cells

Xylem

...port tissues in plants. The cell walls of xylem cells derive most of their strength from lignin , a ch...ube members of the phloem, but they lack companion cells and do not have perforated sides as well as pores at the ends. Tracheids are much narrower cells, ...

X chromosome

...is randomly and permanently inactivated in somatic cells (cells other than egg and sperm cells). This p...syndrome : This results when each of a female's cells has one normal X chromosome and the other sex chromosome is missing or altered. The missing genetic ...

Virus

... A virus is a small particle that infects cells in biological organisms. Viruses are obligate int...n only reproduce by invading and taking over other cells as they lack the cellular machinery for self reproduction. The term virus usually refers to those ...

Virus classification

... in RNA rather than in DNA form. RNA arises in cells as an intermediate when genes are translated into proteins . RNA genomes of viruses can be encoded in two different directions: Either the genes are stored in the 5'->3' direction (positive or + polarity), analogous to the direction in which g...

Vestibular system

...nearacceleration, which in turn deflects the Hair cells (Fig. 4, bottom layer) and thus produces a sensory signal. Most of the utricular signals elicit eye movements, while the majority of the saccular signals projects to muscles that control our posture.While the interpretation of the rotation signals f...

Vacuole

...rane -bound compartments within some eukaryotic cells where they serve a variety of different functions...he central vacuole 3 Vacuoles in budding yeast cells 4 References Vacuoles in protists Some protists and macrophages use food vacuoles ...

Unsolved problems in biology

...anisms of biorhythms and chronobiology ? Why do cells generate electrical fields? Why is magnetite in living creatures? How do animals possess long-range navigation and migration abilities? How was the homing ability developed? How can some animals detect earthquake precursors ? What are organism...

Vaccine

... animal cells, of viral or bacterial DNA . These cells then develop immunity against an infectious agent,...ls, and (2) by recognizing and destroying infected cells before that agent can multiply to vast numbers. Vaccines have contributed to the eradication of S...

Tumor

...ot. Tumors are caused by mutations in DNA of cells . An accumulation of mutations is needed for a tumor to emerge. Mutations that activate oncogenes or repress tumor supressor genes can eventually lead to tumors. Cells have mechanisms that repair DNA and other mechanisms that cause the cell to de...

Transformation

...Transformation is also the process by which normal cells are converted into cells that will continue to divide without limit. Normal cells can divide only a certain number of times b...

Mitosis

...without nuclear or cellular division, resulting in cells with many copies of the same chromosome. This proc...may also be referred to as endoreplication and the cells as endoploid. See also Genetics Meiosis Cell cycle Daughter cell...

T cell

T cells are a subset of lymphocytes that play a large r... There are many types of T cells: Cytotoxic T cells (CD8 + ) destroy infected cells. These cells function as 'killer' or cytotoxic cells because they ...

Systems biology

... parts of a biological system (e.g. organelles , cells , physiological systems , organisms etc.) it is...ins made within a cell . Theproperties of these cells can determine the diseases to which a person issusceptible. Therefore, a given disease exhibited by...

Synapse

Synapses allow nerve cells to communicate with one another through axons a... Synapses are specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands...

Stoma

... produced by photosynthesis in the chlorenchyma cells of the leaf interior exits through these same open...ing and closing of a stoma is controlled by guard cells that surround the opening and involves cellular sodium - potassium pumps. Dicotyledons usually...

Stem cell

Stem cells are primal, undifferentiated cells which have the unique potential to produce any kind of cell in the body. Many medical researchers b...

Spermatogenesis

... occurs in the male gonads or testes . Sperm cells develop initially from germ stem cells known as spermatogonia . As these differentiate they become spermatocytes , spermatids (after me...

Sperm

...n grow into a new organism, such as a human. Sperm cells contain half of the genetic information needed t...XX" (for a female ) or "XY" (for a male ). Sperm cells were first observed by Antoni van Leeuwenhoek in 1679 . Contents showTocToggle("show","hide...

Hans Spemann

...e embryo that directs the development of groups of cells into particular tissues and organs. External link Web page about Hans Spemann...

Species

...earchers have discovered conditions in which pig cells and human cells can fuse together in the body to yield hybrid cells that contain genetic material from both species....

Endoplasmic reticulum

...e net") is an organelle found in all eukaryotic cells . The ER modifies proteins , makes macromolecules , and transfers substances throughout the cell. Prokaryotic organisms do not have organelles and thus do not have an ER. ER's base structure and composition is similar to the plasma membrane , tho...

Skin

...hed by diffusion from the dermis. The main type of cells which make up the epidermis are keratinocytes , with melanocytes and Langerhans cells also present. The epidermis can be further subdivided in to the following strata (beginning with ...

Shigella

...ing on age and condition of the host, as few as 10 cells can be enough to cause an infection. The disease i...la organisms attach to, and penetrate, epithelial cells of the intestinal mucosa. After invasion, they multiply intracellularly, and spread to contiguous ep...

Signal transduction

...s , for example). Signal transduction mediates how cells respond to such stimuli. Most stimuli impinge f... such as the neurotransmitters that allow nerve cells to communicate across synapses , bind to receptor proteins in the membrane and open their ion c...

Semen

... proteins) serve to increase the mobility of sperm cells in the vagina and cervix by creating a less viscous channel for the sperm cells to swim though, and preventing their diffusion out of the semen. Prostaglandin hormones are involve...

Blood plasma

...onent of blood . It is the liquid in which blood cells are suspended. Blood plasma contains proteins ...ared to that of the hemoglobin in the red blood cells ; it may increase under hyperbaric conditions.) It is the storage and transport medium of clotting...

Endoplasmic reticulum

...e net") is an organelle found in all eukaryotic cells . The ER modifies proteins , makes macromolecules , and transfers substances throughout the cell. Prokaryotic organisms do not have organelles and thus do not have an ER. ER's base structure and composition is similar to the plasma membrane , tho...

Endoplasmic reticulum

...e net") is an organelle found in all eukaryotic cells . The ER modifies proteins , makes macromolecules , and transfers substances throughout the cell. Prokaryotic organisms do not have organelles and thus do not have an ER. ER's base structure and composition is similar to the plasma membrane , tho...

Blood type

...tion an individual's characteristics of red blood cells due to substances ( carbohydrates and proteins ...: Individuals with type A blood have red blood cells with antigen A on their surface and produce antibodies against antigen B in their blood serum. Using...

Retrovirus

...iruses ( HTLV 1&2, HIV 1&2) attack CD4 cells .Another feature common to all retroviruses is a lipid coating surrounding their capsid . It is essential for their propagation. This explains why retroviruses can be made inactive by just washing hands. Studies of retroviruses led to the first ...

Recombination

... occurs during the formation of sperm and egg cells (especially in areas of study of biology topics ). It is also referred to as crossing over (or change of phase ). Other possible implication of the term are as follows: In biology , the process of Genetic recombination and meiosis , as ...

Pseudopod

...lse feet) are temporary projections of eukaryotic cells . Cells having this faculty are generally referred...er of different protist groups, but also in some cells ( phagocytes ) in multicellular organisms. Morphology Pseudopods can be classified into severa...

Proteomics

...making proteomics a useful tool for characterizing cells and tissues of interest. To catalog all human proteins and ascertain their functions and interactions presents a daunting challenge for scientists. An international collaboration to achieve these goals is being co-ordinated by the Human Proteome Or...

Protein

...ntial to the structure and function of all living cells and viruses .Many proteins are enzymes or sub...ntous material that makes up the cytoskeleton of cells and much of the structure of animals is also protein: microtubules, actin , intermediate filaments...

Protein biosynthesis

Protein biosynthesis is the process in which cells build proteins . The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with transcription and ending with translation . Protein biosynthesis although very similar, differs ...

Prokaryote

... unicelluar, some are capable of forming groups of cells called colonies . Unlike many eukaryotic multice... It is generally accepted that the first living cells were some form of prokaryote. Fossilized prokaryotes 3.5 billion years old have been discovered, a...

Prion

...his protein is found in the membranes of normal cells (its precise function is not known), but an alter...-containing colony, the prion causes the "invader" cells to die, ensuring that only related colonies obtain the benefit of sharing resources [1] . Prions h...

Plasmolysis

Plasmolysis is the contraction of cells within plants due to the loss of water through ...ue is immersed in a solution of calcium nitrate , cells rapidly lose water by osmosis and the protoplasm of the cells shrinks. This occurs because the ca...

Plant

...ost common. All of these plants have eukaryotic cells with cell walls composed of cellulose , and mos...alled hyphae, which may or may not be divided into cells but contain eukaryotic nuclei . Fruiting bodies, of which mushrooms are the most familiar, are ac...

Cell membrane

...egulation of cell behavior and the organization of cells in tissues. In animal cells, the cell membrane ...--for example, the "apical" surface of epithelial cells that line the vertebrate gut --and limits how far they may diffuse within the bilayer. Finally, ...

Plant

...ost common. All of these plants have eukaryotic cells with cell walls composed of cellulose , and mos...alled hyphae, which may or may not be divided into cells but contain eukaryotic nuclei . Fruiting bodies, of which mushrooms are the most familiar, are ac...

Pigment

...y material resulting in color in plant or animal cells which is the result of selective absorption. Some biological material has so-called structural color, which is the result of selective reflection or iridescence , usually done with multilayer structures. Unlike structural color, pigment color is t...

Physiology

..., what is learned about the physiology of yeast cells can also apply to human cells. The field of an...ysiology addresses the functioning of individual cells . Physiology also has many related and allied fields: Ecophysiology tries to understand how physi...

Alga

...e: Colonial - small, regular groups of motile cells Capsoid - individual non-motile cells embedded in mucilage Coccoid - individual non-motile cells with cell walls Palmelloid - no...

Photosynthesis

...ontained in organelles (compartments within the cells ) called chloroplasts . Although all green parts...st of the energy is produced in the leaves . The cells in the interior tissues of a leaf, called the mesophyll , contain about half a million chloroplasts...

Phosphorylation

...incoming light is processed in the light-sensitive cells of the retina . Signaling networks The netw...s often the stage of their catabolism . It allows cells to accumulate sugars because the phosphate group prevents the molecules from diffusing back across ...

Phloem

...sue consists of sieve-tube members and companion cells . The sieve-tube elements are large, cylindrical ...that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect. ...

Phospholipid

...otic cell membranes , but is mainly present in cells of the nervous system. Amphiphatic character Due to its polar nature, the head of a phospholipid is attracted to water (it is hydrophilic ). The nonpolar head is not attracted to water and is said to be hydrophobic . When placed in water, pho...

Peripheral membrane protein

...purification . An exception to this rule is that cells may attach an "anchor" moiety, such as the fatty-acid anchor of lipid anchored proteins , which makes the purification properties of these proteins the same as integral membrane proteins, although researchers tend not to classify them as such....

Polymerase chain reaction

...ganisms, where it functions to duplicate DNA when cells divide. It works by binding to a single DNA strand and creating the complementary strand. In Mullis's original PCR process, the enzyme was used in vitro (in a controlled environment outside an organism). The double-stranded DNA was separated int...

Patau syndrome

...entage of cases occur when only some of the body's cells have an extra copy of chromosome 13, resulting in a mixed population of cells with a differing number of chromosomes, such cases are called Mosaic Patau syndrome. Patau syndrome...

Paternity testing

...lear DNA in the nucleus, the mitochondria in the cells also have their own genetic material termed the mitochondrial genome . Mitochondrial DNA comes only from the mother, without any shuffling. Proving a relationship based on comparison of the mitochondrial genome is much easier than that based on t...

Passive transport

.... On the other hand, the membrane pores of liver cells are extremely large, to allow a veriety of solutes to pass through and be metabolized. Osmosis Main article: Osmosis Osmosis is basically diffusion of water molecules. Most cell membranes are permeable to water, and since the diffusion of wa...

Oxidative phosphorylation

...ve phosphorylation is a biochemical process in cells . It is the final metabolic pathway of cellular respiration , after glycolysis and the citric acid cycle . The process takes place at a biological membrane . In prokaryotes this is the plasma membrane , and in eukaryotes it is the inne...

Oswald Avery

... information (genes) was thought to be stored in cells in protein molecules. (Before McCarty he had w...se enzymes , which removed the proteins from the cells before the remainder was placed with R strain bacteria. The R strain bacteria transformed, meaning ...

Osmosis

...s by which water is transported into and out of cells . Contents showTocToggle("show","hide") ...f osmosis A practical example of this osmosis in cells can be seen in red blood cells. These contain a high concentration of solutes including salts and...

Origin of life

...esearch involved engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. Origin of organic molecules: Miller and Wchtershuser's theories The " Miller experiments " (including the original Miller&...

Oncogene

...h factors are usually secreted by a few special cells to induce cell proliferation in other cells. If a cell that usually does not produce growth factors suddenly starts to do so (because it developed an oncogene), it will thereby induce its own uncontrolled proliferation ( autocrine loop ), as well ...

Olfaction

...eature of the odor molecule . Odor receptor nerve cells function like a key lock sytem. If the odor molec...t vibrations. The axons from all the thousands of cells expressing the same odor receptor converge in the olfactory bulb . Mitral cells in the olfactory b...

Nutrition

...ally, in humans, the matter which comprises the cells of the body (except those cells produced before birth ) is acquired from food in the digestive system . Not all the food matter in...