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
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