There are many types of plant virus, but often they cause only a loss of yield, and it is not economically viable to try to control them. Plant viruses are often spread from plant to plant by organisms, known as vectors. These are normally insects, but some fungi, nematode worms, and single-celled organisms have been shown to be vectors. When control of plant virus infections is considered economical, for perennial fruits, for example, efforts are concentrated on killing the vectors and removing alternate hosts such as weeds.Plant viruses are harmless to humans and other animals because they can reproduce only in living plant cells.
Bacteriophages are a common and diverse group of viruses and are the most abundant form of biological entity in aquatic environments – there are up to ten times more of these viruses in the oceans than there are bacteria, reaching levels of 250,000,000 bacteriophages per millilitre of seawater. These viruses infect specific bacteria by binding to surface receptor molecules and then entering the cell. Within a short amount of time, in some cases just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins, which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane, and, in the case of the T4 phage, in just over twenty minutes after injection over three hundred phages could be released.
Both the amoeba and the muscle cell share many characteristics: a cell membrane, a nucleus, cytoplasm and organelles. But they are also quite different, because one is a specialised part of a much larger body while the other is a complete organism on its own. This is the major difference between free living cells and the cells of multicelled organisms.The simplest cells are the bacteria. They have genetic material, DNA and RNA, within their cell membranes, but not enclosed in a nucleus. The earliest bacteria were heterotrophs, living on organic molecules around them, engulfing them, absorbing them and then excreting the waste products through their semi-permeable cell membranes. Single celled bacteria are chemical factories capable of producing, from basic materials, all the fats, proteins, carbohydrates and ribonucleic acids that the cells need to function, grow and reproduce.