Cell Structure and Function
Learning Objectives
- Define a cell and distinguish prokaryotic from eukaryotic cell organization.
- Describe the structure and role of the plasma membrane, cytoplasm, and nucleus.
- Explain how mitochondria, the ER, Golgi apparatus, lysosomes, and peroxisomes divide labor inside a cell.
- Relate organelle structure to organelle function (why shape and location matter).
- Identify which organelles are unique to eukaryotes versus present in all cells.
- Apply this knowledge to predict what happens when a specific organelle fails.
Quick Answer
A cell is the smallest unit that can carry out all the processes of life — metabolism, growth, response to stimuli, and reproduction. Every living thing is made of one or more cells, and every cell is bounded by a plasma membrane that separates "living inside" from "non-living outside." Eukaryotic cells (plants, animals, fungi, protists) further divide their interior into membrane-bound organelles, each specialized for a task: the nucleus stores DNA, mitochondria make ATP, the ER and Golgi build and ship proteins, and lysosomes recycle waste. This division of labor is what allows a single eukaryotic cell to be enormously more complex than a bacterial cell, and it's the reason cell structure comes up constantly in genetics, physiology, and disease questions — a broken organelle usually explains a broken phenotype.
What Is a Cell, Really?
Don't think of a cell as a "bag of stuff." Think of it as a self-sufficient, self-repairing chemical factory enclosed by a membrane. It has to import raw materials, export waste, generate its own energy, copy its own instructions, and eventually divide — all without any outside management. Every structure inside a cell exists because it solves one of these problems.
Prokaryotic vs. eukaryotic is the first split you need to make automatically:
- Prokaryotes (bacteria, archaea) have no nucleus and no membrane-bound organelles. Their DNA sits in a region called the nucleoid, free in the cytoplasm. They're small (roughly 1–5 μm) and structurally simple, but metabolically diverse.
- Eukaryotes (animals, plants, fungi, protists) have a true nucleus and a full set of membrane-bound organelles. They're typically 10–100 μm, larger because compartmentalization lets different chemical environments coexist in one cell.
Why does compartmentalization matter? Because many cellular reactions are incompatible with each other if run in the same open space — digestive enzymes that would destroy the cell's own proteins need to be walled off (lysosomes); the reactions of DNA replication need protection from the mechanical chaos of the cytoplasm (nucleus). Membranes let a cell run contradictory chemistry at the same time.
The Plasma Membrane
The plasma membrane is a phospholipid bilayer — two layers of lipid molecules with water-loving (hydrophilic) heads facing out and water-fearing (hydrophobic) tails facing in — studded with proteins. This structure is described by the fluid mosaic model: the membrane isn't a rigid wall, it's a fluid sheet with proteins floating and drifting through it like icebergs in a lipid sea.
Its job is selective permeability: small nonpolar molecules (O2, CO2) slip through the lipid directly; everything else (ions, glucose, large proteins) needs a specific transport protein. This selectivity is exactly what lets a cell maintain an internal composition completely different from its surroundings — for example, keeping potassium concentrated inside and sodium concentrated outside, which is the basis of nerve signaling.
Why it matters: if the membrane were freely permeable, the cell would equilibrate with its environment and die — no concentration gradients, no signaling, no controlled reactions.
Cytoplasm and Cytosol
The cytoplasm is everything inside the plasma membrane but outside the nucleus. The fluid portion is the cytosol; suspended in it are the organelles and the cytoskeleton (actin filaments, microtubules, intermediate filaments), which gives the cell shape, allows organelles to move along tracks, and lets the whole cell crawl or divide.
The Nucleus: The Control Center
The nucleus houses the cell's DNA, organized as chromatin (a loose DNA-protein complex) that condenses into visible chromosomes only during division. It's wrapped in a double membrane (nuclear envelope) studded with nuclear pores that control which molecules — mRNA out, transcription factors in — cross the boundary.
Why double membrane and pores, specifically? Transcription (making RNA from DNA) happens inside the nucleus, but translation (making protein from RNA) happens outside, in the cytoplasm. Separating these two steps lets the cell edit and quality-check RNA (splicing, capping) before it's used to make protein — a level of control prokaryotes, whose transcription and translation are physically coupled, don't have.
The Energy and Manufacturing Organelles
Mitochondria — the ATP generators
Mitochondria have a smooth outer membrane and a highly folded inner membrane (cristae) that dramatically increases surface area for the electron transport chain. The innermost space, the matrix, holds the enzymes of the citric acid cycle. Folding the inner membrane is a direct structure-to-function link: more membrane surface area means more sites for ATP synthase, means more ATP produced per mitochondrion.
Mitochondria also carry their own small circular DNA and can self-replicate — strong evidence for the endosymbiotic theory, which holds that mitochondria were once free-living bacteria engulfed by an ancestral eukaryotic cell roughly 1.5–2 billion years ago.
Endoplasmic reticulum — the assembly line
- Rough ER is studded with ribosomes and manufactures proteins destined for secretion, the membrane, or organelles.
- Smooth ER lacks ribosomes and instead synthesizes lipids, detoxifies drugs (notably active in liver cells), and stores calcium ions for muscle contraction and signaling.
Golgi apparatus — the shipping department
The Golgi receives proteins from the ER at its cis face, chemically modifies them (adding sugar groups — glycosylation — or phosphate groups), sorts them by destination, and packages them into vesicles released from the trans face. Think of the ER as the factory floor and the Golgi as the post office that labels and ships the product.
Lysosomes and peroxisomes — the recycling and detox centers
Lysosomes contain digestive enzymes (kept safely inactive at neutral pH but active at the acidic pH maintained inside the organelle) that break down worn-out organelles, engulfed pathogens, and macromolecules. Peroxisomes oxidize fatty acids and detoxify substances, producing hydrogen peroxide as a byproduct, which they immediately neutralize with the enzyme catalase — keeping this reactive molecule from damaging the rest of the cell.
Real-World Example
In Tay-Sachs disease, a single missing lysosomal enzyme (hexosaminidase A) means a specific lipid can't be broken down. It accumulates in neurons, and the lysosomes swell until the cells die — showing directly how a single organelle malfunction produces a whole-organism disease.
Key Terms
| Term | Definition |
|---|---|
| Prokaryote | Cell type lacking a nucleus and membrane-bound organelles (bacteria, archaea) |
| Eukaryote | Cell type with a true nucleus and membrane-bound organelles |
| Plasma membrane | Phospholipid bilayer with embedded proteins controlling what enters/exits the cell |
| Fluid mosaic model | Model describing the membrane as a fluid lipid sheet with mobile proteins |
| Chromatin | Loosely packed DNA-protein complex found in the nucleus between divisions |
| Cristae | Folds of the mitochondrial inner membrane that increase surface area for ATP production |
| Endosymbiotic theory | Theory that mitochondria and chloroplasts descended from engulfed free-living bacteria |
| Rough/smooth ER | Endoplasmic reticulum with/without ribosomes, for protein vs. lipid synthesis |
| Golgi apparatus | Organelle that modifies, sorts, and packages proteins/lipids from the ER |
| Lysosome | Membrane-bound sac of digestive enzymes for breaking down waste and debris |
| Peroxisome | Organelle that oxidizes fatty acids and detoxifies harmful compounds |
| Cytoskeleton | Network of protein filaments giving the cell shape and enabling movement |
Common Mistakes
Misconception 1: "The nucleus is the 'brain' that actively runs the cell moment to moment." Why it's wrong: The nucleus stores information (DNA) and controls which genes are transcribed, but it doesn't monitor or react to real-time cellular events like a brain does. Correct explanation: The nucleus is more like a reference library — genes are "checked out" (transcribed) when signals from the cytoplasm (via transcription factors) call for them.
Misconception 2: "Mitochondria are only found in animal cells." Why it's wrong: Students often associate mitochondria exclusively with animals because textbooks emphasize the "powerhouse" phrase in that context. Correct explanation: Mitochondria are present in virtually all eukaryotic cells, including plant cells (which also have chloroplasts). Plant cells need mitochondria too, especially in non-photosynthetic tissues like roots.
Misconception 3: "Rough ER and smooth ER are separate, unrelated organelles." Why it's wrong: They're often taught as a list, which makes them feel like two different structures. Correct explanation: Rough and smooth ER are one continuous membrane network; the "rough" regions simply have ribosomes attached and the "smooth" regions don't. Membrane and proteins can move between them.
Comparison and Connections
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (nucleoid region) | Present, membrane-bound |
| Size | ~1–5 μm | ~10–100 μm |
| Membrane-bound organelles | None | Mitochondria, ER, Golgi, lysosomes, etc. |
| DNA | Single circular chromosome | Multiple linear chromosomes |
| Ribosomes | 70S | 80S (70S in mitochondria/chloroplasts) |
| Cell division | Binary fission | Mitosis/meiosis |
| Organelle | Membrane | Primary Job |
|---|---|---|
| Mitochondrion | Double | ATP production |
| Rough ER | Single, continuous with nuclear envelope | Protein synthesis |
| Smooth ER | Single | Lipid synthesis, detoxification |
| Golgi apparatus | Single | Sorting, modifying, packaging |
| Lysosome | Single | Digestion, recycling |
| Peroxisome | Single | Fatty acid oxidation, detox |
Concept Map
Practice Questions
Recall
- What structural feature distinguishes a eukaryotic cell from a prokaryotic cell? Answer guidance: Presence of a true, membrane-bound nucleus and other membrane-bound organelles.
- Name the two faces of the Golgi apparatus and what happens at each. Answer guidance: Cis face receives vesicles from the ER; trans face packages and releases modified products.
Understanding 3. Explain why the inner mitochondrial membrane is highly folded. Answer guidance: Folding (cristae) increases surface area, allowing more ATP synthase complexes and thus more ATP production per organelle. 4. Why does the cell separate rough ER (protein synthesis) from the cytosol instead of having ribosomes just float freely for secreted proteins? Answer guidance: Channeling proteins directly into the ER lumen allows folding, quality control, and tagging for a specific destination (secretion, membrane, lysosome) as they're made — free cytosolic ribosomes make proteins that stay in the cytosol.
Application 5. A drug blocks lysosomal enzyme activity in a patient's cells. What would you expect to accumulate, and why? Answer guidance: Undigested cellular debris and macromolecules (as in lysosomal storage diseases) because the lysosome can no longer break them down. 6. A cell is engineered to lack peroxisomes. What metabolic problem would you predict? Answer guidance: Impaired breakdown of very-long-chain fatty acids and buildup of toxic hydrogen peroxide, since catalase (housed in peroxisomes) would be missing too.
Analysis 7. Compare and contrast the roles of the nucleus and mitochondria in terms of genetic material. Answer guidance: Both contain DNA; the nucleus holds the majority linear genome, mitochondria hold a small circular genome inherited maternally, evidence of their bacterial origin (endosymbiotic theory). 8. Evaluate this claim: "All eukaryotic cells contain the same set of organelles in the same amounts." Is it accurate? Answer guidance: No — organelle abundance reflects cell function. Liver cells have abundant smooth ER for detoxification; muscle cells are packed with mitochondria for ATP; secretory cells have extensive rough ER and Golgi.
FAQ
Q: Do all cells have a nucleus? A: No. Prokaryotic cells (bacteria and archaea) have no nucleus — their DNA is free in the cytoplasm in a region called the nucleoid. Also, mature red blood cells in mammals lose their nucleus during development.
Q: Are mitochondria really "borrowed" bacteria? A: The endosymbiotic theory says yes — mitochondria likely descend from free-living aerobic bacteria engulfed by an early eukaryotic cell. Evidence includes their own circular DNA, double membrane, and 70S ribosomes (bacterial-type, distinct from the 80S ribosomes elsewhere in the eukaryotic cell).
Q: What's the difference between cytoplasm and cytosol? A: Cytoplasm is everything between the plasma membrane and the nucleus, including the organelles. Cytosol is just the fluid part of the cytoplasm, excluding the organelles.
Q: Why do plant cells have both a cell wall and a plasma membrane? A: The plasma membrane controls transport; the rigid cell wall (made of cellulose) provides structural support and prevents the cell from bursting when water enters by osmosis. Animal cells lack a wall and rely on other mechanisms to avoid lysis.
Q: How does the cell know which proteins go to the nucleus, mitochondria, or outside the cell? A: Proteins carry short amino acid sequences called signal or targeting sequences that act like zip codes, directing them to the correct organelle or the secretory pathway.
Quick Revision
- A cell is the smallest independently functioning unit of life.
- Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.
- The plasma membrane is a phospholipid bilayer with embedded proteins (fluid mosaic model); it controls selective permeability.
- Cytoplasm = cytosol + organelles; cytosol is the fluid portion alone.
- The nucleus stores DNA as chromatin and controls gene expression via the nuclear envelope's pores.
- Mitochondria have cristae (folded inner membrane) to maximize ATP production; they carry their own DNA (endosymbiotic theory).
- Rough ER makes and processes proteins; smooth ER makes lipids and detoxifies.
- The Golgi apparatus modifies (glycosylation), sorts, and packages proteins — cis face receives, trans face ships.
- Lysosomes digest waste and debris using enzymes active at acidic pH.
- Peroxisomes oxidize fatty acids and neutralize hydrogen peroxide with catalase.
- Cytoskeleton (actin, microtubules, intermediate filaments) gives shape and enables movement/transport.
- Organelle abundance varies by cell type based on the cell's specialized job.
Related Topics
Prerequisites: Basic biomolecules (lipids, proteins, nucleic acids); the concept of a cell theory.
Related Topics: Cell Membrane and Transport, Cellular Metabolism
Next Topics: Cell Division and Cell Cycle, Cell Signaling and Communication