Lecture 7: Harvesting Energy – Glycolysis & Cell Respiration

COVERS CHAPTER 8

How do we get energy from food?*

• Glucose + O2 >> CO2 + H2O + ATP

• We breathe O2 and eat food (glucose), and convert that to CO2, water and ATP (energy for our cells to function)

The Big Picture*

• Glycolysis and Cell Respiration are both part of a larger CYCLE of life:

• Plants (and other autotrophs) take sunlight and water and CO2 and make glucose from it via photosynthesis. Oxygen is a byproduct of this reaction.

• Humans (and other animals-heterotrophs) take the glucose in (by eating plant material and other foods) and break it down. Water and CO2 are by products.

• In this way, plants give us what we need (O2 and glucose) and we supply the plants with water and CO2.

Photosynthesis Provides the Energy Released by Glycolysis and Cellular Respiration

Fig. 8-1

ATP

H2O O2CO2C6H12O6

glycolysis

photosynthesis

energy from sunlight

cellularrespiration

66 6

What do we use energy for?*

• Most cellular energy in the body is stored in the chemical bonds of ATP

• Cells require a continuous supply of energy to– Run chemical reactions– Grow– Reproduce– Move the body

More big picture*• This lecture describes the reactions that move the energy from energy

STORAGE molecules (like glucose and fat) to energy CARRYING molecules (like ATP, NADH and FADH2)

• About 40% of the energy in glucose can be transferred to ATP, the rest is released as heat.

• Cells break down glucose (the most common energy storage molecule) and give it to energy carrying molecules (ATP) via 2 separate reactions:– Glycolysis (2 steps)

• Glucose activation• Energy Harvest

– Cellular Respiration (3 steps)• Creation of Acetyl CoA• Krebs Cycle• Oxidative Phosphorylation

A Summary of Glucose Breakdown

Fig. 8-2

cellularrespiration

glucose

glycolysis

fermentation2 pyruvate

lactate

ethanol+

CO2

(cytoplasmicfluid)

mitochondrion

ATP

CO2

34or36

ATP2

6 H2O

O2

6

6

If no O2 is availableIf O2 is available

Glycolysis: 2 parts• Takes place in cytoplasm• Starts with a molecule of glucose, ends with PYRUVATE• Is an anaerobic reaction: can happen even in the absence of oxygen• 2 parts

– Glucose activation: a glucose molecule is energized by the addition of TWO high energy phosphates FROM TWO ATP molecules, leaving ADP. (Yes, you have to spend ATP to make it!)

– Energy Harvest: The products of these reactions give high energy phosphates back to 4 ADP molecules, resulting in the creation of 4 ATP molecules (but only a NET production of 2 ATPs.) Also, 2 high energy electrons and a hydrogen ion are added to “empty” electron carrier NAD+ to make NADH. Pyruvate is the end product of glycolysis.

The Essentials of Glycolysis

Fig. 8-3

glucosefructose

bisphosphate

G3P pyruvate

NAD+

ADPATP

2

2 2

22

4 4

2

ADP

NADH

ATP

Energy harvestGlucose activation

CC CCCC CC CC C CC C CCCC

PPP

1 2

Glycolysis*

• One molecule of glucose is transformed into 2 molecules of pyruvate

• Pyruvate then moves into the mitochondrial matrix

Anatomy of a mitochondrion*

• Outer membrane: most small molecules can freely diffuse across

• Intermembrane Space: space between inner and outer membrane

• Inner Membrane: high ratio of proteins to lipids-most molecules must pass through protein channels

• Cristae: internal compartments formed by the inner membrane

• Matrix: space inside inner membrane

Cell Respiration: Big Picture*

• Pyruvate (from glycolysis) is broken down (for every molecule of glucose entering glycolysis, keep in mind TWO pyruvates are made.)

• Energy is extracted (given to energy carrying molecules)• CO2 and H2O are released• Happens in mitochondria• 3 reactions make up cellular respiration:

– Creation of Acetyl CoA (matrix)– Kreb’s Cycle (AKA TCA Cycle, Citric Acid Cycle) (matrix)– Oxidative Phosphorylation (across inner mito membrane from matrix to

intermembrane space)

Creation of Acetyl CoA & Kreb’s Cycle

• Pyruvate >> Acetyl CoA (intermediate) + CO2

• Acetyl CoA >> H2O + 2 ATP + more CO2 + NADH + FADH2 + H ions

• CO2 LEAVES THE MITOCHONDRIA• 2 ATP are made• **High energy electrons and hydrogen ions are

transferred to energy carrying molecules NAD+ (10) and FAD (2), turning them into 10 NADH and 2 FADH2. **follow the NADH, FADH2 and H ions!

Creation of Acetyl CoA

Kreb’s Cycle

Oxidative Phosphorylation• 4 high energy electrons are transferred from NADH and

FADH2 to the ELECTRON TRANSPORT CHAIN, embedded in the inner mito membrane.

• The electrons “jump” from molecule to molecule along the chain, losing small amounts of energy each time. SOME OF THIS ENERGY is used to pump H atoms across inner membrane and into intermembrane space.

• Result is HIGH H+ concentration inside the intermembrane space.

• Electrons reach the “end” of the transport chain and are transferred to oxygen.

• H atoms and oxygen combine to form WATER.

Oxidative Phsophorylation

• FINALLY, H atoms flow DOWN their gradient from intermembrane space BACK to the matrix

• ADP and free phosphate are waiting there, and they combine to form ATP.

• ATP leaves matrix and mitochondria and enter cytoplasm to fuel cell’s processes. (and more ADP moves into matrix to make more ATP.

Oxidative Phosphorylation

Another pic of Oxidative Phosphorylation

ATP Yield*

• One molecule of glucose going through glycolysis can generate TWO molecules of ATP. If the material continues through cell respiration, another 34-36 molecules of ATP can be generated.

Aerobic vs anaerobic reactions

• Glycolysis is an anaerobic reaction: it can happen even in the absence of oxygen

• Cellular Respiration requires oxygen to happen: it is an aerobic reaction.

• (If there is no oxygen available, a cell cannot do cell respiration, and instead performs a fermentation reaction. Lactate-lactic acid- is a byproduct of fermentation.)

Fig. 8-2

cellularrespiration

glucose

glycolysis

fermentation2 pyruvate

lactate

ethanol+

CO2

(cytoplasmicfluid)

mitochondrion

ATP

CO2

34or36

ATP2

6 H2O

O2

6

6

If no O2 is availableIf O2 is available

Can we get energy from other molecules besides glucose?

• Keep in mind that most animals can harvest ATP from a number of different molecules (like glycogen-storage form of glucose- and fats and proteins), but glucose is the PRIMARY source of our ATP.

• (We would have to break down these other molecules and they can enter the cell respiration pathway at different points.)

Proteins and Fats can give us energy!