Chapter 4: Cell Structure & Function (Outline)

Cell Theory Cell Size Prokaryotic Cells Eukaryotic Cells

Organelles Nucleus Endomembrane System Cytoskeleton Centrioles, Cilia and Flagella

Development of Cell Theory In 1665, English Scientist Robert Hooke discovered cells

while looking at a thin slice of cork In 1673, Anton van Leuwenhoek observed pond scum &

discovered single-celled organisms using a handmade microscope

In 1831, English botanist Robert Brown described the nucleus of cells

In 1838, German Botanist, Matthias Schleiden, stated that all plant parts are made of cells

In 1839, German physiologist Theodor Schwann stated that all animal tissues are composed of cells

In 1858, Rudolf Virchow German physician concluded that cells must arise from preexisting cells

Cell Theory

A unifying concept in biology

Originated from the work of biologists Schleiden, Schwann & Virchow

States that: All organisms are composed of cells (Schleiden &

Schwann, 1838-39)

The cell is the basic unit of structure & function in organisms (Schleiden & Schwann, 1838-39)

All cells come only from preexisting cells since cells are self-reproducing (Virchow, 1858)

Cell Size

Most much smaller than one millimeter (mm) Some as small as one micrometer (m) Size restricted by Surface/Volume (S/V) ratio

Surface is membrane, across which cell acquires nutrients and expels wastes

Volume is living cytoplasm, which demands nutrients and produces wastes

As cell grows, volume increases faster than surface

Cells specialized in absorption modified to greatly increase surface area per unit volume

Surface to Volume Ratio

TotalSurfaceArea

(Height Width Number Of Sides Number Of Cubes) 96 cm2 192 cm2 384 cm2

TotalVolume

(Height Width Length x Number Of Cubes)

64 cm3 64 cm3 64 cm3 SurfaceAreaPerCube/VolumePerCube

(Surface Area/ Volume) 1.5/1 3/1 6/1

TotalSurfaceArea

(Height Width Number Of Sides Number Of Cubes) 96 cm2 192 cm2 384 cm2

TotalVolume

(Height Width Length x Number Of Cubes)

64 cm3 64 cm3 64 cm3 SurfaceAreaPerCube/VolumePerCube

(Surface Area/ Volume) 1.5/1 3/1 6/1

Sizes of living things and their component

Prokaryotic Cells Prokaryotes – lack a membrane-bounded nucleus and

are structurally less complicated than the eukaryotes

Prokaryotes are responsible for either all or significant portions of all of the following

Nutrient recycling – mineralization; nitrogen fixing

Decomposition of dead organisms

Disease (infectious) – tuberculoses; anthrax

Commercial uses – foodstuffs; antibiotics; insulin

Prokaryotes are divided into two domains Domain Bacteria

Domain Archaea

Prokaryotic Cells Nuclear body is not bounded by a nuclear membrane

Usually contains one circular chromosome composed of deoxyribonucleic acid (DNA)

The nuclear body is called a nucleoid

Extra chromosomal piece of DNA called plasmid

Structurally simple

Three basic shapes: Bacillus (rod)

Coccus (spherical)

Spirilla (spiral)

Prokaryotic Cells:The Envelope

Cell Envelopes include Glycocalyx

Layer of polysaccharides outside cell wall May be slimy and easily removed, or Well organized and resistant to removal (capsule)

Cell wall Consist of peptidoglycan (amino disaccharide & peptide) Maintains shape of the cell

Plasma membrane Like in eukaryotes – a phospholipid bilayer with proteins Form internal pouches (mesosomes), why?

Prokaryotic Cells:Cytoplasm Cytoplasm - semifluid solution bounded by a

plasma membrane containing Nucleoid – location of the single bacterium

chromosome (coiled)

Plasmid – extrachromosomal piece of circular DNA

Inclusion bodies – Stored granules of various substances

Ribosomes – tiny particles where protein is synthesized (contain RNA & protein in 2 subunits)

Thylakoids – extensive internal membranes found in cyanobacteria, function?

Prokaryotic Cells:Appendages

Appendages are made of protein that include Flagella – the most common form of bacterial

motility (made up of a filament, hook & basal body)

Fimbriae – small, bristle-like fibers that sprout from the cell surface (attach bacteria to a surface)

Conjugation pili – rigid tubular structures used to pass DNA from cell to cell

Prokaryotic Cells: Visual Summary

Eukaryotic Cells

Domain Eukarya Protists

Fungi

Plants

Animals

Eukaryotic cells contain: a true nucleus, bound by a double membrane

a complex collection of organelles

a plasma membrane

Eukaryotic Cells :Organelles Compartmentalization:

Allows eukaryotic cells to be larger than prokaryotic cells

Isolates reactions from others Two classes:

Endomembrane system: Organelles that communicate with one another

via membrane channels and small vesicles Energy related organelles

Mitochondria & chloroplasts Basically independent & self-sufficient

Animal and Plant Cells

Nucleus

Command center of cell, why? Separated from cytoplasm by nuclear envelope

Consists of double layer of membrane Nuclear pores permit exchange of ribosomal subunits &

mRNA between nucleoplasm & cytoplasm

Contains chromatin in semifluid nucleoplasm Chromatin contains DNA of genes Condenses to form chromosomes

Dark nucleolus composed of ribosomal RNA (rRNA) Produces subunits of ribosomes

Anatomy of the nucleus Messenger RNA

(mRNA) carries information about a protein sequence to the ribosome

Transfer RNA (tRNA) assembles the amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis

Ribosomes Serve in protein synthesis Composed of rRNA

Consists of a large subunit and a small subunit Each subunit is composed of protein and rRNA Subunits made in nucleolus Number of ribosomes in a cell varies depending on

function (e.g. pancreatic cells)

May be located: On the endoplasmic reticulum (ER), thereby making it

“rough”, or Free in the cytoplasm, either singly or in groups

called polyribosomes

Ribosome Function

Ribosome binding to the endoplasmic reticulum occurs through a signal peptide on the synthesized protein

Signal peptide combines with a signal recognition particle (SRP)

SRP attaches to SRP receptor, thus allowing protein to enter the lumen of the ER

The signal peptide is removed from the protein (via signal peptidase) in the lumen of the ER

Ribosomal subunits & mRNA break away and protein folds into its final shape

Nucleus, Ribosomes, & ER

Endomembrane System Restrict enzymatic reactions to specific

compartments within cell Consists of:

Nuclear envelope Membranes of endoplasmic reticulum Golgi apparatus Vesicles

Several types Transport materials between organelles of the

system

Endomembrane System:The Endoplasmic Reticulum A membrane network within the cytoplasm of cells

involved in the synthesis, modification and transport of cellular materials

Rough ER Studded with ribosomes on cytoplasmic side Protein anabolism

Synthesizes proteins Modifies proteins - adds sugar to protein (i.e. glycoproteins)

Forms vesicles - transport of large molecules to other parts of cell (i.e. Plasma membrane or Golgi apparatus)

Smooth ER Continuous with rough ER; No attached ribosomes Synthesis of lipids (i.e. phospholipids & steroids)

Endoplasmic Reticulum

Endomembrane System:The Golgi Apparatus

Golgi Apparatus Consists of 3-20 flattened, curved membrane-

bound saccules called cisternae

Resembles stack of deflated balloons

Modifies proteins (i.e. glycosylation) and lipids Packages them in vesicles

Receives vesicles from ER on cis face

Prepares for “export” in vesicles from trans face Within cell

Export from cell (secretion, exocytosis)

Golgi Apparatus

Endomembrane System:Lysosomes Membrane-bound vesicles (common in animal cells

but rare in plant cells) Produced by the Golgi apparatus Low pH Contain hydrolytic enzymes

Digestion of large molecules Recycling of cellular resources Destroying nonfunctional organelles

Lysosomes participate in apoptosis Normal part of development Example: tadpole → frog

Peroxisomes Similar to lysosomes

Membrane-bounded vesicles Enclose oxidative enzymes

However Enzymes synthesized by free ribosomes in cytoplasm

(instead of ER) Active in lipid metabolism Catalyze reactions that produce hydrogen peroxide

H2O2

Toxic molecule Broken down to H2O and O2 by catalase enzyme Alcohol detoxification in liver Germinating seeds oxidize fatty acids to sugars → growth

Peroxisomes & Vacuoles

Energy-Related Organelles:Chloroplast Structure An organelle found within the cells of green plants &

eukaryotic algae

Bounded by a double membrane

Inner membrane infolded

Forms disc-like thylakoids, which are stacked to form grana

Suspended in semi-fluid stroma

Green due to chlorophyll

Chlorophyll absorbs light between the red and blue spectrums and reflects green light, making leaves appear green

Found ONLY in inner membranes of chloroplast

Energy-Related Organelles:Chloroplasts Chloroplasts are a type of plastid & are considered

to have originated as endosymbiotic cyanobacteria Has its own DNA and reproduces independently of

the cell Captures light energy to drive cellular machinery Photosynthesis

Synthesizes carbohydrates from CO2 and H2O Makes own food using CO2 as only carbon source

Chloroplast Structure

Other Plastids Different types of plastids are classified according

to the kinds of pigments they contain

Chromoplasts lack chlorophyll but contain carotenoids responsible for the yellow, orange, & red colors of

some flowers and fruits

Leucoplasts are colorless plastids, which synthesize and store a variety of energy sources in non-photosynthetic tissues Amyloplasts (starch)

Elaioplasts (lipids)

Energy-Related Organelles:Mitochondria Mitochondria are rod-shaped organelles that can be

considered the power generators of the cell Bounded by double membrane

Cristae – Infoldings of inner membrane that encloses matrix, why?

Matrix – Inner semifluid containing respiratory enzymes

Involved in cellular respiration – process by which chemical energy of sugar is converted to ATP

Produce most of ATP utilized by the cell Has its own DNA and reproduces independently of

the cell

Mitochondrial Structure

The Cytoskeleton Maintains cell shape

Assists in movement of cell and organelles

Three types of macromolecular fibers Actin Filaments

Intermediate Filaments

Microtubules

Dynamic, assemble and disassemble as needed Protein phosphorylation (e.g. protein kinases)

Phosphorylation → disassembly Dephosphorylation → assembly

Cytoskeleton Protein Fibers

The Cytoskeleton:Actin Filaments

Extremely thin filaments like a twisted pearl necklace

Dense web just under plasma membrane maintains cell shape

Support for microvilli in intestinal cells Intracellular traffic control

For moving stuff around within cell Cytoplasmic streaming in plant cells

Function in pseudopods of amoeboid cells Pinches off dividing animal cells apart during mitosis Important component in muscle contraction (other is

myosin)

Actin Filaments

Actin Filament Operation

Actin filaments interact with motor molecules (proteins that can attach, detach and reattach to the actin filament)

Myosin pulls actin filaments in the presence of ATP In muscle cells, cytoplasmic myosin tails are bound

to membranes, while heads interact with actin

The Cytoskeleton:Intermediate Filaments Intermediate in size between actin filaments

and microtubules

Rope-like assembly of fibrous polypeptides

Vary in nature (i.e. from tissue to tissue and from time to time)

Functions: Mechanical stability of the plasma- and the

nucleus-membranes

Cell-cell interaction, like those holding skin cells tightly together (keratin)

The Cytoskeleton:Microtubules Hollow cylinders made of two globular proteins

called and tubulin giving rise to structures called dimers

Dimers then arrange themselves into tubular spirals of 13 dimers around

Assembly: Under control of Microtubule Organizing Center

(MTOC) Most important MTOC is centrosome

Interacts with proteins kinesin and dynein to cause movement of organelles

Microtubule Operation

Microtubules Microtubules disassemble and then

reassemble into a spindle during cellular division

Colchicine - a plant toxic (defense mechanism) that inhibits polymerization by binding to tubulin and preventing microtubule assembly

The Cytoskeleton (Summary) Microfilaments regulate:

Cell shape Cell movement

Intermediate filaments effect: The mechanical stability of the plasma- & the

nucleus-membranes Cell-cell interaction

Microtubules effect: Localization and transport of organelles Cell division

Microtubular Arrays:Centrioles

Short, hollow cylinders Composed of 27 microtubules Microtubules are arranged in 9 sets of 3 each

(9 + 0) pattern One pair per animal cell

Located on centrosome of animal cells Oriented at right angles to each other Separate during mitosis (cell division)

May give rise to basal bodies of cilia & flagella Plant cells do not have centrioles

Centrioles

Microtubular arrays:Cilia and Flagella Hair-like projections from cell surface that aid in

cell movement Very different from prokaryote flagella

Outer covering of plasma membrane Inside is a cylinder of 18 microtubules arranged in

9 pairs Two single microtubules run down the centre of

the shaft (9 + 2) pattern found in cilia and flagella In eukaryotes, cilia are much shorter and

numerous than flagella Cilia move in coordinated waves like oars Flagella move like a propeller or cork screw

Cilia and Flagella The pairs of microtubules are connected by short

arms of protein dymein Movement of the cilia or flagella is the result of

sliding movements between microtubule pairs Beneath each cilium of flagellum in the cytoplasm

of the cell is a basal body The two central microtubules of the cilia/flagellum

do not extend into the basal boy. The nine pairs of microtubule do and they are

joined by a third microtubule. Centrioles are needed to create basal bodies in

order to produce cilia and/or flagella