A basal body is similar in structure to a centriole and functions in initiating the assembly of cilia and flagella.
Microfilaments are the thinnest elements of the cytoskeleton, composed of the proteins actin and myosin. They help generate movement and provide mechanical support, being involved in muscle contraction, cell division, and cell locomotion.
Ribosomes are the sites of protein synthesis, consisting of two subunits made separately in the nucleolus. They can be attached to the nuclear membrane and endoplasmic reticulum or free in the cytoplasm. Ribosomes synthesize proteins destined for specific organelles, the plasma membrane, or export from the cell.
“Cell drinking”; movement of extracellular fluid into a cell by infolding of plasma membrane to form a vesicle.
Microvilli are nonmotile, microscopic fingerlike projections of the plasma membrane that greatly increase the surface area of the cell. They are abundant on cells involved in absorption, such as the epithelial cells that line the small intestine.
The Rough ER is continuous with the nuclear membrane and usually folded into a series of flattened sacs. Its outer surface is studded with ribosomes, which are the sites of protein synthesis. Proteins synthesized by ribosomes attached to the Rough ER enter spaces within the ER for processing and sorting.
Secretory vesicles store processed proteins and deliver them to the plasma membrane, where they are discharged by exocytosis into the extracellular fluid.
The centrosome, or microtubule organizing center, is located near the nucleus and consists of a pair of centrioles and the pericentriolar matrix. It plays a critical role in cell division and microtubule formation.
Organelles are specialized structures within the cell that have characteristic shapes and perform specific functions in cellular growth, maintenance, and reproduction.
Digitalis slows the action of the sodium-potassium pumps, causing more Na+ to accumulate inside heart muscle cells. This decreases the Na+ concentration gradient, slowing down the Na+-Ca2+ antiporters and resulting in more Ca2+ remaining inside heart muscle cells, which increases the force of their contractions.
A flagellum is a long, whip-like structure that generates forward motion along its axis by rapidly wiggling in a wavelike pattern. The only example in the human body is a sperm cell’s tail, which propels the sperm toward the oocyte in the uterine tube.
Cytoplasm consists of all the cellular contents between the plasma membrane and the nucleus, and has two components: the cytosol and organelles.
The cytoskeleton is a network of three types of protein filaments—microfilaments, intermediate filaments, and microtubules—that extend throughout the cytoplasm. It serves as a scaffold that helps determine a cell’s shape, organize cellular contents, and aids in the movement of organelles, chromosomes, and whole cells.
Enzymes in the medial cisterns modify proteins to form glycoproteins, glycolipids, and lipoproteins.
Rough ER synthesizes glycoproteins and phospholipids that are transferred into cellular organelles, inserted into the plasma membrane, or secreted during exocytosis.
Cilia move fluids along a cell’s surface, while a flagellum moves an entire cell.
ATP provides the energy needed for the sodium–potassium pump to change shape and move sodium ions out of the cell and potassium ions into the cell.
The Endoplasmic Reticulum (ER) is a network of membranes in the form of flattened sacs or tubules that extends from the nuclear envelope throughout the cytoplasm. It constitutes more than half of the membranous surfaces within the cytoplasm of most cells.
Cilia are numerous, short, hairlike projections that extend from the surface of the cell. Each cilium contains a core of 20 microtubules surrounded by plasma membrane and is involved in cell movement.
The entry (cis) face of the Golgi complex is a cistern that faces the rough ER and is involved in receiving transport vesicles from the ER.
Passive movement of water molecules across a selectively permeable membrane from an area of higher to lower water concentration until equilibrium is reached.
“Cell eating”; movement of a solid particle into a cell after pseudopods engulf it to form a phagosome.
An endosome is a vesicle that fuses with an uncoated vesicle during receptor-mediated endocytosis, where the LDL particles separate from their receptors.
Intermediate filaments are cytoskeletal components that are thicker than microfilaments but thinner than microtubules. They are composed of several different proteins, are exceptionally strong, and help stabilize the position of organelles such as the nucleus and help attach cells to one another.
The Smooth ER extends from the Rough ER to form a network of membrane tubules. Unlike Rough ER, Smooth ER does not have ribosomes on its outer surfaces. It contains unique enzymes that make it functionally more diverse, synthesizing fatty acids and steroids, and helping in the detoxification of lipid-soluble drugs and harmful substances.
Autophagy is the process by which entire worn-out organelles are digested by lysosomes, with the digested components returned to the cytosol for reuse.
The Golgi complex is an organelle consisting of 3 to 20 cisterns, which are small, flattened membranous sacs with bulging edges. It is involved in modifying, sorting, and packaging proteins synthesized by ribosomes attached to the rough ER.
Movement of molecules or ions down a concentration gradient due to their kinetic energy until they reach equilibrium.
Smooth ER detoxifies certain drugs, and repeated exposure to these drugs can increase the amount of smooth ER and its enzymes in liver cells, leading to increased drug tolerance and the need for higher dosages to achieve the same effect.
Exocytosis is a process where materials move out of a cell by the fusion with the plasma membrane of vesicles formed inside the cell.
Receptor-mediated endocytosis is a mechanism by which cells import needed materials by binding them to specific receptors on the cell surface, which then triggers the formation of a vesicle to bring the material into the cell.
The sodium–potassium pump is a primary active transport mechanism that expels sodium ions (Na+) from cells and brings potassium ions (K+) in, maintaining a low concentration of Na+ in the cytosol and a high concentration of K+.
The cytoskeleton is a network of protein filaments that extends throughout the cytosol. It contributes to the structure of the cytoskeleton and other organelles.
Lysosomes are membrane-enclosed vesicles formed from the Golgi complex that contain powerful digestive and hydrolytic enzymes to break down a wide variety of molecules and recycle worn-out cell structures.
Transfer vesicles bud from the edges of the cisterns to move specific enzymes back toward the entry face and move some partially modified proteins toward the exit face.
The pericentriolar matrix surrounds the centrioles and contains hundreds of ring-shaped complexes composed of the protein tubulin. These complexes are organizing centers for the growth of the mitotic spindle and microtubule formation.
Antiporters are transport proteins that move two substances in opposite directions across the membrane.
A clathrin-coated vesicle is a vesicle that forms when clathrin molecules attach to the membrane on its cytoplasmic side, causing the membrane to invaginate and pinch off, containing receptor–ligand complexes.
HIV attaches to a receptor called CD4 on the plasma membrane of helper T cells and enters the cell via receptor-mediated endocytosis.
The cytosol (intracellular fluid) is the fluid portion of the cytoplasm that surrounds organelles and constitutes about 55% of total cell volume. It is 75–90% water plus various dissolved and suspended components.
Transcytosis is an active process where vesicles undergo endocytosis on one side of a cell, move across the cell, and then undergo exocytosis on the opposite side.
Centrioles are cylindrical structures composed of nine clusters of three microtubules (triplets) arranged in a circular pattern. They are part of the centrosome and are crucial for cell division.
Symporters are transport proteins that move two substances in the same direction across the membrane.
Medial cisterns are the sacs located between the entry and exit faces of the Golgi complex. They play a role in the maturation and processing of proteins as they move through the Golgi complex.
Receptor-mediated endocytosis is a highly selective type of endocytosis by which cells take up specific ligands. A vesicle forms after a receptor protein in the plasma membrane recognizes and binds to a particular particle in the extracellular fluid.
A residual body is a vesicle containing undigested materials that remain in the cell after the phagosome fuses with lysosomes and the ingested material is broken down.
An autophagosome is a vesicle formed from a membrane derived from the ER that fuses with a lysosome to recycle cytoplasmic contents.
Microtubules are the largest of the cytoskeletal components, composed mainly of the protein tubulin. They help determine cell shape and function in the movement of organelles, chromosomes during cell division, and specialized cell projections like cilia and flagella.
Smooth ER synthesizes fatty acids and steroids, detoxifies drugs and other harmful substances, removes the phosphate group from glucose-6-phosphate, and stores and releases calcium ions that trigger muscle contraction.
Primary active transport is a process where energy derived from the hydrolysis of ATP changes the shape of a carrier protein, which pumps a substance across a plasma membrane against its concentration gradient.
Active transport is a process where energy is required for carrier proteins to move solutes across the plasma membrane against a concentration gradient.
Peroxisomes are small organelles similar in structure to lysosomes that contain oxidases, enzymes that oxidize various organic substances, including amino acids, fatty acids, and toxic substances like alcohol.
Membrane vesicles deliver their contents to the plasma membrane for incorporation into the membrane, adding new segments of plasma membrane and modifying the number and distribution of membrane molecules.
The entry face receives and modifies proteins produced by the rough ER, while the exit face further modifies the molecules and then sorts and packages them for transport to their destinations.
Secondary active transport uses the energy stored in a Na+ or H+ concentration gradient to drive other substances across the membrane against their concentration gradients. This process indirectly uses energy obtained from the hydrolysis of ATP.
The Na+ gradient, maintained by primary active transport, stores potential energy that can be converted to kinetic energy to transport other substances against their concentration gradients in secondary active transport.
Movement of substances into a cell in vesicles.
Endocytosis is a process where materials move into a cell in a vesicle formed from the plasma membrane.
Nicotine in cigarette smoke paralyzes the movement of cilia, causing smokers to cough often to remove foreign particles from their airways.
Autolysis is the process by which lysosomal enzymes destroy the entire cell that contains them, occurring in some pathological conditions and responsible for tissue deterioration after death.
The sodium-potassium pump maintains a low concentration of Na+ in the cytosol by expelling three Na+ ions into the extracellular fluid and bringing in two K+ ions into the cytosol, which is crucial for maintaining normal cell volume and generating electrical signals.
Coupled active transport of two substances across the membrane using energy supplied by a Na+ or H+ concentration gradient maintained by primary active transport pumps.
Movement of a substance through a cell as a result of endocytosis on one side and exocytosis on the opposite side.
Phagocytosis, or 'cell eating,' is a form of endocytosis where the cell engulfs large solid particles such as worn-out cells, bacteria, or viruses.
Pseudopods are projections of a cell's plasma membrane and cytoplasm that surround and engulf particles during phagocytosis.
Ribosomes are the sites of protein synthesis. These tiny structures are essential for translating genetic information into proteins.
Na+/K+ ATPase is another name for the sodium–potassium pump, an enzyme that hydrolyzes ATP to provide the energy needed to pump sodium and potassium ions across the plasma membrane.
Exocytosis is a process that releases materials from a cell. Membrane-enclosed vesicles called secretory vesicles form inside the cell, fuse with the plasma membrane, and release their contents into the extracellular fluid.
A vesicle is a small, spherical sac that transports a variety of substances from one structure to another within cells.
Active process in which a cell expends energy to move a substance across the membrane against its concentration gradient by transmembrane proteins that function as carriers.
Lysosomes contain many digestive enzymes that break down large protein and lipid molecules. Transport vesicles containing LDL particles bud off the endosome and fuse with a lysosome for degradation.
Proteins synthesized by ribosomes on the rough ER are surrounded by a piece of the ER membrane, which buds from the membrane surface to form transport vesicles that move toward the entry face of the Golgi complex.
The Golgi complex modifies, sorts, packages, and transports proteins received from the rough ER, forms secretory vesicles for exocytosis, forms membrane vesicles for the plasma membrane, and forms transport vesicles for other organelles.
Cisterns are small, flattened membranous sacs with bulging edges that resemble a stack of pita bread. They are part of the Golgi complex and differ in size, shape, and enzymatic activity at different ends of the complex.
Passive movement of a substance down its concentration gradient through the lipid bilayer of the plasma membrane without the help of membrane transport proteins.
Passive movement of a substance down its concentration gradient through the lipid bilayer by transmembrane proteins that function as channels or carriers.
Ligand–receptor complexes trigger infolding of a clathrin-coated pit that forms a vesicle containing ligands.
Pinocytosis, or 'cell drinking,' is a form of endocytosis where the cell takes up tiny droplets of extracellular fluid.
The exit (trans) face of the Golgi complex is a cistern that faces the plasma membrane and is involved in the final stages of modifying, sorting, and packaging proteins for transport out of the cell.
Active process in which a substance moves across the membrane against its concentration gradient by pumps (carriers) that use energy supplied by hydrolysis of ATP.
Movement of substances out of a cell in secretory vesicles that fuse with the plasma membrane and release their contents into the extracellular fluid.
Phagocytes are cells capable of carrying out phagocytosis. The two main types are macrophages and neutrophils.
During the uncoating step, the clathrin-coated vesicle loses its clathrin coat to become an uncoated vesicle. Clathrin molecules either return to the inner surface of the plasma membrane or help form coats on other vesicles inside the cell.
Bulk-phase endocytosis, or pinocytosis, is a form of endocytosis where tiny droplets of extracellular fluid are taken up by the cell without the involvement of receptor proteins.
A phagosome is a vesicle formed when pseudopods surround a particle and the membranes fuse, bringing the particle into the cell.
Lysosomes fuse with the phagosome and release digestive enzymes that break down the ingested material.
