Membrane Structure Overview Pathology > Cellular Biology & Genetics > Cellular Biology & Genetics

PLASMA MEMBRANE

• Phospholipid bilayer: bilayer that comprises mostly phospholipids

• Fluid mosaic: mosaic of proteins embedded within a fluid phospholipid bilayer

• Selectively permeable: some substances move through passively, others use proteins for transport

MEMBRANE COMPONENTS

• Phospholipids

• Proteins

• Cholesterol

• Carbohydrates

PHOSPHOLIPIDS

• Amphipathic: hydrophilic head and hydrophobic fatty acid tails

• Form liposomes in aqueous environment

• Weak hydrophobic interactions = membrane fluidity

• Saturated phospholipids: maximize hydrogens in fatty acid tails, no kinks

• Unsaturated phospholipids: double bond produces kink, increases fluidity

CHOLESTEROL

• Temperature buffer

• Moderate temperature: decreases fluidity, lessens lateral movement

• Low temperature: increases fluidity, prevents solidification

PROTEINS

• Includes transmembrane proteins that span the bilayer (other types exist)

• Proteins provide about half the mass of the membrane

CARBOHYDRATES

• Glycoproteins: branched carbohydrates covalently bound to proteins

• Glycolipids: carbohydrates covalently bound to lipids (extracellular only)

1 / 5 CLINICAL CORRELATION: Blood types

• Carbohydrates on surface of red blood cells must be compatible between donor & recipient in blood transfusion

FUNCTIONS OF THE CELL MEMBRANE

• Cell communication

• Import and export of molecules

• Cell growth

• Cell motility

Eukaryotes have internal membranes within the cell, prokaryotes do not.

FULL-LENGTH TEXT

• Here, we'll learn about the cell membrane, which separates the inside of the cell from its external environment.

• First, start a table to summarize the major features of the plasma membrane.

• Denote that the plasma membrane comprises:

- Phospholipids, which have hydrophilic heads and hydrophobic tails.

- Proteins, which are larger than lipids.

- Cholesterol, which wedges itself between lipids.

- Carbohydrates, which bind to lipids and proteins.

Let's illustrate the membrane, now.

• Sketch a three dimensional rectangle.

• Label the space above it extracellular.

• And the space below it cytosol.

• Bisect the cube, horizontally, and indicate that the cell membrane is a "bilayer," meaning it comprises two layers.

2 / 5 - Indicate that the bilayer is specifically a phospholipid bilayer because the most common of its lipids are phospholipids.

• Draw a phospholipid as a round circle with a pair of tails extending from it.

• Label it as amphipathic (am·fi·path·ic), meaning it has both hydrophilic and hydrophobic components.

• Indicate that the tails represent fatty acids and that they are hydrophobic, meaning they fear water.

• Label the round circle as the "hydrophilic head," meaning it has an affinity for water, and indicate that it links to the hydrophobic tails with a phosphate group, (the "phospho" in phospholipid).

Next, let's illustrate how amphipathic phospholipids arrange themselves in a bilayer.

• Draw a few phospholipids in the top row of our rectangle.

• Then, draw a few upside-down phospholipids in the bottom row.

Let's take a closer look at this arrangement.

• Draw two rows of phospholipids and imagine that we immerse them in an aqueous environment just like our cell.

• Show that in an aqueous environment, this double-layer of phospholipids forms a closed circle: a liposome, which, in three dimensions, is actually a hollow sphere with an aqueous core.

• Phospholipids form liposomes when they are experimentally added to water to insulate their hydrophobic fatty acid tails from water.

Now, let's return to our cell membrane.

• Draw a conveyer belt to show that the molecules in our bilayer move laterally in the membrane, as if on a conveyer belt.

• Indicate that our bilayer is fluid.

- Adjacent phospholipids change positions about ten million times per second!

- Their weak hydrophobic interactions allow them to drift.

3 / 5 Let's elaborate on membrane fluidity.

• Indicate that so far our phospholipids are saturated: they maximize the number of hydrogens in their fatty acid tails and lack kinks.

• Now, illustrate an unsaturated phospholipid with a kink in one of its tails.

- The kink comes from a double bond in the tail.

• Then, draw two rows of unsaturated phospholipid molecules to show that they prevent close-knit packing, which enhances lateral movement and increases membrane fluidity.

• Now, add a few unsaturated phospholipids to our three dimensional membrane.

• Draw a small cholesterol molecule wedged between two phospholipids.

- Like phospholipids, cholesterol is amphipathic.

• Indicate that temperature changes membrane fluidity but cholesterol acts as a "temperature buffer"; it resists these changes (stabilizes the membrane).

• Write that, specifically:

- At moderate temperature, cholesterol decreases fluidity (lessens lateral movement); it stiffens the bilayer.

- At low temperature, cholesterol increases fluidity; it prevents the close packing of molecules, and thus prevents solidification.

Now, let's fill in the rest of our cell membrane.

• Draw a membrane protein that is so large that it protrudes from the bilayer.

- This specific protein is a transmembrane protein, but not every membrane protein associates with the bilayer in this way.

- We describe membrane protein diversity in detail elsewhere.

• Show that the membrane carries many proteins; they provide about half the mass of a plasma membrane.

• Indicate that our membrane is a Fluid Mosaic; it is a mosaic of proteins embedded within a fluid phospholipid bilayer.

4 / 5 • Now, show that glycoproteins are branched carbohydrates that covalently bind to proteins.

• Next, indicate that glycolipids comprise carbohydrates covalently bound to lipids and that they are only found on the extracellular side of the bilayer; extracellular glycolipids and glycoproteins function as cellular flags – they vary between species and between individuals within a species.

• As a clinical correlation, denote that blood types refer to the carbohydrates present on the surface of red blood cells; we use them to distinguish between type A, B, AB and type O blood.

• Denote that in a blood transfusion, the blood types of the donor and the recipient must be compatible.

- If they aren't, the glycoproteins on the donor's red blood cells are recognized as "foreign," and can trigger an immune response in the recipient.

Now, let's fill in the rest of our diagram.

• Shade the rest of the cell membrane to indicate that it is densely packed and three dimensional.

• Denote that the cell membrane is selectively permeable; some substances can pass through passively, while others use proteins for transport.

We elaborate on these mechanisms elsewhere.

Now, let's consider some of the cell membrane's functions beyond its separation of the internal and external environments.

• Denote that they include the following:

- Cell communication

- The import and export of molecules

- Cell growth

- Cell motility

We elaborate on the cell membrane's role in each of these processes elsewhere.

• Finally, write that eukaryotes also have internal membranes within the cell (prokaryotes do not).

- These internal membranes have structures similar to the cell membrane.

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