Source: APK2105C Final Exam Review, University of Florida
Tags: membrane transport, passive transport, active transport, diffusion, facilitated diffusion, osmosis, Na/K pump, electrochemical gradient, vesicular transport, endocytosis, exocytosis, epithelial transport, APK2105C
Difficulty: Intermediate Prerequisites: Parts 1 and 2 of these notes (cell membrane structure, phospholipid bilayer, ions, polarity) and Part 3 (ATP). You need to understand concentration gradients, charge, and how ATP is produced before tackling this material.
Every cell sits behind a phospholipid bilayer that is selective about what passes through. This section covers the rules governing molecular movement across that barrier: the driving forces (chemical and electrical), passive mechanisms that cost no energy (simple diffusion, facilitated diffusion, osmosis), active mechanisms that consume ATP or exploit gradients (primary and secondary active transport), and bulk transport via vesicles (endocytosis, exocytosis). It also covers how epithelial cells move material all the way through, from one side of a tissue to the other. Membrane transport is tested heavily because it underpins how nutrients enter cells, how waste leaves, how nerves fire and how kidneys filter blood.
Molecules cross membranes passively (down their electrochemical gradient, no energy required) or actively (against the gradient, energy required). The phospholipid bilayer blocks most polar and charged molecules, so transport proteins (channels, carriers, pumps) do the work. Vesicular transport handles large molecules. Epithelial transport moves substances entirely through a cell layer via paracellular or transcellular pathways.
Driving force
The sum of forces due to concentration (chemical) and charge (electrical) that determine the net movement of a molecule into or out of the cell.
Chemical driving force
Created by the presence of a higher concentration of a molecule on one side of the membrane. This produces a concentration gradient, and molecules tend to move from the higher-concentration side to the lower (i.e. "down" the concentration gradient). The greater the gradient, the greater the movement.
Electrical driving force
Reflects the influence of the cell's membrane potential on charged ions. Arises from an imbalance of positive and negative ions on either side of the membrane. The direction depends on the sign of the membrane potential and the sign of the ion's charge. The magnitude depends on the size of the membrane potential and the quantity of charge carried by the ion, and it increases as either factor gets larger.
Membrane potential
The net charge difference across the cell membrane. Intracellular fluid has a slight excess of anions (net negative). Extracellular fluid has a slight excess of cations (net positive). The membrane potential is taken as the sign of the net charge inside the cell relative to outside. It is usually negative (about −70 mV).
Electrochemical driving force
The net force combining chemical and electrical driving forces on a charged molecule. If both forces act in the same direction, the electrochemical force acts in that direction. If they oppose, the electrochemical force acts in the direction of the larger force. For an uncharged molecule, only the chemical driving force applies.
Equilibrium potential
The membrane potential at which the chemical and electrical driving forces on a particular ion are equal and opposite, resulting in zero net electrochemical force. For K⁺, the equilibrium potential is about −94 mV. Because the normal resting membrane potential (−70 mV) is less negative than −94 mV, the net driving force moves K⁺ out of the cell.
Flux
The rate at which a substance is transported across a membrane, measured as the number of molecules crossing per unit time. Requires no energy when it occurs passively.
Passive transport
Movement of molecules spontaneously down their concentration gradient, requiring no energy.
Simple diffusion
Molecules move due to their own thermal motion, randomly back and forth through the bilayer. The rate depends on the magnitude of the concentration gradient, the size and shape of the diffusing molecule, membrane permeability, membrane surface area and membrane thickness. The most important factor is the membrane's surface area (the intestinal epithelium and capillary walls have massive surface area for this reason).
Facilitated diffusion
A membrane protein binds a molecule and assists its movement from one side of the membrane to the other. Think of it as accelerated diffusion: transporters let molecules cross much faster than simple diffusion, but no energy is required and movement is still down the concentration gradient.
Glucose transporter
An example of a transmembrane protein used in facilitated diffusion.
Net flux
Depends on the frequency of solute binding to the carrier molecule on the membrane, which is primarily determined by the concentration gradient.
Channels (pores)
Passageways extending across the membrane that facilitate passive transport. Like carriers, they are specific for certain substances. Most transport inorganic ions (sodium, potassium, chloride), though some can transport organic molecules. The movement of sodium through ion channels is passive.
Osmosis
The movement of water across a membrane caused by differences in solute concentration. Water moves toward the side with higher solute concentration because the large solute molecules themselves cannot cross.
Tonicity
Describes the magnitude of osmotic pressure.
Isotonic
Solutions with the same solute concentration on both sides of the membrane.
Hypertonic
The solution with the higher concentration of solutes, relative to the other side.
Hypotonic
The solution with the lower concentration of solutes, relative to the other side.
Water intoxication
A condition caused by excess water retention (e.g. kidney dysfunction). Water moves into cells, causing them to swell. Swelling of brain cells disrupts nervous system function, producing headache, nausea and vomiting.
Severe dehydration
Can cause water to move out of cells (by osmosis), causing them to shrink.
Osmotic pressure
An indirect measure of a solution's solute concentration.
Active transport
Movement of molecules against their concentration or electrochemical gradient, requiring energy. Cells can use up to 40% of their total ATP production for active transport.
Primary active transport
Uses ATP or another chemical energy source directly. The electrochemical gradient is to diffusion as ATP is to active transport.
Secondary active transport
Powered by a concentration gradient or electrochemical gradient previously created by primary active transport. A transport protein couples the movement of one substance to the movement of another. The main difference from primary active transport is the form of energy used (electrochemical gradient vs. ATP).
Pumps
Proteins that actively transport molecules across the membrane. They differ from carrier proteins because they use energy to move molecules in a preferred direction (against the gradient). Most harness energy from ATP and are frequently called ATPases.
Na⁺/K⁺ pump (sodium-potassium pump)
Transports Na⁺ and K⁺ in opposite directions. For each cycle: 3 Na⁺ ions out, 2 K⁺ ions in, via conformational change. It works against two gradients simultaneously (chemical and electrical, i.e. the electrochemical gradient). Its primary role is maintaining Na⁺ and K⁺ concentrations on either side of the membrane.
Aquaporins
Channel proteins through which water moves down its concentration gradient across the membrane.
Co-transport (symport)
A type of secondary active transport where two substances move in the same direction. The classic example is sodium-linked glucose transport: Na⁺ moves down its electrochemical gradient into the cell, releasing energy that pulls glucose in alongside it.
Counter-transport (antiport)
A type of secondary active transport where two substances move in opposite directions. Example: sodium-proton exchange, where Na⁺ flows inward and H⁺ flows outward.
Vesicular transport (cytosis)
Macromolecules enter or leave cells via tiny phospholipid sacs (vesicles). Both endocytosis and exocytosis require energy.
Exocytosis
A vesicle inside the cell fuses with the plasma membrane and releases its contents into the extracellular fluid. Functions: adding components to the plasma membrane, recycling receptors and secreting substances. This is how nerve cells release neurotransmitters. Secretory vesicles are a part of exocytosis.
Endocytosis
Vesicles transport materials into the cell.
Phagocytosis
"Cell eating." The plasma membrane extends around a particle, pinches off to form a phagocytic vesicle, which then fuses with a lysosome to form a phagolysosome. The particle is degraded by lysosomal enzymes. Common in white blood cells. Bacteria engulfed by a white blood cell is an example of phagocytosis.
Pinocytosis
"Cell drinking." An indentation of the plasma membrane forms an endocytotic vesicle in the cytoplasm. It is a nonspecific process; the vesicle contains extracellular fluid with dissolved solutes. A macrophage drinking its surrounding tissue fluid is an example.
Receptor-mediated endocytosis
Highly specific. Receptors in the plasma membrane recognise and bind specific particles. The membrane area is coated with clathrin on its cytosolic surface, indents to form a coated pit, then a coated vesicle.
Transport epithelia
Epithelial cells lining the renal tubules and intestine, capable of moving large volumes of water and solutes between the external environment and the vasculature.
Paracellular pathway
Transport through the gap between adjacent epithelial cells. Access is controlled by tight junctions.
Transcellular pathway
Transport through the epithelial cell itself, requiring specialised transporters and channels in two membranes.
Apical membrane
The portion of the epithelial membrane facing the external environment (toward the lumen).
Basolateral membrane
The membrane facing internally, toward the blood vessels.
Solvent drag
When a high volume of water moving via the transcellular route sweeps organic ions and small solutes along with it through the paracellular pathway.
Transcytosis
A process where macromolecules cross epithelial cells via both endocytosis and exocytosis. The vesicle does not fuse with a lysosome; instead, it travels to the opposite side of the cell and fuses with the plasma membrane to release its contents.
The chemical driving force depends on the concentration gradient. Molecules move from high to low concentration.
The electrical driving force depends on the membrane potential and the ion's charge. The membrane potential is usually about −70 mV (inside negative).
For charged particles (ions), both forces combine into the electrochemical driving force. This net force determines whether an ion moves in or out of the cell.
Potassium (K⁺): more concentrated inside the cell than outside. The chemical driving force pushes K⁺ out. The equilibrium potential for K⁺ is −94 mV. Because −70 mV is less negative than −94 mV, there is still a net outward force on K⁺.
At the equilibrium potential, the electrochemical force is zero: the chemical gradient pulling the ion out is exactly balanced by the electrical gradient pulling it back in.
Simple diffusion: nonpolar, small molecules pass through the bilayer directly. Rate depends mainly on surface area.
Facilitated diffusion: polar or large molecules use carrier proteins or channel proteins to cross. Still no energy required. The glucose transporter is a key example.
Channels/pores: allow specific ions to pass. Most are ion-selective (Na⁺, K⁺, Cl⁻). Sodium moving through ion channels is passive transport.
A greater number of protein carriers would not increase the rate of simple diffusion (simple diffusion does not use carriers).
If a molecule crosses the membrane and is quickly removed by the bloodstream or a metabolic pathway, the diffusion rate will increase (because the concentration gradient is maintained).
Water moves by osmosis toward the side with higher solute concentration.
Isotonic, hypertonic and hypotonic describe relative solute concentrations.
Clinical relevance: water intoxication (cells swell) and severe dehydration (cells shrink).
Primary: uses ATP directly. The Na⁺/K⁺ pump is the classic example.
Secondary: uses an electrochemical gradient set up by primary active transport. Includes symport (co-transport, e.g. Na⁺-glucose) and antiport (counter-transport, e.g. Na⁺-H⁺ exchange).
Up to 40% of a cell's ATP can be spent on active transport.
Used for macromolecules too large for channel or carrier proteins.
Exocytosis: vesicle fuses with membrane, contents released outward (neurotransmitter release).
Endocytosis: membrane engulfs material inward.
Phagocytosis: specific, large particles (bacteria); vesicle fuses with lysosome.
Pinocytosis: nonspecific, extracellular fluid and dissolved solutes.
Receptor-mediated: specific particles, coated pits, clathrin.
Two routes: paracellular (between cells, controlled by tight junctions) and transcellular (through the cell, requiring apical and basolateral transporters).
Solvent drag: water movement through the transcellular route pulls small solutes through the paracellular route.
Transcytosis: endocytosis on one side, vesicle transport across, exocytosis on the other side.
The Na⁺/K⁺ pump is the reason intravenous fluids must be carefully balanced: flooding the blood with too much water (hypotonic solution) would cause cells to swell and potentially burst. Sports drinks are formulated to be roughly isotonic, replacing both water and electrolytes lost through sweat.
Glucose absorption in the intestine is a textbook example of secondary active transport (Na⁺-linked glucose co-transport on the apical side, facilitated diffusion on the basolateral side). This is why oral rehydration therapy for severe diarrhoea includes both salt and sugar: the sodium-glucose co-transporter pulls both into the cell, and water follows by osmosis.
Students sometimes think facilitated diffusion is active transport because a protein is involved. It is not. Facilitated diffusion moves molecules down their concentration gradient and requires no energy.
"Osmosis" is not just "water moving." It specifically refers to water moving across a selectively permeable membrane in response to a solute concentration difference.
The Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per cycle, not equal numbers. This unequal exchange contributes to the membrane potential.
Simple diffusion does not involve carrier proteins. Adding more protein carriers would not speed up simple diffusion (it would speed up facilitated diffusion).
⚠️ Distinguish passive transport (no energy, down gradient) from active transport (energy, against gradient). Be specific about which type: simple diffusion, facilitated diffusion, primary active, secondary active.
⚠️ Know the Na⁺/K⁺ pump stoichiometry: 3 Na⁺ out, 2 K⁺ in, using ATP, via conformational change.
⚠️ Be able to determine the direction of the electrochemical driving force when given the membrane potential, the equilibrium potential and the ion's charge.
⚠️ K⁺ equilibrium potential = −94 mV. Resting membrane potential = −70 mV. Net force on K⁺ is outward.
⚠️ Know the three types of endocytosis (phagocytosis, pinocytosis, receptor-mediated) and be able to give an example of each.
⚠️ Understand the difference between symport (co-transport) and antiport (counter-transport) in secondary active transport.
⚠️ Cells can use up to 40% of their ATP for active transport.
⚠️ Potassium is found in greater concentration inside the cell than outside.
True or false: Facilitated diffusion requires ATP. (False, no energy is required.)
Fill in the blank: The Na⁺/K⁺ pump moves _______ Na⁺ ions out and _______ K⁺ ions in per cycle. (3 out, 2 in.)
True or false: In osmosis, water moves toward the side with lower solute concentration. (False, water moves toward higher solute concentration.)
Fill in the blank: Bacteria engulfed by a white blood cell is an example of _______. (Phagocytosis.)
True or false: The equilibrium potential for K⁺ is more negative than the resting membrane potential. (True, −94 mV vs. −70 mV.)
Q: What is the electrochemical driving force and how is it determined?
A: It is the net force on a charged molecule, combining the chemical driving force (concentration gradient) and the electrical driving force (membrane potential). If both forces act in the same direction, the electrochemical force acts that way. If they oppose, the force acts in the direction of the larger component. For uncharged molecules, only the chemical driving force applies.
Q: Explain why the net driving force on K⁺ is directed out of the cell at resting membrane potential.
A: The chemical driving force pushes K⁺ out because K⁺ is more concentrated inside the cell. The equilibrium potential for K⁺ is −94 mV, meaning the electrical force would need a −94 mV potential to perfectly counterbalance the chemical force. The resting potential is only −70 mV (less negative), so the electrical force pulling K⁺ back in is not strong enough. The net electrochemical driving force therefore pushes K⁺ outward.
Q: Distinguish between primary and secondary active transport, and give one example of each.
A: Primary active transport uses ATP directly as its energy source (e.g. the Na⁺/K⁺ pump). Secondary active transport uses an electrochemical gradient previously established by primary active transport (e.g. Na⁺-linked glucose co-transport, where Na⁺ moving down its gradient provides energy to pull glucose into the cell).
Q: Describe the three types of endocytosis.
A: (1) Phagocytosis: the cell extends its membrane around a large particle, forming a phagocytic vesicle that fuses with a lysosome. (2) Pinocytosis: the membrane indents to capture extracellular fluid and dissolved solutes in a nonspecific manner. (3) Receptor-mediated endocytosis: membrane receptors bind specific particles, the area is coated with clathrin, and a coated vesicle forms.
Q: What is transcytosis?
A: A process in which a macromolecule is taken into an epithelial cell by endocytosis on one side, transported across the cell in a vesicle (without fusing with a lysosome) and released on the other side by exocytosis.
Q: If a positively charged ion is more concentrated outside the cell, in which direction would the electrical force required to balance the chemical gradient be directed, and would the equilibrium potential be positive or negative?
A: The electrical force would need to be directed outward (to prevent the ion from entering). The equilibrium potential for this ion would be positively charged.
Membrane transport connects to nearly everything else in the course. The action potential in nerve physiology depends on Na⁺ and K⁺ channels and the electrochemical gradients maintained by the Na⁺/K⁺ pump. Muscle contraction requires calcium release (stored in the smooth ER/sarcoplasmic reticulum). Kidney function is built on epithelial transport, paracellular pathways and osmosis. The ATP that fuels active transport comes from the metabolic pathways covered in Part 3.
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