Cellular Anatomy: Structure and Function of Eukaryotic Organelles
Cell Structure and Function: The Basic Units of Life
Foundations of Cell Theory
The cell is the basic unit of structure and function of living things. All living things are composed of cells. All cells arise from other cells by the process of cell division. Scientists Schleiden and Schwann established these key tenets of cell theory.
Key Components of the Cell (Organelles)
Plasma Membrane
- Structure: A double layer of phospholipids (a lipid molecule combined with a phosphate group).
- Function: Controls which substances enter and exit the cell, maintaining cellular homeostasis.
Cytoplasm
- Structure: A gel-like substance containing many types of proteins and other molecules.
- Function: Fills the space between the plasma membrane and the organelles inside the cell, providing a medium for metabolic reactions.
Nucleus
- Structure: Surrounded by a nuclear membrane. Contains chromatin (DNA and proteins). The condensed, darker part is the Nucleolus.
- Function: Often called the “brain” of the cell; it stores the DNA and controls cell activities.
Centrioles
- Structure: Cylinders composed of nine triplets of microtubules, held together by supporting proteins.
- Function: Essential for cell division, as they form the spindle fibers.
Golgi Bodies (Golgi Apparatus)
- Structure: Flat, membrane-bound sacs (cisternae), often physically associated with the Endoplasmic Reticulum (ER).
- Function: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
Mitochondria
- Structure: Surrounded by two membranes. The outer membrane separates it from the cytoplasm. The inner membrane folds into shelves (cristae) that stretch across the mitochondria. Contains specialized enzymes.
- Function: Releases energy through cellular respiration, producing ATP (Adenosine Triphosphate).
Cytoskeleton
- Structure: Made of microtubules and microfilaments.
- Function: Helps organize the cell, provides structural support, and facilitates movement.
Vacuole
- Structure: Fluid-filled spaces surrounded by membranes.
- Function: Stores water, food molecules, salts, or pigments (if not contained in plastids).
Plastids
Membrane-bound organelles with specialized functions, primarily found in plant cells:
- Chloroplasts: Contain the green pigment chlorophyll, essential for the capture of sunlight energy during photosynthesis.
- Chromoplasts: Contain red, orange, or yellow pigments that give many flowers and plants their distinctive color.
- Leucoplasts: Synthesize and store starch molecules.
Lysosomes
- Structure: Membrane-bound organelles containing powerful digestive enzymes.
- Function: These enzymes are capable of digesting the cell’s proteins, breaking down and recycling amino acids to make other proteins. Warning: If lysosomes release their contents into the cytoplasm, the enzymes would destroy the entire cell.
Ribosomes
- Structure: Tiny, round organelles composed of nucleic acids and proteins. They consist of two separated RNA-protein complexes: a large subunit sitting on top of a small subunit, with an RNA template sandwiched between the two.
- Function: The site of protein synthesis (translation). Eukaryotic cells that specialize in producing proteins have a particularly large number of ribosomes.
Endoplasmic Reticulum (ER)
- Structure: Large strands of membrane forming a network of tiny canals throughout the cell, connecting the nuclear membrane and the plasma membrane.
- Types and Functions:
- Rough Endoplasmic Reticulum (RER): Has ribosomes attached. Synthesizes proteins destined for secretion or insertion into membranes. These proteins are packed into vesicles for transport and shipped to the Golgi apparatus. Cells whose job is to secrete large amounts of enzymes have abundant RER.
- Smooth Endoplasmic Reticulum (SER): Lacks ribosomes. Synthesizes lipids, transports them through the cell, is in charge of detoxification of medications and poisons, and stores calcium ions.