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Microbes and Pathogens

Microbes are things like bacteria and viruses that are too small to see with the naked eye. They exist on every surface and in every environment on Earth. They’re in the air, in the water, in the soil [more…]

The Parts of the Female Reproductive System

The female reproductive system produces eggs and supports the developing fetus. The reproductive process begins when a sperm cell from a male joins with an egg cell from a female in a process that scientists [more…]

Innate Immunity: Built-In Defenses

You’re usually unaware of all the microbes roaming the world because you can’t see them and because your innate immunity keeps most of them from bothering you. [more…]

The Human Digestive System

Humans have a complete digestive tract: Food enters at one end and wastes exit from the opposite end. Digestion begins in the mouth and continues as food moves through your system: [more…]

The Urinary System

Your body releases wastes that are filtered from your blood and tissue fluids as part of your urine. In particular, urine helps you flush out nitrogenous wastes [more…]

The Inner Workings of the Human Kidney

Like most organs in the human body, the function of a kidney is closely tied to its structure. The outer covering on each kidney, called the capsule, is made of stretchy collagen fibers that help anchor [more…]

Animal Respiration

Respiration is the exchange of life-sustaining gases, such as oxygen, between an animal and its environment. Gas exchange occurs by diffusion, moving necessary gases like oxygen into animals and taking [more…]

Animal Circulatory Systems

Every animal alive possesses a circulatory system that’s in charge of bringing nutrients to cells and removing wastes so they don’t cause disease. While they move fluids around the body, circulatory systems [more…]

Navigate the Human Heart and Circulatory System

The heart and circulatory system of a human, as well as some other mammals, are complex. These large animals need to have a higher blood pressure to push the blood throughout their entire bodies. This [more…]

The Heart and the Cardiac Cycle

Your heart is an impressive little organ. Even though it’s only as big as a clenched adult fist, it pumps 5 liters of blood throughout your body 70 times a minute. Your heart never stops working from the [more…]

Animal Movement with Friction and Gravity

Every type of locomotion, movement from one place to another, requires animals to use energy to overcome the forces of friction and gravity that would otherwise hold them to Earth. [more…]

Biology Basics: Bones and Joints

Joints are structures where two bones are attached. Movable or synovial joints allow bones to move relative to each other. In many areas of the body, strong, fibrous connective tissues called [more…]

How Biologists Classify Life

The figure here represents biologists’ current understanding of the tree of life — the phylogenetic tree that shows relationships among all organisms on Earth. Each of the main branches on the family tree [more…]

Action Potential of Neurons

When a neuron is inactive, just waiting for a nerve impulse to come along, the neuron is polarized —that is, the cytoplasm inside the cell has a negative electrical charge, and the fluid outside the cell [more…]

Biology Basics: Phylogenetic Trees

You can interpret the degree of relationship between two organisms by looking at their positions on a phylogenetic tree.

Phylogenetic trees not only show how closely related organisms are but also help [more…]

How Plants Take Elements from the Soil

Plants get all the carbon, hydrogen, and oxygen they need from carbon dioxide and water, which they use to build carbohydrates during photosynthesis. To build other kinds of molecules they also need elements [more…]

Nervous Systems of Animals

Animals are the only living things on Earth with complex nervous systems that first receive and interpret sensory signals from the environment and then send out messages to direct the animal’s response [more…]

Support from Skeletons and Bones

Skeletons give muscles something to pull against; support the body’s weight; store minerals like calcium and phosphorus; and produce blood cells in the bone marrow. However, not all animals have the same [more…]

Animal Ingestion and Digestion

All animals need to break down food molecules into smaller pieces so they can circulate them around their bodies to all their cells. Their cells take in small food molecules and use them as material for [more…]

Biology Basics: Relationships of Organisms

Biologists draw phylogenetic trees to show the relationships of organisms within a group, much like you’d draw a family tree to show the relationships among your parents, grandparents, and other family [more…]

Discover the Power of DNA Technology

At the heart of a revolution in our understanding of life on Earth is the ability to go to the source — DNA — and directly read the genetic code. Over the past 30 years, this revolution has been taking [more…]

How Scientists Cut DNA with Restriction Enzymes

Scientists use restriction enzymes to cut DNA into smaller pieces so they can analyze and manipulate DNA more easily. Each restriction enzyme recognizes and can attach to a certain sequence on DNA called [more…]

How to Define Biological Evolution

Biological evolution refers to the change of living things over time. Charles Darwin introduced the world to this concept in his 1859 work, On the Origin of Species. [more…]

Biology Basics: Natural Selection Outcomes

The two most extreme outcomes of natural selection for species are extinction and speciation. Natural selection may cause populations of species to change, but exactly how they change depends on the specific [more…]

What Evidence Supports the Theory of Evolution?

Since Darwin first proposed his ideas about biological evolution and natural selection, different lines of research from many different branches of science have produced evidence supporting his belief [more…]


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