Cells contain various specialized subunits, known as organelles, which perform specific tasks to maintain cellular function. Many organelles are enclosed by a lipid bilayer membrane, separating their internal environment from the rest of the cell. This membranous enclosure allows for the compartmentalization of distinct biochemical reactions, creating specialized conditions necessary for various cellular activities.
The Nucleus
The nucleus is the largest organelle within a eukaryotic cell. It is encased by a double membrane, the nuclear envelope, with pores that regulate molecule passage between the nucleus and cytoplasm. Inside, the nucleus contains chromatin, a complex of deoxyribonucleic acid (DNA) that forms the cell’s genetic material. A dense region, the nucleolus, synthesizes ribosomal RNA and assembles ribosomes. The nucleus directs protein synthesis by controlling gene expression.
The Endoplasmic Reticulum and Golgi Apparatus
The endoplasmic reticulum (ER) forms an interconnected network of membrane-bound sacs and tubules throughout the cytoplasm. It has two regions: the rough ER (RER) and the smooth ER (SER). The RER has ribosomes on its surface, where proteins for secretion or membrane insertion are synthesized and folded. These proteins undergo modifications within the RER lumen before transport.
The SER lacks ribosomes and synthesizes lipids. It also detoxifies drugs and poisons. The SER also stores calcium ions, which are released to trigger cellular responses. Proteins and lipids then move from the ER to the Golgi apparatus, an organelle of flattened membrane-bound sacs called cisternae, arranged in stacks.
The Golgi apparatus processes and packages proteins and lipids received from the ER. It adds molecular tags to direct these molecules to their correct destinations, including secretion, plasma membrane insertion, or delivery to other organelles. Vesicles bud off from the Golgi, carrying their cargo to final cellular locations.
Lysosomes, Peroxisomes, and Mitochondria
Lysosomes are spherical membrane-bound organelles containing a diverse array of hydrolytic enzymes. These enzymes function optimally in an acidic environment, which is maintained within the lysosome. Lysosomes are responsible for breaking down waste materials, cellular debris, and foreign particles like bacteria or viruses. They digest worn-out organelles and macromolecules, contributing to the cell’s recycling and waste management processes.
Peroxisomes are small, membrane-bound sacs that contain enzymes involved in various metabolic reactions, notably those that produce hydrogen peroxide as a byproduct. These organelles play a part in detoxifying harmful substances, such as alcohol in liver cells, by converting them into less toxic forms. Peroxisomes also participate in the breakdown of long-chain fatty acids into smaller molecules that can be used for energy production.
Mitochondria are often referred to as the “powerhouses” of the cell due to their primary role in generating adenosine triphosphate (ATP), the cell’s main energy currency. Each mitochondrion possesses a double membrane: a smooth outer membrane and a highly folded inner membrane. The folds of the inner membrane, called cristae, increase the surface area for the chemical reactions of cellular respiration to occur. This complex process converts nutrients into ATP, providing the energy required for nearly all cellular activities.
Chloroplasts and Vacuoles
Chloroplasts are specialized membranous organelles found exclusively in plant and algal cells. They are enclosed by a double membrane, and their internal structure includes stacks of flattened sacs called thylakoids, which are arranged into grana. The fluid-filled space surrounding the grana is known as the stroma. Chloroplasts are the sites of photosynthesis, the process by which light energy is captured and converted into chemical energy in the form of glucose.
Vacuoles are membrane-bound sacs that serve various storage and transport functions within cells. In mature plant cells, a single, large central vacuole typically occupies a significant portion of the cell volume. This central vacuole stores water, nutrients, ions, and waste products, and its turgor pressure helps maintain the cell’s rigidity and support the plant. Animal cells, if they possess vacuoles, usually have several smaller, more transient vacuoles involved in temporary storage, transport of substances, or waste removal.