Chapter 6: A Tour of the Cell - Organelle Notes

Nucleus

Function: Stores DNA, controls essential cell activities including growth, metabolism, and reproduction.
Membrane Structure: Surrounded by a double membrane known as the nuclear envelope, which is perforated with nuclear pores to regulate molecular passage.
Cell Type: Present in both animal and plant cells.
Additional Notes: Contains the nucleolus, a dense region within the nucleus responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomes.

Ribosomes

Function: Primary sites for protein synthesis, where amino acids are assembled into polypeptides based on genetic instructions from mRNA.
Membrane Structure: Not enclosed by a membrane.
Cell Type: Found in both animal and plant cells.
Additional Notes: Ribosomes can exist either free-floating in the cytoplasm, synthesizing proteins that function within the cytosol, or attached to the rough endoplasmic reticulum (ER), synthesizing proteins destined for secretion, insertion into membranes, or delivery to certain organelles.

Smooth Endoplasmic Reticulum (Smooth ER)

Function: Involved in various metabolic processes including: lipid synthesis (e.g., oils, phospholipids, steroids), detoxification of drugs and poisons, and storage of calcium ions, particularly crucial in muscle cells.
Membrane Structure: Consists of a network of interconnected tubules enclosed by a single membrane.
Cell Type: Present in both animal and plant cells.
Additional Notes: Characterized by its lack of ribosomes on its surface, hence the term "smooth."

Rough Endoplasmic Reticulum (Rough ER)

Function: Plays a critical role in the synthesis, folding, modification, and transport of proteins, especially those destined for secretion, insertion into cellular membranes, or delivery to other organelles (like lysosomes or the Golgi apparatus).
Membrane Structure: A network of flattened sacs (cisternae) and tubules, continuous with the outer nuclear membrane, enclosed by a single membrane.
Cell Type: Found in both animal and plant cells.
Additional Notes: Its surface is studded with ribosomes, which are responsible for giving it its "rough" appearance. These ribosomes synthesize proteins directly into or across the rough ER membrane.

Golgi Apparatus (or Golgi Complex/Body)

Function: Acts as a central processing and dispatch station for proteins and lipids synthesized in the ER. It modifies, sorts, and packages these macromolecules into vesicles for secretion or delivery to other cellular destinations.
Membrane Structure: Composed of a stack of flattened membranous sacs called cisternae, enclosed by a single membrane.
Cell Type: Present in both animal and plant cells.
Additional Notes: Plays a key role in the production of lysosomes (by budding off vesicles containing hydrolytic enzymes) and secretory vesicles (which transport materials out of the cell).

Lysosome

Function: The cell's primary digestive and waste recycling center. It breaks down macromolecules (like proteins, fats, polysaccharides, and nucleic acids), old organelles, and foreign substances via enzymatic hydrolysis.
Membrane Structure: A spherical organelle enclosed by a single membrane.
Cell Type: Primarily found in animal cells, though some plant cells may contain lysosome-like vacuoles.
Additional Notes: Contains a variety of powerful hydrolytic enzymes that function optimally in an acidic environment (pH approx. 4.5-5.0), maintained by proton pumps in its membrane.

Vacuoles

Function: Serve diverse storage functions, including water, nutrients, ions, and waste products. In plant cells, the large central vacuole provides structural support (turgor pressure) and is involved in growth by absorbing water. In some protists, contractile vacuoles pump out excess water.
Membrane Structure: Generally enclosed by a single membrane. The membrane surrounding the large central vacuole in plant cells is specifically called the tonoplast.
Cell Type: Present in both animal and plant cells. Animal cells typically have several small vacuoles, while mature plant cells are characterized by a single, large central vacuole that can occupy 30\%-80\% or more of the cell volume.
Additional Notes: Plant vacuoles also store pigments, defense compounds against herbivores, and contribute to cell enlargement.

Mitochondria

Function: The "powerhouses" of the cell, primarily responsible for cellular respiration – the metabolic process that converts chemical energy from food into adenosine triphosphate (ATP), the main energy currency of the cell.
Membrane Structure: Enclosed by a double membrane. The outer membrane is smooth, while the inner membrane is extensively folded into structures called cristae, which increase the surface area for ATP synthesis.
Cell Type: Found in both animal and plant cells.
Additional Notes: Possess their own circular DNA (mtDNA) and ribosomes, suggesting an endosymbiotic origin. They can grow and reproduce independently within the cell.

Chloroplasts

Function: Sites of photosynthesis, the process by which solar energy is captured and converted into chemical energy in the form of glucose and other organic compounds.
Membrane Structure: Enclosed by a double membrane, similar to mitochondria. Internally, they contain a system of interconnected membranous sacs called thylakoids, often stacked into grana.
Cell Type: Found exclusively in plant cells (and other photosynthetic eukaryotes like algae).
Additional Notes: Contain the green pigment chlorophyll, which absorbs light energy. Like mitochondria, chloroplasts have their own circular DNA (cpDNA) and ribosomes, supporting the endosymbiotic theory.

Peroxisomes

Function: Specialized metabolic compartments involved in various oxidative reactions, including the breakdown of fatty acids into smaller molecules that can be transported to mitochondria. They also detoxify harmful substances (e.g., alcohol in liver cells).
Membrane Structure: Small, spherical organelles enclosed by a single membrane.
Cell Type: Present in both animal and plant cells.
Additional Notes: A byproduct of their oxidative reactions is hydrogen peroxide (H2O2), a toxic compound. Peroxisomes contain the enzyme catalase, which efficiently breaks down hydrogen peroxide into water and oxygen, preventing cellular damage.

Cytoskeleton

Function: A dynamic network of protein filaments that extends throughout the cytoplasm, providing structural support to the cell, maintaining its shape, enabling cell movement (e.g., crawling, muscle contraction), and facilitating the intracellular transport of organelles and vesicles.
Membrane Structure: Not membrane-bound; it is a structural framework of protein fibers.
Cell Type: Found in both animal and plant cells.
Additional Notes: Composed of three main types of protein filaments: microtubules (the thickest), microfilaments (also known as actin filaments, the thinnest), and intermediate filaments (intermediate in size). Each type has distinct structures and roles in cell function.