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Membrane Structure and Function

A Cellular Membrane

Is a mosaic of proteins and carbohydrates floating in a fluid bi-layer of phospholipids.

Cellular Membrane

Proteins are embedded in the bilayer or attached to the surface.

Cellular Membrane

Carbohydrates are linked to the proteins and linked to the lipids.

Selective Permeability

A property of biological membranes

Allows some substances to cross the cell membrane more easily than others.

Selective Permeability

small nutrient waste molecules respiratory gases inorganic ions. The plasma membrane regulates the

passage of these substances.

Passive Transport Passive transport is diffusion

across a membrane

Diffusion Diffusion is the spontaneous

movement of a substance down its concentration gradient.

From a more concentrated to a less concentrated area.

Osmosis Osmosis is the passive transport of

water. Water flows across a membrane

from the side with a lesser concentration of solute (hypotonic) to the side with the greater solute concentration (hypertonic).

Cell Survival Depends on Balancing Water Uptake and Loss Cells lacking walls (as in animals and

some protists) are either isotonic with their environments or else have adaptations for osmoregulation.

Plants, prokaryotes, fungi, and some protists have an elastic wall around their cells, which keeps the cells from bursting in a hypotonic environment. Under such conditions, these cells are turgid.

Specific Proteins Facilitate the Passive Transport of Selected Solutes

In facilitated diffusion, transport proteins hasten and help the movement of certain substances across a membrane.

Diffusion, osmosis, and facilitated diffusion are all passive transport processes that do not require the input of energy from the cell.

Active Transport Is The Pumping of Solutes Against Their Gradients

Energy, usually in the form of ATP, is harnessed by specific membrane proteins that perform the active transport.

Some ion pumps generate voltage across membranes.

In co-transport, a membrane protein couples the transport of one solute to another.

Exocytosis and Endocytosis Transport Large Molecules

Exocytosis- intracellular vesicles migrate to the plasma membrane, fuse with it, and release their contents.

Endocytosis- large molecules within vesicles pinch inward from the plasma membrane.

Endocytosis-Transport of Large Molecules into the cell.

Three types of endocytosis are: Phagocytosis- the ingestion of large

particles or whole cells; Pinocytosis- the intake of tiny droplets of

extracellular fluid with all its contained solutes; and

receptor–mediated endocytosis- the ingestion of specific substances that bind to receptor proteins on the membrane.

Specialized membrane proteins Specialized membrane proteins

transmit extracellular signals to the inside of the cell.

Websites

www.bmb.psu.edu/courses/bisci004a/cells/cellstruc.htm