Cell 1

CELL
CELLULAR FUNCTIONS IN SOME SPECIALIZED

CELLS.
• The cell is composed of two basic parts:
Cytoplasm and
Nucleus.
• Individual cytoplasmic components are usually not clearly

distinguishable in common hematoxylin-and-eosin–stained
preparations.
• The cytoplasm is composed of a fluid component, or cytosol, which
contains organelles
Membranous organelle Non-Membranous
organelle
Rough & Smooth Ribosomes
Endoplasmic reticulum
Mitochondria Centrioles
Golgi apparatus Microtubules
Peroxisomes Inclusions (i.e. Glycogen, lipids, crystals,
pigments, hemosiderin etc.)
Lysosomes Cilia - Flagella

Vacuoles & vesicles Microvilli
PLASMA MEMBRANE
• The outermost component of the cell, is the Plasma
membrane or Plasmalemma.
• All eukaryotic cells are enveloped by a limiting

membrane composed of phospholipids, cholesterol,
proteins, and chains of oligosaccharides covalently
linked to phospholipid and protein molecules.
• Membranes range from 7.5 to 10 nm in thickness and

visible only in the electron microscope.
• Cholesterol molecules modulate the fluidity and movement of all
membrane components.
• The lipid composition of each half of the bilayer is different.
• The carbohydrates of the glycoproteins and glycolipids work

as receptors that participate in cell adhesion, recognition,
and response to protein hormones
PROTEINS
• Proteins, are major molecular constituent of membranes, can
be divided into two groups.
Integral proteins :
o Incorporated within the lipid bilayer.
o multipass transmembrane proteins.
Peripheral proteins :
exhibit a looser association with one of the two
membrane surfaces.
GLYCOCALYX
• This layer is made of carbohydrate chains linked to membrane

proteins and lipids.
• role in cell recognition and attachment to other cells and to
extracellular molecules.
TRANSPORTATION
• Some ions, such as Na+, K+, and Ca2+, cross the cell
membrane by passing through integral membrane
proteins.
• passive diffusion:(simple, facilitated, or osmosis).
• active transport via ion pumps using (ATP) (primary,
secondary, symport and antiport).
ENDOCYTOSIS
• Bulk uptake of material.

• involves folding and fusion of membrane to form vesicles which
enclose the material transported.
• Cells show three general types of endocytosis.
1. Phagocytosis
2. Fluid-phase Endocytosis
3.Receptor-mediated Endocytosis
Phagocytosis :
o (“cell eating”) is the ingestion of particles such as
bacteria or dead cell remnants.
o phagosome.
o Fluid-phase Endocytosis :
o smaller invaginations of the cell membrane form and
entrap extracellular fluid.
o Pinocytotic vesicles (about 80 nm in diameter) pinch
off inwardly from the cell surface.
PHAGOCYTOSIS
PINOCYTOSIS
Receptor-mediated Endocytosis :
• Binding of the ligand to its receptor causes receptors to

aggregate in special membrane regions called coated pits.
• several polypeptides, the major one being Clathrin.

• form a coated vesicle carrying the ligand and its receptor.
RECEPTOR-MEDIATED ENDOCYTOSIS
EXOCYTOSIS
• the release of vesicles contents into the extracellular

space without compromising the integrity of the plasma
membrane.
• Exocytosis is triggered in many cells by transient
increase in cytosolic Ca2+.
• process of membrane movement and recycling is called
membrane trafficking.
• reducing blood lipid levels
SIGNAL RECEPTION AND
TRANSDUCTION
• Soluble extracellular signaling molecules bind receptor proteins

only found on their target cells.
• receptor proteins respond to signaling molecules in a specific,

programmed way.
Signaling can take different routes:
Endocrine signaling:
• carried in the blood.
Paracrine signaling :
• act only on local cells very close to the source.
Synaptic signaling :
• neurotransmitters act only on adjacent cells through synapses.
Autocrine signaling :
• signals bind receptors on the same cell type.
HYDROPHILIC SIGNALING
MOLECULES
• Signal transduction.
• the G proteins, binds guanine nucleotides and acts on effectors which
propagate the signal further into the cell.
• Effector proteins are usually ion channels or enzymes that generate large
quantities of small second messenger molecules,
• 1,2-diacyglycerol (DAG),
• cyclic adenosine monophosphate (cAMP),
• inositol 1,4,5-triphosphate (IP3).
MEDICAL APPLICATION
• Several diseases have been caused by defective receptors.

• Pseudohypoparathyroidism and a type of Dwarfism are
caused by nonfunctioning parathyroid and growth hormone
receptors.
• Hydrophobic signaling molecules :
such as small steroid and thyroid hormones, bind reversibly

to carrier proteins in the plasma for transport through the
body.
• intracellular receptor proteins.
NUCLEUS
• blueprint for cell structures and activities encoded in the

DNA of the chromosomes .
• site of (DNA) replication and transcription into precursor
(RNA)molecules.
• It contains enzymes required for replication and repair of
newly synthesized DNA.
• It is enclosed by the nuclear envelope and contains the

nuclear lamina, nucleolus, and chromatin
NUCLEAR ENVELOPE
NUCLEAR LAMINA
• latticelike network of proteins that include Lamins.
• Lamins attach chromatin to the inner membrane of the

nuclear envelope.
• Phosphorylation of the Lamins, (by Lamin kinase)

during prophase of mitosis initiates nuclear disassembly
into small vesicles.
Nuclear Lamina
Functions:
Maintenance of nuclear shape
Spatial organization of nuclear pores within nuclear membrane
Regulation of transcription
Anchoring of interphase heterochromatin
DNA replication.
Lamina
bio.winona.msus.edu/.../ Lec-note/16-new
.htm
NUCLEOLUS :
• responsible for (rRNA) synthesis and ribosome assembly.
• It contains three morphologically distinct zones:
• Granular zone—found at the periphery; contains ribosomal

precursor particles in various stages of assembly.
• Fibrillar zone—centrally located; contains ribonuclear protein
fibrils.
• Fibrillar center—contains DNA that is not being transcribed.
Nucleolus :
• The nucleolus is responsible for ribosomal RNA (rRNA)

synthesis and ribosome assembly.
• It contains three morphologically distinct zones:
• Granular zone—found at the periphery; contains ribosomal

precursor particles in various stages of assembly.
• Fibrillar zone—centrally located; contains ribonuclear protein
fibrils.
• Fibrillar center—contains DNA that is not being transcribed.
CHROMATIN
Chromatin is a complex of DNA, histone proteins, and nonhistone
proteins.
• DNA—a double-stranded helical molecule that carries the genetic
information of the cell.
• B DNA, Z DNA, and A DNA.
• Histone proteins—positively charged proteins enriched with lysine
and arginine residues.
• nucleosomes and solenoid fibers.
• The nucleosomes are the basic repeating units of the chromatin
fiber, having a diameter of approximately 10 nm.
• Nonhistone proteins—enzymes involved in replication, transcription,
DNA repair, and regulation of chromatin function.
• They are acidic or neutral proteins.
Chromatin
6 nucleosomes
become coiled
around an axis,
forming a
solenoid.
Nucleosome, Solenoid model of chromatin, and
chromosome
Nucleoso
me
Forms of Chromatin :
• Heterochromatin :
highly condensed and transcriptionally inactive.
• In a typical eukaryotic cell, approximately 10% of the
chromatin is heterochromatin.
• Euchromatin :
a more extended form of DNA, which is
potentially transcriptionally active.
• In a typical cell, euchromatin accounts for approximate
90%of the total chromatin, although only about 10% is
being actively transcribed.
• In which of the following nuclear structures is DNA
actively transcribed to rRNA?
• (A) Envelope
• (B) Lamina
• (C) Matrix
• (D) Nucleolus
• (E) Pore
• In transmission EM preparations of cells the cell membrane
often appears as a trilaminar structure having two parallel
dark-staining components on either side of an unstained
middle layer. This central poorly stained region of the
membrane is primarily responsible for which of the following
functions?
• a. Creation of a barrier to water-soluble molecules
• b. Binding by cellular receptions to specific ligands
• c. Catalyzing membrane-associated activities
• d. Transport of ions
• e. Connections to the cytoskeleton

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CELL

CELLULAR FUNCTIONS IN SOME SPECIALIZED

• Individual cytoplasmic components are usually not clearly

Lysosomes Cilia - Flagella

• All eukaryotic cells are enveloped by a limiting

• Membranes range from 7.5 to 10 nm in thickness and

• The lipid composition of each half of the bilayer is different.

• The carbohydrates of the glycoproteins and glycolipids work

• This layer is made of carbohydrate chains linked to membrane

• Bulk uptake of material.

• Cells show three general types of endocytosis.

• Binding of the ligand to its receptor causes receptors to

• several polypeptides, the major one being Clathrin.

• the release of vesicles contents into the extracellular

• Soluble extracellular signaling molecules bind receptor proteins

• receptor proteins respond to signaling molecules in a specific,

• Several diseases have been caused by defective receptors.

such as small steroid and thyroid hormones, bind reversibly

• blueprint for cell structures and activities encoded in the

• It is enclosed by the nuclear envelope and contains the

• latticelike network of proteins that include Lamins.

• Lamins attach chromatin to the inner membrane of the

• Phosphorylation of the Lamins, (by Lamin kinase)

Maintenance of nuclear shape

Spatial organization of nuclear pores within nuclear membrane

Anchoring of interphase heterochromatin

• responsible for (rRNA) synthesis and ribosome assembly.

• It contains three morphologically distinct zones:

• Granular zone—found at the periphery; contains ribosomal

• The nucleolus is responsible for ribosomal RNA (rRNA)

• It contains three morphologically distinct zones:

• Granular zone—found at the periphery; contains ribosomal

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