Ldl

Normal liver cell: Cholesterol, as part of low-density lipoprotein (LDL), enters the cell after LDL binds to a receptor.

Liver cells

LDL receptor LDL

Familial Hypercholesterolemia Pictures

Familial hypercholesterolemia: Absence of a functional LDL receptor prevents cholesterol from entering the cells, and it accumulates in the blood.

Familial hypercholesterolemia: Absence of a functional LDL receptor prevents cholesterol from entering the cells, and it accumulates in the blood.

Hypercholesterolemia Ldl Receptors

(b) Cystic fibrosis

Normal cell lining the airway: Cl- leaves the cell through an ion channel. Water follows by osmosis, and moist thin mucus allows cilia to beat and sweep away foreign particles, including bacteria.

channel

Cilia

Normal cell lining the airway: Cl- leaves the cell through an ion channel. Water follows by osmosis, and moist thin mucus allows cilia to beat and sweep away foreign particles, including bacteria.

channel

Cilia

Ion Transporter Mucus

Thin mucus

Thick mucus

Cystic fibrosis: Lack of a Cl- channel causes a thick, viscous mucus to form. Cilia cannot beat properly and remove bacteria; infections can easily take hold.

Mucous Membranes Cilia

Thin mucus

Thick mucus

Thicker mucus

Thicker mucus

17.3 Genetic Diseases of Membrane Proteins The left two panels illustrate normal cell function, while the two right panels show the abnormalities caused by (a) hypercholesterolemia and (b) cystic fibrosis.

The reason for the thick mucus is a defective version of a membrane transport protein, the chloride transporter (Figure 17.3b). In normal cells, this ion channel opens to release Cl- to the outside of an epithelial cell. The resulting imbalance of Cl- ions (because of the channel, there are normally more on the outside of the cell than on the inside) causes water to leave the cell by osmosis, resulting in a moist thin mucus outside the cell. A single amino acid change in the channel protein renders it nonfunctional, leading to thick mucus and the consequent clinical problems.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment