Animal reproduction is necessary for the survival of a species. In the animal kingdom, there are innumerable ways that species reproduce. Asexual reproduction produces genetically identical organisms (clones), whereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from their parents. During sexual reproduction the male gamete (sperm) may be placed inside the female’s body for internal fertilization, or the sperm and eggs may be released into the environment for external fertilization.

As with people, it is vital for individual cells to be able to interact with their environment. In order to properly respond to external stimuli, cells have developed complex mechanisms of communication that can receive a message, transfer the information across the plasma membrane, and then produce changes within the cell in response to the message. In multicellular organisms, cells send and receive chemical messages constantly to coordinate the actions of distant organs, tissues, and cells. The ability to send messages quickly and efficiently enables cells to coordinate and fine-tune their functions.

Cell reproduction is a process of cell division that divides one cell into two identical cells.  In multicellular organisms cell reproduction can be for growth, development or repair, whereas in single cell organisms it is a mechanism of reproduction.  The focus of this content is the cell cycle in eukaryotic cells, regulation of the cell cycle, and consequences of a lack of regulation in the context of cancer. A summary of binary fission in prokaryotic cells is also included.

Your body has many kinds of cells, each specialized for a specific purpose. Just as we use a variety of materials to build a home, the human body is constructed from many cell types. For example, epithelial cells protect the body's surface and cover the organs and body cavities within. Bone cells help to support and protect the body. Immune system cells fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide. Each of these cell types plays a vital role during the body's growth, development, and day-to-day maintenance. In spite of their enormous variety, however, cells from all organisms—even ones as diverse as bacteria, onion, and human—share certain fundamental characteristics.

Plants and animals must take in and transform energy for use by cells.  Plants, through photosynthesis, absorb light energy and form organic molecules such as glucose.  Glucose has potential energy in the form of chemical energy stored in its bonds.  This chapter covers the metabolic pathways of cellular respiration and describes the chemical reactions that use energy in glucose and other organic molecules to form adenosine triphosphate (ATP).  ATP is the cell’s “energy currency” fueling virtually all energy requiring processes.  The chemical reactions of cellular respiration are a series of oxidation- reduction (redox) reactions that are divided into three stages: glycolysis, the citric acid cycle and oxidative phosphorylation.

The core threats to biodiversity are human population growth and unsustainable resource use. Current significant threats to biodiversity include habitat loss, overharvesting/overexploitation, invasive species, and climate change. Conservation biology works to counteract the loss of biodiversity due to these threats. These methods include legislative framework such as the ESA and MBA, setting aside preserves, habitat restoration, and captive breeding programs. 

Ecology is the study of the interactions of living organisms with their environment. One core goal of ecology is to understand the distribution and abundance of living things in the physical environment. Attainment of this goal requires the integration of scientific disciplines inside and outside of biology, such as mathematics, statistics, biochemistry, molecular biology, physiology, evolution, biodiversity, geology, and climatology.

An ecosystem in biology is the first level of organization that includes biotic and abiotic (non-living) components.  Ecosystem types vary widely, and ecologists study their structure and dynamics through field work and computer-based modeling.  Understanding how energy flows through and materials are cycled within ecosystems.

The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask questions about the living world. Its power is that it provides direction for predictions about living things that are borne out in ongoing experiments. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that “nothing makes sense in biology except in the light of evolution.” He meant that the tenet that all life has evolved and diversified from a common ancestor is the foundation from which we approach all questions in biology.

Animal evolution began in the ocean over 600 million years ago with tiny creatures that probably do not resemble any living organism today. Since then, animals have evolved into a highly diverse kingdom. But what is an animal? While we can easily identify dogs, birds, fish, spiders, and worms as animals, other organisms, such as corals and sponges, are not as easy to classify. Animals vary in complexity—from sea sponges to crickets to chimpanzees—and scientists are faced with the difficult task of classifying them within a unified system. They must identify traits that are common to all animals as well as traits that can be used to distinguish among related groups of animals. The animal classification system characterizes animals based on their anatomy, morphology, evolutionary history, features of embryological development, and genetic makeup. This classification scheme is constantly developing as new information about species arises. Understanding and classifying the great variety of living species help us better understand how to conserve the diversity of life on earth. 

Meiosis is the process of cell division that produces haploid gametes.  In sexual reproduction haploid gametes combine through fertilization to form a genetically recombined diploid zygote.  Meiosis includes two successive divisions and processes such as crossing over and independent assortment that increase genetic variability in gametes produced.  Life cycles detail the events between meiosis and fertilization that vary for different multicellular organisms.

The cellular processes of life require energy.  How do living organism obtain energy and how is it used?  This Chapter answers these questions by exploring forms of energy and energy transfer within and between living organisms, as well as the role of enzymes and adenosine triphosphate (ATP) in chemical reactions in cells.

Solute concentrations across semi-permeable membranes influence the movement of water and solutes across the membrane. Osmoregulation and osmotic balance are important bodily functions, resulting in water and salt balance. Osmolarity is measured in units of milliequivalents or milliosmoles, both of which take into consideration the number of solute particles and the charge on them. Some organisms are osmoconformers in that they are isotonic with their environment. The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and maintain the osmolarity of body fluids. Many systems have evolved for excreting wastes that are simpler than the kidney and urinary systems of vertebrate animals. The simplest system is that of contractile vacuoles present in microorganisms. Flame cells and nephridia in worms perform excretory functions and maintain osmotic balance. Some insects have evolved Malpighian tubules to excrete wastes and maintain osmotic balance.

Virtually all life on Earth depends on Photosynthesis. Photosynthesis uses energy in sunlight to form organic molecules such as glucose.  This transformation of light energy to chemical energy provides fuel for the metabolic processes in all organisms.  Photosynthesis also produces oxygen required for aerobic cellular respiration. This chapter covers light energy as part of the electromagnetic spectrum, basic structures involved in photosynthesis and the metabolic pathways of photosynthesis divided into the light-dependent reactions and the Calvin cycle.

In scientific terms, phylogeny is the evolutionary history and relationship of an organism or group of organisms. A phylogeny describes the organism's relationships, such as from which organisms it may have evolved, or to which species it is most closely related. Scientists must collect accurate information that allows them to make evolutionary connections among organisms. It is a highly dynamic field of biology because phylogenetic modeling concepts are constantly changing as new information is collected.  Over the last several decades, new research has challenged scientists’ ideas about how organisms are related. 

Plants are as essential to human existence as land, water, and air. Without plants, our day-to-day lives would be impossible because without oxygen from photosynthesis, aerobic life cannot be sustained. From providing food and shelter to serving as a source of medicines, oils, perfumes, and industrial products, plants provide humans with numerous valuable resources.  When you think of plants, most of the organisms that come to mind are vascular plants. These plants have tissues that conduct food and water, and they have seeds. Seed plants are divided into gymnosperms and angiosperms. Gymnosperms include the needle-leaved conifers susch spruce, fir, and pine. Their seeds are not enclosed by a fleshy fruit. Angiosperms, also called flowering plants, constitute the majority of seed plants. They include broadleaved trees (such as maple), vegetables (such as potatoes), grasses, and plants known for the beauty of their flowers (roses and daffodils, for example).  In this module we will be exploring the different structures found within plants, the different physiological processes performed by plants, the different mechanisms of transport that occur within plants, and various response mechanisms used by plants.

Plants have evolved different reproductive strategies for the continuation of their species. Some plants reproduce sexually, and others asexually, in contrast to animal species, which rely almost exclusively on sexual reproduction. Plant sexual reproduction usually depends on pollinating agents, while asexual reproduction is independent of these agents. Flowers are often the showiest or most strongly scented part of plants. With their bright colors, fragrances, and interesting shapes and sizes, flowers attract insects, birds, and animals to serve their pollination needs. Other plants pollinate via wind or water; still others self-pollinate. This module will explore these ideas as well as their impacts on human ventures such as agriculture and horticulture.

Populations are dynamic entities. Populations consist all of the species living within a specific area, and populations fluctuate based on a number of factors: seasonal and yearly changes in the environment, natural disasters such as forest fires and volcanic eruptions, and competition for resources between and within species. Populations rarely, if ever, live in isolation from populations of other species. In most cases, numerous species share a habitat. The interactions between these populations play a major role in regulating population growth and abundance. All populations occupying the same habitat form a community: populations inhabiting a specific area at the same time. The number of species occupying the same habitat and their relative abundance is known as species diversity. Behavior is the change in activity of an organism in response to a stimulus. Behavioral biology is the study of the biological and evolutionary bases for such changes.

Carl Woese and his colleagues proposed that all life on Earth evolved along three lineages, called domains. Two of the three domains—Bacteria and Archaea—are prokaryotic. Prokaryotes were the first inhabitants on Earth, appearing 3.5 to 3.8 billion years ago. These organisms are abundant and ubiquitous; that is, they are present everywhere. In addition to inhabiting moderate environments, they are found in extreme conditions: from boiling springs to permanently frozen environments in Antarctica; from salty environments like the Dead Sea to environments under tremendous pressure, such as the depths of the ocean; and from areas without oxygen, such as a waste management plant, to radioactively contaminated regions, such as Chernobyl. Prokaryotes reside in the human digestive system and on the skin, are responsible for certain illnesses, and serve an important role in the preparation of many foods.

In more advanced animals, the senses are constantly at work, making the animal aware of stimuli—such as light, or sound, or the presence of a chemical substance in the external environment—and monitoring information about the organism’s internal environment. All bilaterally symmetric animals have a sensory system, and the development of any species’ sensory system has been driven by natural selection; thus, sensory systems differ among species according to the demands of their environments. The shark, unlike most fish predators, is electrosensitive—that is, sensitive to electrical fields produced by other animals in its environment. While it is helpful to this underwater predator, electrosensitivity is a sense not found in most land animals.  In this chapter we will be exploring the different sensory systems found in animals.