Cell Membrane Structure and Transport Processes – Human Physiology Study Notes

Cell Membrane Structure

Overview of the Cell Membrane

The cell membrane, also known as the plasma membrane, is a dynamic structure that surrounds the cell, providing protection and regulating the movement of substances in and out of the cell. It is essential for maintaining cellular integrity and homeostasis.

• Phospholipid Bilayer: The cell membrane is primarily composed of a double layer of phospholipids, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Selective Permeability: The membrane allows certain substances to pass through while restricting others, enabling the cell to control its internal environment.

• Membrane Proteins: Embedded proteins serve various functions, including transport, signaling, and structural support.

• Carbohydrate Chains: Attached to proteins and lipids on the extracellular surface, these chains play roles in cell recognition and communication.

Types of Membrane Proteins

Membrane proteins are crucial for the diverse functions of the cell membrane. They can be classified based on their structure and function:

Type	Function	Example
Channel Proteins	Form pores for passive movement of ions and molecules	Sodium channel
Carrier Proteins	Bind and transport substances across the membrane	Glucose transporter
Enzymes	Catalyze chemical reactions at the membrane surface	ATPase
Structural Support Proteins	Anchor the membrane and maintain cell shape	Actin-binding proteins
Linker Proteins	Connect adjacent cells or extracellular matrix	Cadherins

Transport Processes Across the Cell Membrane

Passive Transport

Passive transport refers to the movement of substances across the cell membrane without the expenditure of cellular energy (ATP). Substances move down their concentration gradient.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or polar molecules via specific membrane proteins (channels or carriers).

• Osmosis: The diffusion of water molecules through a selectively permeable membrane.

Key Equation:

$J = -D \frac{dC}{dx}$ Where: J = flux (rate of diffusion) D = diffusion coefficient dC/dx = concentration gradient

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration gradient. This process is essential for maintaining cellular concentrations of ions and other molecules.

• Primary Active Transport: Direct use of ATP to transport molecules. Example: Sodium-Potassium Pump (Na+/K+ ATPase) moves 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

• Secondary Active Transport: Uses the energy from the movement of one molecule down its gradient to drive the transport of another molecule against its gradient. Example: Na+/Glucose Symporter.

Key Equation (Na+/K+ Pump):

$3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Bulk Transport: Endocytosis and Exocytosis

Large particles and macromolecules are transported across the membrane via vesicular processes:

• Endocytosis: The cell engulfs external substances, forming vesicles that bring materials into the cell.

• Exocytosis: Vesicles fuse with the plasma membrane to release their contents outside the cell.

Summary Table: Transport Processes

Process	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 movement
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Osmosis	No	Down gradient	Water movement
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Yes (indirect)	Against gradient	Na+/Glucose symporter
Endocytosis/Exocytosis	Yes (ATP)	Bulk movement	Phagocytosis, neurotransmitter release

Additional info:

• Membrane proteins can be integral (spanning the membrane) or peripheral (attached to the surface).

• Transport processes are vital for nutrient uptake, waste removal, and signal transduction in cells.