Ch. 7 Cell Structure

Biology

Cytology

• The Study of Cells

History of Cytology

• Begins with the invention of the microscope

• Zacharias Janssen (fix your notes!) credited with the invention of the compound microscope

• Late 1500’s

Robert Hooke

• Coined the term “cell”– Did this while observing cork

cells– Really the cell walls of dead

plant cells from bark

• Published his observations in the book Micrographia

• Influenced many others• Click here for more on

Robert Hooke

Robert Hooke

• Hooke’s microscope was not as good as some (especially those of Leeuwenhoek), but he was still able to make many observations and detailed drawings.

Robert Hooke

• Hooke’s drawings of cork cells

• He also made many other detailed drawings of organisms he viewed with his microscope.– The flea

Anton Van Leeuwenhoek

• Was not the first microscope maker, but made some of the best early microscopes

• Very high magnifications for the time – up to 300x

• 1600s

Anton Van Leeuwenhoek

• Observed first living cells– Pond water, various

bodily fluids that had bacteria swimming about in them, etc.

– Remember that the cells Hooke observed were non-living (cork)

– For more on how his microscope worked, click here.

Anton Van Leeuwenhoek

• Images he drew of bacteria in plaque removed from his teeth.

• These are the first known observations of bacteria

• He called them “animalcules”

Robert Brown• 1st to observe a nucleus

in a cell (1833)– The instrument is

Brown’s microscope– The image below is what

Brown would have seen when he made his discovery. These are cells from an orchid leaf.

• The nuclei are the small dots in each cell

• The big things are stomata

Matthias Schleiden

• Said that all plants are made of cells– 1838

Theodore Schwann

• Said all animals are made of cells

• 1839

Rudolf Virchow

• All cells are made from other cells

• 1855• Before this time it had

been thought that cells could arise spontaneously from non-life

The Cell Theory

• The work of all the scientists listed is compiled into one large idea called The Cell Theory

• Remember – a theory is an explanation for observations that – Is supported by a large body of evidence– Has been tested MANY times– Explains MANY things– It may be revised or improved, but stands as our best

explanation for observations we have.

The Cell Theory

• All living things are composed of cells • Cells are the basic units of structure and

function in living things• All cells come from pre-existing cells

Microscopes

• Compound• Dissecting• Scanning Electron• Transmission Electron

Compound Light Microscope

• Called compound because TWO lenses are combined to magnify the image – not just one.– Eyepiece (ocular)– Objective lens

Limitations of the Compound Microscope

• Images must be small and mounted on a slide

Compound Microscope Parts

• Ocular– Also called eyepiece– Usually magnifies 10x– Needle inside for

pointing to objects

• Body or Arm

Compound Microscope Parts

• Revolving nosepiece– Holds objective lenses– Rotates so that correct

objective can be chosen

• Objective Lens– Usually a “low”,

“medium” and “high” power

Compound Microscope Parts• Attached to the nosepiece• Low Power

– Usually 4x– Widest field of view– Least magnification– Used first to observe– Coarse focus

• High Power– Usually 40x– Narrowest field of view– Most magnification– Use only after low and

medium powers– Fine focus only

More Microscope Parts• Stage• Diaphragm

– Controls the amount of light that strikes the specimen• Focus knobs

– Coarse• Moves the stage in large amounts• Used only with low power

– Fine• Moves the stage in small amounts• Used with high power (but can be used with any)

• Light source• Base

Dissecting Microscope

• Two oculars• Low magnification• Large specimens may be

observed

About Field of View

• Wide field of view– Large area– Specimen appears small– Good for specimens that move a lot and quickly

• Narrow field of view– Small area– Specimen appears large– Good for specimens that are still and for which you want

to see greater detail

Field of View

• Top image– Wide field of view

• Bottom image– Narrow field of view

Scanning Electron Microscope

• VERY large magnification– 1000s of times

• External views• 3-D appearance• Click here to see how

the SEM works and to see images from an SEM

Transmission Electron Microscope

• VERY large magnifications– 1000s of times

• Views cross-sections of images– Inside a cell

• 2-D views• Click here to see how a

TEM works and images from one

Cell Types

• There are TWO major types of cells– Prokaryotic– Eukaryotic

Prokaryotic Cells

• BACTERIA• Simple• No nucleus• No organelles that are

surrounded by membranes

• Most ancient cell type– First to evolve

• Most primitive

Eukaryotic Cells• Everything EXCEPT Bacteria

– Protists, fungi, plants and animals are made up of this type of cell

• Complex• Nucleus present• Organelles surrounded by

membranes• The more recent cell type

– Evolved AFTER prokaryotes• More complex

– More sophisticated parts and functions

Two Major Types of Eukaryotic Cells

• Plant• Animal

Plant Cells

• Plant Cells– Cell wall made of

cellulose– Large central vacuole– Chloroplasts present

Animal Cells• No cell walls• No chloroplasts• Vacuoles are present,

but no large centrally located one.

Cell Size• Cells are small. Why?

– Cells must have a large surface area to volume ratio

– large surface area and small volume allows for adequate amounts of materials to move in and out of the cell to nourish a relatively small volume

– If cell volume were larger, the cell surface size would not increase enough with volume to service the whole cell.

Folds in Cells

• Whenever you see a folded surface in a cell, you can pretty much bet that it is there to increase surface area of the structure in question…

Organelles

• Cell parts or compartments – “little organs”• Separate chemical processes

– Allow chemical processes that are incompatible with each other to occur within a single cell.

Generalized Eukaryotic Cell

Cell Size• Most cells are relatively

small because as size increases, volume increases much more rapidly.

• This makes for a longer diffusion time

Visualizing Cells

Prokaryotic Cells• Simplest organisms

– Cytoplasm is surrounded by plasma membrane and encased in a rigid cell wall composed of peptidoglycan.• no distinct interior compartments

–gram-positive – thick single layer wall that retains a violet dye from Gram stain procedure

–gram-negative – multilayered wall does not retain dye

»Susceptibility of bacteria to antibiotics depends on cell wall structure.

Prokaryotic Cells

• Some use flagellum for locomotion– threadlike structures protruding from cell surface

Bacterial cell wall

Flagellin

Rotarymotor

Sheath

Eukaryotic Cells

• Characterized by compartmentalization by an endomembrane system, and the presence of membrane-bound organelles.– Central vacuole – plants, storage– Vesicles (smaller)– Chromosomes - DNA and protein– Cytoskeleton (internal protein scaffolding)– Cell walls – plants and fungi

Nucleus• Repository for genetic material• Directs activities of the cell• Usually single, some cells several, RBC none

– Nucleolus – ribosome sub-units are made here;• Surface of nucleus bound by two phospholipid

bilayer membranes– nuclear membrane– Nuclear pores – protein gatekeepers

• Usually proteins going in and RNA going out

Nucleus

Chromosomes

• DNA of eukaryotes is divided into linear chromosomes. – exist as strands of chromatin, except during cell

division– associated with packaging histones, packaging

proteins• nucleosomes

Endomembrane System• Compartmentalizes cell, channeling passage of molecules through cell’s interior.– Endoplasmic reticulum

• Rough ER - studded with ribosomes• Smooth ER - few ribosomes

Endoplasmic reticulum• Largest internal membrane• Composed of Lipid bilayer• Serves as system of channels from the nucleus• Functions in storage and secretion• Rough ER is “rough” because of associated

ribosomes (sites of protein synthesis• Smooth ER - lack associated ribosomes –

contained embedded enzymes, catalyze synthesis of carbohydrate and lipid molecules.

Endomembrane System• Golgi apparatus

– collection of Golgi bodies• collect, package, and distribute molecules

synthesized at one location in the cell and utilized at another location

• Front - cis , Back – trans• Cisternae – stacked membrane folds

Vesiclebuddingfrom roughendoplasmicreticulum

Fusionof vesiclewith Golgiapparatus

Migratingtransportvesicle

Protein

Proteins

Transportvesicle

Golgiapparatus

Cisternae

Ribosome

trans face

cis face

Endomembrane System• Vesicles– Lysosomes - membrane-bound vesicles containing

digestive enzymes – from Golgi– Microbodies - enzyme-bearing, membrane-enclosed

vesicles.• Peroxisomes - contain enzymes that catalyze the removal

of electrons and associated hydrogen atoms• Peroxisome – named for hydrogen peroxide produced as a

by-product • Enzyme breaks H2O2 down to water and oxygen

Cytoplasm

PhagocytosisFood

vesicleGolgi

apparatus

Lysosomes

Plasmamembrane

Digestion ofphagocytizedfood particles

or cells

Endoplasmicreticulum

Transportvesicle

Old or damagedorganelle

Breakdownof old

organelleExtracellularfluid

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Ribosomes

• Ribosomes are RNA-protein complexes composed of two subunits that join and attach to messenger RNA.– site of protein synthesis– assembled in nucleoli

Organelles With DNA

• Mitochondria – bounded by exterior and interior membranes– interior partitioned by cristae

• Chloroplasts– have enclosed internal compartments of stacked

grana, containing thylakoids– found in photosynthetic organisms

MitochondriaA. "Powerhouse of the cell" - cellular metabolism

B. Structure- outer and inner membranes, cristae

C. Have their own DNA

Chloroplasts•Chloroplasts are larger and more complex than mitochondria

•Grana – closed compartments of stacked membranes

•Thylakoids – disc shaped structure – light capturing pigment

•Stroma – fluid matrix

Endosymbiosis

• Endosymbiotic theory suggests engulfed prokaryotes provided hosts with advantages associated with specialized metabolic activities.

Theory of Endosymbiosis

Evidence for the endosymbiont theory is that mitochondria and chloroplasts:

- Are appropriate size to be descendants of eubacteria. - Have inner membranes similar to those on prokaryotic

plasma membranes. - Replicate by splitting, as in prokaryotes. - DNA is circular and different from the DNA of the cell's

nucleus. - Contain their own components for DNA transcription and

translation into proteins . - Have ribosomes similar to prokaryotic ribosomes. - Molecular systematics lend evidence to support this

theory. - Many extant organisms are involved in endosymbiotic

relationships.

Cytoskeleton• Network of protein fibers supporting cell shape and

anchoring organelles– Actin filaments

• cell movement– Microtubules

• Hollow tubes• Facilitate cell movement• Centrioles – barrel shaped • organelles occur in pairs – • help assemble animal cell’s microtubules

– Intermediate filaments• Stable - don’t break down

Actin

Microtubules

Intermediate filaments

Cytoskeleton

Plant Cells

• Central vacuole – often found in the center of a plant, and serves as

a storage facility for water and other materials• Cell wall

– primary walls – laid down while cell is growing– middle lamella – glues cells together– secondary walls – inside the primary cell walls

after growth

Plant Cell

Animal Cells• Animal cells lack cell walls.

– form extracellular matrix• provides support, strength, and resilience

Modified from: http://www.coe.unt.edu/ubms/documents/classnotes/ Fall2005/Chapter%205%20-%20Cell%20Structure.ppt