3
Cells: The Living Units: Part B
Membrane Transport: Active Processes
• Two types of active processes:
• Active transport
• Vesicular transport
• Both use ATP to move solutes across a living plasma
membrane
Active Transport
• Requires carrier proteins (solute pumps)
• Moves solutes against a concentration gradient
• Types of active transport:
• Primary active transport
• Secondary active transport
Primary Active Transport
• Energy from hydrolysis of ATP causes shape change in
transport protein so that bound solutes (ions) are “pumped”
across the membrane
Primary Active Transport
• Sodium-potassium pump (Na+-K+ ATPase)
• Located in all plasma membranes
• Involved in primary and secondary active transport of nutrients and ions
• Maintains electrochemical gradients essential for functions of muscle and
nerve tissues
Secondary Active Transport
• Depends on an ion gradient created by primary active transport
• Energy stored in ionic gradients is used indirectly to drive
transport of other solutes
Secondary Active Transport
• Cotransport—always transports more than one substance at a time
• Symport system: Two substances transported in same direction
• Antiport system: Two substances transported in opposite directions
Vesicular Transport
• Transport of large particles, macromolecules, and fluids across
plasma membranes
• Requires cellular energy (e.g., ATP)
Vesicular Transport
• Functions:
•
•
•
•
Exocytosis—transport out of cell
Endocytosis—transport into cell
Transcytosis—transport into, across, and then out of cell
Substance (vesicular) trafficking—transport from one area or
organelle in cell to another
Endocytosis and Transcytosis
• Involve formation of protein-coated vesicles
• Often receptor mediated, therefore very selective
Endocytosis
• Phagocytosis—pseudopods engulf solids and bring them into
cell’s interior
• Macrophages and some white blood cells
Endocytosis
• Fluid-phase endocytosis (pinocytosis)—plasma membrane
infolds, bringing extracellular fluid and solutes into interior of the
cell
• Nutrient absorption in the small intestine
Endocytosis
• Receptor-mediated endocytosis—clathrin-coated pits provide
main route for endocytosis and transcytosis
• Uptake of enzymes low-density lipoproteins, iron, and insulin
Exocytosis
• Examples:
•
•
•
•
Hormone secretion
Neurotransmitter release
Mucus secretion
Ejection of wastes
Summary of Active Processes
• Also see Table 3.2
Membrane Potential
• Separation of oppositely charged particles (ions) across a
membrane creates a membrane potential (potential energy
measured as voltage)
• Resting membrane potential (RMP): Voltage measured in
resting state in all cells
• Ranges from –50 to –100 mV in different cells
• Results from diffusion and active transport of ions (mainly K+)
Generation and Maintenance of RMP
1.The Na+ -K+ pump continuously ejects Na+ from cell and carries
K+ back in
2.Some K+ continually diffuses down its concentration gradient
out of cell through K+ leakage channels
3.Membrane interior becomes negative (relative to exterior)
because of large anions trapped inside cell
Generation and Maintenance of RMP
4.Electrochemical gradient begins to attract K+ back into cell
5.RMP is established at the point where the electrical gradient
balances the K+ concentration gradient
6.A steady state is maintained because the rate of active
transport is equal to and depends on the rate of Na+ diffusion
into cell
Cell-Environment Interactions
• Involves glycoproteins and proteins of glycocalyx
• Cell adhesion molecules (CAMs)
• Membrane receptors
Roles of Cell Adhesion Molecules
• Anchor cells to extracellular matrix or to each other
• Assist in movement of cells past one another
• CAMs of blood vessel lining attract white blood cells to injured
or infected areas
• Stimulate synthesis or degradation of adhesive membrane
junctions
• Transmit intracellular signals to direct cell migration,
proliferation, and specialization
Roles of Membrane Receptors
• Contact signaling—touching and recognition of cells; e.g., in normal
development and immunity
• Chemical signaling—interaction between receptors and ligands
(neurotransmitters, hormones and paracrines) to alter activity of cell
proteins (e.g., enzymes or chemically gated ion channels)
• G protein–linked receptors—ligand binding activates a G protein,
affecting an ion channel or enzyme or causing the release of an internal
second messenger, such as cyclic AMP