Anatomy & Physiology

Anatomy & Physiology Study Guide

Microvilli Many cells have small, finger-shaped projections of the plasma membrane on their exposed surfaces. These projections, called microvilli, greatly increase the surface area of the cell exposed to the extracellular environment. Centrioles All animal cells capable of undergoing cell division contain a pair of centrioles. These are cylindrical structures composed of short microtubules. The microtubules form nine groups with three microtubules in each group. Each of these nine “triplets” is connected to its nearest neighbors on either side. The centrioles form the spindle apparatus associated with the movement of DNA strands during cell division. The centrosome contains these centrioles that facilitate cell division. Cilia Cilia are relatively long, slender extensions of the plasma membrane. They are found on cells lining the respiratory tract, on cells lining the reproductive tract, and at various other locations in the body. Cilia are important because they can “beat” rhythmically to move fluids or secretions across the cell surface. Ribosomes Proteins are produced within cells, using information provided by the DNA of the nucleus. The organelles responsible for protein synthesis are called ribosomes. A functional ribosome consists of two subunits that are normally separate and distinct. One is a small ribosomal subunit and the other a large ribosomal subunit. These subunits contain special proteins and ribosomal RNA. Before protein synthesis can begin, a small and a large ribosomal subunit must join with a strand of messenger RNA. The Endoplasmic Reticulum The endoplasmic reticulum, or ER, is a network of intracellular membranes connected to the nuclear envelope, which surrounds the nucleus. The name endoplasmic reticulum is very descriptive. Endo- means within, -plasm refers to the cytoplasm, and a reticulum is a network. The ER has four major functions: (1) synthesize proteins, carbohydrates, and lipids, (2) store synthesized molecules or materials absorbed from the cytosol, (3) facilitate travel for materials from place to place, and (4) absorb drugs or toxins and neutralize themwith enzymes. The ER forms hollow tubes, flattened sheets, and chambers called cisternae. Two types of ER exist: smooth endoplasmic reticulum (those which do not contain ribosomes) and rough endoplasmic reticulum (those which do contain ribosomes). The Golgi Apparatus When a transport vesicle carries a newly synthesized protein or glycoprotein that is destined for export from the cell, it travels from the ER to an organelle that looks like a stack of plates. This organelle, the Golgi apparatus, typically consists of five or six flattened membranous discs called cisternae. The Golgi apparatus has three major functions: (1) modify and package secretions, such as enzymes, for release through exocytosis, (2) renew or modify the plasma membrane, and (3) package special enzymes within vesicles for use in the cytosol.

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