Cells are the basic building blocks of the body. They were first discovered in the 17th century with the invention of the microscope. The cell theory states that cells are the smallest living units, all new cells come from preexisting cells, and cells contain specialized structures that allow them to carry out life processes. A typical cell contains a plasma membrane, cytoplasm with organelles, and a nucleus containing DNA. The plasma membrane regulates what enters and exits the cell while organelles like the ER, Golgi, mitochondria, and lysosomes carry out specialized functions within the cell.

1. CELL
PHYSIOLOGY
• Prepared by :Shama Dilbar

2. CELLS ARE THE HIGHLY
ORGANIZED, LIVING
BUILDING BLOCKS OF THE
BODY

3. CELL DISCOVERY
• Cells are so small they cannot be seen by the unaided eye. The smallest visible
particle is 5 to 10 times larger than a typical human cell, which averages about 10 to
20 micrometers (mm) in diameter (1 mm 5 1/1,000,000 of a meter). About 100
average-sized cells lined up side by side would stretch a distance of only 1 mm (1
mm 5 1/1000 of a meter; 1 m 5 39.37 in.)
• Until the microscope was invented in the middle of the 17th century, scientists did
not know that cells existed.
• With the development of better light microscopes in the early 19th century, they
learned that all plant and animal tissues consist of individual cells.
• The cells of a hummingbird, a human, and a whale are all about the same size

4. CELL THEORY
• Principles of the Cell Theory
• The cell is the smallest structural and functional unit capable of carrying out life
processes.
• The functional activities of each cell depend on the specific structural properties of
the cell.
• Cells are the living building blocks of all multicellular organisms.
• An organism’s structure and function ultimately depend on the collective structural
characteristics and functional capabilities of its cells.
• All new cells and new life arise only from preexisting cells.
• Because of this continuity of life, the cells of all organisms are fundamentally
similar in structure and function

5. AN OVERVIEW OF CELL
STRUCTURE
• A cell has three major parts
• The plasma membrane which encloses the cell
• The nucleus which houses the cell’s genetic material
• the cytoplasm consists of the cytosol, organelles, and cytoskeleton.
• The cytosol is a gel-like liquid within which the organelles and cytoskeleton are
suspended.
• Organelles are discrete, well-organized structures that carry out specialized
functions.
• The cytoskeleton is protein scaffolding that extends throughout the cell and serves
as the cell’s “bone and muscle.”

7. CELLPLASMA
MEMBRANE
• All cells are enveloped by a plasma membrane, a thin, flexible,
lipid barrier that separates the contents of the cell from its
surroundings
• This barrier contains specific proteins, some of which enable
selective passage of materials. Other membrane proteins are
receptors for interaction with specific chemical messengers in the
cell’s environment. These messengers control many cell activities
crucial to homeostasis.

8. MEMBRANE STRUCTURE
AND FUNCTIONS
• This difference in fluid composition inside and outside a cell is maintained by the
plasma membrane, the extremely thin layer that forms the outer boundary of every
cell and encloses the intracellular contents
• the plasma membrane helps determine the cell’s composition by selectively
permitting specific substances to pass between the cell and its environment. The
plasma membrane controls the entry of nutrient molecules and the exit of secretory
and waste products.
• maintains differences in ion concentrations inside and outside the cell, which are
important in the membrane’s electrical activity.
• The plasma membrane also participates in the joining of cells to form tissues and
organs.

9. THE PLASMA MEMBRANE CONSISTS
MOSTLY OF LIPIDS AND PROTEINS
PLUS SMALL AMOUNTS OF
CARBOHYDRATE. I
• The most abundant membrane lipids are phospholipids
• An estimated 1 billion phospholipid molecules are present in the plasma membrane
of a typical human cell
• phospholipids self-assemble into a lipid bilayer
• Phospholipids have a polar (electrically charged) head containing a negatively
charged phosphate group and two nonpolar (electrically neutral) fatty acid chain
tails .
• The polar end is hydrophilic (meaning “water loving”) because it can interact with
water molecules, which are also polar
• the nonpolar end is hydrophobic (meaning “water fearing”) and will not mix with
water.

11. FUNCTIONS OF LIPID BILAYER
• The lipid bilayer serves the following functions related to its role as a barrier
between a cell’s contents and its surroundings:
• 1. It forms the basic structure of the membrane. The phospholipids can be
visualized as the “pickets” that form the “fence” around the cell.
• 2. Its hydrophobic interior is a barrier to passage of watersoluble substances
between the ICF and ECF. Water-soluble substances cannot dissolve in and pass
through the lipid bilayer. By means of this barrier, the cell can maintain different
mixtures and concentrations of solutes (dissolved substances) inside and outside
the cell.
• 3. It is responsible for the fluidity of the membrane.

12. CHOLESTEROL CONTRIBUTES TO
BOTH THE FLUIDITY AND THE
STABILITY OF THE MEMBRANE
• Cholesterol molecules are tucked between the phospholipid
molecules, where they prevent the fatty acid chains from packing
together and crystallizing, a process that would drastically reduce
membrane fluidity.
• cholesterol molecules also help stabilize the phospholipids’
position. Because of its fluidity, the plasma membrane has
structural integrity but at the same time is flexible, enabling the
cell to change shape.

13. MEMBRANE
PROTEINS
• Membrane proteins are inserted within or attached to the lipid bilayer
• Integral proteins
embedded in the lipid bilayer, with most extending through the entire thickness of the
membrane, in which case they are alternatively called transmembrane proteins (trans
means “across”). Like phospholipids, integral proteins have hydrophilic and
hydrophobic regions.
• Peripheral proteins
Thepolar molecules that do not penetrate the membrane. They only stud the
membrane surface, anchored by weak chemical bonds with the polar parts of integral
membrane proteins or membrane lipids. Peripheral proteins are found more
commonly on the inner than on the outer surface. The plasma membrane has about
50 times more lipid molecules than protein molecules.

14. THE MEMBRANE PROTEINS PERFORM
VARIOUS SPECIFIC MEMBRANE FUNCTIONS.
• Different types of membrane proteins serve the following specialized functions:
• channels,
• Some transmembrane proteins form water-filled pathways, or channels, through the
lipid bilayer
• Some channels are leak channels that always permit passage of their selected ion.
Others are gated channels that may be open or closed to their specific ion as a
result of changes in channel shape .
• Other proteins that span the membrane are carrier, or transport, molecules; they
transfer across the membrane specific substances that are unable to cross on their
own.

15. CELL MEMBRANE

16. THE NUCLEUS

17. STRUCTURE OF NUCLEUS
• . the largest single organized cell component, can be seen as a distinct spherical or oval
structure,
• usually located near the center of the cell.
• t is surrounded by a double-layered membrane, the nuclear envelope, which separates the
nucleus from the rest of the cell.
• The nuclear envelope is pierced by many nuclear pores that allow necessary traffic to move
between the nucleus and the cytoplasm.
• a dense, membrane-less structure composed of RNA and proteins called the nucleolus.
• The nucleolus contains nucleolar organizers, which are parts of chromosomes with
the genes for ribosome synthesis on them. The nucleolus helps to
synthesize ribosomes by transcribing and assembling ribosomal RNA subunits. These
subunits join together to form a ribosome during protein synthesis.

18. • The nucleus houses the cell’s genetic material, deoxyribonucleic acid (DNA) which,
along with associated nuclear proteins, is organized into chromosomes
• consists of a different DNA molecule that contains a unique set of genes.
• Body cells contain 46 chromosomes that can be sorted into 23 pairs on the basis of
their distinguishing features.

19. FUNCTIONS OF DNA
• DNA has two important functions:
• 1. Serving as a genetic blueprint during cell replication. Through this role, DNA
ensures that the cell produces additional cells just like itself, thus continuing the
identical type of cell line within the body. Furthermore, in the reproductive cells
(eggs and sperm), the DNA blueprint passes on genetic characteristics to future
generations.
• 2. Directing protein synthesis. DNA provides codes, or “instructions,” for directing
synthesis of specific structural and enzymatic proteins within the cell. Proteins are
the main structural component of cells, and enzymes govern the rate of all chemical
reactions in the body. By specifying the kinds and amounts of proteins that are
produced, the nucleus indirectly governs most cell activities and serves as the cell’s
control center

20. THE CYTOPLASM
• The cytoplasm is that portion of the cell interior not occupied by the nucleus. It
contains a number of discrete, specialized organelles (the cell’s “little organs”)
• The cytoskeleton (a scaffolding of proteins) dispersed within the cytosol (a
complex, gel-like liquid).

21. ORGANELLS
• Organelles are distinct, highly organized structures that perform specialized
functions within the cell.
• On average, nearly half of the total cell volume is occupied by two categories of
organelles—membranous organelles and non membranous organelles.
• Each membranous organelle is a separate compartment within the cell that is
enclosed by a membrane similar to the plasma membrane. Thus, the contents of a
membranous organelle are separated from the surrounding cytosol and from the
contents of other organelles.
• All human cells contain five main types of membranous organelles—the
endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, and mitochondria.
• The nonmembranous organelles are not surrounded by membrane and thus are in
direct contact with the cytosol.

24. ENDOPLASMIC RETICULUM

25. • The endoplasmic reticulum (ER) is an elaborate fluid-filled membranous system
distributed extensively throughout the cytosol. It is primarily a protein- and lipid-
producing factory.
• Two distinct types of ER—rough and smooth—can be distinguished.
• The rough ER consists of stacks of relatively flattened interconnected sacs,
whereas the smooth ER is a meshwork of tiny interconnected tubules .
• Even though these two regions differ considerably in appearance and function, they
are connected to each other, making the ER one continuous organelle. The relative
amount of rough and smooth ER varies among cells, depending on the activity of
the cell.
• The rough ER synthesizes proteins for secretion and membrane construction.
• The smooth ER packages new proteins in transport vesicles.

26. OVERVIEW OF THE SECRETION PROCESS FOR PROTEINS SYNTHESIZED
BY THE ENDOPLASMIC RETICULUM
1. The rough ER synthesizes proteins to be secreted to the exterior or to be
incorporated into plasma membrane or other cell components.
2. The smooth ER packages the secretory product into transport vesicles, which bud
off and move to the Golgi complex. The transport vesicles fuse with the Golgi
complex, open up, and empty their contents into the closest Golgi sac.
3. The newly synthesized proteins from the ER travel by vesicular transport through
the layers of the Golgi complex, which modifies the raw proteins into final form and
sorts and directs the finished products to their final destination by varying their
wrappers.
4. Secretory vesicles containing the finished protein products bud off the Golgi
complex and remain in the cytosol, storing the products until signaled to empty.
5. On appropriate stimulation, the secretory vesicles fuse with the plasma
membrane, open, and empty their contents to the cell’s exterior. Secretion has
occurred by exocytosis, with the secretory products never having come into
contact with the cytosol.
6. On appropriate stimulation, the secretory vesicles fuse with the plasma
membrane, open, and empty their contents to the cell’s exterior. Secretion has
occurred by exocytosis, with the secretory products never having come into
contact with the cytosol.
7. Lysosomes also bud from the Golgi complex.

27. RIBOSOMES
• Ribosomes, nonmembranous organelles, carry out protein synthesis by translating
mRNA into chains of amino acids in the ordered sequence dictated by the original
DNA code.
• Ribosomes bring together all components that participate in protein synthesis—
mRNA, tRNA, and amino acids—and provide the enzymes and energy required for
linking the amino acids together.
• The nature of the protein synthesized by a given ribosome is determined by the
mRNA being translated. Each mRNA serves as a code for only one protein. A
finished ribosome is about 20 nm in diameter and is made up of two parts of
unequal size, a large and a small ribosomal subunit

28. GOLGI COMPLEX AND
ENDOCYTOSYS
• The Golgi complex, a membranous organelle, is closely associated with the ER.
Each Golgi complex consists of a stack of flattened, slightly curved, membrane-
enclosed sacs.
• The sacs within each Golgi stack do not come into physical contact with one
another.
• Most newly synthesized molecules that have just budded off from the smooth ER
enter a Golgi stack. When a transport vesicle reaches a Golgi stack, the vesicle
membrane fuses with the membrane of the sac closest to the center of the cell. The
vesicle membrane opens up and becomes integrated into the Golgi membrane, and
the contents of the vesicle are released to the interior of the sac-

29. PROCESS
• These newly synthesized raw materials from the ER travel by means of vesicle
formation through the layers of the Golgi stack, from the innermost sac closest to
the ER to the outermost sac near the plasma membrane. Vesicular transport from
one Golgi sac to the next is accomplished through action of membrane-curving coat
protein I (COPI).

30. FUNCTIONS
• During this transit, two important, interrelated functions take place:
• 1. Processing the raw materials into finished products. Within the Golgi complex,
the “raw” proteins from the ER are modified into their final form (for example, by
having a carbohydrate attached). The biochemical pathways that the proteins follow
during their passage through the Golgi complex are elaborate, precisely
programmed, and specific for each final product.
• 2. Sorting and directing the finished products to their final destinations. The Golgi
complex is responsible for sorting and segregating products according to their
function and destination, such as products to be secreted to the cell’s exterior or to
be used for constructing new plasma membrane

31. EXOCYTOSIS
Exocytosis: A secretory vesicle fuses with the plasma
membrane, releasing the vesicle contents to the cell
exterior. The vesicle membrane becomes part of the
plasma membrane.

32. ENDOCYTOSIS
Endocytosis: Materials from the cell exterior are enclosed
in a segment of the plasma membrane that pockets
inward and pinches off as an endocytic vesicle.

33. SECRETORY VESICLES
• How does the Golgi complex sort and direct finished proteins to the proper
destinations?
• Finished products collect within the dilated edges of the Golgi complex’s sacs. The
edge of the outermost sac then pinches off to form a membrane-enclosed vesicle
containing a selected product.
• Each distinct surface protein serves as a specific docking marker (like an address
on an envelope). A vesicle can “dock” lock-and-key fashion and “unload” its selected
cargo only at the appropriate docking-marker acceptor

34. PACKAGING, DOCKING, AND
RELEASE OF SECRETORY
VESICLES

35. Recognition markers in the
membrane of the outermost
Golgi sac capture the
appropriate cargo from the
Golgi lumen by binding only
with the sorting signals of
the protein molecules to be
secreted. The membrane
that will wrap the vesicle is
coated with coatomer, which
causes the membrane to
curve, forming a bud
The membrane closes
beneath the bud,
pinching off the
secretory vesicle.
he vesicle loses its coating,
exposing v-SNARE
docking markers on the
vesicle surface. 3
The v-SNAREs bind only with
the t-SNARE docking-marker
acceptors of the targeted
plasma membrane, ensuring
that secretory vesicles empty
their contents to the cell’s
exterior.

36. LYSOSOMES AND ENDOCYTOSIS
• Lysosomes are small, membrane-enclosed, degradative organelles that break down
organic molecules (lys means “break down”; some means “body”).
• Instead of having a uniform structure, as is characteristic of all other organelles,
lysosomes vary in size and shape, depending on the contents they are digesting.
Most commonly, lysosomes are small (0.2 to 0.5 mm in diameter) oval or spherical
bodies . On average, a cell contains about 300 lysosomes.
• Lysosomes are formed by budding from the Golgi complex. A lysosome contains
about 40 different powerful hydrolytic enzymes that are synthesized in the ER and
then transported to the Golgi complex for packaging in the budding lysosome.

37. • These enzymes catalyze hydrolysis, reactions that break down organic molecules
by the addition of water at a bond site (hydrolysis means “splitting with water). In
lysosomes, the organic molecules are cell debris and foreign material, such as
bacteria, that have been brought into the cell.
• Lysosomal enzymes are similar to the hydrolytic enzymes that the digestive system
secretes to digest food. Thus, lysosomes serve as the intracellular “digestive
system.”
• lysosomes mostly degrade extracellular proteins brought into the cell, whereas most
unwanted intracellular proteins are degraded by the ubiquitin–proteasome
pathway(one of the major destruction ways to control the activities of different
proteins. The function of UPP is to eliminate dysfunctional/misfolded proteins via the
proteasome, and these specific functions enable the UPP to regulate protein quality
in cells.

38. • Extracellular material to be attacked by lysosomal enzymes is brought into the cell
through the process of phagocytosis, a type of endocytosis.
• Endocytosis, the reverse of exocytosis, refers to the internalization of extracellular
material within a cell (endo means “within”)

39. ENDOCYTOSIS CAN BE
ACCOMPLISHED IN THREE WAYS
• Receptor-Mediated Endocytosis
• Pinocytosis
• Phagocytosis

40. RECEPTOR-MEDIATED
ENDOCYTOSIS
• receptor-mediated endocytosis is a highly selective process that enables cells to
import specific large molecules that it needs from its environment.
• Receptor-mediated endocytosis is triggered by the binding of a specific target
molecule such as a protein to a surface membrane receptor specific for that
molecule
• This binding causes the plasma membrane at that site to pocket inward and then
seal at the surface, trapping the bound molecule inside the cell.
• Unfortunately, some viruses can sneak into cells by exploiting this mechanism. For
instance, flu viruses and HIV, the virus that causes AIDS gain entry to cells via
receptor-mediated endocytosis. They do so by binding with membrane receptors
normally designed to trigger the internalization of a needed molecule

42. PINOCYTOSIS
• pinocytosis (“cell drinking”), a droplet of ECF is taken up nonselectively. First, the
plasma membrane dips inward, forming a pouch that contains a small bit of ECF
• The plasma membrane then seals at the surface of the pouch, trapping the contents
in a small, intracellular endocytic vesicle, or endosome. Dynamin, the protein
responsible for pinching off an endocytic vesicle, forms rings that wrap around and
“wring the neck” of the pouch severing the vesicle from the surface membrane.
Besides bringing ECF into a cell, pinocytosis provides a means to retrieve extra
plasma membrane that has been added to the cell surface during exocytosis.

44. PHAGOCYTOSIS
• During phagocytosis (“cell eating”), large multimolecular particles are internalized.
Most body cells perform pinocytosis, many carry out receptor-mediated
endocytosis, but only a few specialized cells are capable of phagocytosis,
• the most notable being certain types of white blood cells that play an important role
in the body’s defense mechanisms.
• When a white blood cell encounters a large particle, such as a bacterium or tissue
debris, it extends surface projections known as pseudopods (“false feet”) that
surround or engulf the particle and trap it within an internalized vesicle known as a
phagosome

45. • A lysosome fuses with the membrane of the phagosome and releases its hydrolytic
enzymes into the vesicle, where they safely attack the bacterium or other trapped
material without damaging the remainder of the cell. The enzymes largely break
down the engulfed material into raw ingredients, such as amino acids, glucose, and
fatty acids, which the cell can use.

47. LYOSOMES REMOVE WORN-OUT
ORGANELLES
• Cells typically live longer than many of their internal components. Lysosomes can fuse with
aged or damaged organelles to remove these useless parts of the cell.
• Lysosomal enzymes degrade the dysfunctional organelle, making its building blocks
available for reuse by the cell.
• This selective self-digestion, known as autophagy (auto means “self ”; phag means “eating”)
makes way for new replacement parts. In most cells, all organelles are renewable
• Some individuals lack the ability to synthesize one or more of the lysosomal enzymes. The
result is massive accumulation within the lysosomes of the compound normally digested by
the missing enzyme. Clinical manifestations often accompany such disorders because the
engorged lysosomes interfere with normal cell activity. More than 50 of these so-called
lysosomal storage diseases have been identified,

48. PEROXISOMES AND
DETOXIFICATION
• Peroxisomes are membranous organelles that produce and decompose hydrogen
peroxide (H2O2) in the process of degrading potentially toxic molecules (peroxi
refers to “hydrogen peroxide”
• Typically, several hundred small peroxisomes about one third to one half the
average size of lysosomes are present in a cell.
• They too arise from the ER and Golgi complex.
• Peroxisomes house oxidative enzymes that detoxify various wastes.
• Like lysosomes, peroxisomes are membrane-enclosed sacs containing enzymes,
but unlike lysosomes, which contain hydrolytic enzymes, peroxisomes house
several powerful oxidative enzymes and contain most of the cell’s catalase.

49. HYDROLYTIC ENZYMES
• Any of the enzymes or catalysts that
act and behave like a hydrolase.
• A hydrolase is an enzyme that speeds
up the hydrolysis(A chemical reaction
in which the interaction of
a compound with water results in
the decomposition of that compound of
a chemical bond).
• These enzymes catalyze
the hydrolysis of a chemical bond of
a compound such as proteins, nucleic
acids, starch, fats, phosphate esters,
and other macromolecular substances.
Oxidative enzymes
• An oxidative enzyme is
an enzyme that catalyses an
oxidation reaction.
• Two most common types of oxidative
enzymes are
• peroxidases, which use hydrogen
peroxide
• oxidases, which use molecular oxygen
• They increase the rate at which ATP is
produced aerobically.

50. • Many enzymes inside the peroxisomes catalyze Redox (reduction-oxidation)
reactions, which will generate hydrogen peroxide (H2O2) as a dangerous byproduct.
• A peroxisomal enzyme, called “Catalase”, can convert H2O2 into water (H2O) and
oxygen (O2) to keep the cell safe.
• Peroxisomes owe their name to hydrogen peroxide generating and scavenging
activities.

51. WHAT DOES PEROXISOME DO?
• Peroxisome is a multifunctional biochemistry laboratory in the cell
• Peroxisomes contain more than 50 different enzymes, which are involved in a
variety of biochemical reactions.
• A variety of substrates are broken down by such oxidative reactions in
peroxisomes, including uric acid, amino acids, and fatty acids.
• The oxidation of fatty acids is a particularly important example since it provides a
major source of metabolic energy.
• The chemical reactions are potentially dangerous to the cells. This is why we need
the peroxisomes to control chemical reactions within a membrane-bound space
separated from the rest of the cells.

52. PEROXISOMES CLOSELY INTERACT WITH OTHER ORGANELLES IN THE CELLS. THE BIOMOLECULES
ARE TRANSPORTED INTO PEROXISOMES FOR SPECIFIC CHEMICAL REACTIONS. THE PRODUCTS ARE
ALSO EXPORTED TO OTHER ORGANELLES FOR BIOLOGICAL FUNCTIONS.

53. • When a reductive reaction happens in the peroxisome, the enzyme takes away
oxygen (in the form of superoxide O2
•−). However, the enzyme cannot hold the
oxygen forever, so the oxygen is transferred to a water molecule. As a result, the
water molecule is oxidized to become a hydrogen peroxide (H2O2).
• The molecules that contained chemically reactive oxygen (like O2•−, •OH, H2O2, and
NO) are called reactive oxygen species (ROS) or free radicals.
• These ROS need to be removed from the cells carefully; otherwise, ROS will
damage the cells by unwanted reactions with DNA, lipid, and proteins.

54. A STABLE ATOM HAS A BALANCED NUMBER OF ELECTRONS, NO MORE, NO LESS. FREE
RADICALS EAGERLY WANT TO STEAL ELECTRONS FROM OTHER ATOMS TO FULFILL THEIR
UNSTABLE STATUS. ANTIOXIDANTS HAVE FREE ELECTRONS THAT CAN GIVE TO FREE RADICALS
TO CALM THEM DOWN

55. • many diseases like cancers and aging originate from the bad effects of ROS in our
bodies. Radiation, tobacco, and drugs also increase the chances of damage by
ROS. Antioxidants or free-radical scavengers can cancel out the effects of ROS.
This is why we are encouraged to eat more healthy food that enriches natural
antioxidants, like vitamins A, C, and E.

56. VAULTS AS
CELLULAR TRUCKS
Vaults, which are nonmembranous
organelles, are shaped like octagonal
barrels
Just like barrels, vaults have a hollow
interior. When open, they appear like pairs
of unfolded flowers with each half of the
vault bearing eight “petals” attached to a
central ring.
A cell may contain thousands of vaults,
which are three times as large as
ribosomes.

57. • Currently, the function of vaults is uncertain, but their octagonal shape and their
hollow interior provides clues. Nuclear pores are also octagonal and the same size
as vaults, leading to speculation that vaults may be cellular “trucks.”
• According to this proposal, vaults would dock at or enter nuclear pores, pick up
molecules synthesized in the nucleus, and deliver their cargo elsewhere in the cell,
Ongoing research supports the role of vaults in nucleus-to-cytoplasm transport, but
their cargo has not been determined.