CHAPTER 6 CELLULAR RESPIRATION

Chemical Energy In Food

Purpose of food: Source of raw materials used to make new

molecules

Source of energy

calorie – the amount of energy needed to raise the temperature of one gram of water one degree Celsius. Cells don’t burn glucose but gradually release

the energy from it.

Glycolysis – chemical pathway that begins the process of releasing the energy in glucose.

Cellular Respiration Overview

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

Contains 3 Pathways:

Glycolysis

Krebs cycle

Electron transport

The equation for cellular respiration is:

6O2 + C6H12O6 6CO2 + 6H2O + ENERGY (ATP)

Cellular Respiration overview

If cellular respiration took place in one step, all the energy would be released at once and most would be lost as heat.

PROBLEM: The cell has to find a way to trap the energy a little bit at a time.

SOLUTION: Each of the three stages of cellular respiration captures some of the chemical energy available in food molecules and uses it to produce ATP or energy.

Step 1: Glycolysis

Takes place in the cytoplasm.

Glycolysis is the process in which 1 molecule of glucose is broken in half, producing 2 molecules of pyruvic acid.

This process has to get things going so it uses 2 stored ATP molecules.

4 ATP molecules are made during glycolysis, so a total of 2 ATP molecules are released by glycolysis as energy.

This is only 2% of the total chemical energy in glucose.

NAD+ is converted to NADH.

Step 1: Glycolysis

Glycolysis is so fast that cells can produce 1000s of ATPs in milliseconds.

Glycolysis does not require oxygen.

OXIDATIVE RESPIRATION

To get the rest of the energy from the food molecules, the cell uses oxygen.

Aerobic – requires oxygen.

The energy-releasing pathways require oxygen, and that is the reason we need to breathe, to respire.

Respiration - process that involves oxygen and breaks down food molecules to release energy.

Step 2: Krebs cycle

The Krebs cycle:

Takes place in the mitochondria.

Also called the Citric acid cycle.

The carbon contained in pyruvic acid (produced by glycolysis) is broken down into carbon dioxide.

This CO2 is considered a waste product and is released from the cell.

Two high energy electron carriers are also generated by the Krebs cycle:

NADH

FADH2

KREBS CYCLE (continued)

The NADH and FADH2 go on to make huge amounts of ATP in the presence of oxygen, and the CO2 is eventually released when you exhale.

Step 3: Electron Transport

Electron transport occurs in the mitochondria.

The high energy electrons are passed from the carriers in the Krebs cycle (NADH, FADH2) to the electron transport chain.

High energy electrons are passed from one carrier protein to the next.

The electron transport chain uses these high energy electrons to convert ADP into ATP or energy.

At the end of the chain, an enzyme combines the electrons with H+ and oxygen to form water, a by-product of electron transport.

ENERGY AND EXERCISE

Aerobic cellular respiration produces 36 total ATP molecules from each glucose molecule (37% efficient).

Remainder of energy from glucose is released as heat (body feels warm after exercise).

Eating food:

Complex carbs are broken down into simple sugars that are converted to glucose.

Lipids and proteins broken down into molecules that enter glycolysis or Krebs cycle.

Breathing and respiration:

Remember, the final electron acceptor for the electrons produced in respiration is oxygen.

Without oxygen, all ATP synthesis in mitochondria stops and body tries to make ATP by glycolysis alone (not sufficient for most cells).

Photosynthesis vs Respiration The processes are almost opposites.

Equations are opposites.

Photosynthesis deposits energy.

Respiration withdraws energy.

Cellular respiration takes place in all eukaryotes and some prokaryotes.

Photosynthesis occurs only in plants, algae, and some bacteria.

Fermentation

Occurs when oxygen is NOT present.

The respiration that occurs in the absence of oxygen is called anaerobic respiration or fermentation b/c it does not require oxygen.

In fermentation, cells perform glycolysis followed by fermentation.

Krebs cycle and electron transport DO NOT occur.

Combination of glycolysis and fermentation produces 2 ATP molecules from one molecule of glucose.

There are two main types of fermentation:

A. Alcoholic Fermentation

B. Lactic Acid Fermentation

Alcoholic Fermentation

Pyruvic Acid + NADH alcohol + CO2 + NAD+

Used by yeast (facultative anaerobes).

Forms ethyl alcohol and carbon dioxide as wastes.

CO2 produced causes bread dough to rise, bubbles in beer.

Lactic Acid Fermentation

Pyruvic Acid + NADH lactic acid + NAD+

This process regenerates NAD+ so that glycolysis can continue to make small amounts of ATP.

Lactic acid fermentation happens in many cells including human muscle cells during rapid exercise. Muscles aren’t getting enough oxygen to produce all

the ATP they need through Krebs cycle and electron transport, so they use lactic acid fermentation to get ATP made.

Lactic acid buildup causes the painful, burning sensation in your muscles after intense activity.

Bacteria undergo lactic acid fermentation to make foods like sour cream, yogurt, buttermilk, and sauerkraut.