[Free Registration/Login may be required to access all resource tools.] The cell membrane is semi-permeable. Sometimes molecules need the help of special transport proteins or an input of energy to help them move across it. When an input...

[Free Registration/Login may be required to access all resource tools.] Cells do eventually wear out. At that point, they need to be replaced. This is one reason that your cells divide. New cells that result after cells divide are also...

[Free Registration/Login may be required to access all resource tools.] Some human genetic disorders are also X-linked or Y-linked, which means the faulty gene is carried on these sex chromosomes. Other genetic disorders are carried on...

[Free Registration/Login may be required to access all resource tools.] Cells are found in all different types of environments, and these environments are constantly changing. For example, one-celled organisms, like bacteria, can be...

[Free Registration/Login may be required to access all resource tools.] All cells have a barrier around them that separates them from the environment and from other cells. This barrier is called the plasma membrane, or cell membrane....

[Free Registration/Login may be required to access all resource tools.] Gene therapy is the insertion of genes into a person's cells to cure a genetic disorder. Could gene therapy be the cure for AIDS? No, AIDS is caused by a virus. Gene...

[Free Registration/Login may be required to access all resource tools.] The process of cell division in eukaryotic cells is carefully controlled. The cell cycle is the life cycle of a cell, with cell division at the end of the cycle....

[Free Registration/Login may be required to access all resource tools.] DNA contains the instructions to create proteins, but it does not make proteins itself. DNA is located in the nucleus, which it never leaves, while proteins are made...

[Free Registration/Login may be required to access all resource tools.] Predation is another mechanism in which species interact with each other. Predation is when a predator organism feeds on another living organism or organisms, known...

[Free Registration/Login may be required to access all resource tools.] Symbiosis describes a close and long-term relationship between different species. At least one species will benefit in a symbiotic relationship. There are three...

[Free Registration/Login may be required to access all resource tools.] When you think of the processes of breathing, the lungs probably come to mind. The lungs are the main organ of the respiratory system. However, many other organs are...

[Free Registration/Login may be required to access all resource tools.] Seedless plants can reproduce asexually or sexually. Some seedless plants, like hornworts and liverworts, can reproduce asexually through fragmentation. When a small...

[Free Registration/Login may be required to access all resource tools.] There are two basic types of cells, prokaryotic cells and eukaryotic cells. The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells...

[Free Registration/Login may be required to access all resource tools.] Energy is the ability to do work. In organisms, this can be physical work, like walking or jumping, or it can be the work used to carry out the chemical processes in...

[Free Registration/Login may be required to access all resource tools.] The life cycle of a plant is very different from the life cycle of an animal. Humans are made entirely of diploid cells (cells with two sets of chromosomes). Our...

[Free Registration/Login may be required to access all resource tools.] Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells. Plant cells have a large central vacuole, are surrounded by a...

[Free Registration/Login may be required to access all resource tools.] Plants are producers, which means they are able to make their own food, as well as "food" for other organisms. Plants are also autotrophs - they collect energy from...

[Free Registration/Login may be required to access all resource tools.] The peripheral nervous system (PNS) contains all the nerves in the body that are found outside of the central nervous system. They include nerves of the hands, arms,...

[Free Registration/Login may be required to access all resource tools.] If pathogens get through the body's first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. The immune...

[Free Registration/Login may be required to access all resource tools.] What does population growth mean? You can probably guess that it means the number of individuals in a population is increasing. The population growth rate tells you...

[Free Registration/Login may be required to access all resource tools.] Biodiversity is a measurement of the amount of variation of the species in a given area. More specifically, biodiversity can be defined as the variety of life and...

[Free Registration/Login may be required to access all resource tools.] For a population to be healthy, factors such as food, nutrients, water and space, must be available. Limiting factors are resources or other factors in the...

[Free Registration/Login may be required to access all resource tools.] The mouth and stomach are just two of the organs of the digestive system. Other digestive system organs are the esophagus, small intestine, and large intestine....

[Free Registration/Login may be required to access all resource tools.] Some of the most important animal behaviors involve mating. Mating is the pairing of an adult male and female to produce young. Adults that are most successful at...