AP Biology Chapter 1 PDF

# Summary Review ## Cellular Functions 1. Differentiation is the process by which cells become specialized. 2. Eight specialized cellular functions are: - movement - conductivity - metabolic absorption - secretion - excretion - respiration - reproduction - com...

# Summary Review ## Cellular Functions 1. Differentiation is the process by which cells become specialized. 2. Eight specialized cellular functions are: - movement - conductivity - metabolic absorption - secretion - excretion - respiration - reproduction - communication ## Structure And Function Of Cellular Components 1. The eukaryotic cell is made of: - plasma membrane - cytoplasm - intracellular organelles. 2. The nucleus is the largest membrane-bound organelle and usually found in the cell’s center. Its chief functions are cell division and control of genetic information. 3. Cytoplasm is the aqueous solution (cytosol) that fills the space between the nucleus and plasma membrane. 4. Organelles suspended in the cytoplasm, are enclosed in biologic membranes. 5. The endoplasmic reticulum (ER) is a network of tubular channels (cisternae) that extend throughout the outer nuclear membrane. It specializes in the synthesis and transport of protein and lipid components of most of the organelles. It is important for protein folding and sensing cell stress. 6. The Golgi complex is a network of smooth membranes and vesicles located near the nucleus. It is responsible for processing and packaging proteins into secretory vesicles that break away from the Golgi and migrate to a variety of intracellular and extracellular destinations, including the plasma membrane. 7. Lysosomes are saclike structures that originate from the Golgi and contain digestive enzymes. These enzymes are responsible for digesting most cellular substances completely to their basic components, such as amino acids, fatty acids, and carbohydrates. Lysosomes also mediate basic cell functions, such as nutrient sensing, metabolic adaptation, and quality control of proteins and organelles. 8. There are four pathways of degradation in lysosomes: - endocytosis - phagocytosis - macropinocytosis - autophagy. 9. Peroxisomes are similar to lysosomes but contain several oxidative enzymes such as catalase and urate oxidase. 10. Mitochondria are found in great numbers in most cells and are responsible for cellular respiration and energy production. The enzymes of the respiratory chain (electron-transport chain), found in the inner membrane of the mitochondria, generate most of the cell’s ATP. 11. Cytosol includes: - intermediary metabolism involving enzymatic biochemical reactions - ribosomal protein synthesis - storage of carbohydrates, fat, and secretory vesicles. 12. The cytoskeleton is composed of a network of protein filaments including microtubules and actin filaments (microfilaments). 13. The plasma membrane encloses the cell and, by controlling the movement of substances across it, exerts a powerful influence on metabolic pathways. 14. The plasma membrane is a bilayer of lipids and proteins. The basic structure of the cell membrane is the lipid bilayer. 15. Membrane functions are determined largely by proteins. These functions include: - recognition and binding units (receptors) for substances moving in and out of the cell - pores or transport channels - enzymes that drive active pumps - cell surface markers, such as glycoproteins - cell adhesion molecules - catalysts of chemical reactions. 16. The information regarding concepts of biologic membranes has changed markedly in the last two decades. 17. Proteins are made from a chain of amino acids known as polypeptides. Proteins move from one compartment to another by gated transport, protein translocation, or vesicular transport. 18. Proteostasis is a state of balanced processes of protein synthesis, folding, and degradation. It is vital to cellular health. 19. Cellular receptors are protein molecules on the plasma membrane, in the cytoplasm, or in the nucleus that are capable of recognizing and binding smaller molecules, called ligands. 20. The ligand-receptor complex initiates a series of protein interactions, causing adenylyl cyclase to catalyze the transformation of cellular ATP to messenger molecules that stimulate specific responses within the cell. 21. The carbohydrate contained within the plasma membrane is generally bound to membrane proteins. ## Cell-to-Cell Adhesions 1. Cell-to-cell adhesions are formed on plasma membranes, thereby allowing the formation of tissues and organs. Cells are held together by three different means: - The extracellular membrane - Cell adhesion molecules in the cells’ plasma membrane - Specialized cell junctions. 2. The extracellular matrix includes: - Collagen - Elastin - Fibronectin. The matrix helps regulate cell growth and differentiation. 3. The basement membrane is a thin layer of connective tissue underlying the epithelium of many organs. It is also called the basal lamina. 4. Cell adhesion molecules (CAMs) are cell surface proteins that bind to an adjacent cell and to components of the extracellular matrix. CAMs include four main protein families: - Integrins - Cadherins - Selectins - Immunoglobulin (Ig) superfamily. 5. The three main types of cell junctions are: - Desmosomes - Tight junctions - Gap junctions. ## Cellular Communication And Signal Transduction 1. Cells communicate in three main ways: - They display plasma membrane-bound signaling molecules (receptors) that affect the receptor proteins inside the target cell, and the signal molecule has to enter the cell to bind to them - They activate the protein channels (gap junctions) that directly coordinate the activities of adjacent cells - They form protein channels (gap junctions) that directly coordinate the activities of adjacent cells. 2. Primary modes of intercellular signaling are: - Contact-dependent - Paracrine - Hormonal - Neurohormonal - Neurotransmitter. 3. Signal transduction involves signals or instructions from extracellular chemical messengers that are conveyed to the cell’s interior for execution. 4. Signal transduction pathways (signaling cascades, relay chains) have several important functions, including: - Physically transferring the signal around the cell. - Amplifying the signal. - Distributing the signal. - Modulating the signal. ## Summary Review – Cont’d 5. Two important second-messenger pathways are cAMP and Ca++. 6. G protein is an intermediary between the receptor and adenylyl cyclase. 7. Phospholipase C, an enzyme protein effector, is bound to the inner side of the membrane. ## Cellular Metabolism 1. The chemical tasks of maintaining essential cellular functions are referred to as cellular metabolism. Anabolism is the energy-using process of metabolism, whereas catabolism is the energy-releasing process. 2. ATP functions as an energy-transferring molecule. Energy is stored by molecules of carbohydrate, lipid, and protein, which, when catabolized, transfer energy to ATP. 3. Oxidative phosphorylation occurs in the mitochondria and is the mechanism by which the energy produced from carbohydrates, fats, and proteins is transferred to ATP. ## Membrane Transport: Cellular Intake And Output 1. Cell survival and growth depend on the constant exchange of molecules with their environment. Simple diffusion across the lipid bilayer of the plasma membrane occurs for important molecules, such as O₂ and CO₂. 2. The majority of molecule transfer depends on specialized membrane transport proteins that span the lipid bilayer and provide private thoroughfares for select molecules. 3. The two main classes of membrane transport proteins are transporters and channels. 4. Water and small, electrically uncharged molecules move through pores in the plasma membrane's lipid bilayer in the process called passive transport. 5. Passive transport does not require the expenditure of energy; rather, it is driven by the physical effects of osmosis, hydrostatic pressure, and diffusion. 6. Larger molecules and molecular complexes (e.g., ligand-receptor complexes) are moved into the cell by active transport, which requires expenditure of energy (by means of ATP) by the cell. 7. Two types of solutes exist in body fluids: electrolytes and nonelectrolytes. Electrolytes are electrically charged and dissociate into constituent ions when placed in solution. Nonelectrolytes do not dissociate when placed in solution. 8. Diffusion is the passive movement of a solute from an area of higher solute concentration to an area of lower solute concentration. 9. Hydrostatic pressure is the mechanical force of water pushing against cellular membranes. 10. Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. 11. The amount of hydrostatic pressure required to oppose the osmotic movement of water is called the osmotic pressure of the solution. 12. The overall osmotic effect of colloids, such as plasma proteins, is called the oncotic pressure or colloid osmotic pressure. 13. Mediated transport can be passive or active. Mediated transport includes the movement of two molecules simultaneously in one direction (symport) or in opposite directions (antiport), or the movement of a single molecule in one direction (uniport). 14. Endocytosis is a cellular internalizing process where a section of the plasma membrane enfolds substances from outside the cell, invaginates, and separates from the plasma membrane forming a vesicle that moves inside the cell. 15. Endocytosis can be subdivided into four categories: - Clathrin-mediated endocytosis - Caveolae-mediated endocytosis - Macropinocytosis - Phagocytosis. Over time, however, these categories may change. 16. Exosomes are small membrane vesicles of endocytic origin containing lipid, protein, and RNA species. They are secreted by nearly all mammalian cell types and confer messages between cells. 17. Exocytosis is the discharge of secretion of material from the intracellular vesicles at the cell surface. Exocytosis has two main functions: - Replacement of portions of the plasma membrane that have been removed by endocytosis - Release of molecules synthesized by the cells into the extracellular matrix. 18. Pinocytosis is a type of endocytosis in which fluids and solute molecules are ingested through formation of small vesicles. 19. Phagocytosis is a type of endocytosis in which large particles such as bacteria are ingested through formation of large vesicles, called vacuoles. 20. All body cells are electrically polarized, with the inside of the cell more negatively charged than the outside. The difference in voltage across the plasma membrane is the resting membrane potential. 21. When an excitable (nerve or muscle) cell receives an electrochemical stimulus, cations enter the cell, causing a rapid change in the resting membrane potential known as the action potential. The action potential “moves” along the cell’s plasma membrane and is transmitted to an adjacent cell. This is how electrochemical signals convey information from cell to cell. ## Cellular Reproduction: The Cell Cycle 1. Cellular reproduction in body tissues involves rounds of cell growth and division. 2. Cellular reproduction in body tissues involves mitosis (nuclear division) and cytokinesis (cytoplasmic division). 3. Only mature cells are capable of division. Maturation occurs during a stage of cellular life called interphase (growth phase). 4. The cell cycle is the reproductive process that begins after interphase in all tissues with cellular turnover. The four phases of the cell cycle are: - G₁phase (G = gap), which is the period between the M phase and the start of DNA synthesis - S phase (S = synthesis), in which DNA is synthesized in the cell nucleus - G₂ phase, in which RNA and protein synthesis occurs, the period between the completion of DNA synthesis and the next phase (M) - M phase (M = mitosis), which includes both nuclear and cytoplasmic division. 5. The M phase (mitosis) involves four stages: - Prophase - Metaphase - Anaphase - Telophase. 6. The mechanisms that control cell division depend on the integrity of genetic, epigenetic, and protein growth factors. 7. Two classes of switches or regulatory molecules that determine a cell’s progress through the cell cycle are cyclin-dependent kinases (Cdks) and cyclins. 8. Organ size and body size depend on cell growth, cell division, and cell survival. 9. A mitogen is a substance that induces or stimulates mitosis. 10. Growth factors stimulate an increase in cell mass, or cell growth, by promoting the synthesis of proteins and other macromolecules and inhibiting their degradation. 11. The DNA damage response occurs when DNA is damaged and several protein kinases are recruited to the site of damage and start a signaling pathway that stops the progression of the cell cycle called cell cycle arrest. ## Tissues 1. Cells of one or more types are organized into tissues, and different types of tissues compose organs. Organs are organized to function as tracts or systems. 2. To form tissue, cells must exhibit: - Intercellular recognition and communication - Adhesion - Memory. 3. The four basic types of tissues are: - Epithelial - Muscle - Neural - Connective tissues. 4. Stem cells are with the potential to develop into many different cell types during early development and growth. Stem cells serve as an internal repair and maintenance system dividing indefinitely. Tissue repair and renewal does not always depend on stem cells. 5. Epithelial tissue covers most internal and external surfaces of the body. The functions of epithelial tissue include: - Protection - Absorption - Secretion - Excretion. 6. Connective tissue binds various tissues and organs together, supporting them in their locations and serving as storage sites for excess nutrients. 7. Muscle tissue is composed of long, thin, highly contractile cells or fibers called myocytes. Muscle tissue that is attached to bones enables voluntary movement. Muscle tissues in internal organs enable involuntary movement, such as the heartbeat. 8. Neural tissue is composed of highly specialized cells called neurons that receive and transmit electrical impulses very rapidly across junctions called synapses. ## Glossary * **Absolute refractory period**: The period after an action potential when a cell cannot initiate a new action potential. * **Active transport**: The movement of a substance across a cell membrane against a concentration gradient that requires energy. * **Amphipathic molecule**: A molecule with both hydrophilic and hydrophobic regions. * **Anabolism**: The energy-using process of metabolism. It involves building up complex molecules from simpler ones. * **Anaerobic glycolysis**: The breakdown of glucose to pyruvate in the absence of oxygen. * **Anaphase**: The stage of mitosis when sister chromatids separate and move to opposite poles of the cell. * **Anion**: A negatively charged ion. * **Antiport**: A type of mediated transport in which two substances move across a cell membrane in opposite directions. * **Ataxia-telangiectasia and rad 3 (ATR)**: A protein kinase involved in the DNA damage response pathway. * **Ataxia-telangiectasia mutated (ATM)**: A protein kinase involved in the DNA damage response pathway. * **Autocrine signaling**: A type of cell signaling where a cell secretes a signaling molecule that binds to receptors on the same cell. * **Autodigestion**: The breakdown of cellular components by lysosomal enzymes. * **Autolysosome (autophagosome)**: A vesicle that contains material that is being degraded by autophagy. * **Autophagy**: A process in which a cell degrades its own components. * **Basal lamina**: A thin layer of extracellular matrix that underlies epithelial tissues. * **Basement membrane (basal lamina)**: A thin layer of extracellular matrix that underlies epithelial tissues. * **Binding site**: The specific region on a molecule where another molecule can bind. * **Cadherin**: A type of cell adhesion molecule that helps cells adhere to one another. * **Catabolism**: The energy-releasing process of metabolism. It involves breaking down complex molecules into simpler ones. * **Cation**: A positively charged ion. * **Caveolae**: Small, flask-shaped invaginations of the plasma membrane that play a role in endocytosis and signal transduction. * **Cell adhesion molecule (CAM)**: A type of cell surface protein that helps cells adhere to one another. * **Cell cycle**: The series of events that a cell goes through as it grows and divides. * **Cell cycle arrest**: The pausing of the cell cycle at a specific point, often due to damage or stress. * **Cell cycle control system**: The set of regulatory proteins that control the progression of the cell cycle. * **Cell junction**: A specialized structure that connects neighboring cells. * **Cell polarity**: The uneven distribution of components within a cell. * **Cell-to-cell adhesion**: The process by which cells adhere to one another. * **Cellular metabolism**: The sum of all chemical reactions that occur within a cell. * **Cellular receptor**: A protein on the cell surface that binds to a specific signaling molecule. * **Centriole**: A small, cylindrical organelle that is involved in the formation of microtubules. * **Centromere**: The region of a chromosome where sister chromatids are attached. * **Channel**: A protein that forms a pore through a cell membrane, allowing the passage of specific molecules. * **Chemical synapse**: A junction between neurons where chemical messengers transmit signals between neurons. * **Chromatid**: One of two identical copies of a chromosome that are joined at the centromere. * **Chromatin**: The genetic material of a cell, consisting of DNA and proteins. * **Chromosome**: A structure composed of DNA and proteins that carries genetic information. * **Cilia**: Short, hair-like projections that extend from the surface of some cells. * **Cisternae**: Flattened, membrane-bound sacs. * **Columnar cell**: A tall, column-shaped epithelial cell. * **Concentration gradient**: The difference in concentration of a substance between two areas. * **Connexon**: A protein channel that forms a gap junction. * **Contact-dependent signaling**: A type of cell signaling where cells must be in direct contact to communicate. * **Cristae**: The folds of the inner membrane of a mitochondrion. * **Cuboidal cell**: A cube-shaped epithelial cell. * **Cyclic adenosine monophosphate (cyclic AMP, cAMP)**: A second messenger molecule that is involved in many cellular processes, including the activation of protein kinases. * **Cyclic guanosine monophosphate (cyclic GMP, cGMP)**: A second messenger molecule that is involved in many cellular processes, including the activation of protein kinases. * **Cyclin**: A protein that regulates the activity of cyclin-dependent kinases. * **Cyclin-dependent kinase (Cdk)**: An enzyme that phosphorylates target proteins, often triggering cell cycle progression. * **Cytochrome**: A protein that contains a heme group and is involved in electron transport. * **Cytokinesis**: The division of the cytoplasm following nuclear division. * **Cytoplasm**: The region of a cell that is enclosed by the plasma membrane and contains the cytoplasm. * **Cytoplasmic matrix**: The cytosol and the organelles of the cell. * **Cytoskeleton**: A network of protein filaments that provides structural support and helps with movement. * **Cytosol**: The fluid within a cell that surrounds the organelles. * **Depolarization**: A decrease in the difference in electrical charge across a cell membrane. * **Desmosome**: A type of cell junction that provides strong adhesion between cells. * **Differentiation**: The process by which a cell becomes specialized to perform a specific function. * **Diffusion**: The passive movement of a substance from an area of higher concentration to an area of lower concentration. * **Digestion**: The breakdown of food molecules into smaller molecules that the body can absorb. * **DNA damage response**: A cellular pathway that is activated when DNA is damaged, often leading to cell cycle arrest or apoptosis. * **E2F**: A transcription factor that is involved in the regulation of genes required for DNA replication. * **Effective osmolality**: The osmotic pressure of a solution that is not affected by the presence of non-permeable solutes. * **Elastic fiber**: A type of protein fiber that is flexible and can stretch. * **Elastin**: A protein that is found in elastic fibers. * **Electrolyte**: A substance that dissociates into ions when dissolved in water. * **Electron-transport chain**: A series of protein complexes that are embedded in the inner membrane of a mitochondrion and are involved in the production of ATP. * **Endocytic vesicle**: A vesicle that is formed during endocytosis. * **Endocytosis**: The process by which a cell takes in substances from its surroundings by engulfing them in membrane-bound vesicles. * **Endoplasmic reticulum (ER)**: A network of membranes that extends throughout the cytoplasm of a eukaryotic cells. * **Endosome**: A vesicle that is formed during endocytosis that carries internalized material to other destinations. * **Equatorial plate (metaphase plate)**: The region of the cell where chromosomes line up during metaphase. * **ER stress**: A condition that occurs when the endoplasmic reticulum is unable to properly fold proteins. * **Eukaryote**: A cell that has a nucleus and other membrane-bound organelles. * **Exocytosis**: The process by which a cell releases substances from its interior by fusing membrane-bound vesicles with the plasma membrane. * **Exosome**: A small vesicle secreted by cells that can carry signaling molecules to other cells. * **Extracellular matrix (ECM)**: The network of molecules that surrounds cells and provides structural support and helps with signaling. * **Fibroblast**: A type of connective tissue cell that produces collagen and other ECM components. * **Fibronectin**: A protein that is found in connective tissues. * **Filtration**: The movement of substances through a membrane from an area of higher pressure to an area of lower pressure. * **First messenger**: A signaling molecule that binds to a receptor on the cell surface. * **G₁ phase**: The first gap phase of the cell cycle, during which the cell grows and prepares for DNA replication. * **G₂ phase**: The second gap phase of the cell cycle, during which the cell prepares for mitosis. * **Go phase**: A resting phase in which cells are not actively dividing. * **G protein**: A type of protein that is involved in signal transduction pathways. * **Gap junction**: A type of cell junction that allows the direct transfer of small molecules between adjacent cells. * **Gated transport**: A type of protein transport in which the movement of a substance is controlled by a gate. * **Gating**: The process of opening or closing a gate. * **Glycolipid**: A lipid molecule that is attached to a carbohydrate. * **Glycolysis**: The breakdown of glucose to pyruvate in the presence of oxygen. * **Glycoprotein**: A protein molecule that is attached to a carbohydrate. * **Golgi complex (Golgi apparatus)**: A series of flattened sacs that are involved in the packaging and modification of proteins. * **Ground substance**: The gel-like material that is found between the fibers of connective tissues. * **Growth factor**: A substance that stimulates cell growth and division. * **Guanosine diphosphate (GDP)**: A nucleotide that is involved in cellular processes. * **Guanosine triphosphate (GTP)**: A nucleotide that is involved in cellular processes. * **Homeostasis**: The maintenance of a stable internal environment. * **Hormonal signaling**: A type of cell signaling where hormones travel through the bloodstream to reach their target cells. * **Hydrolase**: An enzyme that catalyzes the hydrolysis of a chemical bond. * **Hydrophilic**: A molecule that is attracted to water. * **Hydrophobic**: A molecule that is repelled by water. * **Hydrostatic pressure**: The pressure exerted by a fluid. * **Hypertonic solution**: A solution that has a higher concentration of solutes than the cell. * **Hypotonic solution**: A solution that has a lower concentration of solutes than the cell. * **Immunoglobulin superfamily CAM (IgSF CAM)**: A type of cell adhesion molecule. * **Inner membrane**: The inner membrane of a mitochondrion, which is folded into cristae. * **Integrins**: A type of cell adhesion molecule that connects cells to the ECM. * **Intermediary metabolism**: The sum of all chemical reactions that occur between the breakdown of food molecules and the synthesis of macromolecules. * **Intermediate filament**: A type of protein filament that provides structural support and helps with cell shape. * **Interphase**: The stage of the cell cycle between mitosis. * **Ion**: An atom or molecule that has a net electrical charge. * **Isotonic solution**: A solution that has the same concentration of solutes as the cell. * **Junctional complex**: A specialized structure that connects neighboring cells. * **Ligand**: A molecule that binds to a receptor. * **Lipid bilayer**: The double layer of lipid molecules that forms the basis of the cell membrane. * **Lipid raft**: A specialized region of the cell membrane that is enriched in cholesterol and sphingolipids. * **Lysosome**: A vesicle that contains digestive enzymes. * **Macromolecule**: A large molecule, such as a protein, carbohydrate, lipid, or nucleic acid. * **Macropinocytosis**: A type of endocytosis in which cells take in large amounts of extracellular fluid. * **Mechanotransduction**: The process by which cells convert mechanical stimuli into biochemical signals. * **Mediated transport**: The movement of a substance across a cell membrane that is facilitated by a transport protein. * **Membrane protein**: A protein that is embedded in the cell membrane. * **Membrane transport proteins**: Proteins that facilitate the movement of substances across the cell membrane. * **Metabolic pathway**: A series of chemical reactions. * **Metaphase**: The stage of mitosis when chromosomes line up along the equator of the cell * **Microdomain**: A small, specialized region of the cell membrane. * **Micropinocytosis**: A type of endocytosis in which cells take in small amounts of extracellular fluid. * **Microtubule**: A hollow tube that is made of the protein tubulin. * **Microvilli**: Finger-like projections that extend from the surface of some cells. * **Mitochondria**: A double-membrane organelle. * **Mitogen**: A substance that stimulates the growth of cells. * **Mitosis**: The process of nuclear division. * **M phase**: The stage of the cell cycle during which mitosis and cytokinesis occur. * **Multicellular organism**: An organism that is composed of many cells. * **Neurohormonal signaling**: A type of cell signaling where neurons release hormones into the bloodstream. * **Neurotransmitter**: A chemical messenger that is released by neurons to transmit signals between neurons. * **Nonpolar molecule**: A molecule that does not have a separation of electrical charge. * **Nuclear envelope**: The membrane that surrounds the nucleus of a cell. * **Nucleolus**: A region of the nucleus that is involved in the production of ribosomes. * **Nucleoplasm**: The fluid within the nucleus of a cell. * **Oncotic pressure**: The osmotic pressure that is due to the presence of proteins in a solution. * **Organ**: A group of tissues that work together to perform a specific function. * **Organelle**: A specialized compartment within a cell that performs a specific function. * **Osmolarity**: A measure of the total concentration of solute particles in a solution. * **Osmosis**: The movement of water across a semipermeable membrane from an area of higher water concentration to an area of lower water concentration. * **Osmotic pressure**: The pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane. * **Passive transport**: The movement of a substance across a cell membrane that does not require energy. * **Phagocytosis**: A type of endocytosis in which cells take in large particles, such as bacteria or cell debris. * **Phospholipase C**: An enzyme that cleaves membrane phospholipids. * **Pinocytosis**: A type of endocytosis in which cells take in fluids and dissolved solutes. * **Polar molecule**: A molecule that has a separation of electrical charge. * **Poly-peptide**: A chain of amino acids. * **Primary active transport**: A type of active transport that uses energy directly from ATP. * **Prophase**: The stage of mitosis when the chromosomes condense and the nuclear envelope breaks down. * **Proteostasis**: The process of maintaining a balance between protein synthesis, folding, and degradation. * **Receptor**: A protein that binds to a specific signaling molecule. * **Receptor-mediated endocytosis**: A type of endocytosis where a specific ligand binds to its receptor on the cell surface in order to enter the cell. * **Replication**: The process of copying DNA. * **Resting membrane potential**: The difference in electrical charge across a cell membrane when the cell is not stimulated. * **Ribonucleic acid (RNA)**: A nucleic acid that is involved in protein synthesis. * **Ribosome**: A complex of RNA and proteins that is involved in protein synthesis. * **Secondary active transport**: A type of active transport that uses the energy stored in a concentration gradient created by primary active transport. * **Secretory vesicle**: A vesicle that contains substances that are to be released from the cell. * **Selectin**: A type of cell adhesion molecule that is involved in the binding of white blood cells to endothelial cells. * **Signal transduction**: The process by which cells convert extracellular signals into intracellular signals. * **Signal transduction pathway**: A chain of molecular events that begins with the binding of a signaling molecule to a receptor on the cell surface and ends with a change in cell behavior. * **Sister chromatids**: Two identical copies of a chromosome that are joined at the centromere. * **Sodium-potassium pumps**: A type of active transport that pumps sodium ions out of the cell and potassium ions into the cell. * **S phase**: The stage of the cell cycle when DNA is replicated. * **Stem cell**: A cell that has the potential to develop into different types of cells. * **Symport**: A type of mediated transport in which two substances move across a cell membrane in the same direction. * **Synapse**: A junction between two neurons where signaling molecules transmit signals between neurons. * **Telophase**: The stage of mitosis when the chromosomes reach the poles of the cell and the nuclear envelope reforms. * **Tight junction**: A type of cell junction that prevents the leakage of fluids between cells. * **Transcription**: The process of copying a gene's DNA sequence into an RNA sequence. * **Translation**: The process of converting an RNA sequence into a protein sequence. * **Transporter**: A type of membrane transport protein that moves substances across the cell membrane by binding to them. * **Uniport**: A type of mediated transport in which a single substance moves across a cell membrane. * **Vacuole**: A large vesicle that is found in plants and fungi. * **Vescile**: A membrane-bound sac.