Physiology Lecture #1 [Plasma Membrane Function and Structure]

Plasma Membranes are primarily comprised of

Lipids, Proteins, and Carbohydrates

Specialization of the plasma membrane composition defines the

Function and physiology of many cell types

Lipids can be simple-

Containing glycerol and fatty acids

Lipids can be complex-

Containing phospho and sphingo chemical modifying groups

A specific steroid that is an important membrane component

Cholesterol

The Phospholipid bilayer is comprised of a ___ exterior and ___ core and forms an effective ___ to ions and proteins

Polar (hydrophilic), hydrophobic, barrier

The fluidity of the membrane is determined by its ______ composition and ____ content

Describe a saturated fatty acid chain

- Tightly packed lipid molecules

- Less fluid

Describe an unsaturated fatty acid chain

- Imperfect packing

- More fluid

Two common classes of phospholipids found in cellular membranes are

- Phosphoglycerides (most common)

- Sphingolipids

Lipid and embedded protein molecules can diffuse ____

Laterally

Membrane proteins key functions

Transport: Enables the exchange of small molecules across the membrane (channels, transporters, pumps)

Signaling: Receive signals from external cues to regulate cellular processes (GPCRs)

Adhesion: Enables cell-to-cell junctions (Gap junctions, desmosomes, tight junctions)

Other functions of membrane proteins

- Proteins that facilitate exocytosis and endocytosis: entry and exit of large biomolecular complexes

- Membrane-bound enzymes

Types of protein structures

Primary

Secondary

Tertiary

Types of membrane proteins

Peripheral (bound to the surface of the membrane)

Integral (completely span the lipid bilayer, sometimes multiple times)

Peripheral membrane proteins associate with the surface of the membrane via ____

- Ionic or other weak interactions

- May bind to lipid head groups and to other membrane proteins

- Post-translationally modified to enable membrane tethering

Carbohydrates are comprised of ____

Polymerized sugar molecules such as glucose, fructose, and galactose

Carbohydrates are ____ attached to both____ & _____

Covalently, proteins (glycoproteins), lipids (glycolipids)

Describe the glycocalyx

- A carbohydrate-rich layer at the plasma membrane

- Comprised of glycoproteins and glycolipids

- Function to protect cells from mechanical or chemical damage (Particularly in lysosome to protect membrane proteins from proteolysis)

Some viruses bind to cells via

Carbohydrates

Epithelial cells have these domains

Apical and basolateral

The apical and basolateral domains are separated by ___

Tight junctions

In the intestine the Apical domain

Faces the lumen and is involved in bringing nutrients into the cell

In the intestine the basolateral domain

Is involved in transporting nutrients out of cell for distribution in the blood stream

Types of cell adhesion proteins

Desmosomes

Gap Junctions

Tight junctions

Tight junctions

- Act as a barrier (prevent free diffusion of solutes and water between cells)

- Act as a fence (maintain the distribution of membrane lipids and proteins by keeping the apical and basolateral domains separate)

- Provide cell adhesion (helping tissues stick together)

Gap Junctions

- Act as "bolts" that hold cells together

- Allow direct passage of ions, small metabolites, and signaling molecules between cells

Desmosomes

- Act like rivets or spot welds, holding cells together in tissues subject to high mechanical stress

- Provide tensile strength and prevent cells from tearing apart

Small "patches" of membrane with distinct phospholipid composition are found on the cell surface

Lipid rafts

Lipid rafts are often enriched in ____ & ____

Sphingolipids & Cholesterol

Invaginations in plasma membrane. A subset of lipid rafts

Caveolae

Caveolae Structure

- Distinct phospholipid and protein compositions

- Enriched in Cholesterol

- Caveolin is an abundant protein component

Function of Caveolae

- Many proteins involved in cell signaling and cell-cell communication are located in caveolae

- May be sites for internalization of proteins from the plasma membrane into the cell

Mitochondrial Membrane Morphology

Outer membrane is smooth

Inner membrane has elaborate invaginations