Cell structure and function are fundamental to animal physiology. Organelles like the nucleus, mitochondria, and endoplasmic reticulum perform specialized tasks, working together to keep cells alive and functioning properly. These components are essential for energy production, protein synthesis, and cellular communication.
The plasma membrane, cytoskeleton, and cell junctions play crucial roles in maintaining cell shape, facilitating movement, and enabling intercellular interactions. Understanding these structures helps explain how cells respond to their environment and work together in complex organisms.
Cell Organelles
Nucleus: Control Center of the Cell
- Contains the cell's genetic material (DNA) and directs cellular activities
- Surrounded by a double membrane called the nuclear envelope, which separates the nucleus from the cytoplasm
- Nuclear pores in the nuclear envelope allow for selective transport of molecules between the nucleus and cytoplasm
- Nucleolus, a dense structure within the nucleus, is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly
Mitochondria: Powerhouses of the Cell
- Generate most of the cell's supply of adenosine triphosphate (ATP), the primary energy currency of the cell
- Consist of an outer membrane, an inner membrane with cristae (folds), and a matrix containing enzymes for ATP production
- Contain their own DNA, ribosomes, and the machinery necessary for protein synthesis
- Play a role in cell signaling, cell differentiation, and apoptosis (programmed cell death)
Endoplasmic Reticulum and Golgi Apparatus: Protein and Lipid Processing
- Endoplasmic reticulum (ER) is a network of membrane-bound channels and sacs
- Rough ER, studded with ribosomes, is involved in protein synthesis and modification
- Smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage
- Golgi apparatus is a stack of flattened membrane sacs that modifies, packages, and sorts proteins and lipids
- Receives proteins and lipids from the ER, modifies them (glycosylation), and packages them into vesicles for transport to other parts of the cell or for secretion
Lysosomes: Cellular Digestion and Waste Disposal
- Membrane-bound organelles containing hydrolytic enzymes that break down macromolecules, damaged organelles, and foreign particles
- Maintain an acidic internal environment (pH 4.5-5.0) optimal for the function of the hydrolytic enzymes
- Play a role in cellular processes such as endocytosis, phagocytosis, and autophagy (self-digestion of cellular components)
Cell Structure
Plasma Membrane: Selective Barrier and Communication Interface
- Consists of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol
- Selectively permeable, controlling the movement of substances in and out of the cell
- Membrane proteins function as receptors, channels, carriers, and enzymes, facilitating cellular communication and transport
- Glycocalyx, a layer of carbohydrates attached to the outer surface of the membrane, provides cell recognition and adhesion
Cytoskeleton: Cellular Scaffolding and Movement
- Network of protein filaments that provide structure, support, and movement to the cell
- Consists of three main types of filaments:
- Microfilaments (actin filaments) are involved in cell movement, muscle contraction, and cytokinesis
- Intermediate filaments provide mechanical strength and resistance to shear stress
- Microtubules are involved in organelle transport, cell division, and the formation of cilia and flagella
- Motor proteins (kinesins and dyneins) move along microtubules, transporting organelles and vesicles
Cell Junctions: Cellular Connections and Communication
- Specialized structures that connect cells to each other or to the extracellular matrix
- Tight junctions form a seal between adjacent cells, preventing the passage of molecules through the intercellular space
- Adherens junctions and desmosomes provide strong mechanical attachments between cells
- Gap junctions allow for direct communication between cells by forming channels that permit the passage of small molecules and ions