Membrane Structure and Function

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Slide 1

Life at the Edge

The plasma membrane is the boundary that separates the living cell from its surroundings

The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

Slide 2

Fig. 7-1

Slide 3

Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins

Phospholipids are the most abundant lipid in the plasma membrane

Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

Slide 4

Membrane Models: Scientific Inquiry

Membranes have been chemically analyzed and found to be made of proteins and lipids

Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer

Slide 5

Fig. 7-2

Hydrophilic

head

WATER

Hydrophobic

tail

WATER

Slide 6

In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions

In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

Slide 7

Fig. 7-3

Phospholipid

bilayer

Hydrophobic regions

of protein

Hydrophilic

regions of protein

Slide 8

Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer

Slide 9

Fig. 7-4

TECHNIQUE

Extracellular

layer

Knife

Proteins

Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layer

Plasma membrane

Slide 10

The Fluidity of Membranes

Phospholipids in the plasma membrane can move within the bilayer