Cell Structure and Function — Study Guide
Sources: OpenStax Biology 2e sections 4.1–4.6 on cell structure and sections 8.1 and 8.2 on photosynthesis (CC BY), which provide the organelle descriptions and photosynthesis terminology this guide summarizes.
Symbols used: μm is micrometer — prokaryotic cells run about 0.1 to 5 μm, eukaryotic cells 10 to 100 μm. DNA, RNA, mRNA, rRNA, and ATP keep their standard meanings. ER is the endoplasmic reticulum; RER and SER mean rough and smooth ER.
PART 1 — CELL THEORY AND THE TWO BIG DIVISIONS
1.1 Cell theory in three sentences
All living things are made of one or more cells; the cell is the basic unit of structure and function in living things; all cells arise from preexisting cells by division. Every exam question about cells stands on one of those three legs.
1.2 Prokaryotic versus eukaryotic
| Feature | Prokaryote | Eukaryote |
|---|
| Nucleus | No — DNA in a nucleoid region | Yes — double-membrane envelope |
| Membrane-bound organelles | None | Many (ER, Golgi, mitochondria, and the rest) |
| DNA | Usually circular in a nucleoid; DNA-binding proteins present | Nuclear chromosomes linear and histone-packaged |
| Size | About 0.1-5 μm | About 10-100 μm |
| Ribosomes | Smaller (70S) | Larger (80S) |
| Division | Binary fission | Mitosis (and meiosis for gametes) |
| Examples | Bacteria, archaea | Animals, plants, fungi, protists |
The trap in the first row: prokaryotes have no membrane-bound organelles, but they do have ribosomes — ribosomes lack a membrane, so they do not count against prokaryotes. Every living thing makes proteins.
1.3 Animal versus plant in one look
Typical textbook animal cells and photosynthetic plant cells share many eukaryotic organelles. Plant cells also have a cellulose cell wall and a large central vacuole, while chloroplasts occur in photosynthetic tissues rather than in every plant cell. Many animal cells have centrioles and prominent lysosomes, and they lack a cell wall. The comparison tab of this pack carries the full table.
1.4 Part 1 checklist
PART 2 — THE CONTROL CENTER: NUCLEUS AND GENETIC MACHINERY
2.1 Nucleus
The command center: a double membrane (the nuclear envelope) perforated by pores that control what enters and leaves; chromatin (DNA plus proteins) inside; and a dense nucleolus where ribosomal RNA is made and ribosome subunits are assembled. The nucleus stores information — it does not make proteins.
2.2 Ribosomes
Granules of ribosomal RNA and protein — not membrane-bound, so they count as organelles that prokaryotes also own. Free ribosomes synthesize proteins that remain in the cytosol or are imported after translation into destinations such as the nucleus, mitochondria, and peroxisomes; ribosomes directed to the rough ER synthesize secreted, membrane, ER, Golgi, and lysosomal proteins. Both are the site of translation — reading mRNA into a protein chain.
2.3 Rough and smooth ER
Two faces of one network. Rough ER is studded with bound ribosomes: it folds and quality-checks proteins destined for secretion, membranes, or lysosomes. Smooth ER lacks ribosomes and makes lipids and phospholipids, detoxifies drugs and poisons (liver cells are full of it), and stores calcium ions for muscle contraction.
2.4 Golgi apparatus
The post office: flattened stacks with a receiving (cis) face and a shipping (trans) face. It modifies proteins and lipids (adds sugars, cuts chains), sorts them, and packs them into vesicles — some for secretion, some bound for lysosomes or other destinations.
2.5 Lysosomes and vacuoles
Lysosomes are the recycling centers: membrane sacs of acid hydrolases working at about pH 4.5-5 that digest worn organelles, engulfed particles, and macromolecules. Lysosomal membrane damage can contribute to cell injury or regulated death, but rupture is not the normal mechanism of apoptosis. They are characteristic of animal cells; in plants the central vacuole performs much of the same disposal work. Vacuoles are general storage sacs: the plant central vacuole stores water and keeps the cell pressurized and firm (turgor), which is why a dry plant wilts.
2.6 Part 2 checklist
PART 3 — THE ENERGY CENTERS: MITOCHONDRIA AND CHLOROPLASTS
3.1 Mitochondrion
The powerhouse: a double-membrane organelle whose inner membrane folds into cristae and encloses the matrix. Most aerobic respiration runs there after glycolysis in the cytosol: mitochondria use pyruvate-derived fuel and oxygen to produce ATP, CO2, and H2O. Two exam extras: mitochondria carry their own DNA and ribosomes; in humans and many other animals, mitochondrial DNA is usually maternally inherited, and their number tracks the cell's energy appetite — heart muscle and sperm midpieces are packed with them.
3.2 Chloroplast
The plant counterpart (plus algae): the green plastid packed with chlorophyll. Inside the chloroplast, flattened membrane sacs are called thylakoids; thylakoids stack into grana, and the surrounding fluid is the stroma. Photosynthesis runs there — light energy, water, and carbon dioxide in, glucose and oxygen out. Both organelles are semiautonomous: they retain DNA and ribosomes and use fission-like division, but host-cell signals and nuclear-encoded proteins coordinate their biogenesis and division.
3.3 Endosymbiotic theory — the evidence exam
Mitochondria and chloroplasts were once free-living bacteria that moved into a host cell. Four lines of evidence are tested year after year: (1) double membranes — the inner one is the bacterium's own; (2) their own circular DNA; (3) their own ribosomes, and the small bacterial kind; (4) they divide by fission-like mechanisms, although host-cell signals and nuclear-encoded proteins coordinate that division. Each line maps to a bacterial feature — say them as pairs.
3.4 Part 3 checklist
PART 4 — THE FACTORY FLOOR: MEMBRANE, CYTOSKELETON, AND SUPPORT
4.1 Plasma membrane
Every cell's boundary is a phospholipid bilayer — hydrophilic heads out, hydrophobic tails in — studded with proteins, cholesterol (animal cells), and carbohydrates: the fluid mosaic model. It is selectively permeable: small nonpolar molecules pass, ions and large polar molecules need help. The membrane is the gatekeeper of diffusion and osmosis questions.
4.2 Cytoskeleton — three rope sizes
- Microfilaments (actin, thinnest): muscle contraction, cell division's cleavage furrow, cell crawling.
- Intermediate filaments (keratin family): maintain shape and anchor organelles — the scaffold.
- Microtubules (tubulin, thickest): rails for vesicle traffic, the mitotic spindle, and the core of cilia and flagella (the 9+2 arrangement).
Typical animal cells organize spindle microtubules from centrosomes containing centrioles; most higher plant cells build spindles without centrioles.
4.3 Walls and connections
Plants: a cellulose cell wall outside the membrane — rigid box, resists turgor; neighboring plant cells connect through plasmodesmata. Fungi: chitin walls. Animals: no wall — junctions do the anchoring (tight junctions seal, desmosomes rivet, gap junctions pass molecules).
4.4 Part 4 checklist
PART 5 — THE PROTEIN ASSEMBLY LINE (THE ORDERING QUESTION)
5.1 The path of a secreted protein
Nucleus: DNA transcribes to mRNA → mRNA exits a nuclear pore → ribosome bound on rough ER translates it into the ER lumen → RER folds and checks it → transport vesicle buds off → vesicle fuses with the Golgi cis face → Golgi modifies, sorts, packages at the trans face → secretory vesicle heads to the membrane → exocytosis releases the protein. Every step is a classic drag-and-drop exam item; memorize it as a commute with five stations.
5.2 Branch lines on the commute
A Golgi package can instead go to a lysosome (digestive enzymes in, acid-stable), back to the ER, or into the membrane itself. A protein meant for the cell's own cytosol never boards the line at all — a free ribosome makes it in place.
5.3 Structure follows job — the abundance questions
- Pancreas cell shipping digestive enzymes: RER and Golgi stacked high.
- Liver cell clearing drugs: smooth ER multiplied.
- Heart muscle or sperm: mitochondria packed.
- Leaf cell: chloroplasts packed.
- White blood cell that eats bacteria: lysosomes abundant.
Exams invert this: they show you the abundance and ask for the job.
5.4 Part 5 checklist
PART 6 — THE LABELING TOUR (QUICK-ID TABLE)
| Organelle | Look | One-line job | Found in |
|---|
| Nucleus | Largest, double membrane with pores | Stores DNA; command center | Eukaryotes |
| Nucleolus | Dense spot inside nucleus | Builds ribosome subunits | Eukaryotes |
| Ribosome | Tiny dots, no membrane | Protein synthesis (translation) | All cells |
| Rough ER | Membrane sheets studded with dots | Fold and ship proteins | Eukaryotes |
| Smooth ER | Membrane sheets, no dots | Lipids, detox, calcium storage | Eukaryotes |
| Golgi | Stacked flat sacs | Modify, sort, package | Eukaryotes |
| Lysosome | Small sacs of enzymes | Digestion and recycling | Animals mainly |
| Vacuole | Large central sac (plants) | Storage and turgor | Plants large; animals small |
| Mitochondrion | Bean with inner folds (cristae) | Aerobic respiration after glycolysis → ATP + CO2 + H2O | Eukaryotes |
| Chloroplast | Green, stacks of coins | Photosynthesis | Plants, algae |
| Cytoskeleton | Three rope sizes | Shape, movement, transport | All eukaryotes |
| Cell wall | Rigid outer box | Support and protection | Plants (cellulose), fungi (chitin) |
| Plasma membrane | Bilayer skin | Selective gatekeeper | All cells |
Read a row, cover the job column, recite. That is the labeling drill this pack replaces a microscope with.
PART 7 — EXAM PATTERNS AND TRAPS
7.1 The five question patterns
- Identify from description: “vesicles of hydrolytic enzymes” → lysosome; “stacks of flattened sacs” → Golgi.
- Function-to-structure: which organelle folds secreted proteins → RER; which makes lipids → SER.
- Abundance logic: a cell packed with mitochondria does energy-hungry work.
- Order the commute: the protein assembly line, shuffled.
- Big-division sorting: prokaryote vs eukaryote, animal vs plant, and the endosymbiosis evidence list.
7.2 Traps that cost points
- “Prokaryotes have no ribosomes.” False — no membrane-bound organelles, but ribosomes yes.
- “The nucleus makes proteins.” False — it stores the plans; ribosomes translate.
- “Mitochondria and chloroplasts make ATP the same way.” They both make ATP, but respiration consumes oxygen while photosynthesis releases it — opposite oxygen directions.
- “Lysosomes exist only in animal cells.” Standard teaching: characteristic of animals; plants lean on the central vacuole. Expect the textbook answer.
- “DNA lives only in the nucleus.” Mitochondria carry their own DNA, and chloroplasts do as well in photosynthetic eukaryotes.
- “The cell wall is the membrane.” The wall is the outer box; the membrane is the living skin underneath.
- “All plant cells have centrioles.” Most higher plant cells lack centrioles but still build spindles.
APPENDIX A — ONE-PAGE RECALL SHEET
Cell theory: all life is cellular, the cell is the unit, cells come from cells. Prokaryote: no nucleus, no membrane-bound organelles, circular DNA, 0.1-5 μm, fission. Eukaryote: nucleus, organelles, linear DNA on histones, 10-100 μm, mitosis. Nucleus stores; nucleolus builds ribosome parts; ribosomes translate; RER folds export proteins; SER makes lipids and detoxes; Golgi modifies-sorts-packages; lysosomes digest; vacuoles store; mitochondria respire; chloroplasts photosynthesize; cytoskeleton holds and moves; membrane gates; wall boxes. Protein commute: nucleus → RER → vesicle → Golgi → vesicle → membrane. Endosymbiosis evidence: double membranes, own circular DNA, own ribosomes, fission.
APPENDIX B — TRAP LIST
- Ribosomes belong to every cell — membrane-free organelles.
- Free ribosomes serve the cell; bound ribosomes serve export.
- Smooth ER has no ribosomes and never makes proteins.
- Oxygen directions: respiration consumes, photosynthesis produces.
- Most higher plant cells build spindles without centrioles; animal cells maintain shape without cell walls.
- Turgor is the central vacuole's water pressure — wilting is its loss.
- Fluid mosaic means the membrane is a two-dimensional fluid of mobile proteins.
- The Golgi's cis face receives; trans face ships — never the reverse.
These are original HyNote study notes for introductory biology students. They summarize the public OpenStax Biology 2e sections on cell structure and photosynthesis (CC BY) in original wording. They are not exam questions and are not affiliated with any examination board.
