Cell MembraneIn This Lesson:

Cell Membranes and Transport

Today’s Agenda

• The Cell Membrane– Structure– Function• You know…osmosis? Diffusion?

By the end of this lesson…

• You should be able to describe in detail the structure of the cell membrane and link it to its functions.

• You should be able to predict the outcome of an osmotic process.

So now then…

• Okay, you giant piles of cells you…• Where do we go from here?– Cells are going to be turning into cell membranes

and transport – it’s what allows multicellular organisms to be so darn interesting.• And yes, it’s how cells interact with their

environments.

Cell Membrane Structure Overview

• The cell membrane is around 8 nm thick.– For perspective, the thickness of human hair is around 99,000

nm.• It’s composed of:– Lipids

• Mainly phospholipids and some cholesterol.

– Carbohydrates• Signal molecules attached to…

– Proteins• Embedded in the membrane.

• How was it discovered?• TED: Ethan Perlstein – Insights into Cell Membranes Via Dish Detergent

The Phospholipid Bilayer• The cell membrane is primarily

composed of phospholipids.• The cell is sitting in a water-

based environment, therefore:– Each phospholipid has a polar

(hydrophilic) head…• It’s a phosphate group.

– …and a non-polar (hydrophobic) pair of tails.• They’re fatty acids.

• Because it has hydrophilic and hydrophobic parts, it’s amphipathic.

Polar

Non-polar

The Phospholipid Bilayer• These phospholipids are

then arranged in a bilayer.– Key: Don’t confuse a bilayer

for a “double membrane.”– A bilayer is one membrane

with two layers.

Polar

Non-polar

The Phospholipid Bilayer• The bilayer acts as a semi-permeable barrier.• Polar molecules can’t get in or out.

Sugar Salt

WasteLipids

H2O

Non-PolarTails

PolarHeads

PolarHeads

Quick Note: Permeability

• Some things are “impermeable:”– Raincoats, balloons, brick walls.

• Some things are “permeable:”– Air, water.

• Some things are “semi-permeable:”– Nets, gates, cell membranes.• Semi-permeability is sometimes called selective

permeability.

Back to the Phospholipid Bilayer

• Importantly, the composition of the bilayer is not constant.

• A certain percentage is composed of phospholipids with unsaturated fatty acid tails; the rest with saturated tails.– Unsaturated hydrocarbons lead to

increased fluidity.• The lower the temperature, the more

unsaturated the membrane needs to be to prevent freezing.

– Cholesterol is also in the membrane and acts to increase viscosity except at low temperatures.

Another View

Membrane Proteins

• Membrane proteins provide the bulk of the cell-specific (or organelle-specific) functions.

• There are two main types:– Peripheral Proteins• They’re stuck to the outside of the cell.• Example: Antigens (cell markers)

– Integral Proteins• They’re stuck within and usually span the membrane.• Example: Transport Proteins

So why proteins?

• What do you see in the picture to the right?

• What are the blue things with two tails?– Phospholipids

• What’s the yellow thing wedged in there?– Cholesterol

• What are the red squiggly lines?

Polar areasof protein

Non-polar areas of protein

So why proteins?

• amino acids (which are the building blocks of proteins) can be polar or non-polar

• That makes proteins (also amphipathic) a great candidate for transmembrane proteins.

Polar areasof protein

Non-polar areas of protein

Cell Surface Proteins

• Cell surface proteins play a key role in recognition between cells.– This aids in development of

organs and tissues.• Antigens are proteins on the

cell surface that cause a response from the immune system.– They’re how the body “rejects”

cells that are foreign.

Cell Surface Proteins

• Take a look at the image to the right. See those two orangey things?– They’re carbohydrate chains.

• One’s coming from a lipid, making it a glycolipid.

• The other is coming from a protein, making it a glycoprotein.

• These carbohydrate chains make the cell identifiable to other cells.

4 Functions of Cell (Plasma) Membranes:

• Cell signaling• Selective transport• Excretion of wastes• Structural support