Chapter 2 Lesson 2.1 Characteristics of Life

Chapter 2: Basic Biological Principles Lesson 2.1: Characteristics of Life

Is this picture a colorful work of abstract art, or is it something else? Imagine shrinking down to a tiny size, so small you could enter a blood vessel. The picture shows what you might see rushing toward you. Do you know what the red objects are? If you guessed red blood cells, you are right. What about the knobby green objects? Watch out for these! They are viruses that have invaded the blood. When you read this book, you will take an exciting journey into the realm of blood cells, viruses, and just about everything else that is related to life. You will learn how your own body works, what makes living things unique, and what you and viruses have in common.

Lesson Objectives Explain the characteristics of life common to all organisms. Identify the parts that all cells have in common. Compare and contrast prokaryotic and eukaryotic cells.

Vocabulary biology ? development ? eukaryote ? eukaryotic cell ? growth homeostatic mechanism metabolism ? prokaryote ? prokaryotic cell ? reproduction ? virus

Introduction

In this lesson, you will learn about one particular branch of science, the branch called biology. Biology is the science of life. Do you know what life is? Can you define it? Do you know the characteristics of life common to all organisms? Can you describe the characteristics of life that are shared by prokaryotes and eukaryotes?

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CHARACTERISTICS OF LIFE

Look at the duck decoy in Figure 2.1. It looks very similar to a real duck. Of course, real ducks are living things. What about the decoy duck? It looks like a duck, but it is actually made of wood. The decoy duck doesn't have all the characteristics of a living thing. What characteristics set the real ducks apart from the decoy duck? What are the characteristics of living things?

Figure 2.1: This duck decoy looks like it

To be classified as a living thing, an object must have all of the following characteristics: 1. It consists of cell(s) that are organized. 2. It grows and develops. 3. It obtains and uses energy to power all life processes through metabolism. 4. It goes through reproduction: produces offspring (asexually or sexually). 5. It as a universal genetic code (DNA). 6. It has the ability to change over time (evolution). 7. It maintains homeostasis.

Cells and Organization All forms of life are built of cells. A cell is the basic unit of the structure and function of living

things capable of performing all the activities of life (recall this is one part of the cell theory from chapter 1). Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells in Figure 2.2 and onion cells in Figure 2.3. How are they similar? You should view the animation titled Parts of an Animal Cell at: .

Figure 2.2: Human Cells. If you looked at human cells under a microscope, this is what you might see.

Figure 2.3: Onion Cells. If you looked at onion cells under a microscope, this is what you might see.

Some organisms are made up of only one cell and are called unicellular; others are composed of two or more cells and are called multicellular. Complex multicellular organisms have levels of organization as shown in Figure 2.4. Organism is the highest level of organization, made up of organ systems (groups of specialized parts that carry out a certain function in an organism). Organ systems are made up of organs; structures that carry out specialized jobs within an organ system. Organs are made up of tissues; groups of cells that have similar abilities and allow an organ to function. Tissues are made up of cells; these are covered by a membrane, contain genetic information needed for replication, and carry out all cell functions. Within cells we find organelles, tiny structures that carry out functions

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necessary for a cell to stay alive. Organelles contain biological molecules, chemical compounds that provide physical structure, fuel movement, organize energy use, and other cellular functions. Biological molecules are made up of atoms; which are the simplest particle of an element.

Figure 2.4: Complex levels of organization in multicellular organisms.

Growth and Development All living things grow and develop. Growth is the physical change in an organism's size and

weight, while development is the gene-directed process by which an organism matures. Growth occurs through cell division and enlargement of cells, development occurs though cell differentiation (cell becoming different from each other; i.e. some cells become blood cells and others become bone cells) and specialization (adaptation of a cell to perform a specific function). For example, a plant seed may look like a lifeless pebble, but under the right conditions it will grow and develop into a plant. Animals also grow and develop. Look at the animals in Figure 2.5. How will the tadpoles change as they grow and develop into adult frogs? This process of change is called metamorphosis.

Figure 2.5: Tadpoles go through many changes to become adult frogs.

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Obtaining and Using Energy/ Metabolism All living things have an ability to acquire materials for energy to power life processes; such as,

cellular repair, growth, and movement. Metabolic processes are the sum of all chemical reactions that take in and transform energy and materials from an organism's environment for use within their cells. In other words, metabolism is the sum of the chemical reactions through which living things break down or build up materials. Most of us call this eating! We have to be able to convert our food; a form of energy, to chemicals our cells can use through metabolism. Some organisms like plants, algae, and some microorganisms are autotrophs (self-feeders, producers). The autotrophs we are most familiar with are the green plants that use photosynthesis to make their own "food"; by converting light energy into chemical energy. The energy they make is used for cellular respiration. Animals and fungi are heterotrophs (consumers) and capture their food for energy in a variety of ways. Heterotrophs can either use cellular respiration or fermentation to get energy from their food, you will learn more about all of these processes in the chapter called Cells and Cell Processes.

The ability to acquire and use energy is extremely important. Keep in mind without a constant input of usable energy, organisms would quickly become "disorganized" and die.

Reproduction All living things are capable of reproduction. Reproduction is the process by which living things

give rise to offspring and transmit hereditary information (coded in DNA, deoxyribonucleic acid). In sexually reproducing organisms hereditary information recombines from two organisms (parents) of the same species, producing genetically different offspring. In asexually reproducing organisms hereditary information does not come from two different organisms to the offspring, it comes from one organism (parent). Thus the offspring is genetically identical to the parent.

Reproduction is not essential for the survival of an individual organism but it is essential in order for a species to continue and survive as a whole. Reproducing may be as simple as a single cell dividing to form two daughter cells. Generally, however, it is much more complicated. Nonetheless, whether a living thing is a huge whale or a microscopic bacterium, it is capable of reproduction.

Universal Genetic Code (DNA) Living things are based on a universal genetic code, with very small differences between

different living things. All organisms store the information they need to carry out life processes and reproduce in this universal genetic code contained in a DNA (deoxyribonucleic acid) molecule. DNA is made up of four different nucleotides; the order in which the nucleotides are assembled determines the protein that is produced. This is important because proteins give you and every other organism its genetic traits that make living things unique. The genetic code information stored in DNA is copied and passed from parent to offspring during cell reproduction.

Ability to Change Over Time (Evolution) Evolution occurs by a process called natural selection according to the theory of evolution

developed by Charles Darwin in the mid to late 1800s. In natural selection, some populations of living things produce more offspring than others, so they pass more genes to the next generation than others do. The genes that they pass on usually contain a vast variety of characteristics. This variety of characteristics that get passed from one generation to the next can lead to evolution. Evolution is a change in the characteristics of populations of living things over time. The changes in population characteristics arise from genetic mutations which create genetic variations or genetic diversity enabling a population to adapt to changes in their environments and survive. Over many generations, this can lead to major changes in the characteristics of the population of living things. Evolution explains how populations living things are changing today and how modern living things have descended from ancient life forms that no longer exist on Earth. As populations of living things evolve, they generally become better suited for their environment. This is because they evolve adaptations. An adaptation is a characteristic that helps a population of living thing survive and reproduce in a given environment. This

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ability to adapt and survive is known as an organisms' fitness. When we talk about fitness in terms of evolution we are talking about an organisms' ability to survive and reproduce fertile offspring, hence evolution has often been referred to as "survival of the fittest".

Look at the mole in Figure 2.6. It has tentacles around its nose that it uses to sense things by touch. The mole lives underground in the soil where it is always dark. However, by using its touch organ, it can detect even tiny food items in the soil in total darkness. The touch organ is an adaptation because it helps the mole survive in its dark, underground environment. A cartoon depicting the evolution of Homer (Simpson) can be viewed at: .

Figure 2.6: This mole uses its star-shaped nose organ to sense food by touch in the dark.

Maintaining Homeostasis All living things are able to maintain a more-or-less stable internal environment. They keep

things relatively stable on the inside regardless of the conditions around them. The process of maintaining a stable internal environment is called homeostasis, without homeostatic maintenance an organism could die. In human beings, homeostatic mechanisms, regulatory mechanisms that contribute to maintaining a state of equilibrium help us maintain a stable internal body temperature (thermoregulation), regulate our body's water content (water regulation or osmoregulation), and uptake of nutrients by our cells. Organisms maintain homeostasis through feedback loops that regulate their physiological and behavioral responses. A feedback loop utilizes three components in order for homeostatic regulation to occur. The three components are receptors, a control center, and an effector. The receptors monitor and respond to changes in an organism's environment. The control center is the area in the feedback loop that receives information from the receptor. When the control center receives the information it determines if the variable sent by the receptors needs to monitored and adjustments made. In humans, the brain is the control center. Finally the effectors receive information from the control center and correct or made adjustments to maintain homeostasis using either positive or negative feedback loops. For example, if you go outside when the air temperature is below freezing, your body doesn't freeze. Instead, by shivering and other means, it maintains a stable internal temperature. The receptors in your body alert the control center that there is a drop of body temperature in this example and the effectors response by causing you to shiver, which generates body heat. We will look at more in-depth at homeostatic feedback loops in the chapter called Cells and Cell Processes.

Homeostasis, or keeping things constant, is not just a characteristic of living things. It also applies to nature as a whole. Consider the concentration of oxygen in Earth's atmosphere. Oxygen makes up 21% of the atmosphere, and this concentration is fairly constant. What keeps the concentration of oxygen constant? The answer is living things. Most living things need oxygen to survive, and when they breathe, they remove oxygen from the atmosphere. On the other hand, many living things, including plants, give off oxygen when they make food, and this adds oxygen to the atmosphere. The concentration of oxygen in the atmosphere is maintained mainly by the balance between these two processes. A quick overview of homeostasis can be viewed at .

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