Biology Chapter: Diffusion Concepts

MPharm Programme - Cell Science - Cellular Process 1 & 2

• This module covers detailed transport processes in cells.
• Students will understand passive and active transport processes, transport in vesicles, and the movement of substances across membranes.
• Important topics include how different transport processes apply to the nervous system and neurotransmission.

Terminology: Body Fluid Pools

• Intracellular fluid (ICF) makes up 2/3 of total body fluid.
• Located within cells, it's the fluid inside of cells.
• Extracellular fluid (ECF) fills the spaces between cells.
• Interstitial fluid is ECF found between cells.
• Plasma is ECF within blood vessels.
• Lymphatic fluid (lymph) is ECF found in lymphatic vessels.
• Cerebrospinal fluid is ECF found in the brain and spinal cord.

Terminology: Solutions

• A solvent is the liquid that does the dissolving, typically water.
• A solute is the substance dissolved in the solvent (particles or gas).
• Concentration is the amount of solute in a given amount of solvent.
• A concentration gradient is the difference in concentration between two areas of a solution.

Molecule Movement Across Membranes

• Selective permeability is the property of cells that controls which substances can pass through the plasma membrane.
• The difference in chemical concentration from one place to another (inside to outside) is a concentration gradient.
• A difference in electrical charges constitutes an electrical gradient.
• The combined influence of concentration and electrical gradients is called an electrochemical gradient.

Molecule Movement Across Membranes (Gradients)

• An electrical gradient across the plasma membrane is termed the membrane potential.
• Concentration and electrical gradients help move substances across the plasma membrane.
• The combined influence of these gradients on the movement of ions is known as the electrochemical gradient.

Molecule Movement Across Membranes

• Transport processes include passive transport (passive processes), active transport (active processes), endocytosis (phagocytosis, fluid endocytosis (pinocytosis), receptor-mediated endocytosis), and exocytosis.

Types of Passive Transport

• Simple diffusion
• Facilitated diffusion
• Osmosis

The Principle of Diffusion

• Diffusion is the random mixing of particles due to their kinetic energy.
• Diffusion involves the movement of both solutes and solvents.

Factors Influencing Diffusion

• Steepness of the concentration gradient (higher gradient, faster diffusion).
• Temperature (higher temperature, faster diffusion).
• Mass of the diffusing substance (larger mass, slower diffusion).
• Surface area (larger surface area, faster diffusion).
• Diffusion distance (longer distance, slower diffusion).

Simple Diffusion

• No energy is required for simple diffusion.
• It moves from a high to a low concentration gradient, equalizing the gradient.
• Lipid solubility is a deciding factor in determining whether or not certain substances can pass through by simple diffusion.

Facilitated Diffusion

• Solutes that are too polar or highly charged need carriers to cross the plasma membrane by facilitated diffusion, a passive process.
• It requires a carrier in the membrane but not ATP.
• Solutes move down their concentration gradient.
• Facilitated diffusion can occur through ion channels (protein transporters) or carriers (protein transporters).
• Channels are gated (ligand-gated, voltage-gated, mechanically-gated), to regulate movement of ions.
• Channel-mediated facilitated diffusion of potassium ions (K⁺) occurs through a gated K⁺ channel

Facilitated Diffusion (Carriers)

• A carrier (transporter) moves a solute down its concentration gradient.
• Solute binds more often to the carrier on the higher concentration side of the membrane.
• Carriers can become saturated, leading to a maximum transport rate.
• Substances transported via carrier mediated facilitated diffusion include glucose, fructose, galactose, and some vitamins.

Osmosis

• Osmosis is a special form of diffusion concerning water movement across a semi-permeable membrane.
• Water is permeable to the membrane.
• Solutes are impermeable, resulting in water moving to balance solute concentration.
• Water moves from higher to lower concentration areas by moving through the lipid bilayer via simple diffusion or through aquaporin channels (integral membrane proteins).

Osmolarity

• Osmolarity describes the total solute concentration of a solution.
• One osmol equals one mole of solute particles.
• A liter of solution containing one mole of glucose and one mole of sodium chloride has an osmolarity of 3 Osm.
• Higher osmolarity means lower water concentration.

Tonicity

• Tonicity describes a solution's ability to change the shape or tone of cells by affecting their internal water volume.
• Isotonic solutions have the same solute concentration as inside cells, water moves equally in and out.
• Hypertonic solutions have a higher solute concentration than inside cells, leading to water moving out and cell shrinkage.
• Hypotonic solutions have a lower solute concentration than inside cells, leading to water moving in and cell swelling.

Active Transport

• Active transport uses energy (ATP) to move molecules against their concentration gradient.
• Often referred to as pumps, these transporters are able to be saturated.
• Active transport utilizes two primary energy sources: direct ATP use in primary active transport, and electrochemical gradient use in secondary active transport.

Primary Active Transport

• The Na⁺/K⁺-ATPase transporter is necessary for maintaining cellular membrane potential.
• Major primary active transporters in cells include Ca²⁺-ATPase, H⁺-ATPase, and H⁺/K⁺-ATPase.

Secondary Active Transport

• Secondary active transport uses an electrochemical gradient generated by primary active transport—like Na⁺ gradients—to drive the movement of other substances against their concentration gradient.
• There are two binding sites. One for an ion (e.g., Na⁺) and another for the cotransported molecule (e.g., glucose)

Vesicular Transport - Endocytosis

• Endocytosis involves the movement of macromolecules into a cell via vesicles.
• It involves wrapping extracellular materials within the plasma membrane and forming vesicles that travel into the cell.
• Endocytosis is an active process requiring energy.

Vesicular Transport - Exocytosis

• Exocytosis is the movement of macromolecules out of a cell via vesicles.
• Molecules exit the cell after fusing with the plasma membrane.

Types of Endocytosis

• Receptor-mediated endocytosis; a highly selective process, vesicles form after a receptor protein recognizes and binds to a specific extracellular particle.
• Pinocytosis ("cell drinking"); uptake of fluids and dissolved substances.
• Phagocytosis ("cell eating"); uptake of large particles like bacteria or food.

Exocytosis

• Molecules are transported out of the cell by vesicles.
• This process is necessary for replacing membrane components lost via endocytosis, adding new membrane components, and secreting membrane-impermeable substances (hormones, protein hormones).

Exocytosis (process), Exocytosis, Exocytosis with the plasma membrane (Process)

• Exocytosis involves vesicles fusing with the plasma membrane to release contents.
• This process is crucial for nerve cells to release neurotransmitters.

Exocytosis and Nervous System Function

• Nerve cells communicate using neurotransmitters released by exocytosis at synaptic terminals.

Neurotransmission Process

• Action potentials trigger neurotransmitter release.
• Neurotransmitters bind to receptors and initiate a new impulse in the postsynaptic neuron.

Transcytosis

• Used to move substances into, across, and out of cells.
• Vesicles undergo endocytosis.
• The vesicle transports across the cell and then undergoes exocytosis on the opposite side, thereby efficiently moving a substance across the cell.
• This process is frequently seen in moving materials across endothelial cells that line blood vessels.
• Particularly important in fetal circulation.

Transport Summary

• There are various transport processes (passive and active).
• Passive processes include diffusion, simple diffusion, facilitated diffusion, and osmosis.
• Active transport involves ATP use to move substances against the concentration gradient.
• Different types of vesicles are used in transport processes.

Further Reading

• Students should consult provided textbooks for further details on the topics covered in this module.

Test your understanding of diffusion concepts in biology with this quiz. Explore how temperature affects diffusion rates and the factors influencing facilitated and simple diffusion. Additionally, assess your knowledge about ion channels.