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Movement in and out of cells

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________________ Movement in and out of cells. ________________ How substances get in and out of cells (Unit 4 page 26) All cells, need food materials salts and water to live. They also, need, to get rid of toxic substances such as carbon dioxide. Substances may pass through the cell membrane either passively, that means without energy requirement by a process called diffusion, or actively, by some form of active transport. Diffusion: The molecules of a gas, a liquid or a dissolved substance moves from a region where there are a lot of them (i.e concentrated) to regions where there are a few of them. (i.e.less concentrated) until the concentration everywhere is the same. Whether this movement will happen or not depends on whether the cell membrane will let the molecules through. Small molecules such as water, carbon dioxide and oxygen pass through the membrane by diffusion. So diffusion tends to equalize the concentration of these molecules inside and outside the cell all the time. ...read more.


In other cases movement could be speeded up by an enzyme working in the cell membrane. Endo and exocytosis: Cells can take in (endocytose) or expel (exocytose) solid particles or drops of fluids through the cell membrane. Both process occur in all cells, the main difference between them is that endocytosis involves the take in of a substance that can be food or phagocytes. Exocytosis, involves the secretion of a substance inside a vacuole from the inside to the outside of the cell. It takes place for example in some glands. Active transport: The cell membrane has perfectly control over the substances that enter or leave the cell. In some cases, substances are taken into or expelled from the cell, even against the concentration gradient. But this process requires energy. In some cases both process take place at the same time. Osmosis: If a dilute SOLUTION is separated from a concentrated SOLUTION by a partially permeable membrane, water molecules diffuse across the membrane from the dilute to the concentrated solution by a process called Osmosis. ...read more.


Animal cells Water flows into the animal cells by osmosis. The cell membrane is partially permeable to most of the substances dissolved in the cytoplasm. Water entering the cell, will make it swell up, so, unless the extra water is expelled in some way, the cell will burst. On the other hand, if the cell is surrounded by a solution which is more concentrated than the cytoplasm, (i.e there`s a more water outside the cell) water will pass out of the cell by osmosis, and the cell will shrink. So, excessive uptake or loss of water can damage cells. Plant cells: The cytoplasm of a plant cell contains salts, sugars and proteins which effectively reduce the concentration of free water molecules inside the cell. The cell wall is freely permeable to water and dissolved substances, but not the cell membrane. If a plant cell is surrounded by water or a solution more dilute than its contents water will pass by osmosis. The vacuole will expand and press outwards on the cytoplasm and cell wall. The cell is said to be turgid. If vacuoles loose water for any reason, the cell wall will lose their turgor and become flaccid. ...read more.

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Response to the question

Overall a good essay. It explains the different ways that substances move in and out of cells to a high level by including diagrams in relevant places, explaining the concepts of each method and attempting to relate the different concepts ...

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Response to the question

Overall a good essay. It explains the different ways that substances move in and out of cells to a high level by including diagrams in relevant places, explaining the concepts of each method and attempting to relate the different concepts to their functions in the animal. The introduction could be improved upon, and to gain higher marks, the candidate should do deeper research to relate to the higher functions of each method

Level of analysis

The introduction provided is adequate but it does not explain why the cells need basic materials like water in enough detail, or why carbon dioxide in excess can be toxic to the cell. Diffusion is explained and the different concepts expanded on very well. Endo and exocytosis are explained, but I would have liked to have seen images to accompany them so the processes are explained in greater depth and to help the reader clarify the text. The candidate relates these processes to their functions which shows good scientific links. The process of osmosis in relation to plant and animal cells is explained to a good degree, but again diagrams would have been easier to explain and interpret here.

Quality of writing

Lack of commas in certain regions of text and excess in others. Some minor spelling mistakes. Overall spelling, grammar and punctuation is fine apart from minor things mentioned above. The layout of the text is good, with explanations highlighted in clear bold and the different sections split clearly with sub-headings.

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Reviewed by skatealexia 11/07/2012

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