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Chapter 5 · Part 10 of 10
  1. Chapter overview
  2. 1What membranes are made of
  3. 2How membrane molecules move
  4. 3What controls membrane fluidity
  5. 4How cells make membrane lipids
  6. 5Membrane proteins and their sugars
  7. 6How substances cross membranes
  8. 7Osmosis and water balance
  9. 8Channels and transporters
  10. 9Pumps and active transport
  11. 10Exocytosis and endocytosis

Exocytosis and endocytosis

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Channels, transporters, and pumps move ions and small molecules across membranes. Large molecules such as proteins and polysaccharides, and even whole bacteria, are far too big for that, so eukaryotic cells move them in bulk, packed inside membrane vesicles.

Exocytosis

The material a vesicle carries is called its cargo. When a cell packages material into vesicles and releases it outside, the process is exocytosis; when it brings material in, the process is endocytosis. (Exo- means outside, and endo- means inside.)

Cells use exocytosis to release products such as hormones and digestive enzymes. The vesicles usually form at the Golgi apparatus, the organelle that processes and ships proteins:

  1. Cargo is loaded into a forming vesicle as coat proteins assemble on the surface of the Golgi membrane.
  2. The growing protein coat bends the membrane into a bud, and the bud pinches off as a vesicle.
  3. The free vesicle sheds its coat.
  4. The vesicle fuses with the plasma membrane, the way two soap bubbles merge into one, and releases its cargo outside the cell.

Insulin, a polypeptide hormone, leaves the beta cells of the pancreas this way and enters the bloodstream. Other pancreatic cells use exocytosis to release digestive enzymes that act in the small intestine.

Receptor-mediated endocytosis

In endocytosis, the plasma membrane folds inward, or invaginates, to form a vesicle around material from outside the cell. There are three types.

The first brings in specific cargo, chosen by receptors: membrane proteins that each bind one particular kind of molecule. This is receptor-mediated endocytosis:

  1. Cargo molecules bind to their receptors in the plasma membrane, which causes many receptors to gather in one spot.
  2. Coat proteins bind to the membrane there, and the coat makes the membrane fold inward and pinch off as a vesicle.
  3. Inside the cell, the vesicle sheds its coat.
  4. The vesicle usually fuses with an organelle such as a lysosome, the cell’s digestive compartment, and the receptors release their cargo.
  5. The cargo is released into the cytosol, either as is or after being digested into simpler building blocks.

Many nutrients don’t dissolve well in blood, so they travel bound to proteins and enter cells this way. Lipids carried by low-density lipoprotein (LDL) and iron carried by the protein transferrin are two examples.

Cell drinking and cell eating

The other two types of endocytosis are named for what the cell takes in.

Some cells take in droplets of the fluid around them in small vesicles, sampling whatever solutes are dissolved in it. This “cell drinking” is pinocytosis. It’s especially important in cells that absorb nutrients, such as those lining the intestine.

Other cells swallow large particles, such as bacteria, whole. The particle is wrapped in an enormous vesicle called a phagosome (or phagocytic vesicle), and this “cell eating” is phagocytosis. Only certain cells can do it.

Macrophages, cells of the mammalian immune system, are the standard example. They reach around a bacterium with pseudopodia (armlike extensions of the cell) and pull it inside in a phagosome. The phagosome then fuses with a lysosome, whose digestive enzymes destroy the bacterium.

Plants use endocytosis too. Legumes such as peas and beans take nitrogen-fixing bacteria, which turn nitrogen from the air into a form the plant can use, into their root cells. The bacteria live there inside membrane-enclosed compartments, forming root nodules.

Type of endocytosis What it takes in Example
Receptor-mediated endocytosis Specific molecules, picked out by receptors Taking up lipids carried by LDL or iron carried by transferrin
Pinocytosis (“cell drinking”) Droplets of extracellular fluid and whatever is dissolved in them Intestinal cells absorbing nutrients
Phagocytosis (“cell eating”) Large particles, enclosed in a phagosome Macrophages engulfing and destroying bacteria
Receptor-mediatedendocytosis Pinocytosis Phagocytosis Extracellularfluid Cargo Pseudopodia Protein coat Receptor Vesicle Cytosol Phagosome Bacterium Lysosome
Figure 5.8 All three types start with the plasma membrane folding inward. They differ in what they take in: specific cargo, fluid, or whole particles.
Check yourselfA drug stops coat proteins from assembling. Which step would stall in exocytosis, and which in receptor-mediated endocytosis?

Vesicle formation would stall in both. Without a coat, the Golgi membrane can’t bud off vesicles for exocytosis, and the plasma membrane can’t fold inward and pinch off vesicles in receptor-mediated endocytosis.

Check yourselfA macrophage swallows a bacterium. Name the process, the vesicle that forms, and the organelle that destroys the bacterium.

The process is phagocytosis. The bacterium is enclosed in a phagosome, which fuses with a lysosome whose digestive enzymes destroy it.

Practice questions

  1. 1 of 3

    A pancreatic cell packs digestive enzymes into a vesicle that buds from its Golgi apparatus. How do the enzymes finally leave the cell?

  2. 2 of 3

    LDL particles carrying lipids bind to receptors, which gather in one patch of a cell’s plasma membrane. Coat proteins then bind to that patch. What happens next?

  3. 3 of 3

    Cells take in iron bound to the blood protein transferrin while taking in little of the surrounding fluid. What makes this uptake so selective?