Cell Membrane and Transport

Membrane Structure — The Fluid Mosaic Model

Part 1 of 7

The plasma membrane is far more than a passive barrier — it is a dynamic, selectively permeable structure that regulates the flow of materials into and out of the cell. Understanding membrane structure is essential for understanding transport mechanisms.

The modern model of membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972.

Phospholipid Bilayer

The foundation of every biological membrane is a phospholipid bilayer:

Each phospholipid has:

• A hydrophilic head (polar; contains a phosphate group linked to a glycerol backbone)
• Two hydrophobic fatty acid tails (nonpolar; hydrocarbon chains)

In an aqueous environment, phospholipids spontaneously arrange into a bilayer — hydrophilic heads face the water, hydrophobic tails face inward, away from water. This is driven by the hydrophobic effect (maximizing water entropy by minimizing the exposure of nonpolar surfaces to water).

Membrane fluidity is influenced by:

Why "fluid"? Phospholipids can move laterally within their leaflet (~10$^7$ times per second) but rarely flip-flop between leaflets (requires flippase enzymes). Membrane proteins also move laterally, as demonstrated by the Frye-Edidin experiment (1970) using fluorescent labels on human and mouse cells fused into heterokaryons.

Checkpoint — Phospholipid Bilayer

Membrane Proteins — The "Mosaic"

The "mosaic" in the fluid mosaic model refers to the diverse proteins embedded in or attached to the bilayer:

Six major functions of membrane proteins:

1. Transport — channels and carriers move specific molecules across the membrane
2. Enzymatic activity — enzymes catalyze reactions at the membrane surface
3. Signal transduction — receptors bind extracellular ligands and relay signals inside the cell
4. Cell-cell recognition — glycoproteins serve as identification tags (e.g., MHC, blood group antigens)
5. Intercellular joining — tight junctions, desmosomes, gap junctions connect cells
6. Attachment to cytoskeleton/ECM — anchoring proteins maintain cell shape

Asymmetry: The two leaflets of the membrane differ in lipid composition and protein orientation. Carbohydrates are found exclusively on the extracellular face, forming the glycocalyx.

Checkpoint — Membrane Proteins

Selective Permeability

The lipid bilayer is selectively permeable — it allows some substances to cross freely while restricting others:

Substances that cannot cross freely require transport proteins (channels or carriers) to cross the membrane. This selective control allows the cell to maintain an internal environment very different from the exterior.

Key Terms — Membrane Structure

Match the Membrane Component

Exit Ticket — Membrane Structure

Passive Transport — Moving Down the Gradient

Part 2 of 7

Passive transport moves substances down their concentration (or electrochemical) gradient — from high concentration to low concentration. It requires no energy input from the cell because the movement is driven by the second law of thermodynamics (systems tend toward higher entropy).

There are three main types of passive transport:

1. Simple diffusion
2. Facilitated diffusion (via channels or carriers)
3. Osmosis (water movement)

Simple Diffusion

Simple diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration due to random thermal motion, until equilibrium is reached.

Characteristics:

• No protein required
• Only small, nonpolar molecules (O$_2$, CO$_2$, N$_2$) and some small uncharged polar molecules (ethanol) can diffuse through the lipid bilayer
• Rate depends on: concentration gradient, temperature, membrane surface area, and membrane thickness

Equilibrium does NOT mean no movement — at equilibrium, molecules continue to move randomly in both directions, but the net movement is zero because the rates of movement in both directions are equal.

Facilitated Diffusion

Large polar molecules and ions cannot pass through the lipid bilayer by simple diffusion. They require membrane proteins to cross — this is facilitated diffusion.

Two types of transport proteins:

1. Channel Proteins:

• Form a hydrophilic pore through the membrane
• Very fast (up to 10$^8$ ions/second)
• Selective — each channel is specific to particular ions or molecules
• Gated channels can be regulated:
  • Voltage-gated — open/close in response to membrane potential changes (neurons)
  • Ligand-gated — open when a specific molecule binds (neurotransmitter receptors)
  • Mechanically-gated — open in response to physical stretching (touch receptors)
• Example: Aquaporins — channel proteins specific for water; greatly accelerate osmosis

2. Carrier Proteins:

• Bind the solute and undergo a conformational change to transport it across
• Slower than channels (100-1000 molecules/second)
• Specific to particular molecules
• Can be saturated (exhibit Michaelis-Menten kinetics)
• Example: GLUT transporters — facilitate glucose entry into cells

Checkpoint — Passive Transport

Key Concept: Electrochemical Gradient

For uncharged molecules, the concentration gradient alone determines the direction of passive transport. But for ions, two forces act simultaneously:

1. Chemical gradient — ions move from high to low concentration
2. Electrical gradient — ions are attracted to the opposite charge across the membrane

Together these form the electrochemical gradient. An ion might move against its concentration gradient if the electrical force is strong enough (or vice versa).

The membrane potential (typically -70 mV in animal cells, inside negative) means:

• Cations (like K$^+$) have an electrical force pulling them INTO the cell
• Anions (like Cl$^-$) have an electrical force pushing them OUT of the cell

AP Exam Tip: When asked about ion movement, always consider BOTH the concentration gradient AND the electrical gradient.

Key Terms — Passive Transport

Exit Ticket — Passive Transport

Osmosis — Water Follows the Solute

Part 3 of 7

Osmosis is the net movement of water across a selectively permeable membrane from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration).

Water moves through the membrane via:

• Slow diffusion directly through the lipid bilayer
• Rapid flow through aquaporins (water-specific channel proteins)

Osmosis is technically a special case of facilitated diffusion (when aquaporins are involved), but it is traditionally treated as a distinct transport category due to its biological importance.

Tonicity: Predicting Water Movement

Tonicity describes the effect of a surrounding solution on cell volume. It depends on the concentration of non-penetrating solutes (solutes that cannot cross the membrane):

Critical Distinction: Tonicity depends only on non-penetrating solutes. A solution with high urea (which freely crosses membranes) is technically hyperosmotic but effectively isotonic — urea equilibrates and no lasting water movement occurs.

Animal Cells vs. Plant Cells in Different Solutions

In hypotonic solution:

• Animal cell: Water enters, cell swells and may burst (lysis)
• Plant cell: Water enters, cell swells until the rigid cell wall exerts back-pressure (turgor pressure) that stops further water entry — the cell becomes turgid (this is the ideal state for plant cells)

In hypertonic solution:

• Animal cell: Water leaves, cell shrinks and wrinkles (crenation)
• Plant cell: Water leaves, the plasma membrane pulls away from the cell wall (plasmolysis) — this causes wilting

In isotonic solution:

• Animal cell: No net change — cells are maintained (this is why IV fluids are isotonic saline, 0.9% NaCl)
• Plant cell: Cell is flaccid (limp) — not ideal; plants need turgor pressure for structural support

Checkpoint — Osmosis and Tonicity

Water Potential (for AP Biology)

The AP Biology curriculum uses the concept of water potential ($\Psi$, psi) to quantitatively predict the direction of water movement:

$\Psi = \Psi_s + \Psi_p$

Where:

• $\Psi_s$ = solute potential (also called osmotic potential) — always $\leq 0$; more solute makes it more negative
• $\Psi_p$ = pressure potential — can be positive (turgor in plant cells), zero, or negative (tension in xylem)

Water moves from HIGH water potential to LOW water potential.

Solute potential formula:

$\Psi_s = -iCRT$

Where:

• $i$ = ionization constant (number of particles the solute dissociates into; NaCl = 2, glucose = 1)
• $C$ = molar concentration (mol/L)
• $R$ = ideal gas constant (0.0831 L bar/mol K)
• $T$ = temperature in Kelvin

Rules for water potential:

1. Pure water in an open container: $\Psi = 0$ (maximum water potential)
2. Adding solute: $\Psi_s$ becomes more negative, $\Psi$ decreases
3. Adding pressure: $\Psi_p$ increases, $\Psi$ increases
4. Water always moves from higher $\Psi$ to lower $\Psi$

Checkpoint — Water Potential

Key Terms — Osmosis

Exit Ticket — Osmosis

Active Transport — Moving Against the Gradient

Part 4 of 7

Active transport moves substances against their concentration (or electrochemical) gradient — from low to high concentration. This requires energy input, typically from ATP hydrolysis.

Active transport is essential for:

• Maintaining ion gradients across membranes
• Accumulating nutrients inside cells
• Removing waste products
• Generating electrical signals in neurons

Primary Active Transport: The Na+/K+ ATPase

The most important primary active transport protein in animal cells is the sodium-potassium pump (Na$^+$/K$^+$ ATPase):

For each ATP hydrolyzed, the pump moves:

• 3 Na$^+$ ions OUT of the cell
• 2 K$^+$ ions INTO the cell

This creates and maintains steep concentration gradients:

• High Na$^+$ outside, low Na$^+$ inside
• High K$^+$ inside, low K$^+$ outside
• Net export of positive charge → contributes to the negative resting membrane potential (-70 mV)

The pump cycle:

1. 3 Na$^+$ bind to cytoplasmic side of the pump
2. ATP is hydrolyzed; phosphate group is transferred to the pump (phosphorylation)
3. Conformational change exposes Na$^+$ to the extracellular side; Na$^+$ is released
4. 2 K$^+$ bind to the extracellular side
5. Dephosphorylation causes conformational change back
6. K$^+$ is released into the cytoplasm

Energy Cost: The Na$^+$/K$^+$ ATPase consumes approximately 25-30% of total cellular ATP in many animal cells. In neurons, this figure can reach 70%.

Other primary active transport pumps:

• Ca$^{2+}$ ATPase — pumps Ca$^{2+}$ out of the cytoplasm (into ER or extracellular space)
• H$^+$/K$^+$ ATPase — pumps H$^+$ into the stomach lumen (gastric acid secretion)
• H$^+$ ATPase (proton pump) — in plant cell membranes, creates proton gradients for secondary transport

Checkpoint — Primary Active Transport

Secondary Active Transport (Cotransport)

Secondary active transport uses the energy stored in an existing ion gradient (established by primary active transport) to drive the transport of another substance.

Two types:

1. Symport (cotransport):

• Both substances move in the same direction
• The ion moves DOWN its gradient, providing energy for the other substance to move AGAINST its gradient
• Example: SGLT1 — Na$^+$/glucose symporter in the intestinal epithelium uses the Na$^+$ gradient (established by Na$^+$/K$^+$ ATPase) to import glucose against its gradient

2. Antiport (exchange):

• Substances move in opposite directions
• Example: Na$^+$/H$^+$ exchanger — Na$^+$ enters (down its gradient), H$^+$ exits (against its gradient), helping regulate intracellular pH

Key Concept: Secondary active transport is "indirectly" active — it does not directly use ATP, but it depends on a gradient that was created by a primary active transporter that DID use ATP. The energy was stored in the gradient.

In plants: The H$^+$ ATPase (proton pump) in the plasma membrane creates a proton gradient. This gradient powers secondary active transport of sucrose, amino acids, and other nutrients into the cell via H$^+$/solute symporters.

Checkpoint — Secondary Active Transport

Key Terms — Active Transport

Exit Ticket — Active Transport

Bulk Transport — Vesicle-Mediated Movement

Part 5 of 7

Some materials are too large to cross the membrane through channels or carriers (e.g., proteins, polysaccharides, whole cells). These are transported in membrane-bound vesicles through processes called endocytosis (into the cell) and exocytosis (out of the cell).

Both processes require energy (ATP) and involve the dynamic remodeling of the plasma membrane.

Exocytosis — Secretion

In exocytosis, a vesicle fuses with the plasma membrane and releases its contents outside the cell:

1. Material is packaged into a vesicle (often by the Golgi apparatus)
2. The vesicle is transported to the cell surface along the cytoskeleton
3. SNARE proteins on the vesicle (v-SNARE) and target membrane (t-SNARE) interact, bringing the membranes together
4. The vesicle membrane fuses with the plasma membrane
5. Contents are released to the exterior; vesicle membrane becomes part of the plasma membrane

Examples of exocytosis:

• Secretion of neurotransmitters at synapses
• Release of hormones (insulin from beta cells)
• Secretion of digestive enzymes
• Secretion of extracellular matrix components
• Mucus secretion by goblet cells

Membrane Recycling: Exocytosis adds membrane to the plasma membrane. This is balanced by endocytosis, which removes membrane — keeping the total surface area relatively constant.

Endocytosis — Uptake

Endocytosis is the inward folding of the plasma membrane to form a vesicle that brings material INTO the cell:

Three types of endocytosis:

Receptor-mediated endocytosis in detail:

1. Ligands (e.g., LDL particles carrying cholesterol) bind to specific receptor proteins on the cell surface
2. Receptors cluster in clathrin-coated pits — regions of the membrane lined with the protein clathrin on the cytoplasmic side
3. The coated pit invaginates (folds inward) and pinches off as a clathrin-coated vesicle
4. The clathrin coat is removed; the vesicle fuses with an early endosome
5. Contents are processed (e.g., LDL is broken down to release cholesterol)
6. Receptors may be recycled back to the membrane

Clinical Connection: In familial hypercholesterolemia, mutations in the LDL receptor reduce or eliminate receptor-mediated endocytosis of LDL cholesterol. Cholesterol accumulates in the blood, dramatically increasing the risk of cardiovascular disease.

Checkpoint — Bulk Transport

Comprehensive Comparison of All Transport Types

Key Terms — Bulk Transport

Match the Transport Type

Exit Ticket — Bulk Transport

Problem-Solving Workshop — Membrane Transport

Part 6 of 7

This workshop applies membrane and transport concepts to experimental scenarios and calculations commonly tested on the AP Biology exam.

Scenario 1: Dialysis Tubing Experiment

A student fills a dialysis tubing bag (selectively permeable — allows water and small molecules to pass, but not large molecules like starch or protein) with a solution of 5% starch and 2% glucose, then places it in a beaker containing 0% starch and 10% glucose.

After 30 minutes, the student tests:

Analysis:

• Starch molecules are too large to cross the dialysis membrane — they stayed inside
• Glucose molecules are small enough to cross — glucose moved from the beaker (high, 10%) into the bag (low, 2%) by diffusion
• The bag gained mass because water moved in by osmosis (the interior solution had higher total solute concentration initially)

Scenario 1 Questions

Scenario 2: Water Potential Calculation

A student places potato core cylinders into sucrose solutions of different concentrations and measures the percent change in mass after 24 hours:

The approximate isotonic point (0% change in mass) is at ~0.38 M sucrose — this is the molar concentration of solute inside the potato cells.

At the isotonic point, the water potential of the solution equals the water potential of the potato cells.

For the 0.4 M solution at 22 °C: $\Psi_s = -iCRT = -(1)(0.4)(0.0831)(295) = -9.8 \text{ bars}$

Since the potato is in a hypotonic solution at 0.2 M (gains mass), water enters. In a hypertonic solution at 0.6 M (loses mass), water exits.

Scenario 2 Questions

Apply Your Knowledge

Exit Ticket — Workshop

AP Review — Membrane Transport

Part 7 of 7

Comprehensive AP-exam-style questions integrating concepts from all parts of the membrane transport unit.

Key Principles Summary

1. Membranes are selectively permeable — small nonpolar molecules cross freely; large, polar, and charged molecules need transport proteins
2. Passive transport follows the gradient (no ATP); active transport goes against the gradient (requires ATP, directly or indirectly)
3. Water potential determines the direction of osmosis: $\Psi = \Psi_s + \Psi_p$; water moves from high $\Psi$ to low $\Psi$
4. The Na+/K+ ATPase is the foundation for many secondary transport processes and helps maintain the resting membrane potential
5. Bulk transport (endocytosis/exocytosis) handles large molecules and particles via membrane vesicles

AP-Style Questions — Set 1

AP-Style Questions — Set 2

Comprehensive Matching

Final Review

Final Exit Ticket