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Lecture 2
Study of Cells
 Cell shapes and sizes
o Shapes – organs and tissues are often described according to the shapes of
their cells
 Squamous – thin, flat, scaly
 Cuboidal- roughly equal in length, width, and height
 Columnar- distinctly taller than they are wide
 Polygonal- having irregularly angular shapes with four or more
sides
 Stellate- having multiple pointed processes, which give the cells a
somewhat starlike shape
 Spheroid to ovoid- round to oval in shape
 Discoid- disc-shaped
 Fusiform- spindle-shaped; elongated with a thick middle and
tapered ends
 Fibrous- long, slender, and threadlike
o Sizes
 A micrometer is one-millionth of a meter
 Most human cells are about 10 to 15 micrometers wide
 Cell size is limited:
 If a cell were too large, molecules could not diffuse from
place to place fast enough to support metabolism
o Time required for diffusion is proportional to the
square of distance, so if cell size is doubled, the
travel time for molecules within the cell is
quadrupled
 Cells size is limited by the relationship between its volume
and surface area
o The surface area is proportional to the square of its
diameter, while the volume is proportional to the
cube of its diameter, so volume increases much
faster than surface area as diameter increases.
o At some point a cell becomes so large that there
isn’t enough surface area to absorb nutrients and to
get rid of wastes
 The major components of a cell
o Plasma membrane – forms the surface boundary of the cell
o Cytoplasm – material between the cell membrane and the nucleus
 Cytoskeleton – supportive framework of protein filaments and
tubules
 Organelles – diverse structures that perform various functions
 Inclusions- include stored cell products such as lipids and pigments
and foreign bodies such as dust and bacteria
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 Cytosol- a clear gel or fluid inside the cell
o Nucleoplasm – material within the nucleus
Cell Surface
 Plasma membrane – essentially a two-layered lipid film with proteins embedded
in it
o Membrane Lipids
 Phospholipids make up 75% of the membrane lipid molecules
 Two fatty acid tails are hydrophobic, while the phosphatecontaining head is hydrophilic.
o The tails orient away from the water, while the
heads orient towards the intracellular fluid and
extracellular fluid
o The molecules drift from place to place, keeping the
membrane fluid
o Fat soluble substances pass in and out of the cell
through this phospholipids bilayer
o Membrane Proteins
 Proteins may pass all the way through the plasma membrane
(integral proteins), or they may simply adhere to the face of the
membrane (peripheral proteins)
 Roles:
 Receptors – some hormones and neurotransmitters bind to
cells at these proteins to trigger physiological changes
inside the cell
 Enzymes – after chemical messages are received, some
membrane proteins break down those messengers
 Channel proteins – water and solutes may pass through
tunnels made of proteins to enter or leave the cell
membrane
 Cell identity markers – some proteins function as genetic
identification tags to allow the body to determine if cells
belong to the body or are foreign
 Cell adhesion molecules – some proteins allow cells to
stick to each other
 Carriers – some proteins actively bind to a substance on
one side of the membrane and then release it on the other
side
 Membrane Transport – One of the most important functions of the plasma
membrane is to control the passage of materials into and out of the cell
o Filtration – a process in which a physical pressure forces material through
a membrane
 Example: blood pressure forces fluid to seep through the walls of
the blood capillaries into the tissue fluid
 This allows water, salts, and nutrients to be passed from the
blood to the cells surrounding a blood vessel
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o Simple diffusion- the net movement of particles from a greater
concentration to a lesser concentration
 Molecules diffuse through a membrane if the membrane has large
enough pores
 Selectively permeable membranes allow some molecules to pass
through, but not all of them
o Osmosis – the diffusion of water through a selectively permeable
membrane, from the side where water is more concentrated to where water
is less concentrated
o Facilitated diffusion – movement of a solute through a membrane, down
its concentration gradient, with the aid of a carrier
 The carrier binds to the particle on one side, where the solute is
more concentrated, and releases it on the other side, where it is less
concentrated
o Active transport – carrier mediated transport of a solute through a
membrane up its concentration gradient, with the expenditure of adenosine
triphosphate
 ATP is essential to the process because moving particles up a
concentration gradient requires an input of energy
o Vesicular transport – movement of larger particles or droplets of fluid
through the membrane in bubble-like vesicles
 Movement of material into the cell is endocytosis
 Movement of material out of the cell is exocytosis
Glycocalyx – A layer of carbohydrates on the glycoproteins and glycolipids of the
plasma membrane forms a fuzzy, sugary coating
o The coating cushions the plasma membrane and protects it from injury
o The coating functions in cell identity (distinguishing cells from diseased
cells or invading organisms)
o The coating contains cell-adhesion molecules that help bind tissues
together
Microvilli, Cilia, and Flagella
o Microvilli- extensions of the plasma membrane that increase surface area
o Cilia- hairlike processes that extend from cells
 Motile cilia beat in waves that move materials (such as mucus or
egg cells) along the outside surface of the cell
 Non-motile cilia are not well-understood, but some are sensory
o Flagella- long whiplike tails for movement of sperm cells
Intercellular Junctions – arrangements of proteins that link cells together and
attach them to extracellular material
o Tight Junctions- A zipperlike junction between epithelial cells that limits
the passage of substances between them
o Desmosomes-A patchlike intercellular junction that mechanically links
two cells together
o Gap Junctions- A junction between two cells consisting of a pore
surrounded by a ring of proteins in the plasma membrane in each cell
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The pore allows solutes to diffuse from the cytoplasm of one cell
to the next, which makes communication between cells possible
Cytoplasm
 The Cytoskeleton – system of filaments and tubules that provide physical support,
allow cellular movement, and control routing of molecules and organelles to their
destinations within the cell
 Organelles
o The nucleus – round or oval shaped structure near the center of the cell
containing DNA
o Endoplasmic Reticulum – An extensive system of interconnected tubules
or channels enclosed in a membrane
 Rough endoplasmic reticulum – contains ribosomes and
synthesizes proteins for export from the cell
 Smooth endoplasmic reticulum – involved in detoxification,
steroid synthesis, and storage of calcium ions (in muscle cells)
o Ribosomes – Granules composed of ribosomal RNA and enzymes that
read sequences of messenger RNA to assemble sequences of amino acids
to make proteins
o Golgi Complex – Organelle that modifies and packages newly synthesized
proteins and synthesizes carbohydrates
o Lysosomes – Organelles that contain enzymes that are used to digest
foreign matter, pathogens, and expired organelles
o Peroxisomes – Organelles containing enzymes that detoxify drugs and
break down fatty acids, producing hydrogen peroxide in the process
o Mitochondria – Organelles specialized to synthesize ATP (for energy)
o Centrioles- Organelles composed of a short cylinders of microtubules, that
are the origin of the mitotic spindles (used in cell division)
o Inclusions- Any visible object in the cytoplasm of a cell other than an
organelle or cytoskeletal element, such as a dust particle, lipid droplet, or
pigment
Life Cycle of Cells
 Cell Cycle – the life cycle of a cell, extending from the time that a cell is
produced by cell division until it produces daughter cells by cell division.
 Cell Division
o Meiosis – the production of egg and sperm cells (haploid cells)
o Mitosis – produces identical cells for growth or replacement of damaged
cells
 Several phases are involved in mitosis
 Interphase actually precedes replication activities, and it’s
the phase in which most cells remain for long periods of
time. It’s important for mitosis because it’s a phase in
which the DNA is replicated
 Prophase is a phase in which the DNA chromosomes coil
into short, dense rods called chromatids.
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o The chromatids are two genetically identical bodies
joined together at a pinched spot called the
centromere
o The nuclear envelope disintegrates
o Centrioles sprout microtubules called spindle fibers,
which push the centrioles towards opposite poles
Metaphase is a phase in which the chromosomes line-up on
the equator of the cell
o The spindle fibers from opposite poles attach to the
chromatids
Anaphase is a phase in which the two sister chromatids
separate and are pulled by the spindle fibers towards
opposite poles
o The identical chromatids are now called
daughter chromosomes
Telophase is a phase in which the chromosomes are
surrounded by a new nuclear envelope and the DNA
uncoils to return to its dispersed form
Cytokinesis overlaps with telophase and is a process in
which a crease (called a cleavage furrow) begins to form
and eventually pinches one cell into two