Cell Membrane In 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 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 Waste Lipids H2OH2O H2OH2O Non- Polar Tails Polar Heads Polar Heads
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 areas of 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 areas of 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