(4) MICROBIO: Introduction to Cell Structure and Function

theodor schwann (1804-1881) and matthias schleiden (1810-1882)

- both developed the Cell Theory which states that all living things are composed of cells

cell theory

- states that all living things are composed of cells

prokaryotes

-"before nucleus"

- it's distinctive structural feature is not what they have, but what they lack

- they lack internal membrane-bound structures

- they are simpler than eukaryotes

- typically smaller - approximately 1.0 micrometer in diameter, as compared to 10-100 micrometer for eukaryotes (e.g. bacteria and archaea)

eukaryotes

- have membrane surrounding their DNA, forming a nucleus

- have "true nucleus", with membrane bound compartments/organelles

- larger and more complex than prokaryotes (e.g. plants, animals, protozoa, algae, and fungi)

glycocalyses (glycocalyx; singular)

- part of prokayotic cells structure

- a gelatinous, sticky substance surrounding outside the cell (capsule)

- "sugar cup", composed of polysaccharides or polypeptides or both

capsule

- a glycocalyx which is composed of repeating units of organic chemicals firmly attached to the cell surface

- e.g. Streptococcus pneumoniae, Klebsiella pneumoniae

slime layer

- a loose, water soluble glycocalyx, often viscous (Sticky) providing attachments to surfaces

- e.g. oral bacteria (provides lining to our teeth; protect navel)

flagella (flagellum; singular)

- long whip-like structures that extend beyond the surface of the cell and the glycocalyx

- propels the cell

monotrichous

- single flagellum at one pole

amphitrichous

- flagella at both poles

lophotrichous

- tuft of flagella at one pole

peritrichous

- flagella surrounding the entire cell; all over

Amphilophotrichous

- tuft of flagella at both ends

fimbriae

- non-motile extensions

- sticky proteinaceous, bristle-like projections used to adhere to one another and to substances

- shorter than flagella, there may be 100 per cell

- serves as an important function in biofilms (are slimy masses of bacteria adhering to a substance)

pili/pilus

- tubules composed of a protein called pilin

- longer than fimbriae but shorter than flagella

- 1-10/cell (fewer than fimbriae)

- used by bacteria to move across a substrate or towards another bacterium (attachment pili)

- mediates the transfer of DNA from one cell to the other (conjugation)

- DNA can be exchanged through this

prokaryotic cell walls (bacteria)

- provides structure and shape to the cell and protects it from osmotic forces

- assists some cells in attaching to other cells or in eluding antimicrobial drugs

- note that animal cells do not have cell walls

bacterial cell walls

- composed of peptidoglycan, a complex polysaccharide which is composed of two regularly alternating sugars called N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM) which are structurally similar to glucose

- NAG alternates with NAM - these are the "glycan" portions of peptidoglycan

- chains of NAG and NAM are attached to other chains by cross bridges of four amino acids (tetrapeptides) - these cross bridges are the "peptido" portion of the peptidoglycan

gram positive cell walls (purple)

- with thick layer of peptidoglycan (20% of the dry weight of the cell) and unique polysaccharides called teichoic acids which are covalently linked to lipids forming lipoteichoic acids that anchor the peptidoglycan to the cell membrane

- e.g. Mycobacterium have cell walls up to 60% mycolic acid - a waxy lipid (AFB)

gram negative cell wall (pink)

- with thin layer of peptidoglycan (1-2% of the dry weight of the cell)

- outside this layer is a bilayer membrane composed of phospholipids, channel proteins (porins), and lipopolysaccharides (LPS)

lipopolysaccharides (LPS)

- a union of lipid and sugar

- the lipid portion of this is known as lipid A (endotoxin; if released it causes fever, clotting, inflammation, shock)

- released from dead cells when the cell wall disintegrates, and it may trigger fever, inflammation, shock, and blood clotting in humans

periplasmic space

- found between the cell membrane and outer membrane

- contains the peptidoglycan and periplasm

periplasm

- the gel between membrane of gram negative cells

- contains water, nutrients, and substances secreted by the cell (digestive enzymes, and proteins)

Mycoplasma pneumoniae

example of bacteria without cell walls

archael cell wall

- contains a variety of specialized polysaccharides and proteins, but NO peptidoglycan (separate group)

- gram (+) wall have thick wall and stains purple

- gram (-) wall have a layer of protein covering the wall, rather than a lipid membrane in gram (-) bacteria, but they stain pink

prokaryotic cytoplasmic membrane

- beneath the glycocalyx and cell wall

- also referred to as cell membrane or plasma membrane

- composed of lipids and associated proteins

bacterial membranes

- contain phospholipids

- some have sterol-like molecules called hopanoids

- ester linkage

archaeal membranes

- composed of lipids that lack phosphate groups and have branched hydrocarbons linked to glycerol by ether linkage instead of ester linkage

passive transport

- type of transport that require no use of energy by the cell

- the electrochemical gradient provides energy

diffusion

- molecules move down their electrochemical gradient through the phospholipid bilayer of the membrane

- substances transported: oxygen, carbon dioxide, lipid-soluble chemicals

facilitated diffusion

- molecules move down their electrochemical gradient through channels or carrier proteins

- substances transported: glucose, fructose, urea, some vitamins

osmosis

- water molecules move down their concentration gradient across a selectively permeable membrane

- substance transported: water

active transport process

- cells expends energy in the form of ATP to move a substance against its electrochemical gradient

- some processes are linked to a passive process and use the energy of the passive process to carry a substance across the membrane

- ATP-dependent carrier proteins bring substances into cell

- substances transported: Na+, K+, Ca+, H+, Cl-

group translocation

- the substance is chemically altered during transport

- found only in some prokaryotes

- substances transported: glucose, mannose, fructose

endocytosis

- substances are surrounded by pseudopodia and bough into the cell

- phagocytosis involves solid substances

- pinocytosis involves liquids

- substances transported: bacteria, viruses, aged and dead cells, liquid nutrients in extracellular solutions

exocytosis

- vesicles containing substances are fused with cytoplasmic membrane, dumping their contents to the outside

- substances transported: wastes, secretions

cytoplasm of prokaryotes

- general term used to describe the semi-liquid, gelatinous material inside a cell

- it's a semi-transparent fluid, elastic and aqueous

- composed of: cytosol, inclusions, non-membranous organelles

cytosol

- composed of mostly water, but also contained dissolved suspended substances, including ions, carbohydrates, lipids, and wastes

- also contains the cell's DNA (single, circular chromosome) which is not surrounded by a membrane in a region called nucleoid

inclusions

- deposits found within the cytosol

- may include reserve deposits of lipids, starch or compounds containing nitrogen, phosphate and sulfur

- serve as a diagnostic tool for pathogenic bacteria

- e.g. aquatic cyanobacteria (gas vesicles that store gases in protein sacs), magnetobacteria (stores magnetites)

aquatic cyanobacteria

- gas vesicles

- store gases in protein sacs

ribosomes

- site of protein synthesis

- prokaryotic cells have about 1000 of this which gives the cytoplasm a grainy appearance

- approximate size is expressed in Svedbergs (S) which is determined by either their Sedimentation rate

- composed of two subunits, each of which is composed of protein and a type of RNA called rRNA

prokaryotic ribosomes

- 70s ribosomes

eukaryotic ribosomes

80s ribosomes

rod-shaped prokaryotes

- this type of prokaryotes have a simple cytoskeleton

spherical prokaryotes

- this type of prokaryotes lack cytoskeleton

cytoskeleton

- internal network of fibers which play a role in forming a cell's basic shape

- present in rod-shaped prokaryotes, absent in spherical prokaryotes

glycocalyces

- present in animal and protozoan cells but absent in eukaryotic cells that have cell walls

- functions:

- anchor cells to each other

- strengthening the cell structure

- protection against dehydration

- cell to cell recognition and communication

flagella

- shaft is made up of tubulin arrange in chain to form microtubules

- has 9+2 arrangement

- may be single or multiple and are generally found at the end of the pole of the cell

- function: undulate rhythmically rather than rotating; do not move in runs and tumbles

cilia

- hair like structures; shorter and more numerous than flagella

- composed primarily of tubulin microtubules which are arranged in 9+2 arrangement of triplets in their basal bodies

- absent in prokaryotic cells

- a single cell may have this around 100 or even a 1000 which beats rhythmically

- helps cleanse the human respiratory tract from dusts and microorganisms

cell wall

- found in fungi, algae, and plants

- if this is present, glycocalyces are absent

- takes on one of the function of the glycocalyx by providing protection from the environment

- provides shape and support against osmotic pressure

- composed of various polysaccharides, but not the peptidoglycan seen in the walls of most bacteria

plant cell

- its cell wall is composed of cellulose (a polysaccharide; paper/dietary fiber)

fungi

- its cell wall have walls of polysaccharides, including cellulose, chitin, and glucomannan

algae

- its cell wall is composed of variety of polysaccharides and other chemicals such as cellulose, agar, carrageenen, silicates, algin, calcium carbonates, or a combination

cell membrane

- all eukaryotes have this

- plasmalemma for plants and algae

- they contain steroid lipids (sterols), such as cholesterol in animal cells that help maintain fluidity (helps plants not freeze)

- controls the movement of material into and out of the cell

ribosomes

- free or attached to endoplasmic reticulum

- composed of 60s and 40s subunits

cytoskeleton

- composed of network of fibers and tubules

- acts to anchor organelles, functions in cytoplasmic streaming and in movement of organelles within the cytosol, enables contraction of the cell, moves the cell membrane during endocytosis and amoeboid action, and provides the basic shape of many cells

- made up of tubulin microtubules (also found in flagella, cilia, and centrioles), thinner microfilaments composed of actin and intermediate filaments composed of various proteins

centrioles and centrosome

- present in certain kind of eukaryotic cells

- plants, algae, fungi, and prokaryotes lack these

- composed of 9 triplets of microtubules arrange in a way that resembles the 9+0 arrangement seen at the base of eukaryotic flagella and cilia

- play a role in mitosis (nuclear division) cytokinesis (cell division) and in the formation of flagella and cilia

membranous organelles

- not present in prokayotic cells

- includes nucleus, ER, golgi apparatus, lysosome, peroxisomes, mitochondria, chloroplasts

nucleus

- spherical to ovoid

- largest organelle in the cell; some have only one while others have many

- control center of the cell

- contains chromatin (thread like mass of DNA associated with special proteins called histones which play a role in packaging nuclear DNA)

nucleoplasm

- semi-liquid matrix in the nucleus

nucleoli

- a specialized region in the nucleus where RNA is synthesized

chromatin

- a thread like mass of DNA associated with special proteins called histones which play a role in packaging nuclear DNA

nuclear envelope

- surrounds the nucleus

- double membrane

- contains pores that functions to control the import and export of substances

endoplasmic reticulum

- functions as a transport system and is found in 2 forms

Smooth Endoplasmic Reticulum

- plays a role in lipid synthesis as well as transport

rough endoplasmic reticulum

- studded with ribosomes

- proteins produced here are inserted into the lumen (central canal) and transported throughout the cell

golgi apparatus

- the shipping department of the cell

- receives, produces and packages large molecules for export from the cell

- packages secretions in sacs is called secretory vesicles, which then fuse with the cell membrane before dumping their contents outside the cell via exocytosis

lysosomes

- found in animal cells, contain catabolic enzymes that damage the cell if they're released from their packaging into the cytosol

- it's enzymes are used during the self-destruction of old, damaged and diseased cells and to digest nutrients that have been phagocytized

- e.g. white blood cell

peroxisomes

- contains oxidase and catalase that degrade poisonous metabolic wastes (such as free radicals and hydrogen peroxide)

- found in all eukaryotic cells but are specially prominent in the kidney and liver cells of mammals

vesicle and vacuole

- general terms of sacs

- found in plants and alga cells that store starch, lipids, and other substance in the center of the cell

mitochondria

- spherical to elongated structures fond in most eukaryotic cell

- like nuclei, they have 2 membranes, each composed of a phospholipid bilayer

- the inner membrane's surface area is often called the powerhouse of the cell because their cristae produce most of ATP in the cell

chloroplast

- light harvesting structures found in photosynthetic eukaryotes

- like mitochondria and nucleus, chloroplast have 2 phospholipid bilayer membranes and DNA

- can also synthesize a few polypeptide with their own 70s ribosomes

- semi-autonomous

Photosynthetic lamellae

- chloroplasts in photosynthetic prokaryotes

- they have infoldings of their cell membrane