Transport Across Cell Membranes

Principles of Transmembrane Transport

• Critical Necessity of Transport: The movement of nutrients and wastes across cell membranes is a fundamental function required for life. Cells must strictly regulate their internal environment by maintaining very specific ion concentrations both inside and outside the cell.
• Membrane Permeability:     * Simple Diffusion: This process allows only very small, non-ionic molecules to cross the lipid bilayer on their own.     * Semi-permeable Membrane: A physical barrier that allows certain molecules or ions to pass through by diffusion.     * Selectively Permeable Membrane: Cell membranes contain a variety of specific transport proteins that allow precise control over which substances (solutes) can cross.
• Forms of Transport:     * Passive Transport:         * Requires no energy expenditure from the cell.         * Molecules move down their concentration gradients (from an area of high concentration to an area of low concentration).         * Types include simple diffusion, facilitated diffusion, and osmosis.     * Active Transport:         * Requires energy (often in the form of $ATP$) to pump molecules against their concentration gradient (from low concentration to high concentration).

Membrane Transport Proteins: Channels and Transporters

• Protein Specificity: Transport proteins are highly specific for the solute they move. The specific set of transport proteins present in a membrane determines which substances enter or leave, appearing as a characteristic unique to specific cell types or organelle types.
• Main Types of Transport Proteins for Facilitated Diffusion and Active Transport:     * Channels:         * These form physical pores that span the membrane.         * They selectively allow specifically sized and charged molecules to cross.         * Function exclusively for passive transport.     * Transporters:         * Solutes bind to specific binding sites on one side of the membrane.         * The protein undergoes a conformational change to transfer the molecule to the opposite side.         * Transporters can function in both passive and active transport.

Passive Transport: Glucose Transporter and Osmosis

• Passive Glucose Transporter:     * Found in liver cells and various other cell types.     * It possesses two major conformations: one exposing the binding site to the extracellular space and one exposing it to the cytosol.     * Directional flow is strictly concentration-dependent: if $[\text{glucose}]$ is high outside, it moves in; if $[\text{glucose}]$ is high inside, it moves out.
• Osmosis:     * The special form of diffusion specifically for water molecules across a membrane.     * Aquaporins: Specialized transport protein channels that allow water to diffuse rapidly across the membrane.     * Direction of Flow: Water moves from areas of high $\text{H}_2\text{O}$ concentration (low solute concentration/hypotonic) to areas of low $\text{H}_2\text{O}$ concentration (high solute concentration/hypertonic).     * Osmotic Pressure: This is the driving force of osmosis. It can cause cells to swell or shrink depending on the tonicity of the external environment.
• Cellular Control of Osmosis:     * Animal Cells: Utilize the sodium-potassium pump ($Na^+\text{-}K^+$ pump) to maintain proper osmotic balance.     * Plant Cells: Use a rigid cell wall to prevent bursting. As water enters, the cytoplasm expands against the wall, creating turgor pressure, which keeps the cell rigid.     * Protozoa: Freshwater protozoa utilize a contractile vacuole that collects incoming water and سپس contracts to force it out of the cell.

Passive Transport of Ions and the Electrochemical Gradient

• Uncharged Solutes: Move based solely on their chemical concentration gradient.
• Charged Solutes (Ions): Driven by two distinct forces:     1. Chemical Concentration Gradient: Based on the difference in solute concentration.     2. Membrane Potential: The difference in electrical charge across the membrane (measured in volts).
• Electrochemical Gradient: The combination of the chemical concentration gradient and the membrane potential. This combined force determines the direction of passive transport for ions.     * Gradients can work in the same direction (stronger pull) or opposite directions (weaker net driving force).

Active Transport Mechanisms and Gradients

• Biological Importance: Active transport is essential for maintaining intracellular vs. extracellular ionic composition, generating $ATP$ in aerobic respiration and photosynthesis, and maintaining low $pH$ in lysosomes.
• Three Main Mechanisms:     1. Coupled Transporters (Gradient-Driven Pumps): Link the uphill transport of one molecule to the downhill transport of another.     2. ATP-Driven Pumps: Couple uphill transport to the hydrolysis of $ATP$.     3. Light-Driven Pumps: Couple uphill transport to the input of light energy (e.g., bacteriorhodopsin).
• Harnessing Gradients: Cells exploit established gradients to do work. For example, pumping $H^+$ ions into the mitochondrial intermembrane space and harnessing the energy of their return flow to synthesize $ATP$.

The Sodium-Potassium Pump ($Na^+\text{-}K^+$ Pump)

• Function: Specifically pumps sodium ($Na^+$) out of the cell and potassium ($K^+$) into the cell by hydrolyzing $ATP$. It is an $Na^+\text{-}K^+$ ATPase.
• Energy Consumption: Approximately $30\%$ of a cell's total $ATP$ consumption is used by this pump.
• Concentration Gradients: The pump maintains $[Na^+]$ outside at levels $\approx 10\text{-}30 \times$ greater than inside, and $[K^+]$ inside at levels $\approx 10\text{-}30 \times$ greater than outside.
• The Reaction Cycle:     1. Three $Na^+$ ions bind to high-affinity sites on the cytosolic side of the pump.     2. The pump phosphorylates itself by hydrolyzing $ATP$.     3. Phosphorylation triggers a conformational change, ejecting $Na^+$ to the extracellular space.     4. Two $K^+$ ions bind from the extracellular side.     5. The pump dephosphorylates itself (loses the phosphate group).     6. The pump returns to its original conformation, ejecting $K^+$ into the cytosol, and the cycle repeats.

Calcium and Coupled Transporters

• Calcium ($Ca^{2+}$) Transport:     * Cells maintain extremely low intracellular $[Ca^{2+}]$.     * $Ca^{2+}$ acts as a signal; its entry into the cytosol activates proteins for processes like muscle fiber contraction and neurotransmitter release via vesicle fusion.     * $Ca^{2+}\text{ Pumps}$: Found in the plasma membrane and the sarcoplasmic reticulum (SR) / endoplasmic reticulum (ER). These use $ATP$ to pump $Ca^{2+}$ out of the cytosol or into the lumen of the SR.     * A specific mechanism involves the phosphorylation of an aspartic acid residue on the pump protein.
• Coupled Transporters Classification:     * Symport: Moves two different molecules in the same direction across the membrane.     * Antiport: Moves two different molecules in opposite directions.     * Uniport: Moves only one type of substance (not a coupled transporter).

Glucose Symport and Gut Epithelium

• Mechanism: The $Na^+\text{-driven glucose symport}$ uses the electrochemical gradient of $Na^+$ to "drag" glucose into the cell against its gradient.
• Cooperative Binding: Binding of both sodium and glucose is required for the transporter to undergo its conformational change.
• Intestinal Cells:     * Apical Surface (Facing Gut Lumen): Contains $Na^+\text{-driven glucose symports}$ to actively take up every possible glucose molecule from food.     * Lateral and Basal Surfaces: Contain passive glucose uniporters to allow glucose to leave the cell and enter the bloodstream down its concentration gradient.     * Tight Junctions: Prevent the different transporters from mixing between membrane domains.

Hydrogen Ion ($H^+$) Pumps

• Organelle Application: Plants, fungi, and bacteria use $H^+$ pumps instead of $Na^+$ pumps in their plasma membranes.
• Functions: Used for $ATP$ synthesis (mitochondria, chloroplasts), nutrient transport (bacteria), and maintaining acidic acidic environments ($pH$) in lysosomes and plant vacuoles.
• Types: Most are $ATPases$, but some are light-driven (bacteriorhodopsin) or fueled by high-energy electrons.

Ion Channels: Selectivity, Gating, and Potential

• Selectivity: Ion channels contain a selectivity filter lined with amino acid side chains. The pore narrows so that only ions of a specific size and charge can pass, usually one at a time.
• Gating Mechanisms: Channels switch between open and closed states based on stimuli:     * Voltage-gated: Respond to changes in membrane potential (charge across the membrane).     * Ligand-gated: Respond to the binding of a molecule (extracellular or intracellular ligand).     * Stress-gated (Mechanically-gated): Respond to physical mechanical force (e.g., hearing in the ear or the Venus flytrap).
• Membrane Potential: Measured as the voltage across a membrane. In animals, $K^+\text{ leak channels}$ allow $K^+$ to move out down its gradient, creating a voltage that eventually balances the concentration gradient, establishing the resting membrane potential.
• Patch-Clamp Technique: A method to study ion channels by using a glass micropipette to suck up a small piece of membrane and record electrical current (measured in $pA$) over time ($msec$).

Nerve Cell Signaling and Action Potentials

• Neuron Anatomy: Consists of a cell body, dendrites (receiving signals), an axon (conducting signals over distances up to $1\text{ m}$), and nerve terminals.
• Action Potential (Nerve Impulse):     1. Resting State: Membrane is polarized.     2. Depolarization: A stimulus causes voltage-gated $Na^+\text{ channels}$ to open. $Na^+$ rushes into the cell, making the interior more positive ($\approx +40\,mV$).     3. Inactivation: The $Na^+$ channels quickly become inactivated (a refractory period) and closed.     4. Repolarization: Voltage-gated $K^+\text{ channels}$ open, allowing $K^+$ to leave the cell, restoring the negative internal charge.     5. Restoration: The $Na^+\text{-}K^+$ pump restores the ion balance.
• Synaptic Transmission:     * When the action potential reaches the nerve terminal, voltage-gated $Ca^{2+}\text{ channels}$ open.     * The influx of $Ca^{2+}$ triggers the fusion of synaptic vesicles with the plasma membrane, releasing neurotransmitters (ligands) into the synaptic cleft.     * Neurotransmitters bind to transmitter-gated ion channels (ligand-gated) on the postsynaptic cell.     * This binding triggers a change in membrane potential in the next cell, which can be excitatory (promoting an action potential) or inhibitory (preventing one).