Membrane-bound organelles, characteristics of eukaryotic cells

Eukaryotic cells

Eukaryotic cells are characterized by a high level of internal organization that distinguishes them from prokaryotes. A key feature of this compartmentalization is the presence of a nucleus, a membrane-bound compartment that houses the cell’s genetic material (DNA).

This nucleus is surrounded by a nuclear envelope, a double-membrane structure punctuated by nuclear pores that regulate the transport of molecules between the nucleus and the cytoplasm. Inside the nucleus, the nucleolus is the site where ribosomal RNA (rRNA) is transcribed and ribosome assembly begins.

Membrane-bound organelles

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

• Mitochondria:
  • Where energy is produced via oxidative phosphorylation. In this process, ATP synthase harnesses a proton gradient—created as the electron transport chain pumps protons into the intermembrane space—to convert ADP into ATP. Notably, mitochondria contain their own circular DNA and ribosomes, enabling them to replicate independently.
  • Both the inner and outer mitochondrial membranes are phospholipid bilayers. The outer membrane is highly permeable due to porin proteins that allow small molecules to pass through freely. In contrast, the inner membrane is much less permeable and is extensively folded into cristae, which increases the surface area necessary for electron transport and ATP production. The space between these membranes is called the intermembrane space.
• Lysosomes:
  • Vesicles loaded with hydrolytic enzymes that degrade and recycle macromolecules, pathogens, and cellular debris. Materials targeted for degradation are internalized through processes like endocytosis or phagocytosis, and then fused with lysosomes for breakdown.
• The endoplasmic reticulum (ER):
  • Divided into two regions with distinct roles.
• The rough ER, studded with ribosomes, is primarily involved in synthesizing, folding, and modifying proteins destined for secretion or incorporation 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.
• The smooth ER is dedicated to **lipid and steroid synthesis **for the plasma membrane, carbohydrate metabolism, and calcium storage, especially in muscle cells. Vesicles budding from the RER then transport transmembrane proteins to the plasma membrane.

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 to undergo further modification—most notably glycosylation, which attaches sugar molecules to alter 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 house oxidative enzymes essential for fatty acid breakdown and the detoxification of reactive oxygen species. They play a crucial role in lipid metabolism and 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 facilitate intracellular transport:

• Microfilaments, composed of actin, bear tension in the cell and drive contractile events such as cytokinesis.
• Microtubules, built from tubulin, resist compression and form structures like the mitotic spindle, cilia, and flagella; they also guide organelle movement along pathways inside the cell.
• Intermediate filaments, whose composition varies, help stabilize cell shape by bearing tension.

Cilia, flagella, centrioles In eukaryotes, both cilia and flagella originate from microtubules. Cilia can move cells or sweep materials across tissue surfaces, while flagella propel cells like sperm. Centrioles act as microtubule organizing centers during cell division, radiating microtubules out 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.