1. Transport across the cell
membrane
https://www.youtube.com/watch?v=BGeSDI03aaw
https://www.youtube.com/watch?v=oxX2fq2DBBo

3. Passive Transport Active Transport
No ATP is required ATP is required to transport the molecules
and ions
Movement of substances along the
concentration gradient.
Movement of substances against the
concentration gradient.
Not affected by low temperature, absence
of oxygen or metabolic inhibitors like few
poison.
It gets affected by low temperature,
absence of oxygen, and metabolic inhibitors
like cyanides.
Three types:
a. Osmosis
b. Simple Diffusion
c. Facilitated Diffusion
Na/K pump
Ca pump
Na pump
K pump, etc.

4. A few substances can diffuse directly through the lipid bilayer part of the membrane. The
only substances that can do this are lipid-soluble molecules such as steroids, or very small
molecules, such as H2O, O2 and CO2. For these molecules the membrane is no barrier at all.
Since lipid diffusion is (obviously) a passive diffusion process, no energy is involved and
substances can only move down their concentration gradient. Lipid diffusion cannot be
controlled by the cell, in the sense of being switched on or off.
a. Simple Diffusion

5. Oxygen, Carbon dioxide, steroid hormones, lipid soluble drugs, etc. undergo simple
diffusion. Rate of Diffusion depends on:
1. Surface Area of the plasma membrane
2. Molecular Weight of the molecules and ions
3. Thickness of the membrane
4. Polar and nonpolar nature of the molecules and ions
5. Concentration gradient
Rate of Diffusion= Surface Area of plasma membrane*
Concentration gradient/ Thickness of plasma membrane*
molecular weight

6. b. Facilitated Diffusion
The transport of substances across a membrane by a transmembrane or integral membrane protein molecule. The
transport proteins tend to be specific for one molecule. As the name suggests, this is a passive diffusion process, so no
energy is involved and substances can only move down their concentration gradient. There are two kinds of transport
protein:
● Channel Proteins form a water-filled pore or channel in the membrane. This allows charged substances (usually
ions) to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the
entry and exit of ions.
● Carrier Proteins have a binding site for a specific solute and constantly flip between two states so that the site is
alternately open to opposite sides of the membrane. The substance will bind on the side where it is at a high
concentration and be released where it is at a low concentration.

7. Ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-), are important for many
cell functions. Because they are charged (polar), these ions do not diffuse through the membrane.
Instead they move through ion channel proteins where they are protected from the hydrophobic interior
of the membrane. Ion channels allow the formation of a concentration gradient between the
extracellular fluid and the cytosol. Ion channels are very specific, as they allow only certain ions through
the cell membrane. Some ion channels are always open, others are "gated" and can be opened or
closed. Gated ion channels can open or close in response to different types of stimuli, such as electrical
or chemical signals.
Example:
Aquaporins

8. 1. Aquaporins: are channel proteins that allow water to cross the membrane very quickly, and
they play important roles in plant cells, red blood cells, and certain parts of the kidney.
2. Leaky Channels: In neurons it maintains or controls resting potential of the nerve membrane.
A. Potassium ion channel, B. Sodium ion channel
3. Voltage gated ion channel: It requires threshold potential to get open. Eg. In neurons, when
the voltage reaches -50 to -55mV it opens to move positive sodium ions inside the nerve. It is
very important for action potential.
4. Ligand gated ion channel: At neuromuscular junction when the neurons release the
neurotransmitters like acetylcholine it causes opening of this channel as acetylcholine gets
bound to this channel protein. Opening of this channel will allow the passage of sodium ions
into the muscle leading to the muscle contraction.
5. Mechanically gated ion channel: Large amount of pressure stimulates this channel to open.
Eg. smashing of fingers causes mechanical stress. This mechanical stress will causes the pain
receptors of sensory nerves to open and allow the movement of ions into the nerve.

9. Example: Transport of glucose into the
cell.

10. Osmosis
● Movement of Solvent (Water) from the region of its higher
concentration to its lower concentration across a semipermeable
membrane till it reaches equilibrium.
● It is the movement of solvent from a dilute solution (Hypotonic) to
concentrated solution (Hypertonic) till it reaches equilibrium.

11. ● Water potential (Y, the Greek letter psi, pronounced "sy") is simply the effective concentration of water. It is
measured in units of pressure (Pa, or usually kPa), and the rule is that water always "falls" from a high to a
low water potential.
● Osmotic Pressure (OP). This is an older term used to describe osmosis. The more concentrated a solution, the
higher the osmotic pressure. It therefore means the opposite to water potential, and so water move from a low to a
high OP.

13. These are problems that living cells face all the time. For example:
● Simple animal cells (protozoans) in fresh water habitats are surrounded by a
hypotonic solution and constantly need to expel water using contractile vacuoles
to prevent swelling and lysis.
● Cells in marine environments are surrounded by a hypertonic solution, and must
actively pump ions into their cells to reduce their water potential and so reduce
water loss by osmosis.
● Young non-woody plants rely on cell turgor for their support, and without enough
water they wilt. Plants take up water through their root hair cells by osmosis, and
must actively pump ions into their cells to keep them hypertonic compared to the
soil. This is particularly difficult for plants rooted in salt water.

17. 5. Binding of potassium causes
dephosphorylation of protein.
6. Dephosphorylation of protein
triggers changes back to original
conformation, with low affinity for
K+. Potassium ions diffuses into
the cell, and the cycle repeats.

18. The processes described so far only apply to small
molecules. Large molecules (such as proteins,
polysaccharides and nucleotides) and even whole cells
are moved in and out of cells by using membrane
vesicles.
● Endocytosis is the transport of materials into a cell. Materials are enclosed by a fold of the cell membrane,
which then pinches shut to form a closed vesicle. Strictly speaking the material has not yet crossed the
membrane, so it is usually digested and the small product molecules are absorbed by the methods above. When
the materials and the vesicles are small (such as a protein molecule) the process is known as pinocytosis (cell
drinking), and if the materials are large (such as a white blood cell ingesting a bacterial cell) the process is known
as phagocytosis (cell eating).
● Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. Materials to be
exported must first be enclosed in a membrane vesicle, usually from the RER and Golgi Body. Hormones and
digestive enzymes are secreted by exocytosis from the secretory cells of the intestine and endocrine glands.
Sometimes materials can pass straight through cells without ever making contact with the cytoplasm by being taken in

20. Basic Process of Exocytosis
1. Vesicles containing molecules are transported from within the cell to the cell
membrane.
2. The vesicle membrane attaches to the cell membrane.
3. Fusion of the vesicle membrane with the cell membrane releases the vesicle
contents outside the cell.
Exocytosis serves several important functions as it allows cells to secrete waste
substances and molecules, such as hormones and proteins. Exocytosis is also
important for chemical signal messaging and cell to cell communication. In
addition, exocytosis is used to rebuild the cell membrane by fusing lipids and
proteins removed through endocytosis back into the membrane.

22. Phagocytosis

23. Pinocytosis
Pinocytosis is the ingestion of extracellular fluids, i.e. the fluid surrounding the
cell, together with its contents of small dissolved molecules (solutes).

25. Receptor- mediated endocytosis
Endosomes
provide an
environment for
material to be
sorted before it
reaches the
degradative
lysosome.
Clathrin is a
protein that
plays a major
role in the
formation of
coated
vesicles.

26. ● The specified molecule binds to a receptor on the plasma membrane.
● The molecule-bound receptor migrates along the membrane to a region containing a clatherine-coated
pit.
● After molecule-receptor complexes accumulate in the clatherine-coated pit, the pit region forms an
invagination that is internalized by endocytosis.
● A clatherine-coated vesicle is formed, which encapsulates the ligand-receptor complex and
extracellular fluid.
● The clatherine-coated vesicle fuses with an endosome in the cytoplasm and the clatherine coating is
removed.
● The receptor can be enclosed in a lipid membrane and recycled back to the plasma membrane.
● If not recycled, the specified molecule remains in the endosome and the endosome fuses with a
lysosome.
● Lysosomal enzymes degrade the specified molecule and deliver the desired contents to the cytoplasm.
Receptor-mediated endocytosis is thought to be more than a hundred times more efficient at taking in
selective molecules than pinocytosis.
Basic Steps of receptor-mediated endocytosis

27. Conduction of Nerve Impulse

32. Concepts and definitions
Axon – The long, thin structure in which action potentials are generated; the transmitting part of the neuron. After initiation,
action potentials travel down axons to cause release of neurotransmitter.
Dendrite – The receiving part of the neuron. Dendrites receive synaptic inputs from axons, with the sum total of dendritic
inputs determining whether the neuron will fire an action potential.
Spine – The small protrusions found on dendrites that are, for many synapses, the postsynaptic contact site.
Membrane potential – The electrical potential across the neuron's cell membrane, which arises due to different distributions
of positively and negatively charged ions within and outside of the cell. The value inside of the cell is always stated relative to
the outside: -70 mV means the inside is 70 mV more negative than the outside (which is given a value of 0 mV).
Action potential – Brief (~1 ms) electrical event typically generated in the axon that signals the neuron as 'active'. An action
potential travels the length of the axon and causes release of neurotransmitter into the synapse. The action potential and
consequent transmitter release allow the neuron to communicate with other neurons.
Neurotransmitter – A chemical released from a neuron following an action potential. The neurotransmitter travels across the
synapse to excite or inhibit the target neuron. Different types of neurons use different neurotransmitters and therefore have
different effects on their targets.
Synapse – The junction between the axon of one neuron and the dendrite of another, through which the two neurons
communicate.

33. Resting Membrane Potential:
● Membrane potential of a neuron, when it is not transmitting any signal, with respect to its immediate surrounding is called resting potential.
Generally the value of resting potential is -70mV.
● Resting potential exists in all the cells.
-70mV
-70mV

34. Explanation:
Resting membrane potential is negative due to:
1. presence of large number of positive Na ions towards outside of membrane
2. presence of smaller number of positive K ions towards inside of membrane
3. zwitterionic protein molecules of cytoplasm behave as negative ions in presence of
highly charged K
4. Na-K ion pump continuously pumps out three sodium ions while only two
potassium ions are taken inside the cell.

35. 1. Polarization (Resting Potential)
● A neuron at resting is electrically charged but not conducting.
● The Axoplasm or plasma membrane of a resting neuron is negatively charged as
compared to the interstitial fluid.
● The potential difference measured at this stage is called resting potential which is about -
70mV. The interstitial fluid has high concentration of Na+ ion which is about 16 times
higher outside the neuron than inside neuron. Similarly, the axoplasm has high
concentration of K+ ion which is about 25 times higher inside than in outer interstitial
fluids.
● Due to difference in concentration of ions, Na+ ion tends to diffuse into the axoplasm and
K+ ion tends to diffuse outside the axoplasm.
● The membrane of neuron at resting is more permeable to K+ ion than Na+ ion. So, K+
leaves the neuron faster than Na+ enter the neuron.
● The difference in permeability results in accumulation of high concentration of cation (+ve
charged ion) outside the neuron compared to the concentration of cation inside.
● This state of resting neuron is called Polarized state and it is electro-negatively charged.

36. When there is a change in immediate external or internal environment of the body, it
acts as stimulus for neuron. Sodium channels open up to allow diffusion of Na inside
stimulated area, thereby depolarisation of membrane takes place.
It can be described as reversal of negative membrane potential in positive membrane
potential, also called action potential. Impulse transmission can take place only after
the development of action potential from resting potential.
2. Depolarization (Action Potential):
● Any stimulus beyond the threshold can initiate an impulse.
● When such stimulus is applied in the resting neuron, it opens the sodium channel. Now the
permeability of Na+ ion suddenly increases at the point of stimulus causing depolarization.
● The diffusion of Na+ ion increases by 10 times from outside to inside. As a result the axoplasm
become positively charge, which is exact opposite to polarized state, so called as depolarized
state or reverse polarized state.
● The depolarization of the membrane stimulates the adjacent voltage channel, so the action
potential passes as a wave along the length of neuron.

37. 3. Repolarization:
● When the concentration of Na+ ion inside axoplasm increases, the permeability to Na+ decreases and the sodium channel
starts to close.
● The Na-K pump activates, so that Na+ are pumped out and K+ inside until the original resting potential is restored. The
process is known as repolarization and it starts from the same point from where depolarization starts.
● The entire process of polarization, depolarization and repolarization occur within fraction of seconds. Now, again the neuron
is ready for another impulse.
4. Saltatory conduction:
● Transmission of nerve impulses is very rapid. However, nerve impulse conduction along unmyelinated neuron
is slow than that of myelinated neuron. It is because, the myelin sheath act as insulator, so that the impulse
have to jump from one node of Raniver to another.
● This speed up the conduction process, and this type of conduction is known as Saltatory conduction.

38. 1. Resting Nerve Fibre: Outer surface of
membrane is electropositive and inner
surface is electronegative.So, It is polarized.
2. Resting potential: -70mV.

39. Cell Signaling
https://www.youtube.com/watch?v=9sF_h-bAnIE ;
https://www.youtube.com/watch?v=-dbRterutHY
https://www.youtube.com/watch?v=VatdTJka3_M

40. ● Definition: Cell signaling or cell communication is the ability of a cell to receive,
process, and transmit signals with its environment and with itself.
● It is a fundamental property of all cells in every living organism such as bacteria, plants,
and animals.
Why cell signaling is necessary?
● In order to properly respond to external stimuli, cells have developed complex
mechanisms of communication that can receive a message, transfer the information
across the plasma membrane, and then produce changes within the cell in response to
the message.
● In multicellular organisms, cells send and receive chemical messages constantly to coordinate
the actions of distant organs, tissues, and cells.
● Single-celled organisms also communicate with each other:
A. Yeast cells signal each other to aid mating.
B.Some forms of bacteria coordinate their actions in order to form large complexes called
biofilms
C. to organize the production of toxins to remove competing organisms.

41. The major types of signaling mechanisms that occur in multicellular organisms
1. Paracrine: a form of cell signaling in which the target cell is near (para =
near) the signal-releasing cell
2. Endocrine: signals from distant cells that originate from endocrine cells, usually
producing a slow response, but having a long-lasting effect.
3. Autocrine: produced by signaling cells that can also bind to the ligand that is
released: the signaling cell and the target cell can be the same or a similar cell
(prefix auto- means self)
4. Direct signaling: can occur by transferring signaling molecules across gap
junctions between neighboring cells.

43. ● Signals that act locally between cells that are close together are called paracrine signals.
● Paracrine signals move by diffusion through the extracellular matrix.
● These types of signals usually elicit quick responses that last only a short amount of time. In
order to keep the response localized, paracrine ligand molecules are normally quickly
degraded by enzymes or removed by neighboring cells. Removing the signals will
reestablish the concentration gradient for the signal, allowing them to quickly diffuse through
the intracellular space if released again.
● One example of paracrine signaling is the transfer of signals across synapses between
nerve cells. A nerve cell consists of a cell body, several short, branched extensions called
dendrites that receive stimuli, and a long extension called an axon, which transmits signals
to other nerve cells or muscle cells. The junction between nerve cells where signal
transmission occurs is called a synapse. A synaptic signal is a chemical signal that travels
between nerve cells. Signals within the nerve cells are propagated by fast-moving electrical
impulses. When these impulses reach the end of the axon, the signal continues on to a
dendrite of the next cell by the release of chemical ligands called neurotransmitters by the
presynaptic cell (the cell emitting the signal). The neurotransmitters are transported across
the very small distances between nerve cells, which are called chemical synapses. The
small distance between nerve cells allows the signal to travel quickly; this enables an
immediate response.
Paracrine Signaling

44. ● Signals from distant cells are called endocrine signals.
● They originate from endocrine cells.
● In the body, many endocrine cells are located in endocrine glands, such as the
thyroid gland, the hypothalamus, and the pituitary gland.
● These types of signals usually produce a slower response, but have a long-
lasting effect.
● The ligands released in endocrine signaling are called hormones, signaling
molecules that are produced in one part of the body, but affect other body
regions some distance away.
● Hormones travel the large distances between endocrine cells and their target
cells via the bloodstream, which is a relatively slow way to move throughout the
body. Because of their form of transport, hormones get diluted and are present
in low concentrations when they act on their target cells. This is different from
paracrine signaling in which local concentrations of ligands can be very high.
Endocrine Signaling

45. Autocrine Signaling
● Autocrine signaling is a type of cell signaling wherein a cell signal released from the cell binds to the same
cell.
● This means the signaling cell and the target cell can be the same or a similar cell (the
prefix auto- means self, a reminder that the signaling cell sends a signal to itself).
● This type of signaling often occurs during the early development of an organism to
ensure that cells develop into the correct tissues and take on the proper function.
Autocrine signaling also regulates pain sensation and inflammatory responses.
● Further, if a cell is infected with a virus, the cell can signal itself to undergo programmed
cell death, killing the virus in the process.
● In some cases, neighboring cells of the same type are also influenced by the released
ligand. In embryological development, this process of stimulating a group of neighboring
cells may help to direct the differentiation of identical cells into the same cell type, thus
ensuring the proper developmental outcome.

46. ● Gap junctions in animals and plasmodesmata in plants are connections
between the plasma membranes of neighboring cells.
These water-filled channels allow small signaling molecules, called intracellular
mediators, to diffuse between the two cells. Small molecules, such as calcium
ions (Ca2+
), are able to move between cells, but large molecules, like proteins
and DNA, cannot fit through the channels.
● The specificity of the channels ensures that the cells remain independent, but
can quickly and easily transmit signals.
● The transfer of signaling molecules communicates the current state of the cell
that is directly next to the target cell; this allows a group of cells to coordinate
their response to a signal that only one of them may have received.
● In plants, plasmodesmata are ubiquitous, making the entire plant into a giant
communication network.
Direct Signaling Across Gap Junctions

47. Signal signaling pathways
Cell signaling refers to the ability of a cell to perceive information from the extracellular environment and
response appropriately. Signaling molecules are the chemical messengers called as ligand.
Ligands can be:
● Some proteins- Bone morphogenetic proteins (BMP ligands) and hormones like insulin.
● Hydrophobic molecules like steroids
● Ions like calcium ions
● Some gases- Eg. Nitric oxide
Four steps of cell signaling pathway: Cell receptors and the type of signaling molecules
may vary but, a similar pattern is followed by all cells.
1. Reception
2. Induction/Transduction
3. Response
4. Resetting

48. Step 1: Reception
It involves the detection of signaling molecules originating from the extracellular
environment.
Here, A. the ligand is detected and binds to the cell receptor on the cell surface.
Few receptors are specific for given molecules that influence their responses.Few examples
are:
1. Dopamine receptors located on some cells of the nervous system specifically bind
dopamine.
2. Insulin receptors found on the surface of many cells in the body bind insulin.
Few receptors can bind to different types of chemical ligands. These are non-specific: Few
examples are-
3. G-protein-coupled receptors (GPCRs)
4. Enzyme-linked receptors
B. Few ligands pass or diffuse through the cell membrane through ionotropic receptors or ligand
gated channels..example: ions like calcium, sodium, and potassium ions can diffuse across the
membrane and induce the signal. These type of receptors are found in nerve cells, neuromuscular

49. Step 2. Transduction
● Binding of ligand leads to conformational change of receptors which in turn influence a
cascade of events called as signal transduction.

51. Step 3. Response
The third step of cell signaling is concerned with specific cellular
responses to the information presented by the signaling molecule.
The cell response can result either in cell growth and repair or cell
death.
The cell may simply respond by increasing or decreasing the
metabolic process through increased or decreased intake of
glucose.
The response may also involve the regulation of gene expression
where certain genes are activated or de-activated depending on
certain processes.

52. Step 4: Resetting
● Here, the cell is reset back to the normal state. During this final step, the signal
molecule detaches from the cell receptor which in turn stops the series of
events that allow the cell to respond.
● For this reason, the cell machinery involved in transduction reverts to their
original state as they wait for another signal. Therefore, this is an important
step of cell signaling in that it allows the cycle to start over and continue as the
cell receives new signals.
● It also ensures that the cell does not continue to respond when it does not need
to. This regulates various cell processes and prevents abnormal cell functions.

53. Technology
● Given that cell signaling involves the detection and binding of signal molecules to cell
receptors in order to induce specific cellular responses, researchers have taken advantage
of this mechanism to develop drugs and chemicals aimed at influencing given responses.
● In drug development, molecules are developed with the aim of identifying a given target
on the cell surface or within the cell with the goal of influencing given responses.
Therefore, signaling is one of the most important areas in drug discovery.
● Currently, more focus has been directed towards developing drugs that will take advantage
of cell signaling to develop drugs for the treatment of cardiovascular diseases, Alzheimer’s
diseases as well as for wound healing.
● Apart from drug development, new studies are taking advantage of cell signaling for the
manufacture of different types of proteins among other molecules in biotechnology. Here,
cells are influenced to participate in protein synthesis as well as other molecules.
● Essentially, this involves taking advantage of the same mechanism used in vivo. This has
proved to be particularly effective using different types of cells including bacteria and some
protists.