Overview
This lecture reviews the fundamental structure and functions of cells, with a focus on the plasma membrane, cell communication, and transport processes. Understanding these basics is essential for grasping concepts in pathophysiology.
Cell Structure and Organelles
- The plasma membrane surrounds the cell, acting as a selective barrier that regulates the movement of water, electrolytes, nutrients, and waste.
- Lysosomes are small, round organelles containing digestive enzymes that break down cellular waste and damaged components.
- The nucleus, located centrally, contains DNA and controls genetic information and cell division.
- Mitochondria are the cell’s energy producers, generating ATP needed for cellular activities.
- Ribosomes, either free-floating or attached, are responsible for protein synthesis.
- Eukaryotic cells (e.g., human cells) have a nucleus and membrane-bound organelles; prokaryotic cells (e.g., bacteria) lack these structures.
Cell Functions
- Major cellular functions include movement, conductivity (electrical signaling), metabolic absorption, secretion, excretion, respiration, reproduction, and communication.
- The cytoplasm, a gel-like substance, maintains cell shape and keeps organelles separated to ensure proper function.
Plasma Membrane and Transport
- The plasma membrane is a dynamic, semi-permeable structure that maintains cell integrity, controls transport, and enables recognition, communication, and growth regulation.
- According to the fluid mosaic model, the membrane is composed of a phospholipid bilayer with embedded proteins, cholesterol, and other molecules, allowing selective movement of substances.
- Transport mechanisms include:
- Channels: Specific for certain ions or molecules (e.g., potassium channels).
- Enzymes: Facilitate chemical reactions at the membrane.
- Surface Receptors: Bind signaling molecules (ligands) to trigger cellular responses.
- Markers: Identify the cell to others, aiding in recognition and immune response.
- Adhesion Proteins: Help cells stick together and maintain tissue structure.
- The plasma membrane also connects to the cytoskeleton, helping maintain cell shape and internal organization.
- Endocytosis brings substances into the cell via vesicles; exocytosis expels substances, including waste and secretory products.
Cell Communication
- Cells communicate through several mechanisms:
- Contact-dependent (gap junctions): Direct physical contact allows exchange of signals.
- Paracrine: Cells secrete signaling molecules affecting nearby cells.
- Autocrine: Cells release signals that act on themselves.
- Hormonal: Hormones travel through the bloodstream to distant target cells.
- Neurohormonal: Neurons release hormones into the blood to affect distant cells.
- Neurotransmission: Nerve cells release neurotransmitters to adjacent cells.
- Signaling molecules (ligands) bind to cell surface or intracellular receptors, triggering responses such as survival, differentiation, growth, or apoptosis (programmed cell death).
- The plasma membrane’s receptors and channels are essential for receiving and transmitting these signals.
Cell Energy and Metabolism
- ATP (adenosine triphosphate) is the main energy source for cellular functions.
- Energy is produced through glycolysis (anaerobic) and the citric acid (Krebs) cycle (aerobic).
- Without oxygen, cells rely on glycolysis, leading to lactic acid buildup and reduced ATP production.
Electrolytes and Membrane Transport
- Electrolytes (cations: positive; anions: negative) are vital for cell function and fluid balance.
- Body fluids are distributed among intracellular, intravascular, and interstitial compartments, each with its own electrolyte balance.
- Passive transport (diffusion, filtration, osmosis) moves substances without energy input:
- Diffusion: Movement from high to low concentration.
- Filtration: Movement driven by hydrostatic pressure.
- Osmosis: Water moves toward higher solute concentration to balance gradients.
- Passive mediated transport uses membrane proteins to move substances without energy.
- Active transport (e.g., sodium-potassium pump) requires ATP to move substances against their concentration gradients.
- The sodium-potassium pump maintains the cell’s electrochemical balance by exchanging sodium and potassium ions across the membrane.
Membrane Potentials and Action Potentials
- The resting membrane potential is created by unequal charge distribution across the plasma membrane, mainly due to ion gradients.
- Action potential: A rapid change in membrane potential involving:
- Depolarization: Sodium ions rush into the cell.
- Repolarization: Potassium ions exit the cell.
- Refractory period: The cell resets to be ready for the next signal.
- These processes are essential for nerve and muscle cell function.
Tissue Types
- Epithelial tissue: Lines organs and forms protective barriers; can be simple or stratified, and of various shapes.
- Connective tissue: Supports and connects body structures (e.g., bone, cartilage, blood).
- Muscle tissue: Includes smooth (involuntary organs), skeletal (voluntary movement), and cardiac (heart, conducts electricity).
Key Terms & Definitions
- Plasma Membrane: Semi-permeable barrier controlling entry and exit of substances.
- Lysosome: Organelle with digestive enzymes for breaking down cell components.
- Mitochondria: Organelle producing ATP for energy.
- Cytoplasm: Gel-like substance holding organelles in place.
- Endocytosis/Exocytosis: Processes for intake (endo) and release (exo) of substances.
- Electrolyte: Ion conducting electricity in the body (cations: positive, anions: negative).
- Osmosis: Water movement toward higher solute concentration.
- Resting Membrane Potential: Electrical charge difference across the membrane at rest.
- Action Potential: Rapid change in membrane potential for signaling.
- Apoptosis: Programmed cell death.
Action Items / Next Steps
- Review textbook diagrams and descriptions of the plasma membrane, organelles, and transport mechanisms.
- Read the assigned chapter on cellular biology for more detail.
- Be prepared to discuss cell communication and membrane transport in the next class.