Difficulty: Introductory | Prerequisites: Basic chemistry (polarity, hydrophobic/hydrophilic interactions)
This topic sits at the foundation of cell biology. Before you can understand how cells take in nutrients, signal each other or maintain their internal environment, you need to know what the membrane is made of and how things cross it. The plasma membrane is not a static wall; it is a dynamic, selectively permeable barrier built from phospholipids and proteins. If you are coming into this cold, make sure you are comfortable with the idea of polar vs non-polar molecules first.
The plasma membrane is a phospholipid bilayer studded with proteins. Early models (Danielli and Davson's "Sandwich Model") got the bilayer right but placed proteins only on the outside. The current Fluid Mosaic Model (Singer and Nicolson) correctly shows proteins embedded within and spanning the bilayer. Substances cross the membrane either passively (diffusion, osmosis, facilitated diffusion) or actively (using ATP, e.g. the sodium-potassium pump).
Phospholipid bilayer
Two layers of phospholipid molecules arranged with their hydrophobic fatty-acid tails pointing inward and their hydrophilic phosphate heads facing the watery environment on each side. Think of it as two rows of matchsticks pressed tail-to-tail, with the heads facing out.
Amphipathic
A molecule that possesses both a hydrophobic (water-repelling) region and a hydrophilic (water-attracting) region. In simple terms, one end of the molecule likes water and the other avoids it. Phospholipids are the classic example.
Fluid Mosaic Model
The currently accepted model of membrane structure, proposed by Singer and Nicolson (1972). It describes the membrane as a fluid phospholipid bilayer with a mosaic of proteins floating within it, some spanning the entire membrane (transmembrane proteins) and others attached to one surface. In simple terms, the membrane is like a moving sea of lipids with proteins bobbing in it.
Sandwich Model (Danielli-Davson Model)
An earlier model of membrane structure proposed by Danielli and Davson. It depicted two continuous layers of globular protein coating the outer surfaces of a phospholipid bilayer, like bread around a filling. It was rejected because it could not explain transport across the membrane, and because not all membrane proteins are water-soluble.
Transmembrane proteins (integrins)
Proteins that span the full thickness of the phospholipid bilayer. They serve as channels, carriers, receptors and anchors. In simple terms, these are the proteins that poke all the way through the membrane.
Hypertonic solution
A solution with a higher solute concentration than the cell's interior. Water moves out of the cell by osmosis, and the cell shrinks (crenates in animal cells, plasmolyses in plant cells).
Hypotonic solution
A solution with a lower solute concentration than the cell's interior. Water moves into the cell by osmosis, and the cell swells (may lyse in animal cells; turgor pressure builds in plant cells).
Isotonic solution
A solution with the same solute concentration as the cell. There is no net movement of water, and cell volume stays stable.
Facilitated diffusion
A form of passive transport in which transport proteins (channel proteins or carrier proteins) help molecules move down their concentration gradient without the cell spending energy. In simple terms, it is diffusion with a protein escort.
Active transport
Movement of substances across a membrane against their concentration gradient, requiring energy in the form of ATP. In simple terms, the cell spends energy to push molecules the "wrong way" up the gradient.
Sodium-potassium pump (Na+/K+ ATPase)
A transmembrane protein that uses one molecule of ATP per cycle to move 3 Na+ ions out of the cell and 2 K+ ions into the cell. This maintains the electrochemical gradient essential for nerve impulses and cell volume regulation. In simple terms, it is the cell's main pump for keeping sodium out and potassium in.
Danielli-Davson "Sandwich" Model: proposed a phospholipid bilayer coated on both sides by continuous sheets of globular protein. The proteins sat on the surface like slices of bread around a filling.
Rejected because: (1) it could not account for how substances are transported across the membrane, (2) protein composition varies between different membranes (the model assumed uniform protein coats), and (3) not all membrane proteins are water-soluble, so a uniform surface layer is implausible.
Fluid Mosaic Model (Singer and Nicolson, 1972): describes the membrane as a semi-fluid phospholipid bilayer in which proteins are embedded at various depths.
Some proteins span the entire bilayer (transmembrane/integral proteins, e.g. integrins).
Others are loosely attached to one surface (peripheral proteins).
The "fluid" part refers to the lateral movement of lipids and proteins within the plane of the membrane. The "mosaic" part refers to the patchwork of different proteins scattered through the bilayer.
This model accounts for selective permeability, transport mechanisms (channels, carriers, pumps) and cell signalling.
Osmosis is the net movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.
Hypotonic solution (lower solute outside than inside): water enters the cell. The cell swells. Animal cells risk lysis; plant cells become turgid.
Hypertonic solution (higher solute outside than inside): water leaves the cell. The cell shrinks. Animal cells crenate; plant cells plasmolyse.
Isotonic solution (equal solute concentration): no net water movement. Cell volume is stable.
Key exam point: when the question says "the concentration of solutes is less than inside the cell," the external solution is hypotonic relative to the cell, so water moves in.
Both use membrane proteins. The critical differences:
Facilitated diffusion is passive. No energy required. Molecules move down their concentration gradient through channel proteins (e.g. aquaporins for water) or carrier proteins.
Active transport requires ATP. Molecules move against their concentration gradient. The sodium-potassium pump is the textbook example.
Per cycle, using one ATP:
3 Na+ ions are pumped out of the cell
2 K+ ions are pumped into the cell
This creates a net negative charge inside the cell (electrochemical gradient), which is essential for nerve impulse transmission, muscle contraction and maintaining cell volume.
Intravenous (IV) fluids in hospitals are kept isotonic (e.g. 0.9% saline) to avoid lysing or crenating red blood cells. The sodium-potassium pump is the target of cardiac glycosides like digoxin, used to treat heart conditions.
Students often think the Sandwich Model had no phospholipid bilayer. It did. The problem was where it placed the proteins (only on the outside), not the bilayer itself.
Students often confuse "hypotonic" and "hypertonic" by focusing on the water rather than the solute. Remember: the prefix describes solute concentration. Hypo = less solute outside, so water goes in.
Students sometimes think facilitated diffusion requires energy because it uses proteins. It does not. The proteins simply provide a pathway; the driving force is the concentration gradient.
The sodium-potassium pump moves 3 Na+ out and 2 K+ in, not equal numbers. The unequal exchange is what creates the net charge difference.
Expect a question asking you to identify the reason the Sandwich Model was rejected. The answer centres on transport, protein variability and solubility.
Tonicity questions almost always give you a scenario and ask what happens to the cell. Read carefully whether the question describes the solution or the cell's interior.
The sodium-potassium pump ratio (3 Na+ out, 2 K+ in) is a favourite multiple-choice item. Memorise the numbers.
Know the difference between facilitated diffusion (passive, down gradient, no ATP) and active transport (ATP, against gradient).
True or False: The Fluid Mosaic Model was proposed by Danielli and Davson. (False, it was Singer and Nicolson.)
Fill in the blank: A molecule with both hydrophobic and hydrophilic regions is called ________. (Amphipathic.)
True or False: In a hypotonic solution, a cell will lose water. (False, it gains water.)
Fill in the blank: The sodium-potassium pump moves __ Na+ out and __ K+ in per cycle. (3 out, 2 in.)
True or False: Facilitated diffusion requires ATP. (False.)
Q: Which of the following was a primary reason for the rejection of the Danielli-Davson Sandwich Model?
A: It failed to explain how substances are transported across the membrane. The model placed proteins only on the surface, leaving no mechanism for selective transport.
Q: A cell is placed in a solution where the solute concentration is lower than inside the cell. What happens?
A: The solution is hypotonic. Water enters the cell by osmosis, and the cell swells.
Q: In one cycle of the sodium-potassium pump, how many ions of each type are moved?
A: Three sodium ions (Na+) are pumped out of the cell and two potassium ions (K+) are pumped in, using one ATP.
Q: Explain the difference between facilitated diffusion and active transport.
A: Facilitated diffusion is passive transport that uses channel or carrier proteins to move molecules down their concentration gradient without energy. Active transport uses ATP to move molecules against their concentration gradient. Both require membrane proteins, but only active transport consumes energy.
Q: Compare and contrast the Sandwich Model and the Fluid Mosaic Model of the plasma membrane.
A: The Sandwich Model (Danielli and Davson) placed a phospholipid bilayer between two continuous layers of surface protein. The Fluid Mosaic Model (Singer and Nicolson) shows a semi-fluid bilayer with proteins embedded within it at various depths, including transmembrane proteins. The Sandwich Model was rejected because it could not explain transport, the variability of membrane protein composition, or the fact that many membrane proteins are not water-soluble.
This material connects directly to cellular respiration: the inner mitochondrial membrane is where the electron transport chain and chemiosmosis occur, and its selective permeability is what makes the proton gradient possible. It also connects to genetics and biotechnology, where techniques like electroporation deliberately disrupt membranes to introduce DNA.
plasma membrane, cell membrane, phospholipid bilayer, fluid mosaic model, sandwich model, Danielli-Davson, Singer-Nicolson, amphipathic, hydrophobic, hydrophilic, osmosis, tonicity, hypertonic, hypotonic, isotonic, facilitated diffusion, active transport, passive transport, sodium-potassium pump, Na+/K+ ATPase, transmembrane protein, integral protein, peripheral protein, integrins, channel protein, carrier protein, aquaporin, selective permeability, concentration gradient, electrochemical gradient, BIOL 110, intro to biology, Penn State