Overview
Today's lecture began Unit 4, focusing on the structure and function of cells, the plasma membrane, cell junctions, and passive transport mechanisms essential for maintaining homeostasis in the human body.
Introduction to Cells
- The human body is made of trillions of cells, the smallest living units capable of performing all basic life functions.
- Cells in multicellular organisms depend on each other; the actions of one cell can affect others.
- Coordination among cells is necessary to maintain homeostasis.
- Over 250 types of human cells exist, each differing in size, shape, and function.
Cell Theory and Structure
- Cell theory: Cells are the structural and functional unit of life; all cells arise from pre-existing cells.
- How well an organism functions depends on the activities of its cells.
- All cells share three main parts:
- Plasma membrane: Flexible outer boundary.
- Cytoplasm: Intracellular fluid containing organelles.
- Nucleus: Contains DNA, the cell's control center.
Extracellular Materials
- Extracellular fluids:
- Interstitial fluid (surrounds cells)
- Blood plasma (in blood)
- Cerebrospinal fluid (around nervous system organs)
- Cellular secretions: Saliva, mucus.
- Extracellular matrix: Acts as a "glue" to hold cells together.
Plasma Membrane Structure and Function
- Plasma membrane: Separates intracellular and extracellular compartments; acts as a selectively permeable barrier.
- Key components:
- Phospholipid bilayer: Main structure.
- Membrane proteins: Integral and peripheral.
- Cholesterol: Stabilizes membrane.
- Carbohydrates (glycocalyx): Identity markers.
- Functions:
- Physical barrier
- Selective permeability
- Cell communication (via membrane proteins)
- Cell recognition (via glycocalyx)
Membrane Components
- Phospholipids: Form a bilayer with hydrophilic heads facing water and hydrophobic tails inward.
- Cholesterol: Increases membrane stability and fluidity.
- Proteins:
- Integral proteins: Span the membrane; involved in transport, signaling, and enzymatic activity.
- Peripheral proteins: Attached to membrane surface; support, enzymes, and cell connections.
- Carbohydrates:
- Glycocalyx: Short chains of sugars attached to proteins (glycoproteins) or lipids (glycolipids); important for cell recognition and immune response.
Cell Junctions
- Most cells are bound together to form tissues and organs.
- Three main types of cell junctions:
- Tight junctions: Create impermeable seals; prevent passage between cells (e.g., intestines).
- Desmosomes: Anchor cells together, providing mechanical strength (e.g., skin, heart, bladder).
- Gap junctions: Channels allowing ions and small molecules to pass between cells; important for communication (e.g., heart, smooth muscle).
Passive Transport Mechanisms
- Plasma membrane is selectively permeable, allowing only certain substances to cross.
- Passive transport (no ATP required):
- Simple diffusion: Small, nonpolar molecules move directly through the bilayer.
- Facilitated diffusion: Larger or polar molecules move via protein carriers or channels.
- Osmosis: Diffusion of water across the membrane, mainly through aquaporins.
Diffusion and Osmosis
- Diffusion: Movement of molecules from high to low concentration; rate affected by concentration gradient, molecular size, and temperature.
- Simple diffusion: Lipid-soluble/nonpolar molecules (e.g., Oâ‚‚, COâ‚‚) pass directly through the bilayer.
- Facilitated diffusion: Polar or larger molecules (e.g., glucose, ions) use specific protein carriers or channels.
- Carrier-mediated: Protein changes shape to move molecule across.
- Channel-mediated: Ions or water move through protein channels (e.g., aquaporins for water).
- Osmosis: Movement of water from areas of low solute (high water) to high solute (low water) concentration.
- Osmolarity: Measure of solute concentration; water moves to equalize osmolarity.
- Hydrostatic pressure: Outward pressure from increased cell volume.
- Osmotic pressure: Inward pull of water into areas with higher solute.
- Tonicity: Effect of a solution on cell volume:
- Isotonic: No net water movement; cell size unchanged.
- Hypertonic: Water leaves cell; cell shrinks (crenation).
- Hypotonic: Water enters cell; cell swells and may burst (lysis).
Key Terms & Definitions
- Cell Theory: All living things are made of cells; all cells come from existing cells.
- Plasma Membrane: Cell's outer boundary; regulates entry/exit of substances.
- Phospholipid Bilayer: Double layer of phospholipids forming the main structure of cell membranes.
- Integral Protein: Membrane protein spanning the bilayer; involved in transport and signaling.
- Peripheral Protein: Membrane protein attached to the surface; not embedded.
- Glycocalyx: Carbohydrate-rich area on cell surface; important for recognition.
- Tight Junction: Cell connection preventing passage between cells.
- Desmosome: Anchoring junction providing mechanical strength.
- Gap Junction: Communicating channels allowing passage of small molecules.
- Diffusion: Movement of molecules from high to low concentration.
- Osmosis: Diffusion of water across a semipermeable membrane.
- Tonicity: Effect of a solution on cell volume (isotonic, hypertonic, hypotonic).
Action Items / Next Steps
- Review figures 3.1–3.8 and related diagrams in the textbook or PowerPoint for visual understanding.
- Study the definitions of key terms for possible quiz questions.
- Prepare for the next class on active transport and membrane dynamics.
Highlighted Information:
- The plasma membrane's structure (phospholipid bilayer, proteins, cholesterol, glycocalyx) is essential for selective permeability, communication, and recognition.
- Cell junctions (tight junctions, desmosomes, gap junctions) are critical for tissue integrity and communication.
- Passive transport (diffusion, facilitated diffusion, osmosis) maintains cellular homeostasis without energy input.
- Tonicity directly affects cell volume and function—understanding isotonic, hypertonic, and hypotonic solutions is crucial.