Membrane Transport

Structure and Function of Cell Membrane

• Major Function: Regulate the movement of substances into and out of the cell.

  • Achieved through:

  • Passive transport

  • Active transport

Passive Transport

• Occurs when molecules move from an area of higher concentration to lower concentration without energy expenditure.

• Types of Passive Transport:

  • Simple diffusion

  • Facilitated diffusion

  • Osmosis

Simple Diffusion

• Definition: Movement of molecules directly across the phospholipid bilayer from an area of high concentration to low concentration.

• Characteristics:

  • No assistance is needed.

  • Common for gases like oxygen (O) and carbon dioxide (CO extsubscript{2}).

  • Small nonpolar molecules can also pass through.

• Mechanism:

  • Depends on concentration gradients.

  • Continues until equilibrium is reached (equal concentration on both sides).

Facilitated Diffusion

• Definition: Larger or charged molecules that cannot pass directly through the lipid bilayer require assistance from membrane proteins.

• Types of Proteins Involved:

  • Channel proteins

  • Carrier proteins

• Characteristics:

  • Channel proteins act like tunnels and are highly selective.

  • Carrier proteins change shape to transport molecules across the membrane.

• Mechanism:

  • Follows concentration gradients (high to low) but with the aid of proteins.

Osmosis

• Definition: Special form of diffusion concerning the movement of water across a semi-permeable membrane.

• Characteristics:

  • Does not require energy.

  • Water moves from low solute concentration to high solute concentration.

• Explanation:

  • In a solution with solute, there are fewer water molecules than in pure water.

  • Water cannot easily pass through the hydrophobic lipid bilayer, using specialized proteins called aquaporins instead.

• Effects on Cells:

  • Hypotonic Solution:

  • Water enters the cell, causing it to swell or burst (lysis).

  • Isotonic Solution:

  • No net movement of water; cell remains stable.

  • Hypertonic Solution:

  • Water leaves the cell, causing it to shrink (crenation).

Active Transport

• Definition: Movement of molecules against their concentration gradient (from low to high concentration) that requires energy (ATP).

• Sodium-Potassium Pump (Na⁺/K⁺ Pump):

  • Maintains electrical gradient essential for nerve impulses and muscle contractions.

  • Mechanism:

  • Pumps three Na⁺ ions out of the cell and two K⁺ ions into the cell using one ATP molecule per cycle.

  • Functionality:

  • Keeps the inside of the cell negatively charged.

Types of Active Transport

• Primary Active Transport:

  • Directly uses ATP to move ions against their concentration gradients.

• Secondary Active Transport:

  • Does not directly split ATP; instead, it utilizes the gradient setup by primary active transport to drive the movement of other molecules.

  • Mechanism:

  • One molecule flows down its electrochemical gradient while another is pushed up its gradient.

  • ATP is utilized in the earlier steps through primary transport, allowing secondary transporter to function efficiently.

Transport Mechanisms in Secondary Active Transport

• Symporters:

  • Carry two different molecules in the same direction across the membrane.

  • Example: Sodium-glucose symporter.

  • Mechanism:

  • Both molecules bind to the transporter protein on one side of the membrane and are released on the opposite side together.

  • Analogy: Like a tandem turnstile where both a sodium ion and a glucose molecule must be present to enter the cell.

• Antiporters (Exchangers):

  • Carry two different molecules in opposite directions.

  • One substance enters the cell while the other exits or vice versa.

  • Example: Sodium-calcium exchanger in heart cells.

  • Analogy: Like a revolving door where one substance flows in as another flows out.

Bulk Transport

• Definition: Involves membrane remodeling to form vesicles that enclose materials too large for channels and transporters.

• Types of Bulk Transport:

  • Endocytosis

  • Exocytosis

Endocytosis

• Definition: Process where cells engulf substances from the extracellular space into vesicles.

• Purpose:

  • Intake of nutrients, hormones, and microorganisms.

Exocytosis

• Definition: Process of releasing substances into the extracellular space using vesicles.

• Importance:

  • Essential for neurotransmitter release and hormone secretion.

Types of Endocytosis

• Phagocytosis:

  • Also known as “cell eating.”

  • Used for engulfing large particles or microorganisms.

• Pinocytosis:

  • Known as “cell drinking.”

  • Involves the intake of extracellular fluid and dissolved solutes.

• Receptor-Mediated Endocytosis:

  • Specific form of endocytosis where molecules bind to cell surface receptors prior to internalization.