Lab assignment #1

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Microbial cell structure and function

Cell Structure and Function

The four processes of life

The four processes of life that describe the characteristics of all living organisms:

Metabolism

Growth

Responsiveness

Reproduction

Prokaryotic and Eukaryotic Cells: An Overview

Prokaryotes

Lack membrane-bound nucleus (nuclear material), a cytoskeleton, membrane-bound organelles, and internal membranous structures.

Have simple structures compared

to eukaryotes

Composed of bacteria and archaea

Are typically small in size (~1.0 μm in

diameter

Typical prokaryotic cell

Different morphologic features of bacterial cells

Cells with unusual shapes
Vibrios – Resemble rods but are comma shaped.
Spirilla – rigid, spiral shaped cells. Usually with tufts of flagella at each end.
Actinomycetes – typically form filamentous structures. They lie between bacteria and filamentous fungi.
Pleomorphic - bacteria with variable in shape.

External Structures of Bacterial Cells

Glycocalyces

Gelatinous, sticky substance surrounding the outside of the cell

Composed of polysaccharides, polypeptides, or both

Two types of external structures: capsule and slime layer

Capsules

Composed of organized repeating units of organic chemicals

Firmly attached to cell surfaces

Protect cells from drying out

May prevent bacteria from being recognized and destroyed by host immune and phagocytic cells

Enable bacteria to cause diseases (capsules are virulence factors)

External Structures of Bacterial Cells

Slime layer

Loosely attached to cell surface

Protects cells from drying out

Sticky layer allows prokaryotes to attach to surfaces

Water soluble

Slime layers have little or no medical importance/significance

External Structures of Bacterial Cells

Flagella structure and function

Long, whip-like structures that extend beyond surface of the cell

Are responsible for movement: 360º rotation of flagellum propels bacterium through environment (run or tumble)

Rotation can be clockwise or counterclockwise and reversible

Prokaryotes move in response to stimuli:

Positive (stimulus) taxis – organisms move towards food or light;

Negative (stimulus taxis – organisms move away from danger

Flagella are not present on all bacteria

External Structures of Bacterial Cells

Flagellar arrangements

Monotrichous: Cells with a single flagellum

Lophotrichous: Cells with a tuft of flagella at one end of the cell

Amphitichous: Cells with flagella at both ends

Peritrichous: Cells covered with flagella

External Structures of Bacterial Cells

Fimbriae

Non-motile, rod-like proteinaceous extensions

on cell surfaces

Sticky, proteinaceous, bristle-like projections

Used by bacteria to adhere to one another,

to hosts, and to substances in environment

(e.g., Neisseria gonorrhoeae adhering on mucus

membranes)

May be hundreds per cell

Are shorter than flagella

Serve an important function in biofilms formations (slimy masses of bacteria adhering to one another and to a substrate by means of fimbriae and glycocalyces)

External Structures of Bacterial Cells

Pili

Long, hollow tubules composed of pilin

Longer than fimbriae but shorter than flagella

Bacteria typically only have one or two per cell

Also known as conjugation (sex) pili

Bacteria use pili to move across a substrate or towards another bacterium

Pili mediate the transfer of DNA from one cell to another: join two bacterial cells and help transfer DNA (conjugation)

External Structures of Bacterial Cells

Bacterial cell walls

Give bacterial cells characteristic shapes

Protects cell from osmotic forces

Assists some cells in attaching to other cells or other surfaces

Most bacteria have cell walls composed of peptidoglycan. A complex polysaccharide material that covers the entire surface of the cell and is composed of alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

Cell walls help in eluding antimicrobial drugs or resisting antimicrobial drugs (certain antibiotics can target cell walls of bacteria, e.g., penicillin attacks cell wall)

A few bacteria lack a cell walls entirely (e.g., Mycoplasma pneumoniae)

Scientists describe two basic types of bacterial cell walls: Gram-positive and Gram-negative

Possible structure of peptidoglycan

External Structures of Bacterial Cells

Gram-positive bacterial cell walls

Relatively thick layer of peptidoglycan

Contains unique polysaccharides called teichoic acids

Some covalently linked to lipids, forming lipoteichoic acids that anchor peptidoglycan to cell membrane

Peptidoglycan retains crystal violet dye and cells appear purple following Gram Staining Procedure

Acid-fast bacteria contain up to 60% mycolic acid, a waxy lipid

Helps cells survive desiccation and resist stain with regular water-based dyes

Gram-positive bacterial cell wall structure

Gram-negative bacterial cell wall structure

Gram-negative bacterial cell walls

Have only a thin layer of peptidoglycan

Have a bilayer membrane (composed of phospholipid bilayers, channel proteins or porins and lipopolysaccharide or endotoxin) outside of peptidoglycan

Lipid portion (called lipid A) - released from dead and disintegrating cell walls may trigger endotoxic shock (fever, vasodilation, hypotension, inflammation and blood clotting in patients)

May be impediment to the treatment of disease

Following Gram staining procedure, cells appear pink

Gram-negative bacterial cell wall structure

Bacterial Cytoplasmic Membranes

Structure of prokaryotic cytoplasmic membrane

Cytoplasmic membrane (also known as cell membrane or plasma membrane) is a phospholipid bilayer composed of lipids and associated proteins

A phospholipid molecule is bipolar (has a hydrophilic and a hydrophobic ends)

Approximately half the cytoplasmic membrane is composed of proteins (integral proteins, peripheral proteins and glycoproteins)

Protein components of cytoplasmic membranes act as recognition proteins, enzymes, receptors, carriers or channels

Proteins and lipids within membranes flow freely (fluid mosaic model or membrane fluidity) and allow easy passage of substance into and out of the cell

Structure of prokaryotic cytoplasmic membrane

Bacterial cytoplasmic membranes

Functions of cytoplasmic membrane

Energy storage

Controls passage of substances into and out of the cell - selectively permeable (allows some substances to cross it, while preventing the crossing of others)

Naturally impermeable to most substances, but proteins (receptors, channels and carriers) allow substances to cross membrane

Membranes maintain a concentration gradient and electrical gradient - chemicals with concentration gradients across membranes have electrical charges and a corresponding electrical gradient

Chemical and electrical gradients collectively are known as electrochemical gradient

Energy found in electrochemical gradient can be used to transport substances across the membrane

Movement of substances across membranes occurs by passive or active processes of

transport

Electrical potential of a cytoplasmic membrane

Bacterial cytoplasmic membranes

Passive processes of transport

Electrochemical gradient provides a source of energy. The cell does not expend its ATP energy reserve for the following three passive processes of transport:

Diffusion

Facilitated diffusion

Osmosis

Diffusion

Net movement of a chemical down its concentration gradient-from an area of high concentration to an area of low concentration

Requires no energy out put by the cell, a common feature of all passive processes

Chemicals that are small or lipid soluble (e.g., oxygen, CO2, alcohol and fatty acids) can diffuse through the lipid portion of the membrane; larger molecules like proteins and glucose cannot – selectively permeable

Bacterial cytoplasmic membranes

Facilitated diffusion

Integral proteins (channels, carriers etc.) facilitate the diffusion of large or electrically charged molecules through phospholipids bilayer of membranes

Cells expend no energy in facilitated diffusion. Electrochemical gradient provides all the necessary energy

Non-specific channel proteins (common in prokaryotes) allow the passage of a wide range of chemicals with the right size or electrical charge

Specific channel proteins (common among eukaryotic cells) carry only specific substrates. These have specific binding site that are selective for one substance

Passive processes of movement

Solutes, solvents and solutions

Concentration of solutes and solutions

Three classes of solutions according to their concentrations of solutes and solvents:

Isotonic solutions: have the same concentration of solutes and water on either sides of selectively permeable membrane; neither side of membrane experience a net loss or gain of water

Hypertonic solution: contains higher concentration of solutes relative to the solvent

Hypotonic solutions: contains lower concentration of solutes in comparison

Hypotonic and hypertonic refer to the concentration of solute, even though osmosis refers to the movement of the solvent, which in cells is water

Osmosis

Diffusion of water across a selectively permeable (not to all solutes such as proteins, salts, amino acids or glucose) membrane

Water crosses from the side of the membrane that contains a higher concentration of water molecules (lower concentration of solute) to the side that contains a lower concentration of water molecules(higher concentration of solute)

When water pressure is at equilibrium, activity of osmosis stops

Like other chemicals, water moves down its concentration gradient from hypotonic solution into a hypertonic solution

The osmotic movement of water out of a cell and shriveling of its cytoplasm is called plasmolysis

Osmosis

Effects of different solutions on cells

Prokaryotic Cytoplasmic Membranes

Active processes of transport

Require the cell to expend energy (ATP) to move materials across cytoplasmic membranes against their electrochemical gradients

Utilizes trans-membrane carrier proteins. When only one substance is transported at a time, the carrier protein is called a uniport

Simultaneous transport of two chemicals, but in opposite directions (one into the cells and the other out of the cell) at the same time is called antiport

When two substance move together in the same direction across the membrane by means of a single carrier protein, the process of transport is termed symport

Active processes of transport in prokaryotes is by means of carrier proteins and a special process called group translocation (where substances are chemically modified during transport)

Mechanisms of active transport

Group translocation

Eukaryotic Cells

Have nucleus and nuclear membrane surrounding their DNA

Have internal membrane-bound organelles (compartmentalize cellular functions that act like tiny organs)

Eukaryote cells are larger compared to prokaryotes (10-100 μm in diameter)

Have more complex structures than prokaryotes

Comprised of algae, protozoa, fungi, animals, and plants

Nucleolus

Cilium

Ribosomes

Cytoskeleton

Cytoplasmic

membrane

Smooth endoplasmic

reticulum

Rough endoplasmic

reticulum

Transport vesicles

Golgi body

Secretory vesicle

Centriole

Mitochondrion

Lysosome

Nuclear pore

Nuclear envelope

Typical eukaryotic cell

External structure of Eukaryotic cells

Glycocalyces

Eukaryotic cells lacking cell walls have sticky carbohydrate, glycocalyces anchored to their cytoplasmic membranes

Never as organized as prokaryotic capsules

Helps animal cells adhere to each other

Strengthens cell surface

Provide protection against dehydration

Function in cell-to-cell recognition and communication

Eukaryotic cell walls

Fungi, algae, and plants have cell walls but no glycocalyx

Composed of various polysaccharides but not peptidoglycan of most bacteria

Cell wall protects cells from the environment and provide shape and support against osmotic pressure

Cellulose found in plant cell walls and fungal cell walls are composed of polysaccharide, including cellulose, chitin, and/or glucomannan

Algal cell walls composed of cellulose, agar, carrageenan, silicates, algin, calcium carbonate or combination of these

Some protozoa have cell walls composed of various polysaccharides (cellulose and glucomannan)

A Eukaryotic cell wall

External structures of Eukaryotic cells

Flagella

Structure and arrangement

Differ structurally and functionally from prokaryotic flagella

Within the cytoplasmic membrane (Flagella are inside the cell, not extensions outside the cell)

Shaft composed of tubulin arranged to form microtubules

Filaments anchored to cell by basal body AND no hook

May be single or multiple (generally found at one pole of cell)

Do not rotate, but undulate rhythmically

Eukaryotic Flagella and Cilia (movement)

External structures of Eukaryotic cells

Cilia

Some eukaryotic cells move by means of hair-like structures called cilia

Shorter and more numerous than flagella (cover the surface of the cell)

In comparison, no prokaryotic cells have cilia

Cilia in multi-cellular eukaryotes are used to move substances in the local environment past the surface of the cell

Coordinated beating propels cells through their environment

Cilia beat rhythmically and this propels cells through their environment

Eukaryotic Cilia

Eukaryotic cytoplasmic membranes

All eukaryotic cells have cell membrane

Is a fluid mosaic of phospholipids and proteins which act as recognition molecules, enzymes, receptors, carriers or channels

Contains steroid lipids (sterols) such as cholesterol in animal cells to help maintain membrane fluidity

Sterols at high temperature stabilize phospholipid bilayer by making it less fluid and at low temperatures they prevent phospholipid packing, making membrane more fluid

Controls movement of materials into and out of cell

Contain regions of lipids and proteins called membrane rafts

Eukaryotic cytoplasmic membranes are used for passive (diffusion, facilitated diffusion, osmosis) and active processes of transport

Eukaryotic membranes do not perform group translocation, but perform endocytosis (also called phagocytosis if solid substance is brought into the cell and pinocytosis if liquid substance is brought into the cell). Exocytosis enables substances to be exported out of the cell

Eukaryotic Cytoplasmic Membrane

Cytoplasm of Eukaryotes

Cytoplasm of Eukaryotic cells

More complex than that of either bacteria or archaea

Most distinctive difference is the presence of numerous membranous organelles in eukaryotic cells (e. g., Gologi body, rough/smooth endoplasmic reticulum)

Non-Membranous organelles

Ribosomes: Larger than prokaryotic ribosomes (80S versus 70S) and composed of 60S and 40S subunits. Many eukaryotic ribosomes are attached to the membranes of the endoplasmic reticulum

Cytoskeleton: composed of extensive internal network of fibers and tubules

Function in cytoplasmic streaming and in movement of organelles within the cytoplasm

Enables contraction of the cell, provides the basic shape of many cells and anchors organelles

Centrioles and Centrosome: Centrioles play a role in mitosis (nuclear division), cytokinesis (cell division), and in the formation of flagella and cilia. Centrosome – region of cytoplasm where centrioles are found

Cytoplasm of Eukaryotes

Mitochondria and chloroplasts

Mitochondria:

Spherical to elongated structures found in most eukaryotic cells

Have two membranes composed of phospholipid bilayer. Inner membrane is folded into numerous crystae, where most of the cell’s ATP is produced

Interior matrix contains small “prokaryotic” 70S ribosomes and circular molecule of DNA (contains genes for some RNA molecules and for a few mitochondrial polypeptides)

Chloroplasts:

Light-harvesting structures found in photosynthetic eukaryotes

Have two phospholipid bilayer membranes, DNA and have 70S ribosomes

Mitochondrion

Chloroplast

Comparison of prokaryotic and eukaryotic organelles

Comparison of prokaryotic and eukaryotic cells