CK-12 Foundation
Ck 12: Life Science: Keeping Skin Healthy
[Free Registration/Login may be required to access all resource tools.] Your skin is your largest organ and constantly protects you from infections, so keeping your skin healthy is a good idea. Learn more about keeping skin healthy in...
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Ck 12: Life Science: Salamanders
[Free Registration/Login may be required to access all resource tools.] Salamanders are characterized by slender bodies, short legs, and long tails. They are most closely related to the caecilians, little-known legless amphibians. Learn...
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Ck 12: Life Science: Fermentation
[Free Registration/Login may be required to access all resource tools.] Sometimes cells need to obtain energy from sugar, but there is no oxygen present to complete cellular respiration. In this situation, cellular respiration can be...
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Ck 12: Life Science: Importance of Birds
[Free Registration/Login may be required to access all resource tools.] You are probably familiar with birds as food. Around the world, people consume chicken, turkey, and even more exotic birds, like ostriches. Can you think of other...
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Ck 12: Life Science: Blood Vessels
[Free Registration/Login may be required to access all resource tools.] The blood vessels are an important part of the cardiovascular system. They connect the heart to every cell in the body. Arteries carry blood away from the heart,...
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Ck 12: Life Science: Fertilization
[Free Registration/Login may be required to access all resource tools.] The sperm and egg don't look anything like a human baby. After they come together, they will develop into a human being. How does a single cell become a complex...
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Ck 12: Life Science: Nerve Cells and Nerve Impulses
[Free Registration/Login may be required to access all resource tools.] The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages is called a neuron. The messages carried by neurons...
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Ck 12: Life Science: Diversity of Birds
[Free Registration/Login may be required to access all resource tools.] About 10,000 bird species belong to 29 different orders within the class Aves. The diversity among birds is striking. Birds can vary greatly in size and color. Some...
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Ck 12: Life Science: Nails and Hair
[Free Registration/Login may be required to access all resource tools.] Along with the skin, the integumentary system includes the nails and hair. Both the skin and hair contain the tough protein, keratin. The keratin makes your nails...
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Ck 12: Life Science: Plant Hormones
[Free Registration/Login may be required to access all resource tools.] Plants may not move, but that does not mean they don't respond to their environment. Plants can sense gravity, light, touch, and seasonal changes. Learn more about...
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Ck 12: Life Science: Preserving Water Sources
[Free Registration/Login may be required to access all resource tools.] It might seem like there is plenty of water on Earth, but that's not really the case. Water is a limited resource. That means that it is used faster than it is...
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Ck 12: Life Science: Seeds and Seed Dispersal
[Free Registration/Login may be required to access all resource tools.] Plants seem to grow wherever they can. How? Plants can't move on their own. So how does a plant start growing in a new area? Learn more about seeds and seed...
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Ck 12: Life Science: Tracing Evolution
[Free Registration/Login may be required to access all resource tools.] How fast is evolution? Can you actually see evolution happening within your lifetime? Usually evolution takes a long time. So how can we visualize how it has...
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Ck 12: Life Science: Choosing Healthy Foods
[Free Registration/Login may be required to access all resource tools.] Foods such as whole grain breads, fresh fruits, and fish provide nutrients you need for good health. But different foods give you different types of nutrients. You...
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Ck 12: Life Science: Frogs and Toads
[Free Registration/Login may be required to access all resource tools.] Frogs and toads are amphibians in the order Anura. In terms of classification, there is actually not a big difference between frogs and toads. Learn more about frogs...
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Ck 12: Life Science: Human Causes of Extinction
[Free Registration/Login may be required to access all resource tools.] In addition to habitat destruction, other human-caused problems are also threatening many species. These include issues associated with climate change, pollution,...
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Ck 12: Life Science: Human Skeletal System
[Free Registration/Login may be required to access all resource tools.] How important is your skeleton? Can you imagine your body without it? You would be a wobbly pile of muscle and internal organs, and you would not be able to move....
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Ck 12: Life Science: Menstrual Cycle
[Free Registration/Login may be required to access all resource tools.] The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month. Learn more about the menstrual cycle in this...
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Ck 12: Life Science: Segmented Worms
[Free Registration/Login may be required to access all resource tools.] When you think of worms, you probably picture earthworms. There are actually many types of worms, including flatworms, roundworms, and segmented worms. Earthworms...
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Ck 12: Life Science: Circulation and the Lymphatic System
[Free Registration/Login may be required to access all resource tools.] The lymphatic system is a network of vessels and tissues that carry a clear fluid called lymph. The lymphatic system works with the cardiovascular system to return...
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Ck 12: Life Science: Excretion
[Free Registration/Login may be required to access all resource tools.] You might have noticed that sometimes your urine is darker than usual. This can happens when your body is low on water and trying to reduce the amount of water lost...
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Ck 12: Life Science: Human Digestive System
[Free Registration/Login may be required to access all resource tools.] Nutrients in the foods you eat are needed by the cells of your body. How do the nutrients in foods get to your body cells? What organs and processes break down the...
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Ck 12: Life Science: Mammal Characteristics
[Free Registration/Login may be required to access all resource tools.] What is a mammal? They walk, run, swim, and fly. They live in the ocean, fly in the sky, walk on the prairies, and run in the savanna. There is a tremendous amount...
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Ck 12: Life Science: Types of Archaea
[Free Registration/Login may be required to access all resource tools.] The first archaea described could survive in extremely harsh environments in which no other organisms could survive. As a result, archaea are often distinguished by...