Cell Biology Chapter 12

Eukaryotic Cell Structure and Function

• The nucleus contains the genome and is surrounded by cytoplasm, which consists of cytosol and cytoplasmic organelles.
• The cytosol is the main site of protein synthesis and degradation, accounting for more than half of the cell's total volume.
• The endoplasmic reticulum (ER) performs most of the cell's intermediary metabolism, with about half of the total membrane area in a eukaryotic cell enclosing the ER's labyrinthine spaces.

Organelle Functions

• The Golgi apparatus receives lipids and proteins from the ER and dispatches them to various destinations, often covalently modifying them en route.
• Lysosomes contain digestive enzymes that degrade defunct intracellular organelles.
• Mitochondria and chloroplasts generate most of the ATP used by cells to drive reactions requiring an input of free energy.
• Peroxisomes are small vesicular compartments that contain enzymes used in various oxidative reactions.

Protein Sorting and Transport

• Nearly all proteins, except those inside mitochondria and plastids, begin their synthesis on ribosomes in the cytosol.
• The final location of each protein depends on its amino acid sequence, which can contain one or more sorting signals that direct its delivery to different parts of the cell.
• Sorting signals are recognized by complementary sorting receptors that mediate movement between compartments.
• Proteins that do not have any sorting signals remain in the cytosol as permanent residents.

Signal Sequences and Sorting Receptors

• Signal sequences are usually composed of amino acid side chains and come in two general varieties: signal patch and signal sequence.
• Signal sequences direct proteins to different intracellular locations, such as the ER, mitochondria, nucleus, and peroxisomes.
• The signal for protein import into the nucleus is composed primarily of positively charged amino acids.
• A sorting signal for any particular destination needs to be sufficiently distinctive from all other sequences to permit its selective recognition by the appropriate sorting receptor.

Endoplasmic Reticulum (ER)

• The ER membrane typically constitutes more than half of the total membrane in an average animal cell.
• The ER is organized into a netlike labyrinth of branching tubules and flattened sacs that extend throughout the cytosol.
• The ER is specialized in regions that make intimate contacts with other organelles, such as mitochondria, plastids, endosomes, and the plasma membrane.
• The ER can sequester Ca2+ from the cytosol, and its release and reuptake occur in many rapid responses to extracellular signals.

Signal Sequence Discovery

• Signal sequences were discovered in secreted water-soluble proteins that are first translocated across the ER membrane.
• The signal hypothesis was formulated to explain these observations, proposing that the mRNA for the secretory protein codes for a protein that is bigger than the protein that is eventually secreted.

Signal-Recognition Particle (SRP) and Protein Translocation

• The ER signal sequence is guided to the ER membrane by the SRP, which binds to the signal sequence, and an SRP receptor in the ER membrane.
• SRP is a large complex that consists of six different polypeptide chains bound to a single RNA molecule in animal cells.
• The SRP exposes a binding site for an SRP receptor, which brings the SRP-ribosome complex to an unoccupied protein translocator in the ER membrane.
• The translocator transfers the growing polypeptide chain across the membrane, and SRP and SRP receptor are then released, allowing protein synthesis to resume at full speed.