Eukaryotic cells

Eukaryotic cells are defined by a high level of internal organization that sets them apart from prokaryotes. A key part of this compartmentalization is the nucleus, a membrane-bound compartment that contains the cell’s genetic material (DNA).

The nucleus is surrounded by a nuclear envelope, a double-membrane structure with nuclear pores that regulate which molecules can move between the nucleus and the cytoplasm. Inside the nucleus, the nucleolus is where ribosomal RNA (rRNA) is transcribed and where ribosome assembly begins.

Membrane-bound organelles

In addition to the nucleus, eukaryotic cells contain other membrane-bound organelles:

• Mitochondria:
  • The site of energy production via oxidative phosphorylation. In this process, ATP synthase uses a proton gradient - created when the electron transport chain pumps protons into the intermembrane space - to convert ADP into ATP. Notably, mitochondria contain their own circular DNA and ribosomes, which allows them to replicate independently.
  • Both the inner and outer mitochondrial membranes are phospholipid bilayers. The outer membrane is highly permeable because it contains porin proteins that let small molecules pass through freely. The inner membrane is much less permeable and is extensively folded into cristae, which increases surface area for electron transport and ATP production. The space between the two membranes is the intermembrane space.
• Lysosomes:
  • Vesicles filled with hydrolytic enzymes that break down and recycle macromolecules, pathogens, and cellular debris. Materials targeted for degradation are brought into the cell by processes such as endocytosis or phagocytosis, then fused with lysosomes for breakdown.
• The endoplasmic reticulum (ER):
  • A membrane network divided into two regions with distinct roles.
    • The rough ER (RER), studded with ribosomes, synthesizes, folds, and modifies proteins destined for secretion or insertion into membranes. These proteins are directed to the rough ER by signal sequences. Proteins destined for secretion or incorporation into membranes begin with an N-terminal signal sequence that recruits a signal recognition particle, directing the ribosome to the RER; once the protein is synthesized, the signal sequence is removed. Vesicles budding from the RER then transport transmembrane proteins to the plasma membrane via the golgi apparatus.
    • The smooth ER synthesizes lipids and steroids for the plasma membrane, participates in carbohydrate metabolism, and stores calcium, especially in muscle cells.

All regions of the ER are continuous with the nuclear envelope.

• Golgi apparatus- has a stacked, pancake-like structure and transports proteins from the rough ER for further modification - most notably glycosylation, which attaches sugar molecules to alter a protein’s structure, function, and stability. Modified proteins are then packaged into vesicles that bud off from the Golgi and fuse with the plasma membrane via exocytosis to release their contents outside the cell.
• Peroxisomes are small, membrane-bound organelles that contain oxidative enzymes used for fatty acid breakdown and detoxification of reactive oxygen species. They play a crucial role in lipid metabolism and help protect the cell from oxidative damage.

Cytoskeleton

Eukaryotic cells rely on a network of microfilaments, microtubules, and intermediate filaments to maintain shape, enable movement, and support intracellular transport:

• Microfilaments, made of actin, bear tension in the cell and drive contractile events such as cytokinesis.
• Microtubules, built from tubulin, resist compression and form structures such as the mitotic spindle, cilia, and flagella; they also provide tracks that guide organelle movement within the cell.
• Intermediate filaments, which vary in composition, stabilize cell shape by bearing tension.

Cilia, flagella, centrioles

In eukaryotes, both cilia and flagella are built from microtubules.

Cilia can move cells or sweep materials across tissue surfaces, while flagella propel cells such as sperm. Centrioles act as microtubule organizing centers during cell division, with microtubules radiating outward from barrel-shaped structures.

Tissues formed from eukaryotic cells

• Epithelial layers - derived from ectoderm and endoderm, as in skin or the intestinal lining. Malignancies here are known as carcinoma.
• Tissues also arise from connective cells derived from mesoderm, such as muscle or fat, with cancers termed sarcoma.