Membrane transport and cellular homeostasis are crucial for maintaining life. These processes involve the movement of molecules across cell membranes, either passively or actively, to regulate internal conditions. Understanding these mechanisms is key to grasping how cells function and communicate.

This topic delves into the various types of transport, including diffusion, osmosis, and active transport. It also covers ion channels, pumps, and transporters, which play vital roles in maintaining cellular balance. These concepts are essential for comprehending how organs and organ systems work at a cellular level.

Passive vs Active Transport

Energy Requirements and Concentration Gradients

• Passive transport does not require energy input and moves molecules down their concentration gradient
• Active transport requires energy (usually ATP) to move molecules against their concentration gradient
• The concentration gradient is the difference in the concentration of a substance between two regions (intracellular and extracellular spaces)

Types of Passive Transport

• Simple diffusion: Movement of small, nonpolar molecules (oxygen, carbon dioxide) directly through the lipid bilayer
• Facilitated diffusion: Movement of larger or charged molecules (glucose, amino acids) through membrane proteins called channels or carriers
• Osmosis: Movement of water across a selectively permeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration)

Types of Active Transport

• Primary active transport directly uses ATP to power the movement of molecules (Na+/K+ ATPase, Ca2+ ATPase)
• Secondary active transport couples the movement of one molecule against its gradient to the movement of another molecule down its gradient (Na+/glucose cotransporter, Na+/Ca2+ exchanger)
• Endocytosis: Uptake of large particles or macromolecules (proteins, bacteria) by invagination of the cell membrane and formation of vesicles
• Exocytosis: Release of large particles or macromolecules (neurotransmitters, hormones) by fusion of vesicles with the cell membrane

Ion Channels, Pumps, and Transporters

Ion Channels

• Membrane proteins that allow specific ions to pass through the membrane down their electrochemical gradient
• Can be gated by various stimuli (voltage, ligands, mechanical stress) to open or close the channel
• Examples include sodium channels, potassium channels, and chloride channels
• Play a crucial role in generating and propagating electrical signals in excitable cells (neurons, muscle cells)

Ion Pumps

• Use ATP to actively transport ions against their concentration gradient to maintain the proper intracellular and extracellular ion concentrations
• Na+/K+ ATPase: Pumps 3 Na+ out of the cell and 2 K+ into the cell, maintaining the resting membrane potential and cell volume
• Ca2+ ATPase: Pumps Ca2+ out of the cell or into the endoplasmic reticulum, regulating intracellular Ca2+ levels and muscle contraction
• H+/K+ ATPase: Pumps H+ out of the cell in exchange for K+, important for acid secretion in the stomach

Transporters

• Facilitate the movement of specific molecules across the membrane by coupling their transport to the movement of ions down their electrochemical gradient
• Na+/glucose cotransporter (SGLT1): Transports glucose into the cell using the Na+ gradient, important for glucose absorption in the small intestine
• Na+/Ca2+ exchanger (NCX): Removes Ca2+ from the cell in exchange for Na+, crucial for relaxation of cardiac muscle
• Cl-/HCO3- exchanger (AE1): Exchanges Cl- for HCO3-, important for pH regulation and CO2 transport in red blood cells

Osmosis and Cellular Water Balance

Osmosis and Osmotic Pressure

• Osmosis is the movement of water across a selectively permeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration)
• The osmotic pressure of a solution depends on the concentration of solutes that cannot freely cross the membrane (proteins, ions)
• Water moves across the membrane to equalize the osmotic pressure on both sides

Tonicity and Cell Volume Regulation

• Isotonic: Extracellular fluid has the same osmotic pressure as the intracellular fluid, no net movement of water
• Hypotonic: Extracellular fluid has lower osmotic pressure than the intracellular fluid, water enters the cell, causing it to swell
• Hypertonic: Extracellular fluid has higher osmotic pressure than the intracellular fluid, water leaves the cell, causing it to shrink
• Cells have volume-regulated anion channels (VRACs) that allow for the efflux of ions and organic osmolytes to prevent excessive swelling or shrinking

Organismal Adaptations for Water Balance

• Contractile vacuoles in protists: Collect and expel excess water to maintain cell volume in hypotonic environments
• Loop of Henle in the mammalian kidney: Creates a concentration gradient in the medulla, allowing for the production of concentrated urine and water conservation
• Aquaporins: Water channels that facilitate the rapid movement of water across membranes in various tissues (kidney, brain, eyes)

Membrane Potential and Cellular Function

Resting Membrane Potential

• The electrical potential difference across the cell membrane due to the unequal distribution of ions (Na+, K+, Cl-, and organic anions)
• Maintained by the action of ion pumps (Na+/K+ ATPase) and the selective permeability of the membrane to different ions (K+ > Na+)
• Typically ranges from -60 to -90 mV, with the inside of the cell being negative relative to the outside

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative due to the opening of Na+ or Ca2+ channels, important for initiating action potentials
• Hyperpolarization: Membrane potential becomes more negative due to the opening of K+ channels or the activation of Cl- influx, important for inhibitory synaptic transmission
• Graded potentials: Localized, non-propagating changes in membrane potential that vary in amplitude based on the strength of the stimulus (receptor potentials, synaptic potentials)

Action Potentials and Synaptic Transmission

• Action potentials are rapid, all-or-none changes in membrane potential that allow for the propagation of electrical signals along the membrane of excitable cells (neurons, muscle cells)
• Initiated by depolarization to a threshold value, leading to the opening of voltage-gated Na+ channels and further depolarization
• Terminated by the inactivation of Na+ channels and the opening of voltage-gated K+ channels, causing repolarization and a brief hyperpolarization
• Synaptic transmission involves the release of neurotransmitters from the presynaptic cell, which bind to receptors on the postsynaptic cell and cause either depolarization (excitatory) or hyperpolarization (inhibitory)

Cellular Processes Regulated by Membrane Potential

• Muscle contraction: Depolarization of the muscle cell membrane leads to the release of Ca2+ from the sarcoplasmic reticulum, triggering the sliding of myofilaments
• Hormone secretion: Depolarization of endocrine cells (pancreatic beta cells) leads to the influx of Ca2+ and the exocytosis of secretory vesicles containing hormones (insulin)
• Sensory transduction: Changes in membrane potential of sensory receptor cells (photoreceptors, mechanoreceptors) in response to stimuli lead to the generation of graded potentials and the release of neurotransmitters