Endocytosis

Two processes use cellular energy to move substances into or out of a cell without actually crossing the cell membrane. In endocytosis, (enSdo-si-tocsis) molecules or other particles that are too large to enter a cell by diffusion or active transport are conveyed within a vesicle that forms from a section of the cell membrane. In exocy-tosis, (ex-o-si-tocsis) the reverse process secretes a substance stored in a vesicle from the cell.

The three forms of endocytosis are pinocytosis, phagocytosis, and receptor-mediated endocytosis. In pinocytosis, (pic-no-si-tocsis) cells take in tiny droplets of liquid from their surroundings (fig. 3.29). When this happens, a small portion of cell membrane indents (invagi-nates). The open end of the tubelike part thus formed seals off and produces a small vesicle about 0.1 ^m in diameter. This tiny sac detaches from the surface and moves into the cytoplasm.

For a time, the vesicular membrane, which was part of the cell membrane, separates its contents from the rest of the cell; however, the membrane eventually breaks down and the liquid inside becomes part of the cytoplasm. In this way, a cell is able to take in water and the particles dissolved in it, such as proteins, that otherwise might be too large to enter.

Phagocytosis (fag<3o-si-tocsis) is similar to pinocyto-sis, but the cell takes in solids rather than liquid. Certain kinds of cells, including some white blood cells, are called phagocytes because they can take in solid particles such as bacteria and cellular debris. When a phagocyte first encounters such a particle, the particle attaches to the cell membrane. This stimulates a portion of the membrane to project outward, surround the particle, and slowly draw it inside the cell. The part of the

Shier-Butler-Lewis: I I. Levels of Organization I 3. Cells I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

Cell membrane

Fluid

Cell membrane

Cytoplasm

Nucleolus

Fluid

Nucleolus

Cytoplasm

Figure 3.29

A cell may take in a tiny droplet of fluid from its surroundings by pinocytosis.

Fluid-filled vesicle

Fluid-filled vesicle

Cell membrane

Particle

Vesicle

Cell membrane

Particle

Nucleolus Nucleus

Phagocytized particle

Nucleolus Nucleus

Figure 3.30

A cell may take in a solid particle from its surroundings by phagocytosis.

Vesicle

Vesicle

Phagocytized particle

Nucleolus

Nucleus

Phagocytized particle

Digestive products

Digestive products

Residue

Nucleolus

Nucleus

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