Chapter 8

How Cells Release Stored Energy

AKA: Cellular Respiration

• ATP is the prime energy carrier for all cells

• Aerobic Respiration (with oxygen) is the main pathway for energy release from carbohydrates to ATP

How do cells make ATP?

• All energy-releasing pathways start with glycolysis – Glucose is split into two pyruvate molecules– Glycolysis reactions occur in the cytoplasm

• Aerobic Respiration yields 36 ATP

• Anaerobic Respiration (without oxygen) yields 2 ATP

Aerobic respiration route:

C6H12O6 + 6O2 6CO2 + 6H2O

(Reverse equation to photosynthesis)

Overview of Aerobic Respiration

• 1- Glycolysis: is the breakdown of glucose to pyruvate– Small amount of ATP are generate (2 ATP)– Takes place in the cytoplasm

• 2- Kreb Cycle: degrades pyruvate to carbon dioxide, water, ATP, H+ ions and electrons (accepted by NAD+ and FAD)– Takes place in the mitochondrian– Makes 2 ATP

Three steps to aerobic respiration

• 3- Electron Transfer Phosphorylation: processes the H+ ions and electrons to generate high yields of ATP; oxygen is the final electron acceptor– Takes place in the mitochondrion– Yields 32 ATP (this is the real takes place)

Continue…

• 2 ATP is required to start glycosis

• Enzymes in the cytoplasm catalyze several steps in glucose breakdown– Glucose is first phosphorylated in energy-

requiring steps, then the six-carbon intermediate is split to form two molecules of PGAL (which gives a phosphate to make ATP)

– Enzymes remove H+ and electrons from PGAL and transfer them to NAD+ which becomes NADH (used later in the electron transfer)

Glycolysis: First stage of energy-releasing pathways

– By substrate-level phosphorylation, four ATP are produced

• The end product to glycolysis is:– 2 ATP (net gain)– 2 pyruvates– 2 NADH

For each glucose moleucule degraded

Continue…• The pyruvic acid diffuses into the inner

compartment of the mitochondrion where a transition reaction occurs that serves to prepare pyruvic acid for entry into the next stage of respiration:– (a) pyruvic acid acetic acid + CO2 (a waste

product of cell metabolism) + NADH +– (b) acetic acid + co-enzyme A -> acetyl CoA

• Takes place in the inner mitochondria matrix

• Pyruvate enters the mitochondria and is converted to acetyl-CoA, which then joins oxaloacetate already present from a previous “turn” of the cycle.

• During each turn of the cycle, three carbon atoms enter (as pyruvate) and three leave as three carbon dioxide molecules

Second Stage of the Aerobic Pathway: Kreb Cycle

• H+ and e- are transferred to NAD+ and FAD (coenzymes)– Ten coenzymes are loaded with electrons and

hydrogen

• Two molecules of ATP are produced by substrate-level of phosphorlyation

Functions of the second stage

• Most of the molecules are recycled to conserve oxaloacetate for continuous processing of acetyl-CoA

• Carbon dioxide is produced as a by-product

Continue…

• This is where the real work is done

• NADH and FADH2 give up their electrons to transfer (enzyme) system embedded in the mitochondrial inner membrane

Third Stage of the Aerobic Pathway

• According to the chemiosmotic model, energy is released in the passage of electrons through components of the transfer series

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

Electron Transfer Phosphorylation

• Electron transfer 32 ATP

• Glycolysis 2 ATP

• Kreb Cycle 2 ATP

Total 36 ATP

(per glucose molecule)

Summary of the Energy HarvestNet Gain

• Normally, for every NADH produced within the mitochondria and processed by electrons transfer chain, three ATP are produced

• FADH2 produced 2 ATP

Continue…

• NADH from the cytoplasm cannot enter mitochondrian and must transfer its electrons!!– In most cells (skeletal and brain) the electrons

are transferred to FAD and thus yield two ATP (for a total yield of 36)

– But in the liver, heart, and kidney cells, NAD+ accepts the electrons to yield three ATP because two NADH are produced per glucose, this total yield of 38 ATP

Continue…

• Cellular respiration without using oxygen (or very limited)– Pyruvate from glycolysis is metabolized to

produce molecules other than acetyl-CoA

• Example: Single Yeast Cells

Anaerobic Respiration

• With an energy yield of only 2 ATPs

• Glycolysis serves the first stage (just like aerobic respiration)

Fermentation Pathways

• Certain bacteria (as in bacteria) and muscles cells have the enzymes capable of converting pyruvate to lactate– Example: Muscle Cramps

• No additional ATP beyond the net two from glycolysis is produced but NAD+ is regenerated

Lactate Fermentation

• Fermentation begins with glucose degradation to pyruvate

• Cellular enzymes convert pyruvate to acetaldehyde, which then accepts electrons from NADH to become alcohol.

• Yeast are valuable in the baking industry (Carbon dioxide byproduct makes dough “rise”) and in alcoholic beverage production

Alcoholic Fermentation

• Some kinds of bacteria are able to strip electrons from organic compounds and send them through a special electron transfer in their membranes to produce ATP

• Example: Such bacteria include those that reduce sulfate to hydrogen sulfide (foul smelling gas) and those that convert nitrate to nitrite

Anaerobic Electron Transfer

• Excess carbohydrate intake is stored as glycogen in the liver and muscle for future use.

• Free glucose is used until it runs low, then glycogen reserves are tapped

Alternative Energy Sources in the Human Body

• Excess fats (including those made from carbohydrates) are stored away in cells of adipose tissue

• Fats are digested into glycerol, which enters glycolysis, and fatty acids, which enter the Kreb Cycle

• Fatty acids have more carbon and hydrogen atoms, they degraded more slowly and yield greater amounts of ATP

Energy from Fats

• Amino acids are released by digestion and travel in the blood

• After the amino group is removed, the amino acid is removed, the amino acid remanant is fed into in the Kreb Cycle

Energy from Proteins

• Photosynthesis and cellular respiration are intimately connected

• Life is not some mysterious force, but a series of chemical reactions under highly integrated control.

Perspective on the Molecular Unity of LIfe