Comprehensive Human Physiology: Homeostasis, Cell Membranes, and Energy Metabolism

What is the internal environment of the body?

The fluid inside the body that surrounds cells, primarily the extracellular fluid (ECF).

What does homeostasis refer to?

The maintenance of a stable internal environment despite changes in the external environment.

What is the primary function of the gastrointestinal (GI) tract?

To exchange nutrients, water, and electrolytes between the external and internal environments.

What is the role of the kidneys in homeostasis?

To regulate the composition and volume of body fluids through filtration, reabsorption, and secretion.

What is the total body water (TBW) percentage in adult males?

Approximately 60% of body weight.

What is intracellular fluid (ICF)?

Fluid inside cells, making up about 2/3 of total body water.

What is extracellular fluid (ECF)?

Fluid outside cells, making up about 1/3 of total body water.

What is a regulated variable in homeostasis?

A physiological parameter that is kept within a narrow range, such as body temperature or blood glucose.

What is a set point?

The target or desired value for a regulated variable, e.g., body temperature ≈ 37°C (98.6°F).

What is negative feedback?

A control mechanism where a change in a variable triggers responses that oppose the change, keeping the variable near the set point.

What are receptors in a homeostatic control system?

Sensors that detect changes in the regulated variable, such as thermoreceptors or baroreceptors.

What is the role of integrating centers in homeostasis?

To receive input from receptors, compare it to the set point, and decide on a response.

What are effectors in a homeostatic control system?

Cells or organs that produce the response to restore homeostasis, such as muscles or glands.

What is the difference between intrinsic and extrinsic regulation?

Intrinsic regulation is local control within an organ or tissue, while extrinsic regulation is systemic control by neural or hormonal mechanisms.

What are the main components of biomolecules?

Primarily composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and may include phosphorus (P) and sulfur (S).

What are monosaccharides?

Single sugar units that serve as the primary energy source, examples include glucose and fructose.

What are triglycerides?

Glycerol combined with 3 fatty acids, serving as the primary energy storage.

What is the function of phospholipids?

To form cell membranes with hydrophilic heads and hydrophobic tails.

What are amino acids?

Building blocks of proteins, consisting of an amino group, carboxyl group, and a side chain (R group).

What is the role of enzymes in chemical reactions?

Enzymes are biological catalysts that lower the activation energy required to start a reaction.

What is the lock-and-key model?

A model describing substrate specificity where the active site of the enzyme is rigid and fits the substrate exactly.

What is the difference between dehydration synthesis and hydrolysis?

Dehydration synthesis removes water to form bonds, while hydrolysis adds water to break bonds.

What are eicosanoids?

Local signaling molecules derived from arachidonic acid, involved in inflammation, pain, and clotting.

What is the function of nucleotides?

Building blocks of nucleic acids, consisting of a nitrogenous base, pentose sugar, and phosphate group.

What is the role of ATP?

A nucleotide that serves as an energy molecule in cellular processes.

What is positive feedback?

A response that amplifies the original stimulus, continuing until a specific endpoint is reached.

What are antagonistic effectors?

Two effectors with opposite actions that regulate the same variable, allowing fine-tuned control.

What is the Lock-and-Key Model of enzyme action?

The active site is rigid, and the substrate fits exactly into it.

What does the Induced-Fit Model propose about enzyme action?

The active site changes shape upon substrate binding, making it a more accurate model.

What are cofactors in enzymatic reactions?

Inorganic ions such as Mg²⁺ and Zn²⁺ that assist enzyme function.

What are coenzymes?

Organic molecules, often vitamins, that assist enzymes, e.g., NAD⁺ and FAD.

What factors affect enzyme catalytic rates?

Catalytic rate, reactant and product concentrations, affinity, pH, and temperature.

How does substrate concentration affect enzyme activity?

Increased substrate concentration increases the reaction rate until saturation is reached.

What is Km in relation to enzyme affinity?

A lower Km indicates a higher affinity between enzyme and substrate.

What is allosteric regulation?

Regulation where a molecule binds to a site other than the active site, activating or inhibiting the enzyme.

What is covalent regulation in enzymes?

Alteration of enzyme activity by adding or removing chemical groups, such as phosphorylation.

What is feedback inhibition?

A process where the end product inhibits an early step in the pathway to prevent overproduction.

What is feedforward activation?

An early substrate activates a downstream enzyme, increasing pathway efficiency.

What is ATP and its role in biological energy exchange?

ATP (adenosine triphosphate) is the cell's immediate energy currency, storing energy in high-energy phosphate bonds.

What happens during ATP hydrolysis?

ATP is converted to ADP and inorganic phosphate (Pi), releasing energy.

What is the overall reaction for glucose oxidation (aerobic respiration)?

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

What is substrate-level phosphorylation?

ATP is produced directly by transferring a phosphate to ADP, does not require oxygen.

Where does glycolysis occur and what are its products?

Glycolysis occurs in the cytosol and produces 2 pyruvate, 2 ATP, and 2 NADH.

What occurs during the Linking Step (Pyruvate Oxidation)?

Pyruvate is converted to Acetyl-CoA, producing NADH and CO₂, with no ATP produced directly.

What is the Krebs Cycle and where does it occur?

The Krebs Cycle occurs in the mitochondrial matrix, fully oxidizing Acetyl-CoA to CO₂ and producing NADH, FADH₂, and a small amount of ATP.

What is oxidative phosphorylation?

The process where NADH and FADH₂ donate electrons to the electron transport system, producing the majority of ATP.

What is anaerobic glycolysis?

Glycolysis that occurs when oxygen is limited, leading to lactate formation.

What is the Cori Cycle?

A process where lactate is transported to the liver, converted to glucose, and returned to muscle.

What is glycogen and where is it stored?

Glycogen is the storage form of glucose, stored in the liver and muscle.

What is gluconeogenesis?

The synthesis of glucose from non-carbohydrate sources, mainly occurring in the liver.

What is beta oxidation?

The breakdown of fatty acids in mitochondria, producing Acetyl-CoA, NADH, and FADH₂.

What are ketones and when are they produced?

Ketones are produced from excess Acetyl-CoA in the liver and used as an alternative fuel during fasting.

What are essential amino acids?

Amino acids that cannot be synthesized by the body and must be obtained from the diet.

What is nitrogenous waste in protein metabolism?

Amino acid metabolism produces ammonia, which is converted to urea in the liver and excreted by the kidneys.

What is the fluid mosaic model?

It describes the plasma membrane as a dynamic, flexible structure with lipids and proteins moving laterally.

What is the role of cholesterol in the plasma membrane?

Cholesterol modulates membrane fluidity and stability, preventing excessive fluidity at high temperatures and solidification at low temperatures.

What are integral proteins?

Proteins embedded in or spanning the lipid bilayer, functioning as channels, transporters, receptors, and enzymes.

What are peripheral proteins?

Proteins loosely attached to the membrane surface that provide structural support, signaling, and cytoskeletal anchoring.

What is selective permeability in membranes?

Membranes allow some substances to cross easily while restricting others based on size, charge, and polarity.

What is an ion?

A charged particle.

What is a cation?

A positively charged ion (e.g., Na⁺).

What is an anion?

A negatively charged ion (e.g., Cl⁻).

What is membrane potential (Vm)?

The electrical charge difference across the membrane, typically negative inside the cell.

How is membrane potential (Vm) measured?

In millivolts (mV), with resting Vm ≈ −70 mV in neurons.

What principle states that opposite charges attract?

Opposite charges attract and like charges repel.

What determines the direction of electrical force?

It depends on the polarity of the cell and the charge on the particle.

What factors affect the magnitude of electrical force?

The strength of Vm and the amount of charge on the particle.

What is the electrochemical driving force?

The net force determining ion movement, which is the sum of chemical and electrical forces.

What is equilibrium potential?

It exists when electrical force equals chemical force, resulting in no net movement of the ion.

What factors influence passive transport rates across membranes?

Magnitude of the driving force, membrane surface area, and membrane permeability.

What is simple diffusion?

Direct movement through the lipid bilayer without a transport protein.

What are the characteristics of facilitated diffusion?

Passive transport through a carrier that requires a specific binding site and is saturable.

What is primary active transport?

Transport that directly uses energy from a high-energy compound, usually ATP hydrolysis.

What is secondary active transport?

Transport that uses energy released from ion diffusion, indirectly using ATP.

What is osmosis?

The diffusion of water across a semipermeable membrane, moving down its concentration gradient.

What is osmolarity?

Total solute concentration of a solution, expressed as osmoles per liter (Osm/L).

What is the difference between a mole and an osmole?

A mole is the amount of a substance, while an osmole is the number of osmotically active particles.

What is tonicity?

The effect of a solution on cell volume, describing the water gradient across a membrane.

What is the role of Na⁺/K⁺ ATPase in epithelial cells?

It maintains low intracellular Na⁺ levels and creates ion gradients that drive transport.

What is the function of aquaporins?

They facilitate the rapid transport of water across cell membranes.

What is cystic fibrosis?

A defect in the CFTR Cl⁻ channel that impairs Cl⁻ and water secretion, leading to thick mucus.

What is transcytosis?

The transport of large molecules involving endocytosis, vesicular transport, and exocytosis.

What is the relationship between osmolarity and tonicity?

Osmolarity measures total solute concentration, while tonicity describes the effect on cell volume.

What is the significance of epithelial polarity?

It allows for directional transport of substances across the epithelium.

What happens during diffusional equilibrium?

Molecules move equally in both directions, resulting in no net movement.

What factors affect the rate of simple diffusion?

Magnitude of the driving force, membrane surface area, membrane permeability, size, and temperature.

What is the role of GLUT transporters?

They facilitate the diffusion of glucose into the extracellular fluid (ECF).