Cells

Cell Structure and Function

Cells are the basic structural and functional units of all living organisms. Each cell contains specialized organelles that perform distinct functions necessary for life.

• Nucleus: Contains genetic material (DNA) and controls cellular activities. Surrounded by a nuclear envelope, it houses chromatin (uncondensed DNA) and nucleolus (site of ribosome synthesis).

• Cytoplasm: Gel-like substance where organelles are suspended.

• Organelles: Specialized structures such as mitochondria (energy production), endoplasmic reticulum (protein and lipid synthesis), Golgi apparatus (modifies and packages proteins), lysosomes (waste removal), and ribosomes (protein synthesis).

• Plasma Membrane: Encloses the cell, maintaining the internal environment and regulating the movement of substances in and out.

Plasma Membrane: Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, highlighting its dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attach to proteins (glycoproteins) or lipids (glycolipids) for cell recognition and signaling.

• Cholesterol: Stabilizes membrane fluidity.

Membrane Transport Mechanisms

The plasma membrane is selectively permeable, allowing certain substances to pass while restricting others. Transport occurs via passive or active mechanisms.

• Passive Transport: Does not require energy (ATP). Includes simple diffusion, facilitated diffusion, and osmosis. Movement is driven by concentration gradients.

• Active Transport: Requires ATP to move substances against their concentration gradients. Includes primary active transport (e.g., sodium-potassium pump) and secondary active transport. Also includes bulk transport mechanisms such as endocytosis (phagocytosis, pinocytosis) and exocytosis.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, providing communication and structural integrity.

• Tight Junctions: Integral proteins join cells to form impermeable barriers.

• Desmosomes: Cadherin proteins anchor cells together, providing mechanical strength.

• Gap Junctions: Connexon proteins form channels allowing communication and passage of ions and small molecules between cells.

Biomolecules

Basic Chemistry for Anatomy & Physiology

Understanding the chemical basis of life is essential for studying anatomy and physiology. Matter and energy are foundational concepts, with atoms and elements forming the building blocks of biomolecules.

• Matter: Anything that has mass and occupies space; exists as solids, liquids, or gases.

• Energy: The capacity to do work; exists in various forms such as chemical, electrical, mechanical, and radiant.

• Atoms: Smallest units of elements, composed of protons, neutrons, and electrons.

• Elements: Pure substances consisting of one type of atom; major elements in the body include carbon, hydrogen, oxygen, and nitrogen.

Atomic Structure and Chemical Bonds

Atoms combine to form molecules and compounds through chemical bonds, which are determined by the arrangement of electrons.

• Covalent Bonds: Electrons are shared between atoms. Can be nonpolar (equal sharing) or polar (unequal sharing, as in water).

• Ionic Bonds: Electrons are transferred from one atom to another, resulting in charged ions (e.g., NaCl).

• Hydrogen Bonds: Weak attractions between polar molecules, important in the structure of water and biological macromolecules.

Biochemistry: Inorganic and Organic Compounds

Biochemistry focuses on the chemical composition and reactions of living matter. Compounds are classified as inorganic or organic.

• Inorganic Compounds:

  • Water: Most abundant compound in cells; excellent solvent, high heat capacity, involved in hydrolysis and dehydration reactions.

  • Salts: Ionic compounds that dissociate into electrolytes in solution.

  • Acids and Bases: Acids donate protons (H+), bases accept protons; important for maintaining pH balance.

• Organic Compounds:

  • Carbohydrates: Contain C, H, O; include monosaccharides (glucose), disaccharides (sucrose), and polysaccharides (starch, glycogen).

  • Lipids: Contain C, H, O (sometimes P); include triglycerides (fats and oils), phospholipids, and steroids.

  • Proteins: Made of amino acids linked by peptide bonds; serve structural, enzymatic, contractile, and transport functions.

Hierarchy of Biological Organization

Biomolecules combine to form cells, which organize into tissues, organs, organ systems, and ultimately, the organism.

• Hierarchy: Biomolecules → Cells → Tissues → Organs → Organ Systems → Organism

Additional info: This hierarchical organization is fundamental to understanding how structure and function are integrated in the human body.