L3 - Control of the Cellular Environment

Cell Membranes and Transport

The lecture discusses how cells interact with their environment, focusing on the plasma membrane and transport mechanisms.

The Plasma Membrane

• The plasma membrane encases the cell, providing a barrier between the intracellular and extracellular environments.
• It's a phospholipid bilayer consisting of two leaflets of phospholipids.
• Each phospholipid has a polar head group and two hydrophobic fatty acid tails.
• The polar head groups interact with the aqueous solutions inside and outside the cell.
• The hydrophobic tails form a barrier to the movement of charged molecules.
• Electron microscopy reveals two distinct layers representing the cell membranes of adjacent cells.
• The phospholipid bilayer is:
  • Impermeable to most essential molecules.
  • Permeable to water to a certain extent, although it restricts free diffusion.
  • Permeable to small, uncharged molecules like oxygen (O2) and carbon dioxide (CO2).

Impermeability to Specific Molecules

The lipid bilayer is impermeable to:

• Charged molecules (ions): potassium (K^+), sodium (Na^+), calcium (Ca^{2+}), chloride (Cl^−), and bicarbonate (HCO_3^−).
• Small water-soluble molecules: glucose, nucleotides, other sugars, and amino acids.
• Large molecules: proteins, RNA, and intracellular organelles.

Because these molecules must cross the membrane under various circumstances, there exist mechanisms to facilitate their transport.

Diffusion

• Diffusion is the random motion of molecules leading to an even distribution in the available space.
• Diffusion eliminates concentration gradients.

Example

• A droplet of ink in water demonstrates diffusion, eventually resulting in a uniform color.

Osmosis

• Osmosis is the movement of water to equilibrate its concentration across a semipermeable membrane.
• The higher the solute concentration (e.g., sugar or salt) in a given volume, the lower the water concentration.
• Water flows from an area of high water concentration to an area of low water concentration.
• Osmotic pressure, or tonicity, refers to the water gradient between two compartments.

Tonicity

• Isotonic: Solutions with the same solute and water concentrations.
• Hypotonic: A solution with a lower solute concentration.
• Hypertonic: A solution with a higher solute concentration.

Example

• Solution A: 1M glucose.
• Solution B: 1M lactose.
• Solution C: 0.1M glucose.
• A and B are isotonic.
• C is hypotonic compared to A and B.
  • A and B are hypertonic compared to C.

Semi-Permeable Membranes

• If a membrane is impermeable to a solute (e.g., sucrose), no water movement occurs.
• If a membrane is semi-permeable (permeable to water), water moves from an area of lower solute concentration to an area of higher solute concentration to equalize water concentration.
• Unlike fixed semi-permeable membranes, cell membranes are flexible and can expand or contract, allowing the cell to adjust to changes in osmolarity.

Real-World Examples

• Red blood cells maintain their shape under isotonic conditions (300 milliosmols).
• Under hypotonic conditions, water floods into the cells, causing them to burst (hemolysis), leaving behind "red blood cell ghosts".
• Under hypertonic conditions (500 milliosmols), water leaves the cells, causing them to shrink.
• Cells function optimally when their osmolarity is physiologically regulated at equilibrium.

Ion Movement Across Cell Membranes

• Ions move across cell membranes through membrane proteins.
• These proteins include transmembrane channels allowing selective passage of particular molecules or ions.

Ion Channel Selectivity

• The pores of ion channels differ in size and charge distribution.
• Sodium channels are narrower and allow slower movement of sodium ions compared to potassium channels.
• Energy plots show the energy required for ions to traverse the channel, reflecting interactions between the ions and the channel.

Types of Membrane Proteins

• Channels: Facilitate diffusion.
• Transporters:
  • Facilitated diffusion transporters.
  • Active transporters.

Channels vs. Transporters

• Channels:
  • Allow ions to flow down a concentration gradient, very quickly.
• Transporters:
  • Transport one molecule at a time.
  • Involve conformational changes in the protein.

Facilitated Diffusion Transporters

• Molecules move down their concentration gradient (high to low).

Active Transporters

• Use energy, such as ATP hydrolysis, to move solutes against their concentration gradients (low to high).
• Important for maintaining specific ion and nutrient concentrations.

Aquaporins

• Proteins that facilitate the diffusion of water across cell membranes.
• Highly expressed in organs like the kidney and gut, where large volumes of water need to be transported.
• Peter Agre was awarded the Nobel Prize in Chemistry 2003, for discovering their structure utilizing X-ray crystallography.

Types of Ion Movements Across Membranes

• Facilitated diffusion: Ions move through a channel until equilibrium is reached.
• Active transport: ATP hydrolysis drives the movement of molecules against their concentration gradient until all molecules are transported, or another factor influences the equilibrium.

Saturation Kinetics

• The rate of movement through a channel is not always directly proportional to the concentration gradient.
• The interaction between the molecule and the protein channel/transporter influences the rate.
• Saturation occurs due to the affinity of the molecule for the transporter or channel.
• High affinity for a molecule for a transporter may be 100 millimolar. So at 50 to 100 millimolar, Your rate is actually good, but if you're going to 200 millimolar there maybe some drop off, but it will evenatually get there.

Clinical Implications

• In conditions like diabetes, excess glucose is not efficiently transported, leading to prolonged elevated concentrations and detrimental consequences.

The Role of Charge in Ion Movement

• Ion channels conduct charge, which sets up an electrical potential difference across the membrane.
• The electrical potential influences the movement of ions.
• Example: Adding potassium ions to one side of a membrane sets up a charge that repels further movement of the ions.

Patch Clamping

• Technique developed by Bert Sakmann and Erwin Neher. They were awarded the Nobel prize in Physiology or Medicine 1991.
• Uses a tiny electrode to measure the activity of a single ion channel on the cell surface.
• Allows detailed study of ion channel function by measuring the current moving through open and closed states of the channel.
  • A typical cell is 10 microns, with a million ion channels.

Ion Concentrations and Membrane Potential

• Ion concentrations differ significantly between the inside and outside of the cell.
  • Blood plasma: sodium 140 millimolar, potassium 4.6 millimolar, chloride 106 millimolar
  • Extracellular: sodium 1400 millimolar, potassium 5 millimolar
  • Inside cell: sodium 10mm, potassium 150mM.

Sodium-Potassium ATPase

• The sodium-potassium ATPase (sodium pump) actively transports ions against their concentration gradients.
• It expels three sodium ions and uptakes two potassium ions using ATP hydrolysis.
• This sets up concentration gradients across the membrane.
• The unequal movement of charge (3 Na^+ out, 2 K^+ in) leads to a net negative membrane potential.
• Resting membrane potential is typically -60 to -70 millivolts.

Contributing Factors to Negative Membrane Potential

• Excess negative charge inside the cell.
• Presence of negatively charged nucleic acids (RNA).

Goldman-Hodgkin-Katz Equation

• Describes the electrochemical combination determining membrane potential.
• Vm = (RT/F) \cdot ln((PK[K^+]o + P{Na}[Na^+]o + P{Cl}[Cl^-]i) / (PK[K^+]i + P{Na}[Na^+]i + P{Cl}[Cl^-]_o))
  • Where V_m is the membrane potential, R is the gas constant, T is temperature, F is Faraday's constant, [] represents concentrations, P is permeability, o denotes outside the cell, and i denotes inside the cell.

Action Potential

• The action potential is the coordinated movement of ions through ion channels, enabling signal transmission in neurons.
• The Action potential starts usually at resting potential around -70mV.
• A small stimulus from a shift in the membrane potential must happen to reach a -55mV activation of sodium channels.
• Voltage-dependent sodium channels open, allowing sodium ions to flood into the cell.
• This causes the membrane potential to shift to approximately +30 millivolts as the system tries to reach its chemical equilibrium.
• Sodium channels close spontaneously after 1-2 milliseconds.
• Voltage-activated potassium channels open, allowing potassium ions to flood out of the cell.
• The membrane becomes repolarized, returning to the resting membrane potential.

Facilitative Transporters

• Glucose transporters facilitate the movement of glucose across the membrane.
• The sodium gradient can be harnessed by transporters to pull amino acids or sugars through, moving them against their concentration gradient.

Endocytosis and Exocytosis

• Processes for transporting large molecules across the membrane.

Endocytosis

• Molecules accumulate on the extracellular side and are engulfed in vesicles.

Exocytosis

• Vesicles containing molecules fuse with the membrane and release their contents.

Clathrin-Mediated Endocytosis

• Clathrin proteins form a structure on the cell surface.
• Receptors on the vesicle attract target molecules.
• The vesicle buds off, encapsulating the molecules.
• Important for nutrient uptake, signal transduction, and synaptic vesicle recycling.

Phagocytosis

• A cell engulfs larger particles or even another cell.