Nutritional Sciences Lecture: Chemical, Biological, and Physiological Aspects

Levels of Biological Organization in the Human Body

Atoms: The smallest component of an element. Examples include Hydrogen ( $H$ ) and Oxygen ( $O$ ).
Molecules: Formed when chemical bonds join 2 or more atoms together. For instance, water ( $H_2O$ ) is formed by the union of hydrogen and oxygen.
Complex Molecules: These are large molecules, such as carbohydrates, that are diverse in structure and function.
Cells and Organelles: Cells are the fundamental structural and functional units of living organisms. Organelles are specialized structures that carry out specific functions within those cells (e.g., an epithelial cell).
Tissues: Formed when groups of cells come together to carry out specific functions. There are four primary types of tissue in the human body (e.g., epithelial tissue).
Organs: Each organ is composed of two or more different types of tissue. There are over 40 organs in the human body (e.g., the stomach).
Organ Systems: Consist of several organs that together carry out a specific physiological function. There are 11 different organ systems (e.g., the gastrointestinal system).

Atoms, Ions, and Redox Reactions

Fundamental Units of Matter: * Protons: Carry a positive charge. * Neutrons: Carry no charge (neutral). * Nucleus: Composed of protons and neutrons. * Electrons: Carry a negative charge and orbit the nucleus.
Ions: Atoms with an unequal number of protons and electrons, resulting in a net charge. * Cations: Formed when an atom loses electrons, resulting in a net positive charge (e.g., a Sodium atom with 11 protons and 11 electrons loses an electron to become a Sodium ion $Na^+$ with 11 protons and 10 electrons). * Anions: Formed when an atom gains electrons, resulting in a net negative charge. Gaining an electron often involves a name change (e.g., a Chlorine atom with 17 protons and 17 electrons gains an electron to become a Chloride ion $Cl^-$ with 17 protons and 18 electrons).
Reduction-Oxidation (Redox) Reactions: * Oxidation: The loss of electrons from a molecule (e.g., when molecule A transfers electrons to molecule B, molecule A is oxidized). * Reduction: The gain of electrons by a molecule (e.g., when molecule B receives electrons from molecule A, molecule B is reduced).

Chemical Bonds and Essential Elements

Essential Elements: There are 20 elements essential for human health.
Major Elements: Just 6 elements account for $99\%$ of total human body weight: * Oxygen ( $O$ ): $65\%$ of body weight; found in water and all organic molecules. * Carbon ( $C$ ): $18\%$ of body weight; found in all organic molecules. * Hydrogen ( $H$ ): $10\%$ of body weight; found in most molecules, including water. * Nitrogen ( $N$ ): $3\%$ of body weight; a primary component of proteins. * Calcium ( $Ca$ ): $2\%$ of body weight; a component of bones, teeth, and body fluids. * Phosphorus ( $P$ ): $1\%$ of body weight; found in cell membranes and the bone matrix.
Molecules and Notation: * Formed by 2 or more atoms. * Examples: Hydrogen gas ( $H_2$ ), Glucose ( $C_6H_{12}O_6$ ), and 3 water molecules represented as $3H_2O$ .

Assembly and Disassembly of Complex Molecules

Condensation: A reaction that results in the formation of a chemical bond joining molecules together, accompanied by the release of a water molecule ( $H_2O$ ).
Hydrolysis: A reaction that breaks chemical bonds through the addition of a molecule of water ( $H_2O$ ).
Acid-Base Chemistry: * pH Scale: Measures the acidity or alkalinity of a fluid. * Basic (Alkaline): $pH > 7$ ; contains more $OH^-$ than $H^+$ . * Neutral: $pH = 7$ ; contains equal amounts of $OH^-$ and $H^+$ . * Acidic: $pH < 7$ ; contains more $H^+$ than $OH^-$ . * Buffers: Substances that enable fluids to resist changes in pH.

Cellular Anatomy and Transport Mechanisms

Organelles and Environment: Cells contain cytoplasm (cytosol), a gel-like substance containing proteins and electrolytes, and are surrounded by a selectively permeable cell membrane separating the intracellular and extracellular environments.
Key Organelles: * Nucleus: Contains DNA, providing coded instructions for protein synthesis. * Mitochondrion: Produces the majority of energy ( $ATP$ ) used by the cell. * Ribosomes: Located on the Rough Endoplasmic Reticulum; responsible for building and processing proteins. * Rough Endoplasmic Reticulum: Membrane sacs involved in protein synthesis. * Smooth Endoplasmic Reticulum: Involved in lipid synthesis (lacks ribosomes). * Golgi Apparatus: Processes and packages proteins. * Lysosome: Contains digestive enzymes to break down proteins, lipids, and nucleic acids; recycles waste.
Passive Transport Mechanisms (No energy required; high to low concentration): * Simple Diffusion: A substance crosses the membrane directly. * Facilitated Diffusion: A substance crosses the membrane with the assistance of a membrane-bound transport protein. * Osmosis: The movement of water across a membrane toward a higher solute concentration until equilibrium is reached.
Active Transport Mechanisms (Energy required; low to high concentration): * Carrier-Mediated Active Transport: Uses $ATP$ and transport proteins to move substances. * Vesicular Active Transport: * Endocytosis: The cell membrane surrounds a substance to form a vesicle, bringing it into the cell. * Exocytosis: Material packaged in vesicles is released outside the cell.

Tissue Types and Organ Systems

Four Basic Tissue Types: 1. Epithelial Tissue: Covers and lines body surfaces, organs, and cavities. 2. Connective Tissue: Provides structure by binding and anchoring body parts. 3. Muscle Tissue: Contracts and shortens to facilitate movement (Skeletal, Smooth, and Cardiac). 4. Neural Tissue: Facilitates communication by receiving and responding to stimuli.
System Communication and Homeostasis: * Nervous System: Uses neurotransmitters. * Endocrine System: Uses hormones. * Negative Feedback: A common mechanism where the body reverses a change to maintain stability. * Positive Feedback: A rare mechanism that reinforces a change.
The 11 Organ Systems and Functions: * Integumentary: Skin, hair, nails; protection and temperature regulation. * Skeletal: Bones, joints; support, mineral storage, blood cell production. * Muscular: Muscles; voluntary and involuntary movement. * Nervous: Brain, spinal cord; interprets information and controls senses. * Endocrine: Glands; releases hormones for metabolism, growth, and stress response. * Respiratory: Lungs, trachea; gas exchange and acid-base balance. * Circulatory: Heart, blood vessels; transports nutrients, waste, and gases. * Digestive: Mouth, stomach, intestines, liver; breakdowns food and eliminates waste. * Reproductive: Gonads; sexual function and reproduction. * Urinary: Kidneys, bladder; removes metabolic waste and regulates water balance. * Immune: White blood cells, lymph; defense against pathogens.

The Gastrointestinal (GI) System and Digestion

GI Tract Organs: A hollow tube from mouth to anus consisting of the mouth, esophagus, stomach, small intestine, and large intestine.
Accessory Organs: Salivary glands, liver, gallbladder, and pancreas.
Primary Functions: Digestion, absorption, and egestion (elimination).
Tissue Layers of the GI Tract: 1. Mucosa (Innermost): Epithelial tissue; produces secretions for digestion. 2. Submucosa: Connective tissue; contains blood vessels, lymphatic vessels, and nerves. 3. Muscularis: Two layers of smooth muscle (longitudinal and circular) responsible for motility. 4. Serosa (Outermost): Connective tissue; secretes lubricating fluid for protection.
GI Motility: * Transit Time: Generally ranges from 24 to 72 hours. * Segmentation: Circular muscles contract and relax in a "chopping" motion to mix food with secretions. This is a slow movement. * Peristalsis: Rhythmic, wavelike contractions of circular and longitudinal muscles that propel food forward.

Major GI Secretions and Hormones

Secretions: * Mucus: Secreted by mucosal cells; protects and lubricates. * Saliva: From salivary glands; moistens food, aids swallowing, and contains enzymes. * Enzymes: Biological catalysts from salivary glands, stomach, small intestine, and pancreas; chemically break down food. * Bile: Made in the liver, stored in the gallbladder; enables lipid globules to disperse in water. * Pancreatic Juice: Contains bicarbonate ( $NaHCO_3$ ) to neutralize acidic chyme and enzymes for all macronutrients. * Gastric Juice: Contains hydrochloric acid ( $HCl$ ), enzymes (from chief cells), and intrinsic factor (needed for Vitamin $B_{12}$ absorption) from parietal cells.
Hormones: * Gastrin: Produced in the stomach; stimulated by food/alcohol/caffeine; increases gastric motility and juice secretion. * Secretin: Produced in the small intestine; stimulated by chyme; inhibits gastric motility and stimulates pancreatic bicarbonate release. * Cholecystokinin (CCK): Produced in the small intestine; stimulated by protein/fat; stimulates gallbladder contraction and pancreatic enzyme release; slows gastric emptying. * Ghrelin: Produced in the stomach; stimulates hunger; levels are higher during fasting.

Phases of Digestion and Stomach Anatomy

1. Cephalic Phase: Triggered by sight, smell, or thought of food. Includes mastication (chewing) and swallowing (voluntary and involuntary phases). * The epiglottis covers the trachea during swallowing. * The bolus enters the esophagus and passes the gastroesophageal sphincter.
2. Gastric Phase: Food arrives in the stomach. * Stomach Structure: Includes the fundus, body, antrum, and three muscle layers (longitudinal, circular, and diagonal). Folds called rugae allow for expansion. * Pyloric Sphincter: Regulates chyme flow into the small intestine. * Pathologies: * Peptic Ulcers: Erosion of the mucosal lining in the esophagus, stomach, or duodenum. * GERD (Gastroesophageal Reflux Disease): Occurs when the gastroesophageal sphincter weakens, allowing acidic stomach contents to flow back into the esophagus.
3. Intestinal Phase: * Small Intestine (SI): Divided into the Duodenum (receives bile and pancreatic juice), Jejunum, and Ileum. * Absorptive Surface: Increased by circular folds (plica circulares), finger-like projections (villi), and microvilli (the brush border). * Enterocytes: Absorptive cells covering the villi.

Nutrient Circulation and Waste Elimination

Circulation Routes: * Water-Soluble Nutrients: Enter the cardiovascular system and circulate directly to the liver via the hepatic portal vein. * Fat-Soluble Nutrients: Enter the lymphatic system via lacteals in the villi, eventually entering the blood at the thoracic duct.
Exchange: Occurs across capillary walls between plasma and interstitial fluid; oxygen and nutrients enter tissues while $CO_2$ and waste are removed.
Urinary System: * Filtration: Occurs in nephrons within the kidneys to remove waste from blood. * Reabsorption: Returns important substances to circulation. * Excretion: Urine travels via the ureter to the bladder and out through the urethra.
Large Intestine: * Anatomy: Includes the cecum (separated from the ileum by the ileocecal sphincter), ascending, transverse, descending, and sigmoid colon, and the rectum. * Functions: Absorbs fluids and electrolytes; the microbiome breaks down residue to produce Vitamin $K$ , some B vitamins, and lipids; store and eliminate feces. * Movement Abnormalities: Diarrhea (too fast) and constipation (too slow) can lead to dehydration or malnutrition.