Download Cells (Ch3)

CELLS
Chapter 3
Cell Theory
• Cell: structural and functional unit of life
• Organismal functions depend on individual
and collective cell functions
• Biochemical activities of cells dictated by their
shapes or forms, and specific subcellular
structures
• Continuity of life has cellular basis
Cell Diversity
• Over 200 different types of
human cells
• Types differ in size, shape,
subcellular components, and
functions
Plasma Membrane
• Bimolecular layer of lipids and proteins in a
constantly changing fluid mosaic
• Plays a dynamic role in cellular activity
• Separates intracellular fluid (ICF) from
extracellular fluid (ECF)
– Interstitial fluid (IF) = ECF that surrounds cells
Membrane Lipids
• 75% phospholipids (lipid bilayer)
– Phosphate heads: polar and hydrophilic
– Fatty acid tails: nonpolar and hydrophobic
• 5% glycolipids
– Lipids with polar sugar groups on outer membrane surface
• 20% cholesterol
– Increases membrane stability and fluidity
• Lipid Rafts
– ~ 20% of the outer membrane surface
– Contain phospholipids, sphingolipids, and cholesterol
– May function as stable platforms for cell-signaling
molecules
Membrane Proteins
• Integral proteins
– Firmly inserted into the membrane (most are
transmembrane)
– Functions: Transport proteins (channels and carriers),
enzymes, or receptors
• Peripheral Proteins
– Loosely attached to integral proteins
– Include filaments on intracellular surface and
glycoproteins on extracellular surface
– Functions: Enzymes, motor proteins, cell-to-cell links,
provide support on intracellular surface, and form
part of glycocalyx
= protein
= lipid
Extracellular fluid
(watery environment)
Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outwardfacing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Peripheral
Bimolecular
Inward-facing
proteins
lipid layer
layer of
containing
phospholipids
Nonpolar
proteins
tail of
phospholipid
Cytoplasm
molecule
(watery environment)
Figure 3.3
Cell Junctions
• Some cells "free”
• Some bound into communities
– Three ways cells are bound:
• Tight junctions
• Desmosomes
• Gap junctions
Tight Junctions
• Adjacent integral proteins fuse  form
impermeable junction encircling cell
– Prevent fluids and most molecules from moving
between cells
Desmosomes
• Anchor cells together at
plaques (thickenings on
plasma membrane)
– Linker proteins between cells
connect plaques
– Keratin filaments (part of the
cytoskeleton) extend through
cytosol to opposite plaque
giving stability to cell
– Reduces possibility of tearing
Gap Junctions
• Transmembrane
proteins form pores
(connexons) that
allow small molecules
to pass from cell to
cell
– For spread of ions,
simple sugars, and
other small molecules
between cardiac or
smooth muscle cells
Cell Cycle
• Defines changes from formation of cell until it
reproduces
• Includes:
– Interphase
– Cell division (mitotic phase)
Interphase
• Period from cell formation to cell division
• Nuclear material called chromatin
• Three subphases:
– G1 (gap 1)—vigorous growth and metabolism
• Cells that permanently cease dividing said to be in G0
phase
– S (synthetic)—DNA replication occurs
– G2 (gap 2)—preparation for division
© 2013 Pearson Education, Inc.
Interphase
Centrosomes (each
has 2 centrioles)
Plasma
membrane
Nucleolus
Chromatin
Nuclear
envelope
© 2013 Pearson Education, Inc.
DNA Replication
Cell Division
• Meiosis - cell division producing gametes
• Mitotic cell division - produces clones
– Essential for body growth and tissue repair
– Occurs continuously in some cells
• Skin; intestinal lining
– None in most mature cells of nervous tissue,
skeletal muscle, and cardiac muscle
• Repairs with fibrous tissue
Events Of Cell Division
• Mitosis—division of nucleus
– Four stages
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
– Cytokinesis—division of
cytoplasm-by cleavage
furrow
Prophase
Metaphase
Anaphase
Telophase and Cytokinesis
Telophase
Cytokinesis
Nuclear
envelope
forming
Nucleolus forming
Contractile
ring at
cleavage
furrow
Control of Cell Division
• "Go" signals:
– Critical volume of cell when area of membrane
inadequate for exchange
– Chemicals (e.g., growth factors, hormones)
– Availability of space–contact inhibition
• To replicate DNA and enter mitosis requires
– Cyclins–regulatory proteins
• Accumulate during interphase
– Cdks (Cyclin-dependent kinases)–bind to cyclins 
activated
• Enzyme cascades prepare cell for division
– Cyclins destroyed after mitotic cell division
Control of Cell Division
• Checkpoints
– G1 checkpoint (restriction
point) most important
• If doesn't pass  G0–no
further division
– Late in G2 MPF (M-phase
promoting factor)
required to enter M phase
• "Other Controls" signals
– Repressor genes inhibit
cell division
• E.g., P53 gene
Protein Synthesis Overview
Two Components
• Transcription
• Translation
RNA is heavily
involved
Roles of the Three Main Types of RNA
• Messenger RNA (mRNA)
– Carries instructions for building a polypeptide, from
gene in DNA to ribosomes in cytoplasm
• Ribosomal RNA (rRNA)
– A structural component of ribosomes that, along with
tRNA, helps translate message from mRNA
• Transfer RNAs (tRNAs)
– Bind to amino acids and pair with bases of codons of
mRNA at ribosome to begin process of protein
synthesis
Transcription
• Transfers DNA gene base sequence to a
complementary base sequence of an mRNA
• Transcription factor
– Loosens histones from DNA in area to be
transcribed
– Binds to promoter, a DNA sequence specifying
start site of gene to be transcribed
– Mediates the binding of RNA polymerase to
promoter
Transcription
• RNA polymerase
– Enzyme that oversees synthesis of mRNA
– Unwinds DNA template
– Adds complementary RNA nucleotides on DNA
template and joins them together
– Stops when it reaches termination signal
– mRNA pulls off the DNA template, is further
processed by enzymes, and enters cytosol
RNA polymerase
Transcription
Coding strand
DNA
Promoter
region
Template strand
Termination
signal
1 Initiation: With the help of transcription factors, RNA
polymerase binds to the promoter, pries apart the two DNA strands,
and initiates mRNA synthesis at the start point on the template strand.
mRNA
Template strand
Coding strand of DNA
2 Elongation: As the RNA polymerase moves along the template
Rewinding
of DNA
strand, elongating the mRNA transcript one base at a time, it unwinds
the DNA double helix before it and rewinds the double helix behind it.
mRNA transcript
RNA nucleotides
Direction of
transcription
mRNA
DNA-RNA hybrid region
Template
strand
RNA
polymerase
3 Termination: mRNA synthesis ends when the termination signal
is reached. RNA polymerase and the completed mRNA transcript are
released.
Unwinding
of DNA
The DNA-RNA hybrid: At any given moment, 16–18 base pairs of
DNA are unwound and the most recently made RNA is still bound to
DNA. This small region is called the DNA-RNA hybrid.
Completed mRNA transcript
RNA polymerase
Figure 3.35
Translation
• Converts base sequence of nucleic acids into
the amino acid sequence of proteins
• Involves mRNAs, tRNAs, and rRNAs
• Each three-base sequence on DNA is
represented by a codon
– Codon—complementary three-base sequence on
mRNA
– Each codon corresponds to a specific amino acid
What do you notice
about this?
How many stop
codons are there?
Start codons?
Translation
• mRNA attaches to a small ribosomal subunit that moves
along the mRNA to the start codon
• Large ribosomal unit attaches, forming a functional
ribosome
• Anticodon of a tRNA binds to its complementary codon
and adds its amino acid to the forming protein chain
• New amino acids are added by other tRNAs as ribosome
moves along rRNA, until stop codon is reached
Translation
Role of Rough ER in Protein Synthesis
• mRNA–ribosome complex is directed to rough
ER by a signal-recognition particle (SRP)
• Forming protein enters the ER
• Sugar groups may be added to the protein,
and its shape may be altered
• Protein is enclosed in a vesicle for transport to
Golgi apparatus
1 The mRNA-ribosome complex is
directed to the rough ER by the SRP.
There the SRP binds to a receptor site.
ER signal
sequence
2 Once attached to the ER, the SRP is released
and the growing polypeptide snakes through the
ER membrane pore into the cisterna.
3 The signal sequence is clipped off by an
enzyme. As protein synthesis continues, sugar
groups may be added to the protein.
Ribosome
mRNA
Signal
Signal
recognition
sequence
particle Receptor site
removed
(SRP)
Growing
polypeptide
4 In this example, the completed
protein is released from the ribosome
and folds into its 3-D conformation,
a process aided by molecular chaperones.
Sugar
group
5 The protein is enclosed within a
protein (coatomer)-coated transport
vesicle. The transport vesicles make
their way to the Golgi apparatus,
where further processing of the
proteins occurs (see Figure 3.19).
Released
protein
Rough ER cisterna
Cytoplasm
Transport vesicle
pinching off
Coatomer-coated
transport vesicle
Figure 3.39
Membrane Transport
• Plasma membranes are selectively permeable
• Passive Processes
– No cellular energy (ATP) required
– Substance moves down its concentration gradient
• Active Processes
– Energy (ATP) required
– Occurs only in living cell membranes
Passive Processes
• Simple diffusion
– Nonpolar lipid-soluble (hydrophobic) substances
diffuse directly through the phospholipid bilayer
• Facilitated diffusion
– Carrier or channel mediated
• Osmosis
Passive Processes: Facilitated Diffusion
• Some lipophobic molecules (e.g., glucose,
amino acids, and ions) use carrier proteins or
channel proteins, both of which:
– Exhibit specificity (selectivity)
– Are saturable; rate is determined by number of
carriers or channels
– Can be regulated in terms of activity and quantity
Facilitated Diffusion Using Carrier
Proteins
• Transmembrane integral proteins transport specific
polar molecules (e.g., sugars and amino acids)
• Binding of substrate causes shape change in carrier
Facilitated Diffusion Using Channel
Proteins
• Aqueous channels formed by transmembrane
proteins selectively transport ions or water
• Two types:
– Leakage channels
• Always open
– Gated channels
• Controlled by chemical or electrical signals
Passive Processes: Osmosis
• Movement of solvent
(water) across a selectively
permeable membrane
• Water diffuses through
plasma membranes:
– Through the lipid bilayer
– Through water channels
called aquaporins (AQPs)
Passive Processes: Osmosis
• Water concentration is determined by solute
concentration
• Osmolarity: total concentration of solute
particles
• When solutions of different osmolarity are
separated by a membrane, osmosis occurs
until equilibrium is reached
• In Cells: When osmosis occurs, water enters or
leaves a cell
– Change in cell volume disrupts cell function
Tonicity
• Tonicity: The ability of a solution to cause a
cell to shrink or swell
• Isotonic: A solution with the same solute
concentration as that of the cytosol
• Hypertonic: A solution having greater solute
concentration than that of the cytosol
• Hypotonic: A solution having lesser solute
concentration than that of the cytosol
Membrane Transport: Active Processes
• Two types of active processes:
– Active transport
– Vesicular transport
• Both use ATP to move solutes across a living
plasma membrane
Active Transport
• Requires carrier proteins (solute pumps)
• Moves solutes against a concentration
gradient
• Types of active transport:
– Primary active transport
– Secondary active transport
Primary Active Transport
• Hydrolysis of ATP (energy!) causes shape change in
transport protein
• bound solutes (ions) are “pumped” across the
membrane
• Sodium-potassium pump (Na+-K+ ATPase)
– Located in all plasma membranes
– Involved in primary (and secondary) active transport
of nutrients and ions
– Maintains electrochemical gradients essential for
functions of muscle and nerve tissues
Secondary Active Transport
• Depends on an ion gradient created by primary
active transport
• Energy stored in ionic gradients is used indirectly to
drive transport of other solutes
• Cotransport—always transports more than one
substance at a time
– Symport system: Two substances transported in same
direction
– Antiport system: Two substances transported in opposite
directions
Extracellular fluid
Na+-K+
pump
Cytoplasm
1 The ATP-driven Na+-K+ pump
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
Figure 3.11 step 1
Extracellular fluid
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loading
glucose from
ECF
Na+-glucose
symport transporter
releasing glucose
into the cytoplasm
Cytoplasm
1 The ATP-driven Na+-K+ pump
2 As Na+ diffuses back across the
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
membrane through a membrane
cotransporter protein, it drives glucose
against its concentration gradient
into the cell. (ECF = extracellular fluid)
Figure 3.11 step 2
Vesicular Transport
• Transport of large particles, macromolecules,
and fluids across plasma membranes
• Requires cellular energy (e.g., ATP)
• Functions:
– Exocytosis—transport out of cell
– Endocytosis—transport into cell
– Transcytosis—transport into, across, and then out
of cell
– Substance (vesicular) trafficking—transport from
one area or organelle in cell to another
Endocytosis and Transcytosis
• Involve formation of protein-coated (typically
clathrin) vesicles
• Often receptor mediated, therefore very
selective
http://www.biologycorner.com/resources/endocytosis.gif
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
2 Proteincoated
vesicle
detaches.
Plasma
membrane
Cytoplasm
3 Coat proteins detach
and are recycled to
plasma membrane.
Transport
vesicle
Endosome
Uncoated
endocytic vesicle
4 Uncoated vesicle fuses
with a sorting vesicle
called an endosome.
Lysosome
5 Transport
vesicle containing
membrane components
moves to the plasma
membrane for recycling.
6 Fused vesicle may (a) fuse
(a)
with lysosome for digestion
of its contents, or (b) deliver
its contents to the plasma
membrane on the
opposite side of the cell
(transcytosis).
(b)
Figure 3.12
Endocytosis
• Phagocytosis—pseudopods engulf solids and
bring them into cell’s interior
– Macrophages and some white blood cells
Endocytosis
• Fluid-phase endocytosis (pinocytosis)—
plasma membrane infolds, bringing
extracellular fluid and solutes into interior of
the cell
– Nutrient absorption in the small intestine
Endocytosis
• Receptor-mediated endocytosis— highly
selective
– Uptake of enzymes low-density lipoproteins, iron,
and insulin
Vesicle
Receptor recycled
to plasma membrane
(c) Receptor-mediated
endocytosis
Extracellular substances
bind to specific receptor
proteins in regions of coated
pits, enabling the cell to
ingest and concentrate
specific substances
(ligands) in protein-coated
vesicles. Ligands may
simply be released inside
the cell, or combined with a
lysosome to digest contents.
Receptors are recycled to
the plasma membrane in
vesicles.
Figure 3.13c
Exocytosis
• Examples:
– Hormone secretion
– Neurotransmitter release
– Mucus secretion
– Ejection of wastes
Plasma membrane The process
Extracellular
of exocytosis
SNARE (t-SNARE)
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
1 The membrane-
bound vesicle
migrates to the
plasma membrane.
2 There, proteins
at the vesicle
Fused surface (v-SNAREs)
v- and bind with t-SNAREs
t-SNAREs (plasma membrane
proteins).
Fusion pore formed
3 The vesicle
and plasma
membrane fuse
and a pore
opens up.
4 Vesicle
contents are
released to the
cell exterior.
Figure 3.14a
Membrane Potential
• Separation of oppositely charged particles
(ions) across a membrane creates a
membrane potential (potential energy
measured as voltage)
• Resting membrane potential (RMP): Voltage
measured in resting state in all cells
– Ranges from –50 to –100 mV in different cells
– Results from diffusion and active transport of ions
(mainly K+)
1 K+ diffuse down their steep
Extracellular fluid
concentration gradient (out of the cell)
via leakage channels. Loss of K+ results
in a negative charge on the inner
plasma membrane face.
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face.
3 A negative membrane potential
Potassium
leakage
channels
Cytoplasm
(–90 mV) is established when the
movement of K+ out of the cell equals
K+ movement into the cell. At this
point, the concentration gradient
promoting K+ exit exactly opposes the
electrical gradient for K+ entry.
Protein anion (unable to
follow K+ through the
membrane)
Figure 3.15