Cells

Cells

Chapter 3

Outline History of Cell Theory Modern Microscopes Eukaryotic and Prokaryotic Cells Cell Structure and Communication Cell Components Cellular Reproduction Higher Plant Cells Versus Animal Cells

History of Cell Theory 1665 - Cells discovered by Robert Hooke.

Cell Theory - all living organisms are composed of cells and cells form a unifying structural basis of organization.

1831 - Robert Brown discovered nucleus.

1858 - Rudolf Virchow argued no spontaneous generation of cells.– Louis Pasteur experimentally disproved in 1862.

Modern Microscopes Light Microscopes - increase magnification as

light passes through series of transparent lenses made of glass or calcium fluoride crystals

Two main types:– Dissecting (Stereomicroscopes)– Compound Microscopes

Modern Microscopes

Dissecting Microscopes (Stereomicroscopes)• Allow three-

dimensional viewing of opaque objects

• Can magnify up to 30x

Modern Microscopes

Compound Microscopes• Light passes through

thinly sliced material• Can distinguish

organelles 2 micrometers or larger in diameter

• Can magnify up to 1500x

Modern Microscopes Electron Microscopes – use beam of electrons

produced when high-voltage electricity is passed through wire

Two main types:– Transmission Electron Microscopes (TEMs)– Scanning Electron Microscopes (SEMs)

Modern Microscopes

Transmission Electron Microscopes• Up to 200,000x

magnification, but material must be sliced extremely thin

Modern Microscopes

Scanning Electron Microscopes• Up to 10,000x

magnification

• Surface detail can be observed on thick objects.

Modern Microscopes

Scanning Tunneling Microscope• Uses probe that tunnels electrons upon sample

• Produces map of sample surface

• Even atoms can become discernible

• First picture of DNA segment showing helical structure

Eukaryotic Versus Prokaryotic Cells Prokaryotic - cells lack a nucleus• Bacteria and archaea

Eukaryotic - cells contain a nucleus• Plants, animals, fungi and protists• Cell walls - Rigid boundary of cells• Organelles - Membrane-bound bodies found

within eukaryotic cells

Cell Structure and Communication Cell wall surrounds protoplasm.

Protoplasm consists of all living cell components.–Bound by plasma membrane

Cell Structure and Communication Cytoplasm - all cellular components between

plasma membrane and nucleus• Cytosol - fluid within cytoplasm containing

organelles• Organelles - persistent structures of various

shapes and sizes with specialized functions»Most, but not all, bound by

membranes

Cell Structure and Communication

Cell Structure and CommunicationCell Size

• Cells of higher plants vary in length between 10 and 100 micrometers

• Smaller cells have relatively large surface to volume ratios enabling faster and more efficient cellular communication

Cell Structure and CommunicationCell Wall

Main structural component of cell walls is cellulose (long chains of glucose monomers)• Also contain matrix of:−Hemicellulose - holds cellulose fibrils

together−Pectin - gives stiffness (like in fruit jellies)−Glycoproteins - proteins with associated

sugars


Middle lamella - produced when new cell walls are formed• Shared by two

adjacent cells

Flexible primary walls laid down on either side of middle lamella


• Secondary walls produced inside primary walls– Derived from primary

walls by thickening and inclusion of lignin

– Cellulose microfibrils embedded in lignin for strength.

Cell Structure and CommunicationCommunication Between Cells

Fluids and dissolved substances pass through primary walls of adjacent cells via plasmodesmata• Plasmodesmata - cytoplasmic strands that extend

between cells through minute openings

Two adjacent cells connected

by plasmodesmata

Cell ComponentsPlasma Membrane

Plasma Membrane - semipermeable outer boundary of living part of cell• Regulates movement of substances into and out of

cell

Model of plasma membrane

Cell ComponentsNucleus

Nucleus-control center of cell and contains DNA• Sends coded messages from DNA to be used in

other parts of cell• Bound by two membranes constituting nuclear

envelope• Structurally complex pores occupy up to one-third

of total surface area−Permit only certain kinds of molecules to pass

between nucleus and cytoplasm

Cell ComponentsNucleus

Contains fluid nucleoplasm in which are:• Nucleoli - composed

primarily of RNA• Chromatin strands–Composed of DNA

and proteins–Coil and become

chromosomes

Cell ComponentsEndoplasmic Reticulum

Endoplasmic reticulum - enclosed space consisting of network of flattened sacs and tubes forming channels throughout cytoplasm• Facilitates cellular communication and channeling of

materials

• Synthesizes membranes for other organelles and modifies proteins

Cell ComponentsEndoplasmic Reticulum

Rough ER - ribosomes distributed on outer surface of ER• Associated with protein

synthesis and storage Smooth ER - devoid of

ribosomes and associated with lipid secretion

Endoplasmic reticulum and ribosomes

Cell ComponentsRibosomes

Ribosomes - consist of two subunits composed of RNA and proteins• Link amino acids to construct complex proteins

• Subunits assembled in nucleolus

• May occur on outside of rough ER, or in cytoplasm, chloroplasts or other organelles

• No bounding membranes

Cell ComponentsDictyosomes

Dictyosomes - (Golgi bodies in animals) - stacks of flattened discs or vesicles

Dictyosome

Cell ComponentsDictyosomes

Dictyosomes function:• To modify carbohydrates attached to proteins

synthesized and packaged in ER• To assemble polysaccharides and collect them in

small vesicles−Vesicles pinched off from margins of dictyosomes−Vesicles migrate to plasma membrane, fuse with

it, and secrete contents to outside of cell–Contents may include cell wall polysaccharides,

floral nectars, and essential oils in herbs

Cell ComponentsPlastids

Chloroplasts - most conspicuous plastids• Bound by double membrane and contain:

−Grana made up of thylakoids–Thylakoid membranes contain chlorophyll–First steps of photosynthesis occur in

thylakoid membranes−Stroma - matrix of enzymes involved in

photosynthesis−Small circular DNA molecule–Encodes for production of certain proteins

for photosynthesis


TEM and drawing of chloroplast structure


Other types of plastids include:• Chromoplasts

−Synthesize and accumulate carotenoids (yellow, orange, red)

• Leucoplasts−Colorless−May synthesize

starches (amyloplasts)−Or oils (elaioplasts)

Chromoplasts in red pepper cells

Mitochondria - release energy produced from cellular respiration• Bound by two

membranes• Inward membrane

forms numerous folds = cristae

• Increase surface area available to enzymes in matrix

• Matrix also includes DNA and RNA

Cell ComponentsMitochondria

Cell ComponentsMicrobodies

Microbodies - small, spherical bodies distributed throughout cytoplasm containing specialized enzymes• Bound by a single membrane• Peroxisomes - serve in photorespiration• Glyoxisomes - aid in conversion of fat to

carbohydrates

Cell ComponentsVacuoles

In mature cells, 90% of volume may be taken up by central vacuoles• Bounded by vacuolar membranes, tonoplasts• Filled with cell sap - helps maintain pressure

within cell−Contains dissolved substances i.e., salts, sugars,

organic acids, and small proteins−Frequently contains water-soluble pigments

called anthocyanins (red, blue, purple)

Cell ComponentsCytoskeleton

Cytoskeleton - involved in movement within cell and in cell’s architecture• Network of microtubules and microfilaments• Microtubules:

−Control addition of cellulose to cell wall−Involved in movement of flagella and cilia−Found in fibers of spindles and phragmoplasts in

dividing cells−Are thin, hollow, tubelike and composed of tubulins

(proteins)• Microfilaments - role in cytoplasmic streaming

Cellular ReproductionCell Cycle

Cell cycle - orderly series of events when cells divide• Divided into interphase and mitosis

Interphase• Occupies up to 90% of cell cycle• Period when cells are not dividing

−G1 - cell increases in size−S - DNA replication takes place−G2 - mitochondria and other organelles divide, and

microtubules produced

Cellular ReproductionMitosis

Mitosis - process of cellular division• Produces two daughter cells with equal amounts of

DNA and other substances duplicated during interphase

• Each daughter cell exact copy of parent cell• Occurs in meristems• Although a continuous process, divided into 4 phases:

prophase, metaphase, anaphase and telophase


Prophase• Chromosomes condense by coiling and tightening to

become shorter and thicker.−Chromosomes made of two identical chromatids

held together by centromeres–Kinetochore (protein complex) located on outer

surface of each centromere–Spindle fibers (microtubules) attach to

kinetochore and anchored to two poles of cell

• Nuclear envelope fragments and nucleolus disintegrates


Metaphase• Chromosomes align

between poles around circumference of spindle at cell’s equator−Spindle fibers

collectively referred to as spindle

−At end of metaphase, centromeres holding each sister chromatid separate lengthwise.

Prophase

Metaphase


Anaphase• Sister chromatids separate

and pulled to opposite poles, with centromeres leading the way−Spindle fibers shorten as

material is continuously removed from polar ends

−Chromatids after separation are called daughter chromosomes

Anaphase


Telophase• Each group of daughter

chromosomes become surrounded by nuclear envelope

• Daughter chromosomes become longer and thinner and eventually, indistinguishable

• Nucleoli reappear• Spindle fibers disintegrate• Phragmoplast and cell plate

form at equator

Telophase


Cell Plate Formation:• Phragmoplast develops between

daughter cell nuclei −Phragmoplast - complex of

microtubules and ER• Microtubules trap dictyosome-

derived vesicles• Vesicles fuse to form cell plate

−Cell plate grows outward toward mother cell walls

• Portions of ER trapped between vesicles, forming plasmodesmata

Cell plate formation

Higher Plant Cells Versus Animal Cells Plants:• Cell walls• Cell plate and plasmodesmata• Plastids and vacuoles

Animals:• Internal or external skeletons; no cell walls• Plasma membrane called cell membrane• Divide by pinching in two; no cell plate nor

plasmodesmata• Centrioles present during cell division• No plastids nor vacuoles

Review

History of Cell Theory Modern Microscopes Eukaryotic and Prokaryotic Cells Cell Structure and Communication Cell Components Cellular Reproduction Higher Plant Cells Versus Animal Cells

Cells

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