Chapter 3Microscopy and Cell structure

Biology 261

Prof. Santos

• Microscope- tool used to see objects too small to be seen by the naked eye. Types of microscopes used in the study of microorganisms include;

• light microscope• phase contrast microscope• confocal microscope• interference microscope• fluorescence microscope• electron microscope• atomic force microscope.

Terms to know

• Bright field microscopy- lights rays are used to evenly illuminate the field of view.

• Dark field microscopy- light is directed towards the specimen at an angle. This makes it possible for the unstained specimen to appear more visible against a dark background.

Electron Microscope

Electron microscope- uses a beam of electrons to create an enlarged image of the specimen. An electron microscope can magnify a specimen up to 250,000x closer. The wavelength used is less than the wavelength of light, thus the resolution is greater.

2 types

• TEM- transmission electron microscope allows one to see fine detail structures inside the cell.

• SEM- scanning electron microscope allows one to see the surface details of cells like the membrane proteins.

3-Phase contrast microscope- allows you to see unstained cells by altering the background of the cell. This microscope has a device that allows it to amplifies differences in refractive index to create a contrast.

4-fluorescence microscope

Projects ultra violet light causing fluorescent molecules in the specimen to emit a longer wavelength.

5- Confocal microscope

Mirrors are used to scan a laser beam across successive regions and planes of a specimen. A computer program constructs a 3D image.

6- Interference microscope causes the specimen to appear as a 3D image.

8- Atomic force microscope

This very powerful microscope produces a very detailed image of the surface of an specimen by using a very sharp probe (stylus) to move across the surface and “feel” the bumps and valleys of the atoms of the surface.

Prokaryote vs Eukaryote

Structure of prokaryote cell

1-Flagella provides motility. The flagella is made up of three basic parts, filament, hook and basal body.

• Chemotaxis- movement towards a chemical

• Phototaxis- movement towards light

• Aerotaxis- movement towards oxygen

• Magnetotaxis- reaction towards the magnetic field

• Thermotaxis- movement towards a specific temperature

2-Pili proteins that enable the bacterium to adhere to surfaces. Fimbriae allow bacteria to adhere to surfaces and sex pili allow DNA transfer between bacteria.

3-The capsule a viscous and gelatinous layer that surrounds bacteria. It enables bacteria to adhere to certain surfaces and allows organisms to avoid innate defense systems and cause diseases. Ex, Streptococcus pneumoniae.

4- slime layer- gel like layer that is diffuse and irregular. This layer is composed of polysaccharides and enables the bacteria to adhere to surfaces and grow as biofilm.

Ex; Streptococcus mutans grows as biofilm on your teeth to form dental plaque.

5- The cell wall a rigid covering consisting of peptidoglycan that gives the bacterium its shape and protection.

• The type of cell wall distinguishes between 2 types of bacteria; gram negative and gram positive.

• Peptidoglycan is a macromolecule found only in bacteria.

• The basic structure of peptidoglycan is an alternating series of 2 major subunits, N-acetylmuramic acid (NAM) and N- acetylglucosamine (NAG). These subunits are covalently bonded to each other to form a glycan chain.

• Attached to each NAM molecule is a string of 4 amino acids, a tetrapeptide chain. Cross linkages can form between adjacent chains thus joining adjacent glycan chains.

• In gram negative bacteria these tetrapeptides are joined directly.

• In gram positive bacteria they are usually joined indirectly by a peptide interbridge.

• In gram positive bacteria the peptidoglycan layer is thick.

• In gram negative bacteria the peptidoglycan layer is thinner.

Teichoic acid

• In gram positive bacteria there are polymers of teichoic acid present. These teichoic acid polymers are covalently linked to the NAM molecules of the glycan chain. Some are linked to the cytoplasmic membrane and are called lipoteichoic acids.

• These polymers consists of ribitol-phosphates and glycerol phosphates molecules joined together. Sugars and D- alanine may be attached to these polymers providing antigenic determination.

• Teichoic acid provides rigidity to the cell wall and give the cell negative polarity due to the fact that they stick out above the peptidoglycan.

Gram positive

Outer membrane

• In gram negative bacteria there is an outer membrane outside the peptidoglycan. It is a unique lipid bilayer.

• The outer membrane is unlike any other membrane. The outside leaflet consists of Lipopolysaccharides instead of phospholipids.

• The outer membrane is sometimes called the LPS or lipopolysaccharide layer.

• The outer membrane is joined to peptidogylcan by means of lipoproteins

• Two parts are importance for medical reasons;

a- Lipid A is the portion that anchors the LPS in the lipid bilayer. It plays a role in the immune system.

b- O specific polysaccharide is a chain of sugar molecules opposite the Lipid A. Allows for identification.

periplasm

• In gram negative bacteria, it is the space between the inner and outer membrane.

ENDOPLASMIC RETICULUM (ER)

RoughER

SmoothER

Nuclearenvelope

Nucleolus

Chromatin

Plasmamembrane

Ribosomes

Golgi apparatus

LysosomeMitochondrion

Peroxisome

Microvilli

MicrotubulesIntermediate filaments

Microfilaments

Centrosome

CYTOSKELETON:

Flagellum NUCLEUS

Eukaryotic cell

1- Plasma membrane consisting of asymmetrical lipid bilayer.

The cell membrane consists of proteins and lipids.

Internal protein parts

2- Cilia/flagella are protein structures that consists of microtubules in a 9 +2 arrangement. Cilia and flagella function in motility.

3- cytoskeleton consists of proteins such as microtubules, actin filaments and intermediate filaments that function in cell structure/support and act as a molecular monorail.

4- ribosomes are involved in protein production.

5- Chloroplast- double membrane bound organelle involved in photosynthesis

6- Endoplasmic reticulum is a system of canals involved in the production of macromolecules destined to be secreted to other organelles or outside. Two types, smooth and rough.

7- Golgi Apparatus is a system of flat membranes involved in the modification of material made in the ER. The vesicles are coated with special carbohydrates and phosphate groups that signal the vesicles to their specific location.

9- Nucleus is the control center of the cell. Contains the genetic material and is surrounded by a nuclear envelope.

10- Mitochondrion is the site of cellular respiration

11- Lysosomes12- peroxisomes are organelles use to oxidize

substances, breaking down lipids, and detoxifying certain chemicals.