ATP)

• 3) Through a series of reactions the 3C phosphates (PGALs) are chemically rearranged into 2 3C pyruvic acid molecules

• 4) Also – 4 ATP are produced all together, and 2 NADH molecules. (This leaves a net production of 2 ATPs from glycolysis.)

Prior to Krebs Cycle (AKA Citric Acid Cycle)

Pyruvic Acid is converted into Acetyl Coenzyme A (Acetyl CoA) This is called oxidative decarboxylation.

If there is enough O2, the pyruvic acid produced during glycolysis is transported to the mitochondrion.

• A complex system of enzymes converts the pyruvic acid to acetate, and NADH is produced.

• A carrier molecule picks up the acetate to form Acetyl-Coenzyme A (acetyl CoA).

The Acetyl CoA is the compound that delivers the acetate to the Krebs Cycle.

Note the carbon dioxide given off in this process.

Also – see the NADH produced.

IIII) ) Krebs cycleKrebs cycle (Citric Acid Cycle)(Citric Acid Cycle)

It is named for Hans Krebs who discovered it in 1953. (It is also called the citric acid cycle” because citric acid is formed at the start of the cycle.)

During the Krebs cycle Acetyl CoA molecules formed from pyruvic acid molecules, are broken down.

CO2 is given off, and ATP is produced. (1 ATP per each pyruvic acid or each turn of the cycle.)

This happens in the matrix of the mitochondrion. (Aerobic respiration in the mitochondron)

Inner folds called cristae increase surface area for chemical reactions involved in cell respiration.

High energy electrons are carried by special compounds (NADH & FADH2) from Krebs cycle to the Electron Transport Chain.

Krebs Cycle (Citric Acid Cycle) in Depth

1) 2C “Acetyl group” (acetate)

from Acetyl CoA combines with a 4C acid (oxaloacetate) to 6C Citrate (citric acid).

Black balls are carbon atoms

2) After several steps: 6C citric acid is converted to 5C acid (ketoglutarate) Carbon Dioxide is given off, and NADH is formed.

• 3) 5C acid is converted to a 4C acid. Carbon Dioxide is given off, and NADH is formed

• 4) 4C acid is “rearranged” 1ATP, and FADH2 is formed.

• 5) 4C acid becomes oxaloacetate, and NADH is formed, The oxaloacetate is involved in a series of reactions and the cycle begins again. Since each molecule of C6H12O6 produces 2 pyruvic acid molecules in glycolysis, the Krebs cycle completes two turns.

and the cycle begins again

Summary of Products from Krebs

• NADH (needed for the last part of Cell Respiration- E.T. & O.P.)

• FADH2 (needed for the last part of Cell Respiration- (E.T. & O.P.)

• 2 ATP molecules (1 for each pyruvic acid converted to Acetyl CoA)

III) The Electron Transport System (chain) (Let’s call it E.T.) & Oxidative Phosphorylation.

E.T. provides energy to

produce the most ATPs

of cell respiration

It takes place in the mitochondrion’s inner membrane. Electrons are carried and ATPs are produced. It is an aerobic process.

All of this for the E.T. & O.P..oxidative phosphorylation

What Happens During E.T….

• 1) NADH and FADH2 carry Hydrogen atoms (energy rich electrons and protons) to the E.T. system.

• 2) E.T. consists of a series of electron carriers (proteins called cytochromes) embedded in the mitochondrion cristae (folds of inner membrane).

cristae

3) H atoms separate into protons (H+) and electrons (e-).

4) Electrons are passed from one carrier (cytochrome proteins) to the next in E.T. (This is accomplished through a series of oxidation-reduction reactions.)

5) The terminal cytochrome combines electron, protons & oxygen to form water O2 to form water.

6) Some of the free energy of the E.T. carried electrons is used to transport protons from the mitochondrial matrix, across the cristae, to the inter-membrane space.

Due to the proton concentration gradient produced, the protons diffuse back across the membrane to the matrix, through ATP synthase enzyme complex.

• This is called chemiosmosis. • Grand total of ATPS from Glycolysis, Krebs, and ET

system = 38 (34 from E.T.).

Chemiosmosis Energy given off from diffusing protons permits

the enzyme known as ATP synthase, located on the inner mitochondrial membrane, to produce ATP molecules.

ATP synthase

ATP Final Tally from 1 glucose broken down through cell respiration.

• Glycolysis = 2 ATPs

• Krebs Cycle (Citric Acid Cycle) = 2 ATPs

• Electron Transport System & Oxidative Phosphorylation (E.T.O.P.) = 34 ATPs

Total =

38 ATPs