Membranes 0 153

The Plasma Membrane The Plasma membrane is approximately 8-nm thick and is selectively permeable. The Plasma membrane is approximately 8-nm thick and is selectively permeable. Selective Membrane Pore Substances Permeable Permeable Not permeable Selectively permeable means that the membrane is selective as to which substances can pass through it Selectively permeable means that the membrane is selective as to which substances can pass through it The following example will demonstrate this point: The following example will demonstrate this point:

Larger molecules, such as polysaccharides, proteins, and lipids cannot fit, because they are too big. Larger molecules, such as polysaccharides, proteins, and lipids cannot fit, because they are too big.

The Plasma Membrane Cellular membranes are composed primarily of… Lipids(fats) Proteins Carbohydrates (sugars)

In a cell’s plasma membrane, back to back phospholipid molecules form two distinct layers. Outer layer Inner layer

Water is the main component inside and outside the cell Aqueous inside the cell Aqueous environment outside the cell A descriptive word meaning “watery” is the term aqueous.

The outside of the cell is known as the extracellular side. Cytoplasmic side Extracellular side The inside of the cell is the cytoplasmic side.

The two layers together are known as a phospholipid bilayer. Extracellular Side (aqueous) Cytoplasmic Side (aqueous) Phospholipid bilayer

The yellow phosphate heads of the lipids are hydrophilic. Extracellular Side (aqueous) Cytoplasmic Side (aqueous) Phosphate head (hydrophilic) Hydrophilic means “water loving.” Phosphate head (hydrophilic)

The blue colored lipid tails are hydrophobic. Extracellular Side (aqueous) Cytoplasmic Side (aqueous) Phosphate head (hydrophilic) Lipid Tails (hydrophobic) Hydrophobic means “water fearing.” Phosphate head (hydrophilic)

A Phospholipid bilayer is an amphipathic structure, meaning it has both a hydrophilic region and a hydrophobic region. Hydrophilic “water loving” Hydrophilic Hydrophobic “water fearing”

The picture on the right is a tunneling electron micrograph of a cell’s plasma membrane. Phospholipids are the most abundant lipids in most membranes. The diagram on the left is a model which interprets the two lines in the micrograph as a bilayer of phospholipids. Plasma membrane

Membrane Dynamics Phospholipids are very dynamic. They constantly vibrate and make rapid lateral movements many times a second. Phospholipids move laterally at a distance of about 2 μm per second, which is about the length of a typical bacterial cell.

Phospholipids also make transmembrane flip- flops at a rate of about once per month. Cell plasma membranes are usually about as fluid as salad oil.

Extracellular Side (aqueous) Cytoplasmic Side (aqueous) This bilayer also contains many molecules of the steroid cholesterol. Cholesterol Cholesterol slows the movement of phospholipids in cell membranes. At lower temperatures however, cholesterol helps to keep the membrane from solidifying.

The fewer the number of cholesterol molecules, the more fluid the membrane. Unsaturated hydrocarbon tails cause kinks in the lipid bilayer Bilayers with kinks have fewer cholesterol molecules than those without kinks.

The greater the number of cholesterol molecules, the more viscous the membrane. Saturated hydrocarbons have straight tails Bilayers without kinks have more cholesterol molecules than those with kinks.

The phospholipids, steroids and proteins of the membrane form a mosaic of substances that float about. This is known as the Fluid Mosaic Model.

The Fluid Mosaic Model was proposed by Singer and Nicolson in This is a 3-D model of what the membrane might look like.

The website below has an animation which demonstrates how the phospholipids and proteins move about in the membrane.

Parting Shots Lipids have about twice the energy content of both proteins and carbohydrates. Lipids have about twice the energy content of both proteins and carbohydrates. Some lipid derivatives act as hormones (testosterone, estrogen, and progesterone). Some lipid derivatives act as hormones (testosterone, estrogen, and progesterone). Fat under the skin insulates the body against temperature changes (e.g. seal blubber) Fat under the skin insulates the body against temperature changes (e.g. seal blubber) Fat is a hydrophobic storage molecule and thus is lightweight. Fat is a hydrophobic storage molecule and thus is lightweight. Fat acts as a shock absorbent for some organs of the body (e.g. kidneys) Fat acts as a shock absorbent for some organs of the body (e.g. kidneys)

Fluid Mosaic Model