1. Module 5 Intro

1.30. Page 2

Lesson 6

Module 5—Photosynthesis and Cellular Respiration

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Krebs cycle: a metabolic pathway consisting of a series of reactions that break down the end products of glycolysis, producing carbon dioxide and generating a large amount of ATP; also know as the citric acid cycle; named after the 1953 Nobel Prize winning scientist who made the discovery

As you explored in Lesson 5, glycolysis is the first step in cellular respiration. Glycolysis is anaerobic, but cellular respiration can continue aerobically. Under oxygenated conditions, cellular respiration moves from the cytoplasm of the cell to the matrix of the mitochondria. The products of glycolysis—pyruvate and NADH—move into the matrix where they become a part of the Krebs cycle.

 

Aerobic respiration can be summarized by this familiar statement:

 

One glucose plus six oxygen produces 6 carbon dioxide plus 6 water plus ATP in the mitochondria.

 

You will be exploring what happens inside the mitochondria to produce ATP, CO2(g) and H2O(l).

 

Before you continue with this lesson, it will be useful to read “Inside the Mitochondria: Krebs Cycle Preparation” and “The Krebs Cycle” on pages 187 and 188 of the textbook.

 

Self-Check

 

SC 1. Complete this Self-Check activity.

 

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First, have a look at the simplified steps of aerobic respiration involving one molecule of glucose.

  1. Glycolysis:
  • The first step breaks down glucose into pyruvate.

    The diagram represents Glycolysis. A triangle representing glucose is shown on the left, an arrow labelled Enzymes points to two smaller triangles each labelled Pyruvate, and two star shapes labelled ATP.

  • acetyl CoA: an oxidized form of glucose
    In the intermediate step, pyruvate is converted  to acetyl CoA.

  • Reduced NADH is produced as well as CO2(g).

    NADH: a high-energy electron carrier
    It acts as an electron donor to the electron transport system involved in the production of ATP.

    FADH2:     an important coenzyme produced during Krebs cycle

    It acts as an electron donor to the electron transport system involved in the production of ATP.

    A diagram represents the Krebs cycle preparation. A triangle labelled Pyruvate is on the left. An arrow points to a triangle attached to a rectangle. This is labelled Acetyl CoA. Above the arrow there is a u-curved arrow pointing from NAD+ to NADH. After the shape labelled Acetyl CoA, there is a plus sign followed by carbon dioxide.
  1. Krebs Cycle: 
  • acetyl CoA provides electrons for the electron transport chain.

    1. acetyl CoA is oxidized to produce reduced NADH and FADH2.

  • CO2(g) is produced.

  • ATP is produced.

    This diagram represents the Krebs cycle. From left to right: There is a shape labelled Acetyl CoA. An arrow points to a cycle of curved arows representing the Krebs cycle. Citric acid is formed in the Krebs cycle. An arrow points from the Krebs cycle to two shapes labelled ATP. Another arrow points to an octagon labelled CO2. Another arrow points from the Krebs cycle to a circle labeled e- . An arrow points from the circle to two bucket shapes labelled NADH and FADH2. Circles labelled e- are in the buckets.
  1. Electron transport chain:
  • Uses the electrons from NADH and FADH2 to make a lot of ATP

    1. The oxidation of NADH and FADH2 create a proton gradient that powers ATP synthase (chemiosmosis is similar to photosynthesis).

      This is a diagram representing the electron transport chain. The diagram moves diagonally from top left to bottom right. At the top, the bucket shape labelled NADH & FADH2 is shown to be tipped over. The circle shapes labelled e- are falling out of the bucket toward a staircase. The staircase represents the electron transport chain. Electrons move down the electron transport chain (a series of proteins that pass the electrons along). This is indicated by an arrow pointing diagonally down the staircase. At the end of the staircase a circle shape labelled e- is added to a box labelled Oxygen we breathe. An arrow points from the box to an oval labelled Water. There is also a star shape labelled 32 ATP at the end of the staircase. An arrow points from this shape to a circle labelled We use for energy. Electrons are caught by oxygen to make water. 32 ATP are made.

 

Self-Check

 

SC 2.  What compound derived from glucose actually enters the Krebs cycle?

 

SC 3. The carbon atoms derived from glucose are fully oxidized in the Krebs cycle. What becomes reduced during the Krebs cycle? (Remember: LEO the lion goes GER)

 

Check your work.
Self-Check Answers

 

SC 2. Acetyl CoA enters the Krebs cycle.

 

SC 3. NAD+ is reduced to NADH, and FAD is reduced to FADH2.

 

 

Read

 

How much energy does one glucose molecule provide when oxygen is the final electron acceptor?

 

ATP Totals for Aerobic Respiration

 

Glycolysis                           2 ATP

Krebs Cycle                        2 ATP

Electron Transport Chain     32 ATP

                                      36 ATP from 1 glucose

 

 

 

 

 

 

 

 

You have determined that the electron transport chain creates most of the energy for cellular respiration. So, what is the purpose of the Krebs cycle?

 

Watch and Listen

 

The following detailed animation of the Krebs cycle should help you understand its purpose in aerobic respiration.

 

Self-Check

 

SC 4. What is the purpose of the Krebs cycle?

 

SC 5. How does the word catabolism apply to the Krebs cycle?

 

Check your work.
Self-Check Answers

 

SC 4. The purpose of the Krebs cycle is to fully oxidize glucose, provide electrons for the electron transport chain, and create some usable energy for the cell.

 

SC 5. Catabolism means to break down molecules into smaller components and release energy. The Krebs cycle breaks down glucose through oxidation and ATP is synthesized.