unit 2 the cell. the cell is: the most basic structural and functional unit of life. the smallest...
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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 surfaceServe 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
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
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
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.
A. Homeostatic Imbalances
1. Cystic fibrosis
2. Tay-Sachs Disease
3. Mitochondrial myopathies
4. Progeria
5. Werner Syndrome
6. Cancer (carcinogenesis)
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.
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
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
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…
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
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.
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