Basic Characteristics of Cells

1. Basic unit of life-- all life (we know of) depends on cells

2. Cell functions are based entirely on chemical principlesinterconnected chemical reactions allow cells to function

3. Cells take in more energy than they produce-- thermodynamics

4. Cells specialize to optimize their functions for particular purposes

How large are cells?

Few microns

How big is a micrometer (micron)?

1 micron = 1x10-6 meters

10-3

10-2

10-1

common metric prefixes:kilo- = 103

centi- = 10-2

milli- = 10-3

micro- = 10-6

nano- = 10-9

Angstrom = .10 nm = 1x10-10m

light microscopes resolve to ~ 0.2 microns

electron microscopes resolve to ~ 2 nanometers

Average animal cell about 20 microns in diameter

Up to 1 mm (or more) in diameter, meters long!

Major classifications of cells-- Prokaryotic and Eukaryotic

Prokaryote Eukaryote

Eukaryotes:

are generally larger + complex

Membrane bound nucleusMembranes segregate functionEndocytosis and exocytosis

More organized DNAMeiosis+ sexual reproductionExpression of DNA

Have internal cytoskeleton

Organelles

Cell MembraneLipid bilayer with polar heads and hydrophobic interior

found in prokaryotes and eukaryotesforms the limiting boundary of the cell

Cell Membranesemi-permeable membrane-- gasses pass through it, water + ions can't

proteins in the membrane transport specific molecules in or out of the cell

many sugar groups (carbohydrates) are attached to the proteins and lipids

Nucleus"brain" of the cell-- holds theinformation stored as DNA and controls cell functions

Has it's own lipid bilayerwith special "nuclear pores"regulating in and out movement

Place where RNA is synthesizedand processed

Nucleus

DNA is bound to histone proteins in cell-- complex called a chromosome

diploid-- 2 copies of each homologous (very similar) chromatid

each chromatid duplicates and separates during cell division

Nuclei divide first, followed by rest of the cell

Bacteria have to divide too

reproduce by fission-- divide in half -- not as organized as eukaryotes

cytoplasm-- material outside the nucleus inside the cell membrane

Cytoskeleton-- "bones" of a cell-- provides structure and supportalso can act as a highway-- move product from one place to another

Three major fibers form the cytoskeleton:microfilaments - made up of actinform the cleavage furrow during divisioncontractile fibers of musclessmallest fiber - 5-6 nm widefibers have polaritygrow on one end, disassemble on other

intermediate filaments middle size - ~10 nm widemost stable-- change less frequentlycommon at sites of mechanical stressdiagnostic tool in medicine- tissue specific

Cytoskeleton

~ 10 nm

Cytoskeleton

Cytoskeleton

Microtubules - largest of the fiber classes (~ 25 nm wide)made up of 2 protein subunits-- and tubulinlike actin, microtubules are polarized with a + and - endprovides a 'superhighway' in the cell to move in a direction

Cytoskeleton

microtubules make up the spindle fibers that separate chromosomesas well as the flagella that allow motility for many cells

Centrosome

microtubule organizing center

2, generally on opposite sides of nucleus

not found in plant cells

organizes the spindles during mitosis

also important for flagellar organization

Ribosomesfound in both prokaryotes and eukaryotes

location of protein synthesis

complex of proteins and structural RNAs

most numerous organelle in cells

relatively small organelles--

prokaryotes: 25 nm 70S 30S and 50S subunits

eukaryotes: 30nm 80S40S and 60S subunits

Sedimentation Coefficient

Centrifugation is a way of separating substances by density

dense things settle to the bottomlighter things stay toward top

centrifuges measured by G forcesRCF = 1.12r (RPM/1000)2

(r=radius in mm)

sedimentation coefficient is howrapidly a particle moves to the bottom of a centrifugemeasured in Svedberg units (S)

very useful for separating soluble and insoluble materials as well

repeatedly spinning at different speeds can generate multiple fractions

Endoplasmic Reticulum (ER)

ER is a series of foldedmembranes within a cell

smooth ER is site of mostmembrane synthesis

rough ER looks spotteddue to ribosomes boundon the surface

rough ER is location of membrane, secreted orotherwise targettedprotein synthesis

eukaryotes only

Endoplasmic Reticulum (ER)

ER is continuous with the nuclear membrane

amount of ER varies bycellular function

liver cells have lots of ER-used to detoxify chemicals

cells optimized for proteinsecretion have lots of rough ER

Golgi ComplexSeries of flattened membranes thatprocesses proteins, primarilyglycosylation (adding carbohydrates)

vessicles carry proteins from one stackto another, and eventually to their finaldestination

receives proteins to be glycosylatedfrom the endoplasmic reticulum

Vacuoleextremely large and prominent inplant cells

smaller and more numerous in animal cells

in animal cells, primarily storagegranules

in plants, performs storage, somedigestive functions, but primarilyregulates turgor pressure

turgor pressure supplies rigidity

Lysosome

'digestive' organelle-- breaks down storage molecules, endocytosedmaterial, or cellular material that is no longer needed

needs to be isolated to prevent breaking down rest of the cell!

digested particles are released into the cytoplasm where they are recycled

Peroxisome

both generates and degradeshydrogen peroxide

detoxifies certain chemicalsand converts some substances

breaks down long chain fattyacids

also has a role in photorespiration

first isolated as a biochemical fraction

Cell Wallprimary cell wall generally containscellulose and other polysaccharides

secondary cell wall, if present, is morerigid and contains lignin and highercellulose content

plasmodesmata are cytoplasmic linksbetween plant cells passing throughcell walls

Bacterial cell walls containpeptidoglycans instead of cellulose

Extracellular Matrix

Secreted by animal cells, not plants

contains mostly collagen and proteoglycans

elastic network allowing freedom of movement and not cell wall rigidity

aids in support and recognition events

basal lamina- extracellular matrix around muscle cells- important in

muscle cell function and synapse formation

MitochondriaFound in both plants and animals

about the size of a bacteria

2 membranes, inner and outer

cristae-- folds of the innermitochondrial membrane

matrix-- semiliquid material between cristae

oxidizes sugars and other foods for energy

primary site for the generation of adenosine triphosphate, the mainenergy transfer unit of the cell

Chloroplastonly found in plants

huge-- 5-10 microns long

have inner, outer, and thylakoidmembranes

thylakoid-- flattened membranesack

grana-- stacks of thylakoids

chloroplasts are sites of photosynthesis, with carbon fixation taking placewithin the stroma

also use nitrates from soil to make ammonia (NH4) for amino acids

stroma thylakoid

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granum

Chloroplasts and Mitochondria

both have own circular DNA, ribosomes, tRNAs, and internal membranes

ribosomes are more similar to prokaryotic than eukaryotic in size/function

both membrane bound and involved in energy production/utilization

overall size is similar to prokaryotes (about 2 micron in length)

Endosymbiont Theorysuggests that an early one cell organism, protoeukaryotes, developed a sybiotic relationship with a primative bacteria and cyanobacteria

phagocytosis-- property of surrounding something (perhaps a nutrient) within a membrane and pinching it off so it becomes enclosed

algae, diatoms, and others live symbiotically with at least 150 known species of invertebrates and protists, some have even lost their cell walls

even though the earliest life forms are not available for observations, we can still find evidence for some of their characteristics today

Viruses

Obligate cellular parasites--can't replicate without cells

nucleic acid core (DNA or RNA)surrounded by a protein coat

take over cellular machinery toreproduce themselves and eventually kill the host cell

Viruses

Many viruses cause disease

Ebola virus Smallpox virus HIV virus

Viruses

bacteriophage are virusesthat infect bacteria

vital to early molecular biology

Tobacco mosaic virus infects plants

excellent model for studying life cycle

viruses can be used for geneticengineering of plants and animals