Cell Membranes and Transport

The membrane is described as fluid (components move laterally) and mosaic (diverse proteins embedded throughout).

Components

Membrane Fluidity

• Temperature: higher temp → more fluid; lower temp → less fluid
• Cholesterol: moderates fluidity across temperature range
• Unsaturated fatty acids: kinks prevent tight packing → more fluid
• Saturated fatty acids: pack tightly → less fluid

The membrane is selectively permeable — some substances cross freely, others require assistance.

Can cross freely:

• Small nonpolar molecules ($O_2$, $CO_2$, $N_2$)
• Small uncharged polar molecules ($H_2O$ — slowly, or via aquaporins)

Cannot cross without help:

• Large polar molecules (glucose, amino acids)
• Ions ($Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$)

Simple Diffusion

• Movement of molecules from high to low concentration directly through the bilayer
• Driven by the concentration gradient (second law of thermodynamics)
• Rate depends on: gradient steepness, temperature, molecular size, membrane permeability

Facilitated Diffusion

• Movement through channel proteins or carrier proteins — still DOWN the concentration gradient
• Channel proteins: form pores (often gated); selective for specific ions
  • Aquaporins: water channels
  • Ligand-gated channels: open when a signal molecule binds
  • Voltage-gated channels: open in response to membrane potential changes
• Carrier proteins: change shape to shuttle molecules across

Osmosis

• Diffusion of water through a selectively permeable membrane
• Water moves from hypotonic (lower solute) to hypertonic (higher solute) solution
• Isotonic: equal solute concentration on both sides → no net water movement

Effects on cells:

• Animal cell in hypotonic solution: swells and may lyse (burst)
• Animal cell in hypertonic solution: shrivels (crenation)
• Plant cell in hypotonic solution: turgor pressure increases (turgid — ideal)
• Plant cell in hypertonic solution: plasmolysis (membrane pulls from wall)

Water potential ($\Psi$): $\Psi = \Psi_s + \Psi_p$ Water moves from higher $\Psi$ to lower $\Psi$.

Primary Active Transport

• Uses ATP directly to move molecules against the concentration gradient
• Sodium-potassium pump ($Na^+/K^+$ ATPase): pumps 3 $Na^+$ out and 2 $K^+$ in per ATP
  • Maintains the resting membrane potential
  • Essential for nerve impulses and muscle contraction

Secondary Active Transport (Co-transport)

• Uses the gradient created by primary active transport to move another molecule
• Example: $Na^+$/glucose co-transporter in the intestine — Na⁺ flows down its gradient, carrying glucose against its gradient

Electrogenic Pump

• Generates a voltage (charge difference) across the membrane
• $Na^+/K^+$ pump is electrogenic (3+ out, 2+ in → net positive charge outside)
• Proton pump ($H^+$ ATPase): important in mitochondria, chloroplasts, and plant cells

Exocytosis

• Vesicles fuse with the plasma membrane, releasing contents outside the cell
• Used for: secretion of proteins, hormones, neurotransmitters, waste removal

Endocytosis

• Membrane folds inward to engulf material, forming a vesicle inside the cell

In Animal Cells

• Cells need an isotonic environment to maintain homeostasis
• Osmoregulation: kidneys regulate water and solute balance

In Plant Cells

• Cell wall prevents bursting in hypotonic solutions → cells become turgid (ideal)
• Turgor pressure supports the plant structure
• In hypertonic solutions: plasmolysis (membrane detaches from wall) → wilting

Question: A student places red blood cells in three solutions: 0.9% NaCl, 0.1% NaCl, and 5% NaCl. Predict what happens to the cells in each solution. (4 points)

Solution:

0.9% NaCl (isotonic): The solution has the same solute concentration as the cell interior. There is no net movement of water. The cells remain their normal shape.

0.1% NaCl (hypotonic): The solution has a lower solute concentration than the cells. Water moves into the cells by osmosis. The cells swell and may lyse (burst) because animal cells lack a rigid cell wall.

5% NaCl (hypertonic): The solution has a higher solute concentration than the cells. Water moves out of the cells by osmosis. The cells shrink and become crenated (wrinkled).

• Fluid mosaic model: phospholipid bilayer with embedded proteins, cholesterol, and glycoproteins.
• Passive transport (no ATP): diffusion, facilitated diffusion, osmosis — all move down gradients.
• Active transport (ATP): primary (pumps), secondary (co-transport) — against gradients.
• Bulk transport: exocytosis (out) and endocytosis (phagocytosis, pinocytosis, receptor-mediated — in).
• Osmosis: water moves from hypotonic to hypertonic; $\Psi = \Psi_s + \Psi_p$.
• SA:V ratio and membrane composition affect transport efficiency.