Cell Membrane Transport

• The cell membrane acts as a barrier to control which substances enter and exit the cell.
• Movement across the cell membrane can occur passively, without energy, or actively, using energy.
• The cell membrane can engulf particles via endocytosis to bring them into the cell, or exocytosis to move them out.

Passive Movement

• Passive movement of particles or ions across the cell membrane does not require energy.
• Chemical gradient is a difference in the number of particles on either side of the membrane.
• Electrical gradient is a difference in charged particles or ions across the membrane.
• Electrochemical gradient refers to the influence of both chemical and electrical gradients.
• Passive movement includes diffusion, osmosis, and facilitated diffusion.

Diffusion

• Diffusion occurs when molecules move from an area of high concentration to an area of lower concentration until concentrations are equal.
• Lipid-soluble molecules like oxygen, alcohol, carbon dioxide, and fatty acids diffuse easily through the cell membrane.
• Substances can also diffuse through small pores in the membrane.
• The rate of diffusion is affected by the number of openings in the membrane and temperature, as diffusion rates increase with temperature.

Osmosis

• Osmosis involves water movement from an area of low solute concentration to an area of high solute concentration until equilibrium is reached.
• Aquaporins are channels through which water moves.
• Osmotic pressure is the pressure generated by water moving across the membrane.

Facilitated Diffusion

• Facilitated diffusion uses transport proteins to help lipid-insoluble or large molecules pass through the membrane.
• Transmembrane movement of glucose uses this process.
• Glucose binds to transport proteins on the outer surface of the membrane, is moved through, and released into the intracellular space.
• The speed of transport depends on the number of available transport proteins and their binding/release rates.
• Ions like sodium and potassium have electrical charges, making it difficult to pass through the lipid layer without assistance.
• Facilitated diffusion through selective ion channels helps ions move across, which is made of integral proteins spanning the cell membrane.
• Specific stimuli cause the proteins to open a channel or gate, allowing ions to pass through if they match the channel.

Active Transport

• Active transport is used cells use energy to move ions up a concentration gradient.
• The sodium-potassium (Na+/K+)-ATPase pump transports sodium out of the cell, to the extracellular region, and brings potassium into the cell.
• The sodium-potassium pump uses energy from ATP hydrolysis, catalyzed by the enzyme ATPase.
• Without this process, sodium would remain in the cell, causing water to follow and leading to cell swelling.
• Primary active transport directly uses ATP to transport the substance.
• Secondary active transport uses the energy derived from primary active transport, usually of sodium, to transport another substance.
• When sodium leaves the cell via primary active transport, a concentration gradient is created, storing energy that drives secondary active transport.

Cotransport and Counter-transport

• Cotransport (symport) systems cotransport sodium ions and another solute in the same direction.
• Counter-transport (antiport) systems transport sodium ions and another solute in opposite directions.
• Intestinal absorption of glucose and amino acids is an example of cotransport paired with sodium transport.

Transport of Biomacromolecules

• Biomacromolecules, such as proteins, polysaccharides, and nucleic acids, are transported across cell membranes via carrier proteins and special transport methods.
• Endocytosis occurs when a molecule outside the cell is enclosed in an invagination of the cell membrane, forming a vesicle.
• Endocytosis includes pinocytosis and phagocytosis.
• Pinocytosis, or "cell drinking," involves the engulfment of small solid or fluid particles like proteins and electrolytes.
• Phagocytosis means "cell eating," in which the membrane engulfs and destroys microorganisms or other particulate matter.
• After engulfment, a phagosome forms and moves into the cytoplasm, where it fuses with a lysosome for destruction.
• Macrophages and leukocytes (particularly neutrophils) use phagocytosis.
• Exocytosis is the reverse of endocytosis, secreting intracellular substances into the extracellular space.
• Exocytosis removes cellular debris and releases hormones from the cell.

Membrane Potentials

• A membrane potential is the voltage difference between the inside and outside of a cell.
• Membrane potentials are required in nerve and muscle cells and generate nerve impulses, action potentials, and muscular contractions.
• Changes in membrane potential can cause hormone secretion.
• Electrical potential is the ability of separated electrical charges of opposite polarity (+ and -) to do work, measured in volts (V).
• Potential difference refers to the difference between the separated charges.
• Membrane potential is measured in millivolts (mV).
• Extra- and intracellular fluids contain 150-160 mmol/L of positively charged ions and an equal concentration of negatively charged ions.
• Accumulation of ions on the membrane surface contributes to the resting membrane potential (RMP).
• Diffusion potential describes the voltage generated by ions diffusing across the cell membrane.
• For ions to diffuse, the membrane must be permeable to the ion, and a concentration gradient must exist.
• Equilibrium potential occurs when the diffusion and electrical forces are balanced on both sides of the membrane.