Prescotts Microbiology, 9th Edition

5Eukaryotic Cell StructureCHAPTER OVERVIEW

This chapter focuses on eukaryotic cell structure and function. Although prokaryotic organisms are immensely important in microbiology, eukaryotic microorganismssuch as fungi, algae, and protozoaare also prominent members of many ecosystems, and some have medical significance as etiological agents of disease and as model organisms for research. The detailed discussion of eukaryotic microbial structure and function is followed by a comparison of eukaryotes and prokaryotes. LEARNING OUTCOMESAfter reading this chapter you should be able to:

compare and contrast eukaryotic, bacterial, and archaeal cells in terms of their use of membranes, size, morphological diversity, and organelles identify the types of eukaryotic microbes that have cell walls and distinguish them from plant cell wallscompare and contrast the cell envelopes of members of Bacteria, Archaea and Eukarya in terms of their component layers, molecular makeup, and functiondescribe the functions of the cytoplasm identify the three filaments that make up the cytoskeleton of eukaryotic cells and describe their functions differentiate the two types of endoplasmic reticulum in terms of structure and functionoutline the pathway of molecules through the secretory pathway, noting the structures involved and their

role in the processlist the endocytic pathways observed in mammalian cells, noting the structures involved and their role in the process, and noting those pathways that have been observed in eukaryotic microbes

describe the structure of the nucleus, chromosomes, nucleolus, and eukaryotic ribosomescompare and contrast the chromosomes and ribosomes of bacterial, archaeal, and eukaryotic cells

draw a mitochondrion and identify its components compare and contrast mitochondria and hydrogenosomes in terms of their structure and the chemical processes they carry outdraw a chloroplast and identify its components parts

describe the structure of eukaryotic flagella and cilia compare and contrast bacterial, archaeal, and eukaryotic flagella list the types of motility observed in eukaryotic microbes

create a Venn diagram or concept map that clearly distinguishes bacterial, archaeal, and eukaryotic cells in terms of their genome organization, organelles, cell envelopes, ribosome size and component molecules, and cytoskeletondetermine the type of microbe when given a description of a newly discovered microbe CHAPTER OUTLINE

I. A Typical Eukaryotic CellA. Eukaryotic cells have membrane encased nuclei

B. In addition to the nucleus, eukaryotic cells have membrane-bound organelles that perform specific functions within the cell; this allows simultaneous independent control of cellular processes

C. Eukaryotic cells also contain a large intracytoplasmic membrane complex, which provides a large surface area allowing greater respiratory and photosynthetic activity; this complex of membranes also serves as a transport system to move materials to different cell locations

II. Eukaryotic Cell Envelopes

A. Eukaryotic membranes are phospholipid bilayers with sphingolipids and sterols, but the lipids in the outer layer have been shown to differ from those in the inner layer

B. Certain microdomains in the membranes differ in lipid and protein content to suit specific functions

C. The cell wall composition in eukaryotic microbes varies and can include the polymers cellulose, glucan, or chitin, while others have a rigid silica shell

III. Cytoplasm of Eukaryotes

A. The cytoplasmic matrix provides the complex structured environment required for many cellular activities

B. The cytoskeleton is a vast network of interconnected filaments and tubules important for motion and act as a scaffold for maintaining cell organization

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

D. Intermediate filaments (8 to 10 nm), along with microfilaments and microtubules, are major components of the cytoskeleton, and are particularly prominent in nuclear laminaE. Microtubules are hollow cylinders (25 nm) composed of tubulin that help maintain cell shape, are involved (with microfilaments) in cellular movement, participate in intracellular transport of substances, and participate in organelle movements; they also form the mitotic spindle during cell division and are present in cilia and flagella

IV. Organelles of the Secretory and Endocytic Pathways

A. Endoplasmic reticulum

1. The endoplasmic reticulum (ER) is a complex interconnected array of internal membranous tubes and sacs that may have ribosomes attached (rough endoplasmic reticulum; RER) or may be devoid of ribosomes (smooth endoplasmic reticulum; SER)

2. The ER has many important functions:

a. It transports proteins, lipids, and other materials within the cell

b. ER-associated enzymes and ribosomes synthesize lipids and many proteins

c. It is a major site of membrane synthesisd. Proteins often begin their protein folding and transport sorting here.B. The Golgi apparatus

1. The Golgi apparatus is a set of membranous sacs (cisternae) that is involved in the modification, packaging, and secretion of materials; the cisternae exist in stacks called dictyosomes

2. The Golgi apparatus is present in many eukaryotic cells, but many fungi and ciliated protozoa lack it

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

D. Secretory pathway

1. Proteins destined for the cell membrane, lysosomes, or secretion are recognized by amino acid targeting sequences and are transported in vesicles that bud off the ER and join the cis face of the Golgi apparatus; the proteins are modified and transported in vesicles that bud off of the trans face of the Golgi apparatus

2. Transport vesicles move the material to the cell membrane or lysosome; secretory vesicles hold their contents until signaled to release them through fusion with the plasma membrane

3. Proteasomes are a nonlysosomal protein degradation system found in eukaryotic cells that degrades proteins marked with ubiquitin

E. Endocytic pathway

1. 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

a. Phagocytosisendocytosis of large particles by engulfing them into a phagocytic vacuole (phagosome)

b. Pinocytosisendocytosis of small amounts of liquid with its solute molecules; there are three types of pinocytosis: fluid-phase endocytosis; receptor-mediated endocytosis using clathrin-coated pits and vesicles; and a type of endocytosis that forms special vesicles (caveolae), whose contents are not degraded

2. Most endosomes fuse with early lysosomes (newly formed lysosomes) to form late lysosomes; late lysosomes are important to cell functioning

a. Some function as food vacuoles, digesting and releasing nutrients into the cytoplasm

b. Some function to destroy invading bacteria

c. Autophagosomes fuse lysosomes to selectively digest portions of the cells own cytoplasm as part of the normal turnover of cellular components (macrophagy)

3. Undigested materials accumulate in lysosomes called residual bodies

V. Organelles Involved in Genetic Control of the Cell

A. Nucleus

1. Nuclei are membrane-bound structures that house most of the genetic material of the cell

2. Nuclear structure

a. Chromatin is the dense fibrous material seen within the nucleoplasm of the nucleus; the nucleoplasm is the DNA-containing part of the nucleus; when the cell is dividing, chromatin condenses into visible chromosomesb. DNA is organized into chromatin by basic proteins known as histones which allow higher order packaging.c. The nucleus is bound by a double-membrane (two lipid bilayers) nuclear envelope structure which is penetrated by nuclear pores; the nuclear pores allow materials to be transported into or out of the nucleus

d. The nucleolus is a very noticeable structure within the nucleus; it is involved in the synthesis of rRNA and the production of ribosomes

B. Eukaryotic ribosomes

1. Eukaryotic ribosomes are 80S, with 60S and 40S subunits

2. Eukaryotic ribosomes are responsible for synthesis of cellular proteins; they can either be attached to the ER (rough ER) or are free in the cytoplasm a. ER-associated ribosomes synthesize integral membrane proteins or proteins that are secreted out of the cell

b. Free ribosomes synthesize nonsecretory, nonmembrane proteins

VI. Organelles Involved in Energy Conservation

A. The endosymbiotic hypothesis holds that the three energy-conserving organelles are thought to be derived from bacterial cells engulfed by host cells early in evolutionary history

B. Mitochondria

1. Mitochondria are the site of the energy-conserving tricarboxylic acid cycle and the generation of ATP by electron transport and oxidative phosphorylation

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

a. The inner and outer membranes have different lipids and enzymes

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

c. Electron transport and oxidative phosphorylation occur only in the inner mitochondrial membrane

3. Mitochondria use their own DNA and their own ribosomes to synthesize some of their proteins; mitochondrial DNA and mitochondrial ribosomes are similar to bacterial DNA and ribosomes in terms of size and structure; mitochondria reproduce by binary fission

C. Hydrogenosomes are small organelles in some anaerobic protists that conserve energy by fermentation and generate hydrogen; composed of a double membrane they share some features with mitochondria

D. Chloroplasts

1. Chloroplasts are the site of photosynthesis in protists and higher plants

2. Chloroplasts are surrounded by two membranes; the inner membrane encloses a fluid matrix called the stroma; within the stroma is a system of flattened membrane sacs called thylakoids which often form stacks known as grana

a. The formation of carbohydrates from carbon dioxide and water (dark reactions) occurs in the stroma

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

VII. External Structures

A. Cilia and flagella are locomotor structures that differ in length and how they move the cell with cilia like oars and flagella like undulating propellers

B. Cilia and flagella are structurally similar; both are membrane-bound cylinders composed of microtubules, in a 9+2 arrangement, embedded in a matrix; basal bodies have a 9+0 microtubule pattern

VIII. Comparison Bacterial, Archaeal, and Eukaryotic Cells

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

B. Prokaryotes lack a membrane-delimited nucleus and internal membrane-bound organelles; they do not perform endocytosisC. Non-archaeal prokaryotes have a different ribosome structure that sediments at a slightly different rate.D. Despite the significant differences between prokaryotes and eukaryotes, they have remarkable biochemical similarities: the same basic chemical composition, the same genetic code, and some of the same basic metabolic processes

CRITICAL THINKING

1.Describe how the division of labor associated with internal, membrane-delimited organelles enables eukaryotic cells to be more efficient and thereby allows them to grow much larger than prokaryotes grow.

2.Both prokaryotes and eukaryotes are capable of using large particles as nutrient sources. Eukaryotes bring them into the cell by endocytosis. Prokaryotes cannot do this; they export digestive enzymes to digest the particles externally and then transport the resulting small nutrient molecules into the cell. Why is the prokaryotic process much less efficient? Use diagrams to support your answer.

3. The orderly packaging of eukaryotic DNA into chromatin using histones suggests that this packaging may

influence gene regulation. Comment on how loosening up a chromosome may influence the ability of its

genes to be read and expressed.CONCEPT MAPPING CHALLENGE

Construct a concept map that allows you to describe the molecular makeup and function of eukaryotic cell structures. Use the concepts that follow, any other concepts or terms you need , and your own linking words between each pair of concepts in your map.cell wall plasma membrane cytoplasm chloroplast mitochondrion endoplasmic reticulum

Golgi apparatus lysosome nucleus ribosomes cytoskeletonPAGE 1 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.