Membrane Transport

• Membrane transport regulates the passage of solutes like ions through biological membranes.
• The plasma membrane is semi-permeable and limits the cell; it regulates substances entering the cell.
• The plasma membrane structure has a lipid bilayer and proteins.
• Integral and peripheral are the two types of proteins.
• Integral proteins form pumps, carriers, or ion channels.
• The three main types of transport are Passive, Active and Vesicular.

Passive transport

• Passive transport occurs across a concentration gradient and needs no energy.
• Simple diffusion is the movement of any substance along a concentration gradient.
• Facilitated diffusion requires specific carrier proteins.
• Osmosis is the diffusion of water across a semi-permeable membrane.
• Simple diffusion occurs through the interstices of the lipid bilayer or watery channels of large transport (channel) proteins.

Protein Channels

• Open channels are always open, and their selectivity can be varied by pore diameter and electrical charges, as seen in Aquaporins.
• Gated channels' opening is regulated by conformational change in the protein molecule's shape.
• Voltage and chemical gating regulate the gating.
• Voltage gating involves sodium and potassium channels.
• Chemical gating involves acetylcholine channels.

Facilitated Diffusion

• A conformational change in the carrier protein causes movement.
• The diffusion rate has an upper limit because the facilitated diffusion rate cannot exceed the protein molecule's rate of change between its two conformations.
• Glucose transport through GLUT-4 exemplifies facilitated diffusion.

Factors of Passive Transport

• A greater concentration difference results in faster diffusion.
• Smaller molecules diffuse more quickly.
• Lower viscosity results in quicker molecule movement.
• Systemic streaming aids diffusion.
• Lipid-soluble molecules dissolve easily through the phospholipid bilayer, as is seen with oxygen and carbon dioxide.

Active Transport

• Active transport is the movement of a substance against a concentration gradient with the utilization of energy.
• Primary and Secondary active transport are the two types of Active Transport.

Primary Active Transport

• Energy is derived directly from ATP or some high-energy phosphate compounds.
• The Sodium-Potassium Pump is an example of primary active transport.
• In the Sodium-Potassium Pump, sodium ions are pumped out of the cell and potassium ions are pumped into the cell.
• The energy to drive the pump is released by hydrolysis of ATP.

Secondary Active Transport

• Secondary active transport's energy is secondarily derived from the energy stored in the form of ionic concentration differences of secondary molecular or ionic substances between two sides of the cell membrane, created originally by primary active transport.
• Counter transport is one type where an ion and a molecule move in opposite directions, one example being the Sodium-calcium exchanger.
• Co-transport is where the ion and molecule move in the same direction, e.g., SGLT-1.

Vesicular Transport

• Vesicular transport transports macromolecules.

Exocytosis

• Exocytosis involves molecules excreted to the exterior of the cell after a vesicle fuses with the cell membrane.
• Constitutive exocytosis describes quick protein transport to the cell membrane from vesicles with little to no processing.
• Non-constitutive exocytosis sees proteins from the Golgi apparatus enter secondary granules, after which processing of prohormones to mature hormones occurs prior to exocytosis.

Endocytosis

• Endocytosis involves molecules' movement from extracellular to intracellular fluid via the invagination of the cell membrane around the molecule.
• Phagocytosis and Clathrin Mediated Endocytosis are examples of endocytosis.