cell

 Use of this content is subject to the Terms and Conditions of the Evolve web site.

(From Applegate: The anatomy and physiology learning system, ed 4, St Louis, 2011, Saunders.) 600 492 600 492 79 65

Prokaryotic cells lack a defined nucleus and do not contain membrane-bound organelles. Eukaryotic cells have a membrane-enclosed nucleus and a series of membrane-bound organelles that carry out the functions of the cell as directed by the genetic information contained in the nucleus. In other words, prokaryotic cells do not have membrane-bound organelles, whereas eukaryotic cells do. The eukaryotic cell is the more complex of the two cell types.

There are several different organelles functioning in a cell at a given time; only the major ones are considered here.

Nucleus

The first of the organelles is the nucleus, which contains the DNA of the cell in organized masses called chromosomes. Chromosomes contain all of the genetic information for the regeneration (repair and replication) of the cell, as well as all instructions for the function of the cell. Every organism has a characteristic number of chromosomes specific to the particular species.

Ribosomes

Ribosomes are organelles that read the RNA produced in the nucleus and translate the genetic instructions to produce proteins. Cells with a high rate of protein synthesis generally have a large number of ribosomes. Ribosomes can be found in two locations. Bound ribosomes are those found attached to the endoplasmic reticulum (ER), and free ribosomes are those found in the cytoplasm. The two types are interchangeable and have identical structures, although they have slightly different roles.

Endoplasmic Reticulum

The ER is a membranous organelle found attached to the nuclear membrane and consists of two continuous parts. Through an electron microscope, it is clear that part of the membranous system is covered with ribosomes. This section of the ER is referred to as rough ER, and it is responsible for protein synthesis and membrane production. The other section of the ER lacks ribosomes and is referred to as smooth ER. It functions in the detoxification and metabolism of multiple molecules.

Golgi Apparatus

Inside the cell is a packaging, processing, and shipping organelle that is called the Golgi apparatus. The Golgi apparatus transports proteins from the ER throughout the cell.

Lysosomes

Intracellular digestion takes place in lysosomes. Packed with hydrolytic enzymes, the lysosomes can hydrolyze proteins, fats, sugars, and nucleic acids. Lysosomes normally contain an acidic environment (around pH 4.5).

Vacuoles

Vacuoles are membrane-enclosed structures that have various functions, depending on cell type. Many cells, through a process called phagocytosis, uptake food through the cell membrane, creating a food vacuole. Plant cells have a central vacuole that functions as storage, waste disposal, protection, and hydrolysis.

Mitochondria and Chloroplasts

There are two distinct organelles that produce cell energy: the mitochondrion and the chloroplast. Mitochondria are found in most eukaryotic cells and are the site of cellular respiration. Chloroplasts are found in plants and are the site of photosynthesis.

Cellular Membrane

The cellular membrane is the most important component of the cell, contributing to protection, communication, and the passage of substances into and out of the cell. The cell membrane itself consists of a bilayer of phospholipids with proteins, cholesterol, and glycoproteins peppered throughout. Because phospholipids are amphipathic molecules, this bilayer creates a hydrophobic region between the two layers of lipids, making it selectively permeable. Many of the proteins, which pass completely through the membrane, act as transport highways for molecular movement into and out of the cell. Figure 5-4 illustrates the structure of the cellular membrane.

FIGURE 5-4

(From Patton KT, Thibodeau GA: Anatomy and physiology, ed 9, St Louis, 2016, Mosby.) 600 471 600 471 79 62

Generalized cell.

The plasma membrane is made of a bilayer of phospholipid molecules arranged with their nonpolar “tails” pointing toward each other. Cholesterol molecules help stabilize the flexible bilayer structure to prevent breakage. Protein molecules and protein-hybrid molecules may be found on the outer or inner surface of the bilayer—or extending all the way through the membrane.

The Cell The cell is the fundamental unit of biology. There are two types of cells: prokaryotic and eukaryotic cells. Cells consist of many components, most of which are referred to as organelles. Figure 5-3 illustrates a typical cell. FIGURE 5-3

Copyright © 2026 Elsevier Inc., its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.