Organic Compounds in Anatomy & Physiology

Overview of Organic Compounds

Organic compounds are fundamental to all living organisms, forming the molecular basis of life. They are primarily composed of carbon, hydrogen, and oxygen, and are classified into four major groups: carbohydrates, lipids, proteins, and nucleic acids. Each group has unique structures and functions essential for cellular processes and overall physiology.

Carbohydrates

• Structure and Classification:

  • Monosaccharides: Simple sugars with a 1C:2H:1O ratio, often forming ring structures. Examples: glucose, galactose, fructose, ribose, deoxyribose.

  • Disaccharides: Composed of two monosaccharide subunits joined by a polar covalent bond. Examples: lactose, sucrose, maltose.

  • Polysaccharides: Large, highly branched polymers with hundreds of monosaccharide subunits. Example: glycogen.

• Functions:

  • Energy: Glucose is the main fuel for all cells; disaccharides and polysaccharides are broken down to release monosaccharides for energy.

  • Structure: Ribose and deoxyribose are components of nucleic acids.

  • Energy Storage: Glycogen is stored in the liver and skeletal muscle cells for later use.

• Example: Glycogen stored in muscle cells is broken down during exercise to provide glucose for ATP production.

Lipids

• Structure and Types:

  • Triglycerides: Three fatty acids bonded to a glycerol backbone; can be saturated or unsaturated.

  • Phospholipids: Two fatty acids and a phosphate group attached to a glycerol backbone; main component of cell membranes.

  • Steroids: Four-ringed hydrocarbon structure with various modifications; includes hormones and cholesterol.

• Functions:

  • Energy Storage: Triglycerides are stored in adipose (fat) cells and released as needed.

  • Protection and Insulation: Lipids cushion and insulate organs.

  • Structure: Phospholipids form the structural basis of all cell membranes.

  • Regulation: Steroid hormones regulate physiological processes; cholesterol is a key component of cell membranes.

• Example: Cholesterol stabilizes the fluidity of the plasma membrane in animal cells.

Proteins

• Structure and Types:

  • Di- and Polypeptides: Chains of two or 10–50 amino acids, respectively, joined by peptide bonds. Examples: substance P, endorphins, glucagon, calcitonin, insulin.

  • Proteins: Polymers of more than 50 amino acids that fold into specific three-dimensional shapes. Examples: collagen, keratin, amylase, albumin, hemoglobin.

• Functions:

  • Structure: Primary structural molecules in the body (e.g., collagen in connective tissue).

  • Movement: Involved in cell and muscle movement (e.g., actin and myosin).

  • Catalysis: Many proteins function as enzymes to speed up biochemical reactions.

  • Transport: Proteins transport substances throughout the body (e.g., hemoglobin transports oxygen).

  • Defense: Proteins play roles in immune defense mechanisms (e.g., antibodies).

  • Regulation: Many act as hormones or chemical messengers.

• Example: Insulin is a protein hormone that regulates blood glucose levels.

Nucleotides and Nucleic Acids

• Structure and Types:

  • ATP (Adenosine Triphosphate): Consists of the nitrogenous base adenine, ribose sugar, and three phosphate groups.

  • Nucleic Acids: Polymers of nucleotides arranged as a double helix (DNA) or a single strand (RNA).

• Functions:

  • ATP: Main source of chemical energy for the body; drives cellular work.

  • Nucleic Acids: DNA stores genetic information and instructions for protein synthesis; RNA carries out these instructions.

• Example: ATP powers muscle contraction and nerve impulse transmission.

ATP Synthesis and Function

• ATP is synthesized from adenosine diphosphate (ADP) and inorganic phosphate, a process requiring energy, usually derived from glucose catabolism.

• Equation for ATP synthesis:

• Hydrolysis of ATP releases energy for cellular work:

• Continuous ATP production is essential; oxygen is required for efficient ATP synthesis, explaining the necessity of breathing.

Summary Table: Organic Compounds

Additional info: The hydrolysis of ATP is a key exergonic reaction in metabolism, and the continuous regeneration of ATP from ADP and phosphate is fundamental to sustaining life processes.

Cell Structure and Function

Basic Processes of Cells

Cells are the fundamental units of life, carrying out essential processes to maintain homeostasis and support the organism. These processes include metabolism, transport, communication, and reproduction.

• Cell Metabolism: The sum of all chemical reactions in a cell, including:

  • Anabolic reactions: Build complex molecules from simpler ones (e.g., protein synthesis).

  • Catabolic reactions: Break down complex molecules into simpler ones (e.g., cellular respiration).

  • Oxidation-Reduction reactions: Transfer electrons between molecules, crucial for energy production.

• Membrane Transport: Movement of substances into, out of, or within the cell.

• Communication: Cells interact with their environment and other cells via chemical and electrical signals.

• Cell Reproduction: Most cells divide to produce new cells, essential for growth and repair.

Overview of Cell Structure

• Plasma Membrane: The outer boundary that separates the cell from its environment.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing:

  • Cytosol: The fluid portion, also called intracellular fluid (ICF), rich in proteins and solutes.

  • Organelles: Specialized structures that perform specific cellular functions (e.g., mitochondria, ribosomes).

  • Cytoskeleton: A network of protein filaments providing structural support, shape, and transport within the cell.

• Nucleus: The control center of the cell, surrounded by a double membrane (nuclear envelope), containing most of the cell's DNA and the site of RNA synthesis.

Functions of the Plasma Membrane

• Physically isolates the cell from its surroundings.

• Provides structural support.

• Facilitates communication with other cells.

• Regulates transport of substances into and out of the cell.

• Identifies the cell to other cells (cell recognition).

• Defines fluid compartments:

  • Intracellular Space: Contains intracellular fluid (cytosol).

  • Extracellular Space: Contains extracellular fluid (ECF).

The Plasma Membrane

The Phospholipid Bilayer

• The plasma membrane is primarily composed of a double layer of phospholipids, forming a barrier between the cell's interior and exterior environments.

• Phospholipids: Molecules with hydrophilic (water-attracting) phosphate heads and hydrophobic (water-repelling) fatty acid tails.

• In aqueous environments, phospholipids arrange themselves so that the hydrophilic heads face water and the hydrophobic tails face inward, away from water, forming a bilayer.

• This arrangement excludes water from the hydrophobic core, creating an effective barrier.

The Fluid Mosaic Model

• The plasma membrane is described by the fluid mosaic model, which emphasizes its dynamic and heterogeneous nature.

• Membrane components (phospholipids, proteins, cholesterol, carbohydrates) move laterally within the bilayer, contributing to fluidity.

• The 'mosaic' aspect refers to the diverse array of proteins and other molecules embedded in or attached to the membrane.

Membrane Proteins

• Integral Proteins: Span the entire membrane; if they reach both sides, they are called transmembrane proteins.

• Peripheral Proteins: Attached to only one side of the membrane, often anchored by the cytoskeleton or floating within the bilayer.

• Functions of Membrane Proteins:

  • Channels: Allow specific substances to pass through the membrane.

  • Carriers: Transport substances across the membrane, often changing shape in the process.

  • Receptors: Bind signaling molecules (ligands) and trigger cellular responses.

  • Enzymes: Catalyze chemical reactions at the membrane surface.

  • Structural Support Proteins: Maintain cell shape and integrity.

  • Linker Proteins: Connect adjacent cells within a tissue.

Membrane Transport Mechanisms

Passive Transport Processes

• Passive transport moves substances across the membrane without energy input, relying on concentration gradients.

• Simple Diffusion: Movement of small, nonpolar molecules directly through the phospholipid bilayer from high to low concentration.

• Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers.

• Osmosis: Diffusion of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.

  • Water moves through aquaporins (water channels) or between phospholipids due to its small size.

  • Osmosis can change the volume of fluid in cellular compartments.

Tonicity

• Tonicity describes the ability of a solution to cause a cell to gain or lose water, based on solute concentration.

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink (crenate).

• Hypotonic: Lower solute concentration outside the cell; water enters the cell, causing it to swell and possibly burst (lyse).

Active Transport Processes

• Active transport requires energy (usually from ATP) to move substances against their concentration gradients.

• Primary Active Transport: Direct use of ATP to transport molecules via carrier proteins called pumps.

• Sodium-Potassium Pump (Na+/K+ ATPase): Moves 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP hydrolyzed. This maintains essential ion gradients for processes like muscle contraction.

• Equation:

• Secondary Active Transport: Uses the energy from primary active transport to move other substances against their gradients.

• Vesicular (Bulk) Transport: Large particles or volumes of fluid are transported via vesicles, small membrane-bound sacs, in processes that require ATP.

  • Endocytosis: Bringing substances into the cell.

    • Phagocytosis: "Cell eating"; ingestion of large particles (e.g., bacteria) by specialized cells called phagocytes.

    • Pinocytosis: "Cell drinking"; ingestion of fluid and dissolved substances.

    • Receptor-Mediated Endocytosis: Selective uptake of specific molecules via receptor binding.

  • Exocytosis: Release of substances from the cell; also replenishes plasma membrane lost during endocytosis.

  • Transcytosis: Substances are transported into, across, and then out of the cell.

Gradients Across the Membrane

• Chemical Gradient: Difference in concentration of a substance across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

• Electrochemical Gradient: Combined effect of chemical and electrical gradients, influencing the movement of ions.

Summary Table: Types of Membrane Transport

Example: The Na+/K+ pump is essential for maintaining the resting membrane potential in nerve and muscle cells, enabling electrical signaling and contraction.

Additional info: The concepts of tonicity and membrane transport are foundational for understanding fluid balance, nerve impulses, and muscle contraction in human physiology.