Comprehensive Notes on Cell Metabolism and the Human Digestive System

Enzymes and their Role as Biocatalysts

Enzymes are specialized proteins composed of one or more polypeptide chains produced by living cells. They function as biokatalysators, meaning they are capable of making chemical reactions possible, accelerating them, or catalyzing them without being consumed in the process. Structurally, an enzyme is characterized by a specific groove or cavity known as the actief centrum (active site). The substrate molecule fits precisely into this active site, forming an enzym-substraatcomplex. Following the chemical reaction, the products are released, and the enzyme remains unchanged, allowing it to be reused for subsequent reactions. Enzymes are highly reaction-specific and substraat-specifiek, and their primary mechanism of action involves lowering the activeringsenergie (activation energy) required for a reaction to occur.

General Functions and Structure of the Human Digestive System

The primary function of the human digestive system is to break down ingested food into substances that can be utilized by the cells. This involves several specific processes: the inname (intake) of food, the mechanical breakdown of large food fragments, the transport of food through the spijsverteringskanaal via processes like peristaltiek (peristalsis), the chemical breakdown of food components via enzymes found in digestive juices, the absorption (opname) of nutrients into transport systems, and the elimination (verwijdering) of undigested remains through defecatie.

The digestive system consists of the digestive tract, extending from the mouth to the anus, and several auxiliary organs. The wall of the digestive tract is organized into three distinct layers from the inside out: the slijmvlieslaag (mucosa), consisting of epithelial cells and mucus-producing cells; the bindweefsellaag (connective tissue layer), which contains blood vessels, lymph vessels, and nerves; and the spierlaag (muscle layer), comprised of longitudinal and circular muscles that facilitate peristalsis.

Digestion in the Mouth and Esophagus

Digestion begins in the mondholte (mouth), where both mechanical and chemical processes occur. The speekselklieren (salivary glands) secrete between $1$ and $1.5\,L/day$ of saliva. Saliva contains speekselamylase (also called ptyaline), water, mucus, and lysosyme (a substance with antibacterial properties). Speekselamylase is responsible for the breakdown of starch into maltose and glucose via hydrolyse. The pH optimum for amylase is approximately $7$, making the environment in the mouth neutral.

Saliva secretion is a reaction to stimuli such as the sight, smell, or thought of food, while a dry mouth can occur during states of stress or fear. Mechanically, the teeth reduce food size through chewing, which is most effective when prolonged to increase the surface area for enzymes. The tongue assists by mixing food with saliva. Once chewed, the food bolus travels through the slokdarm (esophagus) via peristalsis and enters the stomach.

Digestion in the Stomach

The entry of food into the stomach is regulated by a sphincter muscle. The stomach wall features folds that flatten when the organ fills. Food typically remains in the stomach for $2$ to $3$ hours before passing through the maagportier (pylorus) into the duodenum. Gastric glands produce approximately $2.5\,L/day$ of maagsap (gastric juice).

Gastric juice consists of several critical components. It contains $H_2O$, which acts as a solvent and transport medium. It also contains $HCl$ (hydrochloric acid), which results in a highly acidic pH of $2$. The functions of $HCl$ include activating pepsinogen, killing bacteria, and denaturing proteins. To prevent the acidic environment and enzymes from destroying the cell membranes of the gastric glands, the stomach produces pepsinogeen, an inactive precursor of the enzyme pepsine. Once activated, pepsine acts as an endopeptidase, breaking down protein molecules at internal positions within the polypeptide chain to create smaller polypeptides. Additionally, mucus is secreted to protect the stomach wall from self-digestion. The stomach muscles contribute by mixing, grinding, and transporting the food toward the small intestine.

Digestion in the Duodenum and the Role of Accessory Organs

The twaalfvingerige darm (duodenum) is the site where digestive juices from the liver, gallbladder, and pancreas are introduced. The lever (liver) secretes galsap (bile), a yellow-green fluid containing water, galzouten (bile salts), and the pigment bilirubine (a breakdown product of hemoglobin). The galblaas (gallbladder) stores, concentrates, and releases bile at a rate of approximately $0.7\,L/day$. Bile is not a gland and does not contain enzymes; instead, it acts as an emulgator (emulsifier), breaking large fat droplets into smaller ones to increase the surface area for lipase.

The pancreas (alvleesklier) secretes approximately $1.5\,L/day$ of pancreassap (pancreatic juice), which has a pH that changes from $3$ to $8$ as it neutralizes the acidic stomach contents using $NaHCO_3$ (sodium bicarbonate). Pancreatic enzymes include:

• Pancreasamylase: Splitting remaining starch into glucose and maltose.

• Maltase: Splitting maltose into two glucose molecules.

• Endopeptidase: Breaking proteins into smaller polypeptides.

• Exopeptidase: Cleaving terminal amino acids from a polypeptide chain one by one.

• Lipase: Splitting lipids (triglycerides) into glycerol and free fatty acids.

Digestion in the Small and Large Intestine

In the dunne darm (small intestine), approximately $2\,L/day$ of darmsap (intestinal juice) is produced, maintaining a pH of around $8$. This juice contains enzymes that complete the digestion of nutrients:

• Peptidases and exopeptidases: Cleave terminal amino acids from polypeptide chains.

• Dipeptidase: Splits dipeptides into individual amino acids.

• Maltase: Breaks maltose into two glucose molecules.

• Lactase: Splits lactose into glucose and galactose.

• Sacharase (sucrase): Splits sacharose into glucose and fructose.

The dikke darm (large intestine) does not produce digestive enzymes. Instead, it relies on bacteria to produce Vitamin K, perform limited breakdown of undigestible substances, and produce intestinal gases like $CH_4$ (methane) and $H_2S$ (hydrogen sulfide). The primary functions here are the resorption of water (thickening the feces) and the absorption of $K^+$ ions, which are essential for nerve conduction and muscle contraction.

Importance of Dietary Fiber and Defecation

Undigestible dietary fibers are crucial for health as they retain water (facilitating bowel movements), activate intestinal muscles to ensure smooth transit, absorb cholesterol, provide a substrate for healthy gut flora, and induce a sense of satiety.

Defecatie (defecation) is the removal of feces from the endeldarm (rectum). This occurs when the anus (an internal and external sphincter) relaxes. The urge to defecate typically follows a full meal due to increased peristalsis in the large intestine. On average, defecation occurs $1$ to $3$ times per day. A lack of physical movement can lead to constipation because exercise stimulates peristalsis.

Metabolic Role of Absorbed Nutrients

Nutrients absorbed by the digestive system play vital roles in cell metabolism and structures:

• Glucose: Used in cell respiration to produce $ATP$, with excess stored as glycogen.

• Galactose and Fructose: Converted into glucose by the liver.

• Amino Acids: There are $20$ amino acids, including $12$ non-essential (synthesized via transaminatie) and $8$ essential (must come from diet). They are used for protein synthesis, as an energy source, or converted into fat for long-term storage.

• Glycerol and Fatty Acids: Used as building blocks for phospholipids in cell membranes or burned for energy.

Cellular Digestion by Lysosomes

Digitsion can also occur intracellularly within lysosomen. Lysosomes contain enzymes that break down macromolecules to be reused as building blocks or fuel. There are two primary forms of lysosomal digestion:

• Autofagie: The digestion of the cell's own worn-out organelles.

• Heterofagie: The digestion of external substances brought into the cell. This includes phagocytosis (taking in solid particles) and pinocytosis (taking in liquid droplets).

Following intracellular digestion, residues are removed from the cell through exocytosis. A single cell may contain several hundred lysosomes of varying sizes and shapes.