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`(a) Outline the roles of membranes within cell and at the surface of
(b) Plasma (cell surface) membranes are partially permeable barriers.…read more

Slide 2

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(c) Describe, with the aid of diagrams, the fluid mosaic model of membrane
The Bilayer is 7-10µm thick, making it invisible under a light microscope but visible
using an electron microscope.
Bilayer consists of Phospholipids, proteins, cholesterol, glycolipids and glycoproteins.…read more

Slide 3

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`(d) Describe the roles of the components of the cell membrane;'…read more

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`(e) Outline the effect of changing temperature on membrane structure and
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.
The red colour in beetroot cells is caused by molecules of a red pigment. The
pigment is held in by their cell membranes, which are not permeable to it. However, if
you heat the cells, then their membranes become much more permeable. This happens
because of the effects of a rise in temperature on the phospholipids and proteins in the
cell membranes.
As the phospholipid molecules get hotter, they vibrate more, they move much
more than previously, leaving temporary gaps in the membrane through which the
pigment molecules can pass. The protein molecules, too, vibrate more and more as the
temperature increases. They may vibrate so much that they begin to lose their shapes
and come apart. This, too, leaves gaps in the cell membrane. Very low temperature, on
the other hand, decrease membrane permeability. The phospholipids vibrate much
less, packing together tightly and only rarely providing pathways between themselves
through which molecules might pass. Protein channels remain in place, but transporter
proteins may not work very well, because the low temperatures make it difficult for the
cell to provide ATP needed for active transport. At low temperatures all molecules and
ions will be moving around less, so few of them will hit the membrane and pass through…read more

Slide 5

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`(f) Explain the term cell signalling.'
A cell must stay in contact with its environment and with other cells in order to
survive. Cells must be able to react to changes in their environment. In a
multicellular organism, cells in one part of the body must be able to communicate
with cells in other parts. A cell therefore needs to be able to pick up `signals' at its
surface to which it may need to respond.
signals arrive at the plasma membrane from outside the cell as particular
substances ­ for example, a hormone ­ or changes in electrical potential ­ as
happens in nerve impulses. A receptor in the cell's plasma membrane picks up
these signals and brings about actions within the cell. This process is called cell
signalling.…read more

Slide 6

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Aspirin blocks some important cell-signalling
pathways. One of which involves chemicals called
prostaglandins. These chemicals are produced in
cells after injury, they pass out of the cell and slot into
the receptors of other cells, activating G-proteins and
bringing about various effects in that cell. (e.g. pain
signals in the brain from nerve cells) Aspirin blocks
these receptors, preventing the internal cell
responses, hence why it is widely used as a painkiller.…read more

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