Download anatomy Unit 2

Unit 2
The Cell
The cell is:
 the most basic structural and
functional unit of life.
 the smallest structure capable
of carrying on all vital life
functions.

I. Cell Structure and Function
A. Plasma (cytoplasmic)
membrane
 Envelopes the cell
 Serves as a barrier between
intra- and extra-cellular
environments

Is usually selectively
permeable

Enables the cell to maintain
homeostasis by regulating the
movement of materials into and
out of the cell.
Modifications:
1. Microvilli: slender projections
created by extensive folding of the
free surface
Serve to increase the surface area
of the cell.
Prominent in cells responsible for
absorption.

2. Cilia: slender projections
containing supportive
microtubules.
Serve to move
(redistribute/relocate) body
fluids
3. Flagellum: single “whip like”
projection similar in structure
to cilia, but longer.
Provides the cell with mobility
– propulsion.
Usually moves the entire cell.
B. Cytoplasm
Protoplasmic material
contained within the cell by the
plasma membrane
 Organelles are suspended in the
cytoplasm

C. Organelles
 Distinct structures found in the
cytoplasm
 They play specific roles in the life
process of the cell and, therefore,
the organism.
 Their abundance is determined by
the specific function of each cell.
1. Endoplasmic Reticulum
 An extensive system of
interconnected tubes and
membranes that coil through
the cell connecting the
cytoplasmic and nuclear
membranes
a. Smooth ER
 Enzymes catalyze reactions
involved in:
 metabolism and synthesis of lipids,
 synthesis of steroid-based
hormones,
 detoxification of drugs, some
pesticides, and carcinogens,
breakdown of stored glycogen
to form free glucose
and
 calcium ion storage and release
(specific to muscle cells and
called sarcoplasmic reticulum).

b. Rough ER

The external surface is
abundant in ribosomes which
are small, dark granules made
of proteins and RNA
Ribosomes manufacture all
proteins that are secreted from
cells.
 Some ribosomes float free in the
cytoplasm…and make soluble
proteins that function in the
cytoplasm


Ribosomes can attach to and
detach from the endoplasmic
reticulum as necessary.

The ribosomes on rER
manufactures the proteins and
phospholipids that form all
cellular membranes (so it is
called the “membrane factory”)
2. Golgi Apparatus
Stacked and flattened vesicles
 “Traffic director” for cellular
proteins…
 modifies, concentrates and
packages the proteins and
lipids made at the rER

to aid in their release to the exterior
of the cell
 They also pinch off vesicles that
contain lipids and transmembrane
proteins to send to the plasma (or
other organelle’s) membrane
 Numerous in cells active in
secretion
3. Mitochondria
The energy powerhouse
 Contain large amounts of enzymes
to break down nutrients providing
the cell with energy forming
ATP…aerobic cellular respiration

They contain their own DNA
and RNA
 making them able to reproduce
themselves.


An interesting side note (an IFAF):
Because mitochondria and
mitochondrial DNA are similar to
the purple phylum bacteria, it is
widely accepted that mitochondria
came from bacteria that invaded the
ancient ancestors of plant and
animal cells!
Mitochondria are abundant in
cells with high energy needs
(muscle cells)
and increase in abundance as
energy needs increase.
 The relative density of
mitochondria reflects the energy
requirements of the cell.

4. Lysosomes
“cellular garbage disposals”
 “disintegrator bodies”
 Contain enzymes capable of
breaking down the components
of the cell…worn out or
nonfunctional organelles

Responsible for the digestion of
dead cells
 Digest ingested bacteria, viruses,
and toxins
 Perform metabolic functions such
as the breakdown and release of
glycogen

Break down nonuseful tissues
(i.e.: the webs between fingers
and toes during fetal
development; the uterine lining
during menstruation)
 Break down bone to release
calcium ions into the bloodstream

5. Microtubules
 Stiff, bendable and hollow tubes
made of spherical protein subunits
 determine the overall shape of the
cell and
the distribution of cellular
organelles.
6. Centrosomes and Centrioles
Centrosomes act as a microtubule
organizing center because the
microtubules radiate out from
here.
 Contain paired centrioles


Generates microtubules and
organizes the mitotic spindle in
cell division
Centrioles form the bases of
cilia and flagella
D. The Nucleus

Is surrounded by a nuclear
membrane which contains the
nucleoplasm
Two Vital Functions
Controls and regulates
metabolic activities
 Essential to the process of cell
division

Openings in the nuclear
membrane connect the nucleus
to the ER
 The nucleus manufactures
nucleic acids needed for
protein synthesis

The nucleus contains chromatin
(a nucleoprotein) which
become rod like chromosomes
during cell division.
 Nucleoproteins are
combinations of proteins and
nucleic acids.

The nucleic acid found in
chromatin is deoxyribonucleic
acid…DNA!
 Inside the nucleus are one or
more nucleoli.
 Each nucleolus is a cluster of
protein, DNA and RNA that are
not enclosed by a membrane

It is the site where ribosomal
subunits are assembled and
stored.
 It is also where one type of
RNA is synthesized.


The nucleolus is found in the
nuclei of cells that are not
undergoing cell division; it
disperses and disappears during
cell division and reappears
when division is complete and
new cells are formed.

Nucleoli are abundant in cells
that synthesize large amounts
of proteins such as the liver and
muscles cells.
II. Cell Transport
 Physical processes:
 Entails the net movement of
ions or molecules through a
cell membrane.
 This movement occurs across a
concentration gradient
1. Diffusion: the scattering of
particles…the movement of
molecules from a region of
high concentration to a region
of lower concentration.
2. Osmosis: the diffusion of
(primarily) water through a
selectively permeable
membrane.
3. Active transport: the process by
which substances (ions) are moved
across the plasma membrane due
to the expenditure of energy.
A.T. typically occurs from areas of
low concentration to high (as
opposed to diffusion and osmosis).
4. Transport in vesicles: vesicles are
spherical sacs that “bud” off from
a membrane. They transport
substances from one structure to
another within cells; take in from
(endocytosis), and/or release
substances to (exocytosis),
extracellular fluid
Cell membranes are not
completely permeable to any
substance…
 however, they do allow some
substances to pass through with
more ease than others.

III. Homeostasis And Cells

All of the components of the
cell coordinate to maintain
homeostasis.
A. Homeostatic Imbalances
1. Cystic fibrosis
2. Tay-Sachs Disease
3. Mitochondrial myopathies
4. Progeria
5. Werner Syndrome
6. Cancer (carcinogenesis)
Cellular Disorders
IV. CELL DIVISION

As cells become damaged,
diseased or worn out, they are
replaced through the process of
cell division…
the process by which cells
reproduce themselves.
I. Somatic Cell Division
Somatic cell division replaces
dead or injured cells
 and adds new ones for tissue
growth.

Cells undergo nuclear cell
division …
called Mitosis
 and a cytoplasmic division
called…
cytokinesis


The combination of mitosis and
cytokinesis results in two
identical cells each having the
same number and kinds of
chromosomes as the original
cell.
Somatic cells (with the
exception of sex cells, liver
cells and red blood cells) all
have 23 pairs of chromosomes.
 One member of each pair is
inherited from each parent.

The two chromosomes that
make up each pair are called
homologous chromosomes or
homologs
 They contain similar genes
arranged in the same (or
similar) order

These cells are diploid cells
because they contain two sets
of chromosomes.
 When cell duplication occurs,
all of the chromosomes must be
replicated so that its genes can
be passed on


The cycle of cell division
consists of two major periods:
 Interphase
 Mitotic phase
A. Interphase

There are three subphases of
interphase:
 G1
S
 G2
1. G1 Phase
Also referred to as the
growth phase
 At this time, the cell is
metabolically active

2. S Phase
 Also referred to as the
synthesis phase
 At this time, DNA replication
occurs
 Two identical cells are formed
having the same genetic material
Once this phase begins, the cell
must complete the cycle of
division!
 The centrosome replicates
during the S phase.

3. G2 Phase
 This is the interval between the S
phase and the mitotic phase.
 Cell growth continues
 Enzymes and proteins are
synthesized
 Centrosomes are fully replicated
At the end of the G2 phase, the
cell is ready for division.
 Throughout interphase, the cell
continues to grow and carry on
its normal functions.

B. Mitosis
AKA the M phase
 This is the series of events that
distributes the replicated DNA
of the mother cell to the two
daughter cells.

Mitosis is a continuous process
with the four phases smoothly
transitioning from one to the
next.
 The entire process usually
takes about an hour or less to
take place

1. Prophase
 This is the first, and the longest
phase of mitosis.
 Chromatin fibers condense and
shorten into chromosomes
 Each chromosome is made up of a
pair of identical, double-stranded
chromatids
The central centromere holds the
pairs together.
 Towards the end of prophase, the
centrioles create the mitotic
spindle which pushes the
centrioles farther apart towards
opposite ends of the cell

The nuclear membrane
fragments and disperses to the
ER
 This allows the spindle to
occupy the center of the cell
and interact with the
chromosomes.

Some of the spindles attach to
the chromosome’s centromere.
 The rest of them are linked by
their tips near the center
 So that together they can draw
the chromosomes to the center
of the cell.

2. Metaphase
 Once the microtubules have
aligned the chromosomes in the
center of the cell, the cell is in
metaphase
 Enzymes trigger the separation of
the chromatids as anaphase
begins.
3. Anaphase
This phase begins as soon as
the centromeres of the
chromosomes split
 This is the shortest stage of
mitosis…usually lasting only a
few minutes.

Each chromatid becomes an
individual chromosome
 The spindles attached to the
centromeres pull the
chromosomes towards its pole.
 This makes the chromosomes
look “v” shaped


The other spindles push the two
poles of the cell apart causing it
to elongate.
It is theorized that the
chromosomes are short and
compact to prevent their tangling
during anaphase
 Tangling would result in an
imperfect distribution of genetic
material to the daughter cells.

4. Telophase
As soon as the chromosomes
stop moving, telophase begins.
 This is the reverse of prophase
 The chromosomes at the
opposite poles extend,


a new nuclear envelope (made
from the bits of the old
membrane that have been
stored) forms around each
chromatin mass.
Nucleoli reappear in the nuclei,
 the spindles break down and
disappear
 Mitosis is ended!
 For just a bit…the cell is
binucleate with each nucleus
identical to the original one

C. Cytokinesis

The process of splitting the cell
into two daughter cells begins
during anaphase, and continues
through and after telophase

Cytokinesis occurs as
peripheral microfilaments form
at the cleavage furrow and
squeezes the cell apart.
D. Cell Destiny
 Basically, cells have three
options:
1. To remain alive and
functioning without dividing
2. To grow and divide
3. To die
Homeostasis is maintained by a
balance between cell
proliferation and cell death.
 Signals trigger cell division,
death and stability.

Cell death is regulated…
 The process of apoptosis is an
orderly, genetically
programmed cellular death.
 This is triggered by agents
outside or inside of the cell

Necrosis is a pathological
cellular death that results from
injury.
 Inflammation occurs in
necrosis, but not in apoptosis.

II. Sexual Reproduction
Cell division that occurs in the
gonads
 Each new organism is the result
of the union of two different
gametes…
 one produced by each parent.

Gametes contain a single set of
23 chromosomes
 They are haploid.
 Fertilization restores the cells
to their diploid state.

A. Meiosis
 Occurs in two successive stages:
I and II.
 In meiotic interphase, the
chromosomes of the diploid begin
chromosomal replication similar
to somatic division.
Meiosis I
Begins once chromosomal
replication is complete and
consists of
 Prophase I, Metaphase I,
Anaphase I and Telophase I.

1. Prophase I
Chromosomes shorten, and
thicken
 Nuclear membrane and
nucleoli disappear
 Mitotic spindle forms


In meiotic prophase, the
chromatids of each pair pair off
(synapsis) resulting in a tetrad
(four chromatids)

Also, crossing over frequently
occurs: a process in which parts of
chromatids of homologous
chromosomes can be exchanged
resulting in genetically different
chromatids…

This accounts for much of the
genetic variations among
humans.
2. Metaphase I

Tetrads line up in the middle of
the cell with homologous
chromosomes side by side.
3. Anaphase I
Each homolog separates and is
pulled to opposite poles
 The paired chromatid remain
attached to the centromere.

4. Telophase I
Nuclear envelope forms around
homologs at poles.
 Nucleoli reappear
 Cleavage furrow forms

5. Cytokinesis

Cell divides at cleavage furrow
Meiosis II
Consists of Prophase II,
Metaphase II, Anaphase II and
Telophase II and a second
cytokinesis.
 This results in 4 haploid gametes
that differ from the diploid cell of
origin.
