1. Module 5 Intro

1.25. Page 2

Lesson 5

Module 5—Photosynthesis and Cellular Respiration

Explore

 

Read

 

Anaerobic respiration occurs when oxygen is not present. When oxygen isn’t present, common anaerobic respiration by-products are sulfur, nitrite, methane, and odourless carbon dioxide. The colon contains bacteria that function without oxygen. By-products of this respiration in people are smelly gases. You will explore anaerobic respiration further as you move through this lesson.

 

Consider the following diagram.

 

A circle representing a cell surrounds four text boxes. The top box states “Glycolysis occurs in the cytoplasm; Anaerobic respiration.” Two arrows are pointing away from the top box at angles. The left arrow points to a text box containing the words ”In animals: Lactate fermentation (reversible oxygen debt).” The right arrow points to a textbox containing “In bacteria or chemosynthetic organisms: ethanol/alcohol fermentation (non-reversible).” The third arrow points straight down from the top box to an oval representing the mitochondria. It contains the text ”Aerobic Respiration in the Mitochondria.”

 

glycolysis: a metabolic pathway in which one glucose molecule is broken down to form two, 3-carbon molecules and a small amount of ATP


Glycolysis is the first step in both anaerobic and aerobic respiration.

 

pyruvate: three-carbon molecules produced by glycolysis

 

phosphorylation: the process of adding a phosphate to a molecule; occurs in cellular respiration and photosynthesis

You consume many complex sugars, like lactose (dairy products) and fructose (fruit), which your body breaks down to glucose that can be used at the cellular level.

 

All cellular respiration begins with glycolysis. Glycolysis occurs in the cytoplasm. It starts with a 6-carbon glucose molecule and consists of a series of oxidation reduction reactions that essentially split glucose into two, 3-carbon molecules called pyruvate. Pyruvate is then transported into the mitochondria for further processing and energy harvesting or remains available in the cytoplasm for the process of fermentation.

 

What energy gains are made during glycolysis? Early in glycolysis, two ATP molecules are used to energize glucose to split in two. Glucose is oxidized.

 

Later, four molecules of ADP undergo phosphorylation to produce four molecules of ATP.

 

Electrons are donated to two molecules of NAD+ to generate two molecules of NADH. In glycolysis, there is a net energy yield of 2 ATP and 2 NADH molecules.

 

Glucose and 2 ATP are the input for glycolysis. ATP is broken down into ADP, and glucose is broken down into two pyruvate molecules. NAD+ is reduced to NADH. Two NADH molecules and 2 pyruvate molecules will, if possible, move on to the mitochondrion for anaerobic respiration. Four ATP molecules are produced by glycolysis for a net gain of 2 ATP. These ATP are used in the cell to fuel metabolic pathways.

 

If you need a further explanation of the glycolysis process, read “Outside the Mitochondria: Glycolysis” on pages 183 and 186 of the textbook.

 

Watch and Listen

 

The “How Glycolysis Works” animation shows how NADH, ATP, and pyruvate are produced during glycolysis.

 

Self-Check

 

SC 1. Where does glycolysis occur?

 

SC 2. Does glycolysis occur without oxygen?

 

SC 3. What are three products of glycolysis?

 

Check your work.
Self-Check Answers

 

SC 1. Glycolysis occurs in the cytoplasm.

 

SC 2. Glycolysis occurs without oxygen.

 

SC 3. ATP, NADH, and pyruvate are products of glycolysis.