01 / READINGA selective membrane, not a rigid wall
Phospholipids form a bilayer with hydrophobic regions facing inward and polar heads facing aqueous surroundings. Membrane proteins contribute to transport, signaling and adhesion. Cholesterol modulates bilayer properties rather than simply hardening it at every temperature. Outward-facing carbohydrate chains on glycoproteins and glycolipids contribute to the glycocalyx. The two membrane faces differ in composition and function.
Section references: [1] [3]
02 / READINGFollowing a gradient or using energy
Simple and facilitated diffusion are passive: net movement follows the relevant gradient. Facilitated diffusion uses channels or carriers, but protein involvement does not make it active transport. Ion movement depends on an electrochemical gradient combining concentration and voltage. Primary active transport uses energy directly; secondary active transport uses energy stored in another gradient. Osmosis concerns water movement, while tonicity depends on solutes that do not readily cross the relevant membrane.
Section references: [3]
03 / READINGOrganelles cooperate in integrated pathways
Ribosomes synthesize proteins. Rough-ER-associated ribosomes produce many secreted and membrane-system proteins, which the Golgi modifies and sorts. Lysosomes degrade material in an acidic environment; peroxisomes perform particular oxidative reactions. Mitochondria are central to respiratory ATP production but are not the cell’s only ATP source. Nucleus, cytosol and organelles exchange information and material, so compartmentalization supports coordination rather than complete isolation.
Section references: [1] [2]
04 / READINGCell cycle, chromosomes and cell death
DNA replicates during S phase; mitosis distributes sister chromatids into daughter nuclei. G1 and G2 provide growth and preparation intervals. Cytokinesis divides cytoplasm and differs from nuclear division. A replicated chromosome has two chromatids, but chromosome counting follows centromeres: DNA replication does not automatically double ploidy. Meiosis reduces ploidy during gamete formation. Apoptosis is regulated cellular elimination, not merely a pause in the cell cycle.
Section references: [4]
COMPARISONDistinguish before memorizing
Structures and concepts in this track| Structure or concept | Distinguishing feature |
|---|
| Facilitated diffusion | Carrier or channel; follows the relevant gradient. |
|---|
| Primary active transport | Direct energy use for transport. |
|---|
| Rough ER / Golgi | Synthesis and early processing / modification and sorting. |
|---|
| S phase / mitosis | DNA replication / chromatid separation. |
|---|
Reasoning pitfalls
- Classifying every protein-mediated transport process as active.
- Confusing a chromatid with a homologous chromosome.
- Calling the Golgi the site of protein synthesis.
WORKED QUESTIONSUnderstand the answer and the alternatives
Try answering before opening the explanation. Each example develops a point from the guide.
QUESTION 1A cell must delimit its cytoplasm and selectively control exchange with its surroundings. Which lipid-bilayer-and-protein structure performs this role?
- Plasma membrane
- Phospholipid
- Membrane cholesterol
- Glycocalyx
Show answer and reasoning
Answer: Plasma membrane
Plasma membrane combines a lipid bilayer with proteins and associated components. Together they support selective exchange, signaling and adhesion. The whole structure differs from a single molecule within it.
Why the other options do not fit: Phospholipid and cholesterol are lipid components. Glycocalyx is a carbohydrate-rich outer surface covering rather than the whole bilayer delimiting cytoplasm.
In General review, which term names “An amphipathic molecule with a polar head and two hydrophobic tails”?
- Plasma membrane
- Phospholipid
- Membrane cholesterol
- Glycocalyx
Show answer and reasoning
Answer: Phospholipid
A phospholipid has polar and hydrophobic regions, enabling bilayer organization in water. In the usual glycerophospholipid model, a polar head is associated with two hydrocarbon tails. This property underlies the membrane barrier.
Why the other options do not fit: Plasma membrane is the larger molecular assembly. Cholesterol has a distinct steroid structure rather than the described two-tail model. Glycocalyx is carbohydrate-rich.
Identify the concept described as “A lipid interspersed among phospholipids of animal-cell membranes”.
- Plasma membrane
- Phospholipid
- Membrane cholesterol
- Glycocalyx
Show answer and reasoning
Answer: Membrane cholesterol
Cholesterol intercalates among animal membrane phospholipids and modulates fluidity and permeability. Its effect depends on bilayer conditions rather than simply hardening membranes at every temperature.
Why the other options do not fit: Phospholipids form the bilayer framework. Membrane names the whole structure and glycocalyx the outer carbohydrate-rich layer. None specifically identifies the requested steroid lipid.
ACTIVE RECALLClose the text and reconstruct
- Explain why an ion channel can mediate passive transport.
- Trace a secreted protein from ribosome to extracellular space.
- Compare DNA amount and ploidy before and after S phase.
If a relationship is missing, return to the relevant section. Then practice a short set and save persistent uncertainties in your review notebook.
Start this track’s QUIZ ↗ 