Cell Structure

Main notes

Cells are the basic units of life, and their internal organization determines what an organism can do. This chapter maps the parts of the cell, from the nucleus and organelles to the membranes and the internal skeleton, and explains how they work together. It builds on the chemistry of macromolecules from earlier topics and sets up the next topic, in which the plasma membrane and its transport functions take center stage.

Prokaryotic vs Eukaryotic

All living things are built from cells of two fundamental types, the prokaryotes and the eukaryotes. Prokaryotes, the bacteria and archaea, are small and simple, with their DNA loose in a region called the nucleoid instead of inside a nucleus. Eukaryotes, the protists, fungi, plants, and animals, are larger and far more compartmentalized, with a true nucleus and a full set of membrane-bound organelles. The word "eukaryote" means "true kernel," and "prokaryote" means "before kernel," a reminder that the eukaryotic design appeared later in evolution.

Both cell types share essential machinery. Every cell has a plasma membrane, the boundary layer that encloses the contents, a cytoplasm where the chemistry of life runs, ribosomes that build proteins, and DNA that stores instructions. The two types differ sharply in scale and organization. Prokaryotes are typically about 0.5 to 5 micrometers across, while most eukaryotic cells measure about 10 to 100 micrometers. Prokaryotes usually carry one circular chromosome and often extra rings of DNA called plasmids, whereas eukaryotes keep multiple linear chromosomes. Prokaryotic ribosomes are 70S particles and eukaryotic ribosomes are 80S particles; the S stands for Svedberg units, a measure of how fast a particle settles when spun in a centrifuge.

Common Mistake: Assuming that prokaryotes are "less evolved" because they are small and simple. Prokaryotes are not failed eukaryotes; they dominate the planet in numbers and have thrived for billions of years. They simply run a faster, simpler cellular plan.

ELI-10

Think of a studio apartment versus a house with many rooms. In a studio, everything happens in one open space, just as a bacterium runs its whole life in a single compartment. In a house, each room has a special job, like the kitchen for cooking and the bedroom for sleeping. Eukaryotic cells work the same way, with separate rooms, called organelles, doing separate jobs.

Organelle Functions

Each organelle is a tiny machine with a specific job. The nucleus houses the DNA and acts as the control center; a region inside it, the nucleolus, builds the RNA components of ribosomes. Ribosomes, made of RNA and protein, carry out protein synthesis, translating messenger RNA into proteins. Ribosomes floating free in the cytoplasm make proteins that stay in the cell, while ribosomes anchored to the rough endoplasmic reticulum make proteins destined for membranes or export.

The rough endoplasmic reticulum is studded with ribosomes and folds new proteins. The smooth endoplasmic reticulum makes lipids, stores calcium, and detoxifies drugs. The Golgi apparatus receives proteins and lipids, modifies them, and sorts them into vesicles, small membrane sacs that deliver cargo. Lysosomes are the digestive bags of animal cells, breaking down worn-out parts and engulfed material. Peroxisomes run chemical reactions that produce hydrogen peroxide and then safely destroy that dangerous compound. Vacuoles store water, ions, and wastes, and a single large central vacuole helps plant cells hold their shape. Mitochondria carry out cellular respiration, converting fuel into ATP, the energy currency of the cell. Chloroplasts, found in plants and algae, capture light energy in photosynthesis and make sugars. Together these organelles let a single cell feed, breathe, clean house, and reproduce.

Common Mistake: Believing that plant cells have chloroplasts, so they must not need mitochondria. Plant cells run both machines. Chloroplasts build sugars, and mitochondria burn those sugars for ATP day and night.

ELI-10

Picture a busy factory. The main office holds the blueprints, just as the nucleus holds the DNA. Workstations build products from the blueprints, just as ribosomes build proteins. Delivery vehicles move boxes between stations and out the loading dock, just as vesicles carry materials through the cell. A power plant on the side supplies electricity, just as mitochondria supply the energy for everything the factory does.

The Endomembrane System

The endomembrane system is a coordinated set of compartments that make, modify, and ship proteins and lipids. Its members are the nuclear envelope, the rough and smooth ER, the Golgi apparatus, vesicles, lysosomes, and the plasma membrane. The nuclear envelope is a double membrane around the nucleus, and it is continuous with the rough ER. Newly made proteins enter the ER lumen, where they fold and receive chemical tags. Vesicles then ferry them to the Golgi apparatus, where enzymes finish the job, and finally to the plasma membrane or to other organelles. The system also renews the plasma membrane itself, the boundary layer that encloses every cell.

The path of a secreted protein follows a fixed order:

1. A ribosome on the rough ER builds the protein.
2. The protein folds inside the ER lumen.
3. A vesicle buds off the ER and fuses with the Golgi apparatus.
4. The Golgi modifies and packages the protein.
5. A vesicle delivers the protein to the plasma membrane for release.

Each step depends on the one before it; the ER never sends cargo straight to the plasma membrane. The smooth ER also feeds this system, adding lipids to membranes and helping shape its compartments.

Common Mistake: Thinking the ER, Golgi, and plasma membrane are physically joined like pipes. They are separate compartments that exchange cargo by vesicles, which bud off one membrane and fuse with another. No continuous tube runs between them.

ELI-10

Imagine ordering a package online. The item is built in a workshop, then driven to a sorting center, where it is repackaged and labeled. From there a truck carries it to your door. A cell builds and ships its proteins the same way, with each stop passing the cargo to the next. The package never skips a stop, and neither does a protein.

The Cytoskeleton

The cytoskeleton is a dynamic protein scaffold that gives the cell its shape, anchors organelles, and moves materials, and even whole cells. It is built from three filament systems. Microtubules are hollow tubes of the protein tubulin; they act as tracks for motor proteins, form the mitotic spindle during cell division, and make up the core of cilia and flagella, the whiplike projections that move cells, arranged in a distinctive nine plus two pattern. Microfilaments are thin solid rods of the protein actin that reinforce the cell cortex, drive muscle contraction together with myosin, and power cell crawling such as amoeboid movement. Intermediate filaments are tough rope-like cables, including keratins, that resist stretching and hold organelles in place.

The cytoskeleton is not a fixed frame. It constantly assembles and disassembles, and microtubules grow out from the centrosome, an organizing center near the nucleus. This remodeling lets a cell change shape, crawl, and divide on demand, and it is what separates the chromosomes during mitosis.

Common Mistake: Treating the cytoskeleton as a passive frame, like the steel beams of a building. It is active: motor proteins walk along microtubules, and filaments assemble and fall apart continuously. The cell's shape is a process, not a fixed structure.

ELI-10

Think of a marionette puppet. Stiff rods give the arms and legs their shape, like one set of filaments. Strings pull the limbs to make the puppet move, like motor proteins pulling along their tracks. Extra tough cables keep the head from drooping, like the strongest filaments. A puppet needs all three parts to stand, dance, and stay together, and a cell does too.

Endosymbiotic Theory

The endosymbiotic theory explains how eukaryotes acquired their two energy organelles. Long ago, an ancestral eukaryotic cell engulfed an aerobic bacterium, and the two began to live together; over time the bacterium became the mitochondrion. A later, similar event with a photosynthetic cyanobacterium gave rise to the chloroplast in the ancestors of plants and algae. The theory was championed by Lynn Margulis in the 1960s, building on ideas first proposed in the early twentieth century.

The evidence is strong. Mitochondria and chloroplasts each have a double membrane, and the inner membrane resembles a bacterial membrane more than it resembles the rest of the cell. They carry their own small circular DNA, like a bacterial chromosome, and their own 70S ribosomes, matching bacteria rather than the 80S ribosomes of the eukaryotic cytoplasm. They divide by a process similar to bacterial binary fission. Antibiotics that jam bacterial ribosomes also jam mitochondrial and chloroplast ribosomes. Finally, most mitochondrial genes have moved to the nucleus over time, which is why the organelles can no longer live on their own.

Common Mistake: Assuming that because mitochondria were once bacteria, they can still survive independently today. They cannot; most of their proteins are now encoded by nuclear genes. The partnership is permanent.

ELI-10

Imagine that you invite a houseguest to stay, and over many years the guest becomes part of the household, cooking every meal. Now the household cannot function without the guest, and the guest cannot survive anywhere else. A very long time ago, one cell swallowed another, and the pair grew so dependent on each other that they became a single cell. That is why your cells run on tiny power plants that were once free-living bacteria.

Plant vs Animal Cells

Plants and animals are both eukaryotic, so their cells share the same basic parts, but each cell type carries extras suited to its lifestyle. Plant cells have a cell wall made of cellulose, a stiff outer layer that provides support, while animal cells have only the flexible plasma membrane. Plant cells keep a large central vacuole that stores water and presses against the wall, creating turgor pressure that holds stems upright. Animal cells rely on lysosomes for digestion, while plant cells rarely have them and digest materials in their vacuoles instead. Plant cells contain chloroplasts and usually lack centrioles, the paired structures that help organize microtubules during animal cell division.

Common Mistake: Saying that animal cells have a cell wall. Only plants, fungi, and many prokaryotes have walls; the animal boundary is the plasma membrane alone.

ELI-10

Think of a sandcastle versus a water balloon. The sandcastle keeps its shape because of its stiff walls, just as a plant cell is held firm by its cell wall. The water balloon has no wall, so it wobbles and changes shape freely, just like an animal cell. Both are full of water, and that shared water stands in for the machinery that every cell carries inside.

High-Yield:

• Prokaryotes keep their DNA in a nucleoid and lack membrane-bound organelles; eukaryotes have a true nucleus and a full set of organelles.
• Match each organelle to one main job: the nucleus directs, ribosomes build, the ER and Golgi process and ship, and mitochondria and chloroplasts supply energy.
• A secreted protein always travels rough ER, then Golgi, then vesicles, then plasma membrane, never skipping a stop.
• The cytoskeleton uses three filament types: microfilaments, intermediate filaments, and microtubules.
• Double membranes, circular DNA, and 70S ribosomes in mitochondria and chloroplasts are the fingerprints of endosymbiosis.

Quick Review

• A prokaryotic cell has no nucleus and no membrane-bound organelles; a eukaryotic cell has both.
• Eukaryotic ribosomes are 80S, prokaryotic ribosomes are 70S, and cells range from about 0.5 to 5 micrometers for prokaryotes to about 10 to 100 micrometers for eukaryotes.
• Each organelle has one main job, from the nucleus that directs the cell to the lysosome that digests waste.
• The endomembrane system moves proteins step by step: rough ER, Golgi, vesicles, plasma membrane.
• The cytoskeleton, built from microfilaments, intermediate filaments, and microtubules, shapes the cell and drives movement and division.
• Mitochondria and chloroplasts carry their own DNA and 70S ribosomes, strong evidence for their endosymbiotic origins.
• Plant cells have a cell wall and chloroplasts, while animal cells rely on lysosomes and centrioles.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

See the formulas, procedures, and process blocks in the main notes where applicable.

Common mistakes

See the labeled common-mistake callouts in the main notes where present.