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Cells
The Units of Life
Early Discoveries

Mid 1600s - Robert Hooke observed and
described cells in cork

Late 1600s - Antony van Leeuwenhoek
observed sperm, microorganisms

1820s - Robert Brown observed and
named nucleus in plant cells
Cell Theory

Schleiden and Schwann

Every organism is composed of one or more
cells

Cell is smallest unit having properties of
life

Virchow

All exisiting cells arise from pre-existing
cells.
Cell


Smallest unit of life
Can survive on its own or has potential
to do so

Is highly organized for metabolism

Senses and responds to environment

Has potential to reproduce
Measuring
Cells Vary in Size
Why Are Cells So Small?

Surface-to-volume ratio

The bigger a cell is, the less surface area
there is per unit volume

Above a certain size, material cannot be
moved in or out of cell fast enough
Size is Limited
Ways to Study Cells:
Microscopes


Create detailed images of something
that is otherwise too small to see
Light microscopes


Simple or compound
Electron microscopes

Transmission EM or Scanning EM
Different Types of Light Microscopy: A
Comparison
Limitations of Light Microscopy



Wavelengths of light are 400-750 nm
If a structure is less than one-half of a
wavelength long, it will not be visible
Light microscopes can resolve objects
down to about 200 nm in size
Electron Microscopy



Uses streams of accelerated electrons
rather than light
Electrons are focused by magnets
rather than glass lenses
Can resolve structures down to 0.5 nm
Elecrton Microscopes
Electron micrographs
Ways to Study Cells:
Cell Fractionation
Structure of Cells


Two types of cells

Prokaryotic

Eukaryotic
All cells have:

Plasma membrane

Region where DNA is stored

Cytoplasm
Prokaryotic Cells

No nucleus




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Nucleoid area where DNA resides
No membrane bound organelles.
70s ribosomes
Cell walls contain petidoglycan or
pseudomurein
Prokaryotic Organisms



Archaeobacteria
Eubacteria
Cyanobacteria
A prokaryotic cell
E. coli
Eukaryotic Cells


Have a nucleus and other
organelles
Eukaryotic organisms
Protistans
 Fungi
 Plants
 Animals

Overview of an animal cell
The nucleus and its envelope
Functions of Nucleus


Keeps the DNA molecules of eukaryotic
cells separated from metabolic
machinery of cytoplasm
Makes it easier to organize DNA and to
copy it before parent cells divide into
daughter cells
Cytomembrane System

Group of related organelles in which lipids are
assembled and new polypeptide chains are
modified

Products are sorted and shipped to various
destinations

Components of the cytomembrane system



Endoplasmic reticulum
Golgi apparatus
vesicles
Endoplasmic Reticulum

In animal cells, continuous with nuclear
membrane

Extends throughout cytoplasm

Two regions - rough and smooth
Rough ER




Arranged into flattened sacs
Ribosomes on surface give it a rough
appearance
Some polypeptide chains enter rough ER
and are modified
Cells that specialize in secreting
proteins have lots of rough ER
Smooth ER




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A series of interconnected tubules
No ribosomes on surface
Lipids assembled inside tubules
Smooth ER of liver inactivates wastes,
drugs
Sarcoplasmic reticulum of muscle is a
specialized form
Endoplasmic reticulum (ER)
Golgi Bodies



Put finishing touches on proteins and
lipids that arrive from ER
Package finished material for shipment
to final destinations
Material arrives and leaves in vesicles
The Golgi apparatus
Vesicles

Membranous sacs that
move through the
cytoplasm

Lysosomes

Peroxisomes
Lysosomes
Review: relationships among organelles of the
endomembrane system
The mitochondrion, site of cellular respiration
Organelles with no
Membranes

Ribosomes


Cytoskeleton


Function in protein synthesis
Function in maintenance of cell shape and
positioning of organelles
Centrioles (animals only)

Function during cell division
Ribosomes
Cytoskeleton



Present in all eukaryotic cells
Basis for cell shape and internal
organization
Allows organelle movement within cells
and, in some cases, cell motility
Cytoskeletal Elements
intermediate
filament
microtubule
microfilament
Microtubules

Largest elements

Composed of the protein tubulin

Arise from microtubule organizing
centers (MTOCs)

Polar and dynamic

Involved in shape, motility, cell division
Microtubules

Largest elements

Composed of the protein tubulin

Arise from microtubule organizing
centers (MTOCs)

Polar and dynamic

Involved in shape, motility, cell division
Microfilaments

Thinnest cytoskeletal elements

Composed of the protein actin

Polar and dynamic

Take part in movement, formation and
maintenance of cell shape
Intermediate Filaments

Present only in animal cells of certain
tissues

Most stable cytoskeletal elements

Six known groups

Different cell types usually have 1-2
different kinds
Cell Junctions
tight
junctions
adhering
junction
gap
junction
Cell Membranes
Structure and Function
lipid bilayer
fluid
fluid
one layer
of lipids
one layer
of lipids
Fig. 4.3, p. 52
Lipid Bilayer



Main component of
cell membranes
Gives the
membrane its fluid
properties
Two layers of
phospholipids
Figure 8.2 Two generations of membrane models
The detailed structure of an animal cell’s plasma membrane, in cross section
Functions of Membrane Proteins


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Transport
Enzymatic activity
Receptors for
signal transduction
Figure 3.4.1
Functions of Membrane Proteins

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Intercellular
adhesion
Cell-cell
recognition
Attachment to
cytoskeleton and
extracellular
matrix
Figure 3.4.2
oligosaccharide
cholesterol
groups
phospholipid
EXTRACELLULAR ENVIRONMENT
(cytoskeletal proteins beneatch
ADHESION
the plasma
PROTEIN
membrane)
open
gated
channel channel
protein proten
(open)
gated
channel
proten
(closed)
active
transport
protein
(area of
enlargment)
TRANSPORT PROTEINS
RECEPTOR
PROTEIN
LIPID BILAYER
RECOGNITION
PROTEIN
CYTOPLASM
PLASMA MEMBRANE
Fig. 4.4, p. 53
The fluidity of membranes
Cell Membranes Show
Selective Permeability
O2, CO2, and other small
nonpolar molecules;and H2O
C6H12O6, and other large, polar
(water-soluble) molecules; ions
such as H+, Na+, CI-, Ca++; plus
H2O hydrogen-bonded to them
X
Membrane Crossing
Mechanisms
Diffusion across lipid bilayer
Passive transport
Active transport
Endocytosis
Exocytosis
Diffusion


The net movement of like molecules or
ions down a concentration gradient
Although molecules collide randomly,
the net movement is away from the
place with the most collisions (down
gradient)
Concentration Gradient


Means the number of molecules or ions
in one region is different than the
number in another region
In the absence of other forces, a
substance moves from a region where it
is more concentrated to one one where
it’s less concentrated - “down” gradient
Factors Affecting
Diffusion Rate

Steepness of concentration gradient

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Molecular size

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Smaller molecules, faster diffusion
Temperature

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Steeper gradient, faster diffusion
Higher temperature, faster diffusion
Electrical or pressure gradients
The diffusion of solutes across membranes
Osmosis
A Special Case of Simple Diffusion


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Diffusion of water molecules across a
selectively permeable membrane
Direction of net flow is determined by
water concentration gradient
Side with the most solute molecules has
the lowest water concentration
Tonicity
Refers to relative solute concentration
of two fluids
Hypertonic - having more solutes
Isotonic - having same amount
Hypotonic - having fewer solutes
Tonicity and Osmosis
2%
sucrose
water
10% sucrose
2% sucrose
The water balance of living cells
The contractile vacuole of Paramecium: an evolutionary adaptation for
osmoregulation
Increase in Fluid Volume
compartment
1
compartment
2
HYPOTONIC
SOLUTION
HYPERTONIC
SOLUTION
membrane permeable to
water but not to solutes
fluid volume increases
In compartment 2
Passive Transport

Faciltiated Diffusion

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Flow of solutes through the interior of
passive transport proteins down their
concentration gradients
Passive transport proteins allow solutes
to move both ways
Does not require any energy input
Transport Proteins

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Span the lipid bilayer
Interior is able to open to both sides
Change shape when they interact with
solute
Play roles in active and passive
transport
Two models for facilitated diffusion
Facilitated Diffusion
solute
Active Transport



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Net diffusion of solute is against
concentration gradient
Transport protein must be activated
ATP gives up phosphate to activate
protein
Binding of ATP changes protein shape
and affinity for solute
Active
Transport
High solute concentration
Low solute concentration

P
ATP
ADP

P
P
P
ATP gives up
phosphate to
activate protein
Binding of ATP
changes protein
shape and affinity
for solute
The sodium-potassium pump: a specific case of active transport
Review: passive and active transport compared
Bulk Transport – Another form
of Active Transport
Exocytosis
Endocytosis
The three types of endocytosis in animal cells
Mitosis
How to clone a nucleus
Roles of Mitosis


Multicelled organisms

Growth

Cell replacement
Some protistans, fungi, plants, animals

Asexual reproduction
Chromosome

A DNA molecule & attached proteins

Duplicated in preparation for mitosis
one chromosome (unduplicated)
one chromosome (duplicated)
Cell Cycle



Cycle starts when a new cell forms
During cycle, cell increases in mass and
duplicates its chromosomes
Cycle ends when the new cell divides
Interphase

Usually longest part of the cycle

Cell increases in mass

Number of cytoplasmic components
doubles

DNA is duplicated
Mitosis

Period of nuclear division

Usually followed by cytoplasmic division

Four stages:
Prophase
Metaphase
Anaphase
Telophase
Control of the Cycle

Once S begins, the cycle automatically
runs through G2 and mitosis

The cycle has a built-in molecular brake
in G1

Cancer involves a loss of control over
the cycle, malfunction of the “brakes”
Stopping the Cycle


Some cells normally stop in interphase

Neurons in human brain

Arrested cells do not divide
Adverse conditions can stop cycle

Nutrient-deprived amoebas get stuck in
interphase
Stages of Mitosis
Prophase
Metaphase
Anaphase
Telophase
Early Prophase Mitosis Begins
Duplicated chromosomes begin to condense
Late Prophase


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New microtubules
are assembled
One centriole pair is
moved toward
opposite pole of
spindle
Nuclear envelope
starts to break up
Transition to Metaphase


Spindle forms
Spindle
microtubules
become attached
to the two sister
chromatids of each
chromosome
Metaphase

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All chromosomes
are lined up at the
spindle equator
Chromosomes are
maximally
condensed
Anaphase

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Sister chromatids
of each
chromosome are
pulled apart
Once separated,
each chromatid is
a chromosome
Telophase


Chromosomes
decondense
Two nuclear
membranes form,
one around each set
of unduplicated
chromosomes
Results of Mitosis

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Two daughter nuclei
Each with same
chromosome
number as parent
cell
Chromosomes in
unduplicated form
Cytoplasmic Division

Usually occurs between late anaphase
and end of telophase

Two mechanisms

Cell plate formation (plants)

Cleavage (animals)
Animal Cell Division