Cell Membranes

The cell membrane controls what goes in and out of the cell. Nutrients and oxygen must be able to get into the cell, while wastes and other cellular products must be removed, still other cellular materials must stay right where they are.

Membranes are made of two layers of phospholipids that do not mix with water
Proteins embedded in the membrane allow molecules to move in and out
Some molecular movement requires energy (ATP), some movement does not.

Membranes are also the main way that a cell communicates with other cells. Receptor proteins are perfectly shaped to fit with signal molecules and cause a response.

Some open gates or pumps for moving molecules in or out
Some start signal transduction pathways that turn genes on or off or activate enzymes

The Bouncer

Cell membranes are made of a phospholipid bilayer - the inside of which is non-polar.

Water is polar and does not easily move between the phospholipids.
- Channel proteins called "aquaporins" allow water molecules to pass through
Lipids can squeeze through the bilayer
- Some hormones are lipid-based

Some membrane proteins allow molecules to move in and out without using energy.

Passive Transport does not use energy; it's basically diffusion.
Channel Proteins are little tubes that allow anything smaller than the opening to diffuse through.
- Sorting molecules by size enables osmosis; some solutes have to remain on their side of the membrane.
Carrier proteins act like trapdoors for slightly larger molecules.

Some membrane proteins need energy to perform their function.

Active Transport uses ATP to move molecules that can't move by diffusion alone.
Pumps move molecules from low concentration to high concentration.
Endocytosis (endo - into; cyto- cell) brings large molecules into the cell by surrounding them with a bubble of phospholipids
Exocytosis (exo - out) is a process of surrounding molecules inside the cell with phospholipids and then allowing the bubble to fuse with the cell membrane releasing the contents to the outside.

The Informer

Cell membranes also help control the cell by responding to the environment. Receptor proteins are scattered throughout the membrane that have a shape that is very specific to certain signals.

Some receptors are attached to gateway proteins that let certain molecules in only when they are stimulated.
- Nerve cells have receptors that are stimulated by neurotransmitters released by other nerve cells. When they bind, they open gates that send nervous signals.
Some receptors influence cell behavior
- Growth factors are chemicals that cause a cell to get ready to divide when they bind to specific membrane proteins.
Some receptors turn genes on or off.
- Glucose binds to receptors in pancreas cells that signal the nucleus to activate the genes that make more insulin proteins.
Some receptors activate enzymes inside the cell.
- Liver cells respond to the hormone insulin by activating enzymes that bind glucose molecules together to make glycogen.
A signal transduction pathway describes all of the molecules involved with sending the signal between the receptor protein and the end molecule.