Cellular Respiration: Releasing Chemical Energy

Reactants: Products: glucose carbon dioxide oxygen water (ATP)

Energy is taken out of glucose and this energy transferred to ATP. The carbon dioxide is what is left after the energy is

removed from the sugar. Oxygen is reduced, and as it gains electrons (and hydrogens), it forms water.

Cellular respiration is an exergonic metabolic pathway

External vs. Cellular Respiration• External respiration is breathing.

• Cellular respiration is the breaking down of food molecules to release energy (ATP). During cellular respiration, glucose is gradually oxidized and breaks down into carbon dioxide.

• External respiration is related to cellular respiration, but they are not the SAME thing.

• Cellular respiration occurs within the cytoplasm and mitochondria of cells.

ALL living things need ENERGY!goal of cellular respiration is to make ATP!

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Cellular respiration is biochemical pathway catalyzed by enzymes that provides the

energy cells need to function.

Cellular Respiration Occurs in Three Stages:

1. Glycolysis

2. Krebs Cycle

3. Electron Transport Chain (ETC)

Remember: goal of cellular respiration is to make ATP!

Glycolysis

Glycolysis takes place in the cytoplasm of cells.

Glycolysis- glyco = sugar / lysis = breakThe process that begins the breakdown of sugar. It converts glucose into 2 pyruvic acids (also called pyruvate).

6 carbons 3 carbons

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LOOK AT ALL THOSE STEPS!

Glycolysis Energy Investment• One glucose is broken into two G3Ps

using ATP for the activation energy needed to start the reaction.

Energy Payoff Phase• Electrons and Phosphates (ENERGY) are

stripped from the G3P molecules producing ATP as well as electron carriers, NADH. Additionally, The pyruvates still contain energy.

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Steps of Glycolysis1. Energy Investment:

– 2 ATP are used to add phosphates to glucose energizing the molecule causing it to split into a three carbon sugar (G3P). (Note: some ATP needed to start the process - Activation Energy)

2. Energy Pay-off– Electrons (and hydrogens) are removed by an

electron carrier (NAD+) which becomes reduced (reduction is gain) NAD+ → NADH and the phosphates later get removed forming ATP

Summary on Glycolysis

• Takes place in the cytoplasm

• Glucose is split into pyruvates(2)

• produce some ATP (2)

• NAD+ is reduced to form NADH (2)

Remember: goal of cellular respiration is to make ATP!

When oxygen is present…• Glycolysis does not require

oxygen, but oxygen is needed for the next step of to take place. Aerobic respiration requires oxygen.

• The pyruvate diffuses into the matrix of the mitochondria.

• The matrix is the internal fluid of the mitochondria.

Not this Matrix...

This Matrix...

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ALL Eukaryote Cells have Mitochondria

Mitochondria are everywhere!!animal cells plant cells

Me too!Me too!

• Next, in the matrix, for EACH molecule of pyruvate, a CO2 is removed (pieces of broken down sugar)

• NADH is formed (that’s two more “energy” carriers)

• A coenzyme attaches forming Acetyl CoA (still has some energy left in it)

So, lets continue the journey.....

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The Krebs cycle is a biochemical pathway that further breaks down what is left of sugar.

• It strips energy out of acetyl coA and puts it into ATP and electron carriers, as it does, carbon dioxide is released.

• The Krebs cycle was discovered by Hans Kreb

• It is also called the citric acid cycle because citric acid is the first intermediate formed in the cycle.

• must be done twice (once for each Acetyl CoA)

Krebs Cycle

Acetyl CoA enters the Krebs Cycle

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A more complicated version....

Don’t memorize this!

Steps of the Krebs Cycle there are actually more than 4!

1. Acetyl Co A joins with an oxaloacetate molecule to form citric acid.

2. NAD+ removes electrons (hydrogen) from citrate forming NADH and an intermediate molecule, ketoglutarate. (citric acid is oxidized and NAD+ is reduced)

3. NAD+ removes electrons (hydrogen) from ketoglutarate forming NADH and an intermediate molecule, succinate. This step also removes phosphates producing ATP.

4. Electrons (Hydrogen) removed by electron carrier FAD orm FADH2 and the molecule becomes malate.

5. Malate loses electrons reducing NAD+ into to NADH and reform oxaloacetate acid, which can be used to join with another acetyl CoA.

Krebs Cycle

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The main points of the Krebs cycle• Throughout the entire process of the Krebs cycle

– Sugar is completely broken down (oxidized)– As sugar is broken down, carbon dioxide is

released (6)– Some ATP is produced (2 molecules)– Electrons (Energy) are transferred to NADH (8)

and FADH2 (2) which are used by the an electron transport chain and ATP synthase to produce more ATP.

• Each NADH is worth 3 ATP • Each FADH2 is worth 2 ATP

Energy Summary So Far...

• Glycolysis– 2 ATP– 2 NADH (worth 6 ATP)

• Krebs Cycle (including pyruvate->Acetyl CoA)– 2 ATP– 8 NADH ( worth 24 ATP) – 2 FADH2 (worth 4 ATP)

Remember: goal of cellular respiration is to make ATP!

Electron Transport Chain

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Electron Transport Chain

Electron transport Chain1. Electrons from Krebs Cycle are carried by NADH and

FADH2 to the mitochondrial membrane and a series of proteins call the the electron transport chain. (ETC)

2. Electrons move down the chain of membrane proteins, from high to low energy (redox reactions). Energy is harnessed and used to pump protons (hydrogen ions) from the matrix into the inter-membrane space of the mitochondria. (This is active transport)

3. Oxygen is the “final electron acceptor”, which then bonds with hydrogen ions forming water.

4. The protons act as a battery flowing through ATP synthase which produces ATP through the process of chemiosmosis.

ATP Production

• 38 ATP are produced during cellular respiration per glucose molecule.

2 are produced during glycolysis

2 are produced during the Krebs Cycle

34 are produced by the electron transport chain

Remember: goal of cellular respiration is to make ATP!

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Overview of Cellular Respiration

Overview of Cellular Respiration

But what if oxygen is not present? • When oxygen is not present anaerobic

respiration follows glycolysis.• Pyruvate is still broken down by glycolysis in

the cytoplasm without the use of oxygen. This produces 2 ATP per glucose.

• Glycolysis is then followed by fermentation.

– There are two types of fermentation• Lactic acid fermentation• Alcoholic fermentation

What if you don’t have mitochondria?

Remember: goal is still to make ATP!

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Lactic acid fermentation• In lactic acid fermentation pyruvic acid (pyruvate) is converted to lactic acid.

• When you exercise, often times lactic acid builds up due to limited oxygen. (“feel the burn”)

• The point is to regenerate NAD+ to repeat glycolysis.

Alcoholic fermentation• In alcoholic fermentation pyruvic acid is converted to

ethyl alcohol. Carbon dioxide is also made.• This process is used to produce products like breads,

beer and wine. • The point is to regenerate NAD+ to repeat glycolysis.

A few last notes on respiration...• Amino acids (protein) and lipids

can be broken down during cellular respiration. As they are broken down by digestion/hydrolysis, they are modified into molecules that are part of the Krebs cycle.

• Cellular respiration also releases heat energy that can help organisms maintain temperature homeostasis.

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