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Biol 2430 Anatomy and
Physiology
Lect #2
Muse
5/5/10
THE CELL
The cell is the smallest structural and
functional unit of the body. Most of
the chemical reactions that
sustain life occur inside cells.
An Introduction to Cells
• Sex cells (germ cells)
– Reproductive cells
– Male sperm
– Female oocyte (a cell that develops into an egg)
• Somatic cells (soma = body)
– All body cells except sex cells
An Introduction to Cells
• A cell is surrounded by a watery medium known
as the extracellular fluid (interstitial fluid)
– Plasma membrane (cell membrane) separates
cytoplasm from the extracellular fluid
– Cytoplasm
• Cytosol = liquid
• Intracellular structures collectively known as organelles
Chapter 3
Cells
• vary in size
• possess distinctive
shapes
• measured in
micrometers
Cell functions are similar in all
cells
•
•
•
Cells maintain a selective barrier called the plasma
membrane between their cytoplasm and the extracellular
environment. All substances that enter or leave the cell
must pass across the barrier.
Cells contain hereditary material carrying encoded
instructions for the synthesis of most of the cellular
components. This hereditary material is duplicated prior to
cell reproduction so that each new cell carries a full set of
instructions.
Cells carry out metabolic activities, which are catalyzed
chemical reactions that result in the synthesis and
breakdown of organic molecules.
Components of a cell
•
A generalized body cell has four principal
divisions:
– the plasma membrane
– the cytoplasm
– cytoplasmic organelles
– the nucleus.
A Composite Cell
• hypothetical cell
• major parts
• nucleus
• cytoplasm
• cell membrane
Cell Membrane
•
•
The proteins are divided into two categories: integral and peripheral.
– The integral proteins form the majority of membrane proteins. They
penetrate and are embedded in the bilayer, bound to the nonpolar tail
regions.
• The transmembrane proteins span the bilayer completely and may
form channels (pores) for transport of substances across the
membrane.
• Integral proteins also may lie partly submerged in one side or the
other. They have several functions.
– Some integral proteins serve as cell surface enzymes.
– Integral proteins bound to carbohydrates may form
receptor sites for chemical messages from other cells,
such as endocrine glands.
– Some also function as markers, or antigens, which identify
cell types.
The peripheral proteins are loosely bound to the membrane surface and can
be easily removed from it. Their functions are not as well known as those of
integral proteins. They may be involved in structural support and changes in
membrane shape during cell division or cell movement.
Cell Membrane
• outer limit of cell
• controls what moves in and out of cell
• selectively permeable
•phospholipid bilayer
• water-soluble “heads” form surfaces
• water-insoluble “tails” form interior
• permeable to lipid-soluble substances
• cholesterol stabilizes the membrane
• proteins
• receptors
• pores, channels, carriers
• enzymes
• CAMS (Cellular Adhesion Molecules)
• self-markers
Cell Membrane
The Plasma Membrane
Intercellular Junctions
Tight junctions
• close space between cells
• located among cells that form
linings
Desmosomes
• form “spot welds” between cells
• located among outer skin cells
Gap junctions
• tubular channels between cells
• located in cardiac muscle cells
Cell Adhesion Molecules
• guide cells on the move
• selectin – allows white blood
cells to “anchor”
• integrin – guides white blood
cells through capillary walls
• important for growth of
embryonic tissue
• important for growth of nerve
cells
Ribosomes
• Structure
– Ribosomes are small granules composed of ribosomal RNA and
almost 80 different proteins.
– They occur as individual granules or in clusters called
polyribosomes.
– They may be free in the cytoplasm (free ribosomes) or attached
to the membranes of the endoplasmic reticulum.
• Function
– Ribosomes are the site of protein synthesis.
– Free ribosomes are involved in the synthesis of proteins for the
cell’s own use; for example, in the renewal of enzymes and
membranes.
– Attached ribosomes are the site of synthesis of proteins that are
secretory products to be released from the cell.
Golgi Apparatus
• Function
• The Golgi apparatus is the site of accumulation, concentration,
packaging, and chemical modification of the secretory products
synthesized on the rough ER.
– The transport vesicles pinch off from the ER and carry the secretions to
the Golgi apparatus, where the secretions fuse with its cisternae.
– The large condensing vacuoles concentrate the secretion and package
them to become secretory granules.
– Secretory granules, which are large, densely packed, membranebounded structures, unload their contents via exocytosis upon nervous
or hormonal stimulation.
– The Golgi apparatus also chemically modifies the molecules
synthesized in the ER for incorporation into the plasma membrane. It
adds fatty acid residues to certain proteins to convert them to
lipoproteins, and it synthesizes and attaches carbohydrate side chains
to proteins to form glvcoproteins.
– The Golgi apparatus processes proteins that function intracellularly,
such as the lysosome enzymes.
The Nucleus is the largest
organelle
• It is present in all cells of the body except
mature red blood cells, which lost their
nuclei as they developed.
• Generally, each cell has a single nucleus,
but some giant cells, such as
megakaryocytes of bone marrow,
osteoclasts of bone, and skeletal muscle
cells, may have several nuclei.
Nucleus
•
Structure
– The nuclear envelope consists of a double membrane separated by the
perinuclear space.
• The inner membrane is smooth. The outer membrane often contains
ribosomes and is continuous with the surrounding ER.
• The inner and outer membranes fuse at irregular intervals around the
nucleus to form nuclear pores, which allow for exchange of materials
between the nucleus and the cytoplasm.
– Chromatin appears as irregular clumps or granules material dispersed
throughout the nucleus.
• Chromatin is composed of coiled strands of DNA bound to basic proteins
called histones, varying amounts of RNA, and other nonhistone proteins and
enzyme systems.
• In a dividing cell, the chromatin is condensed and coiled into discrete units,
the chromosomes. Human cells contain 23 pairs of chromosomes.
– The nucleoplasm is the matrix that surrounds the chromatin. It is composed of
proteins, metabolites, and ions.
– The nucleolus is a spherical structure composed of RNA and protein. The size
of the nucleolus and the number present vary in different cell types. It is missing
in cells that do not synthesize protein, such as spermatozoa. It is the site of
ribosome production
The cytoplasm contains a complex
network of structural components
• Microfilaments
– Structure
• Microfilaments are solid thread-like cylinders made of
protein and found in a variety of sites within the cell.
– Function
• Microfilaments are responsible for contractility of cells,
which is a property of all cells but is especially well
developed in muscle cells.
• Contractility is responsible for cell locomotion and
movements associated with phagocytosis, pinocytosis,
and cell division.
Structural Components
• Microtubules
– Structure
• Microtubules are hollow tubes present everywhere
in the cytoplasm in all cells.
• They are composed of protein tubulin molecules.
– Function
• Microtubules contribute to the cytoskeleton, or
supporting elements, of the cell.
• They also are involved in cell division, cell
movements, and the transport of materials from
one area of the cell to another.
Structural Components
• Centrioles
– Structure
• In a nondividing cell, two centrioles are located near the
nucleus and Golgi apparatus in a specialized region called
the centrosome.
– Function
• Centrioles function in cell division and also as the site of the
formation of cilia and flagella.
• Centrioles are self-replicating and divide prior to cell division.
Following replication, each original centriole and its duplicate
migrate to opposite nuclear poles where they induce the
formation of the spindle apparatus during cell division.
Structural Components
• Cilia and flagella
– Structure
• Both cilia and flagella are motile processes that extend out
from the cell surface.
• They are composed of longitudinal microtubules, which are
arranged as two single tubules surrounded by a ring of nine
regularly spaced double tubules.
– Function
• Both cilia and flagella function in movement.
• Cilia are able to move fluid or a layer of mucus over the
surface of the cells on which they occur, while the flagellum
of the sperm cell propels the cell.
Cytoplasmic Organelles
Endoplasmic Reticulum
• connected, membrane-bound
sacs, canals, and vesicles
• transport system
• rough ER
• studded with ribosomes
• protein synthesis
• smooth ER
• lipid synthesis
•added to proteins arriving
from rough ER
• break down of drugs
Ribosomes
• free floating or connected to ER
• provide structural support
Cytoplasmic Organelles
Golgi apparatus
•stack of flattened,
membranous sacs
•modifies, packages and
delivers proteins
Vesicles
•membranous sacs
•store substances
Mitochondria
•membranous sacs with
inner partitions
•generate energy
Cytoplasmic Organelles
Lysosomes
• enzyme-containing
sacs
• digest worn out cell
parts or unwanted
substances
Peroxisomes
• enzyme-containing
sacs
• break down organic
molecules
Centrosome
• two rod-like centrioles
• used to produce cilia and
flagella
• distributes chromosomes
during cell division
Cytoplasmic Organelles
Cilia
• short hair-like projections
• propel substances on cell
surface
Flagellum
• long tail-like projection
• provides motility to sperm
Cytoplasmic Organelles
Microfilaments and microtubules
• thin rods and tubules
• support cytoplasm
• allows for movement of
organelles
Inclusions
• temporary nutrients
and pigments
Cell Nucleus
• control center of cell
• nuclear envelope
• porous double membrane
• separates nucleoplasm from
cytoplasm
• nucleolus
• dense collection of RNA and
proteins
• site of ribosome production
• chromatin
• fibers of DNA and proteins
• stores information for synthesis of
proteins
Movements Into and
Out of the Cell
Passive (Physical) Processes
• require no cellular
energy
• simple diffusion
•facilitated diffusion
• osmosis
• filtration
Active (Physiological) Processes
• require cellular energy
• active transport
• endocytosis
• exocytosis
• transcytosis
Diffusion
• The random movement of particles (molecules or ions)
under the influence of their own thermal energy, from an
area of their higher concentration to an area of their
lower concentration, or “downhill.”
• Diffusion of molecules or ions may take place in a liquid,
gas, or solid or through nonliving or living membranes
that are permeable to them.
– Diffusion in a liquid is the movement of solute and solvent
particles in all directions through a solution, or in both directions
through a permeable membrane.
– Net diffusion is the movement of particles from an area of their
own high concentration to an area of lower concentration; that is,
along their own concentration gradients. Net diffusion means
more particles are diffusing in one direction than in the other.
The rate of net diffusion of particles in a
solution is increased by the following
factors:
• A higher concentration gradient because
there are more particles
• A low molecular weight because large
particles are not as easily moved by
colliding with each other
• An increase in temperature because
higher temperature increases random
particle movement.
Simple Diffusion
• movement of substances from regions of higher concentration
to regions of lower concentration
• oxygen, carbon dioxide and lipid-soluble substances
Osmosis
• movement of water through a selectively permeable
membrane from regions of higher concentration to
regions of lower concentration
• water moves toward a higher concentration of
solutes
Osmosis
Osmotic Pressure – ability of osmosis to generate enough
pressure to move a volume of water
Osmotic pressure increases as the concentration of nonpermeable
solutes increases
• hypertonic – higher
osmotic pressure
• hypotonic – lower
osmotic pressure
• isotonic – same
osmotic pressure
Facilitated Diffusion
• In facilitated diffusion, the carrier substance combines with the
solute molecules to form a solute-carrier complex, which is soluble
in the lipid-bilayer, and thus transports the solute across the
membrane.
• Once on the other side, the solute is released. The carrier breaks
away from the complex, returns to the exterior of the membrane,
and repeats the process.
– The carriers exhibit specificity; i.e. they are highly selective in
distinguishing between closely related molecules.
– Facilitated diffusion can be inhibited by competitive and
noncompetitive inhibitor molecules, which closely resemble the
solute molecules.
– The rate of passage of a solute through facilitated diffusion
depends on:
• its concentration difference on both sides of the membrane
• the number of carrier molecules available
• how rapidly the solute-carrier complex formation takes place.
Facilitated Diffusion
• diffusion across a membrane with the help of a channel or
carrier molecule
• glucose and
amino acids
Filtration
• smaller molecules are forced through porous membranes
• hydrostatic pressure important in the body
• molecules leaving blood capillaries
Active Transport
• carrier molecules transport substances across a membrane
from regions of lower concentration to regions of higher
concentration
• sugars, amino acids, sodium ions, potassium ions, etc.
Endocytosis (endo = inner)
• Means taking into the cell.
• It includes phagocytosis and pinocytosis.
– Phagocytosis (phago = to eat) is the engulfing of large solid
substances by foldings of the plasma membrane to form a
phagocytic vesicle.
• The phagocytic vesicle fuses with a lysosome and the
lysosomal enzymes destroy the contents.
• Specialized phagocytic cells in the body remove
disintegrating cells, foreign matter, and bacteria.
– Pinocytosis (pino to drink) is the engulfing of small drops of
extracellular fluid, which may contain dissolved nutrients,
and incorporating them into the cell.
• Receptor-mediated endocytosis refers to the binding of receptor
molecules on the cell surface with specific substances known as
ligands. The receptor-ligand complex then undergoes endocytosis
for transport into the cell.
Endocytosis
• cell engulfs a substance by forming a vesicle around the
substance
• three types
• pinocytosis – substance is mostly water
• phagocytosis – substance is a solid
• receptor-mediated endocytosis – requires the
substance to bind to a membrane-bound receptor
Endocytosis
Exocytosis
• reverse of endocytosis
• substances in a vesicle fuse with cell membrane
• contents released outside the cell
• release of neurotransmitters from nerve cells
Transcytosis
• endocytosis followed by exocytosis
• transports a substance rapidly through a cell
• HIV crossing a cell layer
Stages of replication
• The two strands of DNA are unwound and separated
(unzipped) by unwinding enzymes, which cause the weak
hydrogen bonds between the paired bases to break.
• The enzyme DNA polymerase, using the four kinds of
complementary nucleotides freely present in the nucleus,
matches and attaches the nucleotides to the exposed bases
on each unzipped, single stranded DNA.
• Two complete DNA double helices are formed, each identical
in nucleotide sequence to the original DNA helix that served
as the templates. Thus, the genetic information is copied
exactly.
• Such replication is termed semiconservative because it
conserves each strand of the original DNA double helix while
each also receives a newly synthesized matching partner
strand.
DNA Replication
• hydrogen bonds break
between bases
• double strands unwind
and pull apart
• new nucleotides pair
with exposed bases
• controlled by DNA
polymerase
The cell cycle and mitosis
• The cell cycle, in cells that are capable of
dividing, refers to the events in a cell’s life span
in the period between the time it was formed by
cell division to the beginning of the next cell
division.
• The greatest portion of the cycle (about 90%) is
devoted to growth and synthesis, called
interphase, with a smaller portion devoted to
nuclear and cell division, or mitosis.
The Cell Cycle
• series of changes a cell
undergoes from the time it
forms until the time it
divides
• stages
• interphase
• mitosis
• cytoplasmic division
The Cell Cycle
Interphase
• very active period
• cell grows
• cell maintains routine functions
• cell replicates genetic material to prepare for nuclear
division
• cell synthesizes new organelles to prepare for cytoplasmic
division
• phases
• G phases – cell grows and synthesizes structures other
than DNA
• S phase – cell replicates DNA
Mitosis
• produces two daughter cells from an original somatic cell
• nucleus divides – karyokinesis
• cytoplasm divides – cytokinesis
• stages
• prophase – chromosomes form; nuclear envelope
disappears
• metaphase – chromosomes align midway between
centrioles
• anaphase – chromosomes separate and move to
centrioles
• telophase – chromatin forms; nuclear envelope forms
Mitosis
Control of Cell Division
• cell division capacities vary greatly among cell types
• skin and blood cells divide often and continually
• neuron cells divide a specific number of times then cease
• chromosome tips (telomeres) that shorten with each mitosis
provide a mitotic clock
• cells divide to provide a more favorable surface area to
volume relationship
• growth factors and hormones stimulate cell division
• hormones stimulate mitosis of smooth muscle cells in uterus
• epidermal growth factor stimulates growth of new skin
• contact (density dependent) inhibition
• tumors are the consequence of a loss of cell cycle control
Stem and Progenitor Cells
Stem cell
• can divide to form two new stem cells
• self-renewal
• can divide to form a stem cell and a progenitor cell
• totipotent – can give rise to every cell type
• pluripotent – can give rise to a restricted number of cell
types
Progenitor cell
• committed cell
• can divide to become any of a restricted number of cells
• pluripotent
Stem and Progenitor Cells
The telomere paradox