Plasma Membrane and Associated Structures

The cell is the basic unit of structure and function in the body. Many of the functions of cells are performed by particular subcellular structures known as organelles.The plasma (cell) membrane allows selective communication between the intracellular and extracellular compartments and aids cellular movement.

Chapter Three

Cells look so small and simple when viewed with the ordinary (light) microscope that it is difficult to think of each one as a living entity unto itself. Equally amazing is the fact that the physiology of our organs and systems derives from the complex functions of the cells of which they are composed. Complexity of function demands complexity of structure, even at the subcel-lular level.

As the basic functional unit of the body, each cell is a highly organized molecular factory. Cells come in a wide variety of shapes and sizes. This great diversity, which is also apparent in the subcellular structures within different cells, reflects the diversity of function of different cells in the body. All cells, however, share certain characteristics; for example, they are all surrounded by a plasma membrane, and most of them possess the structures listed in table 3.1. Thus, although no single cell can be considered "typical," the general structure of cells can be indicated by a single illustration (fig. 3.1).

For descriptive purposes, a cell can be divided into three principal parts:

1. Plasma (cell) membrane. The selectively permeable plasma membrane surrounds the cell, gives it form, and separates the cell's internal structures from the extracellular environment. The plasma membrane also participates in intercellular communication.

Table 3.1 Cellular Components: Structure and Function

Component

Structure

Function

Plasma (cell) membrane

Membrane composed of double layer of phospholipids in

Gives form to cell and controls passage of materials into and

which proteins are embedded

out of cell

Cytoplasm

Fluid, jellylike substance between the cell membrane and

Serves as matrix substance in which chemical reactions

the nucleus in which organelles are suspended

occur

Endoplasmic reticulum

System of interconnected membrane-forming canals and

Agranular (smooth) endoplasmic reticulum metabolizes

tubules

nonpolar compounds and stores Ca2+ in striated muscle cells, granular (rough) endoplasmic reticulum assists in protein synthesis

Ribosomes

Granular particles composed of protein and RNA

Synthesize proteins

Golgi complex

Cluster of flattened membranous sacs

Synthesizes carbohydrates and packages molecules for secretion, secretes lipids and glycoproteins

Mitochondria

Membranous sacs with folded inner partitions

Release energy from food molecules and transform energy

into usable ATP

Lysosomes

Membranous sacs

Digest foreign molecules and worn and damaged organelles

Peroxisomes

Spherical membranous vesicles

Contain enzymes that detoxify harmful molecules and break down hydrogen peroxide

Centrosome

Nonmembranous mass of two rodlike centrioles

Helps to organize spindle fibers and distribute chromosomes during mitosis

Vacuoles

Membranous sacs

Store and release various substances within the cytoplasm

Microfilaments and microtubules

Thin, hollow tubes

Support cytoplasm and transport materials within the cytoplasm

Cilia and flagella

Minute cytoplasmic projections that extend from the cell surface

Move particles along cell surface or move the cell

Nuclear envelope

Double-layered membrane that surrounds the nucleus,

Supports nucleus and controls passage of materials between

composed of protein and lipid molecules

nucleus and cytoplasm

Nucleolus

Dense nonmembranous mass composed of protein and RNA molecules

Produces ribosomal RNA for ribosomes

Chromatin

Fibrous strands composed of protein and DNA

Contains genetic code that determines which proteins (including enzymes) will be manufactured by the cell

Cell Structure and Genetic Control

2. Cytoplasm and organelles. The cytoplasm is the aqueous content of a cell inside the cell membrane but outside the nucleus. Organelles (excluding the nucleus) are subcellular structures within the cytoplasm that perform specific functions. The term cytosol is frequently used to describe the fluid portion of the cytoplasm; that is, the part that cannot be removed by centrifugation.

3. Nucleus. The nucleus is a large, generally spheroid body within a cell. The largest of the organelles, it contains the DNA, or genetic material, of the cell and thus directs the cell's activities. The nucleus also contains one or more nucleoli. Nucleoli are centers for the production of ribosomes, which are the sites of protein synthesis.

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Responses

  • rufino
    Which structures are associated with the membrane?
    6 years ago

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