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Transport across membranes How do substances cross into and out of cells.

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BIOLOGY: Transport across membranes - How do substances cross into and out of cells. If molecules can't get in or out of cells, if a cell can't be provided with new materials /nutrients or if it can't expel waste materials it will die. It is essential therefore cells allow the movement of molecules in and out of cells to ensure the retained health of that cell. However, not all potential incoming material is welcome. This is where the cells membrane selectivity becomes important; its barrier features become crucial. A cells membrane is selectively permeable, it in essence controls what crosses from outside to inside a cell and visa versa. Some molecules are able to cross more freely than others. The exchange/transfer of molecules between the cell and its environment is essential to life. All living cells are encased and bound by a cell membrane; it houses all the cells contents. These membranes are primarily made up of phospholipids and cholesterols with inter spread proteins which make up a bi-layer on a cells outer boundary. The cell membrane is thin and elastic in structure and is normally between 7-10nm thick. It is the cell membrane which ultimately defines the boundaries of all cells and protects it from environmental changes. ...read more.


Though it does differ to diffusion because it employs the use of protein channels at different point on a cell membrane. Some molecules and ions are not able to pass through the lipid bi-layer of the cell membrane, molecules and ions such as glucose, sodium ions and chloride ions and amino acids need assistance to diffuse across the cells membrane as they are larger molecules than those molecules which can diffuse unaided- such as water or oxygen molecules. The cell membrane contains many different types of protein channels to allow certain molecules to cross and enter or exit cells. The proteins on the membrane form water filled channels across a membrane. This will allow only hydrophilic molecules to pass through. Only the presence of a precise and specific molecule will enable transport across a particular channel. Each protein channel will only allow one kind of molecule or ion to pass through it. 'If the particular molecule is not present the channel remains closed' (p55 Toole, G 2008). From this we can ascertain that the cell membrane has some regulatory control over the entry and exit of molecules/ions but protein channels provide specific molecules with corridors to enable transport across the membrane (Source:http://en.wikipedia.org/wiki/Facilitated_diffusion) This diagram shows protein channels and carrier protein sites for transporting molecules through facilitated diffusion. ...read more.


The small sac or pockets appear in the cell membrane which is then pinched off to form a vesicle inside the cell. This vesicle now inside a cell contains substances from outside the cell. Endocytosis can be further explored, it involves phagocytosis and pinocytosis. Phagocytosis is when solid materials are brought into the cell by the active process of endocytosis. Cells specialising in this are called phagacytes. The phagocytic vesicles fuse with a lysosome in a cell which digests the contents. Pinocytosis essentially means cell drinking and is when fluid is brought into the cell via the same means as above, by endocytosis. These pinocycotic vesicles allow liquids to be brought into a cell and spread within it. Exocytosis is the reverse of endocytosis and is the process whereby materials are removed from a cell. Large molecules such as proteins maybe transported out of a cell by exocytosis. Vesicles housing protein (within a cell) ready for export break from the golgi apparatus in the cytoplasm and travel to the cell membrane. Here it fuses with the cell membrane so that the contents are transported outside the cell. The contents are secreted from the cell. Vesicles containing the material ready to be expelled once fused with the cell membrane release and secrete their contents and become part of the surface membrane. (Source:http:users.rcn.com). Bib Toole, G & Toole, S 2008 AQA Biology for AS level. Surrey. ...read more.

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Response to the question

Overall good basic correct essay. Basic but accurate reply in response to the question and explores each method of transport to a detailed and accurate level. However, to progress the candidate should relate the different types of transport to real ...

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Response to the question

Overall good basic correct essay. Basic but accurate reply in response to the question and explores each method of transport to a detailed and accurate level. However, to progress the candidate should relate the different types of transport to real life scenarios where the types of transport are used and be a lot more concise with the report wording to gain higher marks.

Level of analysis

The report structure could be better defined to be more phenotypically appealing. The introduction appears to tell the story of cells in a discussion tone which is not appropriate for a scientific essay unless the rest of the tone is going to be set out that way, but for a piece of scientific A level coursework, I would say do not use this method. The sentences could be a lot more concise and straight to the point rather than including words and information that is not too relevant. Considers each method of crossing membranes to a basic scientific level that I would expect from A level candidates, however the words are not concisely presented as mentioned before which should be practiced throughout. In facilitated diffusion where osmosis is discussed the candidate should also mention the other methods by which water can cross membranes e.g. via aquaporins or Na+ glucose co-transportation with solvent drag to achieve a higher level of analysis. Good use of diagrams. The candidate could also relate each type of diffusion to a real life scenario where it is used. No conclusion and the provision of a bibliography should not use shortened words.

Quality of writing

Spelling, grammar and punctuation all fine.

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Reviewed by skatealexia 24/03/2012

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