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4 Eucaryotic Cell Structure and Function

 

CHAPTER OVERVIEW

This chapter focuses on eucaryotic cell structure and function. Although procaryotic organisms are immensely important in microbiology, eucaryotic microorganisms-such as fungi, algae, and protozoa-are also prominent members of many ecosystems, and some have medical significance as etiological agents of disease as well. The chapter concludes with a comparison of eucaryotic and procaryotic cells.

 

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o discuss the various elements of the cytoskeleton (microfilaments, intermediate filaments, microtubules) with regard to their structure and various functions within the cell

o discuss the composition, structure, and function of each of the internal organelles, such as the endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, mitochondria, chloroplasts, nucleus, and nucleolus

o discuss the mechanism of endocytosis and the difference between phagocytosis and pinocytosis

o compare mitosis and meiosis

o compare and contrast procaryotes and eucaryotes

 

CHAPTER OUTLINE

I. An Overview of Eucaryotic Cell Structure

A. Eucaryotic cells have membrane-delimited nuclei

B. Eucaryotic cells have membrane-bound organelles that perform specific functions within the cells; this allows simultaneous independent control

C. The large membrane surface area of eucaryotic cells allows greater respiratory and photosynthetic activity

II. The Cytoplasmic Matrix, Microfilaments, Intermediate Filaments, and Microtubules

A. The cytoplasmic matrix, although superficially featureless, provides the complex environment required for many cellular activities

B. Microfilaments (4 to 7 nm) may be scattered throughout the matrix or organized into networks and parallel arrays; they play a major role in cell motion and cell shape changes

C. Microtubules are hollow cylinders (25 nm) that help maintain cell shape, that are involved (with microfilaments) in cellular movement, and that also participate in intracellular transport of substances

D. Microtubules also form the mitotic spindle during cell division and are present in cilia and flagella

E. Intermediate filaments (8 to 10 nm) are major components of the cytoskeleton, an intricate network of interconnected filaments that helps maintain cell shape and contributes to cellular movement

III. The Endoplasmic Reticulum (ER)-a complex set of internal membranes that may have ribosomes attached (rough or granular endoplasmic reticulum; RER or GER), or that may be devoid of ribosomes (smooth or agranular endoplasmic reticulum; SER or AER)

A. The ER transports proteins, lipids, and other materials within the cell

B. The ER is a major site of cell membrane synthesis

C. Lipids and many proteins are synthesized by ER-associated enzymes and ribosomes

D. New ER is produced through expansion of old ER

IV. The Golgi Apparatus-a set of membrane sacs (cisternae) that is involved in the modification, packaging, and secretion of materials; they exist in stacks called dictyosomes

A. Materials move to the cis (forming) face of the Golgi apparatus from the ER

B. These materials are then transported from the cis to the trans (maturing) cisternae by vesicles that bud off from the cisternal edges and move to the next sac

C. As the materials move through the Golgi apparatus, they undergo further modification

D. Finally, they bud off from the trans face for transport to their final destination

V. Lysosomes and Endocytosis

A. Lysosomes are membrane-bound vesicles that contain enzymes needed for intracellular digestion of all types of macromolecules

1. Primary lysosomes are newly formed lysosomes

2. Secondary lysosomes (food vacuoles) result from the fusion of primary lysosomes with phagocytic vesicles (endosomes)

3. Residual bodies are lysosomes that have accumulated large quantities of indigestible material

B. Endocytosis is the process in which the cell takes up solutes or particles by enclosing them in vesicles (endosomes) pinched off from the plasma membrane

1. Phagocytosis-endocytosis of large particles by engulfing them into a phagocytic vacuole

2. Pinocytosis-endocytosis of small amounts of liquid with its solute molecules

C. Lysosomes fuse with endosomes in order to digest the materials within the endosome

D. Lysosomes join with phagosomes for defensive purposes as well as to acquire nutrients

E. Autophagic vacuoles are lysosomes that selectively digest portions of the cell's own cytoplasm as part of the normal turnover of cellular components

VI. Eucaryotic Ribosomes-generally larger and more complex than procaryotic ribosomes; however, like their procaryotic counterparts, they are responsible for the synthesis of cellular proteins

A. Ribosomes may be attached to the ER or they may be free

B. ER-associated ribosomes synthesize integral membrane proteins or proteins that are secreted out of the cell

C. Free ribosomes synthesize nonsecretory, nonmembrane proteins

D. Molecular chaperones aid the proper folding of proteins after synthesis and also assist the transport of proteins into eucaryotic organelles such as mitochondria

VII. Mitochondria-the site of tricarboxylic acid cycle activity and the generation of ATP by electron transport and oxidative phosphorylation

A. Mitochondria have both an inner membrane and an outer membrane enclosing a fluid matrix

B. The inner and outer membrane have different lipids and enzymes

C. The enzymes of the tricarboxylic acid cycle and the b-oxidation pathway for fatty acids are located within the matrix

D. Electron transport and oxidative phosphorylation occur only on the inner mitochondrial membrane

E. Mitochondria use their own DNA and their own ribosomes to synthesize some of their proteins

VIII. Chloroplasts-the site of both the light and the dark reactions of photosynthesis

A. Chloroplasts have an outer membrane and an inner membrane system of flattened sacs called thylakoids that often form stacks known as grana; the fluid matrix compartment is called the stroma

B. The formation of carbohydrate from carbon dioxide and water (dark reaction) occurs in the stroma

C. The trapping of light energy to generate ATP, NADPH, and oxygen (light reaction) occurs in the thylakoid membranes of the grana

IX. The Nucleus and Cell Division

A. Nuclei are membrane-bound structures that house the chromatin (genetic material) of the cell

1. Euchromatin is loosely organized and genetically active

2. Heterochromatin is tightly coiled and contains dormant genes

B. The nuclear envelope is a double-membrane structure penetrated by nuclear pores that allow materials to be transported into or out of the nucleus

C. The nuclear lamina, a network of intermediate filaments, lies against the inner surface of the nuclear envelope and supports it

D. The nucleolus is a highly active region of the chromatin involved in the synthesis of ribosomes

E. Mitosis is a process of nuclear division in which the (duplicated) genetic material is distributed equally to two daughter nuclei so that each has a full set of chromosomes and genes

F. Meiosis is a complex, two-stage process of nuclear division in which the number of chromosomes in the resulting daughter cells is reduced from the normal (diploid) number to one-half of that number (haploid)

G. Cytokinesis is the process by which the cytoplasm and its components are distributed to the new daughter cells; it usually occurs in association with, but independently from, mitosis and/or meiosis

X. External Cell Coverings

A. Some cells have a rigid cell wall

B. Other cells, such as some protozoa, have a pellicle, which is a rigid layer of components just within the plasma membrane

XI. Cilia and Flagella-complex locomotor structures composed of a series of microtubules (the axoneme) located within the matrix of a membrane-bound hairlike appendage; they are very different from procaryotic flagella

XII. Comparison of Procaryotic and Eucaryotic Cells

A. Eucaryotes have a membrane-delimited nucleus and many complex membrane-bound organelles, each of which perform a separate function for the cell

B. Procaryotes lack a membrane-delimited nucleus and internal membrane-bound organelles; they are functionally simpler and do not undergo mitosis, meiosis, endocytosis, and other complex activities performed by many eucaryotes

C. Despite the significant differences between procaryotes and eucaryotes, they have remarkable biochemical similarities: the same basic chemical composition, the same genetic code, and the same basic metabolic processes

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