Cellular Level of Organization

Cellular Level of Organization

Introduction

  • Lecture Presentation by Chasity O’Malley, Palm Beach State College.

  • Notes prepared by Lori Garrett, Parkland College.

Plasma Membrane

  • Definition and Function:

    • Acts as a barrier separating the cytosol (the intracellular fluid) and extracellular fluid (ECF).

    • Coordinates cellular activity with the extracellular environment.

  • Permeability:

    • Determines which substances can cross the membrane:

    • Freely Permeable: Any substance can pass without difficulty.

    • Selectively Permeable: Only certain substances can cross. Plasma membranes are selectively permeable, allowing the passage of some materials while preventing others.

    • Impermeable: No substances can pass through. While specific cells may be impermeable to certain substances, no living cell has a completely impermeable membrane.

Membrane Transport

  • General Concept:

    • The plasma membrane acts both as a barrier and a gateway between the cytoplasm and ECF.

    • Selective Permeability: Allows some substances like nutrients and wastes to pass through while typically preventing others.

Methods of Transport

  • Classification:

    • Two overlapping classifications for moving substances in and out of a cell:

    • Passive vs. Active Transport

    • Carrier-mediated vs. Non-carrier mediated transport.

Passive Transport

  • Characteristics:

    • Does not require ATP.

    • Movement occurs down a concentration gradient.

    • Examples include:

    • Simple Diffusion: Movement of molecules from an area of high concentration to an area of low concentration.

    • Facilitated Diffusion: Uses membrane proteins for transport with no energy expenditure.

    • Osmosis: The diffusion of water across a selectively permeable membrane.

Active Transport

  • Characteristics:

    • Requires ATP to function.

    • Movement occurs against a concentration gradient (from low concentration to high concentration).

    • Examples include:

    • Active Transport: Direct use of ATP to move substances.

    • Vesicular Transport: The movement of materials via vesicles is an active process.

Carrier-mediated Transport

  • Mechanism:

    • Utilizes membrane proteins to transport substances across cell membranes.

    • Includes:

    • Facilitated Diffusion: Passive transport using a carrier protein.

    • Active Transport: Utilizes ATP to move substances against their concentration gradient.

Factors Influencing Diffusion Rates

  • Distance: Inversely related to diffusion rates (greater distance slows diffusion).

  • Molecule Size: Inversely related (smaller molecules diffuse faster).

  • Temperature: Directly related (higher temperatures increase molecular movement).

  • Gradient Size: Directly related; a larger difference in concentration accelerates diffusion.

  • Electrical Forces: Affects the movement of ions (attraction/repulsion due to charge).

Diffusion

  • Definition:

    • Continuous random movement of ions or molecules in a liquid or gas leading to even distribution.

  • Concentration Gradient:

    • The difference in concentration across a space; diffusion occurs until dynamic equilibrium is achieved, where molecular motion continues but net movement ceases.

Example of Diffusion in Water

  • Demonstration:

    • A colored sugar cube is placed in water, establishing a steep concentration gradient.

    • As time elapses, sugar and dye molecules spread through the solution, achieving an even distribution.

Key Points on Substance Movement Across the Plasma Membrane

  • Large molecules that can't pass through the membrane channels must be transported via a carrier mechanism.

  • Lipids, lipid-soluble molecules, and soluble gases (e.g., O2 and CO2) can diffuse across the lipid bilayer. Small water-soluble molecules and ions pass through membrane channels.

Osmosis

  • Definition:

    • Net diffusion of water across a selectively permeable membrane, balancing solute concentrations.

  • Osmotic Pressure:

    • Indicates the force of pure water moving into a solution with a higher solute concentration.

  • Hydrostatic Pressure:

    • The fluid force; can estimate osmotic pressure when applied to halt osmotic flow.

Tonicity

  • Definition:

    • Describes the effect of osmotic solutions on cell volume. Three possible effects include:

    • Isotonic: No net osmotic flow across the membrane (equal concentration).

    • Hypotonic: Causes osmotic flow into the cell (can lead to hemolysis in red blood cells - cell bursting due to large influx of water).

    • Hypertonic: Causes osmotic flow out of the cell (can lead to crenation in red blood cells - cell shrinking as water leaves).

Importance of Tonicity

  • Relevant in clinical settings, such as administering fluids to patients.

  • Normal saline solution: 0.9% NaCl (isotonic with blood).

Carrier-mediated Transport

  • General Mechanism:

    • Hydrophilic or large molecules must be transported via carrier proteins.

    • Types include:

    • Facilitated Diffusion: Passive transport requiring no energy, but limited by available carrier proteins.

    • Active Transport: Requires ATP, independent of the concentration gradient. Example: sodium-potassium pump.

Vesicular Transport

  • Definition and Mechanisms:

    • Materials move across cell membranes in small membranous sacs (vesicles).

    • Two major types:

      • Endocytosis: Intake into the cell using endosomes.

      • Exocytosis: Discharge of materials into the ECF.

Types of Endocytosis

  • Pinocytosis: Known as “cell drinking,” involves the ingestion of liquid.

  • Phagocytosis: Known as “cell eating,” this process is performed by phagocytes/macrophages.

Receptor-mediated Endocytosis

  • Involves materials in the ECF binding to specific receptors on the membrane surface, forming endosomes, which then fuse with lysosomes for processing.

Phagocytosis Mechanism

  • Starts with pseudopodia surrounding the object to form a phagosome. This fuses with lysosomes, and released nutrients are absorbed while residues get ejected through exocytosis.

Cell Life Cycle

  • Definition:

    • The process where a single cell divides to produce two daughter cells.

  • Types of Cell Division:

    • Mitosis: Produces 2 daughter cells, each with 46 chromosomes.

    • Meiosis: Produces 4 sex cells, each with 23 chromosomes.

Mitosis Overview

  • Pairs of daughter cells formed are half the original size, growing to match the original size before dividing and maintaining identical chromosome copies.

  • Ends with cytokinesis, followed by interphase.

Interphase Stages

  • G1 Phase: Normal functions, preparations for division, growth, and organelle duplication.

  • S Phase: DNA replication occurs, including synthesis of histones and proteins.

  • G2 Phase: Last-minute protein synthesis and centriole replication.

  • G0 Phase:** Non-dividing phase where certain cells may remain indefinitely (e.g., skeletal muscle cells).

DNA Replication Mechanics

  • Strands unwind, DNA polymerase binds to form new strands. Each strand grows in one direction and splices through ligases.

Mitosis Phases

  • Prophase: Chromosomes coil and centrioles move to poles.

  • Metaphase: Chromosomes align at the metaphase plate.

  • Anaphase: Chromatids pull apart, drawn along spindle apparatus.

  • Telophase: Cells prepare to re-enter interphase, nuclear membranes reform, and chromosomes uncoil. Cytokinesis begins with the cleavage furrow formation.

Conclusion

  • Understanding cellular transport mechanisms and the cell life cycle is essential for grasping basic biological concepts associated with living organisms.