Zoology: Chapter 3

Hemidesmosomes

Occur at the base of cells and anchor them to underlying connective tissue layers.

Adhesion junctions

Occur just beneath tight junctions. These anchoring junctions are similar to tight junctions in that they encircle the cell. They are different form tight junctions in that they do not seal adjacent cells to each other.

Facilitated diffusion or facilitated transport

One kind of mediated transport mechanism. A transporter assists a molecule to diffuse through the membrane that it cannot otherwise penetrate. Requires no metabolic energy.

Protoplasm

Organized living substance; cytoplasm and nucleoplasm of the cell.

Isotonic or isoosmotic

Pertaining to solutions having the same or equal osmotic pressure.

Plastids

Plants cells contain these. Some of them are photosynthetic organelles, and plant cells bear a cell wall containing cellulose outside the cell membrane.

Pinocytosis

Small areas of the surface membrane are invaginated into cells to from tiny vesicles. The invaginated pits and vesicles are called caveolae. Pinocytosis functions for intake of at least some vitamins, and similar mechanisms may be important in translocating substances from one side of a cell to the other, as occurs during some capillary exchange. This variation of pinocytosis is often called transcytosis. Some signal molecules, such as some hormones or growth factors, are internalized by pinocytosis.

Stages of Mitosis

Prophase, metaphase, anaphase, and telophase.

Syncytium

Sometimes a multinucleate mass is formed by cell fusion rather than nuclear proliferation.

Organelle

Specialized part of a cell; a subcellular structure that performs functions analogous to organs of multicellular animals. A cell's interior is composed of numerous cellular organelles within a network of membranes. The components of a cell are highly organized.

Gap junctions

Provide a means of intercellular communication. They form tiny canals between cells, so that their cytoplasm becomes continuous, and small molecules and ions can pass from one cell to another.

Hypertonic

Refers to a solution that contains a greater concentration of dissolved particles than that of another solution with which it is compared; contrasts with hypotonic.

Hypoosmotic

Refers to a solution whose osmotic pressure is less than that of another solution with which it is compared or taken as a standard; contains a lesser concentration of dissolved particles and loses water during osmosis; contrasts with hyperosmotic.

Phagocytosis "Cell eating"

A common method of feeding among protozoa and some lower metazoa, such as sponges, cnidaria, and flatworms. It is also the way in which white blood cells engulf cellular debris and invading mircrobes or other pathogens in the blood.

Fluid-mosaic model

A phospholipid bilayer, two layers of phospholipid molecules, all oriented with their water-soluble (hydrophilic) ends toward the outside and their fat-soluble portions (hydrophobic) ends toward the inside of the membrane. The phospholipid bilayer is fluidlike, giving the membrane flexibility and allowing the phospholipid molecules to move sideways freely within their own monolayer.

Apoptosis

A programmed cell death, which is in many cases necessary for continued health and development of vertebrates.

Clathrin

A protein forming a lattice structure lining the invaginated pits during receptor-mediated endocytosis. Hence, they are called clathrin-coated pits.

Endosome

A remaining vesicle

Mediated transport system

A second way that a substance may enter across a cell membrane. The substance binds to a specific site on a transmembrane protein that assists it across the membrane.

Hypotonic

A solution that contains a lesser concentration of dissolved particles than that of another solution with which it is compared; contrasts with hypertonic.

Receptor-mediated endocytosis

A specific mechanism for bringing large molecules within a cell. Proteins of the plasma membrane specifically bind particular molecules (termed ligands) which may be present in the extracellular fluid in very low concentrations.

Cytoskeleton

A system of tubules and filaments form this. Eukaryotic cells have this. These provide support and maintain the form of cells, and in many cells, they provide a means of cellular locomotion and movement of macromolecules and organelles within the cell. The cytoskeleton is composed of microfilaments, microtubules, and intermediate filaments.

Cell theory

A theory stating that all living organisms are composed of cells.

Endocytosis

A third way that a substance may enter across a cell membrane. The substance is enclosed within a vesicle that forms from the membrane surface and detaches inside the cell.

Plasma membrane

Acts as a selective gatekeeper for entrance and exit of many substances involved in a cell metabolism. Some substances can pass through with ease, others enter slowly and with difficulty, and still others cannot enter at all.

Zygote

All cells originate from the division of a single cell, a zygote, which is the product of union (fertilization) between an egg and a sperm (gametes or germ cells).

Mitochondria

Are conspicuous organelles present in nearly all eukaryotic cells. They are diverse in size, number, and shape; some are rodlike, and others are nearly spherical. They may be scattered uniformly throughout the cytoplasm or localized near cell surfaces and other regions of high metabolic activity. A mitochondrion is composed of a double membrane. The outer membrane is smooth, whereas the inner membrane is folded numerous platelike or fingerlike projections called cristae, which increase the internal surface area where chemical reactions occur. Mitochondria are often called "powerhouses of cells," because enzymes located on the cristae catalyze the energy-yielding steps of aerobic metabolism. The majority of a cell's ATP, the most important energy-transfer molecule of all cells, is produced in this organelle.

Microtubules

Are larger than microfilaments and are tubular structures composed of a protein called tubulin. Each tubulin molecule is actually a doublet, or dimer, composed of two globular proteins. Microtubules play a vital role in moving chromosomes toward daughter cells in cell division, and they are important in intracellular architecture, organization, and transport. Microtubules form essential parts of the structures of cilia and flagella.

Intermediate filaments

Are larger than microfilaments but smaller than microtubules. There are six biochemically distinct subtypes of intermediate filaments, and their composition and arrangement depend on the cell type in which they occur. These filaments resist cell stretching, and help to hold adjacent cells together.

Ribosomes

Are sites of polypeptide or protein synthesis. They perform this function free within the cytoplasm when manufacturing polypeptides for use in the cytoplasm or nucleus. Alternately, they become attached to endoplasmic reticulum (ER) when manufacturing polypeptides destined for the plasma membrane, for lysosomes, or for export from the cell.

Desmocomes

Are small ellipsoid discs occur at various points beneath tight junctions and adhesion junctions, within the plasma membrane in each cell. From each desmosome a tuft of intermediate filaments extends into the cytoplasm, and transmembrane linker proteins extend through the plasma membrane into the intercellular space to bind the discs of adjacent cells together. Desmosomes are not seals but some to increase the strength of the tissue.

Nucleoli

Are specialized parts of certain chromosomes that stain in a characteristically dark manner. They carry multiple copies of DNA information used to synthesize ribosomal RNA. After transcription from DNA, ribosomal RNA combines with protein to form the two subunits of ribosomes, which leave the nucleolus and pass to the cytoplasm through pores in the nuclear envelope.

Microfilaments

Are thin, linear structures, observed distinctly in some protozoan groups, where they faciliate cellular locomotion and in muscle cells, where they are responsible for the ability of a cell to contract. They are made of a protein called actin. Several dozen other proteins are known that bind with actin and determine its configuration and behavior in particular cells.

Prophase

At the beginning of prophase, centrosomes (and their centrioles) replicate, the nuclear envelope disintegrates, and the two centrosomes migrate to opposite poles of the cell. At the same time, microtubules are manufactured between the two centrosomes to form an oval-shaped spindle. Other microtubules radiate outward from each centrosome to form asters. The asters will develop into the microtubular portion of the cytoskeleton in each new daughter cell formed during cell division. At this time the diffuse nuclear chromatin condenses to form visible chromosomes. These actually consist of two identical sister chromatids formed by DNA replication during interphase and joined at their centromere. Dynamic spindle microtubules repeatedly extend from and retract toward each centrosome. When a microtubule encounters a kinetochore, it binds to the kinetochore, ceases extending and retracting, and is now called a kinetochore microtubule.

Cytoplasm

Cellular material located between the cell membrane and nuclear envelope. Within the cytoplasm are many organelles, such as mitochondria, Golgi complexes, centrioles, and endoplasmic reticulum.

Chromosomes

Chromatin is organized into a number of discrete linear bodies called chromosomes (color bodies). In cells that are not dividing, chromatin is loosely organized and dispersed. They are varied at lengths and shapes, some bent and some rodlike. During mitosis, chromosomes shorten further and become increasingly condensed and distinct, and each assumes a shape partly characterized by the position of constriction, the centromere.

Nuclei

Contain linear chromosomes suspended in nucleoplasm. The chromosomes are normally loosely condensed, flexible strands of chromatin, composed of a complex of DNA, and DNA-binding proteins. Chromosomal DNA carries the genetic information encoding cellular RNA and protein molecules. The linear chromosomes become condensed and visible as discrete structures only during cell division.

Hyperosmotic

Contains a greater concentration of dissolved particles and gains water through a selectively permeable membrane from a solution containing fewer particles; contrasts with hypoosmotic.

Cleavage furrow

During the final stages of nuclear division a cleavage furrow appears on the surface of a dividing cell and encircles it at the midline of the spindle. The cleavage furrow deepens and pinches the plasma membrane. Finally, the infolding edges of the plasma membrane meet and fuse, completing cell division.

Endocytosis

Endocytosis, the ingestion of material by cells, is a collective term that describes three similar processes: phagocytosis, pinocytosis, and receptor-mediated endocytosis. They are pathways for specifically internalizing solid particles, small molecules and ions, and macromolecules. All require energy and thus are forms of active transport.

Plasma membrane

Eukaryotic cells are enclosed within this thin, selectively permeable (what can and can't go through) membrane.

Tight junction

Formed when the membranes of two cells next to each other appear to fuse. They are formed from rows of transmembrane proteins that bind tightly between adjacent cells. Tight junctions function as seals to prevent the passage of molecules between cells from one side of a layer of cells to another.

Smooth endoplasmic reticulum

Functions in synthesis of lipids and phospholipids and as a detoxification site within cells.

Asexually

In animals that reproduce asexually, mitosis is the only mechanism for the transfer of genetic information from parent to progeny, and thus the progeny are genetically identical to the parents.

Sexually

In animals that reproduce sexually, the two gametes that fuse to form the zygote must contain only half the usual number of chromosomes, so that progeny formed by the union of gametes will not contain double the parental number of chromosomes. Production of these gametes requires a special type of reductional division called meiosis.

Bulk-phase endocytosis

In phatocytosis, pinocytosis, and receptor-mediated endocytosis, some amount of extracellular fluid is necessarily trapped in the vesicle and nonspecifically brought within the cell.

Somatic cells

In the formation of body cells or somatic cells the process of nuclear division is called mitosis. By mitosis, each "daughter cell" is ensured a complete set of genetic instructions.

Lysosomes

Intracellular organelle consisting of a membrane enclosing several digestive enzymes that are released when the lysosome fuses with vescicles or endosomes produced by endocytosis. Enzymes that they contain are involved in the breakdown of foreign material, including bacteria engulfed by a cell. Lysosomes also destroy injured or diseased cells and worn-out cellular components. Their enzymes are so powerful that they kill the cell that formed them if enough of the lysosome membranes rupture.

Diffusion

Is a principle way that a substance may enter across a cell membrane. Along with a concentration gradient. It's a movement of particles or molecules from an area of higher concentration to an area of lower concentration of the particles or molecules, thus tending to equalize the concentration throughout the area of diffusion.

Golgi complex

Is composed of a stack of membranous cisterae that function in modification and packaging of polypeptide and protein products produced by rough ER.

Osmosis

Is the diffusion of water molecules across a membrane.

Ameboid movement

Many cells move this way, using pseudopodia.

Endoplasmic reticulum membranes

May be covered on their outer surfaces with ribosomes and are thus designated rough ER. This is the site of polypeptide synthesis. When ER lacks a ribosomal covering, it is called smooth ER. Polypeptides synthesized on rough ER enter the ER cisternae or membrane and are destined for incorporation into the plasma membrane for export from the cell or for use by the lysosomes.

Lysosomal vesicles

May pour their enzymes into a larger membrane-bound body containing an ingested food particle, a food vacuole or phagosome.

Centrosome

Microtubules radiate from here, the microtubule oranizing center, which is near the nucleus. Centrosomes are not membrane bound. Within centrosomes are found a pair of centrioles, which are themselves composed of microtubules. Each centriole of a pair lies at right angles to the other and is a short cylinder of nine triplets of microtubules. They replicate before cell division.

Stages of cell division

Mitosis (Nuclear division of chromosomes) and Cytokinesis (Division of cytoplasm)

Selectively permeable membrane

Most cell membranes are permeable, that is, permeable to water but variably permeable or impermeable to solutes.

Metaphase

The condensed sister chromatids are moved to the middle of the nuclear region, called the metaphase plate. The centromeres line up precisely in this region with the arms of the sister chromatids trailing off randomly in various directions.

Cyclindependent kinases (cdk's)

The events in cell cycles are exquisitely regulated. Transitions during cell cycles are mediated by enzymes called CDK'S and regulatory preotein subunits that activate them, called cyclins.

Centromere

The location of the kinetochore, a disc of proteins that binds with microtubules of the spindle that forms during mitosis.

Nucleus

The most prominent organelle, which is enclosed within two membranes to form a double-layered nuclear envelope.

Nuclear envelope

The outer membrane of the nuclear envelope is continuous with a cytoplasmic endomembrane system composed of the endoplasmic reticulum and the Golgi complex, lysosomes, the plasma membrane, and the vesicles that pass between them. The space between the membranes of the nuclear envelope communicates with the space between the ER membranes.

Osomotic pressure

The pressure that resists the flow of water into the cytoplasm.

Active transport

The second kind of mediated transport mechanism. Energy is supplied to the transporter system to transport molecules in the direction opposite to a concentration gradient.

Interphase

The stage between nuclear divisions. Replication occurs at this stage. Replication of DNA occurs during a phase called the S phase (period of synthesis). The S phase is preceded and succeeded by G1 and G2 phases, during which no DNA synthesis is occurring. During G1, transfer RNA, ribosomes, messenger RNA, and many enzymes are synthesized. During G2, spindle and aster proteins are synthesized in preparation for chromosome separation during mitosis.

Anaphase

The two sister chromatids separate to become two independent chromosomes. The chromosomes move toward their respective poles, pulled by their kinetochore microtubules. As chromosomes are pulled apart, the centrosomes are moved farther apart, so the cell becomes elongated.

Microvilli

They are small, fingerlike projections consisting of tubelike evaginations of the plasma membrane with a core of cytoplasm containing bundles of actin microfilaments.

Cilia and flagella

They cover the surface of a structure or line a tube or cavity. They are motile extensions of a cell surface that sweep materials past the cell. Flagella provide the means of locomotion for male reproductive cells (sperm) of most animals and many plants. Cilia are proposed to play an integral role in cell signalling, both during development and in the adult organism. They are composed of a long cylinder of nine pairs of microtubules enclosing a central pair. At the base of each cilium or flagellum is a basal body (kinetosome), which is identical in structure to a single centriole.

Gated channels

They require a signal to open or to close them. Gated ion channels may open or close when a signaling molecule binds to a specific binding site on the transmembrane protein (chemically-gated ion channels) when the ionic charge across a plasma membrane changes (voltage-gated ion channels) or when the membrane is distorted (mechanically-gated ion channels)

Transporters

Transmembrane protein carriers that move molecules across a membrane. Transporters enable solute molecules to cross the phospholipid bilayer. Transporters are usually quite specific, recognizing and transporting only a limited group of chemical substances or perhaps even a single substance.

Aquaporins

Water channels that have several different types. They are especially important in the digestive system for absorption of water from food. and the kidney for water reabsorption during urine formation.

Telophase

When daughter chromosomes reach their respective centrosomes or poles, telophase has begun. At this time, the daughter chromosomes are crowded together and stain intensely with histological stains. Spindle fibers disappear as the microtubules are disassembled and the chromosomes lose their identity, reverting to a diffuse chromatin network characteristic of an interphase nucleus. Finally, nuclear membranes reappear around the two daughter nuclei.

Exocytosis

When the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium. It occurs in various cells to remove undigestible residues of substances brought in by endocytosis, to secrete substances such as hormones, to recycle membrane receptors and membranes, and to transport a substance completely across a cellular barrier (transcytosis)

Myosin

Whose interaction with actin causes contraction in muscle and other cells.