Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, bonds, polarity), macromolecule structure (Ch. 5).
Every living cell is wrapped in a plasma membrane that decides what gets in and what stays out. This chapter covers how that membrane is built (phospholipid bilayer, proteins, cholesterol) and the two historical models that tried to explain its architecture. It then walks through how substances cross the membrane, from simple diffusion all the way to ATP-powered pumps. If you understand polarity and the basics of protein structure, you have what you need to start here.
The plasma membrane is a selectively permeable phospholipid bilayer studded with proteins. Small, nonpolar molecules slip through on their own (passive transport), while polar or large molecules need protein channels or ATP-driven pumps (active transport). Two models tried to explain this: the Sandwich model failed; the Fluid Mosaic model (Singer and Nicolson) is the accepted one, featuring transmembrane proteins in a semi-fluid lipid sea.
Plasma membrane
The outer boundary of a cell, composed of a phospholipid bilayer with embedded proteins. In simple terms, it is the cell's gatekeeper.
Phospholipid bilayer
Two layers of phospholipid molecules arranged tail-to-tail, forming the structural backbone of the membrane. Think of it as two rows of lollipops facing inward, with their sticks (fatty acid tails) touching.
Amphipathic
Having both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region within the same molecule. This is what allows phospholipids to self-assemble into a bilayer in water.
Selective permeability
The membrane's ability to allow some substances through while blocking others. In simple terms, it picks and chooses what crosses.
Fluid Mosaic Model
The accepted model of membrane structure (Singer and Nicolson), describing a semi-fluid phospholipid bilayer with a mosaic of embedded and peripheral proteins. Think of it as proteins floating in a lipid sea.
Sandwich Model
An earlier, rejected model (Danielli and Davson) proposing that proteins coat the outside of the phospholipid bilayer like bread on a sandwich. It failed because it could not explain transport across the membrane.
Integral (transmembrane) proteins
Proteins that span the entire membrane, providing channels or carriers for transport. In simple terms, they are tunnels or ferries built into the membrane wall.
Diffusion
The net movement of molecules from high concentration to low concentration, requiring no energy. Think of it as a drop of dye spreading evenly through a glass of water.
Osmosis
The diffusion of water across a selectively permeable membrane, from a region of lower solute concentration to higher solute concentration. In simple terms, water chases the solute.
Tonicity
The ability of a surrounding solution to cause a cell to gain or lose water. This is what determines whether a cell swells, shrinks, or stays the same.
Hypotonic
A solution with lower solute concentration than the cell interior, causing water to rush in. The cell swells (animal cells may lyse; plant cells become turgid).
Hypertonic
A solution with higher solute concentration than the cell interior, causing water to leave. The cell shrivels (animal cells crenate; plant cells plasmolyse).
Isotonic
A solution with equal solute concentration to the cell interior, producing no net water movement.
Facilitated diffusion
Passive transport assisted by membrane proteins (channels or carriers) that speed movement down the concentration gradient. In simple terms, molecules hitch a ride through a protein doorway, still going downhill.
Aquaporins
Channel proteins specifically dedicated to the rapid transport of water across the membrane. Think of them as dedicated water pipes.
Active transport
Movement of substances against the concentration gradient, requiring energy (ATP). In simple terms, the cell pays energy to push molecules uphill.
Sodium-potassium pump (Na+/K+ ATPase)
A carrier protein that pumps 3 Na+ out and 2 K+ in per cycle, using one ATP. This maintains the cell's negative resting membrane potential.
Osmoregulation
The active control of water balance inside a cell or organism, essential for survival in hypertonic or hypotonic environments.
Plasmolysis
The shrinking of a plant cell's cytoplasm away from the cell wall when placed in a hypertonic solution. In simple terms, the plant cell dries out inside its rigid box.
Apoptosis
Programmed cell death, a controlled process in which the cell dismantles itself. Relevant here because membrane changes signal apoptosis.
Selectively permeable: allows nonpolar molecules to pass freely, blocks most polar and charged molecules
Fluid in nature: lipids and many proteins move laterally within the bilayer
Composed of lipids, proteins, and carbohydrates
Each phospholipid is amphipathic: a hydrophilic phosphate head faces the watery environment, while two hydrophobic fatty acid tails face inward
Built from triacylglycerols (three-ring fat structures): lipids linked to a phosphate group
The bilayer spontaneously forms in water because the hydrophobic tails cluster together to avoid contact with water
Sandwich Model (Danielli and Davson)
Proposed that proteins form continuous sheets coating both surfaces of the phospholipid bilayer, like bread around a filling.
Rejected for three reasons:
Could not explain how substances are transported across the membrane
Membrane size and protein composition varies between cell types
Many membrane proteins are not soluble in water, which contradicts a surface-coating arrangement
Fluid Mosaic Model (Singer and Nicolson) – the accepted model
Retains the phospholipid bilayer but adds transmembrane (integral) proteins that span the membrane, creating channels and transport pathways.
Includes both hydrophobic and hydrophilic protein regions, similar to the Sandwich model
Adds integrins and other transmembrane proteins embedded within the bilayer, allowing substance transport
The membrane is semi-fluid: lipids and proteins drift laterally, giving it a mosaic appearance
Additional components visible in the membrane: cholesterol (regulates fluidity), glycoproteins and glycolipids (cell recognition), peripheral proteins (loosely attached to the surface), and cytoskeletal microfilaments anchored underneath
Transport: move specific substances across the membrane (channels and carriers)
Enzymatic activity: catalyse reactions at the membrane surface
Signal transduction: receive external signals (e.g. hormones) and relay them inside the cell
Cell-cell recognition: glycoproteins act as identity tags
Intercellular joining: connect cells to their neighbours
Attachment to the cytoskeleton and ECM: anchor the membrane to internal and external structures
All passive transport moves substances down their concentration gradient.
Diffusion
Molecules spread from areas of high concentration to low concentration until evenly distributed. No membrane proteins are needed for small, nonpolar molecules (O2, CO2). This is the simplest form of transport.
Osmosis
The diffusion of water specifically, across a selectively permeable membrane. Water moves from the region of lower solute concentration toward higher solute concentration.
Tonicity describes how a solution affects a cell's water balance:
Hypotonic solution: solute concentration outside is lower than inside the cell. Water rushes in. Animal cells lyse (burst); plant cells become turgid (firm, which is normal for them).
Hypertonic solution: solute concentration outside is higher than inside the cell. Water leaves. Animal cells shrivel (crenate); plant cells plasmolyse (cytoplasm pulls from the wall).
Isotonic solution: solute concentration is equal on both sides. No net water movement. Animal cells are normal; plant cells are flaccid.
Osmoregulation is the cell's or organism's mechanism for controlling water balance, and it is essential for survival in non-isotonic environments.
Facilitated diffusion
Still passive (no ATP), but membrane proteins speed up transport for molecules that cannot cross the lipid bilayer on their own. Two protein types are involved:
Channel proteins: form a pore (tunnel) through the membrane. Aquaporins are channel proteins dedicated to rapid water transport.
Carrier proteins: bind to a specific molecule, change shape, and release it on the other side.
Moves substances against the concentration gradient, from low to high concentration. Requires ATP.
Nonpolar molecules can pass the bilayer without help, but polar and charged molecules cannot, so transport proteins are essential.
Transport proteins allow the passage of hydrophilic substances that would otherwise be blocked by the hydrophobic interior.
Carrier proteins in active transport bind molecules and undergo conformational (shape) changes powered by ATP to ferry ions across.
The sodium-potassium pump (Na+/K+ ATPase)
The classic example of active transport. Each cycle consumes one ATP and moves 3 Na+ ions out of the cell while bringing 2 K+ ions in. Because the exchange is unequal (3 out, 2 in), it generates a net negative charge inside, maintaining the cell's resting membrane potential. The pump goes through repeated conformational changes to shuttle ions against their gradients.
Students often confuse the direction of water movement in osmosis. Water moves toward the higher solute concentration (where there is less free water), not away from it.
Hypotonic and hypertonic are frequently mixed up. A useful mnemonic: "hypo" means under (less solute outside, so water floods in and the cell swells); "hyper" means over (more solute outside, so water leaves and the cell shrinks).
The phospholipid head is hydrophilic and the tail is hydrophobic, not the other way round. The source study guide itself has this reversed in one place; be careful.
Facilitated diffusion is still passive transport. The presence of a protein helper does not make it active. Active transport is defined by requiring ATP, not by requiring a protein.
⚠️ The essay question explicitly asks you to compare the Fluid Mosaic Model with the Sandwich Model and explain why the Sandwich Model was rejected. Know all three reasons for rejection.
⚠️ You must be able to define and distinguish every mode of transport: diffusion, osmosis, facilitated diffusion, and active transport. The essay prompt lists each one.
⚠️ The sodium-potassium pump is called out by name. Know the ratio (3 Na+ out, 2 K+ in), that it requires ATP, and that it maintains membrane potential.
⚠️ You will not be tested on bare definitions, but the exam essays require you to define terms in context to build a full concept essay. Practise weaving definitions into explanatory paragraphs.
⚠️ Be ready to draw and label the phospholipid bilayer and a diagram of the Fluid Mosaic Model for the essay.
True or False: Facilitated diffusion requires ATP. (False, it is passive.)
Fill in the blank: In the Fluid Mosaic Model, ______ proteins span the entire membrane and allow transport of substances. (Transmembrane / integral)
True or False: In a hypertonic solution, an animal cell will swell and burst. (False, it will shrivel. Swelling happens in a hypotonic solution.)
Fill in the blank: The sodium-potassium pump moves __ Na+ ions out and __ K+ ions in per cycle. (3, 2)
True or False: The Sandwich Model was proposed by Singer and Nicolson. (False, it was Danielli and Davson.)
Q: Compare the Sandwich Model and the Fluid Mosaic Model. Why was the Sandwich Model rejected?
A: The Sandwich Model (Danielli and Davson) proposed that proteins coat both outer surfaces of the phospholipid bilayer. It was rejected because it could not explain transport across the membrane, protein size and composition vary between membranes, and many membrane proteins are insoluble in water. The Fluid Mosaic Model (Singer and Nicolson) replaced it by embedding transmembrane proteins within the bilayer, allowing transport and reflecting the membrane's semi-fluid, mosaic nature.
Q: Define active and passive transport and give one example of each.
A: Passive transport moves substances down the concentration gradient without energy. Example: diffusion of O2 across the membrane. Active transport moves substances against the concentration gradient and requires ATP. Example: the sodium-potassium pump, which expels 3 Na+ and imports 2 K+ per ATP consumed.
Q: A red blood cell is placed in a hypotonic solution. What happens and why?
A: The solution has lower solute concentration than the cell interior, so water moves into the cell by osmosis. The cell swells and may lyse (burst), because animal cells lack a rigid cell wall to resist the internal pressure.
Q: What is the role of aquaporins in the plasma membrane?
A: Aquaporins are channel proteins embedded in the membrane that provide a dedicated, rapid passageway for water molecules. They facilitate osmosis as part of passive transport, meaning they speed water movement without requiring energy.
Q: Explain how the sodium-potassium pump maintains membrane potential.
A: The pump uses one ATP per cycle to move 3 Na+ ions out of the cell and 2 K+ ions in. Because more positive charges leave than enter, the interior becomes relatively negative, establishing and maintaining the resting membrane potential.
This material connects directly to Chapter 8 (Metabolism): the ATP that powers active transport is produced by metabolic pathways, and enzymes discussed in Ch. 8 are themselves membrane-associated proteins in many cases.
It also connects to Chapter 9 (Cellular Respiration): the electron transport chain depends on the inner mitochondrial membrane's selective permeability and uses chemiosmosis (a proton gradient across a membrane) to generate ATP.
Understanding tonicity and osmoregulation here lays groundwork for later topics in physiology, including kidney function and how organisms maintain homeostasis.
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