Cell Membrane: Structure and Function

Plasma Membrane

Boundary that separates the living cell from its surroundings

What supports a membrane’s structure and functions?

Lipids (mostly phospholipids), proteins, and carbohydrates

Phospholipids

Most abundant lipid in plasma membrane, amphipathic with hydrophilic head facing outward and hydrophobic tails facing inward

Creates a bilayer, differs in fatty acid chain length, degree of saturation, and polar groups present

Fluid Mosaic Model

The membrane is a fluid structure with a mosaic of various embedded proteins and lipids (types of lipids vary)

Lipids form main fabric, proteins determine most functions

What studies supported fluid mosaic model?

Freeze fracture, specialized preparation that splits a membrane along the middle of the phospholipid bilayer

Fluidity of Membranes

Must be fluid to work properly, temperature and unsaturated fatty acid amounts can determine the fluidity

held together by hydrophobic interactions

What does fluidity affect?

Affects permeability and the ability for membrane proteins to move.

Membranes that are too fluid cannot support protein function either, must find perfect fluidity.

Movement of Phospholipids

Phospholipids are able to move within the bilayer, lipids (and some proteins) drift laterally and occasionally flip-flop

Very rapid

Cholesterol

A steroid that affects the membrane depending on temperature

if it’s warmer, cholesterol restricts excess fluidity, and if it’s colder cholesterol maintains fluidity

Membrane Proteins

Most are amphipathic and can insert within the bilayer, maximizes contact with water and extracellular fluid

Proteins are associated in specialized functions

Movement of Proteins

Can occasionally drift along cytoskeletal fibers in the cell

Others are immobile due to attachment to matrix or cytoskeleton

Peripheral proteins

Bound to the surface of the membrane and lack hydrophobic groups (on the hydrophilic head)

Integral proteins

Proteins that are partly embedded or penetrate the hydrophobic core

These proteins have hydrophobic regions that consist of one or more stretches of nonpolar amino acids, often in alpha helices.

Six major functions of membrane proteins

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to extracellular matrix and cytoskeleton

Cell-Cell Recognition

Cells recognize each other by binding onto surface molecules (carbs) on the plasma membrane, important for sorting and defense

How does diverse carbohydrates help cells?

Due to differing carbs from as far as cell to cell, they function as markers to distinguish one cell from another.

Membrane Carbohydrates

Short, branched chains of around 15 sugar units, sometimes covalently bonded to lipids and proteins (glycolipids/proteins)

Proteoglycan - proteins with more and longer carbohydrates bonded to it

Cell Joining

Cells adhere to one another through interactions between cell surface carbohydrates and proteins

Membrane Faces

Distinct inside and outside faces, lipids differ in lipid composition and proteins have directional orientation

How is the membrane built?

Asymmetrical arrangement of proteins, lipids and carbs built by ER and Golgi apparatus, creating membrane sidedness

Transmembrane protein

Extend through the bilayer, have domains with different functions on the inner and outer sides of the membrane

Selective Permeability

Allows some substances to cross it more easily than others

Regulates cell’s molecular traffic and exchanges materials with surroundings