campbell book

Another theme seen in previous posts (Exploring Levels of Biological Organization) is the idea that the appropriate form of the function, which you'll recognize from everyday life, for example a screwdriver is suited to tighten or loosen screws, a hammer to pound nails. How a device works is correlated with its structure. AppJied to biology, this theme is a guide to
the anatomy ofJife at all its structural levels. An example from Figure 1.4 is seen in the leaf: Its thin, flat shape maximizes the amount of sunlight that can be captured by its chloroplasts.
Analyzing a biological structure gives us dues about what it does and how it works. Conversely, knowing the function of something provides insight into its construction. An example from the animal kingdom, the wing of a bird, provides additional instances ofthe structure-function theme (Figure 1.6), In exploring life on its different structural levels, we discover
functional beauty at every turn.

Wing bones have a honeycombed internal structure that is strong but lightweight


The flight muscles are controlled by neurons (nerve cells). which transmit signals. With long extenSions, neurons are espeCially well structured for communication within the body


The flight muscles obtain energy in a usable form from organelles called mitochondria. A mitochondrion has an inner membrane with many infoldings. Molecules embedded in the inner membrane carry out many of the steps in energy produdion, and the Illfoldings pack a large amount of this membrane into a small container.


A bird's wings have an aerodynamically efficient shape


source: Campbell and Reece book

Systems Biology
Asystem is simply a combination ofcomponents that function together. A biologist can srudy a system at any level of organization. A single leaf cell can be considered a system, as can a frog, an ant colony, or a desert ecosytem_ To understand how such systems work, it is not enough to have a "parts list", even a complete one. Realizing this, many researchers are now complementing the reductionist approach with new strategies for studying whole systems. This changing perspective is analogous to moving from ground level on a street corner to a helicopter high above a city, from which you can see how variables such as time of day, construction projects, accidents, and traffic-signal malfunctions affect traffic throughout the city.
The goal of systems biology is to construct models for the dynamic behavior of whole biological systems. Successful models enable biologists to predict how a change in one or more variables will affect other components and the whole system. Thus, the systems approach enables us to pose new kinds of questions. How might a drug that lowers blood pressure affect the functions of organs throughout the human body? How might increasing a crop's water supply affect processes in the plants, such as the storage of molecules essential for human nutrition? How might a gradual increase in atmospheric carbon dioxide alter ecosystems and the entire biosphere? The ultimate aim of systems biology is to answer big questions like the last one.
Systems biology is relevant to the study of life at all levels. During the early years of the 20th century, biologists studying animal physiology (functioning) began integrating data on how multiple organs coordinate processes such as the regulation of sugar concentration in the blood. And in the 1960s, scientists investigating ecosystems pioneered a more mathematically sophisticated systems approach with elaborate models diagramming the network of interactions between organisms and nonliving components of ecosystems such as salt marshes. Such models have already been useful for predicting the responses of these systems to changing variables. More recently, systems biology has taken hold at the cellular and molecular levels, as we'll describe later when we discuss DNA.

Theme: Organisms interact with their environments, exchanging matter and energy
Turn back again to Figure 1.4, this time focusing on the forest. In this or any other ecosystem, each organism interacts continuously with its environment, which includes both nonliving factors and other organisms. A tree, for example, absorbs water and minerals from the soil, through its roots. At the same time, its leaves take in carbon dioxide from the air and
use sunlight absorbed by chlorophyll to drive photosynthesis, converting water and carbon dioxide to sugar and oxygen. The tree releases oxygen to the air, and its roots help form soil by breaking up rocks. Both organism and environment are affected by the interactions between them. The tree also interacts with other organisms, such as soil microorganisms associated with its roots and animals that eat its leaves and fruit.

Ecosystem Dynamics
The operation ofany ecosystem involves two major processes. One process is the cycling of nutrients. For example, minerals acquired by a tree will eventually be returned to the soil by organisms that decompose leaf litter, dead roots, and other organic debris. The second major process in an ecosystem is the one-way flow of energy from sunlight to producers to consumers. Producers are plants and other photosynthetic organisms, which use light energy to make sugar. Consumers are organisms, such as animals, that feed on producers and other consumers. The diagram in Figure 1.5 outlines the two processes acting in an African ecosystem.


Energy Conversion
Moving, growing, reproducing, and the other activities of life are work, and work requires energy. The exchange of energy between an organism and its surroundings often involves the transformation of one form ofenergy to another. For example, the leaves ofa plant absorb light energy and convert it to chemical energy stored in sugar molecules. When an animal's muscle
cells use sugar as fuel to power movements, they convert chemical energy to kinetic energy, the energy of motion. And in all these energy conversions, some ofthe energy is converted to thermal energy, which dissipates to the surroundings as heat. In contrast to chemical nutrients, which recycle within an ecosystem, energy flows through an ecosystem, usually entering
as light and exiting as heat (see Figure 1.5).
source: Campbell and Reece book

Biology is a subject of enormous scope, and anyone who follows
the news knows that biological knowledge is expandingat an ever-increasing rate. Simply memorizing the factual details of this huge subject is nota reasonable option. How, then, can you, as a student, go beyond the facts to develop a coherent view of life? One approach is to fit the many things you learn into a s et of themes that pervade all ofbiology-ways of thinking about life that will still apply decades from now. Focusing on a few big ideas will help you organize and make sense of all the information you'll encounter as you study biology. To help you, we have selected seven unifying themes to serve as touchstones as you proceed through this book.

Evolution, the Overarching Theme of Biology
Evolution is biology's core theme-the one idea that makes sense of everything we know about living organisms. Life has been evolving on Earth for billions of years, resulting in a vast diversity of past and present organisms. But along with the diversity we find many shared features. For example, while the sea horse, jackrabbit, hummingbird, crocodile, and penguins in Figure 1.3 look very different, their skeletons are basically similar. The scientific explanation for this unity and diversity-and for the suitability of organisms to their environments-is evolution: the idea that the organisms living on Earth today are the modified descendants ofcommon ancestors. In other words, we can explain traits shared by two organisms with the idea that they have descended from a common ancestor, and we can account for differences with the idea that heritable changes have occurred along the way. Many kinds of evidence support the occurrence of evolution and the theory that describes how it takes place. We'll return to evolution later in the chapter, after surveying some other themes and painting a fuller picture of the scope of biology.

Theme: New properties emerge at each level in the biological hierarchy
The study of life extends from the microscopic scale of the molecules and cells that make up organisms to the global scale of the entire living planet. We can divide this enormous range into different levels of biological organization.
Imagine zooming in from space to take a closer and closer look at life on Earth. It is spring, and our destination is a forest in Ontario, Canada, where we will eventually explore a maple leaf right down to the molecular level. Figure 1.4 (on the next two pages) narrates this journey into life, with the circled numbers leading you through the levels of biological organization illustrated by the photographs.

Emergent Properties
If we now zoom back out from the molecular level in Figure 1.4, we can see that novel properties emerge at each step, properties that are not present at the preceding level. These emergent properties are due to the arrangement and interactions of parts as complexity increases. For example, if you make a testtube mixture of chlorophyll and all the other kinds of molecules found in a chloroplast, photosynthesis will not occur. Photosynthesis can take place only when the molecules are arranged in a specific way in an intact chloroplast. To take another example. if a serious head injury disrupts the intricate architecture ofa human brain, the mind may cease to function properly even though all of the brain parts are still present. Our thoughts and memories are emergent properties of a complex network of nerve cells. At a much higher level ofbiological organization-at the ecosystem level-the recycling of chemical elements essential to life, such as carbon, depends on a network ofdiverse organisms interacting with each other and with the soil, water, and air.
Emergent properties are not unique to life. We can see the importance of arrangement in the distinction between a box of bicycle parts and a working bicycle. And while graphite and diamonds are both pure carbon, they have very different properties because their carbon atoms are arranged differently. But compared to such nonliving examples, the unrivaled complexity ofbiological systems makes the emergent properties of life especially challenging to study.

The Power and Limitations of Reductionism
Because the properties of life emerge from complex organization, scientists seeking to understand biological systems confront a dilemma. On the one hand, we cannot fully explain a higher level of order by breaking it down into its parts. Adissected animal no longer functions; a cell reduced to its chemical ingredients is no longer a cell. Disrupting a living system interferes with its functioning. On the other hand, something as complex as an organism or a cell cannot be analyzed without taking it apart.
Reductionism-the reduction of complex systems to simpler components that are more manageable to study-is a powerful strategy in biology. For example, by studying the molecular structure of DNA that had been extracted from cells, James Watson and Francis Crick inferred, in 1953, how this molecule could serve as the chemical basis of inheritance. The central role of DNA in cells and organisms became better understood, however, when scientists were able to study the interactions of DNA with other molecules. Biologists must balance the reductionist strategy with the larger-scale, holistic objective of understanding emergent properties-how the parts of cells, organisms, and higher levels of order, such as ecosystems, work together. At the cutting edge of research today is the approach called systems biology.






source: Campbell and Reece book

The flower featured on the cover of this book and in Figure 1.1 is from a magnolia, a tree ofancient lineage that is native to Asian and American forests. The magnolia blossom is a sign of the plant's status as a living organism, for flowers contain organs of sexual reproduction, and reproduction is a key property oCHfe, as you will learn later.


Like all organisms, the magnolia tree in Figure 1.2 is living in close association with other organisms, though it is a lone specimen far from its ancestral forest. For example, it depends on beetles to carry pollen from one flower to another, and the beetles, in turn, eat from its flowers. The flowers are adapted to the beetles in several ways: Their bowl shape allows easy access, and their multiple reproductive organs and tough petals (see Figure 1.1) help ensure that some survive the voracious beetles. Such adaptations are the result of evolution, the process of change that has transformed life on Earth from its earliest beginnings to the diversity oforganisms living today. As discussed later in this chapter, evolution is the fundamental organizing principle of biology and the main theme of this book.
Although biologists know a great deal about magnolias and other plants, many mysteries remain. For instance, what exactly led to the origin of flowering plants? Posing questions about the living world and seeking science-based answersscientific inquiry-are the central activities ofbiology, the scientific study of life. Biologists' questions can be ambitious.

They may ask how a single tiny cen becomes a tree or a dog, how the human mind works, or how the different forms of life in a forest interact. Can you think ofsome questions about living organisms that interest you? When you do, you are already starting to think like a biologist. More than anything else, biology is a quest, an ongoing inquiry about the nature of life.
Perhaps some of your questions relate to health or to societal or environmental issues. Biology is woven into the fabric of our culture more than ever before and can help answer many questions that affect our lives. Research breakthroughs in genetics and cell biology are transforming medicine and agriculture. Neuroscience and evolutionary biology are reshaping psychology and sociology. New models in ecology are helping societies evaluate environmental issues, such as global warming. There has never been a more important time to em·bark on a study of life.

But what is life? Even a small child realizes that a dog or a plant is alive, while a rock is not. Yet the phenomenon we call life defies a simple, one-sentence definition. We recognize life by what living things do. The following figure highlights some of the properties and processes we associate with life.


Order. This close-up of asunflower illustrates the highly Qrdered structure that characterizes life.


Evolutionary adaptation. The appearance of this pygmy sea horse camouflages the animal in this enviroment. Such adaptations evolve over many generations by the reproductive success of those individuals with heritable traits that are best suited to their environments.


Reproduction. Organisms (living things) reproduce their own kind. Here an emperor penguin protects Its baby.


Energy processing. This hummingbird obtains fuel in the form of nectar from flowers. The hummingbird will use chemical energy stored in its food to power flight and other work.

The picture above reminds us that the magical world of life varies. How do biologists make sense of this diversity and complexity? This opening chapter sets up a framework for answering this question. The first part of the chapter provides a panoramic view of the biological "landscape;' organized around some unifying themes. We then focus on bioJogy's overarching theme, evolution, with an introduction to the reasoning that led Charles Darwin to his explanatory theory. Finally, we look at scientific inquiry how scientists raise and attempt to answer questions about the natural world.