Betsy T., Keogh J. Microbiology Demystified

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sis. There are two kinds of endocytosis. These are phagocytosis and pinocytosis.

Phagocytosis engulfs solid substances (large molecules) while pinocytosis

engulfs liquid substances (small molecules). Exocytosis is the process that cells

use to remove large substances, which is the way waste products and useful mate-

rial as hormones and neurotransmitters are expelled from a cell through vesicles.

Cytosol and Cytoplasm

The cytosol is the intracellular fluid of a prokaryotic cell that contains proteins,

lipids, enzymes, ions, waste, and small molecules dissolved in water, com-

monly referred to as semifluid. Substances dissolved in cytosol are involved in

cell metabolism.

The cytosol also contains a region called the nucleoid, which is where the

DNA of the cell is located. Unlike human cells, a prokaryotic microorganism has a

single chromosome that isn’t contained within a nuclear membrane or envelope.

Cytosol is located in the cytoplasm of the cell. Cytoplasm also contains the

cytoskeleton, ribosomes, and inclusions.

Ribosomes

A ribosome is an organelle within the cell that synthesizes polypeptide. There

are thousands of ribosomes in the cell. You’ll notice them as the grainy appear-

ance of the cell when viewing the cell with an electron microscope.

A ribosome is comprised of subunits consisting of protein and ribosomal

RNA, which is referred to as rRNA. Ribosomes and their subunits are identified

by their sedimentation rate. Sedimentation rate is the rate at which ribosomes

are drawn to the bottom of a test tube when spun in a centrifuge. Sedimentation

rate is expressed in Svedberg (S) units. A sedimentation rate reflects the mass,

size, and shape of a ribosome and its subunits. It is for this reason why the sed-

imentation rates of subunits of a ribosome do not add up to the ribosome’s

sedimentation rate.

Ribosomes in prokaryotic cells are uniquely identified by the number of pro-

teins and rRNA molecules contained in the ribosome and by sedimentation rate.

Prokaryotic ribosomes are relatively small and less dense than ribosomes of

other microorganisms. For example, bacterial ribosomes have a sedimentation

rate of 70S compared to the 80S sedimentation rate of a eukaryotic ribosome,

which you’ll learn about later in this chapter.

Ribosomes and their subunits are targets for antibiotics that kill a bacterium

by inhibiting the bacterium’s protein synthesis. These antibiotics only kill cells

that have a specific ribosome sedimentation rate. Cells with a different ribosome

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sedimentation rate are unaffected by the antibiotic. This enables the antibiotic to

kill bacterium and not the body that is infected by the bacterium.

For example, erythromycin and chloramphenicol, popular antibiotics, kill

bacteria whose subunits have a sedimentation rate of 50S. Streptomycin and

gentamycin affect bacteria whose subunits have a 30S sedimentation rate.

Inclusions

An inclusion is a storage area that serves as a reserve for lipids, nitrogen, phos-

phate, starch, and sulfur within the cytoplasm. Scientists use inclusions to iden-

tify types of bacteria. Inclusions are usually classified as granules.

•

Granule inclusion. Membrane-free and densely packed, this type of inclu-

sion has many granules each containing specific substances. For example,

polyphosphate granules, also known by the names metachromatic gran-

ules and volutin, have granules of polyphosphate that are used to synthe-

size ATP and are involved in other metabolic processes. A polyphosphate

granule appears red under a microscope when stained with methylene blue.

•

Vesicle inclusion. This is a protein membrane inclusion commonly found

in aquatic photosynthetic bacteria and cyanobacteria such as phytoplank-

ton, which suspends freely in water. These bacteria use vesicle inclusions

to store gas that give the cell buoyancy to float at a depth where light, car-

bon dioxide (CO

), and nutrients—all required for photosynthesis—are

available.

Eukaryotic Cells

A eukaryotic cell (Fig. 4-6) is larger and more complex than a prokaryotic cell

and found in animals, plants, algae, fungi, and protozoa. When you look at a

eukaryotic cell with a microscope you’ll notice a highly organized structure of

organelles that are bound by a membrane. Each organelle performs a specialized

function for the cell’s metabolism. Eukaryotic cells also contain a membrane-

bound nucleus where the cell’s DNA is organized into chromosomes.

Depending on the organism, a eukaryotic cell may contain external projec-

tions called flagella and cilia. These projections are used for moving substances

along the cell’s surface or for moving the entire cell. Flagella move the cell in a

wavelike motion within its environment. Cilia move substances along the cell’s

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surface and also aid in movement of the cell. Flagella and cilia are comprised of

axoneme microtubules. An axoneme microtubule is a long, hollow tube made

of protein called a tubulin.

CELL WALL

Many eukaryotic cells have a cell wall. The composition of the cell wall differs

with each organism. For example, the cell walls of many fungi are composed of

chitin cellulose. Chitin is a polysaccharide, which is a polymer of N-acetylglu-

cosamine (NAG) units. The cell wall of other fungi is made of cellulose, which

is also a polysaccharide. Cellulose is also found in the cell wall of plants and

many algae. Yeast has a cell wall composed of glucan and mannan, which are

two polysaccharides.

In contrast, protozoa have no cell wall and instead have a pellicle. A pellicle

is a flexible, proteinaceous covering. Eukaryotic cells of other organisms (such

as animals) that lack a cell wall have an outer plasma membrane that serves as

an outside cover for the cell. The outer plasma membrane has a sticky carbo-

CHAPTER 4 Prokaryotic and Eukaryotic Cells

RER

SER

Fig. 4-6. A eukaryotic cell.

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hydrate called glycocalyx on its surface. Glycocalyx is made up of covalently

bonded lipids and proteins in order to form glycolipid and glycoprotein in the

plasma membrane. Glycolipid and glycoprotein anchor the glycocalyx to the cell,

giving the cell strength and helping the cell to adhere to other cells. Glycocalyx

is also a molecular signature used to identify the cell to other cells. White blood

cells use this to identify a foreign cell before destroying it.

A eukaryotic cell lacks peptidoglycan, which is critical in fighting bacteria

with antibiotics. A bacterium is a prokaryotic cell. Peptidoglycan is the frame-

work of a prokaryotic cell’s cell wall. Antibiotics such as penicillin attack

peptidoglycan resulting in the destruction of the cell wall of a bacterium.

Eukaryotic cells invaded by the bacterium remain unaffected because eukary-

otic cells lack peptidoglycan.

PLASMA MEMBRANE

The plasma membrane is a selectively permeable membrane enclosing the

cytoplasm of a cell. This is the outer layer in animal cells. Other organisms

have a cell wall as the outer layer and the plasma membrane is between the

cell wall and the cell’s cytoplasm. The cell wall is the outer covering of most

bacteria, algae, fungi, and plant cells. In eubacterium, which is a prokaryotic micro-

organism, the cell wall contains peptidoglycan.

The plasma membrane surrounds a eukaryotic cell and serves as a barrier

between the inner cell and its environment. The cytoplasmic membrane is com-

posed of proteins and lipids. Carbohydrates are used to uniquely identify the

cell to other cells. Lipids, known as sterols, help prevent the destruction of the cell

when there is an increase in osmotic pressure and are mainly used for stability.

Lysis is the destruction of a cell. Prokaryotic cytoplasm lacks certain features

that are found in eukaryotic cytoplasm, such as a cytoskeleton. In a eukaryotic

microorganism, the cytoskeleton provides support and shape for cells and helps

transport substances through the cell.

The plasma membrane of a eukaryotic cell functions like the plasma mem-

brane of a prokaryotic cell, which you learned about previously in this chapter.

That is, substances enter and leave the cell through the cytoplasmic membrane

by using simple diffusion, facilitated diffusion, osmosis, and active transport.

Eukaryotic cells extend parts or sections of plasma membrane. The exten-

sions of the plasma membrane are called pseudopods. The word pseudopod

means “false foot,” and these “feet” enable the cell to have amoeboid motion.

An amoeboid motion consists of muscle-like contractions that move the cell over

a surface. Pseudopods are used to engulf substances and bring them into the cell,

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which is called endocytosis (a type of active transport). There are two types of

endocytosis. These are phagocytosis (eat) and pinocytosis (drink). In phago-

cytosis, solid particles are engulfed by the cell. An example is when a white

blood cell engulfs and destroys a bacteria cell. In pinocytosis, liquid particles are

brought into the cell. An example is when extracellular fluid containing a sub-

stance is destroyed by the cell.

CYTOPLASM AND NUCLEUS

The cytoplasm of a eukaryotic cell contains cytosol, organelles, and inclusions,

which is similar to the cytoplasm of the prokaryotic cell. Eukaryotic cytoplasm

also contains a cytoskeleton that gives structure and shape to the cell and assists

in transporting substances throughout the cell.

The nucleus of a eukarytoic cell contains DNA (hereditary information) and

is contained within a nuclear envelope. DNA is also found in the mitochondria

and chloroplasts. Depending on the organism, there can be one or more nucleoli

within the nuclear envelope. A nucleolus (little nucleus) is the site of ribosomal

RNA synthesis, which is necessary for ribosomes to function properly.

In the nucleus, the cell’s DNA is combined to form several proteins called

histones. The combination of about 165 pairs of DNA and nine molecular of his-

tones make up the nucleosome. When a eukaryotic cell is not in the reproduc-

tion phase, the DNA and its proteins look like a threaded mass called chromatin.

When the cell goes through nuclear division, the strands of chromatin condense

and coil together, producing rod-shaped bodies called chromosomes.

A eukaryotic cell uses a method of cell division during reproduction called

mitosis. This is the formation of two daughter cells from a parent cell.

ENDOPLASMIC RETICULUM

The endoplasmic reticulum contributes to the mechanical support and distribu-

tion of the cytoplasm and is the pathway for transporting lipids and proteins

throughout the cell. The endoplasmic reticulum also provides the surface area

for the chemical reaction that synthesizes lipids, it stores lipids and proteins until

the cell needs them.

The endoplasmic reticulum consists of cisterns, which are a network of flat-

tened membranous sacs. The end of these cisterns can be pinched off to become

membrane-enclosed sacs called secretory vesicles. Vesicles transport synthe-

sized material in the cell.

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There are two kinds of endoplasmic reticula.

•

Rough endoplasmic reticulum. Covered by ribosomes, which are the sites

for synthesizing protein.

•

Smooth endoplasmic reticulum. Not covered by ribosomes, this is the site

for synthesizing lipids.

GOLGI COMPLEX

The Golgi complex is considered the “Fedex System” of the cell because it

packages and delivers proteins, lipids, and enzymes throughout the cell and to

the environment. The Golgi complex contains cisterns stacked on top of each

other. A cistern is a sac or vessel and is filled with proteins or lipids (packaged),

detached from the Golgi complex, and transported to another part of the cell.

LYSOSOME

A lysosome is a sphere in animal cells that is formed by, but is separate from, the

Golgi complex, it contains enzymes used to digest molecules that have entered

the cell. Think of lysosomes as the digestive system of the cell. For example,

lysosomes in a white blood cell digest bacteria that is ingested by the cell dur-

ing phagocytosis.

MITOCHONDRION

The mitochondrion is an organelle that is comprised of a series of folds called

cristae that is responsible for the cell’s energy production and cellular respira-

tion. Chemical reactions occur within the center of the mitrochondrion, called

the matrix; it is filled with semifluid in which adenosine triphosphate (ATP) is

produced. ATP is the energy molecule in the cell. The mitochondrion is the

powerhouse of the cell.

CHLOROPLAST

Eukaryotic cells of green plants and algae contain plastids, one of which is chloro-

plast. Chloroplasts are organelles that contain pigments of chlorophyll and

carotenoids used for gathering light and enzymes necessary for photosynthesis.

Photosynthesis is the process that converts light energy into chemical energy. The

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pigment is stored in membranous sacs called thylakoids that are arranged in stacks

called grana.

CENTRIOLE

A centriole is a pair of cylindrical structures near the nucleus that is comprised

of microtubules and aids in the formation of flagella and cilia. The centriole also

has a part in eukaryotic cell division.

Quiz

1. A gram-positive cell wall has

(a) many layers of peptidoglygan, which repels the crystal of violet dye

when the cell is stained

(b) many layers of peptidoglygan, which retains the crystal of violet dye

when the cell is stained

(d) one layer of peptidoglygan, which repels the crystal of violet dye

when the cell is stained

2. A cytoplasmic membrane is

(a) a membrane that provides a selective barrier between the nucleus

and the cell’s internal structures

(b) the cell wall

structures

(d) a membrane that provides a selective barrier between the cell wall and

the cell’s internal structures

3. Amphipathic means

(a) that the cytoplasmic membrane of a cell is bilayered and contains

both polar and nonpolar parts

(b) that the cytoplasmic membrane of a cell is unilayered and contains

polar parts

nonpolar parts

(d) that the cytoplasmic membrane of a cell is resistant to all substances

residing outside the cell

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4. What is the function of the nonpolar part of the cytoplasmic membrane?

(a) The cytoplasmic membrane does not have a nonpolar part.

(b) The nonpolar part of the cytoplasmic membrane prevents extra-

cellular fluid from leaving the cell and intracellular fluid from enter-

ing the cell.

prevents extracellular fluid from entering the cell and intracellular

fluid from exiting the cell.

(d) The nonpolar part of the cytoplasmic membrane is to position organ-

elles within the cell.

5. The function of the transport protein is

(a) to regulate cell division

(b) to regulate the positioning of organelles within the cell

(d) to give a cell its color

6. The function of the channel protein is

(a) to direct movement of a cell through channels in its environment

(b) to channel substances among organelles within the cell

permit the flow of molecules through the cytoplasmic membrane

(d) to form pores (called channels) in the nucleus membrane that permit

the flow of molecules through the nuclei of the cell

7. Passive transport is

(a) the process of moving substances through the cytoplasmic mem-

brane without expending energy by using a concentration gradient

(b) the process of moving substances through the cytoplasmic mem-

brane without expending energy by using a transport protein

brane by expending energy by using a concentration gradient

(d) the process of moving substances through the nucleus membrane

without expending energy by using a concentration gradient

8. What is facilitated diffusion?

(a) A passive transport process in which molecules or ions of a sub-

stance move from a region of lower concentration to a region of

higher concentration without the assistance of an integral protein

(b) A passive transport process in which molecules or ions of a sub-

stance move from a region of lower concentration to a region of

higher concentration with the assistance of an integral protein

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stance move from a region of higher concentration to a region of

lower concentration with the assistance of an integral protein

(d) A passive transport process in which molecules or ions of a sub-

stance move from a region of higher concentration to a region of

lower concentration with the assistance of an integral protein

9. Active transport is

(a) the movement of a substance across the cytoplasmic membrane in

the direction of the gradient by using energy provided by the cell

(b) the formation of pores (called channels) in the cytoplasmic mem-

brane that permit the flow of molecules through the cytoplasmic

membrane

against the gradient by using energy provided by the cell

(d) is a process in which molecules or ions of a substance move from a

region of higher concentration to a region of lower concentration

with the assistance of an integral protein

10. What are the two types of endocytosis?

(a) Facilitated diffusion and passive transport

(b) Phagocytosis and pinocytosis

(d) Nucleoid and cytosol

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