Source: Unit 1 Review, Introduction to Biology I
Difficulty: Introductory to Intermediate | Prerequisites: Chemical bonds and macromolecules (especially lipids and proteins).
This set of notes covers how the cell membrane is built, how it controls what enters and leaves the cell, how proteins fold into functional shapes, and how enzymes speed up reactions. These topics sit at the intersection of chemistry and cell biology: you need the macromolecule foundations (lipids, proteins, chemical bonds) from earlier in Unit 1. If you are comfortable with amphipathic molecules, R groups, and the difference between polar and nonpolar, you are ready. Membrane transport and enzyme regulation are among the most exam-heavy topics in introductory biology.
The cell membrane is a phospholipid bilayer studded with proteins, described by the fluid mosaic model. Small nonpolar molecules pass freely; everything else needs a transport protein or energy input (ATP). Proteins fold through four structural levels driven by R-group interactions, and enzymes are proteins that catalyse reactions, with their activity regulated by competitive inhibitors, non-competitive inhibitors, and allosteric effectors.
Phospholipid
A lipid molecule with a glycerol backbone, a phosphate-containing head group (hydrophilic), and two fatty acid tails (hydrophobic). Phospholipids self-assemble into bilayers in water.
Think of it as having a water-loving head and two water-fearing tails, which is why it naturally forms a two-layered sheet.
Fluid mosaic model
The accepted description of membrane structure: a dynamic, fluid phospholipid bilayer with proteins, cholesterol, and carbohydrates embedded in or attached to it, forming a mosaic pattern.
In simple terms, the membrane is not rigid; its components drift and move laterally, like icebergs floating in a sea of lipids.
Integral protein
A membrane protein permanently embedded in the lipid bilayer, typically spanning the membrane (transmembrane). It has hydrophobic regions that sit within the bilayer and hydrophilic regions exposed to the aqueous environment on either side.
Peripheral protein
A membrane protein that associates temporarily with the membrane surface, usually by binding to integral proteins or to the polar heads of phospholipids. It does not penetrate the hydrophobic core.
Osmosis
The diffusion of water across a selectively permeable membrane, from a region of higher water concentration to a region of lower water concentration (equivalently, from lower solute concentration to higher).
In simple terms, water moves towards wherever there is more "stuff" dissolved.
Hypertonic solution
A solution with a higher solute concentration (lower water concentration) than the cell's interior. Water leaves the cell, causing it to shrivel (crenation in animal cells, plasmolysis in plant cells).
Hypotonic solution
A solution with a lower solute concentration (higher water concentration) than the cell's interior. Water enters the cell, which may swell and burst (lyse).
Active transport
Movement of molecules across a membrane against their concentration gradient, requiring energy input, usually from ATP hydrolysis.
In simple terms, the cell spends energy to push molecules from where there are fewer to where there are more.
Primary structure (protein)
The linear sequence of amino acids in a polypeptide chain. This sequence alone determines how the protein will fold.
Tertiary structure (protein)
The overall three-dimensional shape of a single polypeptide, produced by interactions between R groups (and between R groups and the backbone). These interactions include hydrogen bonds, ionic bonds, van der Waals forces, hydrophobic packing, and disulphide bridges.
Competitive inhibitor
A molecule that competes with the substrate for binding at the enzyme's active site. Its effect can be overcome by increasing substrate concentration.
Non-competitive inhibitor
A molecule that binds to an allosteric site (not the active site), changing the enzyme's shape and reducing its activity regardless of how much substrate is present.
Allosteric regulation
Regulation of enzyme activity by a molecule binding at a site other than the active site, causing a conformational change that either activates or inhibits the enzyme. Feedback inhibition is a common example.
The membrane is built from phospholipids arranged in a bilayer: hydrophilic heads face outward (towards water), hydrophobic tails face inward (away from water).
Phospholipids can self-assemble into three arrangements:
Micelle: a single-layer sphere with tails inward, formed in low concentrations.
Bilayer: a flat two-layer sheet, the basis of cell membranes.
Liposome: a bilayer sphere enclosing an aqueous interior.
The fluid mosaic model describes the membrane as dynamic: lipids and many proteins move laterally within the plane of the bilayer.
Membrane fluidity depends on two main factors:
Tail length and saturation: shorter tails and more C=C double bonds (unsaturated) increase fluidity. Longer, saturated tails pack more tightly and reduce fluidity.
Cholesterol: cholesterol is amphipathic and inserts between phospholipids. At high temperatures it restrains movement (reduces fluidity); at low temperatures it prevents tight packing (maintains fluidity). It acts as a buffer.
Integral (transmembrane) proteins are permanently embedded in the bilayer. They typically span the entire membrane and have:
Hydrophobic regions that sit within the lipid core.
Hydrophilic regions exposed on one or both surfaces.
Peripheral proteins sit on the membrane surface, associating temporarily with the polar head groups or with integral proteins. They are involved in signalling and structural support.
Membrane proteins carry out a range of functions: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton.
The membrane is selectively permeable. Small, nonpolar molecules (O2, CO2) and small uncharged polar molecules (water, to a degree) can pass through the lipid bilayer. Large, polar, or charged molecules cannot and require transport proteins.
Molecules move down their concentration gradient, from high concentration to low.
Simple diffusion: molecules pass directly through the lipid bilayer. On a graph of transport rate vs. concentration, the line is linear and does not plateau.
Facilitated diffusion: molecules move through channel proteins or carrier proteins.
Channel proteins form a hydrophilic tunnel selective for particular ions or molecules.
Carrier proteins bind the molecule and undergo a conformational change to shuttle it across.
On a graph, facilitated diffusion reaches a plateau (saturation) because there are a finite number of transport proteins. The rate does not continue to rise beyond that point.
Osmosis is the diffusion of water across a selectively permeable membrane, driven by differences in solute concentration.
Water moves from high water concentration (low solute) to low water concentration (high solute).
Hypertonic environment: more solute outside the cell. Water leaves. Cell shrivels.
Hypotonic environment: less solute outside the cell. Water enters. Cell swells and may burst.
Isotonic environment: solute concentration is equal. No net water movement.
Molecules are moved against their concentration gradient, from low concentration to high. This requires ATP.
On a graph, active transport can push the internal concentration beyond equilibrium.
Primary active transport: ATP is hydrolysed directly by the transport protein. The Na+/K+ pump (sodium-potassium pump) is the classic example: it pumps 3 Na+ out and 2 K+ in per ATP.
Secondary active transport: ATP is used indirectly. First, primary active transport establishes an ion gradient (often a proton gradient). The potential energy stored in that gradient then drives transport of another molecule against its own gradient.
Protein folding is driven by the hydrophobic effect and by interactions between R groups. The process is spontaneous: the folded state has lower free energy than the unfolded state because hydrophobic R groups are buried away from water and additional stabilising bonds form.
Hydrophobic R groups: nonpolar hydrocarbon chains or uncharged carbon rings. They cluster in the protein interior, away from water, stabilised by van der Waals interactions (temporary charge attractions from asymmetrical electron distributions).
Hydrophilic R groups: polar or charged.
Acidic R groups lose a proton and become negatively charged.
Basic R groups gain a proton and become positively charged.
Charged R groups form ionic bonds with each other, which influences how proteins associate.
Primary structure: the linear amino acid sequence. This is the blueprint: it determines everything that follows.
Secondary structure: local folding patterns stabilised by hydrogen bonds between atoms in the polypeptide backbone (not the R groups). Two common forms: alpha helices and beta pleated sheets.
Tertiary structure: the overall 3D shape of a single polypeptide chain, produced by interactions between R groups and between R groups and the backbone. This includes hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and disulphide bridges.
Quaternary structure: the arrangement of two or more polypeptide subunits (each with its own tertiary structure) into a functional multi-subunit protein. Haemoglobin (four subunits) is a standard example.
Peptide bond formation: the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of another in a condensation (dehydration) reaction, releasing water.
Enzymes are biological catalysts, almost always proteins. They lower the activation energy of a reaction without being consumed.
Competitive inhibitors bind to the active site, directly competing with the substrate for access. Increasing substrate concentration can outcompete them.
Non-competitive inhibitors bind to an allosteric site (a site other than the active site). This changes the enzyme's shape so the active site no longer fits the substrate properly. Increasing substrate concentration does not overcome this type of inhibition.
Allosteric enzymes can be regulated by molecules that bind at allosteric sites, switching the enzyme between active and inactive conformations. Feedback inhibition is a key example: the end product of a metabolic pathway binds to an enzyme earlier in the pathway, shutting it down when enough product has accumulated.
The Na+/K+ pump is essential to nerve impulse transmission. Every time a neuron fires, sodium and potassium ions flow through channels, and the pump restores the resting gradient afterwards. Drugs that target this pump (such as digoxin for heart conditions) exploit its central role.
Feedback inhibition is how your body regulates cholesterol synthesis. Statins, one of the most widely prescribed drug classes, are competitive inhibitors of HMG-CoA reductase, the enzyme that catalyses a key step in cholesterol production.
Students often think osmosis moves solute. It does not. Osmosis is the movement of water, not solute.
A common mistake is confusing hypertonic and hypotonic by focusing on solute rather than water. Remember: water moves towards the hypertonic side (more solute, less water).
Students sometimes think competitive inhibitors permanently disable an enzyme. They do not; they are outcompeted by adding more substrate.
Students frequently confuse secondary and tertiary structure. Secondary structure involves hydrogen bonds along the backbone (alpha helices, beta sheets). Tertiary structure involves R-group interactions that fold the whole chain into its 3D shape.
Students often say cholesterol always makes membranes more rigid. Cholesterol is a fluidity buffer: it reduces fluidity at high temperatures and increases it at low temperatures.
⚠️ Know the graph shapes for simple diffusion, facilitated diffusion, and active transport. Simple diffusion is linear. Facilitated diffusion saturates (plateaus). Active transport can push concentration beyond equilibrium.
⚠️ Be able to predict what happens to a cell in a hypertonic, hypotonic, or isotonic solution.
⚠️ Distinguish between primary and secondary active transport, and explain how the Na+/K+ pump works.
⚠️ Know the four levels of protein structure and which types of bonds or interactions stabilise each level.
⚠️ Be ready to explain the difference between competitive and non-competitive inhibition, including what happens when you increase substrate concentration for each.
⚠️ Understand cholesterol's dual role in membrane fluidity (buffer at high and low temperatures).
True or false: Large, charged molecules can pass freely through the phospholipid bilayer.
Fill in the blank: In a hypertonic solution, water ______ the cell, causing it to ______.
True or false: Facilitated diffusion requires ATP.
Fill in the blank: The ______ structure of a protein is determined by interactions between R groups.
True or false: A non-competitive inhibitor can be overcome by adding more substrate.
(Answers: 1. False, they require transport proteins. 2. Leaves, shrivel. 3. False, it is passive transport. 4. Tertiary. 5. False, non-competitive inhibitors change the enzyme's shape regardless of substrate concentration.)
Q: Explain why cholesterol is described as a "fluidity buffer" for the cell membrane.
A: Cholesterol is amphipathic and inserts between phospholipids. At high temperatures, it restricts phospholipid movement, reducing fluidity. At low temperatures, it disrupts tight packing of fatty acid tails, preventing the membrane from becoming too rigid. The net effect is that cholesterol stabilises membrane fluidity across a range of temperatures.
Q: A cell is placed in a solution where solute concentration outside the cell is higher than inside. Describe what happens to the cell and name the type of solution.
A: The solution is hypertonic. Water moves out of the cell by osmosis (from higher water concentration inside to lower water concentration outside), and the cell shrivels.
Q: Compare the graph shapes of simple diffusion and facilitated diffusion as substrate concentration increases. Explain the difference.
A: Simple diffusion produces a linear graph: rate increases proportionally with concentration because there is no protein involved. Facilitated diffusion produces a curve that levels off at a maximum rate (saturation), because once all available channel or carrier proteins are occupied, adding more substrate does not increase the rate.
Q: A student claims that secondary structure is stabilised by R-group interactions. Is this correct? Explain.
A: No. Secondary structure (alpha helices and beta sheets) is stabilised by hydrogen bonds between atoms in the polypeptide backbone, specifically between the carbonyl oxygen of one amino acid and the amino hydrogen of another. R-group interactions stabilise tertiary structure.
Q: How does feedback inhibition work, and why is it useful for the cell?
A: In feedback inhibition, the end product of a metabolic pathway binds to an allosteric site on an enzyme earlier in the pathway, inhibiting its activity. This prevents the cell from overproducing the product when enough has already accumulated, conserving resources and energy.
Membrane transport connects directly to cellular respiration and photosynthesis, where proton gradients (built by active transport) drive ATP synthesis via chemiosmosis.
Protein folding and enzyme regulation tie into genetics and gene expression: a mutation that changes a single amino acid can alter an R group, disrupt folding, and abolish enzyme function, which is the molecular basis of many genetic diseases.
The fluid mosaic model reappears in cell signalling, where receptor proteins embedded in the membrane detect external signals and trigger internal responses.
Cell membrane, phospholipid bilayer, fluid mosaic model, membrane fluidity, cholesterol, saturated fatty acid, unsaturated fatty acid, integral protein, transmembrane protein, peripheral protein, selective permeability, diffusion, simple diffusion, facilitated diffusion, channel protein, carrier protein, osmosis, hypertonic, hypotonic, isotonic, active transport, passive transport, Na+/K+ pump, sodium-potassium pump, primary active transport, secondary active transport, proton gradient, electrochemical gradient, protein folding, primary structure, secondary structure, alpha helix, beta sheet, tertiary structure, quaternary structure, R group, side chain, hydrophobic, hydrophilic, van der Waals, ionic bond, peptide bond, enzyme, catalyst, active site, substrate, competitive inhibitor, non-competitive inhibitor, allosteric site, allosteric regulation, feedback inhibition, conformational change