Regulation provides for the maintenance of complexity, order, and the characteristics of life. Regulatory mechanisms manage and control life at various levels of biological organization. Another, more recognized term for stability, is homeostasis. Homeostasis is generally used in reference to the constancy of an organism's internal environment. Without proper regulation, chaos and disorder will ensue, and life will be disrupted.

Molecules and Cells

• Enzymes: Reaction Regulators

Enzymes are proteins that alter the rate of a reaction. Enzymes lower the activation energy so reactions can occur faster.

• Plasma Membrane: Cell Regulator

The plasma membrane is the membrane on the outside of both animal and plant cells. The components control which substances may enter and leave the cell. The purpose of cell regulation is to have an equilibrium between the cell and its environment.

The plasma membrane is a phospholipid bilayer with proteins and cholesterol. The phopsholipid consists of 2 fatty acids and a phosphate. The bilayer is two layers of phospholipids. Small, non-polar molecules are transported through the membrane by the process of diffusion.

Embedded in the plasma membrane in between phospholipids are proteins. There are three types of proteins which help to regulate substances entering and leaving cells:

1. Channel proteins - allow molecules too big for diffusion to pass freely through the membrane
2. Gated channel proteins - with selective opening and closing, the types of substances as well as the flow is controlled
3. Ion pumps - transport of substances against concentration gradients, energy required (e.g. sodium-potassium pump)

The plasma membrane transports water by osmosis. Water molecules are small enough to easily get through the bilayer, but the flow of water is regulated by internal and external factors. When there is a steep concentration gradient between the inside and outside of the cell, and substances are unable to pass through the membrane, the water will move. Water will flow via osmosis into or out of the cell in order to reach a dynamic equilibrium.

**Understand what it means for a cell to be hypotonic, hypertonic, and isotonic with its external environment.

• Na⁺ and K⁺ : Osmoregulators

K⁺ (Potassium) is an example of an osmoregulator in plants.

Potassium is a vital part of controlling transpiration and gas exchange through the stomates on the underside of plant leaves. Regulating the size of the stomate opening is important in preventing excessive water loss and controlling carbon dioxide flow into the leaf. Both water and carbon dioxide are key factors in the process of photosynthesis. A drastic loss of either could have adverse effects on photosynthesis, a process depended upon by every organism.

Potassium regulates the flow of water into and out of the guard cells, which consequently regulates the opening and closing of the stomate. The opening and closing of the stomates depends on the concentration of water in the plant and the plant's need for carbon dioxide; both are important in photosynthesis. Here are two scenarios:

1. The plant needs carbon dioxide for photosynthesis:

Potassium will flow into the guard cells, thus causing an influx of water. This causes the guard cells to open the stomates, allowing carbon dioxide to enter, and water to transpire.

2. The plant needs to conserve water:

Potassium will not flow into the guard cells, so the cells will not fill with water. There is no stomate opening to allow for water to transpire or carbon dioxide to enter.

Na⁺ (Sodium) is an example of an osmoregulator in animals.

Sodium is a primary osmoregulator in the nephron. It directs the flow of water into and out of the convoluted tubule.

Sodium is also important in the electrical potential along a neuron membrane.

• Cell Cycle: Cell Regulator

Regulating the period of growth and division of a cell is important in maintaining homeostasis within an organism. For example, while nerve cells never replicate, blood cells need to be replenished often. The cell cycle, the repeated phase of a cell's life, as well as some hormones and enzymes, help the cell to regulate the different stages of the cell's life.

Interphase, mitosis, and cytokinesis are the three main stages in the cell cycle. However, some cells do not replicate. After the growing phase of the interphase stage, the cell reaches the R-point, the restriction point. This point determines whether a cell divides or not. If growing stops before the R-point, the cell will not divide, but if the cell reaches this point, it must procede through mitosis.

A regulatory factor, the phosphorylation of Mitosis Promoting Factor (MPF), may occur at the R-point. MPF is a kinase that phosphorylates proteins needed for cell division. This means that MPF will give phosphates to these proteins, therefore initiating the start of mitosis. If MPF is present in the cell, division may occur. However, if there is no MPF in the cell, vital proteins will not be activated and division will not be able to procede.

Other regulators of the cell cycle include cyclins, oncogenes, tumor-supressor genes, surface : volume ratio, and apoptosis.

Heredity and Evolution

• Operons: Regulating Gene Expression in Prokaryotes

Genes code for specific proteins. Prokaryotes use all of the genes in their DNA strand, but not all the coded proteins are needed by the cell at all times. The functional unit of a prokaryotic cell is an operon. Each operon contains those genes that code for a specific set of proteins, as well as genes that regulate the proteins' production. Certain external factors such as the environment, chemical changes, and hormones cause individual proteins to either begin gene expression (inducible operon) or to halt gene expression (repressible operon.) The presence of such factors in the cell will stimulate the regulatory genes of the operon. By determining which proteins are necessary at one time, homeostasis is maintained in the cell.

Parts of an operon:

• Regulatory Gene
• Repressor
• Promotor Gene
• Operator Gene
• Structural Gene(s)
• Regulatory Protein

• Introns and Exons: Regulating Gene Expression in Eukaryotes

In prokaryotes, all of the genes in a cell are used at some point, but in a multicellular eukaryotic organism, cells are specialized. Even though every somatic cell contains every gene used in the entire organism, not every cell uses all the genes. Depending on where a cell is located, it may produce proteins vital only to that part of the organism. For example, cells in your feet do not need to be producing salivary amylase that breaks down starch in the mouth. Cell differentiation begins when the embryo forms, and acts as a regulatory mechanism for the organism. If every cell were to make every protein needed, the organism would be disorganized and chaotic.

Eukaryotes do not contain operons, but do have transcriptional units containing:

• Promoter
• Enhancer
• Structural Gene(s)
• Regulatory Protein

There are three critical points when gene expression can be regulated in a eukaryotic organism.

1. Transcription - Certain regulatory proteins present in the cell affect a gene linked to that protein. Transcription will procede according to the level of regulatory protein in the cell.
2. RNA Clean-up - Introns and exons are separated by spliceosomes. The extraneous introns are discarded.
3. Translation - Proteins control when mRNA is translated. Hormones control how much and for how long the mRNA is translated.

• Predator-Prey Relationships: Regulating Evolution

The relationship of a predator and its prey is imporant in keeping populations of both species at a tolerable level. The proliferation of one species could be hazardous to the environment, causing unnecessary food loss and displacement of other species. Even though balance is necessary, each species will evolve over time to better their survival rate. The predators best suited to capture prey will survive and pass on their beneficial traits. Likewise, prey with adaptations allowing them to escape predators, will pass on their traits to the next generation. Natural selection and evolution become a large part of the predator-prey relationship. Advantageous traits are selected for, and species evolve.

• DNA

Deoxyribonucleic acid (DNA) is the functional unit that passes traits on from one generation to the next. DNA has many properties that regulate the passage of traits and the formation of proteins.

1. During DNA replication, enzymes are used to edit the replicated strand, correcting any mistakes made by DNA polymerase. Without this regulatory factor, a large number of mutations would be present in gametes, and passed on to offspring. Although mutation is necessary for evolution, it may be hazardous if too common.
2. At the end of transcription, spliceosomes cut out the extraneous introns in the gene sequence. If these sequences were not extracted, the sequence of base pairs that code for proteins would be interrupted. The proper amino acids would not be translated. Having non-functional proteins made from these wrong amino acids would be a waste of energy and could increase disorder.
3. DNA determines the characteristics of all offspring and inherited traits. Because DNA is the code for all living things, it is a regulatory mechanism in and of itself.

• Spontaneous Abortions: Regulating the Effects of Mutations

Mutations are common in the DNA of all cells. This means that mutations in germ cells can be passed on to gametes and offspring. Mutation is common in meiosis when the tetrads are formed. The exchange of chromosomal segments when the tetrads are aligned is known as crossover. A potentially fatal mutation present in a embryo's DNA may cause the body to spontaneousely abort the embryo. This mechanism prevents the spread of deadly mutations throughout the population.

Organisms and Populations

• Plant and Animal Hormones: Chemical Regulation of an Organism

Plant Hormones integrate growth, development, and metabolism. A hormone is a substance that is produced in one tissue, and transported and affects another tissue. Six plant hormones with selected functions are:

1. Auxins - Promotes cell division and elongation at the tips, apical dominance in the roots and stem, inhibits lateral growth
2. Gibberellins - Promotes hyper-elongation in the stems
3. Cytokinins - Promotes cell division and lateral growth, generally produced in the root
4. Ethylene Gas - Produced by ripe fruits, promotes the break-down of cell walls and threrefore the ripening of fruits
5. Abscissic Acid - Stress hormone that is normally associated with the transport of potassium in and out of guard cells
6. Florigen - Proposed hormone (under investigation), promotes flowering

Hormones are the basis for homeostasis in an animal. Hormones balance the chemical make-up of an organism, and may influence whole organs and systems. The endocrine system is dedicated to maintaining a chemical order in an animal, with the help of hormones. Hormones are chemical messages excreted by ductless glands and are sent from one part of the body to another. Some important glands are:

• Pituitary gland - One of the most significant glands, it is located at base of the head and is controlled by the nearby hypothalamus. The hypothalamus is the part of the brain that gathers information about the chemical nature of the organism from blood or neurons. If necessary, it sends releasing factors or inhibiting factors to the pituitary gland. The pituitary then releases hormones accordingly.

Anterior pituitary - Produces and secretes hormones

Posterior pituitary - Only holds and secretes two hormones

• Adrenal Glands - Located above the kidney, two different areas
1. Adrenal Cortex - Secretes steroids
2. Adrenal Medulla - Secretes epinephrine and nonepinephrine (hormones), activates quick responses
• Others include the thyroid gland, parathyroid glands, pancreas, and gonads.

**The endocrine system and the nervous system may work together to provide homeostatic conditions.

• Temperature Regulation

External temperature may have an effect on the growth and development of a plant. Large increases or drops in a plant's optimal temperature could be hazardous to the plant's health.

Controlling internal temperature as part of establishing homeostasis in animals is called thermoregulation. In some animals, temperature is regulated by a negative-feedback mechanism. This process involves three steps:

1. Information about the ambient and internal temperature is gathered
2. The hypothalamus searches for deviations from normal body temperature.
3. If a change has occured, there is a response by the body's muscles or glands.

Responses may include shivering or goosebumps when the temperature is below average, and sweating or redness (see below) when the temperature is above average.

Blood plays a part in controlling the internal body temperature by carrying heat to different parts of the body. Heat can be released to the external environment through the skin, or it can be retained in the body. When the internal temperature is too high, blood rushes to the surface of the body so heat can escape, thus cooling the organism. This causes a rosy complexion.

Homeotherms are organisms that have a constant body temperature. These animals can increase their metabolism to create heat, or slow their metabolism to lose heat. Note that increasing metabolism is an energy expensive process, causing these animals to consume a lot of food so glucose can be converted to ATP.

Endotherms are animals who generate their own heat.

Poikilotherms are organisms that have a changeable body temperature.

Ectotherms are animals that are affected by the external environment. These animals must seek the heat of the sun to warm themselves, or escape the sun in order to cool themselves.

• Population Dynamics

The size of a population is regulated by environmental resistance, which limits biotic potential. The point at which the environment limits the population is called the carrying capacity. This environmental effect prevents the population from exhausting the available resources. Some environmental resistance is characterized by limiting factors - those substances in the smallest amount. Factors limiting population growth:

Density - Independent (does not depend on the # of individuals)

1. Climate changes = weather
2. Soil nutrients
3. Water levels (droughts/floods)
4. Human activities (pesticides/pollutants)

Density - Dependent (effect is greater as population increases)

1. Competition
2. Parasitism/Predation
3. Emmigration/Immigration
4. Physiological and Psychological control
5. Disease
6. Stress (anything that prohibits population increase)

• Vertebrate Systems

The Nervous System

The nervous system is the control center for regulating bodily functions. The system detects changes in the external or internal environment and triggers a stabilizing reaction. Examples of responses include sweating, changing heart rates, and reflexes.

The Endocrine System

As well as the nervous system, the endocrine system is a primary regulatory agent. The system controls changes in the metabolic activity of certain cells. The endocrine system detects and corrects any chemical changes in the organism by releasing hormones. This system provides homeostasis, controls reproductive development, and can respond to stimuli fom the nervous system.

The Respiratory System

The respiratory system maintains a balance of oxygen and carbon dioxide in the body. Oxygen and carbon dioxide have an important role in sustaining a system of buffers to help maintain the pH of body fluids.

The major buffering system: CO₂ + H₂O -------> H₂CO₃ -------> HCO₃^- + OH^-

The Excretory System

The excretory system helps preserve the uniformity of body fluids. The concentrations of metabolic wastes, salt, and water are regulated.