Membrane Transport

• Membrane transport is the movement of particles across or through a membranous barrier.

Materials Transported by Cell Membrane

• Small, hydrophobic molecules like O₂, CO₂, and N₂ are membrane permeable.
• Small, uncharged polar molecules like H₂O, urea, and glycerol are mostly permeable; some pass by diffusion or osmosis.
• Large, uncharged polar molecules like glucose and sucrose are mostly impermeable, requiring transport proteins to pass through the membrane.
• Ions like Na+, K+, Cl-, and Ca2+ are completely impermeable to the membrane and require transport proteins.

Factors Affecting Membrane Transport

• Membrane Permeability
• Size and charge of solute
• Transmembrane solute concentration

Types of Membrane Transport

• Passive Transport: Diffusion, Osmosis, Facilitated diffusion
• Active Transport: Primary active, Secondary active, and Bulk transport

Passive Transport

• Does not require the use of ATP (energy).
• The driving force is usually kinetic energy in the form of a concentration gradient (high concentration to low concentration.)
• Passive transport moves molecules from an area of high concentration to an area of low concentration, like moving downhill.

Simple Diffusion

• Solute molecules pass directly through the membrane from a region of high concentration to an area of low concentration.
• Small, uncharged molecules and lipid-soluble molecules use simple diffusion.
• Gas exchange in alveoli of the human body happens through simple diffusion.

Osmosis

• Diffusion of solvent molecules through a semi-permeable membrane happens because of osmosis.
• Solvent molecules travel from an area of low solute concentration to an area of high solute concentration.

Types of Solutions and Osmosis

• Isotonic Solution: Water molecules move into and out of the cell at the same rate; the cell volume remains constant.
• Hypotonic Solution: Water enters the cell by osmosis, causing the cell to swell and potentially lyse (expand and burst).
• Hypertonic Solution: Water exits the cell by osmosis, causing the cell to shrink (crenation).
• Osmoregulation in marine fish: Osmotic water loss through gills and other body parts. Gain of water and salt ions from food as well as water and salts from drinking seawater. Excretion of salt ions through gills and small amounts of water in scanty urine from kidneys.
• Osmoregulation in freshwater fish: Osmotic water gain through gills and other body parts. Gain of water and some ions in food and the uptake of salts through gills. Excretion of salt ions and large amount of water in dilute urine from kidneys.

Facilitated Diffusion

• Molecules move through a semi-permeable membrane from an area of high concentration to an area of low concentration with a transport molecule.
• Polar molecules, ions, and small nonpolar molecules use facilitated diffusion.

Channel Proteins

• Channel proteins are membrane proteins that form pores or channels, serving as passageways for ions (charged particles) to cross the membrane.
• Aquaporins are water channel proteins.

Gated-Channel Proteins

• These membrane proteins open a gate, allowing molecules to pass through the membrane.
• These can be opened or closed in response to stimuli such as voltage, mechanical stress, or ligands.

Carrier Proteins

• Carrier proteins carry ions or molecules across the membrane by undergoing a conformational change upon binding.
• Glucose and amino acids use carrier proteins.

Active Transport

• Active transport requires the use of ATP (energy).
• Molecules move against the concentration gradient (from low concentration to high concentration), like moving uphill.

Primary Active Transport

• Uses energy, usually in the form of ATP.
• It is facilitated by membrane protein transporters such as ion pumps, ion channels, and ATPases.
• This includes Na+, K+, Mg2+, and Ca2+ including the sodium-potassium pump.

Secondary Active Transport

• Uses potential energy, often from an electrochemical potential difference as ions are pumped in and out of the cell.
• Also known as couple transport or cotransport.

Types of Transporter Molecules

• Symporter: Substrates move in the same direction.
• Antiporter: Substrates move in the opposite direction.

Bulk Transport

• The movement of large materials into or out of the cell by way of vesicles.

Endocytosis

• Cells take in substances from outside the cell by engulfing them in a vesicle.
• Purposes include taking in nutrients, capturing pathogens or other substances, and disposing of old and damaged cells.

Phagocytosis

• A cell engulfs large particles using pseudopodia and encloses them in a vesicle called a "phagosome."

Pinocytosis

• A cell "gulps or drinks" the extracellular fluid with dissolved small molecules.
• The cell membrane encloses around the fluid and seals itself to form vesicles.

Receptor-Mediated Endocytosis

• The cell membrane contains receptor proteins that engulf specific target molecules.

Exocytosis

• Secretory vesicles within the cell fuse with the plasma membrane, releasing their contents into the extracellular space.
• The purpose of exocytosis is for secretion of cell products and removal of toxins and other waste products.