Cell transport

Cell Transport

Overview

  • Cell transport refers to the mechanisms through which substances move across the cell membrane and enter or exit the cell.

Concentration Gradient

  • Definition: The difference in the concentration of a chemical between one side of the plasma membrane and the other side is referred to as a concentration gradient.

  • Electrical Gradient: Represents the difference in electrical charge across the membrane, affecting how substances move.

Movement of Substances Across Plasma Membrane

Types of Membranes

  • Structure: Composed mainly of a phospholipid bilayer, proteins, and carbohydrates.

Solutions' Effects on Cells

  • Hypotonic Solutions: Lower concentration outside the cell; can cause cells to swell.

  • Hypertonic Solutions: Higher concentration outside the cell; can cause cells to shrink.

  • Isotonic Solutions: Equal concentration; no net movement of water.

Transport Mechanisms

  1. Passive Transport: Movement across the membrane without energy input.

    • Simple Diffusion: Movement of molecules from high to low concentration directly through the membrane.

    • Facilitated Diffusion: Uses carrier proteins to assist in the movement of certain molecules.

    • Osmosis: Specifically refers to the diffusion of water across a semipermeable membrane.

  2. Active Transport: Requires energy (ATP) to move substances against their concentration gradient.

    • Movement Against Concentration Gradient

    • Expenditure of Energy: Cells actively transport substances requiring ATP.

Cell Membrane Structure

  • Functions:

    1. Controls what enters and exits the cell, maintaining homeostasis.

    2. Provides protection and support for the cell.

Key Components of the Cell Membrane

  1. Lipid Bilayer: Two layers of phospholipids.

    • Phosphate Head: Polar (hydrophilic or water-loving).

    • Fatty Acid Tails: Non-polar (hydrophobic or water-fearing).

  2. Proteins: Embedded in the membrane to assist with transport and send signals.

Characteristics of the Cell Membrane

  • Pores: Membranes contain pores that contribute to selective permeability, allowing some molecules to cross while restricting others.

    • Selectively Permeable: Some molecules pass while others are blocked.

    • Factors Affecting Permeability:

      • Size (macro or micro)

      • Charge (+ or -)

      • Solubility (lipid-soluble or not)

  • Small, non-charged, lipid-soluble molecules can easily cross cell membranes.

Types of Cellular Transport

  • Passive Transport: Does not require energy; molecules move from high to low concentration.

    1. Diffusion: Movement from high to low concentration directly.

    2. Facilitated Diffusion: Requires transport proteins to help larger or charged molecules move across the membrane.

    3. Osmosis: Molecules of water moving across the membrane.

  • Active Transport: Requires energy; used to move substances against their concentration gradients.

    • Bulk Transport: Larger quantities of materials transported into (endocytosis) or out of (exocytosis) the cell.