Membrane Transport Notes

Membrane Permeability and Selectivity

• Cells regulate their internal composition by controlling the passage of materials across membranes.
• Membrane transport is regulated because membranes are both semi-permeable and selective.
  • Semi-permeability: The non-polar lipid bilayer restricts movement of hydrophilic molecules and acts as a barrier to large polar or charged substances (like ions).
  • Selectivity: Integral membrane proteins enable transport of hydrophilic molecules and can adopt ‘open’ or ‘closed’ conformations to regulate molecular transport.
• Implications: Transport control affects nutrient uptake, waste removal, and overall cellular homeostasis.

Deterrents to Cross-M bilayer and Transport Proteins

• Non-polar, lipophilic molecules freely cross the bilayer (e.g., O$2$, CO$2$, steroids).
• Very small polar molecules can freely cross (e.g., water, urea).
• Large polar molecules cannot freely cross.
• Ions and other charged molecules cannot freely cross the bilayer.
• Examples by category:
  • Non-polar: O$2$, CO$2$, steroids
  • Small polar: H$_2$O, urea
  • Large polar: glucose
  • Charged ions, ATP (and other charged molecules)
• Summary hint: Large or charged = not easily transported by the bilayer alone.

Types of Transport Across Membranes

• Passive Transport
  • Movement along a concentration gradient (no ATP energy required).
  • Does not require ATP hydrolysis; relies on gradient and membrane properties.
  • Includes simple diffusion and facilitated diffusion (via proteins).
• Active Transport
  • Movement against a concentration gradient (low to high).
  • Requires energy (ATP hydrolysis).
  • Requires transport proteins (pumps) to move substances across membranes.

Passive Transport, Diffusion, and Osmosis

• There are three main passive transport mechanisms:
  • Simple Diffusion: Small or lipophilic molecules freely cross the membrane (e.g., O$2$, CO$2$, steroid hormones).
  • Facilitated Diffusion: Large polar or charged molecules require membrane proteins to cross (e.g., ions, glucose).
  • Osmosis: Water movement is determined by relative solute concentrations inside and outside the cell.
• Observations: Passive transport does not use ATP; transport relies on concentration gradients and membrane properties.

Simple Diffusion Across Membranes

• Simple diffusion is the net movement of molecules from higher concentration to lower concentration until equilibrium is reached (along a concentration gradient).
• Key concept: No energy input, molecules move down their gradient until equilibrium.

Channel vs Carrier Proteins in Facilitated Diffusion

• Some substances cannot freely cross membranes (ions, large polar macromolecules); they rely on proteins for transport.
• Channel Proteins:
  • Have hydrophilic internal pores that allow ion movement.
  • May be gated with a selectivity filter to regulate transport.
• Carrier Proteins:
  • Undergo conformational changes to translocate material across the membrane.
• Examples: Ions typically use channels; glucose uses carrier proteins.

Water Movement Across Membranes: Osmosis

• Osmosis is the net movement of free water molecules across a semi-permeable membrane from a region of lower solute concentration to higher solute concentration.
• Conceptual directions:
  • Low solute levels (high free water) to high solute levels (low free water).
• Important interplay: Solute concentration gradients drive water movement, not just membrane permeability.

Osmosis in Solutions: Hypertonic, Hypotonic, Isotonic

• Solute concentration categories:
  • Hypertonic: Higher solute concentration outside the cell; water tends to move out.
  • Hypotonic: Lower solute concentration outside the cell; water tends to move in.
  • Isotonic: Equal solute concentrations inside and outside; water movement is balanced.
• Osmotic outcomes (before and after osmosis):
  • Hypertonic solution → water loss from the cell (cell shrinks).
  • Hypotonic solution → water gain by the cell (cell swells, may Lyse).
  • Isotonic solution → stable water content.
• Visual cues: Hypertonic → crenation tendency in animal cells; hypotonic → swelling; isotonic → normal shape.

Effects of Water Movement in Cells: Animal vs Plant Cells

• Cells without cell walls (animal cells):
  • Hypertonic: crenation (membrane wrinkling due to water loss).
  • Hypotonic: lysis (cell bursts due to excess water).
  • Isotonic: normal cell shape.
• Cells with cell walls (plant, fungi, bacteria):
  • Hypertonic: plasmolysis (membrane pulls away from cell wall).
  • Hypotonic: turgor (membrane pushes against the rigid cell wall, maintaining rigidity).
• Key terms: crenation, lysis, plasmolysis, turgor.

Osmosis in Erythrocytes and Onion Cells (Practical Comparisons)

• Animal erythrocytes in hypertonic solutions may crenate; in hypotonic solutions may swell or lyse.
• Onion cells (plant cells) show plasmolysis in hypertonic solutions and turgor in hypotonic solutions.
• Practical takeaway: Animal cells lack rigid walls; plant cells rely on cell wall to resist osmotic pressure.

Medical Isotonicity and Applications

• Isotonicity is essential for cell survival; surrounding fluids must be isotonic to maintain cell integrity.
• Natural and artificial strategies to maintain isotonicity:
  • Living adaptations: contractile vacuoles in unicellular organisms regulate water.
  • Multicellular organisms: aquaporins regulate water intake and release.
  • Medical applications: isotonic solutions for procedures and organ transplant; intravenous isotonic fluids restore fluid balance.

Contractile Vacuoles and Water Regulation in Unicellular Organisms

• Contractile vacuoles expel excess water to maintain osmotic balance:
  • Water enters the vacuole by osmosis and fills it.
  • The vacuole fuses with the plasma membrane and contracts, expelling water (diastole and systole).
• This mechanism helps single-celled organisms survive hypo- or hyperosmotic conditions.

Aquaporins: Water Channels in Membranes

• Aquaporins are integral membrane proteins that act as selective water channels.
• They enable faster water transport in response to changes in solute concentration.
• Expression levels of aquaporins can be regulated by gene expression to maintain osmotic homeostasis in multicellular organisms.
• Visual concept: Aquaporin-facilitated water transport accelerates osmosis beyond simple diffusion.

Active Transport and Pumps

• Active Transport moves substances against their concentration gradient (low to high).
• Energy source: ATP hydrolysis powers pump activity.
• Mechanism:
  • A molecule binds to a specific protein pump.
  • ATP is hydrolyzed, causing a conformational change in the pump.
  • The molecule is translocated across the membrane.
  • The pump returns to its original conformation, ready for another cycle.
• Concept: Pumps enable accumulation of nutrients and maintenance of membrane potential in cells.

Vesicular Transport (Bulk Transport)

• When the bilayer cannot accommodate material, membranes can break and reform to enclose material in vesicles.
• Endocytosis: Internalisation of material into the cell via vesicle formation.
• Exocytosis: Export of material packaged into vesicles by the Golgi complex.
• Summary: Vesicular transport allows large particles, fluids, or bulk materials to cross membranes.

Topic Connections and Cross-References

• Translocation (AHL): Water potential changes impact translocation rates in plants (D2.3.11).
• Systems Integration: Neurons use active transport to establish membrane potentials (C3.1.8).
• Water Properties: Osmosis is a consequence of the solvent properties of water (D2.3.1).
• Excretory System (AHL): Osmoregulation in the kidneys involves aquaporins (D3.3.10).
• Real-world relevance: Understanding isotonic solutions is essential for medical procedures, fluid therapy, and organ transplantation.

Summary of Key Concepts

• Semi-permeability vs selective transport: lipid bilayer blocks most hydrophilic/charged substances, while transport proteins enable controlled passage.
• Types of transport: passive (along gradient) vs active (against gradient, energy-dependent).
• Diffusion, facilitated diffusion, and osmosis as main passive mechanisms; channel vs carrier proteins define facilitated diffusion.
• Osmosis drives water movement and is governed by solute concentrations; hypertonic, hypotonic, isotonic states determine cell volume changes.
• Cell-wall presence changes osmotic outcome: plasmolysis vs turgor.
• Isotonicity is crucial for health; aquaporins and contractile vacuoles illustrate biological solutions to osmoregulation.
• Vesicular transport enables bulk movement of materials; the Golgi apparatus plays a central role in exocytosis.
• Interconnectedness of membrane transport with physiology, plant biology, excretory systems, and clinical applications.

Quick Reference: Key Terms

• Semi-permeable membrane
• Selectivity; transport proteins; open/closed conformations
• Simple diffusion; Facilitated diffusion; Channel proteins; Carrier proteins
• Osmosis; Hypertonic; Hypotonic; Isotonic
• Crenation; Lysis; Plasmolysis; Turgor
• Aquaporins; Isotonic solutions
• Contractile vacuoles; Diastole; Systole
• Endocytosis; Exocytosis; Golgi complex