Chapter 8

Cellular respiration

Why Energy

• All active organisms use energy

• Carbon dioxide and water are both metabolic by-products produced by living cells

How do cells make ATP?

• ATP is the prime energy carrying molecule for all cells

• Aerobic respiration (with oxygen) is the main pathway for releasing energy from carbohydrates

• All energy releasing pathways begin with glycolysis– Glucose is split into two pyruvate molecules

– Glycolysis happens in the cytoplasm without the use of oxygen

Aerobic Respiration

• Yields 36 ATP from every glucose; fermentation yields only 2

• C6H12O6 + 6O2 6CO2 + 6H2O• 3 series of reactions are needed for aerobic

respiration– Glycolysis the breakdown of glucose to pyruvate, small amounts

of ATP are made– Krebs cycle- degrades pyruvate to carbon dioxide, water, ATP,

H+, and electrons– Electron transfer phosphorylation- processes H+ and electrons to

generate high yields of ATP, oxygen is the final electron acceptor

Aerobic respiration

Glycolysis

• Enzymes in cytoplasm catalyze several steps in the breakdown of glucose– Glucose is immediately phosphorylated and is split to

form two molecules of PGAL

– Enzymes remove H+ from PGAL and transfer them to NAD+ which is changed to NADH

– 4 ATP are produced

• End products are 2 pyruvates, 2ATP, 2 NADH

Glycolysis

Krebs Cycle

• Pyruvate enters the mitochondria and is converted to AcetylCoEnzyme-A which joins oxaloacetate that is already present

• During each turn of the cycle, 3 carbons enter as pyruvate and three leave as carbon dioxide

• H+ and electrons are transferred to NAD+ and FAD• 2 molecules of ATP are produced• Most of the molecules are recycled• CO2 is produced

Krebs cycle

Third stage

• NADH and FADH2 give electrons to transfer systems in the mitochondria

• Energy from electrons is released and is used to pump hydrogen out of the mitochondria

• The flow of hydrogen ions through ATP synthase channels joins ADP + P to make ATP

• Oxygen joins with the “spent” H+ to yield water

Electron transport chain

Summary of energy harvest

• Electron transfer yields 34 ATP• Glycolysis yields 2 ATP• Krebs yields 2 ATP• For every NADH=3ATP, FADH2=2ATP• NADH from cytoplasm cannot enter mitochondria

and must transfer electrons• In most cells the electrons are transferred to FAD and

yield 2 ATP• In liver, heart, and kidney cells NAD+ accepts the

electrons and yields 3 more ATP for a total of 38

Anaerobic respiration or fermentation

Fermentation

• Anaerobic respiration- happens without oxygen- pyruvate is metabolized to produce molecules other than ACoA

• Energy yields are only 2 ATP- so is limited to single-celled orgs. Or multicelled for limited times

• Begins with glycolysis- just like aerobic

Lactic Acid fermentation

• Certain bacteria (like those in milk) and muscle cells have enzymes capable of converting pyruvate to lactate

• No additional ATP is made, but NAD+ is regenerated

• Lactic acid is a byproduct

Alcoholic fermentation

• Begins with glucose degradation to pyruvate

• Enzymes convert pyruvate to acetaldehyde, which accepts electrons from NADH to become alcohol

• Yeasts are valuable in the baking industry

• By-products are CO2 and alcohol

Anaerobic electron transfer

• Some bacteria are able to strip electrons from organic compounds and send them through special electron transfer to produce ATP

• Examples include those that can reduce sulfate to hydrogen sulfide and those that can convert nitrate to nitrite. These are the bacteria that make swamps smell like rotten eggs

Alternative energy sources• Excess carbohydrates are stored as glycogen in the

liver and muscle for future use• Free glucose is used until it runs low, then glycogen

is used• Excess fats are stored in cells of adipose tissue• Fats are digested into glycerol which enters

glycolysis and fatty acids that enter Krebs cycle• Fats have more carbon and are degraded slowly• Amino acids in proteins are released during

digestion.• After the amino group is broken down, the amino

acid remnant enters Krebs cycle

Photosynthesis and Respiration

• The two reactions are essentially the mirror images of each other. The products of one are the reactants in the other

• Life is not a mysterious force, but a series of chemical reactions under tight control