Unit 2 Biology
Chromatin
The substance distributed in the nucleus of a cell that condenses to form chromosomes during cell division. It consists mainly of DNA and proteins called histones.
Organ system
A group of organs that work together to perform one or more functions. Each does a particular job in the body, and is made up of certain tissues.
Vacuoele
A cavity within the cytoplasm of a cell, surrounded by a single membrane and containing fluid, food, or metabolic waste. Vacuoles are found in the cells of plants, protists, and some primitive animals. In mature plant cells, there is usually one large vacuole which occupies a large part of the cell's volume and is filled with a liquid called cell sap. The cell sap stores food reserves, pigments, defensive toxins, and waste products to be expelled or broken down. In the cells of protists, however, there may be many small specialized vacuoles, such as digestive vacuoles for the absorption of captured food and contractile vacuoles for the expulsion of excess water or wastes.
Organ
A distinct part of an organism that performs one or more specialized functions. Examples of organs are the eyes, ears, lungs, and heart of an animal, and the roots, stems, and leaves of a plant.
Tissue
A large mass of similar cells that make up a part of an organism and perform a specific function. The internal organs and connective structures (including bone and cartilage) of vertebrates, and cambium, xylem, and phloem in plants are made up of different types of tissue.
Chloroplast
A plastid in the cells of green plants and green algae that contains chlorophylls and carotenoid pigments and creates glucose through photosynthesis. In plants, chloroplasts are usually disk-shaped and can reorient themselves in the cell to vary their exposure to sunlight. Chloroplasts contain the saclike membranes known as thylakoids, which contain the chlorophyll and are arranged in stacklike structures known as grana. Besides conducting photosynthesis, plant chloroplasts store starch and are involved in amino acid synthesis. Like mitochondria, chloroplasts have their own DNA that is different from the DNA in the nucleus, and chloroplasts are therefore believed to have evolved from symbiont bacteria, their DNA being a remnant of their past existence as independent organisms.
Flagella
A slender whiplike part extending from some single-celled organisms, such as the dinoflagellates, that moves rapidly back and forth to impart movement to the organism.
Plasmid
A small, circular unit of DNA that replicates within a cell independently of the chromosomal DNA and is most often found in bacteria. Certain plasmids can insert themselves into chromosomes in places where there is a common sequence of nucleotides. Plasmids contain a few genes, which usually code for proteins, especially enzymes, some of which confer resistance to antibiotics. Plasmids are used in recombinant DNA research, especially to transform bacterial cells.
Ribosome
A sphere-shaped structure within the cytoplasm of a cell that is composed of RNA and protein and is the site of protein synthesis. Ribosomes are free in the cytoplasm and often attached to the membrane of the endoplasmic reticulum. Ribosomes exist in both eukaryotic and prokaryotic cells. Plastids and mitochondria in eukaryotic cells have smaller ribosomes similar to those of prokaryotes.
Chromosome
A structure in all living cells that consists of a single molecule of DNA bonded to various proteins and that carries the genes determining heredity. In all eukaryotic cells, the chromosomes occur as threadlike strands in the nucleus. During cell reproduction, these strands coil up and condense into much thicker structures that are easily viewed under a microscope. Chromosomes occur in pairs in all of the cells of eukaryotes except the reproductive cells, which have one of each chromosome, and some red blood cells (such as those of mammals) that expel their nuclei. In bacterial cells and other prokaryotes, which have no nucleus, the chromosome is a circular strand of DNA located in the cytoplasm.
Mithochondria
A structure in the cytoplasm of all cells except bacteria in which food molecules (sugars, fatty acids, and amino acids) are broken down in the presence of oxygen and converted to energy in the form of ATP. Mitochondria have an inner and outer membrane. The inner membrane has many twists and folds (called cristae), which increase the surface area available to proteins and their associative reactions. The inner membrane encloses a liquid containing DNA, RNA, small ribosomes, and solutes. The DNA in mitochondria is genetically distinct from that in the cell nucleus, and mitochondria can manufacture some of their own proteins independent of the rest of the cell. Each cell can contain thousands of mitochondria, which move about producing ATP in response to the cell's need for chemical energy. It is thought that mitochondria originated as separate, single-celled organisms that became so symbiotic with their hosts as to be indispensible. Mitochondrial DNA is thus considered a remnant of a past existence as a separate organism.
Centriole
Either of a pair of cylinder-shaped bodies found in the centrosome of most eukaryotic organisms other than plants. During cell division (both mitosis and meiosis), the centrioles move apart to help form the spindle, which then distributes the chromosomes in the dividing cell.
Organelle
A structure or part that is enclosed within its own membrane inside a cell and has a particular function. Organelles are found only in eukaryotic cells and are absent from the cells of prokaryotes such as bacteria. The nucleus, the mitochondrion, the chloroplast, the Golgi apparatus, the lysosome, and the endoplasmic reticulum are all examples of organelles. Some organelles, such as mitochondria and chloroplasts, have their own genome (genetic material) separate from that found in the nucleus of the cell. Such organelles are thought to have their evolutionary origin in symbiotic bacteria or other organisms that have become a permanent part of the cell.
Solvent
A substance that can dissolve another substance, or in which another substance is dissolved, forming a solution. Water is the most common solvent.
Solute
A substance that is dissolved in another substance (a solvent), forming a solution.
Pseudopods
A temporary footlike extension of a one-celled organism, such as an amoeba, used for moving about and for surrounding and taking in food.
Cilia
A tiny hairlike projection on the surface of some cells and microscopic organisms, especially protozoans. Cilia are capable of whipping motions and are used by some microorganisms, such as paramecia, for movement. Cilia lining the human respiratory tract act to remove foreign matter from air before it reaches the lungs.
Lipid bilayer
A two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends facing outward.
Eye spots
An area that is sensitive to light and functions somewhat like an eye, found in certain single-celled organisms as well as many invertebrate animals.
Nucleus
An organelle in the cytoplasm of eukaryotic cells (all cells except prokaryotes) that contains nearly all the cell's DNA and controls its metabolism, growth, and reproduction. The nucleus is surrounded by a pair of membranes called the nuclear envelope, which can be continuous in places with the membranes of the endoplasmic reticulum. The membranes of the nuclear envelope have interconnected pores that allow the exchange of substances with the cell's cytoplasm. The nuclear DNA is wrapped around proteins (called histones) in strands of chromatin, which exists in a matrix known as nucleoplasm (analogous to the cytoplasm outside the nucleus). Just prior to cell division, the chromatin condenses into individual chromosomes, which contain the cell's hereditary information. The nucleus also contains at least one spherical nucleolus, which mainly contains RNA and proteins and directs the construction of the cell's ribosomes.
Eukaryote
An organism whose cells contain a nucleus surrounded by a membrane and whose DNA is bound together by proteins (histones) into chromosomes. The cells of eukaryotes also contain an endoplasmic reticulum and numerous specialized organelles not present in prokaryotes, especially mitochondria, Golgi bodies, and lysosomes. The organelles are enclosed in a three-part membrane (called a unit membrane) consisting of a lipid layer sandwiched between two protein layers. All organisms except for bacteria and archaea are eukaryotes.
Stem cells
An unspecialized cell found in fetuses, embryos, and some adult body tissues that has the potential to develop into specialized cells or divide into other stem cells. Stem cells from fetuses or embryos can develop into any type of differentiated cells, while those found in mature tissues develop only into specific cells. Stem cells can potentially be used to replace tissue damaged or destroyed by disease or injury, but the use of embryonic stem cells for this purpose is controversial. Also called progenitor cell.
Prokaryote
Any of a wide variety of one-celled organisms of the kingdom Monera (or Prokaryota) that are the most primitive and ancient known forms of life. Prokaryotes lack a distinct cell nucleus and their DNA is not organized into chromosomes. They also lack the internal structures bound by membranes called organelles, such as mitochondria. At the molecular level, prokaryotes differ from eukaryotes in the structure of their lipids and of certain metabolic enzymes, and in how genes are expressed for protein synthesis. Prokaryotes reproduce asexually and include the bacteria and blue-green algae. Also called moneran.
Isotonic
Denoting or relating to a solution having the same osmotic pressure as some other solution, especially one in a cell or a body fluid.
Hypertonic
Having a higher osmotic pressure than a particular fluid, typically a body fluid or intracellular fluid.
Concentration
The amount of a particular substance in a given amount of another substance, especially a solution or mixture.
Cell
The basic unit of living matter in all organisms, consisting of protoplasm enclosed within a cell membrane. All cells except bacterial cells have a distinct nucleus that contains the cell's DNA as well as other structures (called organelles) that include mitochondria, the endoplasmic reticulum, and vacuoles. The main source of energy for all of a cell's biological processes is ATP.
Chemotaxis
The characteristic movement or orientation of an organism or cell along a chemical concentration gradient either toward or away from the chemical stimulus. Bacteria exhibit chemotaxis when they move toward a source of nutrients.
Cell specialization
The differentiation of cells as they develop, depending on their location in a tissue or organ, and therefore their primary roles within that tissue, organ, and ultimately, organism.
Nuclear envelope
The double-layered membrane enclosing the nucleus of a eukaryotic cell. The nuclear envelope has pores that allow the passage of materials into and out of the nucleus. Also called nuclear membrane.
Cytoplasm
The jellylike material that makes up much of a cell inside the cell membrane, and, in eukaryotic cells, surrounds the nucleus. The organelles of eukaryotic cells, such as mitochondria, the endoplasmic reticulum, and (in green plants) chloroplasts, are contained in the cytoplasm. The cytoplasm and the nucleus make up the cell's protoplasm.
Osmosis
The movement of a solvent through a membrane separating two solutions of different concentrations. The solvent from the side of weaker concentration usually moves to the side of the stronger concentration, diluting it, until the concentrations of the solutions are equal on both sides of the membrane. ◇ The pressure exerted by the molecules of the solvent on the membrane they pass through is called osmotic pressure. Osmotic pressure is the energy driving osmosis and is important for living organisms because it allows water and nutrients dissolved in water to pass through cell membranes.
Diffusion
The movement of atoms or molecules from an area of higher concentration to an area of lower concentration. Atoms and small molecules can move across a cell membrane by diffusion.
Active transport
The movement of ions or molecules across a cell membrane in the direction opposite that of diffusion, that is, from an area of lower concentration to one of higher concentration. Active transport requires the assistance of a type of protein called a carrier protein, using energy supplied by ATP.
Cell wall
The outermost layer of cells in plants, bacteria, fungi, and many algae that gives shape to the cell and protects it from infection. In plants, the cell wall is made up mostly of cellulose, determines tissue texture, and often is crucial to cell function.
Facilitad diffusion
The process of spontaneous passive transport of molecules or ions across a cell's membrane via specific transmembrane integral proteins.
Cleavage
The series of mitotic cell divisions by which a single fertilized egg cell becomes a many-celled blastula. Each division produces cells half the size of the parent cell.
Equilibrium
The state of a body or physical system that is at rest or in constant and unchanging motion. A system that is in equilibrium shows no tendency to alter over time. ◇ If a system is in static equilibrium, there are no net forces and no net torque in the system. ◇ If a system is in stable equilibrium, small disturbances to the system cause only a temporary change before it returns to its original state.
Homeostasis
The tendency of an organism or cell to regulate its internal conditions, such as the chemical composition of its body fluids, so as to maintain health and functioning, regardless of outside conditions. The organism or cell maintains homeostasis by monitoring its internal conditions and responding appropriately when these conditions deviate from their optimal state. The maintenance of a steady body temperature in warm-blooded animals is an example of homeostasis. In human beings, the homeostatic regulation of body temperature involves such mechanisms as sweating when the internal temperature becomes excessive and shivering to produce heat, as well as the generation of heat through metabolic processes when the internal temperature falls too low.
Plasma membrane (cell membrane)
The thin membrane that forms the outer surface of the protoplasm of a cell and regulates the passage of materials in and out of the cell. It is made up of proteins and lipids and often contains molecular receptors. The membranes of organelles within the cell are made of the same basic material as the cell membrane. In plant cells, the cell membrane is surrounded by a rigid cell wall.
Cell theory
a basic tenet of modern biology, first stated by Matthias Schleiden and Theodor Schwann in 1838-39, that cells are the basic units of structure and function in living organisms.
Contractile vacuole
a membrane-enveloped cellular organelle, found in many microorganisms, that periodically expands, filling with water, and then contracts, expelling its contents to the cell exterior: thought to be important in maintaining hydrostatic equilibrium.
Phototaxis
movement of an organism toward or away from a source of light.
Osmotic pressure
the force that a dissolved substance exerts on a semipermeable membrane, through which it cannot penetrate, when separated by it from pure solvent.
Turgor pressure
the pressure exerted on a plant cell wall by water passing into the cell by osmosis Also called hydrostatic pressure