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Biol. 2304 - Human Anatomy
CELL STRUCTURE & FUNCTION REVIEW
Atoms are the building blocks of molecules, cells are the building blocks of living organisms.
They are the basic unit of life!
PLASMA MEMBRANE (all cells have a plasma membrane)
1. FUNCTIONS:
¾ gives shape to a cell; encloses & protects the cell
¾ some membrane proteins & carbohydrates provide receptor sites or binding sites (functions
described below)
¾ some membrane proteins function as enzymes that catalyze chemical reactions
¾ some membrane proteins help change membrane shape during processes such as cell division,
locomotion, & endocytosis
¾ regulates the entrance & exit of materials (it is very selective about what it lets in & out; this is
called selective permeability)
2. STRUCTURE: Fluid Mosaic Model (what does this mean? why is this important?)
Some Membrane Lipids:
¾ phospholipid bilayer - hydrophobic fatty acid tails & hydrophilic phosphate heads - review chemistry
handout on phospholipids.
¾ cholesterol - its ring structure keeps the phospholipid fatty acid tails from packing together to keep
the membrane more fluid; in other words, cholesterol molecules create space.
Some Membrane Proteins: (proteins float in the fluid lipid bilayer)
¾ Integral proteins - inserted in the bilayer (they span the entire width of the cell membrane; mainly
involved in transport; channel proteins - proteins with a channel through which water and small,
polar substances move across the membrane; carrier proteins - bind to specific substances &
transport them across the membrane
¾ Peripheral proteins - usually appended to exposed parts of integral proteins on the outside of the
membrane; some are enzymes; others are involved in the changes in cell shape that occur during cell
division & contraction of muscle cells.
Membrane Carbohydrates:
Glycoproteins & Glycolipids - branching chains of sugars are attached to proteins and lipids in the
membrane; these carbohydrate chains form a glycocalyx on the outside of the membrane (cells are
"sugar coated" and sticky or gummy on the outside); some of the functions of the glycocalyx:
¾ determine the ABO blood groups
¾ recognition of the egg by sperm
¾ can serve as receptors for hormones
¾ can serve as receptors to trigger endocytosis
¾ act as “markers” - id. a cell as being a certain type; also id the cell as belonging to “self” (not a
foreigner like a bacterium)
¾ help anchor the cell in place
MICROVILLI - minute, fingerlike projections of the cell membrane; their function is to increase the surface
area of the cell membrane; most often found on the surface of absorptive cells such as columnar
epithelial cells in the intestine; different from cilia, which are hair-like projections made of protein.
MEMBRANE TRANSPORT
A. PASSIVE TYPES OF TRANSPORT ACROSS THE PLASMA MEMBRANE
1. Most passive transport processes depend on the process of DIFFUSION
¾ Definition - the net movement of particles from a greater concentration to a lower concentration
(down a concentration gradient) to distribute the particles uniformly; it's a passive process molecules move by their own kinetic energy - requires no energy expenditure in the form of ATP by
the cell.
¾ Diffusion through the cell membrane - The lipid interior of the cell membrane is a barrier to simple
diffusion; most polar molecules (polar molecules get "stuck" in the nonpolar fatty acid tails). Small,
nonpolar, lipid soluble molecules like fats, carbon dioxide, oxygen, & alcohol move easily through
the cell membrane by simple diffusion.
2. OSMOSIS - a special case of diffusion; the movement of water across a semipermeable membrane –
water moves from a high water concentration to a low water concentration (or from a low solute
concentration to a high solute concentration); water moves across cellular membranes through pores
in channel proteins or through momentary openings in the membrane.
Tonicity: describes the relative concentrations of solute in two fluids, such as the fluid inside &
outside a cell; you can then describe the relative concentrations of water in the two fluids; 3 cases:
1.) isotonic solutions ("iso" = same) - two or more solutions that have equal concentrations of
solute.
2.) hypotonic solution ("hypo" = less) - one solution has less solute (more water) than the other; a
cell that is in a hypotonic environment will lyse (burst); ex. placing a cell in distilled water would
cause the cell to lyse - water would move into the cell to where the water concentration is lower.
3.) hypertonic solution ("hyper" = more) - one solution has more solute (less water) than the
other; a cell that is in a hypertonic environment will crenate (shrink), because the water in the cell
moves out of the cell to an area of lower water concentration; ex. placing a cell in water with a
high salt or sugar concentration would cause the cell to crenate – water would move out of the
cell to where the water concentration is lower.
Note: The above examples describe the environment that the cell is in (i. e., the solution is
hypotonic or hypertonic to the cell). You can also talk about the cell in relation to its environment
(i. e., the cell is hypertonic or hypotonic to its environment). You have to make this distinction!!
The cells in our bodies try to maintain the isotonic condition so that they are not in danger of
lysing or crenating.
3. Facilitated Diffusion - Again, only small, nonpolar molecules readily diffuse across the cell
membrane. Polar & charged molecules get "stuck" in the fatty acid part of the lipid bilayer. Small,
polar molecules, like water, and some ions can diffuse through channel proteins. Most biologically
important molecules, however, are polar & are much larger an water (ex. glucose) and cannot fit
through channel proteins. Special selective carrier proteins are located in the membrane to
transport molecules like glucose. In facilitated diffusion, carrier proteins move molecules from a high
concentration to a low concentration like in simple diffusion; it is believed that changes in the shape of
the carrier protein allow it to envelop and then release the transported substance.
B. ACTIVE TYPES OF TRANSPORT ACROSS THE CELL MEMBRANE
These processes use energy (ATP)!!!
1. Active Transport - Carrier proteins move molecules move from low concentration to high
concentration (against the concentration gradient). Examples:
Sodium-Potassium Pump – solute pump important in the propagation of nerve impulses; maintains
a higher sodium concentration outside the nerve cell & a higher potassium concentration inside the
nerve cell.
2. Vesicle Mediated Transport - vesicles from the cell membrane, the endoplasmic reticulum (smooth
or rough), or the golgi complex form around large molecules; this type of transport can down the
gradient or against the gradient. Two types:
¾
Exocytosis - substances are exported out of the cell in vesicles formed from the golgi complex or the
e. r.; these vesicles move to the surface of the cell & fuse with the cell membrane, expelling their
contents to the outside; ex. hormones made in the smooth e. r. are transported to the golgi complex
for further processing - the golgi complex then transports the hormone to the surface of the cell in a
vesicle & the hormone is released to the outside.
¾
Endocytosis - substances are imported into the cell; vesicles are formed from the cell membrane,
sometimes in response to the triggering of a receptor membrane protein (called receptor-mediated
endocytosis); the cell membrane envelopes the substance to be imported & pinches off to form a
vesicle that moves into the cytoplasm; many endocytic vesicles fuse with enzyme-containing vesicles
called lysosomes to digest their contents; Examples:
1.) Phagocytosis ("cell eating") - solids are imported into the cell; ex. a white blood cell
engulfing a bacterium.
2.) Pinocytosis ("cell drinking") - liquids are imported into the cell; ex. absorption by columnar
epithelial cells in the intestine.
PARTS OF A GENERALIZED ANIMAL CELL: (what's inside the plasma membrane)
(Note: no single cell will have this exact structure)
A. NUCLEUS ("The Control Center of the Cell")
Structure:
¾ nuclear envelope - double membrane with nuclear pores that surrounds the nucleus.
¾ nucleoplasm - like cytoplasm
¾ chromatin - genetic material composed of DNA & associated proteins; chromatin condenses & forms
chromosomes during cell division.
¾ nucleolus (usually two or more per nucleus) - spherical bodies composed of proteins, DNA, & RNA;
where RNA is constructed into ribosomes; not membrane-bound.
Function: carrier of the hereditary information, which exerts a continuing influence over the ongoing
activities of the cell through protein synthesis. The only thing your cell can make directly from DNA is
protein! So how would the cell make a lipid if the only thing it can make from DNA is protein?
B. CYTOPLASM (or cytosol) - Thick, semitransparent, elastic fluid inside the cell's cell membrane &
external to the nucleus; it contains organelles.
C. CYTOPLASMIC ORGANELLES (organelles – “little organs” inside the cell that perform specific
functions)
1. RIBOSOMES (may be free in the cytoplasm or attached to rough endoplasmic reticulum & the
nucleus)
¾ structure - not membrane-bound; made up of RNA & protein.
¾ function - sites of protein synthesis (amino acids are assembled into polypeptides).
2. ENDOPLASMIC RETICULUM
¾ structure: interconnecting flattened sacs, tubes, & channels.
¾ types & functions: (both types support the cytoplasm & provide more surface area inside the cell
for chemical reactions to take place):
a.) Rough E. R. - (ribosomes are attached to it) - function: initial modification of proteins; process:
polypeptide chains are formed at the ribosome & some of them are transported to the lumen of the e.
r. for modification; if further modification is necessary, the polypeptides are then packaged in
transport vesicles (a piece of the e. r. pinches off around the polypeptide); these vesicles transport
the polypeptides to the golgi complex for further modification of the polypeptides into a proteins.
b.) Smooth E. R. - (no ribosomes attached) - function: main site of lipid synthesis, including sex
hormone production and glyceride production (lipids are sent to the golgi body in transport vesicles
for further modification & distribution); in liver cells it is involved in the detoxification of blood; in the
muscle cells it stores & release calcium for muscle contraction; in muscle and liver cells it stores
glycogen.
3. GOLGI COMPLEX ("the Wal-Mart Distribution Center")
¾ structure - 4 to 8 flattened, membrane-bound sacs loosely stacked on top of one another &
surrounded by vesicles; looks like a stack of pancakes.
¾ function - final modification of proteins & lipids; process: transport vesicles from the E. R. fuse with
the golgi complex; the contents of the vesicles are processed in the golgi complex; the finished
product is pinched off in a piece of golgi membrane (another vesicle) & is transported to the part of
the cell where it is needed; the golgi complex processes, packages, & distributes the material the cell
manufactures.
4. VESICLES
¾ structure - membrane-bound sacs that could be pinched off pieces of golgi complex, E.R., or cell
membrane
¾ function - transport material within the cell & into & out of the cell.
¾ some specialized vesicles:
1.) Lysosomes - large vesicles that pinch off from the golgi complex; contain enzymes for breaking
down proteins, lipids, etc. (digestion within the cell); they fuse with other vesicles (such as
phagocytic vesicles) to degrade or digest their contents.
2.) Peroxisomes - large vesicles that contain enzymes that break down toxic hydrogen peroxide into
water & oxygen.
3.) Exocytic & Endocytic vesicles - already discussed
5. MITOCHONDRIA ("The Powerhouse of the Cell")
¾ structure - usually shown oval shaped; double membrane: smooth outer membrane & a folded inner
membrane (folds provide more surface area for chemical reactions to take place).
¾ function - break down energy containing organic molecules (ex. carbohydrates) & repackage the
energy into smaller units (ATP) that can be used by the cells;
6. CYTOSKELETON
¾ structure - network of filamentous protein structures called microtubules & microfilaments.
¾ functions - give the cell shape (support), anchor the organelles, transport substances through the
cell, involved in cell division, involved in cell motility (flagella), involved in the contraction of muscle
cells.
7. CENTRIOLES
¾ structure - paired cylindrical structures composed of protein filaments.
¾ function - during cell division, organize a microtubule network, called spindle fibers; spindle fibers
are responsible for moving the chromosomes around in the cell (we’ll talk more about this in mitosis
section).
8. CILIA & FLAGELLA
¾ Cilia - short, hairlike, motile cellular extensions that occur on the surfaces of certain cells; ex. ciliated
columnar epithelial cells line the respiratory tract propel mucus to trap foreign debris (dust, bacteria,
etc.) upward from the lungs to the back of the throat where it can be coughed up and swallowed.
¾ Flagellum - in humans, the single, long, hairlike cellular extension that occurs in sperm cells;
involved in sperm motility.
MITOSIS
A. GENERAL
1. Different Types of Cellular Division Briefly Defined:
a. Mitosis & Meiosis (nuclear division)
b. Cytokinesis (cytoplasmic division)
c. Uses of Mitosis & Cytokinesis in humans: tissue growth & repair.
d. Uses of Meiosis & Cytokinesis in humans: gamete (sperm & egg) formation in the testes
and ovaries for sexual reproduction.
B. A LITTLE ABOUT CHROMOSOMES
1. Chromosome Defined - In human cells, DNA (deoxyribonucleic acid) & special proteins form
linear structures called chromosomes. Sections of the DNA (called genes) code for the
production of specific proteins.
2. How many chromosomes? All individuals of the same species have the same number of
chromosomes in their body cells or somatic cells. Human somatic cells or body cells each have
46 chromosomes (23 pair). Human gametes (sperm & egg) each have 23 chromosomes.
3. More on Mitosis & Meiosis:
a. Mitosis is a type of nuclear division that maintains the parental number of chromosomes for
daughter cells (after mitosis, the daughter cells have the same number of chromosomes as
the original parent cell). Mitosis occurs in the somatic cells. Mitosis in a single cell results in
two daughter cells that are identical to the parent cell.
b. Meiosis is a type of nuclear division that reduces the parental chromosome number by half
(after meiosis, the daughter cells have half the number of chromosomes as the original
parent cell). Meiosis occurs in the reproductive organs to produce sperm & egg - the sperm
& egg will each have only 23 chromosomes, so that when the sperm & egg nuclei fuse
during fertilization, the resulting zygote will have 46 chromosomes. Meiosis in a single cell
results in 4 cells that are different from each other & from the parent cell.
C. THE CELL CYCLE: (Involves 3 major phases: interphase, mitosis, & cytokinesis)
1. INTERPHASE - growth & replication phase of the cell cycle.
a. The cell increases in size.
b. All organelles & molecules increase in number. Chromosomal (DNA) replication takes place.
The genetic material (DNA) is in the threadlike form called chromatin; each strand of DNA
replicates itself; the replicated strand & the original strand remain attached to one another at
a central point called the centromere - the two strands are now called sister chromatids &
are identical to one another.
c. The chromatin begins to coil & condense into thicker, rodlike forms called chromosomes;
this process is called chromosome condensation.
Why do you think condensation occurs?
2. MITOSIS = division of the nucleus; 4 phases (PMAT):
a. Prophase - the nucleoli disappear; the nuclear envelope begins to disappear; chromatin
condensation is completed; the centriole pairs move to opposite poles of the cell (one pair at
each pole); these centrioles organize protein filaments to form the spindle apparatus,
which will help move the chromosomes around during division.
b. Metaphase - the nuclear envelope has completely disappeared; spindle fibers from both
poles attach to the centromere region of each chromosome; the spindle fibers arrange the
chromosomes at the equator or midline of the cell.
c. Anaphase - sister chromatids of each chromosome are separated from each other & are
drawn to opposite poles by the spindle fibers when the fibers contract.
d. Telophase - the spindle apparatus disperses; new nuclear envelopes reform around the two
sets of chromosomes; nucleoli begin to reappear; the chromosomes decondense to form
chromatin again.
3. CYTOKINESIS - "division of the cytoplasm" usually begins during late anaphase or telophase;
the cell is divided into two equal parts; a furrow appears in the cell membrane & deepens into a
groove.
Lab Objectives:
Id. the stages of mitosis on a slide (metaphase, anaphase, telophase). Make sketches of what you
observe. Look in your lab manual and/or textbook for great photomicrographs to guide you.
Also look at the models of mitosis. Identify the following:
¾ Cell Cycle: Interphase, Mitosis, Cytokinesis
¾ Mitotic Phases: Prophase, Metaphase, Anaphase, Telophase (don’t worry about specifically
identifying early, middle, late)
¾ Spindle Fiber Apparatus
¾ Centriole Pairs
¾
¾
¾
¾
¾
Centromere
Chromososmes
Chromosome Condensation
DNA Replication
Sister Chromatids