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

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.

Secondary Active Transport

Transport of second solute (e.g., glucose) is powered by the sodium gradient created by the Na/K Pump.   Glucose is "co-transported" with glucose – glucose is moving up its concentration gradient, sodium is moving down its concentration gradient.   If direction of  movement is the same it is called symport, if direction of movement is in opposite directions it is called anti-port.

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.

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.

 

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 ribosomes are often not considered organelles because they are not membranous.  Instead they are termed inclusions or macromolecular complexes.

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.

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: ribosomes, melanin granules, glycogen deposits, and lipofuscin granules.

Genetics and Cell Function

Nucleus

Contains genetic material -- DNA, chromosomes.

Chromosomes are condensed chromatin (DNA with proteins).

Controls workings of cell. Double membrane (nuclear envelope) with pores -- pores are for transport of large molecules (RNA, receptors) between nucleus and cytoplasm.

Nucleolus -- assembly point for ribosomes. rRNA transcribed here.

Nucleus, and the ER

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."

Transcription/translation overview, general and molecular views

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

1.  Initiation: AUG is the start codon -- matches a UAC tRNA anticodon with amino acid methionine attached.

2.  Elongation--additional tRNAs with amino acids attached are brought to ribosome, peptide bonds are formed between adjacent amino acids. One tRNA stays attached (the one with the amino acid chain), the other leaves. The mRNA then moves down one codon along the ribosome. The process continues and the amino acid chain grows longer.

3. Termination: at some point there is a "termination codon"--it is a tRNA with no amino acid attached -- the growth of the protein ceases.

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

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.

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 snynapses, 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.