Transport Proteins

Channels

Transport proteins are essential for moving substances across cell membranes. Channels are a type of transport protein that form pores in the membrane, allowing specific molecules or ions to pass through by diffusion.

• Gating: Some channels require a trigger (such as a change in voltage or binding of a molecule) to open or close, a process known as gating.

• Passive Transport: Channels facilitate passive transport, meaning substances move down their concentration gradient without energy input.

• Example: Ion channels in nerve cells allow sodium and potassium ions to move across the membrane during nerve impulses.

Carrier Proteins

Carrier proteins bind to specific molecules (such as sugars or amino acids) and undergo conformational changes to transport them across the membrane.

• Passive Transport: Like channels, carrier proteins can mediate passive transport (facilitated diffusion).

• Directionality: Movement is typically down the concentration gradient.

• Specificity: Carrier proteins are specific for the molecules they transport.

• Example: GLUT1 is a glucose transporter that moves glucose into cells.

• Cellular Concentration: Carrier proteins can help concentrate molecules or ions inside the cell if coupled to other processes.

Active Transporters (Pumps)

Active transporters (or pumps) move substances against their concentration gradients, requiring energy input, usually from ATP hydrolysis.

• Energy Input: Active transport requires energy, often provided by ATP.

• Example: The Na+/K+ ATPase pump moves sodium ions out of the cell and potassium ions into the cell, maintaining essential gradients.

• Binding Ability: Pumps have specific binding sites for the ions they transport, and their affinity can change during the transport cycle.

• Gradient Maintenance: In mammalian cells, Na+ is typically 10 times higher outside the cell, while K+ is 30 times higher inside.

Summary of Membrane Transport

Membrane transport can be classified based on energy requirements and the direction of movement relative to concentration gradients.

Membrane fluidity affects permeability, and not all transport requires ATP hydrolysis.

Energy & Chemical Reactions

Role of Enzymes

Enzymes are biological catalysts that use energy to drive the chemical reactions necessary for life.

• Energy: Defined as the ability to do work.

• Enzyme Function: Enzymes lower the activation energy required for reactions, increasing reaction rates.

• Example: Digestive enzymes break down food molecules into absorbable units.

Energy Transformations

Energy can exist in different forms, such as kinetic (movement) and potential (stored in chemical bonds).

• 1st Law of Thermodynamics: Energy is neither created nor destroyed; it simply changes form.

• Example: Mechanical energy can be converted to heat or sound energy.

Potential Energy in Cells

Potential energy in cells is often stored in the bonds of molecules. The type of bond (polar vs. nonpolar) affects the amount of potential energy.

• Bond Strength: Nonpolar bonds (e.g., C-H) are longer and weaker, storing more potential energy than polar bonds (e.g., O-H).

• Key Concept: Potential energy can be released in chemical reactions.

• Spontaneity: Whether a reaction occurs spontaneously depends on energy changes.

Spontaneity of Chemical Reactions

Criteria for Spontaneous Reactions

Reactions tend to be spontaneous if they result in products with lower potential energy and greater disorder (higher entropy).

• Lower Potential Energy: If products have lower potential energy than reactants, energy is released (exothermic reaction).

• Enthalpy (H): The total energy in a molecule; a negative change in enthalpy () favors spontaneity.

• Greater Disorder (Entropy, S): If products are more disordered, entropy increases ( positive), favoring spontaneity.

• 2nd Law of Thermodynamics: Entropy tends to increase in the universe.

• Example: Diffusion of molecules from high to low concentration increases entropy.

Free Energy and Reaction Spontaneity

The change in free energy () determines whether a reaction is spontaneous.

• Formula:

• : Change in free energy

• : Change in enthalpy

• : Change in entropy

• T: Temperature in Kelvin

• Spontaneity: A negative indicates a spontaneous reaction.

• Important Notes: Spontaneity does not indicate reaction speed or rate.

Questions to Consider:

• Can a spontaneous reaction have a positive ?

• Can a nonspontaneous reaction have a positive ?

Additional info:

• Carrier proteins and channels both facilitate diffusion, but carriers undergo conformational changes while channels form open pores.

• Active transport is essential for maintaining cellular ion gradients, which are critical for processes like nerve impulse transmission.

• Enzymes do not change the overall free energy of a reaction; they only lower the activation energy required to reach the transition state.