CELLS
Cell Theory: Key concepts (also touched on in historical section)
1.All living things are made of cells.
2.Cells are the basic units of life -- the structural and functional units
3.Cells come only from other cells.
Cell Components:
Nucleus, Cytoplasm, and Plasma Membrane.
Plasma membrane: defines "inside and outside" of cell -- intracellular
and extracellular
Membranes are selective
barriers
Basic structure is a phospholipid bilayer.
Hydrophobic inside, hydrophilic outside. Lipid soluble compounds pass through
membranes easily.
However, lipids alone cannot explain all the functions of a cell membrane
-- proteins are involved.
Proteins are asymmetrically distributed.
Fluid Mosaic Model of the Cell Membrane
Proteins
in membrane serve:
1) to give structural support; anchoring sites.
2) to selectively
transport materials across the membrane (carriers and channels)
3) as enzymes which control surface reactions
4) as receptors
for hormones and other regulatory molecules
5) as cell "markers" or recognition proteins (e.g., major
histocompatability proteins); cell adhesion molecules.
Passive and active movements across membranes
Diffusion:
Solute concentration dependent.
Simple Diffusion: passive, follows concentration gradient.
Example: oxygen and carbon dioxide diffusion between blood and air in
the lungs.
Facilitated Diffusion: like simple diffusion, but requires protein carrier
molecules in membrane.
Protein carrier is like an enzyme in that it can be saturated, and can be denatured
by temperature and pH changes.
Example: glucose carriers in cell membranes.
Osmosis:
Solvent (water) movement from an area of low solute concentration to high
solute concentration across a semi-permeable membrane.
Water is moving down its concentration gradient.
Osmosis depends on the number of solute molecules in solution.
Potential for osmosis is measured as osmolality or osmolarity; milliosmole
= 1/1000 of an osmole.
Examples:
Red blood cells (RBC). Plasma is isotonic to RBCs (no net movement of water
into or out of cells)
Place RBCs in distilled water (a hypotonic solution), water enters cells,
cells then swell and burst. (Hemolysis)
Place RBCs in high salt solution (a hypertonic solution) and the cells shrivel
because water leaves the cells. (Crenation)
Active Transport:
Solutes are transported
from low to high concentration areas.
Requires energy and a membrane protein carrier. ATP provides energy.
Example: Sodium-Potassium
Pump
Another Animation of sodium-potassium pump. And Another Animation.
Flash Animation of sodium-potassium pump.
Sodium has a high concentration
outside the cell, Potassium has a high concentration inside the cell.
An ATP-driven pump in the cell membrane maintains disparity.
The pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell.
This helps to generate a charge
disparity across the membrane.
Both active transport and
facilitated diffusion are examples of "carrier-mediated transport"
The carriers are membrane proteins. Protein structure allows systems to have:
1) Specificity--process
can be selective for 1 kind of molecule
2) Saturation--carriers can be saturated
3) Competition--similar molecules may compete for same carrier.
Co-Transport or Secondary Active Transport
Flash Animation of Secondary Active Transport
Sodium-Potassium pump creates the sodium gradient (primary active transport). The sodium gradient provides the energy to power the importation of glucose (an uphill, energy requiring process).
Cell movements:
Fluidity of membrane is important in cells that move by extending pseudopodia
or "false feet".
Some white blood cells move via pseudopodia.
Intracellular movements: Active (requires energy), bulk movements.
Endocytosis -- brings external environment inside the cell
a) Phagocytosis: "cell eating"
b) pinocytosis: cell drinking
c) receptor mediated endocytosis
Exocytosis -- components
of cell exit; secretion.
Endocytosis
and exocytosis — advanced
CYTOPLASM AND ORGANELLES
Cytoplasm:
Area of cell minus the nucleus
Composed of "particulate phase" (organelles, lipid droplets, inclusions) and
"cytosol" (fluid phase). Fluid phase has dissolved organic and inorganic molecules.
Organelles:
Endoplasmic reticulum (ER): structurally, a series of channels (cisternae). Used for transporting and processing proteins and lipids. Point of attachment for ribosomes (rough ER = RER). Smooth ER (SER) has no ribosomes -- smooth appearance.
Ribosomes: Site
of protein synthesis. Made up of protein and a special type of RNA -- ribosomal
RNA (rRNA). Clusters of ribosomes are called polyribosomes or polysomes. Proteins
made by ribosomes
are attached to ER.
(Note: Some texts do not include ribosomes as organelles since they are do not
contain membranes).
Golgi Apparatus: similar to ER in structure. Proteins in ER are shipped to Golgi where they are modified, made into glycoproteins. Proteins that pass thru Golgi are generally secreted.
Lysosomes: Bags of digestive enzymes. Enclosed in a membrane. Fuses with components in cell or can be discharged out of cell.
Mitochondria:
"powerhouse of the cell"
Double membrane structure -- internal and external membranes.
Folds in membrane are called cristae.--they increase surface area for ATP production
Produces 95% of cell's ATP for metabolic activities; 5% is produced in cytoplasm.
Site of "aerobic respiration" -- oxygen required for metabolism; carbon
dioxide produced.
Two related processes occur in mitochondria -- Krebs (tricarboxylic cycle) and
electron chain transport.
Organelles in MIT's Biology Hypertextbook
The Cytoskeleton is made up of:
Centrioles: cylindrical, tubular (bundles of microtubules) structures. Involved in mitosis (cell reproduction).
Cilia
and Flagella:
Cilia
are small, appear in large groups and are used to move objects along pathways
in the body -- trachea, oviduct (fallopian tube).
Flagella are usually long, single structures -- sperm
tails.
Microtubules make up internal structure.
Cytoplasmic inclusions: generic category for inclusions that are not commonly thought of as organelles: melanin granules, glycogen deposits, and lipofuscin granules.
Glycogen granules (EM): go to slide 74
Genetics and Cell Function
Contains genetic material -- DNA, chromosomes.Structure of the Nucleus -- check out images of pores!
PROTEIN SYNTHESIS
DNA in nucleus is the code
for proteins (& RNA).
A sequence of nucleic acids (genes)
will be deciphered into a sequence of amino acids (proteins).
Regulation: which specific
genes get decoded is under the control of a variety of factors, such as hormones.
The process of forming RNA from a DNA template is called Transcription.
Messenger RNA is a "transcript"
of DNA. Single stranded mRNA is formed under the direction of DNA, using RNA
polymerase. This happens in nucleus.
mRNA leaves nucleus via pores in nuclear envelope.
The process of forming a protein is called Translation.
mRNA links up with ribosome in cytoplasm. Ribosome brings mRNA and amino acid carriers together. The amino acid carriers are "transfer RNA," or tRNA, molecules.
The flow of genetic information from DNA to RNA to protein is called the "central dogma."
Sequence of 3 nucleic acids on mRNA, a codon, will match up with 3 complementary nucleic acids on tRNA, the anticodon. tRNA has a specific amino acid attached.
Genetic code is a "triplet"
code. This means that a series of 3 nucleic acids is the code for an amino acid.
How many possible codes? 4 X 4 X 4 = 64. More than enough for 20 amino acids.
If it was only a doublet code -- 16 (4 X 4) possible codes -- not enough for
20 amino acids.
Three Phases of
Translation
Transcription/Translation Tutorial with Animations
Genetic mutations:
one or more nucleic acid is deleted, inserted, or rearranged. If a sequence
is thrown off, protein synthesis is affected -- wrong amino acid would be inserted
or, if stop codon is placed in wrong position, translation could be stopped
in wrong place. Some mutations are "silent" -- no effect on translation
or function of finished protein.
Sickle Cell Anemia is a classic example of how a difference in 1 amino acid of a large protein can affect health.
Heredity: We inherit a set of genes (our genotype), but our traits (phenotype) depend on the expression of these genes.
The Cell Cycle
The process of cell division
marks the beginning and end of the cell cycle, or life span, of a single cell.
The cell
cycle is that period from the beginning of one cell division to the beginning
of the next cell division.
DNA Replication
When DNA
replicates, a copy is made to ensure that each new (daughter) cell receives
the same genetic information as the parent cell.
New strand is complementary structure of old strand
Interphase: Interphase is the period of the cell cycle during which the normal activities of the cell take place: growth, cellular respiration, protein synthesis.
Mitosis: Mitosis distributes genetic material from the parent cell to the daughter cells equally. It includes the actual division of replicated DNA. Mitosis is divided into 4 sequential stages.
Prophase: chromatin
becomes dense -- forms chromosomes
centrioles move to the poles of nucleus
mitotic spindle formed
nuclear membrane dissolves
nucleolus disappears
Metaphase: chromosomes line up at center of spindle
microtubules attach near centromere
Anaphase: Sister chromatids split
Chromatids move toward opposite poles
Telophase: Chromosomes (single chromatids) arrive at poles.
Spindle dissolves
Nuclear membrane and nucleolus reform
Chromosomes uncoil to from chromatin
Mitosis: the
movie. Ebert and Roper give it two thumbs up!
Mitosis is usually followed by Cytokinesis
Cytokinesis = division of the cytoplasm. Final production of two daughter
cells from parent cell.
Mitotic Rates differ between different tissues.
Epithelial tissues: high mitotic rates
Cancer cells: high mitotic rates or unregulated mitosis.
Some cells stop dividing shortly before or after birth, e.g., muscle cells and
neurons.
Meiosis:
How eggs and sperm do it! (Divide up their DNA, that is).
Meiosis is the process by which eggs and sperm reduce their normal complement
of chromosome pairs (diploid state or 2N, e.g., 23 pairs in humans) to a "half
set" (haploid state or 1N, e.g., 23 single chromosomes). Upon fertilization,
the two half sets are united to reform the normal diploid state.
There are two phases in
meiosis — meiosis I and meiosis II.
DNA replicates during interphase just as in mitosis.
Meiosis 1:
Prophase I . Homologous chromosomes pair and form synapses, a step unique to
meiosis.
The paired chromosomes are called bivalents, and the formation of chiasmata
caused by genetic recombination becomes apparent.
Chromosomal condensation allows these to be viewed in the light microscope.
Note that the bivalent has two chromosomes and four chromatids, with one chromosome
coming from each parent.
The nuclear membrane disappears.
One kinetochore forms per chromosome rather than one per chromatid, and the
chromosomes attached to spindle fibers begin to move.
Metaphase I
Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase
plate.
The orientation is random, with either parental homologue on a side.
This means that there is a 50-50 chance for the daughter cells to get either
the mother's or father's homologue for each chromosome.
Anaphase I
Chiasmata separate. Chromosomes, each with two chromatids, move to separate
poles. Each of the daughter cells is now haploid (23 chromosomes), but each
chromosome has two chromatids.
Telophase I
Nuclear envelopes may reform, or the cell may quickly start meiosis 2. Chromosomes
do not decondense to form chromatin.
Cytokinesis
Analogous to mitosis where
two complete daughter cells form.
Meiosis 2 is similar to mitosis (with Pro-, Meta-, Ana, and Telophase II’s)
but there is no "S" phase (DNA synthesis phase). The chromatids of each chromosome
are no longer identical because of recombination.
Meiosis II separates the chromatids producing two daughter cells each with 23
chromosomes (haploid), and each chromosome has only one chromatid.
How many types of cells are there, anyhow? Click here to find out!
Good overview sites related to the cell.
