Biology - Movement Across Cell Membranes

These are complete notes copied out of my textbook completed for homework, on movement across cell membranes. Diagrams included. It is based on the OCR specification for AS Biology. Happy revising! =P

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  • Created on: 13-01-13 00:18
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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 are actively moved by the cell, which uses energy to make them move up
their concentration gradients. This is called active transport.
Particles are constantly moving around randomly. They hit each other and bounce off in
different directions. Gradually, this movement results in the particles spreading out
evenly throughout the space within which they can move. This is diffusion.
If there are initially more particles in one place than another, we say there is a
concentration gradient for them.
Diffusion is the net movement of molecules or ions down their concentration
gradient ­ that is, from a place where they are in a high concentration to a place
where they are in a lower concentration.
There are usually a large number of different kinds of particles bouncing around inside and
outside a cell, on both sides of its plasma membrane. Some of these particles hit the
plasma membrane.
If they are small ­ like oxygen and carbon dioxide molecules ­ and do not have an
electrical charge, they can easily slip through the phospholipid bilayer.
Oxygen enters a cell like this. Inside the cell, aerobic respiration constantly uses up
oxygen, so the concentration of oxygen inside the cell is low. If there is more oxygen
outside the cell, then there is a concentration gradient for oxygen.
Oxygen molecules on both sides of the plasma membrane are moving freely around, and
some of them hit the plasma membrane and pass through it.
This happens in both directions, but because they are more oxygen molecules in a given
volume outside the cell than inside, more of them will pass through the membrane from
outside to inside rather than in the opposite direction.
The overall effect is for oxygen to move from outside the cell, through the plasma
membrane, into the cytoplasm.
Facilitated diffusion
Oxygen and carbon dioxide have small molecules with no electrical charge, and can pass
easily through the phospholipid bilayer.
However, any other molecules or ions may be too big, or too highly charged, to do this.
For example, chloride ions, Cl-, have an electrical charge and cannot pass through the
phospholipid bilayer.
Cells therefore need to provide special pathways through the plasma membrane which
will allow such substance to pass membrane which will allow such substance to pass

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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 just like ordinary diffusion, except that
the molecules or ions only get through the membrane if they happen to bump into a
channel.…read more

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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 water enters the cell
by osmosis. If the water potential gradient is
very high, so much water may enter that the
cell bursts.…read more

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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' direction. This is usually the case with sodium
ions and potassium ions.…read more

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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 vesicle fuses with the plasma membrane, emptying the vesicle's contents outside
the cell.
Moving substances into a cell in this way is called endocytosis.…read more

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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 happen?
The red colour in beetroot cells id caused by molecules of a red pigment. The pigment is
held in by their cell membranes, which are not permeable to it.…read more


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