Membrane Transport of Small Molecules and the Electrical Properties of Membranes

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Membrane Transport of Small Molecules and the Electrical Properties of Membranes

Introduction

The plasma membrane is a selectively permeable barrier that regulates the movement of small molecules and ions into and out of the cell. This chapter explores the mechanisms of membrane transport, the types of transport proteins, and the electrical properties that arise from ion gradients across membranes.

Comparison of Inorganic Ion Concentrations Inside and Outside a Typical Mammalian Cell

Cells maintain distinct concentrations of ions across their plasma membranes, which is essential for various physiological processes, including electrical signaling and osmoregulation.

Ion	Cytoplasmic Concentration (mM)	Extracellular Concentration (mM)
Na+	5–15	145
K+	140	5
Mg2+	0.5	1–2
Ca2+	10–4	1–2
H+	7 × 10–5 (pH 7.2)	4 × 10–5 (pH 7.4)
Cl–	5–15	110

Key Point: The cell must remain electrically neutral, so the total positive and negative charges are balanced.
Additional info: Most cell constituents are negatively charged (e.g., proteins, nucleic acids, metabolites).

Permeability of the Lipid Bilayer

The lipid bilayer is selectively permeable, allowing some molecules to cross more easily than others.

Hydrophobic molecules (e.g., O2, CO2, N2, steroid hormones) diffuse freely across the membrane.
Small uncharged polar molecules (e.g., H2O, urea, glycerol, NH3) cross less readily.
Large uncharged polar molecules (e.g., glucose, sucrose) and ions (e.g., H+, Na+, K+, Ca2+, Cl–, Mg2+) have very low permeability and require transport proteins.

Types of Membrane Transport Proteins

Transport proteins facilitate the movement of specific molecules across the membrane.

Transporters (Carriers)

Bind a specific solute at a binding site.
Undergo a conformational change to move the solute from one side of the membrane to the other.
Highly specific for their substrates.

Channels

Form hydrophilic pores in the membrane.
Less specific; allow multiple ions or water molecules to pass through simultaneously.
Transport is much faster than via transporters.

Mechanisms of Membrane Transport

Transport across membranes can be passive or active, depending on whether energy input is required.

Passive Transport

Occurs down a concentration or electrochemical gradient.
Includes simple diffusion, channel-mediated, and transporter-mediated diffusion.

Active Transport

Moves solutes against their concentration or electrochemical gradients.
Requires energy input (e.g., ATP hydrolysis, light, or coupling to another gradient).

Types of Active Transport

Active transport is essential for maintaining ion gradients and cellular homeostasis.

Coupled Transporters: Use the energy from the movement of one solute down its gradient to drive the transport of another solute against its gradient.
ATP-Driven Pumps: Hydrolyze ATP to transport ions or molecules (e.g., Na+/K+ pump).
Light-Driven Pumps: Use light energy to drive transport (mainly in some bacteria and archaea).

Coupled Transport: Uniport, Symport, and Antiport

Uniport: Transports a single type of molecule in one direction.
Symport: Transports two different molecules in the same direction.
Antiport: Transports two different molecules in opposite directions.

Electrochemical Gradients and Membrane Potential

The movement of ions across membranes generates an electrochemical gradient, which is a combination of the concentration gradient and the electrical potential difference across the membrane.

Membrane potential is the voltage difference across a membrane, typically negative inside the cell relative to outside.
The electrochemical gradient determines the direction and rate of ion movement.

Equation for Nernst Potential:

Where E is the equilibrium potential, R is the gas constant, T is temperature, z is the charge of the ion, and F is Faraday's constant.

Summary Table: Types of Membrane Transport

Type	Energy Requirement	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Channel-mediated	No	Down gradient	Ion channels
Transporter-mediated	No	Down gradient	Glucose transporter
Active Transport	Yes	Against gradient	Na+/K+ pump

Key Examples and Applications

Na+/K+ Pump: Maintains high K+ and low Na+ inside the cell by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed.
Glucose Symporter: Couples the downhill movement of Na+ to the uphill transport of glucose into the cell.
Aquaporins: Channel proteins that facilitate rapid water movement across membranes.

Additional info:

Membrane transport is fundamental for nutrient uptake, waste removal, and signal transduction in all cells.
Disruption of transport processes can lead to diseases such as cystic fibrosis, diabetes, and neurological disorders.