Microbial cell structure and functions

© 2015 Pearson Education, Inc.

Microbial cell structure and functions

Course_ Gneral Microbiology

Lec. no_ 2nd lecture

2nd Grade – Fall Semester 2021-2022

Instructor´s name_ Heshu Jalal Instructor´s email_ Heshu.jalal @tiu.edu.iq


II. Cells of Bacteria and Archaea

• 2.5 Cell Morphology

• 2.6 Cell Size and the Significance of Being Small


2.5 Cell Morphology

• Morphology = cell shape

• Major cell morphologies (Figure 2.11)

• Coccus (pl. cocci): spherical or ovoid

• Rod: cylindrical shape (bacillus/bacilli)

• Spirillum: spiral shape

• Cells with unusual shapes

• Spirochetes, appendaged bacteria, and filamentous

bacteria

• Many variations on basic morphological types

© 2015 Pearson Education, Inc. Figure 2.11

Coccus

Rod

Spirillum

Spirochete

Stalk Hypha

Filamentous bacteria

Budding and appendaged bacteria


2.6 Cell Size and the Significance of Being

Small

• Surface-to-volume ratios, growth rates, and

evolution

• Advantages to being small (Figure 2.13)

• Small cells have more surface area relative to cell volume

than large cells (i.e., higher S/V)

• Support greater nutrient exchange per unit cell

volume

• Tend to grow faster than larger cells


III. The Cytoplasmic Membrane and Transport

(aka Plasma Membrane)

• 2.7 Membrane Structure

• 2.8 Membrane Functions

• 2.9 Nutrient Transport


2.7 Membrane Structure

• Cytoplasmic membrane

• Thin structure that surrounds the cell

• Vital barrier that separates cytoplasm from environment

• Highly selective permeable barrier; enables

concentration of specific metabolites and excretion of

waste products



• Composition of membranes

• General structure is phospholipid bilayer (Figure 2.14)

• Contain both hydrophobic and hydrophilic components

• Fatty acids point inward to form hydrophobic

environment; hydrophilic portions remain exposed to

external environment or the cytoplasm


Fatty acids

Phosphate

Ethanolamine

Fatty acids

Hydrophilic

region

Hydrophobic

region

Hydrophilic

region

Glycerophosphates

Fatty acids

Glycerol



• Cytoplasmic membrane (Figure 2.15)

• Embedded proteins

• Stabilized by hydrogen bonds and hydrophobic

interactions

• Mg2+ and Ca2+ help stabilize membrane by forming ionic

bonds with negative charges on the phospholipids


Phospholipids

6–8 nm

Integral

membrane

proteins

Hydrophilic

groups

Hydrophobic

groups

Phospholipid

molecule

Out

In



• Membrane proteins

• Outer surface of cytoplasmic membrane can interact with a

variety of proteins that bind substrates or process large

molecules for transport

• Inner surface of cytoplasmic membrane interacts with

proteins involved in energy-yielding reactions and other

important cellular functions

• Integral membrane proteins

• Firmly embedded in the membrane

• Peripheral membrane proteins

• One portion anchored in the membrane



• Archaeal membranes

• Ether linkages in phospholipids of Archaea

• Bacteria and Eukarya that have ester linkages in

phospholipids


2.8 Membrane Function

• Permeability barrier (Figure 2.18)

• Polar and charged molecules must be transported

• Transport proteins accumulate solutes against the

concentration gradient

• Protein anchor

• Holds transport proteins in place

• Energy conservation

• Generation of proton motive force


Functions of the cytoplasmic membrane

Permeability barrier:Prevents leakage and functions as agateway for transport of nutrients into,and wastes out of, the cell

Protein anchor:

Site of many proteins that participate in

transport, bioenergetics, and chemotaxis

Energy conservation:

Site of generation and dissipation of the

proton motive force


2.9 Nutrient Transport

• Three major classes of transport systems in

prokaryotes (Figure 2.20)

• Simple transport

• Group translocation

• ABC system

• All require energy in some form, usually proton

motive force or ATP


Out In

R~

2

3

Transported

substance

P

P

ATP ADP + Pi

Simple transport:

Driven by the energy

in the proton motive

Force (H+ gradient)

Group translocation:

Chemical modification

of the transported

substance driven by

phosphoenolpyruvate

ABC transporter:

Binding

proteins are involved

and energy comes

from ATP.

1


2.9 Nutrient Transport

• Three transport events are possible: uniport,

symport, and antiport (Figure 2.21)

• Uniporters transport in one direction across the

membrane

• Symporters function as co-transporters

• Antiporters transport a molecule across the membrane

while simultaneously transporting another molecule in

the opposite direction


IV. Cell Walls of Bacteria and Archaea

• 2.10 Peptidoglycan

• 2.11 LPS: The Outer Membrane

• 2.12 Archaeal Cell Walls


2.10 Peptidoglycan-the Wonder Wall

• Species of Bacteria separated into two groups

based on Gram stain reaction

• Gram-positives and gram-negatives have different

cell wall structure (Figure 2.24)

• Gram-negative cell wall

• Two parts: LPS and peptidoglycan

• Gram-positive cell wall

• One part: peptidoglycan


2.10 Peptidoglycan

• Gram-positive cell walls (Figure 2.27)

• Can contain up to 90% peptidoglycan

• Common to have teichoic acids (acidic substances)

embedded in their cell wall

• Lipoteichoic acids: teichoic acids covalently bound to

membrane lipids


Peptidoglycan

cable

Teichoic acid Peptidoglycan Lipoteichoic

acid

Cytoplasmic membrane

Wall-associated

proteinTeichoic acid

and

Lipoteichoic

acid-

Structural-

linkers and

stiffeners

Wall proteins

may be

enzymes,

receptors or

attachment

molecules


2.10 Peptidoglycan

• Prokaryotes that lack cell walls

Mycoplasmas

• Group of pathogenic bacteria

Species of Archaea


2.11 LPS: The Outer Membrane

• Total cell wall contains ~10% peptidoglycan

• Most of cell wall composed of outer membrane,

aka lipopolysaccharide (LPS) layer

• LPS replaces most of phospholipids in outer half of outer

membrane (Figure 2.29)

• Endotoxin: the toxic component of LPS


O-specific

polysaccharideCore polysaccharide

Lipid A Protein

Lipopolysaccharide

(LPS)

Phospholipid

Porin

Lipoprotein

8 nm

Out

Cell Wall or

Envelope

Outer

membrane

Periplasm

Cytoplasmic

membrane

Peptidoglycan

In

Figure 2.29


2.11 LPS: The Outer Membrane

• Periplasm: space located between cytoplasmic

and outer membranes (Figure 2.29)

• Contents have gel-like consistency

• Houses many proteins

• Porins: channels for movement of hydrophilic

low-molecular-weight substances


2.12 Archaeal Cell Walls

• Some Archaea have cell walls, some do not

• When present, walls are different from Bacterial

cell walls



• S-Layers

• Most common cell wall type among Archaea but may

also be found in Bacteria

• Consist of protein or glycoprotein

• Network of proteins/glycoproteins attached to outer

surface of plasma membrane

• Has openings or “pores”



• No peptidoglycan (no Wonder Wall)

• Typically no outer membrane

• Pseudomurein aka pseudopeptidoglycan

• Polysaccharide similar to peptidoglycan


V. Other Cell Surface Structures and Inclusions

• 2.13 Cell Surface Structures

• 2.14 Gas Vesicles

• 2.15 Endospores


2.13 Cell Surface Structures

• Capsules and slime layers

• Polysaccharide layers (Figure 2.32)

• May be thick or thin, rigid or flexible

• Assist in attachment to surfaces

• Protect against phagocytosis

• External to cell

• In Gram + and –

• Not part of LPS outer membrane


Cell Capsule


2.13 Cell Surface Structures• Fimbriae

• Filamentous protein (curlin, adhesin) structures for

attachment

• Enable organisms to stick to surfaces


2.13 Cell Surface Structures

• Pili

• Filamentous protein (pilin) structures

• Typically longer than fimbriae

• Facilitate genetic exchange between cells (conjugation)

• Some types are involved in motility.


Virus-

covered

pilus


2.14 Gas Vesicles

• Gas vesicles

• Confer buoyancy in

planktonic cells

• Spindle-shaped, gas-filled

structures made of protein

• Impermeable to water


2.16 Endospores

• Endospores

• Highly differentiated cells resistant to heat, harsh

chemicals, and radiation

• “Dormant” stage of bacterial life cycle.

• may persist for decades in environment (soil)

• Ideal for dispersal via wind, water, or animal gut

• Present only in some gram-positive bacteria


VI. Microbial Locomotion

• 2.17 Flagella and Swimming Motility

• 2.18 Gliding Motility

• 2.19 Chemotaxis and Other Taxes


2.17 Flagella and Swimming Motility

• Flagella: structures that assists in swimming

• Different arrangements: (a) peritrichous, (b) polar,

(c) lophotrichous

• Helical organization

• Very different from a eukaryotic flagellum


Peritrichous Polar Lophotrichous


2.17 Flagella and Swimming Motility

• Flagellar structure of Bacteria

• Consists of several components (Figure 2.51)

• Filament composed of flagellin

• Move by rotation

• Flagellar structure of Archaea

• Half the diameter of bacterial flagella

• Composed of several different proteins

• Move by rotation


15–20 nm

Filament

Flagellin

L

P

MS

HookOuter

membrane

(LPS)

L

Ring

P

Ring

Rod

PeptidoglycanPeriplasm

MS Ring

Basal

body

C Ring

Cytoplasmic

membrane

Fli proteins

(motor switch)

Mot proteinMot protein

45 nm

Rod

MS Ring

C Ring

Mot

protein

+ + + + + + + +

− − − − − − − −


2.18 Gliding Motility

• Gliding motility

• Flagella-independent motility (Figure 2.56)

• Slower and smoother than swimming

• Movement typically occurs along long axis of cell

• Requires surface contact

• Gliding proteins


2.19 Chemotaxis and Other Taxes

• Taxis: directed movement in response to chemical

or physical gradients

• Chemotaxis: response to chemicals

• Phototaxis: response to light

• Aerotaxis: response to oxygen

• Osmotaxis: response to ionic strength

• Hydrotaxis: response to water

Microbial cell structure and functions

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