The Structure of the Plasma Membrane

Membrane Structure

• The plasma membrane is 8 nm thick.

• It is mainly made up of phospholipids and proteins.

Membrane Structure

Phospholipid bilayer

protein

cholesterolc

carbohydrate

Membrane phospholipids

• Phospholipids have a hydrophilic head and two hydrophobic tails.

Membrane phospholipids

• The phospholipids form a bilayer- The hydrophilic heads face outward and the

hydrophobic tails face inward

Hydrophobictails

Membrane Proteins

• Membrane proteins are found within the phospholipid bilayer

• They can extend all the way through the bilayer – integral proteins

• They can be found embedded on either surface – peripheral proteins

Integral Protein

Phospholipid bilayer

Peripheral protein

Functions of Membrane Proteins

• Some proteins transport substances across the membrane by forming channels or by physically moving them across the membrane.

Membrane Proteins

Functions of membrane proteins include:• Transport of substances across the membrane• Join cells to one another• Act as enzymes• Bind to chemical messengers from other cells• Act as identification tags so cells can recognize one another• Attach to cytoskeleton and the extracellular matrix

Membrane cholesterol and carbohydrates

Cholesterol• Cholesterol is found within the

phospholipid bilayer• It helps stabilize the

phospholipids

Carbohydrates• Located on outer layer of

membrane only• Bound to proteins (glycoproteins)

or phopholipids (glycolipids)• Act as identification tags for cell

cell recognition

Fluid Mosaic Model

Fluid Mosaic Model

• The membrane is a fluid mosaic• Fluid – the phospholipids and the proteins drift within the

membrane• Mosaic – the diversity of proteins found within the

membrane and the arrangement of these proteins in the membrane

Lecture Review 11/281. What is the membrane mainly composed of?2. How are the phospholipids arranged in the

membrane?3. What are the 2 types of membrane proteins and

why are they different?4. What is the role of cholesterol?5. What is the role of carbohydrates?6. What are 2 functions of the proteins in the

membrane?7. Why is the current model called fluid mosaic?