Eukaryotic Cell Structure and Function

Core Concepts

The nucleus and nuclear envelope

The nucleus contains most of the cell's DNA and is the site of DNA replication and transcription. The nuclear envelope — a double membrane — separates the nucleus from the cytoplasm. Nuclear pores, large protein complexes that span the envelope, regulate the passage of molecules (such as mRNA and proteins) between the nucleus and cytoplasm.

The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosomal subunits are assembled. It is not membrane-bound.

Ribosomes

Ribosomes are the sites of protein synthesis (translation). They consist of two subunits (large and small), each composed of rRNA and proteins. Ribosomes can be:

• Free ribosomes: Suspended in the cytosol; synthesize proteins that function in the cytosol.

• Bound ribosomes: Attached to the rough endoplasmic reticulum; synthesize proteins destined for secretion, insertion into membranes, or transport to lysosomes.

The endomembrane system

The endomembrane system is a network of membranes and compartments that work together to synthesize, modify, package, and transport proteins and lipids. It includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane. (The plasma membrane is functionally connected to the endomembrane system through vesicle trafficking, although structurally it is part of the cell boundary.)

Endoplasmic reticulum (ER)

The ER is a network of membranous tubules and sacs continuous with the nuclear envelope.

• Rough ER: Studded with ribosomes. Proteins synthesized on bound ribosomes enter the ER lumen, where they are folded, modified (e.g., glycosylation — adding sugar groups), and packaged into transport vesicles.

• Smooth ER: Lacks ribosomes. Functions include: synthesis of lipids (including phospholipids and steroids), detoxification of drugs and poisons (especially in liver cells), calcium ion storage (important for muscle contraction and cell signaling), and carbohydrate metabolism.

Golgi apparatus

The Golgi apparatus consists of flattened membranous sacs (cisternae). It receives vesicles from the ER at its cis face, modifies their contents (further glycosylation, sorting), and dispatches vesicles from its trans face to their destinations: the plasma membrane (for secretion), lysosomes, or other organelles. The Golgi acts as the cell's "shipping and receiving center."

Lysosomes

Lysosomes are membrane-bound vesicles containing hydrolytic enzymes (digestive enzymes) that work best at acidic pH (~5). Functions include:

• Digestion of macromolecules

• Breakdown of damaged organelles (autophagy)

• Destruction of engulfed bacteria or viruses (in phagocytic cells of the immune system)

The lysosomal membrane protects the rest of the cell from these enzymes. If a lysosome ruptures, the enzymes are largely inactive at the neutral pH of the cytosol.

Peroxisomes

Peroxisomes are small, membrane-bound organelles that contain enzymes for oxidative reactions, including the breakdown of fatty acids and the detoxification of hydrogen peroxide (H2O2). The enzyme catalase converts toxic H2O2 to water and oxygen.

Vacuoles

Vacuoles are large, membrane-bound storage compartments.

• Central vacuole (in mature plant cells): Stores water, ions, nutrients, and waste products; contributes to turgor pressure (the pressure of the cell contents against the cell wall). The central vacuole can occupy most of the cell's volume.

• Contractile vacuoles (in some freshwater protists): Pump excess water out of the cell to prevent bursting.

• Food vacuoles: Form by phagocytosis and fuse with lysosomes for digestion.

Vesicles

Vesicles are smaller, membrane-bound sacs that transport materials between organelles and to and from the plasma membrane. Vesicle trafficking is a dynamic process involving budding, transport along cytoskeletal tracks, and fusion with target membranes.

Mitochondria

Mitochondria (singular: mitochondrion) are the sites of cellular respiration — the process that converts the chemical energy of food molecules into ATP. Key structural features:

• Double membrane: Outer membrane (smooth) and inner membrane (highly folded into cristae).

• Cristae: Folds of the inner membrane that increase surface area for the electron transport chain and ATP synthase.

• Matrix: The fluid-filled interior containing enzymes for the citric acid cycle, mitochondrial DNA, and ribosomes.

Mitochondria have their own DNA (circular, like bacterial DNA) and their own ribosomes (similar to bacterial ribosomes). They reproduce independently of the cell by binary fission. These observations are key evidence for the endosymbiotic theory.

Chloroplasts

Chloroplasts are found in plants and algae and are the sites of photosynthesis. Key structural features:

• Double membrane.

• Thylakoids: Flattened, membrane-bound sacs containing chlorophyll and other photosynthetic pigments. Stacks of thylakoids are called grana (singular: granum).

• Stroma: The fluid-filled interior containing enzymes for the Calvin cycle, chloroplast DNA, and ribosomes.

Like mitochondria, chloroplasts contain their own DNA and ribosomes and reproduce by binary fission — further evidence for the endosymbiotic theory.

Endosymbiotic theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Rather than being digested, these prokaryotes established a mutually beneficial relationship with their host: the engulfed cell provided ATP (in the case of the mitochondrial ancestor) or fixed carbon (in the case of the chloroplast ancestor), and the host cell provided protection and nutrients.

Evidence for the endosymbiotic theory includes

1. Mitochondria and chloroplasts have their own circular DNA, similar to bacterial DNA.

2. Their ribosomes are more similar to prokaryotic (70S) ribosomes than to eukaryotic (80S) ribosomes.

3. They reproduce independently of the cell by binary fission.

4. They have double membranes, consistent with engulfment (the inner membrane is derived from the original prokaryote's membrane; the outer membrane is derived from the host's membrane).

5. The inner membrane of mitochondria contains enzymes and transport systems similar to those in the plasma membrane of modern prokaryotes.

The cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, enables cell movement, anchors organelles, and facilitates intracellular transport. It consists of three main types of fibers:

Centrosomes and centrioles

The centrosome is the microtubule-organizing center in animal cells. It contains a pair of centrioles (cylindrical structures composed of microtubule triplets). The centrosome organizes the mitotic spindle during cell division. Plant cells lack centrioles but still organize microtubules.

Cilia and flagella

Cilia (singular: cilium) and flagella (singular: flagellum) are microtubule-based projections that extend from the cell surface. Both have a core structure of nine microtubule doublets arranged in a ring around two central microtubules (the "9 + 2" arrangement). Cilia are typically shorter and more numerous; flagella are longer and usually single or few in number. Both are powered by the motor protein dynein.

• Motile cilia: Beat rhythmically to move fluid across the cell surface (e.g., in the respiratory tract).

• Primary cilia: Non-motile; function as sensory antennae in many cell types.

Extracellular matrix (ECM): Network of proteins (mainly collagen) and polysaccharides between cells. Provides structural support; regulates cell behavior. Integrins connect ECM to cytoskeleton.

Cell walls

Cell walls are rigid structures outside the plasma membrane found in plants, fungi, and many prokaryotes (but NOT in animal cells).

• Plant cell walls: Composed primarily of cellulose. Provide structural support, protect against mechanical stress, and maintain cell shape. The central vacuole presses the plasma membrane against the cell wall, creating turgor pressure that supports non-woody plants.

• Fungal cell walls: Composed of chitin.

Cell junctions

Cell junctions connect cells to each other and to the ECM.

• Tight junctions: Seal adjacent cells together, preventing leakage of fluid between cells (e.g., in the lining of the intestine).

• Desmosomes: Anchor cells together, providing mechanical strength (e.g., in skin and heart muscle).

• Gap junctions: Channels that allow direct passage of ions and small molecules between adjacent cells (e.g., in heart muscle, enabling coordinated contraction).

• Plasmodesmata (plants): Channels through cell walls that connect the cytoplasms of adjacent plant cells.

Plant vs. animal cells: Plants have cell walls (cellulose), chloroplasts, and large central vacuoles; lack centrioles; store starch. Animal cells lack cell walls and chloroplasts; have centrioles, common lysosomes; store glycogen.

Organelle cooperation: Secretory protein: Nucleus → Ribosome/Rough ER → Transport vesicle → Golgi → Secretory vesicle → Plasma membrane (exocytosis).

ELI Example

Mitochondria and chloroplasts are like permanent houseguests who moved in billions of years ago. A larger cell swallowed a bacterium, found it useful (it made ATP), and kept it. Over time, the bacterium lost unnecessary genes and became a mitochondrion. Same story for chloroplasts. Evidence: they still have their own DNA, ribosomes, and divide like bacteria.

Do Not Confuse

Lab Link

In the microscopy laboratory, you can observe many of the structures discussed in this chapter. The nucleus is visible as a dark circle in stained cells. Chloroplasts are visible as green ovals in plant cells (e.g., Elodea). The central vacuole appears as a clear, large space in plant cells. Amyloplasts (starch-storing organelles) can be seen in potato cells stained with iodine. Mitochondria require specialized stains to be visible. The relationship between what you can see and what requires electron microscopy provides important perspective on the scale of cellular structures.

High-Yield Memory Anchors

• Nucleus = information. ER = synthesis + modification. Golgi = sorting + shipping.

• Mitochondria = cellular respiration. Chloroplasts = photosynthesis.

• Endosymbiotic theory: mitochondria and chloroplasts = former free-living bacteria.

• Cytoskeleton = structure + transport + movement.

• Organelles cooperate; they do not work in isolation.

Quick Check

Q1 (Foundational): Name the organelle that performs each function: (a) synthesizes proteins, (b) modifies and packages proteins for secretion, (c) produces most of the cell's ATP, (d) digests macromolecules and old organelles.

Q2 (Application): A pancreatic cell specializes in secreting digestive enzymes. Which organelles would you expect to be especially abundant in this cell, and why?

Q3 (Comparison/Reasoning): Explain two pieces of evidence supporting the endosymbiotic theory. Why is this theory classified as a "theory" rather than a hypothesis?

Quick Check Answers

A1: (a) Ribosomes. (b) Golgi apparatus. (c) Mitochondria. (d) Lysosomes.

A2: A pancreatic cell secreting digestive enzymes would have: (1) Abundant rough ER — the enzymes are proteins synthesized on bound ribosomes and enter the ER lumen for folding and modification. (2) Prominent Golgi apparatus — the enzymes must be further modified, sorted, and packaged into secretory vesicles. (3) Many mitochondria — ATP is needed to power protein synthesis, vesicle transport, and secretion. (4) Many secretory vesicles — near the plasma membrane, ready to release enzymes by exocytosis.

A3: Two pieces of evidence: (1) Mitochondria and chloroplasts contain their own circular DNA that resembles bacterial DNA more than eukaryotic nuclear DNA. (2) Mitochondria and chloroplasts have 70S ribosomes, similar to prokaryotic ribosomes rather than the 80S ribosomes found in the eukaryotic cytoplasm. The endosymbiotic theory is classified as a theory because it is a well-substantiated, broad explanatory framework supported by multiple independent lines of evidence — it explains the origin of two entire classes of organelles and makes testable predictions that have been repeatedly confirmed.

Chapter Summary

Eukaryotic cells are compartmentalized: nucleus (DNA), ribosomes (protein synthesis), ER (modification), Golgi (sorting), lysosomes (digestion), mitochondria (ATP), chloroplasts (photosynthesis). The endosymbiotic theory explains the origin of mitochondria and chloroplasts from engulfed prokaryotes. Plant and animal cells differ in cell wall, chloroplasts, and central vacuole.

Common Mistakes

Mistake: "All eukaryotic cells have every organelle listed in the textbook."

Reality: Organelle composition varies by cell type. Red blood cells (mammalian) lose their nucleus and organelles during maturation. Muscle cells have abundant mitochondria. Pancreatic cells have extensive rough ER. Form follows function.

Mistake: "Mitochondria and chloroplasts are part of the endomembrane system."

Reality: Mitochondria and chloroplasts are NOT part of the endomembrane system. They are autonomous organelles with their own DNA and membranes, derived from endosymbiosis. They do not receive vesicles from the ER or Golgi.

Mistake: "Lysosomes are present in all eukaryotic cells."

Reality: Lysosomes are common in animal cells but rare in plant cells. Plant cells use vacuoles for some of the functions that lysosomes perform in animal cells.

Mistake: "The Golgi apparatus makes proteins."

Reality: The Golgi modifies, sorts, and packages proteins — it does not synthesize them. Ribosomes make proteins. The ER folds and modifies them.