Overview
This lecture covered the fundamentals of cells, their structure, cellular environments, organelles, and key transport processes essential for cell function and physiology.
Cell Theory and Basics
- All living things are composed of cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells—no spontaneous generation.
- Each cell in the body descends from the original zygote (fertilized egg).
- Differentiation is the process by which cells become specialized into various types (e.g., skin, liver).
- The human body contains about 75–100 trillion cells, with red blood cells making up around 25 trillion.
Cellular Environment and Fluid Compartments
- Two-thirds of body fluid is intracellular (inside cells), called intracellular fluid (ICF).
- One-third is extracellular fluid (ECF), outside cells.
- ECF is split into blood plasma (20–25%) and interstitial fluid (ISF, 75–80%), which fills spaces between cells.
- ICF is rich in potassium ions; ECF is rich in sodium ions.
Homeostasis
- Cells maintain a relatively stable internal environment despite changes, a process called homeostasis.
- Homeostasis is central to physiology, involving regulation of factors like temperature, blood pressure, and chemical concentrations.
Cell Structure: Main Components
Plasma Membrane
- Also known as the cell membrane (PM); a dynamic, selectively permeable barrier controlling entry and exit.
- Composed mainly of a phospholipid bilayer with embedded proteins (transmembrane and peripheral), glycolipids, glycoproteins, and cholesterol.
- Follows the fluid mosaic model: constantly shifting components.
Cytoplasm and Organelles
- Cytoplasm is a semi-liquid (gel-like) substance containing organelles.
- The fluid part is cytosol, site of most metabolic reactions (metabolism = sum of all chemical reactions).
- Catabolism breaks down molecules; anabolism builds molecules.
Key Organelles
- Endoplasmic Reticulum (ER):
- Rough ER (with ribosomes) synthesizes proteins.
- Smooth ER synthesizes lipids/carbohydrates and stores calcium ions.
- Golgi Apparatus: Modifies, sorts, and packages proteins/lipids; forms lysosomes (for intracellular digestion) and secretory vesicles (for releasing substances).
- Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP (energy) and produces CO₂ and water.
Nucleus
- Stores genetic information (DNA, chromosomes, genes).
- Mitochondria also contain their own DNA.
Cellular Transport Processes
- Non-carrier mediated (Passive) Transport:
- Includes diffusion (movement from high to low concentration) and osmosis (water movement); requires no energy.
- Carrier-mediated Transport:
- Uses embedded proteins to transport substances; can be:
- Active transport (requires ATP, moves against gradients).
- Facilitated diffusion (no ATP, uses carrier proteins).
- Vesicular (Bulk) Transport:
- Endocytosis (moves substances into cells) and exocytosis (moves substances out); requires energy (ATP).
Key Terms & Definitions
- Cell Theory — the principle that all living things are made of cells, and all cells come from pre-existing cells.
- Homeostasis — maintaining a stable internal environment.
- Intracellular Fluid (ICF) — fluid inside cells.
- Extracellular Fluid (ECF) — fluid outside cells.
- Plasma Membrane — the cell’s outer boundary, composed of a phospholipid bilayer.
- Cytoplasm — the gel-like substance inside the cell containing organelles.
- Organelle — specialized structure within a cell (e.g., mitochondria, ER).
- Metabolism — all chemical reactions in an organism/cell.
- Catabolism — breaking down molecules for energy.
- Anabolism — building complex molecules from simpler ones.
- Diffusion — passive movement of substances from high to low concentration.
- Osmosis — diffusion of water across a membrane.
- Active Transport — movement of substances against a gradient using energy.
- Facilitated Diffusion — protein-assisted passive transport.
- Endocytosis/Exocytosis — energy-requiring processes for bulk transport into/out of cells.
Action Items / Next Steps
- Review textbook/web diagrams of cellular structures and membranes.
- Prepare for the next lecture on tissues and embryology.
- Understand key terms for future discussions on physiology.