CHAPTER 9

CELLULAR RESPIRATION

9-1 Chemical Pathways

FOOD:• Food is the main energy source of

living things. • One gram of the sugar glucose, when

burned in the presence of oxygen, releases 3811 calories of heat energy.

• A calorie is the amount of energy needed to raise the temperature of 1 gram water up one Celsius degree.

• A food Calorie (capital C) is not the same as a calorie (lowercase c).

- A food Calorie is a kilocalorie, which equals 1000 calories.

• Glucose (food) is gradually broken down, and energy is released within cells.

- Glycolysis starts the process. - When oxygen is present, glycolysis

is followed by the Krebs cycle and the electron transport chain. If oxygen is not present, a different pathway is taken.

Cellular Respiration• Cellular Respiration is the process that

releases energy by breaking down food molecules in the presence of oxygen.

• Glycolysis, the Krebs cycle and the electron transport chain make up the process of cellular respiration.

Equation:Oxygen + glucose carbon dioxide +

water + energy

Glycolysis

• Glycolysis takes place in the cytoplasm of a cell.

• When oxygen is present, Glycolysis is followed by the Krebs cycle and the Electron Transport Chain. They take place inside the mitochondria.

• Glycolysis is the process in which one molecule of glucose is broken in half. This produces 2 molecules of pyruvic acid and a 3-carbon compound.

• Glycolysis starts by using the energy of 2 ATP molecules. At the end of glycolysis, 4 ATP molecules have been produced. It has a net gain of 2 ATP molecules.

• NAD+ is present during glycolysis. It is the electron carrier nicotinamide adenine dinucleotide.

• NAD+ main function in glycolysis is to carry high-energy electrons which becomes NADH and holds the electrons until they can be transferred to other molecules (to the electron transport chain)

• The process of glycolysis does not require oxygen to occur, therefore it can supply energy to the cells when oxygen is not present.

• If oxygen is not present, glycolysis is followed by fermentation (anaerobic reaction).

Fermentation

• Fermentation is the process in which cells convert NADH and NAD+ by passing high-energy electrons back to pyruvic acid.

• It produces a steady supply of ATP. • There are 2 main types:– Alcoholic– Lactic acid

• Yeasts and some microorganisms use alcoholic fermentation.

• The equation:pyruvic acid + NADH alcohol +

carbon dioxide + NAD+• Lactic acid fermentation converts glucose into

lactic acid. • The equation:

pyruvic acid + NADH lactic acid +NAD+

• During excessive exercise, your muscles produce ATP by lactic acid fermentation. This occurs AFTER they have used up all of the ATP produced by cellular respiration.

Ch 9 Section 2

The Krebs Cycle and Electron Transport

An Overview:

• The pathways of cellular respiration are said to be aerobic because they require oxygen.

• At the end of glycolysis, 90% of the energy in glucose is still unused.

• The cell wants to use all of the energy, so it uses oxygen to accept the high energy electrons.

• Krebs Cycle and Electron Transport Chain MUST HAVE oxygen to complete these final steps of cellular respiration.

Rolling and Folding???• Some of the steps in cellular

respiration take place in the membrane inside the cell structure called the mitochondrion, which has a folded inner membrane. What purpose do these folds serve?

Psst…don’t write this, let’s just think a little bit!

Glucose(C6H1206)

+Oxygen

(02)

Glycolysis KrebsCycle

ElectronTransport

Chain

Carbon Dioxide

(CO2)+

Water(H2O)

Cellular Respiration

The Krebs Cycle

• Discovered in 1937 by British biochemist Hans Kreb.

• Hans Kreb won the Nobel Prize in 1953 for his discovery of the Krebs cycle.

• Krebs cycle is also known as the citric acid cycle.

• It is the second stage of cellular respiration.

• The pyruvic acid produced in glycolysis is passed to the second stage of cellular respiration, Krebs cycle.

• Pyruvic acid is broken down into carbon dioxide in a series of reactions.

• Citric acid is the first compound formed in the reactions. (AKA…citric acid cycle)

Stages of Krebs Cycle:

STAGE 1 – Citric Acid Production• Pyruvic Acid enters the mitochondria (where the

rest of cellular respiration occurs)• A carbon is removed, forming carbon dioxide• Electrons are removed, changing NAD+ to NADH • Coenzyme A joins the 2-carbon molecule to form

acetyl-CoA• Acetyl-CoA then adds the 2-carbon acetyl group

to a 4-carbon compound, forming citric acid

STAGE 2 – Energy Extraction• Citric Acid (from stage 1) is broken down

into a 5-carbon compound, then into a 4-carbon compound.

• 2 molecules of carbon dioxide are released

• Electrons join NAD+ and FAD to form NADH and FADH2

• One molecule of ATP is generated• THE TALLY from one molecule of pyruvic

acid = 4 NADH, 1 FADH2 and one molecule of ATP.

What happens to the Krebs Cycle products?

• CO2 released is the source of CO2 in your breath

• Each exhale you expel the CO2

produced by the Krebs cycle

• ATP produced during the Krebs Cycle can be used for cellular activities

• NADH (electron carriers) can be used to generate huge amounts of ATP.

Electron Transport• Electrons produced in the Krebs Cycle are passed to

the electron transport chain by the electron carrier molecules NADH and FADH2.

• The electron transport chain uses the high-energy electrons from the Krebs Cycle to convert ADP to ATP.

• This couples the movement of high-energy electrons with the production of ATP.

• Three stages:– Electron transport– Hydrogen Ion Movement– ATP production (ATP synthase uses energy from H+ ions

to convert ADP to ATP.

Totals:

• The complete breakdown of glucose through cellular respiration, including glycolysis, results in the production of 36 molecules of ATP.

Quick Energy vs. Long-Term Energy

• Energy and Exercise: At the start of a race, your muscles contain ATP to last only a few seconds. By the 50 meter mark, most of the ATP is produced by lactic acid fermentation. At the end of the race, you are breathing deeply. Why? To resupply your body with oxygen!!!

• If you exercise for more than 90 seconds, ATP is produced through cellular respiration. This process releases energy more slowly so you can “pace yourself”.

• Your body stores energy in muscle in a carbohydrate called glycogen. Glycogen is broken down during the first 15 minutes of activity, then your body breaks down fat for energy.

A SUMMARY

• Photosynthesis: deposits energy and occurs in plants, algae, and bacteria

• Cellular Respiration: withdraws energy and takes place in all eukaryotes and some prokaryotes