Chapter 3: Movement of Substance In and Out of Cells

Overview of Substance Movement in Cells

  • The movement of substances in and out of cells is controlled by the cell membrane, which functions as a selectively permeable barrier that regulates the exchange of materials between the internal and external environments.

  • Selective Permeability & Homeostasis: This property allows the cell to maintain a stable internal environment through specific transport processes, including:

    • Diffusion

    • Osmosis

    • Active transport

Structure of the Cell Membrane

  • General Composition: The cell membrane is very thin and is composed primarily of proteins, lipids, and carbohydrates.

  • Phospholipid Bilayer:

    • Lipids are arranged into thin outer layers known as a phospholipid bilayer.

    • Hydrophilic (water-loving) heads: Face outwards toward the external fluid and internal cytoplasm.

    • Hydrophobic (water-hating) tails: Face inwards toward each other, forming a semi-permeable barrier.

  • Membrane Proteins:

    • Protein components are found embedded inside or attached to the double layer of phospholipids.

    • Integral proteins: Embedded inside the phospholipid bilayer.

    • Peripheral proteins: Surface-level proteins located on the membrane boundary.

    • Functions: Function in molecular transport, cellular signaling, and providing structural support.

  • Carbohydrate Chains:

    • Carbohydrate chains are attached to surface proteins, forming glycoproteins (glocoptoteins).

    • Functions: Play a critical role in cell recognition and cellular communication.

  • Cholesterol Molecules:

    • Cholesterol molecules are scattered within the phospholipid bilayer in animal cells.

    • Functions: Maintain membrane fluidity and structural stability under varying temperature conditions.

Cell Membrane Illustration


Cell Membrane Structure Diagram

  • Phospholipid Bilayer: Arranged with outward-facing hydrophilic heads and inward-facing hydrophobic tails.

  • Integral Proteins: Span across or are deeply embedded within the bilayer.

  • Peripheral Proteins: Bound to the surface of the cell membrane.

  • Cholesterol: Distributed inside the hydrophobic core of the bilayer.

  • Glycoproteins / Carbohydrate Chains: Extending from membrane proteins outward into the external environment for recognition.

Properties of the Cell Membrane

  • Selective Permeability: The membrane allows certain substances to pass through while blocking others, directly helping the cell control its internal environment.

  • Flexibility and Fluidity: The membrane is soft and flexible, which lets the cell change shape, move, and repair itself without breaking.

  • Amphipathic Structure: The membrane is constructed from phospholipids with water-loving (hydrophilic) heads and water-facing/water-hating (hydrophobic) tails that self-arrange into a bi-layer, creating a stable chemical barrier.

  • Protein Content: Contains various functional proteins that assist in transporting molecules, receiving external signals, and providing structural support.

  • Polarity: The membrane possesses regions with different electrical charges (it is polar), which are important for cell signaling and electrical balance.

Processes That Move Substances In and Out of Cells

  • Diffusion:

    • Definition: The passive movement of particles (molecules or ions) from an area of high concentration to an area of lower concentration until they are evenly spread.

  • Simple Diffusion:

    • Definition: A form of diffusion where molecules pass directly through the phospholipid bilayer of the cell membrane without any protein assistance.

    • Types of Transported Substances: Typically carries small, non-polar molecules across the membrane, such as oxygen (O2O_2) and carbon dioxide (CO2CO_2).

  • Facilitated Diffusion:

    • Definition: Diffusion where molecules move down their concentration gradient but require help from specific membrane proteins (channel proteins or pores) because they are large, polar, or charged.

    • Examples: Glucose and charged ions.

Additional Factors Influencing Transport

  • Surface Area: Transport rates depend on the available surface area across which exchange occurs.

  • Distance: The physical distance over which substances must travel affects the duration of transport.

  • Size of Molecules: Larger molecules face greater physical constraints passing across membrane barriers.

  • Membrane Characteristics: The specific composition and thickness of the transport layer dictate permeability.