Unit 3 is about what cells are made of and how their parts work together to keep the cell alive. After it, you should be able to name the major organelles and their jobs, compare prokaryotic and eukaryotic cells, and explain how cells hold their shape and move.
Read the overview first, then work through the review notes in order and answer each checkpoint before you peek at the answer.
This unit zooms in from whole organisms to the single cell. You'll learn how scientists figured out that cells are the basic unit of life, then take apart a cell piece by piece to see what each structure does.
Honors Biology Unit 3 covers cell structure and function. It starts with eukaryotic cell structures, including ribosomes and organelles such as the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, chloroplasts and vacuoles. It also traces how the ER and Golgi process and transport proteins. It then looks at prokaryotic cells, which have a nucleoid and 70S ribosomes but no nucleus. Many have plasmids, and most bacteria have a peptidoglycan cell wall. Next comes the cytoskeleton (microfilaments, intermediate filaments and microtubules), plus cilia and flagella. The unit closes with cell theory, its history from Hooke in 1665 to Virchow in 1855, and the difference between unicellular and multicellular life. Exact order varies by school, and many courses teach the cell membrane and transport right after this unit.
This is cell theory, built over about 200 years of better microscopes. It explains why new cells arise from existing cells, rather than appearing spontaneously under present-day conditions. It does not explain how the first cells originated.
The nucleus stores DNA, ribosomes on the rough ER build proteins, the ER and Golgi process and ship them, lysosomes digest waste, and mitochondria make ATP. Separate compartments let incompatible jobs, like acidic digestion and protein building, happen in the same cell at once.
Bacteria and archaea have a cell membrane, ribosomes and DNA, but their DNA sits in an unenclosed nucleoid. They are usually about 0.1-5 µm across. Their small size helps materials move through the cell over short distances.
Almost every fact in this unit follows this pattern. Mitochondria fold their inner membrane into cristae to fit in more ATP-making surface, cells lining your airway carry hundreds of cilia to sweep mucus, and bacteria stay small to keep a high surface area-to-volume ratio. When you forget what a structure does, look at its shape and ask what that shape would be good for.
Eukaryotic cells split their work among organelles, and each one's structure matches its job.
Prokaryotes (bacteria and archaea) have no nucleus and no membrane-bound organelles, but they still have everything a cell needs to live.
The cytoskeleton is a network of protein fibers that gives a eukaryotic cell its shape, holds organelles in place and moves things around.
Cell theory has three parts.
Open this guide for a closer review of the topic.
Eukaryotic cells split their work among organelles, and each one's structure matches its job. The nucleus stores DNA behind a double-membrane nuclear envelope, and nuclear pores let mRNA out to the cytoplasm. The nucleolus inside makes ribosomal RNA.
Ribosomes build proteins. Free ribosomes make proteins that stay in the cytoplasm or enter structures such as the nucleus and mitochondria. Ribosomes on the rough ER make proteins headed for membranes, lysosomes or export. The smooth ER has no ribosomes and instead makes lipids, breaks down drugs and stores calcium.
Follow a secreted protein like insulin and you trace the endomembrane system in order. It goes from the rough ER into a transport vesicle, through the Golgi apparatus (where it gets modified and sorted), into a secretory vesicle, and out the plasma membrane. The Golgi also sends digestive enzymes to compartments called endosomes, which can mature into lysosomes. Lysosomes use enzymes that work at acidic pH to digest worn-out parts and engulfed bacteria.
Mitochondria release energy from food as ATP through cellular respiration. Plant cells have mitochondria too. Many also have chloroplasts for photosynthesis, and typical plant cells have a cellulose cell wall and a large central vacuole that helps keep the cell firm. Mitochondria and chloroplasts have their own DNA, often represented as circular, and divide by splitting. These features support the endosymbiotic theory that they descended from engulfed bacteria. They still rely on genes in the nucleus, so they can't survive outside the cell.
Check yourself: A pancreas cell exports large amounts of digestive enzymes. Which two organelles would you expect it to have a lot of, and why? (Answer: Rough ER to make the enzyme proteins and Golgi apparatus to modify and package them into vesicles for export.)
Prokaryotes (bacteria and archaea) have no nucleus and no membrane-bound organelles, but they still have everything a cell needs to live. Their chromosome or chromosomes sit in the nucleoid, a region of cytoplasm with no membrane around it. Many prokaryotes have one circular chromosome, but there are exceptions. Many also carry plasmids, small extra DNA circles that often hold genes for antibiotic resistance and can pass between cells.
Prokaryotic ribosomes are 70S, smaller than the 80S ribosomes in eukaryotic cytoplasm. Antibiotics such as tetracycline can target bacterial ribosomes while largely sparing cytoplasmic ribosomes. Mitochondria have bacterial-like ribosomes, so some antibiotics can also affect mitochondrial protein synthesis. Outside the membrane, most bacteria have a cell wall made of peptidoglycan that keeps them from bursting when water rushes in. Archaea have cell walls too, but without peptidoglycan. Some bacteria add a sticky capsule, hair-like pili for attaching, or flagella for swimming.
Worked example. How does size affect a cell's surface area-to-volume ratio? Model a cell as a cube. A cube 1 µm on a side has surface area µm² and volume µm³, so its surface area divided by its volume is 6 µm⁻¹. A cube 10 µm on a side has surface area µm² and volume µm³, so its ratio is 0.6 µm⁻¹. Making the cube 10 times wider cuts its surface area per unit of volume by a factor of 10. A high ratio gives a small cell more membrane area per unit of volume for exchanging materials.
Check yourself: Why can a drug that targets bacterial ribosomes affect bacteria more than protein synthesis in your liver cell cytoplasm? (Answer: Bacteria have 70S ribosomes, while liver cell cytoplasm has 80S ribosomes. Mitochondria have bacterial-like ribosomes, so some drugs can affect mitochondrial protein synthesis too.)
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Examples | Bacteria, archaea | Animals, plants, fungi, protists |
| Typical size | 0.1-5 µm | 10-100 µm |
| DNA location | Nucleoid, no membrane | Nucleus with a double membrane |
| DNA shape | Usually one circular chromosome, often plus plasmids | Several linear chromosomes |
| Membrane-bound organelles | None | Mitochondria, ER, Golgi, lysosomes and more |
| Ribosomes | 70S | 80S in the cytoplasm |
| Cell wall | Usually present (peptidoglycan in bacteria) | Plants (cellulose) and fungi (chitin); none in animals |
The cytoskeleton is a network of protein fibers that gives a eukaryotic cell its shape, holds organelles in place and moves things around. It has three types of filaments.
Microfilaments are the thinnest, about 7 nm wide, and are made of actin. They support the cell's outer edge, push out pseudopodia so cells like amoebas can crawl, slide past myosin to contract muscle, and pinch an animal cell in two during cytokinesis. Intermediate filaments (8-12 nm) are made of tougher proteins like keratin and act as the cell's ropes, resisting pulling forces. Microtubules are the widest, about 25 nm, and are hollow tubes of tubulin. In many animal cells, they grow out from the centrosome. They form the spindle that separates chromosomes and act as tracks for motor proteins such as kinesin and dynein.
Many motile eukaryotic cilia and flagella have microtubules in a 9+2 pattern, with 9 outer pairs around 2 central microtubules. Many primary cilia, which often act as sensors, have a 9+0 pattern instead. Cilia are short and numerous, like the ones sweeping mucus out of your airways. Flagella are long and few, like the single flagellum on a sperm cell.
Worked example. Kinesin carries cargo along microtubules at roughly 1 µm per second. A motor neuron from your spine to your foot can be 1 m long, which is µm. Travel time is s, and days. Without microtubule tracks, simple diffusion over that distance would take far longer.
Check yourself: A drug stops microtubules from forming. Name one process that would fail in a dividing cell. (Answer: The mitotic spindle could not form, so chromosomes would not separate. Vesicle transport along microtubule tracks would also stall.)
| Filament | Protein | Diameter | Main jobs |
|---|---|---|---|
| Microfilament | Actin | About 7 nm | Cell shape at the edges, crawling, muscle contraction, cytokinesis |
| Intermediate filament | Keratin, lamin and others | 8-12 nm | Resists tension, anchors organelles, supports the nucleus |
| Microtubule | Tubulin | About 25 nm | Spindle in cell division, transport tracks, cilia and flagella |
Cell theory has three parts. All living things are made of one or more cells. The cell is the basic unit of structure and function. All cells come from pre-existing cells.
The theory grew with the microscope. In 1665 Robert Hooke looked at cork and named the tiny boxes "cells," though he was seeing only the walls of dead plant cells. In the 1670s Antonie van Leeuwenhoek used stronger lenses to see living bacteria and protists. Matthias Schleiden (plants, 1838) and Theodor Schwann (animals, 1839) concluded that all plant and animal tissue is made of cells. In 1855 Rudolf Virchow popularized the idea that every cell comes from another cell.
Unicellular organisms, like E. coli or an amoeba, are one cell doing every life process. Multicellular organisms, like you, are made of specialized cells that depend on each other. A muscle cell and a neuron usually have the same genome but use different genes, so they look and work differently. Many familiar multicellular organisms are eukaryotes, but some bacteria also form multicellular structures.
Worked example. A microscope has a 10x eyepiece and a 40x objective, so its optical magnification is x. Suppose a cell measures 8 mm across in an image whose final magnification is known to be 400x. Its actual size is mm, or 20 µm. That size is typical of many eukaryotic cells, but size alone cannot identify the cell. A photo may be resized, so use its final magnification or a scale bar rather than assuming it matches the microscope's optical magnification.
Check yourself: Which part of cell theory did Virchow popularize, and what older idea did it argue against? (Answer: That cells come from pre-existing cells, which argued against spontaneous generation under present-day conditions. Robert Remak had presented evidence for cell division before Virchow popularized the idea.)
| Term | Definition |
|---|---|
| Organelle | A specialized structure inside a cell that does a specific job, usually enclosed by a membrane in eukaryotes. |
| Endomembrane system | The connected membranes of the nuclear envelope, ER, Golgi, vesicles, lysosomes and plasma membrane that build and ship proteins and lipids. |
| Endosymbiotic theory | The idea that mitochondria and chloroplasts descended from bacteria engulfed by an ancient cell, supported by their own DNA, ribosomes and double membranes. |
| Nucleoid | The unenclosed region of a prokaryotic cell that holds its chromosome or chromosomes. |
| Plasmid | A small, separate circle of DNA in many bacteria that often carries genes like antibiotic resistance. |
| Peptidoglycan | The sugar-and-amino-acid mesh that makes up bacterial cell walls. |
| Cytoskeleton | The network of microfilaments, intermediate filaments and microtubules that shapes a eukaryotic cell and moves things inside it. |
| Motor protein | A protein like kinesin, dynein or myosin that uses ATP to move along cytoskeletal fibers. |
| Surface area-to-volume ratio | A cell's membrane area divided by its volume. For cells with the same shape, the ratio decreases as size increases, which can limit the exchange of materials. |
| Cell theory | The principle that all living things are made of cells, cells are the basic unit of life, and all cells come from existing cells. |
Because animal cells "get" mitochondria and plant cells "get" chloroplasts in many diagrams, it's easy to think plants skip respiration. Plant cells have mitochondria, and many plant cells also have chloroplasts. Chloroplasts use light energy to help make sugars, while mitochondria break down sugars to make ATP, day and night.
Prokaryotes lack membrane-bound organelles, not all cell structures. They still have a cell membrane, DNA, cytoplasm and 70S ribosomes. Whether you call a ribosome an organelle depends on your textbook, but either way prokaryotes have them, because every cell has to make proteins.
The shared name hides a big difference. Many motile eukaryotic flagella have microtubules in a 9+2 pattern and bend in waves. Bacterial flagella are made of a different protein and spin like a propeller, driven by a rotary motor in the cell membrane.
In plants, fungi and bacteria, the cell wall sits outside the cell membrane, and both are present. The wall gives support and keeps the cell from bursting. The membrane is the selective barrier that controls what goes in and out, so every cell has one, wall or not.