2.B.2 Membrane Transport Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.

Membrane Transport Active Transport Protein Pumps Na + /K + PumpEndocytosisPhagocytosisPinocytosisExocytosis Passive Transport DiffusionOxygen, CO2 Facilitated Diffusion OsmosisGlucoseIon Channels

Passive transport does not require the input of metabolic energy. Membrane Transport Active Transport Passive Transport

Diffusion is the tendency for molecules to spread out evenly into the available space.

The net movement of molecules is down their concentration gradient: from an area of high concentration to an area of low concentration.

At dynamic equilibrium, molecules move in both directions at the same rate; there is no net movement of molecules in any particular direction.

Passive transport plays a primary role in the import of resources and the export of wastes.

Membrane proteins play a role in facilitated diffusion of charged particles and polar molecules through a membrane.

Example: Glucose transport

Example: Na + and K + Transport (Ion Channels)

Osmosis is the diffusion of water across a semipermeable membrane. Osmoregulation, the control of water balance, is a necessary adaptation for life.

External environments can be hypotonic, hypertonic or isotonic to internal environments of cells.

Water will diffuse down its concentration gradient, from an area with more water molecules to an area with less water molecules.

A solution is hypertonic to a cell if it contains more solute than in the cell’s internal environment. More WaterLess Water More SoluteLess Solute Tonicity Animal Cell Shriveled Plant Cell Plasmolyzed

A solution is hypotonic to a cell if it contains less solute than in the cell’s internal environment. More SoluteLess Solute Less WaterMore Water Tonicity Animal Cell Lysed Plant Cell Turgid (normal)

A solution is isotonic to a cell if it contains the same amount of solute as in the cell’s internal environment. Equal Solute Equal Water Animal Cell Normal Plant Cell Flaccid Tonicity

Water Potential (  ) Water will flow down its concentration gradient from and area of higher water potential to an area of lower water potential.

Active transport requires free energy to move molecules from regions of low concentration to regions of high concentration. Membrane Transport Active Transport Passive Transport

Active transport uses free energy (often provided by ATP) to transport molecules and/or ions across the membrane.

Active transport is used to establish and maintain concentration gradients. It is used whenever a molecule needs to be pumped against its concentration gradient.

Phosphorylation of a carrier protein induces a conformational (shape) change. Hydrolysis of the bound phosphate group then restores the carrier to its original conformation.

Example: the Sodium-Potassium Pump

Cytoplasmic Na + binds to the sodium-potassium pump. Na + binding stimulates phosphorylation by ATP. Phosphorylation causes the protein to change its shape. Na + is expelled to the outside. K + binds on the extracellular side and triggers release of the phosphate group. Loss of the phosphate restores the protein’s original shape. K + is released, and the cycle repeats.

Bulk transport across the plasma membrane occurs by endocytosis and exocytosis.

In exocytosis, internal vesicles fuse with the plasma membrane to secrete large macromolecules out of the cell.

In endocytosis, the cell takes in macromolecules and particulate matter by forming new vesicles derived from the plasma membrane.

Phagocytosis is a form of endocytosis by which cells engulf solid material to ingest. “Cell Eating”

Pinocytosis is a form of endocytosis by which cells engulf dissolved particles. “Cell Drinking”

Learning Objectives: LO 2.12 The student is able to use representations and models to analyze situations or solve problems qualitatively and quantitatively to investigate whether dynamic homeostasis is maintained by the active movement of molecules across membranes. [See SP 1.4]