MB501T Cell Biology

Virtual University · Cell Biology · Fridoon Jawad Ahmad · BS& PhD (UW-Madison, USA) · Professor of Biomedical Sciences, · King Edward Medical University, · Lahore, Pakistan.

The majority of the contents of this document were taken from the following two sources.

Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A, Lewis J, et al. New York: Garland Science; 2002. Life: The Science of Biology, Seventh Edition, by William K. Purves, David Sadava, Gordon H. Orians, and H. Craig Heller. Mars today is a cold, dry place, not suitable for life as we know it. But 3 billion years ago, it was warmer and wetter. An orbiting probe from Earth recently photographed a huge dry lake bed, the size of New Mexico and Texas combined, on the Martian surface. Another probe found evidence of water trapped just below the icy surface of the Martian polar region. These discoveries by geologists have sparked the interest of biologists, for where there is water, there can be life. There is good reason to believe that life as we know it cannot exist without water. Animals and plants that live on Earth’s land masses had to evolve elaborate ways to retain the water that makes up about 70 percent of their bodies. Aquatic organisms living in water do not need these water-retention mechanisms; thus biologists have concluded that the first living things originated in a watery environment. This environment need not have been the lakes, rivers, and oceans with which we are familiar. Living organisms have been found in hot springs at temperatures above the usual boiling point of water, in a lake beneath the frozen Antarctic ice, in water trapped 2 miles below Earth’s surface, in water 3 miles below the surface of the sea, in extremely acid and extremely salty water, and even in the water that cools the interiors of nuclear reactors. With 20 trillion galaxies in the universe, each with 100 billion stars, there are many planets out there, and if our own solar system is typical, some of them have the water needed for life. As biologists contemplate how life could originate from nonliving matter, their attention focuses not just on the presence of water, but on what is dissolved in it. A major discovery of biology is that living things are composed of the same types of chemical elements as the vast nonliving portion of the universe. This mechanistic view— that life is chemically based and obeys universal physicochemical laws—is a relatively recent one in human history. The concept of a “vital force” responsible for life, different from the forces found in physics and chemistry, was common in Western culture until the nineteenth century, and many people still assume such a force exists. However, most scientists adhere to a mechanistic view of life, and it is the cornerstone of medicine and agriculture.

A Grander Canyon on Mars

This false color image from the Mars Global Surveyor shows in blue the dry remains of what was once a huge lake on Mars. Just as the Colorado River carved Earth’s Grand Canyon, torrents of water from the lake probably carved the mile-deep canyon that is visible as a thin blue line just north of the lake bed.

Part One • THE CELL

Before describing how chemical elements are arranged in living creatures, we will examine some fundamental chemical concepts. We will first address the constituents of matter: atoms. We will examine their variety, their properties, and their capacity to combine with other atoms. Then we will consider how matter changes. In addition to changes in state (solid to liquid to gas), substances undergo changes that transform both their composition and their characteristic properties. Then we will describe the structure and properties of water and its relationship to acids and bases. We will close the chapter with a consideration of characteristic groups of atoms that contribute specific properties to larger molecules of which they are part, and which will be the subject of Chapter 3.

Water and the Origin of Life’s Chemistry

Astronomers believe our solar system began forming about

4. 6 billion years ago when a star exploded and collapsed to

form the sun, and 500 or so bodies called planetesimals collided with one another to form the inner planets, including Earth. The first chemical signatures indicating the presence of life here are about 4 billion years old. So it took 600 million years, during a geological time frame called the Hadean, for the chemical conditions on Earth to become just right for life. Key among those conditions was the presence of water. Ancient Earth probably had a lot of water high in the atmosphere. But the new planet was hot, and this water evaporated into space. As Earth cooled, it became possible for water to remain on its surface, but where did that water come from? One current view is that comets—loose agglomerations of dust and ice that have orbited the sun since the planets formed—struck Earth repeatedly and brought not only water but other chemical components of life, such as nitrogen. As Earth cooled, chemicals from the rocks dissolved in the water and simple chemical reactions took place. Some of these reactions could have led to life, but impacts by large comets and rocky meteorites would have released enough energy to heat the developing oceans almost to boiling, thus destroying any early life. These large impacts eventually subsided, and life gained a foothold about 3.8 to 4 billion years ago. The prebiotic Hadean was over (Figure 2.1). The Archean had begun, and there has been life on Earth ever since. In Chapter 3 we will return to the question of how the first life could have arisen from inanimate chemicals. But before doing so, we need to understand what the chemistry of life entails. Like the rest of the chemical world, living things are made up of atoms and molecules.

Atoms: The Constituents of Matter

More than a trillion (1012) atoms could fit over the period at the end of this sentence. Each atom consists of a dense, positively charged nucleus, around which one or more negatively charged electrons move (Figure 2.2). The nucleus contains one or more protons and may contain one or more neutrons. Atoms and their component particles have volume and mass, which are properties of all matter. Mass measures the quantity of matter present; the greater the mass, the greater the quantity of matter. The mass of a proton serves as a standard unit of measure: the atomic mass unit (amu), or dalton (named after the English chemist John Dalton). A single proton or neutron has a mass of about 1 dalton (Da), which is 1.7 × 10–24 grams (0.0000000000000000000000017 g). The mass of an electron is 9 × 10–28 g (0.0005 Da). Because the mass of an electron is negligible compare to the mass of a proton or a neutron, the Present

Billions of years ago Phanerozoic: Current life

Proterozoic: More complex life

Archean: Early life

Hadean: Chemical evolution Cambrian “explosion” (see Chapter 22)

Atmospheric oxygen (O2)

Earliest life

Earth forms

2.1 A Geological Time Scale

The Hadean encompasses the time from the formation of Earth (about 4.6 billion years ago) until the earliest life appeared (about 3.8 billion years ago).During the Hadean chemical conditions evolved that were conducive to life,which was able to gain a foothold once the rain of comets and meteorites ended. Each neutron has a mass of 1 and no charge. Each proton has a mass of 1 and a positive charge.

Each electron has negligible mass and a negative charge. Nucleus

2.2 The Helium Atom

This representation of a helium atom is called a Bohr model. It exaggerates the space occupied by the nucleus. In reality, although the nucleus accounts for virtually all of the atomic mass, it occupies only 1/10,000 of the atom’s volume.