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Cells!
Muse 2430 Lecture #2
5/8/13
Chapter 3
Cells
• vary in size
• possess
distinctive
shapes
• measured in
micrometers
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
Cell Membrane
The Plasma 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.
Structure of the Plasma
Membrane
Intercellular Junctions
Tight junctions
• close space between
cells
• located among cells
Desmosomes
that
form
linings
• form
“spot
welds” between cells
• located among outer skin cells
Gap junctions
• tubular channels between cells
• located in cardiac muscle cells
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.
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
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.
Cytoplasmic Organelles
Microfilaments and
microtubules
• thin rods and tubules
• support cytoplasm
• allows for movement of
Inclusions
organelles
• temporary
nutrients and
pigments
Movements Into and
Out of the Cell
Passive (Physical)
Processes
• require no
cellular energy
• simple diffusion
•facilitated
diffusion
• osmosis
Active (Physiological) Proc
• require cellular energy
• active transport
• endocytosis
• exocytosis
• transcytosis
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
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.
Filtration
• smaller molecules are forced through porous m
• 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.
Transport in Vesicles
• Vesicle - a small spherical sac formed by budding off
from a membrane
• Endocytosis - materials move into a cell in a vesicle
formed from the plasma membrane
three types: receptor-mediated endocytosis
phagocytosis
bulk-phase endocytosis (pinocytosis)
• Exocytosis - vesicles fuse with the plasma membrane,
releasing their contents into the extracellular fluid
• Transcytosis - a combination of endocytosis and
exocytosis
Phagocytosis
Endocytosis
Transcytosis
• endocytosis followed by exocytosis
• transports a substance rapidly through a cell
• HIV crossing a cell layer
Exocytosis
• reverse of endocytosis
• substances in a vesicle fuse with cell membran
• contents released outside the cell
• release of neurotransmitters from nerve cells
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
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, membrane-bounded
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.
Golgi Complex
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
Stem and Progenitor Cells
Mitosis = cell copying
The
End