Eukaryotic Cells

Overview of Eukaryotic Cell Structure

Eukaryotic cells are distinguished from prokaryotic cells by their complex internal organization, including a nucleus and various membrane-bound organelles. These features allow for compartmentalization and specialization of cellular functions.

• Larger Size: Eukaryotic cells are generally larger than prokaryotic cells.

• Genome Enclosed by Nuclear Envelope: The genetic material (DNA) is contained within a double-membraned nucleus.

• Membrane-Bound Organelles: Organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus perform specialized functions.

• Cytoskeleton: A network of protein filaments and motor proteins provides structural support and facilitates movement within the cell.

Example: Human cells, such as pancreatic beta cells, are eukaryotic and contain all these features.

The Nucleus

Structure and Function of the Nucleus

The nucleus is the defining organelle of eukaryotic cells, responsible for storing and protecting genetic information and regulating gene expression.

• Nuclear Envelope: A double membrane that separates the nucleus from the cytoplasm.

• Nuclear Pore Complexes: Large multiprotein structures embedded in the nuclear envelope that control the movement of molecules between the nucleus and cytoplasm.

• Nucleolus: A dense region within the nucleus where ribosomal RNA is synthesized and ribosome assembly begins.

• Lamins: Intermediate filament proteins that provide structural support to the nuclear envelope.

Key Point: The ratio of surface area to volume decreases as cell size increases, which influences the need for internal membranes and compartmentalization.

Nuclear Transport and Localization Signals

Proteins and other molecules are selectively transported into and out of the nucleus via nuclear pore complexes, guided by specific signal sequences.

• Nuclear Localization Signal (NLS): A short amino acid sequence that directs proteins to the nucleus.

• Nuclear Export Signal (NES): Directs proteins out of the nucleus.

• Example: Pyruvate kinase is normally found in the cytosol, but when engineered to contain an NLS, it is transported into the nucleus.

Additional info: Transport through the nuclear pore complex is regulated by recognition of these signals by transport proteins.

The Endomembrane System

Components and Functions

The endomembrane system is a network of interrelated membranous organelles that divide the cell into functional and structural compartments, facilitating synthesis, modification, transport, and breakdown of biomolecules.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Functions:

  • Synthesis, modification, transport, and secretion of proteins

  • Synthesis of lipids and detoxification of toxins

  • Transportation and breakdown of biomolecule-containing particles

• Membrane Similarity: Membranes of the endomembrane system are similar and can fuse or exchange portions.

Example: The nuclear envelope is considered part of the endomembrane system due to its continuity with the ER.

Evolution of the Endomembrane System

It is hypothesized that the endomembrane system evolved from infoldings of the plasma membrane in ancestral cells, leading to the formation of internal compartments.

• Origin: Internal membranes may have originated from the plasma membrane.

• Compartmentalization: Allowed for increased specialization and efficiency of cellular processes.

Protein Synthesis and Targeting in the Endomembrane System

Insulin Synthesis and Secretion

Insulin is a peptide hormone produced by beta cells in the pancreas. Its synthesis and secretion illustrate the secretory pathway in eukaryotic cells.

• Function: Insulin regulates glucose levels by signaling cells to take in glucose.

• Medical Relevance: Diabetes mellitus results from defective insulin production or insensitivity to insulin.

The Secretory Pathway

Proteins destined for secretion or for other parts of the endomembrane system follow a specific pathway involving several organelles.

1. Synthesis in Rough ER: Proteins are synthesized by ribosomes associated with the rough ER, where they are folded and modified.

2. Transport to Golgi Apparatus: Proteins are packaged into vesicles and transported to the Golgi for further modification and sorting.

3. Processing in Golgi: Proteins may have sugars added or removed, or other chemical modifications.

4. Sorting and Secretion: Mature proteins are sorted into vesicles for delivery to the plasma membrane or other destinations.

5. Exocytosis: Secretory vesicles fuse with the plasma membrane to release their contents outside the cell.

Targeting Proteins to the ER

Proteins destined for the endomembrane system or secretion contain an ER signal sequence that directs their translation to the rough ER.

• Signal Recognition Particle (SRP): Binds to the ER signal sequence and directs the ribosome to the ER membrane.

• Translocon: A protein channel in the ER membrane through which the growing polypeptide enters the ER lumen.

Additional info: The first part of the protein translated is the ER signal sequence, which is recognized by the SRP.

Processing of Insulin

Insulin is synthesized as a precursor (preproinsulin), which is processed through several steps before becoming active insulin.

• Preproinsulin: Initial translation product with a signal peptide.

• Proinsulin: Signal peptide is removed, and the protein is folded with disulfide bridges formed.

• Insulin: In the Golgi, the C peptide is removed, and mature insulin is sorted into secretory vesicles.

Summary Table: Secretory Pathway Steps

Key Terms and Concepts

• Eukaryotic Cell: Cell with a nucleus and membrane-bound organelles.

• Nucleus: Organelle containing genetic material.

• Endomembrane System: Network of organelles involved in synthesis, modification, and transport of biomolecules.

• Rough ER: Endoplasmic reticulum with ribosomes, site of protein synthesis.

• Golgi Apparatus: Organelle for processing and sorting proteins and lipids.

• Exocytosis: Process of vesicle fusion with the plasma membrane to release contents.

• Signal Sequence: Short peptide that directs proteins to specific cellular locations.

Equations and Ratios

Surface Area to Volume Ratio: As a cell increases in size, its surface area to volume ratio decreases, which impacts transport and metabolic efficiency.

Additional info: This ratio is critical for understanding why cells are small and why larger cells require internal membranes for efficient function.