Chapter 3: Cell metabolism

Reactions are bidirectional-what pushes them in either direction?

Law of Mass Action) 

Forward 🡺 Reactants transformed into products

Reverse 🡺 Products transformed into reactants

Hydrolysis reaction

• What goes in? → Water (H₂O)

• What happens? → Water breaks a bond

• What comes out? → Smaller molecules

• Purpose: Breaks down larger molecules

• Type: Catabolic

• Energy: Catabolic reactions generally release energy

Condensation reaction

• What goes in? → Smaller molecules

• What happens? → A water molecule is removed and a bond forms

• What comes out? → Larger molecule + water

• Purpose: Builds larger molecules

• Type: Anabolic

• Energy: Usually requires/uses energy

Phosphorylation & Dephosphorylation

What types of enzymes catalyze each of these reactions?

Phosphorylation

• What happens? → A phosphate group (Pi) is added to a molecule.

• Purpose: Often helps store or transfer energy or change a molecule's activity.

• Enzyme: Kinase

Remember: Kinase = adds phosphate

Dephosphorylation

• What happens? → A phosphate group is removed from a molecule.

• Purpose: Can release energy or change a molecule's activity.

• Enzyme: Phosphatase

Remember: Phosphatase = removes phosphate

Oxidation-Reduction. What does it mean to be oxidized?

Oxidation is a loss of electrons. And reduced is to gain an electron. 

Reduction and Electron Transfer

In a reduction reaction, electrons can be transferred in two ways:

1. Directly as electrons (e⁻)

2. As part of a hydrogen atom (H)

NAD⁺ → NADH

NAD⁺ accepts 2 electrons total:

• 1 e⁻ is transferred directly to NAD⁺

• 1 e⁻ is transferred as part of a hydrogen atom (H)

So:

NAD⁺ + 2e⁻ + H⁺ → NADH

Energy

The capacity to do work

Facts

Kinetic & Potential Energy

• Kinetic energy = energy of motion

• Potential energy = stored energy

• Energy can be converted between kinetic and potential forms.

• In cells, energy can also be stored in molecules such as ATP.

ADP + Pi ⇌ ATP

• ADP + Pi → ATP: energy is stored in ATP.

• ATP → ADP + Pi: stored energy is released for cellular work.

If energy isn't captured...

If energy released from a reaction is not captured and stored in another molecule, it is released as heat.

Laws of Thermodynamics

1) First law of Thermodynamics – Energy cannot be created or destroyed

2) Second law of Thermodynamics – Processes proceed in the direction that spreads out energy 

Larger molecules breaking to smaller ones (catabolic)

Movement of molecules from high concentration to low concentration (diffusion)

 Which releases energy? Catabolic. Which uses/stores energy? Anabolic

Catabolic=products have less energy than the reactants. And anabolic: products have more energy than the reactants.

What affects reaction rates? How do they affect reaction rates?

1) Concentration of reactants or products (Law of Mass Action) (Figure 3.2). More reactants mean more collisions mean a faster reaction. More products can push the reaction in the reverse direction. 

2) Temperature: Higher temp means molecules move faster, so more collisions and a faster reaction, and vise versa. 

What else affects reaction rates? How do they affect reaction rates?

3)

Activation energy height (Figure 3.3 & 3.4): Decreased activation energy height=increased reaction rate (so easier to overcome and a faster reaction), and a higher height means harder to overcome, so the reaction is slower. 

How is it normally overcome? By increasing the energy of the molecules. Such as by increasing the temperature. 

Last factor affecting reaction rate:

Enzymes: Enzymes decrease the activation energy barrier so the reaction rate can increase. They also help reduce the height.

Are they consumed by reactions?- No. They are not. 

 Are they changed by reactions? (Figure 3.6)No, they are not, and they can be used again, 

Specificity –enzymes are specific to particular substrates.

 The Induced-Fit model allows reactions to go in reverse (Figure 3.7). When the substrate binds, the enzyme changes shape slightly to better fit the substrate. Enzymes can catalyze reactions in either direction, 

What are cofactors?

Cofactors: Non-protein molecules or ions that help enzymes function.

Examples: Mg²⁺, Cu²⁺, Zn²⁺, Fe²⁺/Fe³⁺

Role: Help substrates bind and maintain enzyme shape.

Coenzymes – What are they?

Coenzymes: Organic cofactors that help enzymes function.

Example: NAD⁺, FAD, CoA

Role: Transfer small chemical groups between reactions.