Sexual Reproduction - Biology Encyclopedia

Sexual reproduction is a method for producing a new individual organism while combining genes from two parents. A single sperm and egg fuse during fertilization, and their genomes combine in the new zygote.

Sexual Reproduction, Evolution of - Biology Encyclopedia

The most basic way to reproduce is to make more copies of one's self, a process called asexual reproduction. In contrast, sexual reproduction involves the union of specialized sex cells (eggs and sperm) from two parents to produce genetically unique offspring.

Sexual Selection - Biology Encyclopedia

English naturalist Charles Darwin revolutionized scientific thinking when he proposed that species evolve over time to become adapted to their environments by means of natural selection in his On the Origin of Species (1859). He was initially puzzled, though, by the seemingly useless exaggerated characters often found in animals, particularly males.

Sexually Transmitted Diseases - Biology Encyclopedia

A sexually transmitted disease (STD) is any disease whose primary (though not necessarily only) mode of transmission is some form of sexual contact. STDs may be viral, bacterial, protistan (protozoan), or fungal.

Shoots - Biology Encyclopedia

The shoot is the production center for a plant. It is the organ system that gives rise to stems, leaves, and flowers.

Signaling and Signal Transduction - Biology Encyclopedia

All cells are able to sense and respond to substances present in their external environments. For example, motile bacteria will move toward a source of sugar but away from a toxic chemical such as phenol.

Skeletons - Biology Encyclopedia

Everyone is familiar with the human skeleton and its role in supporting the body. Less familiar is the variety of skeletons in other animals and the additional functions they provide.

Skin - Biology Encyclopedia

How is the skin, the largest organ in the body, constructed? The skin has two layers: the upper layer is the epidermis, and the lower layer is the dermis.

Sleep - Biology Encyclopedia

Sleep is a very important process and is characterized by a stereotypical posture, little movement, and a decrease in response to stimuli. These characteristics might also describe coma, but in sleep, unlike in coma, the characteristics are reversed each morning.

Slime Molds - Biology Encyclopedia

There are two major unrelated phyla of slime molds. The Myxomycota are the true (plasmoidal) slime molds, and the Dictyosteliomycota are the cellular slime molds.

Smoking and Health - Biology Encyclopedia

Smoking cigarettes sets into motion a chain reaction of changes that set the stage for infection, degenerative diseases, cardiovascular disease, and cancer. Smoke contains thousands of chemicals, but it is nicotine that causes the powerful addiction that compels a person to continually deliver the other harmful chemicals to the respiratory system.

Social Behavior - Biology Encyclopedia

Social behavior is defined as interactions among individuals, normally within the same species, that are usually beneficial to one or more of the individuals. It is believed that social behavior evolved because it was beneficial to those who engaged in it, which means that these individuals were more likely to survive and reproduce.

Sociobiology - Biology Encyclopedia

Sociobiology is the study of the biological basis of social behavior using evolutionary principles. The term was coined by a prominent entomologist E.

Soil - Biology Encyclopedia

One of the first distinctions made by a soil scientist is that "soil" and "dirt" are not the same. Dirt is what collects on the car or in the corner of the bedroom when it has been months since the last time it was vacuumed.

Speciation - Biology Encyclopedia

Speciation refers to the genesis of a new species from an ancestral species. There are two basic ways this can happen.

Species - Biology Encyclopedia

There is little agreement among scientists about the definition of the word "species." However, most biologists would agree that a species is a detectable, naturally occurring group of individuals or populations that is on an evolutionary path independent from other such groups. Several more detailed definitions have been articulated over the years; two that have gained prominence are the biological species concept (BSC) and the phylogenetic species concept (PSC).

Spinal Cord - Biology Encyclopedia

The spinal cord is a bundle of nerve fibers, no thicker than the human thumb, that links the brain with the rest of the body. The spinal cord is protected by the vertebral column, and together with the brain it comprises the central nervous system.

Stress Response - Biology Encyclopedia

The stress response is the human body's reaction to anything that throws off the balance inside it—injury, infection, fear, exercise, or pain. The body reacts with an alarm phase, then a resistance phase, during which it tries to fix the imbalance, and then, if that fails, an exhaustion phase.

Structure Determination - Biology Encyclopedia

Determining the structure of a molecule, especially a protein, is an important step in determining its function. In many diseases, changes in molecular structure are involved in the pathologic process, and understanding these changes can help in the design of therapy.

Symbiosis - Biology Encyclopedia

A wide array of interactions among plants, animals, and microorganisms occurs in nature. Some of these relationships are characterized by a close physical association among species that persists for a significant period of the life cycle.

Synaptic Transmission - Biology Encyclopedia

Synaptic transmission is the process whereby one neuron (nerve cell) communicates with other neurons or effectors, such as a muscle cell, at a synapse. A typical neuron has a cell body (soma), branching processes specialized to receive incoming signals (dendrites), and a single process (axon) that carries electrical signals away from the neuron toward other neurons or effectors.

T Cells - Biology Encyclopedia

Vertebrate animals have two immune system mechanisms to specifically identify and remove infectious agents from the body. In humoral immunity, proteins called antibodies bind to the foreign invader, targeting it for destruction by nonspecific defenses.