Cell Structure and Function Chapter 9

Metabolism

All the chemical reacts in a cell

Anabolic Pathways

• Synthesize cellular components, often polymers such as starch and glycogen

• Increase in order and a decrease in entropy

• Endergonic

Catabolic Pathways

• Breakdown of cellular constituents such as the hydrolysis of glucose

• Decrease in order and increase in entropy

• Exergonic 

• Release free energy

• Can be aerobic or anaerobic

3 Important Chemical Players

• ATP

• NAD+/NADH

• Glucose

What is the role of ATP?

Primary energy currency

Power cell movement, transport of molecules/ions, and enzyme catalyzed reactions.

Phosphoanhydride Bonds

Energy Rich Bonds

Hydrolysis of ATP to ADP and Pi is exergonic because?

• Charged Repulsion between the adjacent negatively charged phosphate groups

• Resonance Stabilization of both products of hydrolysis

• Increases entropy and solubility of the products of hydrolysis

Resonance Stabilization

When a Phosphoester bond is formed between a phosphate and alcohol group, the extra electrons are delocalized over only three oxygen atoms instead of four

Less resonance stabilized and more energy

Increase in Entropy

Once the phosphate group gets removed from ATP it is no longer in a fixed position. The spatial randomization of ADP and Pi after hydrolysis decreases their free energy and makes the reaction more exergonic.

This in turn makes them become more soluble and their increased interactions with water molecules decreases their free energy

Summary: Phosphate group removed - more entropy. Randomization/entropy decreases free energy and makes them exergonic. More exergonic means more soluble. 

Catabolic Fuels

Highly reduced compounds like lipids and sugars

Oxidation

Removal of electrons

Dehydrogenation

Removal of a proton and an electron (i.e. hydrogen)

Cellular reacts involving organic molecules are almost always dehydrogenation reactions

Coenzymes

Function along with enzymes, serving as carriers of electrons or small functional groups

Electrons or Hydrogens removed during biological oxidation are transferred to them 

NAD+ (nicotinamide adenine dinucleotide)

Most common coenzyme serving as an electron acceptor.

Adds 2 electrons and a proton to its aromatic ring, generating NADH plus a proton

Glucose

• its oxidation is a highly exergonic process

• Oxidized to CO2 and Oxygen reduced to Water

Aerobic Respiration

• Oxidation of glucose in the presence of oxygen 

• Yields 36 ATP/glucose

Anaerobic Respiration

• Yields 2 ATP/glucose

• Via glycolysis

Fermentation

Electrons removed during glucose oxidation are returned to an organic molecule later in the same pathway

Obligate aerobes

absolute requirement for oxygen

Obligate anaerobes

cannot use oxygen as an electron acceptor; oxygen is toxic to these organisms.

Facultative organisms

can function under aerobic or anaerobic conditions.

Glycolysis

Ten step reaction sequence that converts one glucose molecule into 2 molecules of pyruvate 

• glucose goes to 2 three-carbon pyruvates

• ATP and NADH produced

Occurs in cytosol

Important Features of the Glycolytic Pathway

• Initial input of 2 ATP

• Sugar splitting reaction in which glucose is split into 2 three carbon molecules

• Oxidative event that generated NADH

• 2 Steps at which the reaction sequence is coupled to ATP generation

Glycolysis Mechanism