The Plasma Membrane – Gateway to the Cell

The Plasma Membrane is Semipermeable

Small molecules and larger hydrophobic molecules move through.

Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

The physical properties of phospholipids account for membrane assembly and many of its properties.

Plasma Membrane Functions

Maintain a high concentration of materials in the cell.

Keep harmful materials out.

Control the movement of materials into and out of the cell.

Let the cell sense its environment.

Phospholipids

Cholesterol

Proteins (peripheral and integral)

Carbohydrates

Membrane Components

Proteins Are Critical to Membrane Function

Transport Processes - Diffusion

Solutes move down a concentration gradient until they are evenly distributed. This is diffusion.

Another way of saying this is that solutes move from a region of higher concentration to a region of lower concentration until there is no difference in concentration.

Three Forms of Transport Across the Membrane

Three Forms of Transport Across the Membrane

Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out.

Three Forms of Transport Across the Membrane

Examples: Glucose or amino acids moving from blood into a cell.

An nerve electrical impulse results from opening protein channels for ions that move by facilitated diffusion.

Three Forms of Transport Across the Membrane

Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.

Moving the “Big Stuff”

Large molecules move in via one of three forms of endocytosis.

Pinocytosis

This is the most common form of endocytosis.

Pinocytosis takes up most proteins and other large molecules.

Pinocytosis

pinocytic vesicles forming mature transport vesicle

Transport into a capillary cell (blue).

Receptor-Mediated Endocytosis

Receptor proteins make this a highly specific form of transport.

Cholesterol is taken-up this way.

Receptor-Mediated Endocytosis

Cholesterol Delivered to Cells is Carried to Cells in a Low-Density Lipoprotein (LDL) Particle

The LDL particle is taken-up by receptor-mediated endocytosis.

Endocytosis – Phagocytosis Transports Large Particles

In Preparation for Phagocytosis

The Threshold of Phagocytosis - Capture of a Yeast Cell (yellow) by Membrane Extensions of an Immune System Cell (blue)

Moving the “Big Stuff”

Molecules are moved out of the cell by vesicles that fuse with the plasma membrane.

Exocytosis: moving things out.

This is how many hormones are secreted and how nerve cells communicate with one another.

Exocytosis

Exocytic vesicle immediately after fusion with plasma membrane.

Exocytosis and Nervous System Function

A nerve cell communicates to another cell by releasing chemicals via exocytosis at the synaptic terminal.

Exocytosis and Chemical Communication at the Synapse

The synapse is the region where a nerve cell and its target cell are closely apposed.

Black Widow Spider Venom and Exocytosis

Black widow spider venom causes massive exocytosis of neurotransmitter into the synapse.

Botulism and Exocytosis

Botulism is caused by botulinim toxin – a protein produced by a bacterium that sometimes contaminates foods.

Botulinim toxin blocks exocytosis at the synapse and causes paralysis.

Mechanism of botulinum toxin.

Mechanism of botulinum toxin web site.