Cellular Transport: Passive and Active Mechanisms

Posted on Jan 3, 2025 in Biology

Transport Across Membranes

  • Plasma membranes separate the inner cellular environment (cytoplasm) from the extracellular environment (interstices- interstitial or extracellular space).
  • Plasma membranes act as barriers to most, but not all, molecules.
  • Plasma membranes are differentially permeable (semi-permeable).
  • Materials move into and out of cells through passive transport or active transport.

1. Passive Transport

  • Requires no energy from the cell.
  • Requires a concentration gradient: unequal concentrations of a substance inside and outside of the cell.
  • Passive Transport includes:
    • Simple Diffusion – e.g. O2 and CO2
    • Osmosis – e.g. H2O
    • Facilitated Diffusion – e.g. glucose

1.1. Simple Diffusion

  • Natural random movement of molecules across a concentration gradient.
  • Rate of Diffusion depends on:
    • Steepness of the concentration gradient
    • Temperature
    • Charge
    • Diameter of diffusing molecules
  • Diffusing molecules may reach homeostasis: a state of equilibrium when they are distributed equally throughout the system.

1.2. Osmosis

  • Special type of diffusion.
  • Refers to the diffusion of water through the cell membrane across a concentration gradient.
  • Osmosis is an important process in all living organisms because water is an essential component of, and with many functions in, all cells.
  • Although water is a polar molecule, it is able to pass through the lipid bilayer of the plasma membrane.
  • Water passes by diffusion from a region of higher concentration of water to a region of lower concentration of water.
  • Water is never transported actively; that is, it never moves against its concentration gradient.
  • However, the movement of water in and out of the cell can be controlled the concentration gradient of water can be altered by active transport of solutes.
  • Examples are:
    • Movement of water into roots.
    • Food preservation (e.g. salt draws water out of meat or fish).
    • Filtering urine through kidneys.
    • Movement of water across intestine walls.
    • Dialysis / kidney machine.

1.3. Facilitated Diffusion

  • Highly selective transmembrane proteins allow specific molecules to cross the membrane.
  • These proteins facilitate diffusion down the concentration gradient.
  • Glucose enters most cells by facilitated diffusion through carrier transmembrane proteins.
  • Aquaporins:
    • Channel transmembrane proteins
    • Form hydrophilic channels
    • Greatly facilitate osmosis of water

2. Active Transport

  • Requires energy and includes:
    • 2.1. Carrier Assisted Transport: the movement of molecules against a concentration gradient.
    • 2.2. Vesicle Mediated Transport: the movement of special “packages”, which contain large molecules, into or out of the cell.

2.1. Carrier Assisted Transport

  • Sodium-potassium pump in nerve cells.
  • Inside the cell: Na+ is maintained at low concentrations and K+ is maintained at high concentrations.
  • Outside the cell: Na+ is maintained at high concentrations and K+ is maintained at low concentrations.
  • When a nerve message is propagated, the Na+ and K+ ions pass across the membrane.
  • After the message has passed, the ions must be actively transported back to their “original starting position” across the membrane.

2.2. Vesicle-Mediated Transport

  • Vesicle or vacuole “packages” fuse with the cell membrane to transport chemicals into the cell (endocytosis) or out of the cell (exocytosis).
  • Used to carry molecules that cannot cross the membrane individually.
  • Both endocytosis and exocytosis require energy from the cell.

2.2.1. Endocytosis

  • (Endo – means to bring in)
  • Requires energy.
  • Plasma membrane:
    • Surrounds molecules
    • Forms a pocket
    • Becomes a vacuole in the cytoplasm
  • Three forms of endocytosis:
    • Pinocytosis (pino = drink) for bringing in liquids.
    • Phagocytosis (phago = eat) when solids brought in.
    • Receptor-mediated endocytosis.

2.2.2. Exocytosis

  • (Exo – means to take out)
  • Requires energy.
  • A membrane-bound vesicle or vacuole:
    • Containing large molecules
    • Joins with the cell’s plasma membrane
    • Makes an opening
    • Squeezes the molecules out into interstitial space

Common Characteristics of All Cells

  • Genetic material
    • Located in a specific nuclear region.
    • Responsible for maintaining cell & reproduction of new cells.
  • Plasma (aka Cell) membrane
    • Envelope that separates and protects the cell from its environment.
    • Acts as a selective barrier for import and export of material.
  • Cytoplasm
    • Rest of material of cell within the plasma membrane, excluding the nuclear region.
    • Consists of: cytosol (a fluid portion), organelles and inclusions (suspended chemical substances).
  • Ribosomes
    • Organelles on which protein synthesis takes place.