What is a cell?
Some basic concepts about cells:
1. Fundamental survival unit of life, may
reproduce (replicate), most carry genetic
information
2. A machinery which performs function(s).
Complicated components inside the cell to
achieve homeostasis
3. Differentiated cells form tissue. Different
tissues form an organ (larger, more complex
machines). Different organs
“communicate” with each other in an
organism to achieve multi-functions and
overall homeostasis
Homeostasis: a very important concept in
Physiology
Maintain a relatively stable internal environment
despite fluctuations (changes) in the external
environment.
The stable internal environment is for the health
of the cell or the body.
Therefore, blood glucose, blood pressure, body
temperature, blood Na level etc can be
maintained in a predictably and relatively stable
level.
Inter-cellular signaling
Neuron
neurotransmitter
Neuron
Target
eg. muscle
Endocrine cells
Hormones
Hormones
(autocrine)
Adjacent
Cells
(paracrine)
Endocrine cells
Target cells
eg kidney cells
Body: 60% water
2/3 of the body water is inside the cell
Therefore, 1/3 of body water is extracellular (outside the
cell)
Extracellular fluid: 80% is interstitial fluid (fluid between
cells); 20% is plasma
Life begins/proliferate in aqueous (water)
environment.
But how to “house” or keep a cell’s own
stuff from losing into the environment?
How to prevent external unwanted
things from going into the cell?
Water and Oil Do NOT MIX!!!
Saline solution
oiloil
water
A film of
lipid
The cell’s
“properties”
are kept
inside
cell
An
aqueous
(“watery”)
interior
But there has to be some exchange between
the cell and the environment!
glucose
transporters
Ca2+
Down the gradient
Down the gradient
Amino
acid
“channels”
Charged
Substances
Difficult to go
thru
K+
Organic
Substances
Easily go thru
Product eg. Insulin in b-cells
O2
CO2
Free diffusion
water
Only slightly
permeable
The cell has to sense and respond to the
environment!
Signal eg. hormone, neurotransmitter,
chemical, nutrient, drug
Signal: eg. Light, odour,
mechanical stress
receptor
receptor
Intracellular Signaling
secretion
Response!!!
Shape changes
Cell division
(growth)
Contraction
Cell death (necrosis,
apoptosis)
A cell is always exchanging
materials and information
with the environment, and
will take actions accordingly
in a specific and desirable
manner.
The cell is like a fortress,
and the membrane is like
the fortress wall.
An unhealthy membrane
will result in permeability
to substances that
otherwise would not enter
the cell.
Trypan blue exclusion
(cell viability) test: dead
cells are stained with the
dye trypan blue, easily
observed under
microscope.
The cell membrane has “workers” that, like soldiers at the fortress wall, identify,
select and transport who/what can enter/leave the cell.
Here, we look closely at some basic machines which work in
a piece of healthy membrane.
glycolipid
Different cell types differ in their lipid-to-protein ratio and their
unique set of membrane proteins
Note: role of cholesterol? Keeping membrane rigid. Should not be too much,
not too less.
Protein molecules can move around but never “flip-flop”
Bilayer is asymmetric
Eg. In red blood cells
PPC at outer leaflet
While PPI, PPS and PPE
at inner leaflet
Phosphatidyl inositol
Phosphatidyl choline
Phosphatidyl serine
Phosphatidyl ethanolamine
receptor
PLC
G protein
Phosphatidyl inositol 4,5 bisphosphate (a
phosphorylated derivative of PPI and a minor
lipid at the inner leaflet cleaves, upon hormone
stimulation, into inositol 1,4,5 trisphosphate (IP3) and
Diacylglycerol (DG). These two are released into the
cytosol as important INTRAcellular messengers
DG
Protein kinase C
Protein
phosphorylation
IP3
Ca
(facilitated transport)
No ATP
Involved.
But ATP has
already been
spent in maintaining the
gradient shown
in red.
Na
Na
Ca
K
K
Ca
Ca
ER in non-muscle cells
SR (sarcoplasmic
reticulum) in muscle
cells
ATP-driven active transport (pumps)
Models showing how active transport might operate.
The transported solute binds to the protein
as it is phosphorylated (ATP expense).
Figure 4-7
The opening and closing of ion channels results
from conformational changes in integral proteins.
Discovering the factors that cause these
changes is key to understanding excitable cells.
Difference between passive diffusion &
facilitated transport
1.Facilitated transport is much faster as the
transported molecules never traverse the
hydrophobic core of the membrane.
2.Facilitated transport is specific.
3.Facilitated transport shows saturation.
Has maximum transport rate.
Figure 4-9
In simple diffusion,
flux rate is limited
only by the
concentration
gradient.
(facilitated transport)
In carriermediated
transport,
the number
of available
carriers
places an
upper limit
on the
flux rate.
Glut1 is an example of uniporter
Insulin promotes glucose uptake into cells such as skeletal
muscle cells, hence lowering glucose level in blood. Here we
see the cross-talk between receptor and transporter. Insulin
receptor failure causes type II diabetes.
Sodium/glucose Symporter
Sodium/Ca antiporter (exchanger)
Na
glucose
Na
Ca
Figure
4-13
Secondary active transport uses the energy in
an ion gradient to move a second solute.
Diverse examples of carrier-mediated transport.
Figure 4-15
The concept of osmolarity: hypoosmolarity and hyperosmolarity.
Cells in hypertonic solutions shrink, while in hypotonic solution swell. Only true in
certain cells having water channel!
Hypotonic solution
Fig 1. Agre’s experiment with cells containing or
lacking aquaporin. Aquaporin is necessary for
making the 'cell' absorb water and swell.
Water channels are only in
certain cells, notably red
blood cells and epithelial cells
of the renal collecting duct.
Neuron
Generation of membrane potential:
1.Driven by ATP (energy), Na/K
ATPase (Na/K pump) establishes
(K leak
channels)
the gradients for Na and K.
2. Membranes are most permeable to
K, only slightly permeable to Cl and Na.
3. Negatively charged protein (A-)
are immobile and therefore do not
cross the membrane.
4. A negative charge is established at
the cytosolic side of membrane.
5. Further K outward movement will
increase the negative charge. The
latter will become eventually big
enough to counteract K movement, and
an equilibrium potential is reached:
membrane potential
*Concept of electrochemical gradient
th
2.3RT/zF: ~60 mV at biological temperatures (monovalent cations).
For K at physiological setting: Ek = -92 mV
If [K] is the same at both side (ie. No gradient), Ek = 0 mV
Adopted from Stephen Wright, Ph.D
Note:
If membrane only permeable to K, then membrane potential will be VK
If membrane only permeable to Na, then membrane potential will be VNa
If membrane only permeable to Cl, then membrane potential will be VCl
However, membranes are usually permeable to Na, K and Cl
Note that permeability to these 3 ions differ:
For example, in neurons, PK is about 10 times of PNa or PCl
Goldman equation
Cl channels are so few at the membrane that they
do not contribute much to the resting membrane
potential
Some predictions:
Opening of K channels causes hyperpolarization
Opening of Na channels causes depolarization
Opening of Cl channels causes hyperpolarization
What is the purpose of the membrane potential?
The K and Na gradients represent a form of
STORED ENERGY
Symporter
Sodium channel (voltage-gated)
opening initiates the action
potential (AP). AP is the
activation signal that spreads
along the neuron.
The Ca ion gradient is extremely steep!!!
Ca conc. in cytosol very low. Indeed an intracellular signal.
1-2 mM Ca
--
- - [Ca] = 50-200 nM
Ca
Modified from Dr Tomoko Kamishima
Department of Human Anatomy and Cell Biology
University of Liverpool
Two important second messengers
(intracellular signals):
1. Ca2+: activates Ca2+/calmodulin-dependent protein kinase
which phosphorylate a number of proteins.
2. cAMP
Some hormone receptors are coupled to Gs (stimulatory
G protein), which in turn activates adenylate cyclase (AC).
AC converts ATP to cyclic AMP. cAMP activates protein
Kinase A, which cause protein phosphorylation.
Figure 5-5
Binding of ligands to membrane-spanning receptors
activates diverse response mechanisms.
Figure 5-5a
Binding of the ligand to the receptor
alters the receptor’s shape, which
then opens (or closes) an ion channel.
Figure 5-5b
Binding of the ligand to the receptor alters the
receptor’s shape, which activates its enzyme
function, phosphorylating an intracellular protein.
Figure 5-5c
Binding of the ligand to the receptor alters
the receptor’s shape, which activates an
associated enzyme function,
phosphorylating an intracellular protein.
Figure 5-5d
Binding of the ligand to the receptor alters the
receptor’s shape, which activates an associated
G-protein, which then activates effector proteins,
i.e., enzyme functions or ion channels.
Figure 5-6
The cyclic AMP second messenger system.
Amplification is the key concept here!!!
Figure 5-8
The cAMP system rapidly amplifies the response
capacity of cells: here, one “first messenger” led
to the formation of one million product molecules.
Figure 5-9
Cells can respond via
the cAMP pathways
using a diversity
of cAMP-dependent
enzymes, channels,
organelles, contractile
filaments, ion pumps, and
changes in gene expression.
This receptor-G-protein complex is linked to and
activates phospholipase C, leading to an increase
in IP3 and DAG, which work together to activate
enzymes and to increase intracellular calcium levels.
Not all responses to
hydrophilic signals
are immediate:
Increases in gene expression
can occur, and the resulting
proteins can increase the
target cells’ response.
This hydrophobic signal
requires a carrier protein
while in the plasma …
… but at the target cell
the signal moves easily
through the membrane
and binds to its receptor.
Figure 5-4
Let’s look at the major organelles
The nucleus is the largest
organelle of the cell.
Double-membraned: Two lipid
bilayers.
Nucleus keep the genetic
material: DNA.
The nuclear pore allows
exchange between nuclear
content and the cytosol eg.
mRNA, made thru transcription
of the DNA, is exported from the
nucleus thru the nuclear pore
into the cytosol for protein
synthesis.
Nucleolus is the place where
rRNA is made and ribosomal
proteins are added to rRNA.
Figure 3-16
NUCLEUS
The DNA code is
“transcribed” into
mRNA.
RIBOSOMES
The mRNA is
“translated” to
give instructions
for proteins
synthesis.
Figure 3-17
(note: mRNA intermediate not shown)
GENES “CODE FOR” PROTEINS
The “triplet code” of DNA determines
which amino acid will be placed in
each position of the protein.
Glucose
ATP
Glycolysis
In the cytosol
Pyruvate
Oxidative phophorylation
ATP
“Cleaner” inside the cell
The lumen is acidic which facilitates the digestion.
Digestion of engulfed foreign particles.
Digestion of protein and peptides by proteases
Digestion of RNA, DNA by nucleases.
Cytoskeleton
(tubulin as subunits)
(microfilament)
Function: Scaffolding (structural stability)
Cell shape changes, Movements
The cytosol itself is composing of a lot of
proteins, a lot being enzymes responsible
for various metabolism. In some cells
glucose is stored as polymer (glycogen).
In specialized cells (adipocytes) fat is
stored in large amount as triacylglycerides.