Chapter 9

Hunger can present a variety of different symptoms in different people• Ultimately, the cause is the same,

your body has a need for food

Food provides living things with energy• It is used as the chemical building

blocks needed for growth and reproduction

One gram of sugar (C6H12O6) when burned in the presence of oxygen releases 3811 calories of heat energy!

Calorie – the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius• Is represented as kilocalories (C) on food

labels

Cells don’t actually “burn” the glucose molecules• Instead energy is gradually released

The releasing of energy starts with a processes called Glycolysis• In the presence of oxygen, glycolysis

proceeds to 2 other pathways• In the absence of oxygen, glycolysis is

followed by another pathway

Cells don’t actually “burn” the glucose molecules• Instead energy is gradually released

The releasing of energy starts with a processes called Glycolysis• In the presence of oxygen, glycolysis proceeds

to 2 other pathways• In the absence of oxygen, glycolysis is followed

by another pathway

In the presence of oxygen, glycolysis is followed by the Krebs Cycle and Electron Transport Chan (ETC)• They are included in the process of cellular

respiration

Cellular Respiration – the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen

There are 2 sites where cellular respiration takes place

1.Glycolysis – in the cytoplasm2.Krebs Cycle & ETC – in the

mitochondria

6O2 + C6H12O6 -> 6CO2 + 6H2O + Energy

Oxygen + Glucose -> Carbon Dioxide + Water + Energy

The process requires oxygen and food and gives off carbon dioxide, water and energy

This process occurs in multiple steps• If there was only 1 step, all of the energy from

glucose would be released at once• Cells need to control energy usage by storing

it and releasing it little bits at a time This is done by creating ATP

Glycolysis – the process in which one molecule of glucose is broken in half, producing 2 molecules of pyruvic acid, a 3 carbon compound

is the 1st set of reactions in cellular respiration

Even though glycolysis is an energy - releasing process, it requires energy to start it

2 molecules of ATP are used to start glycolysis, and ultimately when the process is over, 4 ATP molecules are produced

*Glycolysis gives a net gain of 2 ATP molecules *

One reaction of glycolysis moves 4 high energy electrons using an electron carrier NAD+

• NAD+ + 4 electrons -> NADH

With the use of NAD+, energy can be passed to other pathways of the cell

Glycolysis is so fast that thousands of ATP’s can be produced in milliseconds!

The cell requires the use of NAD+ to pass on the high energy electrons to keep NAD+ free for more glycolysis to occur

1. Created energized ATP molecules

2. Passed on high energy electrons (w/ NADH)

3. Created pyruvic acid (which gets used in the Krebs Cycle)

When glycolysis occurs and there is no oxygen present, it proceeds to the pathway of fermentation

Fermentation – process that releases energy from food molecules by producing ATP in the absence of oxygen

Anaerobic – does not require oxygen

During fermentation, cells convert NADH to NAD+ by passing the high energy electrons back to pyruvic acid• Keeps NAD+ free to accept more electrons• Allows more ATP energy to be produced

1. Alcoholic Fermentation • Done by some yeasts and microorganisms

to produce alcohol and breads

pyruvic acid + NADH -> alcohol + CO2 + NAD+

2. Lactic Acid Fermentation• Occurs in muscles during rapid exercise

when the body cannot supply enough of oxygen to the tissues

• Lactic Acid Fermentation can produce ATP to supply muscles with additional energy

• Too much lactic acid buildup can cause a painful, burning sensation which is associated with muscle fatigue

pyruvic acid + NADH -> lactic acid + NAD+

In addition of supplying overused muscles with energy, lactic acid fermentation is also utilized for making different food products• Done with the use of bacteria

Ex.) cheeses, yogurts, pickles, sauerkraut

At the end of glycolysis, about 90% of the chemical energy that was available in the glucose is still unused• It is trapped in high energy electrons and

pyruvic acid

To extract the rest of the energy, the cell is going to use oxygen• Because cellular respiration requires oxygen

it is an aerobic process

In the presence of oxygen, the pyruvic acid produced in glycolysis is passed 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• Because a molecule called Citric Acid is one of

the first compounds formed, Krebs Cycle is also referred to as the Citric Acid Cycle

As seen in glycolysis and photosynthesis, the Krebs Cycle also produces high energy electrons which require special electron carriers

Along with carbon dioxide as a product, the highly energized compounds NADH, ATP and FADH2 are also produced

OR

The Krebs Cycle’s highly energized electrons are passed to the ETC the energized carrier molecules NADH and FADH2

The purpose of the ETC is to use the energized electrons to convert ADP into ATP

How much energy does cellular respiration yield from 1 glucose molecule?

Remember, glycolysis had a net gain of 2 ATP molecules • Which is all that can be made in the absence of

oxygen (anaerobic)

With the addition of the Krebs Cycle and ETC, the cell can make an additional 34 ATPs for a grand total of 36 ATPs per glucose

The 36 ATP molecules the cell makes per glucose represents about 38% of the total energy of glucose• May not seen efficient, but the cell is more

efficient at using chemical fuel than the average engine is at using gasoline fuel

The remaining 62% of glucose energy is released as heat• Which is why the body feels warmer after

vigorous exercise

Quick Energy• Cells contain small amounts of ATP

made through glycolysis and cellular respiration This ATP is enough to last about 90 seconds

of intense activity

• After this small amount of ATP is used, lactic acid fermentation starts, causing lactic acid buildup in the muscles The only way to rid the body of this is with

oxygen, which is why heavy breathing starts soon after activity

Long-Term Energy• For exercise longer than 90 seconds,

cellular respiration is the only way to generate a continuing supply of ATP

• Cellular respiration releases ATP more slowly than fermentation does Why athletes have to pace themselves

• The body is also able to utilize carbohydrate glycogen in muscles and other tissues Can supply energy for 15 to 20 minutes of

activity