Biological Functions are Extremely Sensitive to pH

• H+ and OH- ions get special attention because they are very reactive
• Substance which donates H+ ions to solution = acid
• Substance which donates OH- ions to solution = base
• Because we deal with H ions over a very wide range of concentration, physiologists have devised a logarithmic unit, pH, to deal with it
  • pH = - log [H⁺]
  • [H⁺] is the H ion concentration in moles/liter
  • Because of the way it is defined a high pH indicates low H ion and a low pH indicates high H ion- it takes a while to get used to the strange definition
  • Also because of the way it is defined, a change of 1 pH unit means a 10X change in the concentration of H ions
    • If pH changes by 2 units the H+ concentration changes by 10 X 10 = 100 times

• Human blood pH is 7.4
  • Blood pH above 7.4 = alkalosis
  • Blood pH below 7.4 = acidosis
• Body must get rid of ~15 moles of potential acid/day (mostly CO₂)
  • CO₂ reacts with water to form carbonic acid (H₂CO₃)
  • Done mostly by lungs & kidney
• In neutralization H⁺ and OH^- react to form water
• If the pH changes charges on molecules also change, especially charges on proteins
  • This changes the reactivity of proteins such as enzymes
• Large pH changes occur as food passes through the intestines.

The small intestine

Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi, which line the inner surface of the small intestine.

This scanning electron micrograph (courtesy of Keith R. Porter) shows the villi carpeting the inner surface of the small intestine.

The crypts at the base of the villi contain stem cells that continuously divide by mitosis producing

• more stem cells
• cells that migrate up the surface of the villus while differentiating into
  1. columnar epithelial cells (the majority). They are responsible for digestion and absorption.
  2. goblet cells, which secrete mucus;
  3. endocrine cells, which secrete a variety of hormones;
• Paneth cells, which secrete antimicrobial peptides that sterilize the contents of the intestine.

All of these cells replace older cells that continuously die by apoptosis.

The villi increase the surface area of the small intestine to many times what it would be if it were simply a tube with smooth walls. In addition, the apical (exposed) surface of the epithelial cells of each villus is covered with microvilli (also known as a "brush border"). Thanks largely to these, the total surface area of the intestine is almost 200 square meters, about the size of the singles area of a tennis court and some 100 times the surface area of the exterior of the body.

Incorporated in the plasma membrane of the microvilli are a number of enzymes that complete digestion:

• aminopeptidases attack the amino terminal (N-terminal) of peptides producing amino acids.
• disaccharidasesThese enzymes convert disaccharides into their monosaccharide subunits.
  • maltase hydrolyzes maltose into glucose.
  • sucrase hydrolyzes sucrose (common table sugar) into glucose and fructose.
  • lactase hydrolyzes lactose (milk sugar) into glucose and galactose.

Fructose simply diffuses into the villi, but both glucose and galactose are absorbed by active transport.

• fatty acids and monoglycerides. These become resynthesized into fats as they enter the cells of the villus. The resulting small droplets of fat are then discharged by exocytosis into the lymph vessels, called lacteals, draining the villi.

Ingestion: Food taken in the mouth is

• ground into finer particles by the teeth,
• moistened and lubricated by saliva (secreted by three pairs of salivary glands)
• small amounts of starch are digested by the amylase present in saliva
• the resulting bolus of food is swallowed into the esophagus and
• carried by peristalsis to the stomach.

Characteristics of Facilitated Diffusion & Active Transport - both require the use of carriers that are specific to particular substances (that is, each type of carrier can 'carry' one type of substance) and both can exhibit saturation (movement across a membrane is limited by number of carriers & the speed with which they move materials

Membrane Structure & Function

Cell Membranes

• Cell membranes are phospholipid bilayers (2 layers)
• Bilayer forms a barrier to passage of molecules in an out of cell
• Phospholipids = glycerol + 2 fatty acids + polar molecule (i.e., choline) + phosphate
• Cholesterol (another lipid) stabilizes cell membranes
• the hydrophobic tails of the phospholipids (fatty acids) are together in the center of the bilayer. This keeps them out of the water

Membranes Also Contain Proteins

• Proteins that penetrate the membrane have hydrophobic sections ~25 amino acids long
• Hydrophobic = doesn't like water = likes lipids
• Membrane proteins have many functions:
  • receptors for hormones
  • pumps for transporting materials across the membrane
  • ion channels
  • adhesion molecules for holding cells to extracellular matrix

cell recognition antigens

Contractility : Means ability of cardiac muscle to convert electrical energy of action potential into mechanical energy ( work).
The excitation- contraction coupling of cardiac muscle is similar to that of skeletal muscle , except the lack of motor nerve stimulation. 

Cardiac muscle is a self-excited muscle , but the principles of contraction are the same . There are many rules that control the contractility of the cardiac muscles, which are:

1. All or none rule: due to the syncytial nature of the cardiac muscle.There are atrial syncytium and ventricular syncytium . This rule makes the heart an efficient pump.

2. Staircase phenomenon : means gradual increase in muscle contraction following rapidly repeated stimulation..

3. Starling`s law of the heart: The greater the initial length of cardiac muscle fiber , the greater the force of contraction. The initial length is determined by the degree of diastolic filling .The pericardium prevents overstretching of heart , and allows optimal increase in diastolic volume.

Thankful to this law , the heart is able to pump any amount of blood that it receives. But overstretching of cardiac muscle fibers may cause heart failure.

Factors affecting  contractility ( inotropism)

I. Positive inotropic factors:

1. sympathetic stimulation: by increasing the permeability of sarcolemma to calcium.
2. moderate increase in temperature . This due to increase metabolism to increase ATP , decrease viscosity of myocardial structures, and increasing calcium influx.
3. Catecholamines , thyroid hormone, and glucagon hormones.
4. mild alkalosis
5. digitalis
6. Xanthines ( caffeine and theophylline )

II. Negative inotropic factors:

1. Parasympathetic stimulation : ( limited to atrial contraction)
2. Acidosis
3. Severe alkalosis
4. excessive warming and cooling .
5. Drugs ;like : Quinidine , Procainamide , and barbiturates .
6. Diphtheria and typhoid toxins.