Mitochondria and Cellular Energy Production

Question 1

Define catabolism.

Answer 1

The breakdown of cellular marcomolecules.

With respect to cellular energy, catabolism breaks down macromolecules and releases the energy stored within them. This released energy is transferred to other molecules and is ultimately stores as adenosine triphosphate (ATP) for use as the cells primary source of energy.

Question 2

Define anabolism.

Answer 2

The production of cellular macromolecules.

Anabolic reactions consume this ATP as small moleculels are built into macromolecules that the cell needs.

Question 3

Describe the structure of ATP.

Answer 3

ATP is composed of an adensine molecule, a ribose sugar, and a chain of three phosphates.

The phosphates are important as a lot of energy is stored in the bond between 2 and 3 phosphates.

When the 3rd phosphate is removed ADP is formed and energy is released, which can be used in cellular processes.

GTP is identical to ATP, except the adenosine is replaced with guanosine. GTP is another primary cellular energy source.

Question 4

Name 2 high energy molecules.

Answer 4

1. NAD+
   NAD+ is changed to its high energy form by the addition of an H+ ion and two electrons, producing NADH
2. FAD
   FAD is changed to its high energy form by the addition of 2 H+ ions and 2 electrons producing FADH2

Question 5

Describe the structure of the mitochondria.

Answer 5

The structure of the mitochondria is critical for their function. Mitochondria are double membranes organelles. The inner membrane appears to be folded in on itself o form structures called cristae.

The inside of the mitochondria is called the matrix and this is where macromolecules are converted into small, high energy compounds such as NADH. These compounds are then converted into ATP within the cristae.

Mitochondria even contain their own DNA to help them produce some of the proteins necessary for ATP production.

Question 6

How does glocuse enter cells?

Answer 6

Glucose first enters the bloodstream from ingested foods, the formation of complex molecules in the body from simpler molecules, or from the breakdown of glycogen stores. Once in the blood, glucose circulates and is available for cells to use.

The most common way to get glucosse into cells is by what are called glucose transporters (GLUT) found in most mammalian cells.

Question 7

Outline the 10 steps of glycolysis.

Answer 7

Steps 1 -5

• Converts 1 glucose molecule into 2 glyceraldehyde 3-phosphate (G3P)
• Requires an investment of 2 ATP in the first reaction
• Net energy is -2ATP

Steps 6-7

• Each G3P molecule is converted into a 3-phosphoglycerate
• For each G3P, this stage produces 1 ATP and 1 NADH
• Net energy is 0 ATP + 2 NADH

Steps 8-10

• each 3-phosphoglycerate molecule is converted to pyruvate
• The creation of pyruvate generates 1 ATP

1 Glucose + 2ADP + 2NAD+ = 2 pyruvate + 2ATP + 2 NADH

Question 8

What is anaerobic metabolism?

Answer 8

In plant cells and yeast, pyruvate undergoes fermentation where pyruvate is converted into ethanol in a process oxidizing the high energy molecule NADH into NAD+.

In most animal cells and bacteria, pyruvate is reduced so that NADH can be oxidized to NAD+ and lactate is formed. Some bacteria use this pathway to ferment milk and make cheese. In either case, the NADH produced during glycolysis is consumed resulting in a net yield of 2 ATP per glucose.

Question 9

When do you think lactate is produced in humans?

Answer 9

During heavy exercise, oxygen delivery to skeletal muscles is not sufficient, so anaerobic metabolic processes occur and lactate is produced.

Question 10

Outline the steps of Aeobic Respiration.

Answer 10

1. The conversion of Pyruvate to Acetyl CoA
   - At the end of glycolysis, pyruvate is in the cytosol of the cell and must be transported into the mitochondria
   - A carrier molecule on the inner mitochondrial membrane bring pyruvate into the matrix
   - In the matrix, pyruvate is decarboxylated by the pyruvate dehydrongenase complex
   - This complex takes the pyruvate, NAD+ and CoA and converts them into acetyl-CoA, NADH and CO2
   - Net energy of this stage is 2 NADH
2. The Krebs Cycle
   - The Krebs cycle is a complex series of chemical reactions to complete the oxidation of carbohydrates, proteins, and fats to produce the substrates for cellular energy production
   - Total energy produced by each Acety-CoA that enters the Krebs cycle is 4ATP, 10NADH and 2FADH2
3. The Electron Transport Chain
   - The ETC is a collection of protein complexes and electron carriers that are found within the mitochondrial inner membrane
   - The purpose of the ETC is to use the energy carrier produced by the Krebs Cycle to pump protons in to the intermembrane space
   - NADH and FADH2 enter the ETC at different point which menas they they pump different amount of protons
   - NADH generates the most ATP
4. Formation of ATP by ATP Synthase
   - The ETC concentrates protons within the intermembrane space, which leads to the generation of ATP in a chemiosmotic reaction
   - Located within the inner membrane is the enzyme ATP synthase
   - ATP Synthase is activated by the flow of protons back into the matrix - this flow of protons causes movement within the enzyme that creates kinetic energy which is converted to potential energy as ADP is converted to ATP.

Question 11

Outline the steps of the Krebs Cycle.

Answer 11

1. Acetyl-CoA enters the cycle by combining with oxaloacetate to form citrate, a six carbon molecule. Recall that we started with glucose, a six cabon molecule, so you should already be thinking that a lot of energy is not stored in citrate, we just need to get it out. At this stage, CoA is released and can be recycled to produce more acetyl-CoA
2. Citrate is broken down by several chemical reations to remove two of the carbons and create succinate. This removal of carbons transfers energy to two NAD+ to form two NADH and releases the carbon in the form of two CO2. A GTP is also produced.
3. To complete the cycle, succinate is converted back to oxaloacetate by multiple processes. These processes release the high energy molecules NADH + FADH2

Question 12

What are the 4 complexes of the ETC?

Answer 12

1. NADH-coenzyme Q oxidoreductase uses electrons from NADH to pump protons from the matrix to the intermembrane space
2. Succinate-coenzyme Q oxidoreductase passes electrons from FADH2 to the elctron transport chain, releasing protons into the matrix
3. Coenzyme Q-cytochrome c oxidoreductase uses electrons from both NADH and FADH2 to pump protons from the matrix to the intermembrane space
4. Cytochrom c oxidase uses electrons from complex III to pump protons from the matrix to the intermembrane space

Question 13

Describe Fat Metabolism.

Answer 13

Fat metabolism occurs primarly in the mitochondria but fatty acids cannot cross biological membranes without help.

• Fatty acids circulating in the blood stream are called free fatty acids and are tkane up into cells by a family of fatty acid transport proteins
• Once inside the cytosol of a cell, the fatty acid needs to be ‘activated’ in order for it to be transported into the mitochondria (this activation requires 2 ATP per FFA)
• -> activation is combing with coenzyme A too make a fatty acyl-CoA ester by consumer 2 ATP
• Inside the mitochondria, fatty acids are metabolized by beta-oxidation to produce high energy molecules
• -> Beta-oxidation breaks down fatty acids 2 carbons at a time, releasing FADH2 and NADH

Question 14

How many ATP can be produced from a 10 carbon fatty acid

Answer 14

64

Question 15

Calculaye how many ATP can be produced from an 18 carbon fatty acid

Answer 15

120

Question 16

How are proteins metabolised?

Answer 16

All molecules that are used for generating ATP are comprised only of carbon, hydrogen, and oxygen, so the nitrogen on the amino acids need to be removed by a process called deamination. This nitrogen is converted into a urea group and excreted in the urine.

Depending on the original structure of the amino acid there are different possibilities once the nitrogen has been removed:

• Some result in three carbon molecules that can be used to synthesize glucose
• Some result in the formation of acetyl-CoA that can diectly enter Krebs Cycle
• Some result in products that can enter as intermediaries of the Krebs Cycle

Question 17

When does the body use glucose, fats, or proteins to generate ATP?

Answer 17

Not every cell in the body has the same energy demands. Some cells are very active and require more energy than lower activity cells. Cells also have preferences for using carbohydrates, proteins, or fats as energy sources and even within a cell it may vary over time.

Question 18

Are there preferences by different organs for the use of glucose, fats or proteins to generate ATP?

Answer 18

Yes! Each tissue has its own preference.

Question 19

How is energy stored in the body?

Answer 19

Energy is stored in the body packaged in different types of biological molecules: carbohydrates, fats and proteins.

Question 20

Why isn’t energy just stored as ATP?

Answer 20

It is much more efficient to store potential energy as carbohydrates, fats and proteins than ATP itself.

If we think in terms of potential energy per gram, fat contains more energy than glycogen and glycogen than ATP itself.

Question 21

What happens when too much ATP is produced?

Answer 21

Because so many cellular processes use ATP, it is usually found in high concentrations within cells. The problem is that ATP is not very stable and requires constant production to prevent a cell from running out. In general, there is typically no direct storage of ATP for later use. The potential energy in excess ATP, however, can be transferred to another molecule that is more stable. This molecule is called phosphocreatine or creatine phosphate. This pathway for ATP formation is particularly important for nerve and muscle cells.