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Explanation:
Cell membranes are barriers which protect and regulate the movement of substances in and out of the cells. It is a semi-permeable and selective structure.
In the passive transport system, substances move along a concentration gradient and it doesn't require energy. The examples of passive transports include; simple diffusion which transports carbon dioxide and oxygen, osmosis and facilitated diffusion which transports charged molecules like ions, sucrose etc.
In active transport, substances move against the concentration gradient. This requires energy. The two main types are: primary active transport, which uses direct metabolic energy e.g Na+/K+ pump, and the secondary active transport system which couples the transported molecules with another molecule moving along the gradient, example the Na+/glucose transporter.
We could say that there are five steps in scientific method: observation, hypothesis, prediction, experiment and conclusion. Observation is the first step and we could also call it research. In this step we must understand the problem in the right way. The hypothesis is the possible answer, based on the research. Prediction is even more specific than the hypothesis: on this stage, you must know how will you demonstrate that your hypothesis is true. Experiment tests your hypothesis and shows whether your hypothesis was true or false. After that you can proceed to making conclusion, which summarizes the experiment's results, and tells do the results match up to hypothesis and in which way.
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