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Origins of Life on Earth
Origins of Life on Earth

Earth was created around 4.5 billion years ago and life began not long after. Primitive life likely possessed the elemen..

Cambial cells
Plant Tissues

Plant organs are comprised of tissues working together for a common function. The different types of plant tissues are m..

Direct and indirect physiologic effects of growth hormone
Animal Growth Hormones

Hormones are produced in the endocrine glands of animals. The pituitary gland and hypothalamus are the most impor..

Human gastrointestinal tract
Human Biology – Food and Digestion

This tutorial recognizes the importance of food as a source of energy that will fuel many biological processes. A good d..

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Sleep and Dreams – Neurology

While learning and intelligence are associated with the functions of a conscious mind, sleep and dreams are activities o..

Freshwater aquatic plankton
Freshwater Communities & Plankton

Planktons are microscopic organisms that live suspended in aquatic habitats. There are two groups: the phytoplanktons an..