Cell Respiration

3.7, 8.1

Assessment statements

3.7.1 Define cell respiration.3.7.2 State that, in cell respiration, glucose in the

cytoplasm is broken down by glycolysis into pyruvate, with a small yield of ATP.

3.7.3 Explain that, during anaerobic respiration, pyruvate can be converted in the cytoplasm into lactate, or ethanol and carbon dioxide, with no further yield of ATP.

3.7.4 Explain that, during aerobic cell respiration, pyruvate can be broken down in the mitochondrion into carbon dioxide and water with a large yield of ATP.

Cell respiration

• The controlled release of energy from organic compounds in cells to form ATP

1st step of cell respiration: Glycolysis1. Glucose enters cell through membrane and

floats in cytoplasm2. An enzyme modifies the glucose slightly, then

a second enzyme modifies this molecule even more.

3. Series of reactions cleave the 6-carbon glucose into two 3-carbon molecules called pyruvate

4. Some of the energy released from the breaking of covalent bonds in the glucose is used to form 4 ATP molecules

http://library.thinkquest.org/27819/media/glycolysis.gif

Anaerobic respiration (Fermentation)• Alcoholic

– Glycolysis– Pyruvates converted to ethanol

– CO2 released

– Ex. yeast

• Lactic acid– Glycolysis– Pyruvates converted into lactic acid

– CO2 produced

– Allows glycolysis to continue in absence of oxygen

– Benefit?

Aerobic respiration

• Most efficient

• Performed by cells with mitochondria and oxygen

Steps of aerobic respiration

1. Begins with glycolysis2. Pyruvates enter mitochondrion3. Each pyruvate loses a CO2 molecule and

becomes acetyl-CoA4. Acetyl-CoA enters into series of reactions

called the Krebs cycle5. 2CO2 produced from each pyruvate6. Series of other reactions through electron

transport chain7. Water and large amt. of ATP produced8. More efficient b/c glucose is completely

oxidized

Assessment statements

8.1.1 State that oxidation involves the loss of electrons from an element whereas reduction involves a gain of electrons; and that oxidation frequently involves losing oxygen or gaining hydrogen.

8.1.2 Outline the process of glycolysis, including phosphorylation, lysis, oxidation and ATP formation.

8.1.3 Draw and label a diagram showing the structure of a mitochondrion as seen in electron micrograph

8.1.4 Explain aerobic respiration, including the link reaction, the Krebs cycle, the role of NADH + H+, the electron transport chain and the role of oxygen.

8.1.5 Explain oxidative phosphorylation in terms of chemiosmosis.

8.1.6 Explain the relationship between the structure of the mitochondrion and its function.

Oxidation and reduction

Oxidation Reduction

Loss of electrons Gain of electrons

Gain of oxygen Loss of oxygen

Loss of hydrogen Gain of hydrogen

Many C-O bonds Many C-H bonds

Results in a compound with lower potential energy

Results in a compound with higher potential energy

Oxidation and reduction cont.

• Cellular respiration is a catabolic pathway which contains both oxidation and reduction reactions– Glucose is oxidized b/c electrons are

transferred from it to oxygen; protons follow the electrons to produce water

– Oxygen atoms that occur in the oxygen molecules on the reactant side of the equation are reduced; large drop in the potential energy on the product side

Oxidation and reduction cont.

• Always occur together

• Referred to as redox reactions

• Reduced form always has more potential energy than the oxidized form of the molecule

Glycolysis – “sugar splitting”

• Thought to have been one of the first biochemical pathways to evolve

• Uses no oxygen

• No required organelles

• Occurs in both prokaryotic and eukaryotic cells

• A hexose, generally glucose, is split in the process

Three stages of glycolysis

1. Two molecules of ATP are used to begin glycolysis. The phosphates from the ATPs phosphorylate glucose to form fructose-1, 6-bisphosphate

6-carbon glucose

P P

2 ATP

2 ADP

2. The 6-carbon phosphorylated fructose is split into two 3-carbon sugars called glyceraldehyde-3 (G3P). This process involves lysis.

P P

P P

Fructose-1, 6-bisphosphate

Glyceraldehyde-3-phosphate Glyceraldehyde-3-phosphate

Step 3a. Once the two G3P molecules are formed, they enter

an oxidation phase involving ATP formation and production of the reduced coenzyme NAD.

b. Each G3P undergoes oxidation to from a reduced molecule of NAD+, which is NADH.

c. As NADH is being formed, released energy is used to add an inorganic phosphate to the remaining 3-carbon compound.

d. This results in a compound with two phosphate groups.

e. Enzymes then remove the phosphate groups so they can be added to ADP to produce ATP.

f. The end result is the formation of four molecules of ATP, two molecules of NADH and two molecules of pyruvate (the ionized form of pyruvic acid)

P

2P

P P

G3P

pyruvate

2 NAD+

2 NADH

4 ADP

4 ATP

2

2

2

Summary of glycolysis1. 2 ATPs are used to start process2. Total of 4 ATPs are produced (net gain of 2

ATPs)3. 2 molecules of NADH are produced4. Involves substrate-level phosphorylation, lysis,

oxidation and ATP formation5. Occurs in the cytoplasm of the cell6. Metabolic pathway controlled by enzymes;

when ATP is high, feedback inhibition will block first enzyme slowing or stopping the process

7. 2 pyruvate molecules are present at the end of the pathway

Mitochondria

• Place where the rest of cell respiration takes place in the presence of oxygen

The link reaction

1. Pyruvate enters the matrix of the mito. via active transport

2. Pyruvate is decarboxylated to form the 2-carbon acetyl group

3. Removed carbon is released as CO2

4. The acetyl group is then oxidized with the formation of reduced NAD+

5. The acetyl group combines with coenzyme A (CoA) to form acetyl CoA

Krebs cycle (tricarboxylic acid cycle)• If cellular ATP levels are low, the acetyl

CoA enters the Krebs cycle

• Occurs within the matrix of the mito.

• Called a cycle b/c it begins and ends with the same substance

• Step 1– Acetyl CoA combines with a 4-carbon

compound called oxaloacetate to form a 6-carbon compound called citrate

• Step 2– Citrate is oxidized to form a 5-carbon

compound– Carbon combines with oxygen to form CO2

– NAD+ forms NADH

• Step 3– 5-carbon compound is oxidized to form a 4-

carbon compound– Carbon combines with oxygen to form CO2

• Step 4– 4-carbon compound undergoes various

changes resulting in another NADH, FADH2, and ATP

• The 4-carbon compound is changed during these steps to re-form the starting compound of the cycle, coxaloacetate

• The Krebs cycle will run twice for each glucose molecule entering cellular respiration

Krebs Cycle outcomes

• 2 ATP

• 6 NADH

• 2 FADH2

• 4 CO2

Electron Transport Chain

• Occurs on the inner mitochondrial membrane and on the membranes of the cristae

• Embedded in the membranes are molecules that are easily reduced and oxidized

• These carriers of electrons are close together and pass the electrons from one to another due to an energy gradient

• Each carrier molecule has a slightly different electronegativity and a different attraction for electrons

• Most of these carriers are proteins with heme groups and are referred to as cytochromes.

ETC cont.

• In the process, small amts. of energy are released

• Sources of the electrons are the coenzymes NADH and FADH2 from the Krebs cycle and link reactions

• Electrons step down in potential energy as they pass from one carrier to another

• At the end of the chain, the de-energized electrons combine with available oxygen (final electron acceptor)

• Two hydrogen ions from the aqueous surrounds combine as well forming water

Chemiosmosis and Oxidative Phosphorylation• Chemiosmosis involves the movement of

protons (hydrogen ions) to provide energy so that phosphorylation can occur

• Because this type of phosphorylation uses an electron transport chain, it is called oxidative phosphorylation

Review of interior structure of mitochondrion• Matrix – place where

Kreb’s cycle takes place

• Cristae – large surface area for ETC to function

• Membranes – barrier allowing for proton accumulation on one side; ATP synthase

So, what just happened?

1. Electrons provide energy needed to pump protons from matrix to intermembrane space

2. Difference in concentration of hydrogen ions exists

3. Ions passively move into the matrix through a channel in ATP synthase

4. The enzyme harnesses available energy and phosphorylates ADP

Summary of ATP production in cellular respiration• Glucose → NADH/FADH2 → ETC →

chemiosmosis → ATP

Process ATP used ATP produced Net ATP gain

Glycolysis 2 4 2

Krebs cycle 0 2 2

ETC and chem. 0 32 32

Total 2 38 36

• Only about 30 ATP is generated in reality• Accounts for 30% of energy present in the chemical

bonds of glucose• Where does the rest go?