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Anatomy of Cells
Overview of the
Cellular Basis of Life
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A cell is the basic structural and functional unit
of living organisms. When you define the
properties of a cell, you are in fact defining the
properties of life.
The activity of an organism is dependent on
both the individual and collective activity of the
cells.
According to the principle of complimentarity,
the biochemical activities of cells are
determined and made possible by the specific
subcellular structures of cells.
The continuity of life has a cellular basis.
Properties of a Cell
Cells are chemically composed chiefly of
carbon, hydrogen nitrogen, oxygen, and
trace amounts of several other elements.
Size
• Diameters range from about 2 micrometers
(1/12,000 of an inch) in to 10cm or more in the
largest cell. The typical human cell is about 10
micrometers. The largest, the fertilized egg, is
nearly 100 micrometers in diameter.
• Lengths range from a few micrometers to a
meter or more. Some skeletal muscle cells are
30cm long, and the nerve cells that cause your
foot muscles to contract run from the end of your
spinal cord to your foot.
Shape
• A cell’s shape reflects its function. Flat, tile-like
epithelial cells that line your cheek fit closely
together, forming a living barrier that protests
underlying tissues from bacterial invasion.
Examples:
• Spherical (fat cells)
• Disk-shaped (red blood cells)
• Branching (nerve cells)
• Cube-like (kidney tubule cells)
3 Major Parts
• Nucleus: Control center
• Cytoplasm: packed with and supports the
organelles
• Plasma membrane: forms the external cell
boundary
The Plasma Membrane
The plasma membrane is a thin, stable structure composed
of a double layer, or bilayer, of phospholipids molecules
with protein molecules dispersed in it. It is called the
fluid mosaic model because the molecules are able to
slowly float around the membrane.
The lipid bilayer forms the basic “fabric” of the
membrane and is relatively impermeable to most water
soluble molecules.
Membrane proteins imbedded in the lipid bilayer are
responsible for most specialized functions of the plasma
membrane.
Receptor Proteins: react to the presence of hormones or
other regulatory chemicals and trigger metabolic
changes in the cell
Marker Proteins: (glycoproteins) allows cells to recognize
each other for immune or developmental purposes
Transport Proteins: either channel or transport needed
chemicals through the membrane that can not otherwise
pass through the lipid bilayer
There are 2 distinct protein
populations:
• Integral proteins: embedded
into the membrane
• Peripheral proteins: attach to
integral membrane proteins, or
penetrate the peripheral
regions of the lipid bilayer
• Branching sugar groups are
attached to most of the
proteins that abut the
extracellular space. The term
glycocalyx is used to describe
these attachments.
Functions of glycocalyx
1. Determining the ABO and other blood
groups.
2. Provide binding sites for other toxins.
3. Recognition of the egg by sperm.
4. Determining cellular life span.
5. Serving in the immune response.
6. Helping to guide and direct embryonic
development.
Cytoplasm and Organelles
The cytoplasm is the gel-like internal substance (cytosol) of
cells that contains many tiny suspended structures called
organelles (little organs). Organelles are classified as
either membranous or nonmembranous.
Membranous organelles
• Endoplasmic reticulum (ER): rough Er
synthesizes proteins; smooth ER synthesizes
lipids, steroid hormones, and carbohydrates
used in forming glycoproteins
Membranous organelles
• Golgi apparatus: synthesizes carbohydrates to
combine with proteins; packages products to be
sent out of the cell
Membranous organelles
Lysosomes: bags of digestive enzymes
break down worn cell parts and ingested
particles; a cell’s digestive system
Membranous organelles
Peroxisomes: contain enzymes that detoxify
harmful substances
Membranous organelles
Mitochondria: Catabolism; ATP synthesis; a
cell’s power plant
Nonmembranous
• Ribosomes: site of protein synthesis; a
cell’s protein factory
Nonmembranous
• Cytoskeleton: acts as a framework to support the cell
and its organelles; functions in cell movement; forms cell
extensions (microvilli, cilia, flagella) Cell fibers
*Microfilaments: smallest cell fibers; cellular
muscles; run parallel to the long axis of the cell; found
and function mostly in muscle cells
*Intermediate filaments: slightly thicker fibers;
form much of the supporting framework in most cells
*Microtubles: thickest fibers; form tubes;
“engines of the cell” because they move things around in
the cell or move the cell itself
Cytoskeleton
Nonmembranous
• Centrosome
Nonmembranous
• Cell Extensions
Microvilli: minute, fingerlike extensions of the
plasma membrane that project from a free, or
exposed, cell surface; they increase the surface
area of the membrane
Cilia: hair-like extension of the membrane
used in moving
substance
Flagella: tail-like extensions used for
movement
Cell Nucleus
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Nucleolus: produces ribosomes
Chromatin: uncoiled DNA
Chromosomes: coiled DNA
Chromatids: Duplicated DNA
Specializations of the Plasma
Membrane
Membrane junctions help to bind cells closely to form
tissues.
• Tight junctions: protein molecules in adjacent
plasma membranes fuse together tightly like a
zipper, obliterating the intercellular space and
forming an impermeable junction; ex. Epithelial cells
of the digestive tract
Membrane junctions
• Desmosomes: act as mechanical couplings or
adhesion junctions that prevent separation of tissue
layers – membranes do not actually touch
Membrane junctions
• Gap junctions: provide for direct passage of chemical
substances between adjacent cells – the cells are connected by
hollow cylinders composed of transmembrane proteins, called
connexons – embryonic cells prior to development of
circulatory system and in adults in electrically excitable tissues
such as heart and smooth muscle, and also between some
nerve cells