Movement Thru Membrane

The Cell Membrane Transport Study Notes

Overview of Membrane Transport

• Movement of substances across the plasma membrane into and out of the cell.

• Involves two primary types of transport:

  • Passive Transport

  • Active Transport

Passive Versus Active Transport

Definitions

• Passive Transport: Movement of substances along the concentration gradient from an area of high concentration to low concentration without energy input.

• Active Transport: Movement of substances against the concentration gradient from an area of low concentration to high concentration, requiring energy (ATP).

Key Distinction

• Passive Transport = No energy usage, movement with gradient.

• Active Transport = Requires energy, movement against gradient.

Mechanisms of Passive Transport

Diffusion and Osmosis

• Diffusion:

  • Net movement of molecules from an area of higher concentration to an area of lower concentration.

  • Molecules are in constant motion, leading to mixing.

  • Movement results from a concentration gradient, which is the difference between high and low concentrations of substances.

• Osmosis:

  • Specifically refers to the diffusion of water across a selectively permeable membrane.

Types of Passive Transport Mechanisms

1. Simple Diffusion:

   • Substances move directly through the membrane down the concentration gradient.

   • Example: Dissolving a colored sugar cube in water, where sugar molecules spread out to eliminate concentration gradients over time.

2. Facilitated Diffusion:

   • Passive movement of substances with the help of carrier proteins due to size limitations of molecules (e.g. glucose, amino acids).

   • Mechanism: Molecule binds to a receptor site on the carrier protein, altering protein shape to allow the molecule passage.

Factors Affecting Diffusion

• Temperature: Higher temperatures increase molecular motion.

• Distance: Shorter distances promote faster diffusion.

• Concentration Gradient: Greater differences enhance diffusion rates.

• Size of Molecule: Smaller molecules diffuse more quickly.

• Electrical Forces: Charged particles may be attracted or repelled.

• Phase of Substance: Solid, liquid, or gas phases affect diffusion.

Solute, Solvent, Solution, and Concentration Gradient

• Solution: Mixture consisting of a solute dissolved in a solvent.

• Solute Concentration: Amount of solute in a given volume of solvent, also known as osmotic concentration.

• Concentration Gradient: Change in solute concentration across an area.

Osmosis and Tonicity

• Osmosis: Movement of water across a selectively permeable membrane influenced by solute concentration.

  • More solute = less water concentration.

• Tonicity: Describes how a solution affects cell volume:

  • Isotonic Solution: Same solute concentration as the cell, no change in cell size.

  • Hypotonic Solution: Lower solute concentration than the cell, can cause cell swelling and rupture (hemolysis).

  • Hypertonic Solution: Higher solute concentration than the cell, can cause cell shrinkage (crenation).

Osmotic and Hydrostatic Pressure

• Osmotic Pressure:

  • Pressure required to prevent osmosis across a semipermeable membrane.

  • Opposes the movement of water out of blood vessels (blood colloidal osmotic pressure).

• Hydrostatic Pressure:

  • Force exerted by the fluid against the walls of the blood vessels.

Passive Transport: Filtration

• Filtration: Separation of fluid and solutes aided by gravity and hydrostatic pressure, such as in kidney function filtering blood.

Active Transport

• Requires energy (ATP) and protein carriers.

• Movement from an area of low concentration to high concentration against the gradient.

Types of Active Transport

1. Na/K Pump:

   • Active transport using ATP to move sodium out and potassium into the cell.

2. Vesicular Transport:

   • Involves the use of vesicles (small membranous sacs) for transportation.

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

   • Endocytosis: Vesicles form from the membrane to intake substances from the exterior; further categorized into:

     • Phagocytosis (Cell Eating): Engulfing large particles, digesting them via lysosomes.

     • Pinocytosis (Cell Drinking): Intake of fluid from extracellular environment.

     • Receptor-Mediated Endocytosis: Specific molecules bind to receptors, forming vesicles once enough ligands are collected.

Types of Endocytosis Explained

• Pinocytosis: Cells absorb extracellular fluid.

• Phagocytosis: Cells engulf large particles or pathogens (e.g. macrophages).

• Receptor-Mediated Endocytosis: Specific ligand binding, requiring all receptors to be filled for efficiency.

Summary of Transport Mechanisms

Summary: Passive vs Active Transport