The groups of modern primates
At about the same time as the dinosaurs became extinct, about 65 million years ago, the primitive primates diverged quickly to give rise to two main suborders; the prosimians(meaning “before apes”) and anthropoids(meaning “ape fom”). The prosimians are represented today by lemurs, lorises, and tarsiers, and the anthropoids by monkeys, apes, and humans.
Monkeys are distinguished from apes in having long tails, and the forelimbs are not usually longer than the hindlimbs. They are believed to have evolved from two different groups of lemur-like animals which became isolated when continental drift separated Eurasia from North America. The North American group evolved into New World monkeys which died out in North America but somehow colonised South America. The Eurasia group gave rise to Old World monkeys, from which apes and humans evolved. There are several differences between Old World monkey and New World monkeys which show their separate evolution. For example, the nostrils of monkeys from South America are wide open and far apart, and New World monkeys have a long tail that is prehensile (adapted for grasping branches); the nostrils of monkeys from Africa and Asia are narrow and close together and no old World monkey has a prehensile tail.
Photosynthesis: An Overview
Most plants have no structures for ingesting and digesting food. They have no mouth and no alimentary canal, yet plant material is rich in carbohydrates, proteins and fats. Instead of obtaining their food from other organisms, plants make it for themselves using simple ingredients. They are autotrophs(self-feeders).
What is photosynthesis?
A typical plant takes in carbon dioxide (from the air) and water (from the soil) and builds these up into sugars and other complex substances. Oxygen is released as a waste product. The energy in the chemical bonds of the raw materials carbon dioxide and water is less than the energy in the chemical bonds of the products. Therefore the reaction is endergonic and requires an external source of free energy. This energy is supplied by sunlight that falls on the plant. A green substance, chlorophyll, enables the plant to trap light energy and use it to make sugars. The process of using sunlight to build up complex substances from simpler ones is called photosynthesis.
Photosynthesis is a complex process which takes place in a series of small steps. There are two main stages in photosynthesis: a light-dependentstage in which water is broken down into hydrogen and oxygen using light energy; and light-independent stage in which the hydrogen reacts with carbon dioxide to form a carbohydrate. Water is re-formed in this reaction. The light-dependent stage happens only in the light; the light-independent stage happens both when it is light and when it is dark.
Дата добавления: 2015-11-04; просмотров: 568;