Cell Structure and Function: The MDCAT Unit 1 Guide
Every organelle, every membrane, every high-yield fact you need from MDCAT Biology's first — and most frequently tested — chapter.
If there is one chapter that quietly decides how comfortable the rest of your MDCAT Biology preparation feels, it is Cell Structure and Function. Almost every later chapter — genetics, physiology, biotechnology — assumes you already know your way around a ribosome, a Golgi apparatus, and a plasma membrane. Get this chapter solid now, and the rest of the syllabus stops feeling like new information and starts feeling like the same ideas, repeated in new contexts.
This guide walks through the chapter the way MDCAT actually tests it: not as a list of organelle names to memorize, but as a set of structure-function relationships, comparisons, and "except" questions that examiners return to year after year.
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60 real MDCAT-style MCQs from this chapter, a live 60-minute timer, and instant scoring with a full answer review.
What Makes This Chapter High-Yield
Cell Structure and Function typically contributes several direct questions to the MDCAT paper every year, but its real weight is hidden: concepts from this chapter reappear inside questions labeled "Biomolecules," "Cell Cycle," "Support and Movement," and even "Reproduction." Examiners are especially fond of three question formats here:
- Comparison questions — plant vs animal cell, prokaryotic vs eukaryotic, primary vs secondary lysosome.
- "Except" questions — "All of the following are true except…" — which reward students who know boundaries, not just definitions.
- Structure-to-function questions — given a structure (e.g., cristae, tonoplast), identify what it does or where it's found.
Prokaryotic vs Eukaryotic Cells
Every cell on Earth fits into one of two categories, and MDCAT loves testing the line between them.
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (nucleoid region only) | Present, membrane-bound |
| Histones | Absent | Present |
| Ribosomes | 70S | 80S (cytoplasmic); 70S in mitochondria/chloroplast |
| Cell division | Binary fission | Mitosis / meiosis |
| Respiration site | Mesosomes | Mitochondria |
| Example | Bacteria | Plant, animal, fungal cells |
A frequently tested fact: ribosomes are the one structure common to both prokaryotic and eukaryotic cells — every other membrane-bound organelle is exclusive to eukaryotes.
Cell Wall & Middle Lamella
The cell wall gives plant, fungal, and bacterial cells their shape and protects them from osmotic bursting — but its chemistry differs sharply between kingdoms, and MDCAT tests exactly this difference:
- Plant cell wall: cellulose microfibrils, cemented by lignin (in secondary walls) and pectin.
- Fungal cell wall: chitin.
- Bacterial cell wall: peptidoglycan (murein) — the entire structure is sometimes called the sacculus.
Between two adjacent plant cells lies the middle lamella, composed mainly of calcium pectate — it is the "glue" that holds neighboring cell walls together, not part of either individual cell's own wall.
Plasma Membrane & the Fluid Mosaic Model
The plasma membrane is the single most tested structure in this chapter, largely because the fluid mosaic model (Singer & Nicolson, 1972) packs several testable ideas into one concept:
- The membrane is a phospholipid bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward on both surfaces.
- It is asymmetrical — the two halves of the bilayer differ in protein and lipid composition.
- It is selectively permeable, not freely permeable — small nonpolar molecules and gases cross easily, while ions need channel proteins.
- Cholesterol, wedged among the phospholipids, buffers membrane fluidity against temperature change.
- Membrane proteins act as channels, carriers, receptors, and enzymes — for example, adenylate cyclase functions as an enzyme embedded in the membrane, not a receptor itself.
Cytoplasm, Ribosomes & the Endomembrane System
The cytoplasm is more than empty filler — it's the site of glycolysis, protein synthesis, and cyclosis (the streaming movement that helps distribute organelles like mitochondria through the cell). Within it, the endomembrane system works as a coordinated production line:
- Ribosomes (free or RER-attached) synthesize proteins. Free ribosomes make proteins used inside the cell (e.g., insulin, hemoglobin); RER-bound ribosomes make proteins for export or membrane insertion.
- Rough Endoplasmic Reticulum (RER) processes and packages these proteins, sending them onward to the Golgi apparatus.
- Smooth Endoplasmic Reticulum (SER) handles lipid metabolism, calcium storage, and drug/toxin detoxification in liver cells — it does not synthesize proteins.
- Golgi apparatus modifies, sorts, and packages materials arriving from the RER. Its cis (forming) face is convex, and its trans (maturing) face is concave — a detail MDCAT has asked directly.
The secretory pathway for an exportable protein, such as a pancreatic digestive enzyme, therefore follows: Ribosome → RER → Golgi apparatus → secretory vesicle.
Mitochondria, Plastids & Lysosomes
Mitochondria
Mitochondria are the cell's energy centers, and MDCAT tests their unique, semi-autonomous nature:
- Contain their own circular, double-stranded DNA and 70S ribosomes — evidence for the endosymbiotic theory.
- The inner membrane folds inward to form cristae, increasing surface area for ATP synthase.
- Krebs cycle reactions occur in the matrix (inner compartment); the electron transport chain occurs on the cristae.
- Their number per cell rises and falls with a tissue's physiological (metabolic) activity — muscle and liver cells are mitochondria-rich.
Plastids
All plastids develop from a common precursor, the proplastid, and can interconvert between types:
- Chloroplasts — photosynthesis; the Calvin cycle occurs in the stroma, light reactions on the thylakoid membranes.
- Chromoplasts — carotenoid pigments that color petals and fruits, aiding pollination and seed dispersal.
- Leucoplasts — colorless, storage plastids (amyloplasts store starch, elaioplasts store lipids, aleuroplasts store proteins).
Lysosomes
Membrane-bound sacs of hydrolytic enzymes, produced by budding from the Golgi apparatus. A primary lysosome has not yet fused with material to digest; a secondary lysosome (e.g., a phagocytic or digestive vacuole) has fused with its target. Enzyme deficiencies here cause lysosomal storage disorders — Tay-Sachs disease is the classic MDCAT example.
The Nucleus
As the cell's control center, the nucleus is tested through its boundary and its contents:
- The nuclear envelope is double-membraned and porous, studded with nuclear pores that regulate exchange between nucleoplasm and cytoplasm — it is permeable to ribosomal subunits but not freely permeable the way a simple membrane would be.
- The nucleolus assembles ribosomal subunits and contains the rRNA-coding DNA.
- Chromatin condenses into chromosomes during division — this is the defining structural output of the nucleus.
Common Mistakes MDCAT Students Make
Ready to Test Your Recall?
This chapter is best locked in through active recall, not re-reading. The attached live test pulls 60 exam-style MCQs from this exact chapter — cell wall, plasma membrane, cytoplasm, organelles, and the nucleus — with a live 60-minute timer and an instant, question-by-question answer review.
Open the 60-MCQ Live Test →
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