Chapter 9

Cellular Respiration

9-1 Cellular Respiration:An Overview

Chemical Energy and FoodFood provides living things with the chemical building blocks they need to grow and reproduce.

Food is the source of raw materials from which cells can synthesize new molecules

Most importantly, food is a source of energy.

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

Glucose is a source of energy.

Cells release the energy from glucose and other food compounds.

Cells don’t simply burn food and release energy as heat.

Instead, they break down food molecules gradually, capturing a little bit of chemical energy at key steps.

This begins with glycolysis. Glycolysis releases only a small amount of energy.

Mitochondrion Structure Living things get the energy they need from food. Both plant and animal cells carry out the final stages of cellular respiration in the mitochondria.

Overview of Cellular Respiration Cellular Respiration is the process that releases

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

6O2 + C6H12O6 → 6CO2 + 6H2O + Energy

(oxygen + glucose → carbon dioxide + water + energy)

This equation does not fully explain cellular respiration.

Cellular respiration takes place in 3 stages in order to control the energy released.

Energy has to be released a little at a time or it would be explosive.

The cell traps these little bits of energy by using them to make…

ATP

Stages of Cellular Respiration

Glycolysis

Oxygen Present No Oxygen

High Energy Limited Energy Release Release

Small energy yield

Fast process

Can produce 1,000’s of ATP molecules in a few milliseconds

Does not require oxygen

Glycolysis:

At the end of glycolysis about 90% of the energy that was available in the glucose is still unused

To extract the rest of that energy, the cell turns to one of the most powerful electron acceptors → oxygen

Oxygen is required for the final steps of respiration and because of this these steps are said to be aerobic.

The 3 main Stages of Cellular Respiration

Glycolysis takes place in the cytoplasm

Krebs Cycle and the Electron Transport Chain take place inside the mitochondria

The 3 main Stages of Cellular Respiration

9.2 The Process of Cellular Respiration

Glycolysis is the process in which one molecule of glucose is broken in half, producing two molecules ofpyruvic acid (a 3-carbon compound).

Glycolysis

ATP ProductionA little energy is needed to get

the process of glycolysis going.

2 ATP molecules are needed to produce 4 ATP molecules through glycolysis.

This gives the cell a net gain of 2 ATP molecules.

NADH ProductionNAD+ is an electron carrier.

When NAD + accepts a pair of high-energy electrons, it is known as NADH.

NADH holds the electrons until they can be transferred to other molecules.

Therefore, NAD + helps to transfer the energy from glucose to other pathways in the cell.

The Krebs Cycle In the presence of oxygen, pyruvic

acid produced in glycolysis passes to the second stage of cellular respiration -The Krebs Cycle

During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions.

At the end of the Krebs Cycle, CO2 is released, ATP is produced and a number of high-energy electrons are captured by NAD+ making NADH and other electron carrier molecules.

Electron Transport Chain The cell uses the high energy

electrons from the Krebs cycle to convert ADP to ATP

Cellular Respiration Totals Glycolysis = 2 ATP Krebs Cycle = 2 ATP Electron Transport Chain = 32 ATP Total 36 ATP/Glucose molecule or about

38% of the total energy in the molecule. The rest is released as Heat.

9.3 Fermentation (Anaerobic Respiration)

Fermentation follows glycolysis when oxygen is not present.

Fermentation releases energy from food molecules in the absence of oxygen.

During fermentation, NADH converts back to NAD+ by passing high-energy electrons to pyruvic acid.

This allows glycolysis to continue producing a steady supply of ATP.

Because fermentation does not require oxygen, it is anaerobic.

Anaerobic means “not in air”

The 2 main types of fermentation are alcoholic fermentation and lactic acid fermentation.

Alcoholic Fermentation Produces carbon dioxide (CO2) as

well as alcohol.

pyruvic acid + NADH → alcohol + CO2 + NAD+

Examples: Yeasts (Breads, etc.) Beverages

Lactic Acid FermentationIn cells, pyruvic acid that

accumulates as a result of glycolysis can be converted to lactic acid.

This type of fermentation is called lactic acid fermentation.

This process regenerates NAD+ so that glycolysis can continue

pyruvic acid + NADH → lactic acid + NAD +

Lactic acid is produced in your muscles during rapid exercise when the body cannot supply enough oxygen to the muscle tissues.

The larger muscles (arms and legs) quickly run out of oxygen and ATP

Muscles rapidly produce ATP by lactic acid fermentation.

The build up of lactic acid causes a painful, burning sensation.

Energy and Exercise Quick Energy

Cells normally contain a small amount of ATP produced from glycolysis (enough for a few seconds of activity)

After a few seconds, cells produce energy by lactic acid fermentation

Lactic acid requires extra oxygen to get rid of it. This is why you keep breathing heavily after the race is over.

For long term energy, the body uses a combination of both aerobic and anaerobic respiration.

That is why athletes pace themselves in a race.

Comparing Photosynthesis and Cellular Respiration

Photosynthesis is the process that deposits the energy

Respiration is the process that withdraws the energy.

The equations of photosynthesis and respiration are the opposite of each other.

Photosynthesis removes CO2 from the air and respiration puts it back.

The products of cellular respiration are the reactants of photosynthesis.

Comparing Photosynthesis and Cellular Respiration

Photosynthesis Cellular Respiration

Function

Location

Reactants

Products

Equation