Familial Hypercholesterolemia

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The significance of LDL receptors and receptor-mediated endocytosis is illustrated by a hereditary disease called familial hypercholesterolemia.31 People with this disease have an abnormally low number of LDL receptors. Their cells therefore absorb less cholesterol than normal, and the cholesterol remains in the blood. Their blood cholesterol levels may be as high as 1,200 mg/dL, compared to a normal level of about 200 mg/dL. People who inherit the gene from both parents typically have heart attacks before the age of 20 (sometimes even in infancy) and seldom survive beyond the age of 30.

Endothelial cells also imbibe insulin by receptor-mediated endocytosis. Insulin is too large a molecule to pass through channels in the plasma membrane, yet it must somehow get out of the blood and reach the surrounding cells if it is to have any effect. Endothelial cells take up insulin by receptor-mediated endocytosis, transport the vesicles across the cell, and release the insulin on the other side, where tissue cells await it. Such transport of a substance across a cell (capture on one side and release on the other side) is called transcytosis32 (fig. 3.23). Receptor-mediated endocytosis is not always to our benefit; hepatitis, polio, and AIDS viruses "trick" our cells into engulfing them by receptor-mediated endocytosis.

31 familial = running in the family; hyper = above normal + cholesterol + emia

= blood condition

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

114 Part One Organization of the Body

Muscle cell Tissue fluid

Familial Hypercholesterolemia

Figure 3.23 Transcytosis. An endothelial cell of a capillary imbibes droplets of blood plasma at sites indicated by arrows along the right. This forms pinocytotic vesicles, which the cell transports to the other side. Here, it releases the contents by exocytosis at sites indicated by arrows along the left side of the cell. This process is especially active in muscle capillaries and transfers a significant amount of blood albumin into the tissue fluid.

Why isn't transcytosis listed as a separate means of membrane transport, in addition to pinocytosis and the others?

Capillary endothelial cell

Intercellular cleft Capillary lumen

Pinocytotic vesicles

Muscle cell Tissue fluid

Figure 3.23 Transcytosis. An endothelial cell of a capillary imbibes droplets of blood plasma at sites indicated by arrows along the right. This forms pinocytotic vesicles, which the cell transports to the other side. Here, it releases the contents by exocytosis at sites indicated by arrows along the left side of the cell. This process is especially active in muscle capillaries and transfers a significant amount of blood albumin into the tissue fluid.

Why isn't transcytosis listed as a separate means of membrane transport, in addition to pinocytosis and the others?

Exocytosis (fig. 3.24) is the process of discharging material from a cell. It occurs, for example, when endothe-lial cells release insulin to the tissue fluid, breast cells secrete milk, gland cells release hormones, and sperm cells release enzymes for penetrating an egg. It bears a superficial resemblance to endocytosis in reverse. A secretory vesicle in the cell migrates to the surface and "docks" on peripheral proteins of the plasma membrane. These proteins pull the membrane inward and create a dimple that eventually fuses with the vesicle and allows it to release its contents.

The question might occur to you, If endocytosis continually takes away bits of plasma membrane to form intracellular vesicles, why doesn't the membrane grow smaller and smaller? Another purpose of exocytosis, however, is to replace plasma membrane that has been removed by endocytosis or become damaged or worn out. Plasma membrane is continually recycled from the cell surface into the cytoplasm and back to the surface.

Table 3.3 summarizes the mechanisms of transport we have discussed.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. What is the importance of filtration to human physiology?
  2. What does it mean to say a solute moves down its concentration gradient?
  3. How does osmosis help to maintain blood volume?
  4. Define osmolarity and tonicity, and explain the difference between them.
Secretory Vesicles Electron Micrograph

Figure 3.24 Exocytosis. (a) A secretory vesicle approaches the plasma membrane and docks on it by means of linking proteins. The plasma membrane caves in at that point to meet the vesicle. (b) The plasma membrane and vesicle unite to form a fusion pore through which the vesicle contents are released. (c) Electron micrograph of exocytosis.

Figure 3.24 Exocytosis. (a) A secretory vesicle approaches the plasma membrane and docks on it by means of linking proteins. The plasma membrane caves in at that point to meet the vesicle. (b) The plasma membrane and vesicle unite to form a fusion pore through which the vesicle contents are released. (c) Electron micrograph of exocytosis.

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 3 Cellular Form and Function 115

Table 3.3 Methods of Membrane Transport

Transport Without Carriers

Filtration

Simple Diffusion Osmosis

Carrier-Mediated Transport Facilitated Diffusion

Active Transport

Primary Active Transport Secondary Active Transport

Cotransport

Countertransport

Uniport

Symport

Antiport

Vesicular (Bulk) Transport

Endocytosis Phagocytosis Pinocytosis

Receptor-Mediated Endocytosis Exocytosis

Movement of material without the aid of carrier proteins

Movement of water and solutes through a selectively permeable membrane as a result of hydrostatic pressure

Diffusion of particles through water or air or through a living or artificial membrane, down their concentration gradient, without the aid of membrane carriers Simple diffusion of water through a selectively permeable membrane

Movement of material through a cell membrane with the aid of carrier proteins

Transport of particles through a selectively permeable membrane, down their concentration gradient, by a carrier that does not directly consume ATP Transport of particles through a selectively permeable membrane, up their concentration gradient, with the aid of a carrier that consumes ATP Direct transport of solute particles by an ATP-using membrane pump

Transport of solute particles by a carrier that does not in itself use ATP but depends on concentration gradients produced by primary active transport Transport of two solutes simultaneously in the same direction through a membrane by either facilitated diffusion or active transport Transport of two different solutes in opposite directions through a membrane by either facilitated diffusion or active transport

A carrier that transports only one solute, using either facilitated diffusion or active transport A carrier that performs cotransport A carrier that performs countertransport

Movement of fluid and particles through a plasma membrane by way of vesicles of plasma membrane; consumes ATP

Vesicular transport of particles into a cell

Process of engulfing large particles by means of pseudopods; "cell eating"

Process of imbibing droplets of extracellular fluid in which the plasma membrane sinks in and pinches off small vesicles containing droplets of fluid Phagocytosis or pinocytosis in which specific solute particles bind to receptors on the plasma membrane, and are then taken into the cell in clathrin-coated vesicles with a minimal amount of fluid Process of eliminating material from a cell by means of a vesicle approaching the cell surface, fusing with the plasma membrane, and expelling its contents; used to release cell secretions, replace worn-out plasma membrane, and replace membrane that has been internalized by endocytosis

  1. Define hypotonic, isotonic, and hypertonic, and explain why these concepts are important in clinical practice.
  2. What do facilitated diffusion and active transport have in common? How are they different?
  3. How does the Na+-K+ pump exchange sodium ions for potassium ions across the plasma membrane? What are some purposes served by this pump?
  4. How does phagocytosis differ from pinocytosis?
  5. Describe the process of exocytosis. What are some of its purposes?

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Responses

  • James
    Why isnt transcytosis listed as a separate means of membrane transport?
    4 years ago

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