Download the essence of life

the cell
biology 1
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How we investigate cells
Cell types and size
A tour of the cell
The nucleus
The endomembrane system
Specialized organelles
How we investigate cells
• Microscopy
– Light microscope (Hooke 1665)
• Magnification x1000
• Limited by resolution of light energy (0.2 µm)
– Electron microscope (since 1950s)
• Uses beam of electrons to resolve 0.2nm)
– Transmission electron microscope (TEM) examines internal
structure
– Scanning electron microscope (SEM) examines surface
features (3d)
• Other techniques include cell fractionation
(via centrifuge)
Cell types and sizes
• First cells observed in 1665 by Hooke
were cork cells
• Further work by Leeuwenhoek, Schleiden
and Schwann led to proposal of cell
theory, which states
– Organisms are composed of one or more
cells
– Cells are the smallest unit of life
– Cells arise only by division of previously
existing cells
• Cell types include prokaryotic and
eukaryotic cells
Prokaryotic
Eukaryotic
(pro=before; karyon=kernel)
(eu=true; karyon=kernel)
Monera
(Cyanobacteria and bacteria)
Protista, Plantae, Fungi, and
Animalia
No true nucleus, lacks nuclear
envelope
True nucleus bound by nuclear
envelope
Genetic material found in
nucleoid region
Genetic material found in
nucleus
No membra ne bound organelles Contains cytoplasm with cytosol
and membra ne bound
organelles
Size = 1–10µm
Size = 10–100µm
The importance of surface area:volume ratio
• Let amount of metabolism that occurs within a cell
be a function of cell volume
• Let rate of metabolism (which is a function of
supply of reactants, and removal of products) be a
function of surface area
• Therefore,surface area:volume ratio (SA:Vol)
dictates the efficiency of a cell
– Small cells have high SA:Vol, and are therefore efficient
– Large cells tend towards a low SA:Vol ratio, and are
therefore inefficient
How to maximize SA:Vol
1
5
1
5
1
Original Cell:
SA = 1x1x6 = 6 units2
Vol = 1x1x1 = 1 unit3
SA:Vol = 6:1
Grow large? NO
SA = 5x5x6 = 150 units2
Vol = 5x5x5 = 125 unit3
SA:Vol = 1.2:1
1
5
1
1
5
Divide? YES
SA = 5x5x5x6x1x1 = 750 units2
Vol = 5x5x5 = 125 unit3
SA:Vol = 6:1
5
5
SA:Vol – the limiting factor in cell size
• Prokaryotes, because of their smaller size, have
optimum SA:Vol
• Single-celled eukaryotes which are larger cells
compensate by an internal membrane system that
partitions cells into compartments
– Folded surface maximizes surface area for reaction
– Membranes incorporate some enzymes that participate
in reactions
– Compartments provide localized environment for
reactions
• For larger biomasses, eukaryotic strategy is
instead to divide = multicellular organisms, with cell
specialization
A tour of the cell
• All cells have at least three components
– Nucleus or nucleoid region
– Plasma membrane
– Cytoplasm
• In addition, eukaryotic cells have membrane
bound organelles, and endomembrane system
• Prokaryotic cells have an additional boundary the cell wall, primarily a peptidoglycan matrix
(review Prokaryotes, pp.84-85)
• Some eukaryotic cells (e.g., plants) also have a
cell wall, which instead is cellulose based
Eukaryotes: the Nucleus
• Most, but not all eukaryotic cells have a nucleus
• Nucleus contains a majority of the genetic information
of the cell
• Contains a mixture of DNA and proteins complexes
known as histones—this mixture is known as
chromatin
• Enclosed by a nuclear envelope—a double bilipid
layer membrane
– Outside membrane is continuous with the cell’s endomembrane system
– Inside membrane may be bound to a nuclear matrix
• Nucleolus is the site of rRNA production
The endomembrane system
• Divides cell into compartments
• Includes
– endoplasmic reticulum (ER)
• Rough ER (with ribosomes)
• Smooth ER (without ribosomes)
– Golgi apparatus
• Golgi bodies
– (Lysosomes, vacuoles)
• Responsible for production of various
macromolecules, including proteins and some
lipids
The ER’s role in protein production
• mRNA, transcribed from DNA, enters the cell’s
cytoplasm from the nucleus through a nuclear pore
• mRNA binds with a ribosome, and migrates to the
surface of the ER
• As polypeptide chain is assembled, it is passed into
the lumen of the ER
• Polypeptide is passed along ER and budded off into
vesicle that travels to the cis face of a golgi body
• Final assembly of molecule occurs in Golgi body,
vesicle budded off of trans face
• Transport vesicle containing final assembled protein
travels to destination
Smooth ER
• Synthesis of lipids, phospholipids and
steroids
• Participates in cellular metabolism
• Detoxifies drugs and poisons
(drug tolerance?)
• Stores Ca2+ ions necessary for muscle
contraction
is an extension of
Nuclear envelope
is confluent with
Rough ER
Smooth ER
Membrane and secretory
proteins produced in
ER are transported in
Vesicles
Fuse with the
forming cis
face of
Golgi Apparatus
Pinches off
maturing
(trans) face
Vesicles
Gives rise to
Lysosomes
And Vacuoles
Fuse with and add to
plasma membrane and
may release cellular products
to outside
Plasma
Membrane
Specialized organelles
• Mitochondria are sites of cellular
respiration
– Number of mitochondria correlates with
metabolic activity of cell
– Enclosed by a double membrane
– Features internal folds termed cristae
– Internal fluid is termed mitochondrial matrix
– Evidence for Endosymbiotic theory?
• Size
• Have own DNA
• Plastids are specialized membrane bound
organelles found in plants, including
• Amyloplasts (store starch)
• Chromoplasts (store pigments other than
chlorophyll)
• Choloroplasts (store chlorophyll, site of
cellular photosynthesis)
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Enclosed by double membrane
Contains stacks (grana) of thylakoids
Fluid inside of chloroplast is termed stroma
Evidence for Endosymbiotic theory?
• Size
• Have own DNA
Cell walls
• Some cells have an additional,
structural boundary outside of the
plasma membrane
• In plants, the cell wall is cellulose based
• In bacteria, the cell wall is peptidoglycan
based
– Gram positive and negative bacteria
• Structural rigidity of cell wall allows cells
to take on more water than an animal
cell could
The cytosol
• Contains cytoplasm and cytoskeleton
• Cytoskeleton = network of fibers throughout
the cytoplasm that forms a dynamic
framework for support and movement.
Constructed from:
– Microtubules (e.g., cilia, flagella, centrioles)
– Intermediate filaments (e.g., the cellular scaffold
– Microfilaments (actin: muscle contraction,
localized contractions of portions of cell)
The plasma membrane
• Just one example of a lipid bilayer
membrane found in cells
• Controls passage of molecules in and
out of the cell
• Like all membranes, represents a
complex interactions of phospholipids,
proteins and carbohydrates
Membrane theory
• Davson-Danielli model suggested
phospholipid bilayer sandwiched
between two layers of globular protein
– Suggests a symmetrical geometry - in fact
not the case (inside and outside face)
– Some membranes looked different, and
had different functions
– Since most proteins are hydrophobic, woul
not be stable
The Fluid Mosaic Model
• Proposed by Singer-Nicolson
• Membrane is a mosaic of proteins
‘bobbing’ in a fluid phospholipid bilayer
• Hydrophilic portions of phospholipid and
proteins are maximally exposed to
aqueous interface, ensuring stability
• Most lipids and some proteins drift
laterally across surface of membrane fluidity is aided by addition of kinked
hydrocarbon tails
Molecules found in the
phospholipid bilayer matrix
• Integral proteins
– Unilateral or transmembrane
• Peripheral proteins
• Carbohydrates (cell-cell recognition)
– Oligosaccharides
– Glycolipids
– Glycoproteins
Permeability - the ultimate
control of metabolism?
• Membranes display selective permeability
– Solubility characteristics of the phospholipid bilayer
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Nonpolar molecules dissolve in membrane
Small polar molecules pass easily
Large polar molecules do not pass easily
Ions are usually pumped across
– Presence of specific integral transport proteins
• Hydrophilic tunnel through membrane?
• May bind to substance and move it across
Types of transport
• Passive transport systems
– Diffusion - movement of molecules down a
concentration gradient (high conc. to low conc.).
Relies on intrinsic kinetic energy of molecules
– Osmosis (the diffusion of water) - the movement of
water from a low concentration solute (hypertonic
solution) to a high concentration solute (hypotonic
solution)
– Facilitated diffusion. Uses transport proteins
• Active transport
– Requires energy (ATP) to pump molecules against
concentration gradient
Large molecule transport
across a membrane
• Exocytosis (out of) and endocytosis
(into the cell)
– Phagocytosis (engulfment of particles
using pseudopodia)
– Pinocytosis (engulfment of fluid using
pseudopodia)
– Receptor-mediated endocytosis
Cell Communication
• How do cells talk to each other? There
are four general methods (Raven and
Johnson, Figure 7.3:
• Direct contact
• Paracrine signalling (short-lived, local)
• Endocrine signalling (hormones)
• Synaptic signalling (neurotransmitters)
A general model of
communication...
• Signal production
• Signal Transmission
• Signal Reception
How are signal received? Proteins play a
key role - their 3-dimensionality
provides specificity to signal types
– Intracellular
– Cell surface
Intracellular receptors
• Lipid soluble or small molecule signals pass directly
through cell membrane to receptor
• Receptors are almost always enzymes-addition of
chemical signal either inhibits or activates enzyme
activity by altering shape of protein
• Effected enzymes may be in the cytoplasm or
nucleus (DNA transcription)
• For example, Nitric Oxide activates the enzyme
Guanylyl cyclase, which catalyzes the production of
cyclic guanosine monophosphate, causing relaxation
of smooth muscles around vessels, increasing blood
flow
Cell Surface receptors
• Chemically gated ion channels
• Enzymic receptors
• G-protein linked receptors
Raven and Johnson Figure 7.6
Chemically gated ion channels
• Multipass protein whose core provides
passage to ions if gate binds with signal
• Channels are specific to particular ions,
depending on geometry
• Influx of ions (e.g., Ca2+, K+, Na+) has
variable effects according to cell type
Enzymic receptors
• Binding of signal to exterior of transmembrane protein reconfigure enzyme
to be active on inside of cell
• In most cases the enzyme is a kinase,
causing phosphorylation
G-Protein linked receptors
• Guanosine triphosphate (GTP)-binding
protein
• Trans-membrane multipass protein (7
times) creates channel through
membrane
• Addition of signal causes protein to bind
GTP, activating it. GTP-protein complex
then diffuses out to target
How does the receptor effect
a change in the cell?
• Typically, the resultant change will effect
secondary signal - commonly Ca2+ or
cAMP (cyclic adenosine
monophosphate)
• An amplification system causes the
signal to proliferate throughout the cell
How to cells stick together
• Tight junctions
• Anchoring junctions (cadherin-mediated
and integrin mediated links)
– Hemidesmosome
– Desmosome
• Communicating junctions
– Gap junction
– Plasmodesmata