5.1 Structure of Cell Membranes

Anchors

a membrane protein that attaches to other proteins and helps to maintain cell structure and shape

Fluid Mosaic Model

a model that proposes that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of two types of molecules, lipids and proteins

Cholesterol

an amphipathic lipid that is a major component of animal cell membranes

Liposome

an enclosed bilayer structure spontaneously formed by phospholipids in environments with neutral pH, like water

amphipathic

having both hydrophilic and hydrophobic regions

Fluid

when describing membranes, the ability of membrane lipids to move in the plane of the membrane

What are two ways in which proteins associate with membranes?

Proteins can associate with membranes in the following ways. First, integral membrane proteins are permanently associated with the membrane and cannot be removed without destroying the membrane itself. Most integral membrane proteins span the cell membrane, so they have both hydrophilic and hydrophobic regions. Second, peripheral membrane proteins are temporarily associated with the membrane and can easily be experimentally separated. These proteins can be associated with either the internal or external side of the membrane. They are mostly hydrophilic and interact with the polar heads of the lipid bilayer, or the hydrophilic regions of integral membrane proteins.

The interior region of a phospholipid bilayer is characterized as: A) hydrophilic B) hydrophobic C) polar D) hydrophilic and polar

B) hydrophobic

A protein that is temporarily associated with a biological membrane is a(n): A) integral membrane protein B) peripheral membrane protein C) transmembrane D) transmembrane protein and an integral membrane protein

B) peripheral membrane protein *Consider which type of protein is more likely to be weakly tethered to the membrane: a membrane that is deeply embedded or a protein that is not deeply embedded

The plasma membrane is composed of a phospholipid bilayer and associated proteins. What else is commonly found in the plasma membranes of animal cells? A) amino acids B) ethanol C) cholesterol D) nucleic acids

C) cholesterol

The lipid components of cellular membranes often include: A) phospholipids B) phospholipids and fatty acids C) phospholipids and cholesterol D) fatty acids and cholesterol

C) phospholipids and cholesterol

Like cell membranes, many fats and oils are made up in part of fatty acids. Most animal fats (like butter) are solid at room temperature, whereas plant fats (like canola oil) tend to be liquid. Can you predict which type of fat contains saturated fatty acids, and which type contains unsaturated fatty acids?

Saturated fatty acids are less mobile within the membrane compared to unsaturated fatty acids. As a result, saturated fatty acids tend to be solid at room temperature, whereas unsaturated fatty acids tend to be liquid. Butter and many other animal fats contain saturated fatty acids and are solids, whereas many plant (and fish) oils contain unsaturated fatty acids and are liquids at room temperature.

Lipid Rafts

lipids assembled in a defined patch in the cell membrane

Transporters

membrane proteins that move ions or other molecules across the cell membrane

How do lipids with hydrophilic and hydrophobic regions behave in an aqueous environment?

In an aqueous environment, the polar hydrophilic head group readily interacts with the polar water molecules. In contrast, the nonpolar hydrophobic tail does not readily interact with water, but instead interacts with other nonpolar tail groups or hydrophobic molecules. For example, a micelle forms when the polar head group of a lipid interacts with water and they hydrophobic tails of the lipids interact with each other, excluding the water. Lipids can also form bilayers and liposomes.

micelles

a spherical structure in which lipids with bulky heads and a single hydrophobic tail are packed

bilayer

a two-layered structure of the cell membrane with hydrophilic "heads" pointing outward toward the aqueous environment and hydrophobic "tails" oriented inward away from water

What would happen in the FRAP experiment if proteins did not move in the plane of a membrane?

An experiment showing that proteins move in membranes uses the fluorescence recovery after photobleaching (FRAP) technique. First, the proteins embedded in the cell membrane are labeled with a fluorescent dye. A laser is then used to bleach a small area of the cell surface so that it no longer fluoresces. Eventually, the fluorescently labeled proteins from other parts of the cell move into the bleached area and cause it to fluoresce once again. If the proteins did not move in membranes, that area would stay bleached over time.

If cells had single-layer membranes like micelles, how would the structures of transmembrane proteins be affected? A) transmembrane proteins would only possess hydrophilic regions B) transmembrane proteins would possess a hydrophilic region in the cell interior and a hydrophobic region in the extracellular space C) transmembrane proteins would only possess hydrophobic regions D) transmembrane proteins would possess a hydrophobic region in the cell interior and a hydrophilic region in the extracellular space E) the structures of transmembrane proteins would remain the same as if cells had lipid bilayers

D) transmembrane proteins would possess a hydrophobic region in the cell interior and a hydrophilic region in the extracellular space

Receptors

a molecule on cell membranes that detects critical features of the environment. Receptors detecting signals that easily cross the cell membrane are sometimes found in the cytoplasm

Enzymes

a protein that functions as a catalyst to accelerate the rate for a chemical reaction; enzymes are critical in determining which chemical reactions take place in a cell

Integral Membrane Proteins

a protein that is permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself

Peripheral Membrane Proteins

a protein that is temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions

Transmembrane Proteins

proteins that span the entire lipid bilayer most integral membrane proteins are transmembrane proteins