cellular respiration

Cellular Respiration

Releasing Chemical Energy

Chapter 6

BIOCHEMICAL REACTIONS

• All living organisms require a constant supply of energy to sustain life.

• Cellular respiration - the chemical energy stored in glucose is converted into a more usable form – ATP – Requires the presence of oxygen and the correct enzymes – Carbon dioxide, water and heat are also released as by-products

of this reaction.

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat)

glucose + oxygen → carbon + water + energy dioxide

Why Cell “Respiration”?

1. This process requires oxygen, which is supplied by breathing

2. The mechanical movement of air or water through the lungs/gills is often referred to as ventilation to distinguish it from respiration.

3. What about plants?

• Cellular respiration – slow, controlled release of energy (max. harvest of energy from food)

Review of ATP

Review of ATP• ATP is the “energy

currency/rechargeable batteries” of cells• When energy is harvested from a chemical

reaction or sunlight, it is stored when a phosphate group is attached to an ADP to form ATP.– Called phosphorylation

• When the ATP is broken back down to ADP, stored chemical energy is released to do work in a cell – Called dephosphorylation– Some energy is lost as heat

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat)

glucose + oxygen → carbon + water + energy

dioxide

OXIDATION REACTIONS

• Oxidation begins in the cytoplasm and is completed in the mitochondria

• 3 parts to cellular respiration (each an enzyme-controlled pathway)

– Glycolysis – Krebs cycle – Electron transport

system

Overview

• Animation: http://www.qcc.cuny.edu/BiologicalSciences/Faculty/DMeyer/respiration.html

1. Glycolysis - a 6C glucose molecule is broken into 2 3C molecules of pyruvate (pyruvic acid)

a. Occurs in the cytoplasm of the cell – near the mitochondria

b. Yields: + 2 ATP (4 ATP – 2ATP - used to phosphorylate glucose when it enters cell)

+ 2 NADH (NAD+ is reduced to NADH)

c. This process is anaerobic (without oxygen)…can happen even if there is an insufficient O2 level to carry out the rest of cellular respiration

If there is O2 present, respiration continues. Each 3C pyruvate will…

1. Lose atoms of carbon and oxygen – CO2 released

(…it is now called an acetyl group)

2. Join to a molecule of coenzyme A (which is a B vitamin) – Acetyl CoA

- CoA acts as a shuttle, carrying acetyl groups

3. NAD+ (coenzyme that shuttles around hydrogen and electrons) is reduced to NADH.

4. These reactions are often called the ‘Intermediate Reactions’

2. Kreb’s Cycle (Citric Acid Cycle)a. Acetyl CoA enters the mitochondrion and 2C

acetyl group bonds to a 4C compound (oxaloacetate) to form a 6C compound called Citric Acid (citrate)

b. The 6C compound is broken down to a 5C compound

1 CO2 is produced

1 NAD+ is reduced to NADH

c. 5C compound is broken down into a 4C compound1 CO2 is produced

1 NAD+ is reduced to NADH

d. Oxaloacetate is regenerated (4C 4C)

This yields:1 ATP (ADP ATP)

1 FADH2 (FAD FADH2)

1 NADH (NAD+ NADH)

e. So, the total yield of just the Kreb’s cycle is:2 ATP

6 NADH

2 FADH2 per glucose

3. Electron transport system (ETS) makes ATPa. Electrons from

reduced coenzymes NADH and FADH2 are transferred through a series of redox reactions until the electrons are accepted by oxygen to make water.

b. Mitochondrial structure

1) Double membrane-bound organelle

2) Inner membrane folded into christae a) Increase surface

area for reactions

b) ETS located here• Intermembrane

space • Matrix - Kreb’s

cycle

c. ATP synthesis 1. H atoms from coenzymes

dropped off at ETS (inner membrane)

2. H atoms split into a proton (H+) and an electron (e-)

- Electrons go through ETS

- Energy from electrons is used to pump the H+ out into the intermembrane space

3. H+ concentration in this space increases

4. The H+ RUSH back into the matrix (because of concentration gradient) through an H+ channel (ATP synthetase complex) making ATP

5. Called chemiosmosis

• Animation:

http://vcell.ndsu.nodak.edu/animations/atpgradient/movie.htm

d. Happy endings…1. H+ and e- (now low energy) are rejoined2. H atoms bond to available oxygen atoms and form water:

H+ + e- + O2 H2O

This is why you breathe!! The O2 is merely a hydrogen dump!

O2 allows the continual movement of H+ through the ATP synthetase

No O2, no rushing H+ movement, no ATP, no life!

e. ETS produces: (per glucose)

2 NADH (from glycolysis)

2 NADH (from intermediate reactions) +

6 NADH (Krebs cycle)______________

10 NADH x 3 ATP/NADH = 30 ATP

2 FADH2 x 2 ATP/ FADH2 = 4 ATP____

for a total 34 ATP/glucose from ETS

Cellular Respiration Energy Summary

34 ATP/glucose from ETS +2 ATP (glycolysis) +2 ATP (Krebs cycle)_______________

38 ATP per glucose!!!38 ATP per glucose!!!

• Prisoners’ explanation

• Current applications

• Other nutrients can be used for energy– Lipids fatty acids,

enter Krebs Cycle– Proteins amino

acids• NH3 removed urea• Carbon portions enter

Krebs Cycle as oxaloacetate

– Carbon skeletons can be used for biosynthesis of amino acids, nucleic acids and fatty acids

Alternatives to Aerobic Respiration

What if there’s not enough oxygen?

Glycolysis still happens (since it’s anaerobic anyway…).

- Yield is 2 ATP + 2 NADH + 2 pyruvic acid (3 C molecule).

- Fate of the pyruvic acid depends on what type of organism you are…

If you are a plant or yeast cell…Pyruvic acid will become ETHANOL in a process

called alcoholic fermentation.

If you are a bacterial cell…Your pyruvic acid can be fermented to vinegar or to start the process of cheesemaking.

If you are an animal cell…

Your pyruvic acid becomes LACTIC ACID in a process called lactic acid fermentation.

Photosynthesis and Cellular Respiration

• Cellular respiration and photosynthesis share several features:– They are enzyme-controlled

biochemical pathways.– They make use of ATP for energy transfer– They use an Electron Transport

System to help make ATP.

Photosynthesis and Cellular Respiration

Photosynthesis and Cellular Respiration

Light + 6 CO2 + 6 H2O → C6H12O6 + 6

O2

CC66HH1212OO66 + 6 O + 6 O22 → 6 CO → 6 CO22 + 6 H + 6 H22O + O + energy energy (ATP + heat) (ATP + heat)

• End of cellular respiration!

Acetyl CoA

NADH

NADPH

FADH

CELLULAR RESPIRATION

• Breakdown of glucose molecules in the presence of oxygen.

• The oxidation of glucose (by many enzymes) results in carbon dioxide and water.

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat)

glucose + oxygen → carbon + water + energy

dioxide

A Definition of Cellular Respiration

The energy stored in glucose (with the presence of oxygen and the correct enzymes) is converted into a more usable form – ATP. Carbon dioxide and water are also released as by-products of this reaction.

C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP

[Read the last paragraph on page 131]

GLYCOLYSIS

• Glucose (6 carbons) is broken into two 3 carbon molecules called pyruvate (pyruvic acid).

• This makes enough energy to make 2 ATP molecules.

• In addition, an NADH molecule is also made and transferred to the electron transport chain.

3. Electron transport system (ETS), located in the membranes of mitochondria (and chloroplasts) makes ATP.– High-energy electrons are

passed stepwise through a series of oxidation-reduction reactions from one carrier molecule to another.

• Every time the electron is passed, some of its energy is released and can be used to make ATP

• The rest of the energy is released as heat

How much energy do you get from 1 molecule of glucose?

Glycolysis – 2ATP and 2 NADH (each x3)Intermediate – 2 NADH (each x3)Kreb’s Cycle – 2 ATP, 6 NADH (each x3),

and 2 FADH2 (each x2)The ETS yields 8 ATP from glycolysis, 6

ATP from the Intermediate Reactions, 24 ATP from the Kreb’s Cycle

For a total of…38 ATP per initial molecule of glucose

Throughout the process, coenzymes are being reduced so, in the end, they

can all be oxidized (so ATP can be generated!) – sort of like POKER!

CELLULAR RESPIRATION SUMMARY

• Glucose is broken down to carbon dioxide and water, making 4 ATPs directly and another 32 ATP via the electron transport system.