Chapter 8

HOW CELLS RELEASE ENERGY

(hyperlink on title)Jill before teaching inbed these hyperlinks

Define CR: it is all the biochemical pathways necessary to

extract energy from nutrient molecules in the presence of oxygen – leaving CO2 as a metabolic WASTE

Who does it? ALL living organisms!! Even those without a

respiratory system, including plants and fungus

Anyone who has mitochondria and EVEN if they DON’T

Reverse of photosynthesis – moving energy from carbs into ATP

All cells (prokaryotic & eukaryotic) require energy to:• combat entropy• carry out day-to-day functions• repair/replace worn out organelles• reproduce

What form of energy do cells use?

ATPLink on picture

How do cells obtain ATP?All cells must make their own ATP

from nutrients they have either synthesized (autotrophs) or consumed (heterotrophs).

Most cells break down nutrients to make ATP in TWO ways:

• Cellular respiration (aerobic process)

• Fermentation (anaerobic process)

General equation for Aerobic cellular respiration of glucose:

C6H12O6 + 6O2 6CO2 + 6H2O + 30 ATP

General wquation for Fermentation (anaerobic cellular respiration)

Not so well defined!

Cellular respiration occurs in 3 stages:

Glycolysis

Krebs Cycle

Electron Transport

Chain

Eukaryotic cells

Cytoplasm

Mitochondria

Prokaryotic cells

Cytoplasm

Cell membrane

Cool StuffWhen ATP ADP, 7.5 kcal/mole of

energy released. How does this compare to the peanut?

Extra Credit:calculate kcal/gram for ATPConsider peanut: 1.5 to 2.5 Kcal/gHow many ATP’s per peanut?Formula for ATP C120H16N70O208P93

WHY a biochemical pathway – why not realease energy

in ONE step?Glucose molecule = 686

Kcal/moleThis is 3.81 Kcal/g (more than twice as much as in an entire peanut!)

Instant cell death!!!

Efficient?

Not very!Each step will loose some energy

via heatNot all bad – this helps maintain

internal temperature of organism

Glycolysis (“glucose-splitting”) (hyperlink in pink) Glucose (6C) is split into two pyruvate (3C) molecules.

(aka pyruvic acid)• does not require oxygen• energy harvested/glucose:

2 ATP (via substrate-level phosphorylation)2 NADH (actively transported into mitochondria of eukaryotic cells for use by the electron transport chain)

• 1st half: activates glucose – 2 ATP’s used – no ATP gained

• 2nd half: extracts a little energy• Takes place in cytoplasm (cytosol)• Adding phosphates prevent glucose from migrating

out f the cytoplasm

First half of glycolysis activates glucose by investing 2 ATP molecules.

Second half of glycolysis extracts energy by releasing 4 ATP molecules.

Where the big stuff happens!

Intermediary step required before Kreb can happen: Pyruvic acid must be converted to Acetyl CoA before it can enter Krebs cycle. PA moves into mitochondrial matrix. PA looses a CO2 (when NAD reduced to NADH) and becomes Acetyl CoA ( 2 Carbon molecule)

2. Krebs Cycle (aka. citric acid cycle)• Acetyl Co A enters the Kreb and

combines with oxaloacetate to form citric acid.

• cells use carbon skeletons of intermediates to produce other organic molecules (amino acids).

• Enormous quantities of CO2 produced• energy harvested per acetyl CoA: Per

GLUCOSE molecule (half each cycle) 2 ATP (via substrate-level

phosphorylation)

6 NADH 2 FADH2

Thus far, how much useable energy has been produced from the breakdown of 1 glucose molecule?

4 ATPsThe electron transport chain is

needed to harvest the potential energy in NADHs & FADH2s.

Electron Transport Chain (ETC)Series of proteins & electron carriers embedded

in the cristae,inner mitochondrial membrane (eukaryotes) or cell membrane (prokaryotes).

• O2 is the final electron acceptor• Uses the energy trapped in NADH and FADH2

in other steps (become NAD and FAD)• H2O is the final product (electrons + H ions +

O = water)• Meantime the H ion released from NADH and

FADH begin to fill the intermembrane compartment (space between outer mictochondrial membrane and the cristae)

Electron Transport, con’t• Hydrogen ions (protons) slide into a channel

of ATP synthase• When channel is stimulated, ADP is

phosphoralated to produce ATP. • Net ATP varies hugely – estimates are around

26• energy harvested/NADH: 2.5 ATPs (via

chemiosmotic phosphorylation) • energy harvested/FADH2: 1.5 ATPs (via

chemiosmotic phosphorylation)

How many ATPs can 1 glucose yield?

Can cells use proteins & lipids to produce energy?

FermentationBiochemical pathways that try to

extract energy from nutrients, in the absence of oxygen.

Glycolysis produces pyruvic acid which is broken down in fermentation

Alcoholic fermentationPyruvic acid is broken down to ethanol

and carbon dioxide.Ex. yeast (used in production of baked

goods & alcoholic beverages)

NET ATP???

Lactic acid fermentationPyruvic acid is

broken down to lactic acid.

Examples: • certain

bacteria (used in production of cheese & yogurt)

• human muscle cells in oxygen debt

Photosynthesis, glycolysis & cellular respiration are interrelated.

Many tutorials at ONE site:Biology Animations to choose from