Bulk Transport

Polypeptides and proteins, as well as many other molecules, are too large to be transported through a membrane by the carriers described in previous sections. Yet many cells do secrete these molecules—for example, as hormones or neurotransmitters—by the process of exocytosis. As described in chapter 3, this involves the fusion of a membrane-bound vesicle that contains these cellular products with the plasma membrane, so that the membranes become continuous (fig. 6.20).

The process of endocytosis resembles exocytosis in reverse. In receptor-mediated endocytosis (see fig. 3.4), specific molecules,

Interactions Between Cells and the Extracellular Environment

Lumen of kidney tubule or small intestine

Epithelial cells of kidney tubule or small intestine

Glucose Na+ (lower

(higher concentration) concentration)

Tight — junction

Epithelial cells of kidney tubule or small intestine

Transport of Na+ down its concentration gradient provides energy for glucose to be moved against its concentration gradient

Tight — junction

Na+ ^ I ^ K+ Primary active transport

Glucose

Na+ ^ I ^ K+ Primary active transport

Glucose

ATP used to move both Na+and K+ against their concentration gradients

Blood

■ Figure 6.19 Transport processes involved in the epithelial absorption of glucose. When glucose is to be absorbed across the epithelial membranes of the kidney tubules or the small intestine, several processes are involved. (1) Primary active transport (the Na+/K+ pumps) in the basal membrane use ATP to maintain a low intracellular concentration of Na+. (2) Secondary active transport uses carriers in the apical membrane to transport glucose up its concentration gradient, using the energy from the "downhill" flow of Na+ into the cell. Finally, (3) facilitated diffusion of glucose using carriers in the basal membrane allows the glucose to leave the cells and enter the blood.

such as protein-bound cholesterol, can be taken into the cell because of the interaction between the cholesterol transport protein and a protein receptor on the plasma membrane. Cholesterol is removed from the blood by the liver and by the walls of blood vessels through this mechanism.

Exocytosis and endocytosis together provide bulk transport out of and into the cell, respectively. (The term "bulk" is used because many molecules are moved at the same time). It should be noted that molecules taken into a cell by endocytosis are still separated from the cytoplasm by the membrane of the endocytotic vesicle. Some of these molecules, such as membrane receptors, will be moved back to the plasma membrane, while the rest will end up in lysosomes.

Test Yourself Before You Continue

1. List the three characteristics of facilitated diffusion that distinguish it from simple diffusion.

2. Draw a figure that illustrates two of the characteristics of carrier-mediated transport and explain how this type of movement differs from simple diffusion.

3. Describe active transport, including primary and secondary active transport in your description. Explain how active transport differs from facilitated diffusion.

4. Discuss the physiological significance of the Na+/K+ pumps.

Fox: Human Physiology, I 6. Interactions Between I Text I I © The McGraw-Hill

Eighth Edition Cells and the Extracellular Companies, 2003

Environment

Chapter Six

Extracellular fluid

Cytoplasm -

Invagination * /

Endocytosis

Formation of pouch

Formation of vesicle

Invagination * /

Formation of pouch

Formation of vesicle

Cytoplasm -

Extracellular substances now within vesicle

Extracellular substances now within vesicle

Exocytosis

Exocytosis

Secretion now in extracellular fluid

■ Figure 6.20 Endocytosis and exocytosis. Endocytosis and exocytosis are responsible for the bulk transport of molecules into and out of a cell.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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