Cells: Structure and Function
Cells are small. There are 100 trillion cells in the body.
They range in size from 7.5 µm = micrometers (micrometer is 1 millionth of a meter) to
250 µm, which is visible to the naked eye.
There are thousands of types of cells, each is specialized for a task: skin, liver, kidney,
etc. Each cell has specialized structures for their function.
Every cell has three things in common:
1. Metabolic functions (using nutrients such as sugars and oxygen, and creating
waste products)
2. Responds to its environment
3. Capable of maintaining homeostasis within itself and within the body.
a. HOMEOSTASIS is maintaining a constant and appropriate internal
environment, such as temperature, pH, and glucose levels.
All cells have several main components
• Plasma membrane (actually, a type of organelle)
• Cytoplasm and cytosol
• Nucleus
• Organelles (are surrounded by a membrane)
• Ribosomes (are not surrounded by a membrane)
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CYTOPLASM: the watery liquid inside and outside the organelles, but outside the nucleus.
NEUCLEOPLASM: the liquid inside the nucleus.
CYTOSOL: another liquid that is thicker than water, and is NOT inside the organelles. It
is only found outside of the organelles and nucleus.
Cytosol contains the following:
a. Mostly water
b. Things dissolved in water (amino acids, sugars like glucose, nucleic
acids, and ATP, which is a molecule used for energy).
c. Cytoskeleton: made up of long protein fibers, extend throughout cytosol.
Function of cytoskeleton:
1) Maintains cell shape
2) Movement (such as muscle cell contraction, organelles within
the cell, or the cell itself moving around).
CELL COMPONENTS
1. PLASMA (CELL) MEMBRANE: The cell membrane is semi-permeable to
allow only certain things into and out of the cell.
Functions of the Plasma Membrane:
a. Movement of materials into and out of cell, and acts as a barrier to the
external environment.
b. Acts as a site for receiving signals from the rest of the body.
c. Holds the cell in place.
The plasma membrane is made up of two layers of molecules = PHOSPHOLIPIDS.
• The plasma (cell) membrane is made up of two layers of molecules =
PHOSPHOLIPIDS.
• It’s therefore called a phospholipid bilayer
• Phospholipids are amphipathic molecules.
• That means they have one end that has an affinity for something and another end
that does not have an affinity to that substance. In this case, the affinity is to
water.
• A substance that likes water is called HYDROPHILIC (likes water).
• A region of a molecule that is hydrophilic is called a POLAR region.
• A substance that dislikes water is called HYDROPHOBIC (afraid of water).
• A region of a molecule that is hydrophobic is called a NON-POLAR region.
• Therefore, the phospholipids, being amphipathic, will have a polar region and a
non-polar region.
• The polar region is the PHOSPHATE HEADS
• The non-polar region is the FATTY ACID TAILS.
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The cell membrane is like a film of oil on water. Is oil flexible? (yes) Is oil strong? (no)
But it prevents materials from going across into the water.
The plasma membrane has proteins in it that are made in the RIBOSOMES and
transported to the cell membrane in this case (other proteins are carried elsewhere).
Ribosomes carry out the three functions (above) of the plasma membrane.
• Around each organelle is a membrane identical to the plasma membrane except
for the proteins. Each cell has hundreds of membranes. Ribosomes are not
organelles because they do not have a plasma membrane.
2. ENDOPLASMIC RETICULUM (ER): The ER is a network of channels.
• Two types:
• Rough ER: contains ribosomes
 Function of ribosomes is to make proteins.
• Smooth ER: no ribosomes
 Function is to detoxify chemicals that enter the cell.
ROUGH ENDOPLASMIC RETICULUM (endoplasmic = within cytoplasm;
reticulum = network; rough = surface of membrane covered with ribosomes. This is
an organelle, but the ribosomes are not.
Function of RER is the synthesis (making) of proteins:
a. Membrane proteins
b. Proteins for export (such as digestive system enzymes)
c. Proteins for use within the cell
SMOOTH ENDOPLASMIC RETICULUM (no ribosomes)
Function of SER
SER is continuous with the rough ER, but lacks ribosomes and has several functions
1) Detoxifies harmful substances (alcohol, drugs, medicines)
NOTE: in CSI, when they suspect poisoning, they first look at the SER in the liver.
2) Stores calcium
3) Involved in lipid production (lipid bodies)
The ER forms LIPID BODIES which can also store lipids in the cell (in addition to a
regular storage vesicle).
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Fun Fact: There is a link between life span of a cell and lipids (cholesterol,
triglycerides and fatty acids).
The accumulation of breakdown products of lipids impairs many of the cell’s
stress responses.
Calorie-rich diets cause an increase in lipid bodies, decreasing the life span of the
cell.
Low-calorie diets alter the way fats are processed in cells, increasing the life span
of the cell.
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3. GOLGI COMPLEX
Functions of Golgi complex:
• When the proteins have finished their journey in the RER, their edges are
exposed, and are vulnerable to oxidative damage. Therefore, they first go to the
Golgi complex, which puts chemical bonds on the ends of the proteins.
• Thus, in the Golgi complex, the proteins are modified and prepared for
transport out of the cell.
• The Golgi complex is like a Fed-Ex center that packages and ships the proteins
that were made in the ribosomes.
VESICLES (vacuoles): are bubble-like containers for various substances. Some are
created by the end of the Golgi complex: a piece of membrane pinches off, leaving a
protein in the vesicle, which carries the protein to the cell membrane, where it merges
with the cell membrane, pops, and releases its contents outside of the cell. Other vesicles
are storage containers for food or enzymes.
Vesicles: a sphere of membrane with something in it. This is an organelle. Many types:
a. LYSOSOMES: are sacs of powerful digestive enzymes to dissolve an old
organelle, bacteria, or foreign debris. They are also used to commit cell suicide
(APOPTOSIS is the term for programmed cell death). When bacteria enter a cell,
the lysosome will fuse with the bacteria and release its enzymes on them to
destroy them.
a. TRANSPORT VESICLES: when material needs to move from RER to
Golgi complex, or from Golgi complex to cell membrane, etc.
b. STORAGE VESICLES: one vesicle may store carbohydrates, one may store
lipids, one may store enzymes.
Disorder of Lysosomes
Tay–Sachs disease is a genetic disorder that causes deterioration of mental and physical
abilities that commences around six months of age and usually results in death by the age
of four. It is caused by insufficient activity of an enzyme needed by lysosomes to break
down phospholipids. The lipids accumulate in the brain.
5. MITOCHONDRIA
These are considered the smallest living units in the body because they can make their
own energy (ATP). Cells have hundreds of mitochondria.
Function of mitochondria is to make most of the cell’s ATP, which is cellular energy
(ATP is an energy source).
Some ATP is made in the cytosol, but most is made in the mitochondria.
NOTE: Mitochondria must have OXYGEN to convert nutrients to ATP for energy.
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Mitochondria – generate most of the cell’s energy (ATP); most complex
organelle.
Contains curves known as cristae that can be seen under a microscope.
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Fun Facts: Mitochondrial DNA (mtDNA)
• Nuclear and mitochondrial DNA are thought to be of separate evolutionary origin,
with the mtDNA being derived from the DNA of the bacteria that were engulfed
by the early ancestors of today's eukaryotic cells.
• mtDNA is inherited from the mother (maternally inherited).
• This enables researchers to trace maternal lineage far back in time.
• Using mtDNA can be a useful tool in genealogical research into a person's
maternal line (matrilineage).
• Biologists can determine and then compare mtDNA sequences among different
species and use the comparisons to build an evolutionary tree for the species
examined.
• Studies have used mtDNA to trace the ancestry of domestic dogs to wolves.
• However, they have recently found that the Sabre-tooth tiger is not the ancestor of
the domestic cat.
6. NUCLEUS: Usually the largest structure in a cell. It does not contain cytoplasm; it is
called nucleoplasm. The nuclear membrane contains pores, called nuclear pores. These
allow certain materials into and out of the nucleus.
Functions of the nucleus:
a. Stores DNA (chromosomes are made up of DNA)
b. Makes RNA (RNA is the code for making a protein. It is copied from DNA).
REVIEW OF GENETICS
Our nucleus contains 23 pairs of chromosomes. A chromosome is a double-stranded string of
DNA. DNA is made of a string of molecules called nucleic acids. There are only 4 different
nucleic acids: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Each A, T, G, or C on
one strand of DNA is paired to its counterpart on the other strand of DNA. Adenine (A) only
pairs with Thymine (T), and Guanine (G) only pairs with Cytosine (C). When they pair up, they
are called base pairs. There are about 250 million base pairs of nucleic acids on one
chromosome! The double strand of DNA looks like a ladder. It is then twisted into a shape called
a helix. Therefore, DNA is a double-stranded helix.
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When the body needs a particular protein, the double-stranded DNA helix unwinds, just
in the segment that contains the nucleic acid sequence (called a GENE) for that protein.
The gene is copied in the nucleus and the copy is taken to the cytoplasm, then taken to a
ribosome, which reads the nucleic acid sequence. Every three nucleic acids code for one
particular amino acid. These amino acids are then linked in the proper order in the
ribosome, and the protein is made. When a person has a genetic defect, it is because the
nucleic acids are not in the exact right order. There may be one nucleic acid substituted
for another. There may be a new nucleic acid inserted. This will displace the rest of the
nucleic acid sequence. There may be a nucleic acid deleted. This will also displace the
rest of the nucleic acid sequence. Sometimes, just one amino acid in the wrong order will
cause death in a person before they are born.
A gene is a particular sequence of nucleic acids on the DNA strand of the chromosome. The
function of the genes on the DNA is to tell RNA to tell a ribosome how to make a particular
protein. Proteins carry out most of the functions of the body.
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TRANSCRIPTION is the process of DNA communicating with RNA.
• During transcription, mRNA (messenger RNA) SYNTHESIS occurs. The gene
on the DNA is like my hand. I want to duplicate my hand, so I make a clay mold
of it. The clay mold is the messenger RNA molecule.
• This occurs in the nucleus.
• The mRNA then exits the nucleus through a pore and goes to the cytoplasm.
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TRANSLATION is the process of RNA communicating with a ribosome to tell it what
type of protein to make. Therefore, translation is characterized by PROTEIN
SYNTHESIS.
• This occurs in the cytoplasm.
• During translation, the mRNA (clay mold of my hand) has already left the nucleus
and is now in the cytoplasm. The RNA presents its “hand imprint” to the
ribosome. The ribosome fills the hand imprint with “plaster” to make a positive
cast, or a duplicate of the original gene.
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If you want a construction worker (or ribosome) to build a house (or a protein),
you don’t send the original blueprint (or gene) to the construction worker at the
construction site; you send a copy of the blueprint (or the RNA), and keep the
original in a safe (chromosome) within your own house (the nucleus).
When your body wants a new protein, the DNA helix is unwound at the point
(gene) that codes for the desired protein. The exposed gene sequence of nucleic
acids attracts its matching nucleic acids that are floating around in the nucleus.
When each nucleic acid in the exposed region finishes binding to its matching
nucleic acid like a positive cast (they are now called base pairs), the newly formed
segment detaches and the DNA helix closes back up. The newly formed segment
is the mRNA. The mRNA nucleic acid sequence is the exact opposite of the
desired gene sequence, like a negative cast (clay mold of my hand).
The mRNA exits the nucleus, and threads its sequence through a ribosome. New
nucleic acids floating around will sense that the mRNA nucleic acids are exposed
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and not paired up, so the floating ones will bind with the exposed mRNA
sequence. When the new sequence detaches from the mRNA, its form is the exact
copy of the original gene. Now we are ready to take this gene and create a protein.
The ribosome then reads the gene (the nucleic acid sequence). Every group of
three nucleic acids is called a CODON. Each codon codes for one amino acid.
For example, if the first three nucleic acids are G, C, T, when you check that code
in a manual, you find that means the first amino acid is Alanine. If the next three
nucleic acids are C, C, G, that codes for Proline. Therefore, the ribosome links
alanine to proline, and so on, until the entire amino acid sequence is finished. This
new protein is placed in an envelope for protection, and dumped into the
endoplasmic reticulum. During its journey in the RER and then in the Golgi
complex, protective molecular groups are placed around the delicate ends and side
groups of the protein. After that, it is ready to start functioning.
Interesting Dilemma!
• If all proteins are made by the ribosomes, and the ribosomes are a protein
themselves, where did the first ribosome come from??
NUCLEOLUS
• Within a nucleus there are areas that are darker. These are regions of condensed
RNA. Their function is to carry copies of the genes for proteins to the ribosomes.
• The nucleolus is NOT an organelle, but the nucleus is. Don’t get “nucleolus”
mixed up with the word “nucleus” on the test. The nucleolus does not contain the
DNA; the nucleus does. The nucleolus is within the nucleus, but it does NOT
contain DNA.
• The nucleolus contains RNA, which is important for protein synthesis.
• Do not get nucleus and nucleolus mixed up!
7. CENTRIOLES
Centrioles are filaments within the cell that function during mitosis.
• When the cell goes from metaphase to anaphase of mitosis, the chromatids
separate and follow the spindles of the centrioles towards the opposite ends of the
cell.
SOME SPECIAL CELL STRUCTURES
FLAGELLUM
• Some cells have a flagellum, which is a whip-like tail used to help them move
(locomotion).
• An example is a sperm cell.
MICROVILLI
• Some cells have microvilli, which increase the surface area of cells by
approximately 600 fold, thus facilitating absorption and secretion.
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CILIA
• Some cells have cilia, which are small, hair-like structures that can wave back and
forth, causing substances to move along across the top of the cell.
 For example, the cells of the lungs are lined with cilia, which move
mucous up from the lungs so it can be coughed up and swallowed.
CELL CYCLE: the life cycle of a cell. Some cells never divide (neurons).
When getting ready to divide, undergoes MITOSIS to make two nuclei.
Then cell divides in two = CYTOKINESIS.
Some cells divide rapidly (every few days), some rarely (every 1-2 months), some never.
Stem Cells
Cells are always dying: wash hands, scratched skin, move, digest foods, etc.
A population of cells are always available to replace the cells that died = STEM CELLS.
Muscle stem cells give rise to new muscle cells.
Bone marrow stem cells give rise to new blood cells.
Embryonic stem cells give rise to any type of cells, including neurons (adults don’t have
neural stem cells) and pancreatic cells (diabetics don’t have pancreatic stem cells).
Stem cells are named by type + suffix: BLAST
Erythrocyte = RBC. Erythroblast = stem cell that gives rise to erythrocyte.
Review of Mitosis
 All cells in our body divide by duplicating their chromosomes and then splitting
into two cells, a process called mitosis
 Mitosis produces two daughter cells with the same number and kind of
chromosomes as the parent cell.
 If a parent cell has 46 chromosomes prior to mitosis, how many
chromosomes will the daughter cells have?
 Answer = 46.
 This condition is called diploid (2n).
Sex Cells (Gametes; egg and sperm cells)
 After mitosis, sex cells undergo another cell division without duplicating the
chromosomes. This is called meiosis: each daughter cell has only half of the
chromosomes.
 In males, it produces the cells that become sperm
 In females, it produces the cells that become eggs.
 The sperm and the egg are the sex cells, or gametes.
 GAMETES contain half the number of chromosomes compared to the rest of
the body cells (23 chromosomes).
 This condition is called haploid (n).
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Mitosis Stages
 Interphase: Chromosomes duplicate stage)
 Prophase: Chromosomes shorten and thicken.
 Metaphase: Chromosomes line up in the middle of the cell
 Anaphase: Chromosomes pull apart
 Telophase: Cytoplasm divides in two, forming two daughter cells
MEIOSIS
 Meiosis only occurs in the testes and ovaries when they are ready to make an egg
cell or a sperm cell.
 First, mitosis occurs as normal.
 But right after that, the two daughter cells divide again (meiosis), but this time
there is no reproduction of the chromosomes.
 During meiosis, when the second cell division is at the metaphase stage, the
chromosomes touch each other and exchange a few genes.
 The exchange of genetic material between chromatids is called crossing-over.
 That is what allows for genetic variation.
 Meiosis results in four daughter cells, each having half the number of
chromosomes as the parent cell.
 The daughter cells are not genetically identical, and neither is identical to the
parent cell.
 For example, in MEIOSIS, if the parent cell has 46 chromosomes, the
GAMETE will have 23.
 It will be haploid (n).
 When a sperm and egg (gametes) combine and contribute their chromosomes, the
fertilized egg (called a zygote) will now have 46 chromosomes again.
 It will be diploid (2n).
 Chromosomes can become abnormal if the sister chromosomes do not separate
properly during meiosis. This is called nondisjunction.
The rate of cell division is close to the rate of cell death. 200 billion erythrocytes die
every day, so 200 billion erythrocytes have to be made every day.
Too few = anemia; too many is also a problem.
Body needs to do two things:
1. Control the rate of cell division
2. Control the rate of cell death (apoptosis)
Too many cells can be a TUMOR (an abnormal growth from excess cells). Two types of
tumors:
1. BENIGN (“harmless”, although can cause harm by pressing on vital structure)
2. MALIGNANT (cancerous). These are dangerous because the cells in the
tumor METASTASIZE (leave original site, go elsewhere and grow).
Cancer is hundreds of diseases, each with a different cause, symptoms, treatment, and
prognosis. Any cell type can become malignant, producing different types of cancer.
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FOUR TYPES OF CANCER
1. CARCINOMA: epithelial tissue
2. SARCOMA: Connective tissue (bones, muscles)
3. LYMPHOMA: lymph nodes
4. LEUKEMIA: Blood or blood-forming tissues
How do you distinguish between cancers? If there’s a tumor in the lung, BIOPSY (take a
sample of cells, examine under a microscope to see what kind of cells they are). If
pancreas cells are in lung tumor, indicates pancreatic cancer.
THE REST OF THIS TRANSCRIPT WILL NOT BE ON ANY TEST OR QUIZ
FUN FACTS ABOUT GENES
What causes dimples? All you need is one dominant gene to inherit a particular trait, including dimples,
freckles, a widow's peak, a cleft chin, and bushy eyebrows.
Does a blood transfusion change your DNA? White blood cells do contain DNA. However, most blood
transfusions involve only red blood cells, which do not contain DNA. And even in a rare whole blood
transfusion, no traces of foreign DNA from the white blood cells have been detected in a recipient's blood.
On the other hand, there is at least one situation in which a transplant can change your DNA. After a bone
marrow transplant, the DNA in a blood sample may actually reflect the donor rather than the recipient.
That's because in this case, blood stem cells are transferred. The recipient will produce blood that contains
the donor's cellular elements but almost none of his or her own DNA. And yes, this would affect DNA
blood evidence (though there are other ways to test for DNA.)
Why am I right-handed, but my brother is left-handed?
Only 5% to 30% are left-handed. No one's sure if left-handedness is genetic, a learned behavior, caused by
prenatal or birth trauma, or if it's some combination of these factors.There is only an indirect genetic cause
for left-handedness.If both parents are right-handed or if only the father is left-handed, a child has a 1 in 10
chance of being left-handed. If only the mother is left-handed, the odds rise to 2 in 10. If both parents are
left-handed, the child has a 4 in 10 chance of being left-handed. So, while left-handedness might be passed
from parent to child, the chances are still greater that a child will be right-handed. Identical twins have
identical genes and, if handedness were wholly genetic, both twins would be either right- or left-handed.
But in studies of left-handed twins, only 76% of the pairs were both left-handed. Others studies suggest that
men are 1.5% to 4% more likely to be left-handed than women.
Do identical twins have identical DNA?
Identical twins, formed when one fertilized egg splits, are the only people in the world with identical DNA.
Fraternal twins, on the other hand, are formed when two different eggs are fertilized. Genetically speaking,
fraternal twins are no closer than normal siblings, sharing only about 50% of their genes. Identical twins
have the same genotype, or DNA, but they have different phenotypes, meaning that the same DNA is
expressed in different ways. Traits determined by phenotype include fingerprints and physical appearance.
Thus, a DNA test can't determine the difference between identical twins, while a simple fingerprint can.
Chromosomal abnormalities
Mary was a very attractive young woman who liked to play ice hockey. Her high school gym teacher
took an interest in her ability and gave her extra coaching. She hoped that one day Mary would play on an
Olympic team. But something was wrong. Mary was sixteen and still not menstruating. Her parents decided
to have her undergo a series of medical tests. Much to the surprise of everyone, Mary had an X and Y
chromosome in the nucleus of her cells. She was a chromosomal male. The doctor explained to Mary and
her parents that Mary had testicular feminization syndrome. She has internal testes that produce
testosterone but her cells won’t respond to it. Her genitals are like those of a female and she has welldeveloped breasts. However, she will never be able to have children. Mary will be able to go on and play
hockey in the Olympics but she has to always carry a letter explaining her condition. Otherwise she will be
disqualified because of her sex chromosomes. This is an example of various syndromes that occur when
people have chromosomal abnormalities.
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