Cell Biology | Passive & Active Transport | Endocytosis & Exocytosis

Summary: Membrane Transport

The lecture surveys how substances cross the cell membrane. Its central distinction is whether transport moves substances down a concentration gradient without direct energy use, uses ATP to move them against a gradient, or moves them in vesicles.

1. Simple diffusion

• Simple diffusion is passive: it does not directly use ATP.
• Molecules move from higher to lower concentration and pass directly through the membrane, without a channel or carrier protein.
• Examples include:
  • Oxygen and carbon dioxide
  • Steroid hormones, such as testosterone, estrogen, progesterone, aldosterone, and cortisol
  • Fat-soluble drugs
• The membrane’s phospholipid bilayer helps explain this selectivity: its interior is nonpolar, so small, nonpolar, or lipid-soluble substances cross more readily than charged or strongly polar substances.
• Four factors affect diffusion rate:
  • Greater surface area increases diffusion.
  • A steeper concentration gradient increases diffusion.
  • Greater membrane thickness decreases diffusion.
  • Greater molecular weight decreases diffusion.

2. Facilitated diffusion

• Facilitated diffusion is also passive: substances move down their concentration gradients without ATP being used directly.
• Unlike simple diffusion, it requires a membrane protein, either a channel or a carrier, to help a substance cross.
• It is especially important for charged, polar, or larger substances that cannot readily pass through the membrane’s nonpolar interior.

Osmosis

• Osmosis is water movement from an area of higher water concentration to lower water concentration—equivalently, toward an area of higher solute concentration.
• Aquaporins are channels that allow water to cross the membrane.

Ion channels

The lecture describes several types of channels that allow ions to move down their gradients:

• Leak channels: Often open, including potassium leak channels in neurons. Potassium movement through these channels contributes to the resting membrane potential.
• Voltage-gated channels: Open when the membrane reaches a particular voltage. Sodium and calcium channels are important in neuronal action potentials.
• Ligand-gated channels: Open when a particular chemical binds. At the neuromuscular junction, acetylcholine binding opens a channel that allows sodium entry, contributing to an action potential and muscle contraction.
• Mechanically gated channels: Open in response to physical force. The lecture uses pressure on a finger as an example: ion movement can activate pain-sensing nerves and initiate a signal toward the central nervous system.

Carrier-mediated facilitated diffusion

• Carrier proteins transport substances down their concentration gradients.
• GLUT transporters facilitate glucose uptake into cells. The lecture highlights GLUT4 in muscle and adipose tissue; insulin increases its presence at the cell membrane, promoting glucose entry.

3. Primary active transport

• Primary active transport uses ATP directly to move substances against their concentration gradients.
• ATPases break down ATP, and the released energy powers the transport.

Major examples

• Sodium-potassium ATPase: Moves three sodium ions out of the cell and two potassium ions in per cycle, against their gradients.
  • The lecture says insulin and thyroid hormones can increase its activity.
  • It also connects the pump to digoxin, which inhibits it and is used in heart failure.
• Calcium ATPases: In muscle cells, calcium pumps move calcium from the cytoplasm into the sarcoplasmic reticulum, helping muscle relaxation. The lecture describes sympathetic stimulation as increasing this calcium uptake, allowing more calcium to be available for a subsequent contraction.
• Gastric proton pump: Parietal cells use an ATP-dependent hydrogen-potassium pump to secrete acid into the stomach. Proton pump inhibitors reduce acid secretion and are used for conditions such as reflux and peptic ulcer disease.

4. Secondary active transport

• Secondary active transport moves a substance against its gradient by coupling it to another substance moving down its gradient.
• It does not use ATP directly at the transporter. Instead, it depends on gradients—especially the sodium gradient—maintained by the sodium-potassium ATPase.
• Two transport arrangements are described:
  • Symport: Both substances move in the same direction.
  • Antiport: The substances move in opposite directions.

Examples and clinical connections

• Sodium-glucose cotransporter (SGLT): Sodium moves into the cell down its gradient and helps carry glucose into the cell against its gradient. The lecture discusses kidney SGLT2 inhibitors, which reduce glucose reabsorption so more glucose leaves in the urine, lowering blood glucose.
• Sodium-potassium-2 chloride cotransporter (NKCC): Moves sodium, potassium, and two chloride ions in the same direction. In the kidney, loop diuretics, such as furosemide (Lasix), inhibit this transporter. The lecture explains that this increases salt and water loss in urine and can help reduce fluid overload.
• Sodium-proton exchanger: Sodium moves into the cell while protons move out. The lecture links altered renal proton handling with metabolic acid-base changes and discusses aldosterone as a regulator.
• Sodium-calcium exchanger: Sodium moves into heart muscle cells while calcium moves out. The lecture connects digoxin’s inhibition of the sodium-potassium ATPase to reduced sodium-gradient-driven calcium export, increased calcium inside heart cells, and stronger contraction.

5. Vesicular transport

Vesicular transport moves material in membrane-bound packages. Endocytosis brings material into the cell; exocytosis releases material from it.

Endocytosis

The lecture discusses three forms:

• Pinocytosis (“cell drinking”):

  • The membrane folds inward and encloses extracellular fluid and dissolved substances in a vesicle.
  • Microtubules and motor proteins, including kinesins and dyneins, can move vesicles within the cell.
  • Intestinal cells are an example of cells that take up fluid and dissolved solutes.
  • Vesicle formation and movement require cellular energy, distinct from the lecture’s description of ATP-powered membrane pumps.

• Phagocytosis (“cell eating”):

  • White blood cells, especially neutrophils and macrophages, engulf large particles such as bacteria.
  • Actin helps form pseudopods, which surround the particle and enclose it in a phagosome.
  • The phagosome is acidified, then fuses with a lysosome to form a phagolysosome. Lysosomal enzymes digest the captured material.
  • Remaining material may be released from the cell by exocytosis.

• Receptor-mediated endocytosis:

  • Specific molecules bind to receptors, allowing selective uptake.
  • The lecture’s example is LDL uptake by liver cells: LDL binds LDL receptors, and clathrin helps form a clathrin-coated pit that buds inward as a vesicle.
  • The vesicle becomes an endosome. Acidification separates LDL from its receptor; receptors can return to the cell surface, while LDL is processed.
  • The lecture connects disrupted LDL uptake or processing with familial hypercholesterolemia and increased cardiovascular risk.

Exocytosis

• In exocytosis, a vesicle moves to and fuses with the cell membrane, releasing its contents outside the cell.
• It can remove cellular waste and secrete substances such as:
  • Neurotransmitters
  • Hormones, including insulin
  • Mucin from goblet cells
• The lecture outlines a secretory pathway for proteins: DNA is transcribed into mRNA; proteins are made and modified in the rough endoplasmic reticulum; vesicles carry them to the Golgi apparatus for further processing; and mature vesicles then travel toward the cell membrane.
• Microtubules and motor proteins help move vesicles. V-SNARE proteins on vesicles interact with T-SNARE proteins on the target membrane to promote fusion. The lecture notes that calcium is important for this release process.

Speakers and sources featured

• Professor Zach Murphy — lecturer and presenter, identified in the video context.
• No other speakers are featured in the subtitles.