Science and Life PDF

# Science and Life ## 1.1 Introduction Even if you are a novice to biology, you can easily recognize that a butterfly, cactus plant, and lion are all living things while the rock is not. Also, you already know a lot about one living thing (yourself). What does it mean to be living? Besides, what...

# Science and Life ## 1.1 Introduction Even if you are a novice to biology, you can easily recognize that a butterfly, cactus plant, and lion are all living things while the rock is not. Also, you already know a lot about one living thing (yourself). What does it mean to be living? Besides, what are the criteria used to define the basic features of life? In other words, what are the fundamentals that characterize living things and distinguish them from nonliving matter? Biology is the science of life that helps us understand everything surrounding us. Biologists study life by testing hypotheses and theories to learn about, develop new concepts and ideas, and assist us in distinguishing between living things from non-living objects on the planet. Living organisms and non-living things have some characteristics in common. For instance, both are made up of fundamental natural substances, such as carbon and hydrogen. However, all living things share some features that distinguish them from non-living world, these features are: * An organism is composed of cells. Cells are the smallest units in all living organisms; they vary in size and appearance and carry genetic material or DNA. * All organisms utilize energy and materials; organisms take energy and use it to perform various activities. Our cells use the energy and raw materials in food to build and operate in ways that keep us alive. * A living organism responds to stimuli. For example, plants grow toward a source of light because a living organism interacts with its environment and responds to environmental changes. * Living organisms maintain homeostasis. The ability of an organism to automatically maintain a constant internal condition regardless of the external environment-is called homeostasis (staying the same); for example, body temperature remains stable even during changes in outside temperature. * All organisms can reproduce, grow, and develop, and these processes are conducted by instruction from the DNA, the genetic material. * Living organisms evolve and adapt to the environment. The organization of life extends through several levels, starting with the smallest level of organization developed into much larger and more complex levels. The hierarchical levels of biological organization are: * **Atoms:** is the smallest units of a chemical element. All matter is composed of atoms. * **Molecules and macromolecules** form when the atoms bond with each other. * **Cells:** molecules and macromolecules join together to form larger structures such as membranes. * **Tissues:** are formed when many specialized cells of the same type are connected, only in multicellular organisms such as plants and animals. * **Organs:** are composed of two or more tissue types. For example, the heart comprises several tissues (including muscle, nervous, and connective tissue) - only in multicellular organisms. * **Organ system:** a group of biological functions is performed by collaborative groups of tissue and organs. * **Organism:** all living things can be called organisms. * **Population:** is a group of organisms of the same species that occupy the same environment * **Community:** is an assemblage of populations of different species. The types of species found in a community are determined by the environment and the interactions of species with each other. * **Ecosystems:** are formed by interactions of a community of organisms with their physical environment. * **Biosphere:** includes all Earth's ecosystems together. ## 1.2 Cells All living organisms consist of a small structure called a cell. Cells are the most basic units of life and smallest building blocks in the body of a living being and are responsible for all life activities. Some organisms are elementary and consist of only one cell, such as bacteria; it is called unicellular. In contrast, some organisms, such as animals and plants, are complex in their structure and consist of more than one cell; they are multicellular. Since cells are small, they cannot be seen with a naked eye. This tiny structure can be studied using a microscope. ### The cell theory Because of its small size, the cell was not studied until discovering the microscope in the 17th century. The cell discovery was credited to the scientist Robert Hooke, who examined a piece of cork using a simple microscope. He noticed clustered structures resembling a honeycomb, and he named them cells because they reminded him of the cells where monks lived in a monastery. The discovery journey did not stop; German scientists Matthias Schleiden and Theodor Schwann confirmed that all animals and plants contain cells. Two decades later, the German scientist Rudolph Virchow developed a vital part of the cell theory, where he found that each cell originates from another cell. Scholars Schwann, Schleiden, and Virchow contributed to developing a cell theory, which states that: * all organisms are composed of one or more cells, * the cell is the structural and smallest unit of life, and * all cells arise from a pre-existing cell by cell division. ## 1.3 Cell Differentiation – Specialized cells In humans and other multicellular organisms, the cells assemble and work together; there are many different types of cells with a specific shape, distribution, and function; for example, some cells respond to environmental signals, and others are responsible for carrying oxygen. Cells inside the body are specialized and formed from undifferentiated cells. These cells can divide and supply the cells that compose the bodies of multicellular organisms such as humans and animals by cell differentiation. **Cell differentiation** is an irreversible process by which cells become specialized into particular types. The human body contains approximately 210 specialized (or differentiated) cells organized into diverse and complex structures. Distinguishable features for specialized cells include the following: * Each cell contains unique materials. * Specialized cells have a distinctive shape. * Each type of specialized cell performs a particular role. ### Examples of specialized cells are blood cells. Blood cells are produced from tissues inside the bones called bone marrow which contains stem cells that can change (differentiate) into red blood cells, white blood cells, and platelets. * **Red blood cells:** These cells look like biconcave discs, and lack a nucleus to increase the cell's surface area. Red blood cells are considered some of the most abundant cells in human blood, in which twenty drops of blood contain approximately 5 million red blood cells. The primary function of red blood cells is to carry and transport oxygen throughout our body and, to a lesser degree, carbon dioxide in the blood. * **White blood cells:** These cells have nuclei. These cells play an essential role in maintaining balance by eliminating harmful germs like bacteria and viruses. There are different types of white blood cells essential for killing pathogens. * **Platelets:** Platelets are produced from the bone marrow and play an essential role in blood clotting. Like red blood cells, platelets lack nuclei and some organelles; this makes them unable to divide. Moreover, these cells are enclosed by sticky substances, making them stick to irregular surfaces such as ruptured blood vessels or sclerotic plaques that form at the injury site. ## 1.4 Stem cells - Cell Replacement Therapy Stem cells are unspecialized cells that can differentiate into various cell types. An unlimited number of cells are found in all multicellular organisms. Upon cell division, one of the sister cells remains as a stem cell while the other develops into specialized cells such as liver cells, muscle cells, etc. Stem cells continue to divide throughout life and give rise to specialized cells. Moreover, this mechanism is essential for regenerating cells with a limited lifespan in mammals, such as skin cells and red blood cells. Stem cells are distinguished in terms of: * **Self-renewal:** the ability of stem cells to produce copies by symmetric or asymmetric division. * **Differentiation:** it can differentiate into one or more types of specialized cells. The two daughter cells produced can have different fates, where one stem cell may remain undifferentiated, and the other can differentiate into a specialized cell. ## 1.5 Free Radicals Free radicals are harmful, naturally occurring chemicals that can be described as atoms or groups of atoms with an unstable electron and are formed when oxygen reacts with specific molecules. Free radicals are produced in the body due to some cellular process and can also be introduced into our body exogenously by smoking cigarettes, radiation, drinking alcohol, air and water pollution, certain gases, and even sunlight. Free radicals affect the human body where they can cause significant damage to living cells. They affect the structure and composition of DNA, enzymes, and cell membranes, resulting in cell damage. Furthermore, the studies show that some free radicals are involved in the buildup of cholesterol in artery walls. Despite the harmful effect of free radicals, they can kill invading cells such as bacteria inside our body. Free radicals are eliminated through molecules that can donate electrons, and this process can be performed through substances called (antioxidants). Antioxidants include vitamin E, vitamin C, and beta-carotene found in vegetables and fruit. In addition to many plant compounds known as flavonoids; This is one of the reasons why a diet rich in fruits and vegetables is good for our health.