Biology 5.3B Cellular Respiration

Cellular Respiration

Cellular Energy

Most of the foods we eat contain energy stored in proteins, carbohydrates, and fats.

Before our cells can use this energy it must be transferred to ATP within the body’s cells.

Cells transfer the energy in organic compounds to ATP through a process called cellular respiration.

Process cellular respiration

Cellular Energy

Some ATP is made with oxygen. Some ATP is made without oxygen.

Each metabolic process has a different name.

1. Metabolic processes that require oxygen are called aerobic processes.

2. Metabolic processes that require NO oxygen are called anaerobic processes.

Cellular Energy: Respiration

The energy-producing process in living things is called respiration.

Respiration is the release of energy by combining oxygen with digested food (glucose).

Carbon dioxide and water are also produced as byproducts. They are the waste products of respiration.

A simple formula to show respiration looks like this:

Glucose + oxygen carbon dioxide (waste) + water (waste) + energy

Stages of Cellular Respiration

Cellular respiration occurs in three major stages

1. Glucose is converted to pyruvate doubling the amount of ATP present.

2. Aerobic respiration occurs: Pyruvate goes through the Krebs Cycle. Energy originally stored in glucose-pyruvate is transferred to NADH & FADH2 compounds.

3. In an electron transfer chain, electrons and hydrogen atoms form H2O as a waste product. A phosphate group gets added to ADP and large amounts of ATP result from this.

Stage 1: Breakdown of Glucose

The primary fuel for cellular respiration is glucose which is formed when carbohydrates such as starch and sucrose are broken down.

In the first stage of cellular respiration, glucose is broken down in the cytoplasm during a process called glycolysis.

Stage 1: Breakdown of Glucose

Glycolysis is an enzyme-assisted anaerobic (requires no oxygen) process that

breaks down one six carbon molecule of glucose

to two three-carbon pyruvate ions

Stage 1: Breakdown of Glucose: Glycolysis

Glycolysis can be summarized as follows:

Stage 1: Breakdown of Glucose

2 ATP molecules go through a series of reactions.

The result is 2 3-carbon pyruvate ions are produced plus 4 ATP molecules.

The amount of ATP is doubled from 2 to 4, a net gain of 2 ATP

Detail of steps of chemical reactionglycolysis

Stage 1: Breakdown of Glucose: Glycolysis

Summary: Stage 1

1. Glycolysis uses two ATP molecules

2. but produces four ATP molecules in return.

3. Thus, we gain two ATP molecules for a gain ratio of 2 to 1.

Stage 1: Breakdown of Glucose: Glycolysis

Glycolysis is followed by another set of reactions that uses the energy temporarily stored in NADH to make more ATP.

This next reaction is called the Krebs Cycle or Citric Acid Cycle.

Stage 2: Production of ATP

Stage 2: Krebs Cycle

In stage 2, “when oxygen is present”, the pyruvate ions enter mitochondria and go through a series of chemical reactions.

The result of this reaction is the creation of one molecule of CO2 (carbon dioxide as a waste product) and one molecule of the compound Acetyl-CoA.

This Acetyl-CoA compound than goes through a series of reactions called the Krebs Cycle.

Stage 2: Production of ATP: Krebs Cycle

Summary: Stage 2

After the Krebs Cycle, NADH and FADH2 now contain much of the energy that was previously stored in glucose and pyruvate.

When the Krebs Cycle is completed, the four-carbon compound that began the cycle has been recycled, and acetyl-CoA can enter the cycle again.

Stage 3: Electron Transfer Chain

Summary: Stage 3

In aerobic respiration, electrons donated by NADH and FADH2 pass through an electron transport chain.

1. In this electron transfer chain, hydrogen ions get created.

2. At the end of the chain, spent electrons and hydrogen ions combine with oxygen, forming water molecules (H2O) as a waste product.

3. Hydrogen ions diffuse back into the inner compartment through a carrier protein that adds a phosphate group to ADP, making ATP.

4. Up to 34 more ATP molecules are produced at this point by this transport chain.

Electron Transfer Chain

Production of ATP: Summary

The total amount of ATP that a cell is able to harvest from each glucose molecule that enters glycolysis depends on the presence or absence of oxygen.

Cells use energy most efficiently when oxygen is present.

In the first stage of cellular respiration, glucose is broken down to pyruvate during glycolysis. Glycolysis is an anaerobic process (no oxygen required), and it results in a gain of two ATP molecules.

In the second stage of cellular respiration, the pyruvate passes through either aerobic respiration (requires oxygen) or fermentation. When oxygen is not present, fermentation occurs instead.

Production of ATP: Summary

Most of a cell’s ATP is made during aerobic respiration. (requiring oxygen)

For each molecule of glucose that is broken down, as many as two ATP molecules are made directly during the Krebs cycle,

and up to 34 ATP molecules are produced later by the electron transport chain.

Computer Lab Activities

Go to the following Web-based activities and watch and complete both activities including the quiz at the end of the second site

video 1 cellular respiration

Aerobic respiration UK version

Aerobic Respiration