Membrane Transport: Passive, Active, and Vesicular Processes
Properties of Membranes: Read from Slides
Transport:
Transport is the controlled movement of specific ions and molecules across a membrane by membrane proteins.
Passive transport moves ions and molecules with the concentration gradient – from the side with a higher concentration to the side with a lower concentration; it is a type of diffusion; Diffusion involves a net movement of molecules or ions.
Simple Diffusion
Diffusion through the lipid part of a biological membrane depends solely on molecular size and lipid solubility. Nonpolar inorganic gases (O2, N2, and CO2) and organic molecules are transported by simple diffusion. Water (a polar molecule) is small enough to slip between the hydrocarbon tails of phospholipid molecules in a fluid bilayer.
Facilitated Diffusion
Diffusion of polar and charged molecules through transport proteins in the hydrophobic lipid bilayer. Facilitated diffusion is specific: Membrane proteins transport certain polar and charged molecules (such as water, amino acids, sugars, and ions), but not others.
Channel Proteins
Integral membrane proteins that form hydrophilic channels in the membrane through which water and ions can pass.
Aquaporins; Channel proteins that transport water.
Ion Channels; Facilitate transport of ions (Na+, K+, Ca2+, Cl–).
Most are gated channels.
Gated Channels; Switch between open, closed, or intermediate states.
Carrier Proteins
Bind a specific single solute and transport it across the lipid bilayer (uniport transport). Undergo conformational changes that move the solute-binding site from one side of the membrane to the other.
Osmosis is the diffusion of water across a selectively permeable membrane in response to concentration gradients. Movement of water by osmosis develops forces that can cause cells to swell and burst, or shrink and shrivel up. Animal cells expend energy to counteracting the inward or outward movement of water by osmosis. Water moves across a semipermeable membrane from the region with less solute (higher water concentration) to the region with more solutes (lower water concentration). Association of water molecules with solute molecules reduces the amount of water available to cross the membrane. The concentration of free water molecules is lower on the solute side than on the pure water side.
If the concentrations of solutes inside and outside the cell are balanced (equal), the two solutions are isotonic. Animal cells must constantly use energy to actively transport Na+ from inside to outside in order to keep fluids on either side of the plasma membrane isotonic.
If the solution surrounding a cell contains nonpenetrating solutes at lower concentrations than in the cell, the solution is hypotonic to the cell; Water enters and the cell swells.
If the solution surrounding a cell contains nonpenetrating solutes at higher concentrations than in the cell, the solution is hypertonic to the cell; Water leaves and the cell shrinks.
Active Transport moves ions or molecules against the concentration gradient; from lower to higher concentration – uses energy directly or indirectly from ATP.
Three main functions of active transport
- Uptake of essential nutrients from fluid surrounding cells, even when concentrations are higher in cells.
- Removal of secretory or waste materials from cells or organelles, even when concentrations are higher outside.
- Maintenance of intracellular concentrations of H+, Na+, K+, and Ca2+.
Active transport of ions contributes to an electrical potential difference (voltage) across the plasma membrane, called a membrane potential. Neurons and muscle cells use membrane potential in response to a stimulus, when their membrane potential changes rapidly and transiently.
Example: In nerve cells, active transport is the basis for transmission of a nerve impulse.
The Na+/K+ pump (Na+/K+-ATPase) in the plasma membrane of all animal cells moves 3 Na+ out of the cell and 2 K+ into the cell in the same pumping cycle.
Exocytosis and Endocytosis:
Eukaryotic cells import and export larger particles and molecules such as proteins by exocytosis and endocytosis. Both exocytosis and endocytosis require energy from ATP.
In exocytosis, secretory vesicles bud from the Golgi complex move through the cytoplasm and contact the plasma membrane. The vesicle membrane fuses with the plasma membrane, releasing the contents of the vesicle to the cell exterior. The vesicle membrane becomes part of the plasma membrane.
In endocytosis, proteins and other substances are trapped in pitlike depressions that bulge inward from the plasma membrane and pinch off as an endocytic vesicle.
Endocytosis occurs by one of two pathways: bulk endocytosis or receptor-mediated endocytosis.
Pinocytosis is nonspecific – it takes in any solutes present in ECF (Extracellular fluid) because the membrane lacks surface receptors for specific molecules. Receptor-Mediated Endocytosis is specific and also called phagocytosis.