The light reactions supply ATP and reducing equivalents in the form of NADPH.

Note: one inorganic phosphate (Pi) becomes attached to the sugar that is made, so there are only 17 Pi at the end of the balanced equation.

Oxygen is produced as a byproduct of the light reacti

ons by photosystem II. The absorption of light at 680 nm by the chlorophyll a molecule allows it to split water into oxygen and hydrogen. The electrons donated from this reaction are carried through various electron carriers to photosystem I and ultimately to NADPH for use in the dark reactions.

Sugar is produced during the dark reactions, also known as the Calvin cycle. Note: to produce one six carbon sugar, there must be two revolutions of the cycle.

One O₂ is generated for every CO₂ fixed. You need 6CO₂ and 12NADPH to generate 1 sugar molecule. This requires splitting 6O₂ to generate enough electron reducing equivalents.

The stages of mitosis are prophase, metaphase, anaphase, telophase and cytokinesis.

Prophase: nucleoli and nuclear envelope disappear, chromatin condenses, centriole pairs move away from each other toward the poles. Mitotic spindle forms.

Metaphase: centrioles arrive at the poles, chromosomes bind to the spindle apparatus, and the chromosomes migrate to the metaphase plate, where they align themselves along it, with one chromatid on either side of the metaphase plate and the centromere on the plate.

Anaphase: the centromeres split, and the sister chromatids part.  Each sister chromatid moves toward the opposite pole.

Telophase: the chromosomes arrive at the poles, the nuclear membrane and the nucleolus reform, and the chromatin uncoils.

Cytokinesis: a cleavage furrow forms between the daughter cell nuclei, and the cytoplasm divides.

Note: each chromosome prior to anaphase is made up of two chromatids. Once the chromatids separate at anaphase, each is now called a chromosome. Chromatin is the DNA and the proteins which make up the chromosomes.

illustration from: www.accessexcellence.com/AB/GG/mitosis.html

Inhibition of microtubule polymerization inhibits mitosis because microtubule polymerization is required for formation of the spindle apparatus.  If the spindle apparatus doesn't form, then the chromosomes cannot separate and mitosis does not occur.

Diploid refers to any cell which contains two complete sets of chromosomes, one inherited from each parent during sexual reproduction.

The centromere holds the duplicated chromosomes together until they segregate at anaphase. It also provides the binding site for the chromosome to attach to the spindle fibres, and the division point for the separation and movement of the chromosome toward the cell polls during anaphase.

Prophase I (leptotene, zygotene, pachytene, diplotene, diakinesis), metaphase I, anaphase I, telophase I, interkinesis, prophase II, metaphase II, anaphase II, telophase II.

Illustration from: www.accessexcellence.com/AB/GG/meiosis.html

Meiosis is a reductional division which generates 4 haploid daughter cells from a diploid parent cell. The haploid cells are genetically different from one another. Mitosis conserves chromosomal number, and generates two daughter cells which are genetically identical to each other and to the original parent cell.

1. Ten chromosomes would be present in a typical skin cell.
    
2. Ten many chromosomes would be present in a cell at mitotic prophase.
   
   
3. Twenty many chromatids would be present in a cell at meiosis I.
   
   
4. Ten many chromatids would be present in a cell at prophase II.
   
   
5. Five many chromosomes would be present in each gamete.
    

Crossing over creates a greater level of genetic diversity - it allows genes that are carried on the same chromosome to be recombined between parental chromosomes to generate new combinations of genes.

illustration from: http://www.accessexcellence.com/AB/GG/comeiosis.html

Increased genetic diversity.

Chiasmata are the physical attachment points which result from cross over events between homologous chromosomes.

Polar bodies are the cells which result from the uneven distribution of cellular resources and cytoplasm during oogenesis.  Polar bodies remain on the egg surface until they are reabsorbed.

Cell division during spermatogenesis is even - both cells receive  half of the available cytoplasm and organelles. Uneven divisions during oogenesis result in one larger cell with extra growth reserves to support the growth of the zygote. This is necessary to compensate for the fact that sperm contribute little cytoplasm or organelles to the zygote.

Meiosis is a reductional division - this means that during cell division, the chromosome number is reduced from 2N to 1N. Thus, when gametes combine, the resulting zygote is diploid once again. (1N + 1N = 2N).

When a neuron reaches threshold level, an action potential is propagated down the axon.

A resting potential is developed by the action of the Na⁺ K⁺ ATPase which helps to maintain the osmotic balance across the cell membrane. This enzyme uses the energy of ATP hydrolysis to pump 3 Na⁺ out of the cell (to maintain low Na+ levels inside the cell) and 2K⁺ into the cell (to maintain a high K⁺ level inside the cell).

Action potentials propagate down the axon in several steps.

1. A stimulus causes a small depolarization of the cell.
2. Voltage gated Na⁺ channels open, allowing Na⁺ to flow into the cell.
3. More voltage gated Na⁺ channels open in response to the increased depolarization resulting from the Na⁺ influx. Note: Na⁺ channels can only stay open briefly - after that they close automatically and are inactive until the resting potential returns. Some neurons can reset more quickly because voltage gated K+ channels open in response to the influx of Na⁺ (but after a short delay). This decreases membrane depolarization to the resting potential more quickly.
4. The depolarization of one point on the axon provides the depolarization stimulus to the next area of the axon and the action potential propagates  as a wave down the axon.

The action potential can't go backwards up the axon because the Na⁺ channels are temporarily inactivated after they open until that area of the axon returns the resting potential. Only in the closed state are they able to open in response to a stimulus.

A neurotransmitter is a small signaling molecule which is secreted into a synaptic cleft that serves to modify the activity of the post synaptic neuron and pass a signal between the two neurons.

When the action potential reaches the end of the axon, it causes voltage gated Ca²⁺ channels to open in response to the membrane depolarization which is occurring. This allows an influx of Ca²⁺ into the cell (Note: cells normally maintain an internal concentration of Ca²⁺ that is 1000 times lower than the external environment). The influx of Ca²⁺ causes secretory vesicles containing neurotransmitter to fuse with the presynaptic membrane and release the neurotransmitter into the synaptic cleft.

1. The action potential depolarizes the membrane.
2. Voltage gated Ca²⁺ channels open.
3. Secretory vesicles fuse with presynaptic membrane in response to Ca²⁺ influx.
4. Neurotransmitter is released and diffuses across the synaptic cleft.
5. If the synapse is inhibitory:
   1. transmitter gated chloride channels open and the production of action potentials is inhibited.
   If the synapse is excitatory:
   1. transmitted gated Na⁺ channels open.
   2. membrane becomes slightly depolarized.
   3. if enough channels open, an action potential results

An antigen is any molecule capable of generating an immune response. Most are greater than 10000 daltons in molecular weight (1 dalton = molecular weight of hydrogen), and they are usually either proteins or polysaccharides.

An antigen binds to the antigen binding sites at the ends of the arms of the antibody.

The Fc portion of the antibody binds to immune cells - it can be used to anchor antibodies to the surface of B cells, and may also help antigens coated with antibodies to be taken up by macrophages.

Immunity is humoral if a soluble antigen interacts directly with antibodies. B cells amplify the antibody response. Cell mediated immunity is when the antigens are intracellular, and the immune response depends on the production and amplification of T cells.

A cell which engulfs an antigen and displays its digested parts on the cell surface.  The antigen presenting cell (APC) can be contacted by a T cell, which may become activated in response to the antigen fragments being "presented" in the context of other cell surface markers.

A cytokine is a soluble immune response modulator produced by leukocytes (white blood cells). In response to cytokine production, immune cells may be attracted to the area, proliferate, etc. depending on the situation.

The theory of clonal selection relates to antibody production in that rather than always having large amounts of every possible antibody around or large numbers of cells that could make every possible antibody, the body has a few B cells of every type around. These B cells do not reproduce in the absence of the appropriate antigen. The antigen then "selects' the correct B cell clones to amplify - those cells whose surface antibody binds to the antigen. These cells can then generate a large population of effector cells which secrete large amounts of the specific type of antibody needed.

Intracellular pathogens require a cell mediated immune response.  An antigen presenting cell such as a macrophage will engulf the antigen and display digested parts of it on the cell surface in combination with the MHCII protein complex (Major Histocompatibility Complex). The MHCII protein complex is a cell surface marker for immune cells.
The activated macrophage will bind to T cells with receptors that recognize the antigen fragments, causing the macrophage to secrete the cytokine IL-1 (interleukin 1). This will cause T helper (T_H)and T cytotoxic (T_cyt)cells to proliferate. The T_cyt cells will recognize infected cells by looking for pieces of virus on the surface of infected cells in combination with MHCI (a protein complex present on the surface of all nucleated cells in the body which allows the immune system to recognize them as "self"). When T_cyt cells bind to infected cells they release perforin, a protein complex which causes the infected cells to lyse.

An extracellular pathogen requires a humoral response.  The antigen will enter the blood, coming in contact with a B cell which has antibodies on its surface which match the antigen. The B cell will grow and multiply in response to contact with the antigen, and most of the progeny cells will differentiate into plasma cells (antibody secreting cells which can secrete up to 2000 antibodies per second for their 4 - 5 d lifespan).

The antibodies produced by the plasma cells will tag the antigen (form antigen-antibody complexes). At this point, the complexes may be phagocytosed by macrophages, may cause agglutination (formation of very large, crosslinked complexes) which may be removed by the kidneys or phagocytosed, or the antigen-antibody complexes may activate the complement cascade, which will result in lysis of the antigen cells.

The skin can be considered to be part of the immune system because it represents a physical barrier to the microbial colonization of the interior of the body.  It is a comparatively dry, salty, low pH environment due to the products of the sebaceous (sweat) glands.  In addition, the normal flora of the skin (those bacteria, etc. that are normally present on healthy human skin) will compete with pathogens for resources such as space and nutrients.

Steroid hormones are fat soluble molecules whose structure is based on that of cholesterol.  Peptide hormones are short chains of amino acids - they may be as short as 3 amino acids or as long as 200 amino acids.

Steroid hormones pass through the cell membrane of target cells by passive diffusion, and they bind to protein receptors in the nucleus. The hormone-receptor complex binds to a specific site on the DNA (the hormone response element) and alters gene expression.

Peptide hormones bind to protein receptors on the cell surface. The receptor interacts with a second messenger system. The second messenger produces a signal that elicits a cellular response, from proteins that already exist in the cell.

A hormone is a molecule synthesized and secreted by an endocrine gland into the blood which travels to a distant site and elicits a specific biological response from the target cell.

Functions of hormones include: sexual differentiation; regulation of blood sugar levels, Ca²⁺ levels, growth, milk production, blood osmolarity, metabolic rate, etc.

1. prolactin: stimulation of milk production and secretion in humans.
2. thyroid stimulating hormone: stimulates the thyroid gland to secrete hormones such as T₃ (triiodothyronine) and T₄ (thyroxine) which govern metabolic rate.
3. estrogen: initiates building of the uterine lining, develops and maintains female sexual characteristics.
4. growth hormone: stimulates general growth, especially of skeleton, and affects metabolism.

1. glucocorticoids: increase blood sugar by altering many aspects of carbohydrate metabolism.
2. mineralocorticoids: promote the readsorption of Na⁺ and excretion of K⁺ by the kidneys. 

Prostaglandins are modified fatty acids which act as local regulators. E.g. they may be involved in relaxing/contracting blood vessels (PG E/PG F), or in uterine contractions during labour.

Cyclic AMP (cAMP) is a second messenger for hormone action.  When the hormone binds to its receptor, the receptor activates the enzyme adenylate cyclase which converts ATP to cAMP and pyrophosphate.  The rise in internal cAMP levels causes the cellular response, often in the form of an enzyme cascade. Because cAMP has a short half-life in the cell, it breaks down quickly and allows this system to respond rapidly to changes in the levels of hormone acting at the receptor.

Insulin has a receptor made up of α chains and β chains. The insulin binds to the α chains, which causes the activation of the tyrosine kinase of the β chains (which autophosphorylate themselves). This causes other enzymes to phosphorylate other proteins which result in:
            a) increased uptake of glucose by cells (liver and muscle).
            b) increased synthesis, and decreased breakdown, of glycogen.
            c) increase in glycolysis activity
            d) increased fatty acid and triacylglycerol synthesis.

The pituitary gland has two functional parts:

1. the anterior pituitary which responds to signals from hypothalmic hormones to release ACTH, TSH, FSH, LH, GH, etc.
2. the posterior pituitary which contains the axon termini of many hypothalmic neurons. This area also secretes hormones including oxytocin, vasopressin and ADH.
   The pituitary mediates signals from the brain to the rest of the endocrine system.

Zinc finger proteins have 3 distinct regions - a transcription activation sequence, a DNA binding element and a hormone binding element.
The DNA binding element is the zinc finger region. The ends of the "fingers" interact with the bases in the DNA by inserting themselves into the major groove of the double helix of DNA. Following the binding of the hormone-receptor complex to the hormone response element, the transcription activation domain alters the expression levels of the adjacent genes.