Cellular Transport: Mechanisms of Movement Across the Plasma Membrane

Cellular Transport

Introduction

Cellular transport refers to the movement of substances across the plasma membrane, a critical process for maintaining cellular homeostasis. This topic covers the physical principles underlying molecular movement and the specific mechanisms by which cells regulate the entry and exit of molecules.

Molecules and Movement

Kinetic Energy and Temperature

• Kinetic energy is the energy of motion possessed by all molecules.

• Temperature is a measure of the average kinetic energy of molecules in a substance.

• The phases of matter—solid, liquid, and gas—differ in the kinetic energy of their molecules:

  • Solids: molecules have the least kinetic energy.

  • Liquids: molecules have more kinetic energy than solids.

  • Gases: molecules have the most kinetic energy.

• Absolute zero is the temperature at which all molecular motion theoretically stops (0 K or -273.15°C).

Membrane Transport

Plasma Membrane Properties

• The plasma membrane is a selectively permeable barrier, allowing some substances to pass while excluding others.

Basic Methods of Transport

• Passive transport: Movement of substances across the membrane without the use of cellular energy (ATP).

• Active transport: Movement of substances across the membrane that requires ATP energy.

Passive Transport

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, down a concentration gradient.

• Driven by the kinetic energy of molecules.

• The steeper the concentration gradient, the faster the rate of diffusion.

• Diffusion continues until dynamic equilibrium is reached.

Factors Influencing Diffusion Rate

• Concentration gradient

• Molecular size

• Temperature

Conditions for Diffusion

• Molecules are small enough to pass through membrane pores.

• Molecules are lipid-soluble.

• Molecules are assisted by a transport protein (facilitated diffusion).

Types of Diffusion

• Simple diffusion: Unassisted movement of lipid-soluble or small molecules (e.g., fats, fat-soluble vitamins, oxygen, carbon dioxide) through the phospholipid bilayer or membrane pores.

• Osmosis: Simple diffusion of water through a selectively permeable membrane, often via specialized channels called aquaporins.

• Facilitated diffusion: Movement of lipid-insoluble or large/charged substances via channel or carrier proteins, always down the concentration gradient (e.g., glucose transport).

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the effect of a solution on cell volume.

• Isotonic solutions: Same solute and water concentrations as the cell; no net water movement, no visible cell change.

• Hypertonic solutions: Higher solute concentration than the cell; water leaves the cell, causing it to shrink (crenation).

• Hypotonic solutions: Lower solute concentration (more water) than the cell; water enters the cell, causing it to swell or even burst (lysis).

Note: Tonicity terms are used when comparing two areas, such as intracellular fluid (ICF) vs. extracellular fluid (ECF).

Table: Effects of Tonicity on Cells

Solution Type	Relative Solute Concentration	Water Movement	Effect on Cell
Isotonic	Equal to cell	No net movement	No change
Hypertonic	Higher than cell	Out of cell	Cell shrinks (crenates)
Hypotonic	Lower than cell	Into cell	Cell swells (may lyse)

Facilitated Diffusion

• Used for lipid-insoluble, large, or charged substances.

• Transported via channel proteins or carrier proteins.

• Always moves substances down their concentration gradient.

• Example: Glucose transport into cells.

Summary of Passive Transport

• Does not require ATP energy.

• Examples of substances: glucose, oxygen, carbon dioxide.

• All move down their concentration gradients.

Active Transport

Overview

Active transport requires ATP to move substances across the membrane, typically against their concentration gradients or when substances are too large or insoluble in lipids.

• Types include pumps, endocytosis, and exocytosis (vesicular transport).

Pumps (Primary Active Transport)

• Specific protein carriers (pumps) transport amino acids, sugars, and ions.

• Energy comes directly from ATP hydrolysis.

• Phosphorylation of the pump protein changes its shape, allowing transport.

• Example: Sodium-potassium pump (Na+/K+ pump):

  • Necessary for nerve impulses.

  • Transports 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP hydrolyzed.

  • Both ions are moved against their concentration (electrochemical) gradients.

$\text{Na}^+/\text{K}^+\ \text{ATPase}:\ 3\ \text{Na}^+_{\text{(in)}} \rightarrow \text{out},\ 2\ \text{K}^+_{\text{(out)}} \rightarrow \text{in},\ \text{per ATP hydrolyzed}$

Secondary Active Transport (Cotransport)

• Uses a cotransport protein.

• One solute moves down its concentration gradient, providing energy for another solute to move up its gradient.

• Primary active transport (e.g., Na+ pump) creates the gradient used by secondary transport.

• The second solute is "dragged along" (cotransported).

• Analogy: Like water behind a dam being released to generate electricity.

Vesicular Transport

• Bulk movement of substances across the membrane using vesicles.

• Types:

  • Endocytosis: Bringing substances into the cell by engulfing them in a vesicle.

  • Exocytosis: Expelling substances from the cell by fusing a vesicle with the plasma membrane.

• Mechanisms can selectively collect hormones, mucus, waste products, etc.

• Examples: Hormone secretion, neurotransmitter release, uptake of nutrients.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction (Gradient)	Examples
Simple Diffusion	No	Down	O2, CO2, fat-soluble vitamins
Facilitated Diffusion	No	Down	Glucose, ions
Osmosis	No	Down	Water
Primary Active Transport	Yes (ATP)	Up	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient from ATP)	Up (for one solute)	Glucose/Na+ cotransport
Vesicular Transport	Yes (ATP)	Bulk	Endocytosis, exocytosis

Key Terms

• Concentration gradient: Difference in concentration of a substance across a space or membrane.

• Dynamic equilibrium: State where concentrations are equal and movement continues but with no net change.

• Selective permeability: Property of membranes that allows some substances to pass while blocking others.

• Aquaporins: Channel proteins that facilitate water movement across membranes.

• Cotransport: Coupled transport of two substances across a membrane.

Additional info: The sodium-potassium pump is essential for maintaining the resting membrane potential in excitable cells such as neurons and muscle cells. Facilitated diffusion is crucial for the uptake of glucose in most body cells, especially after meals.