The cell is the fundamental unit of all living organisms. Specialized, membrane-enclosed structures called organelles perform specific tasks, allowing the cell to maintain life and execute complex functions. This division of labor enables cells to operate with greater efficiency. The focus here is on eukaryotic cells (animal, plant, and fungal cells), which contain a true nucleus and other internal membrane-bound compartments. These structures manage genetic information, manufacture components, generate energy, and dispose of waste.
Cellular Boundaries and Environment
Every cell must be separated from its external surroundings. This boundary is the plasma membrane, a flexible barrier composed of a phospholipid bilayer with embedded proteins. It functions as a selectively permeable gate, regulating the passage of molecules into and out of the cell, and facilitates communication via receptor proteins.
Inside the plasma membrane lies the cytoplasm, encompassing all material between the membrane and the nucleus. The cytoplasm consists of organelles and the cytosol, a gel-like aqueous solution. The cytosol provides the medium for many metabolic reactions.
Compartmentalization via membrane-bound organelles allows for simultaneous, specialized chemical reactions. The cytoskeleton, a network of protein filaments, provides structural support and serves as a track for moving vesicles and organelles.
The Nucleus and Genetic Command
The nucleus serves as the cell’s information processing center. It is enclosed by the nuclear envelope, a double membrane perforated by nuclear pores that control molecular traffic. The nucleus holds nearly all of the cell’s genetic material, deoxyribonucleic acid (DNA).
Within the nucleus, DNA is organized with proteins, particularly histones, forming chromatin. When the cell prepares to divide, chromatin condenses into chromosomes. The primary function of the nucleus is to maintain the integrity of this genetic information and control cell activities by regulating gene expression.
This regulation begins with transcription, where specific DNA segments are copied into messenger RNA (mRNA) templates. The nucleolus synthesizes ribosomal RNA (rRNA) and assembles ribosome subunits. Once formed, these subunits are exported through the nuclear pores to the cytoplasm, ready to execute genetic instructions.
Manufacturing Protein and Lipid Resources
Protein synthesis occurs on ribosomes, molecular machines composed of rRNA and protein. Some ribosomes float freely in the cytosol, while others are attached to the Endoplasmic Reticulum (ER).
The Rough Endoplasmic Reticulum (RER) has attached ribosomes, giving it a studded appearance. Proteins destined for secretion or membrane insertion are synthesized here and threaded into the RER’s internal space (lumen). Within the lumen, polypeptide chains are folded and modified before being sent onward.
The Smooth Endoplasmic Reticulum (SER) lacks ribosomes and handles lipid synthesis, including phospholipids and steroids. It is also crucial for detoxification of drugs and metabolic byproducts. Both ER types package materials into transport vesicles that travel to the next destination.
This destination is the Golgi apparatus, a stack of flattened sacs called cisternae. The Golgi receives vesicles from the ER at its cis face. Materials are modified, sorted, and packaged into new vesicles as they move through the stack to the trans face, which are then shipped to their specific destinations.
Energy Generation and Waste Recycling
Energy is primarily generated by the mitochondria. These organelles feature two membranes, including a highly folded inner membrane that maximizes surface area. The final steps of cellular respiration, known as oxidative phosphorylation, take place on this inner membrane. Mitochondria use oxygen and digested products to generate adenosine triphosphate (ATP), the cell’s main energy currency, which fuels cellular activities.
Cells manage waste and degrade old components using specialized digestive organelles. Lysosomes are spherical vesicles containing hydrolytic enzymes that break down worn-out organelles, macromolecules, and ingested material. These enzymes work best in the acidic environment the lysosome maintains.
Peroxisomes contribute to detoxification by containing enzymes, such as catalase, that neutralize harmful substances like reactive oxygen species. They also play a significant role in the breakdown of very long-chain fatty acids.