Cell Transport

Plasma Membrane

• It is the boundary separating living cells from their surroundings.
• Exhibits selective permeability, allowing some substances to cross more easily than others.
• Phospholipids, the most abundant lipid, are amphipathic molecules with hydrophobic and hydrophilic regions.
• The fluid mosaic model describes the membrane as a fluid structure with a mosaic of embedded proteins.
• Proteins aren't randomly distributed within the membrane.
• Phospholipids can move within the bilayer; lipids and some proteins drift laterally.
• Lipids rarely flip-flop transversely across the membrane.

Membrane Proteins & Function

• Proteins determine most of a membrane's specific functions.
• Peripheral proteins are bound to the membrane surface.
• Integral proteins penetrate the hydrophobic core.
• Transmembrane proteins are integral proteins that span the membrane.
• Hydrophobic regions of integral proteins often consist of nonpolar amino acids coiled into alpha helices.
• Major functions include transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and ECM.

Membrane Carbohydrates

• Cells recognize each other via molecules, often carbohydrates, on the plasma membrane's extracellular surface.
• Membrane carbohydrates are covalently bonded to lipids(glycolipids) or more commonly to proteins(glycoproteins).
• Carbohydrates on the external side of the plasma membrane are variable among species, individuals, and cell types.

Synthesis & Sideness

• Membranes have distinct inside/outside faces.
• The asymmetrical distribution of proteins, lipids, and carbs is determined during membrane construction in the ER and Golgi apparatus.

Selective Permeability

• Cells exchange materials with their environment through the plasma membrane.
• Plasma membranes are selectively permeable, regulating molecular traffic.
• Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer and cross easily.
• Hydrophilic molecules, including ions and polar molecules, do not cross easily.

Cell Membrane Transport Summary

• Passive transport includes: simple diffusion, osmosis, and facilitated diffusion
• Active transport includes: pumps, exocytosis, and endocytosis
• Endocytosis includes: Pinocytosis, phagocytosis, and receptor mediated

Passive Transport

• It is diffusion across a membrane without energy investment.
• Diffusion is the tendency of molecules to spread evenly into available space.
• A population of molecules may diffuse directionally, though each molecule moves randomly.
• At dynamic equilibrium, the number of molecules crossing the membrane is the same in both directions.
• Substances diffuse down the concentration gradient where the density of a chemical substance increases or decreases.
• No work is done in passive transport because no energy is expended by the cell.

Osmosis

• It is the diffusion of water across a selectively permeable membrane.
• Water diffuses from lower solute concentration to higher solute concentration until the solute concentration is equal on both sides.

Tonicity of Solutions

• Tonicity is a solution's ability to cause a cell to gain or lose water.
• Isotonic solution: Solute concentration is the same inside and outside the cell, resulting in no net water movement.
• Hypertonic solution: Higher solute concentration outside the cell, causing the cell to lose water.
• Hypotonic solution: Lower solute concentration outside the cell, causing the cell to gain water.
• Hypertonic or hypotonic environments create osmotic problems for organisms.
• Osmoregulation is the control of solute concentrations and water balance, essential for life in such environments.

Filtration

• The process forces small molecules across the plasma membrane with the aid of hydrostatic (water) pressure.
• Blood pressure forcing wastes out of blood vessels and into kidney tubules during urine formation is an example.

Facilitated Diffusion

• Transport proteins speed the passive movement of molecules across the plasma membrane.
• Channel proteins provide specific corridors for molecules or ions to cross the membrane.
• Aquaporins facilitate water diffusion.
• Ion channels facilitate ion diffusion.
• Gated channels open or close in response to a stimulus.
• Carrier proteins change shape to translocate the solute-binding site across the membrane.

Active Transport

• It moves substances against their concentration gradients requiring energy, usually in the form of ATP.
• Specific embedded proteins in the membranes perform it.
• Uniporters transport a single type of molecule.
• Symporters transport two molecules or ions in the same direction.
• Antiporters transport two molecules or ions in opposite directions.
• Allows cells to maintain concentration gradients differing from their surroundings.
• The sodium-potassium pump is one type of active transport system.

Cotransport

• Occurs when active transport of a solute indirectly drives transport of other substances.

Exocytosis & Endocytosis

• Small molecules and water enter or leave the cell through the lipid bilayer or transport proteins.
• Large molecules (polysaccharides, proteins) cross the membrane in bulk via vesicles.
• Bulk transport requires energy.
• During exocytosis, transport vesicles migrate to the membrane, fuse, and release their contents outside the cell; secretory cells export products.
• During endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane.
• Motor proteins like kinesin and dynein are involved in vesicle transport.