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Reianna Shakil L6EFBiology10/11/12



Movement across cell membranes
Many substances move into and out of cells through their plasma membranes. Some of
these substances move passively ­ that is, the cell does not have to use energy to make
them move. Passive processes include diffusion, facilitated diffusion and osmosis.
Other substances…

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Reianna Shakil L6EFBiology10/11/12



Such pathways are provided by channel proteins. These proteins lie in the membranes,
stretching from one side to the other, forming a hydrophilic channel through which ions
can pass.
The ions pass through by diffusion, down their concentration gradient.
This process is called facilitated diffusion. It is…

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Reianna Shakil L6EFBiology10/11/12




Osmosis and animal cells

Figure 2.8 shows what happens when animal cells are place in solutions with water potentials
higher or lower than the water potential of the cytoplasm inside the cells. If the solution
outside the cell has a higher water potential than the cytoplasm, then…

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Reianna Shakil L6EFBiology10/11/12


plasma membrane is often damaged in this process. A cell in this state is said to be
plasmolysed. The cell usually dies.




Active transport

There are many instances where a cell needs to take up, or get rid of, substances whose
concentrations gradient is in the `wrong'…

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Reianna Shakil L6EFBiology10/11/12




Exocytosis and endocytosis

Moving substances out of a cell in this way is called exocytosis. The substance to be
released from the cell is contained in a tiny membrane-bound sac called a vesicle. The
vesicle is moved to the plasma membrane along microtubules. The membrane around
the…

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Reianna Shakil L6EFBiology10/11/12



How temperature affects membrane permeability

If you cut some pieces of beetroot, wash them and place them in water, the water will
remain colourless. If, however, you heat the beetroot pieces, then some of their red
colour comes out and the water goes red. Why does this…

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