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Biology revision – Transport across cell membranes (Unit 4)
Structure of the cell-surface membrane
- All membranes have same structure and are known as plasma membranes
- Cell-surface membrane is the plasma membrane that surrounds cells and forms the
boundary between cell cytoplasm and the environment
- Allows different conditions to be established inside and outside a cell
- Controls movement of substances in and out of the cell
Phospholipids
- Form a bilayer along with cholesterol
- Important components of cell-surface membranes because:
o Hydrophilic heads of both phospholipid layers point to the outside of the cell-
surface membrane attracted by water on both sides
o Hydrophobic tails of both phospholipid layers point into the centre of the cell
membrane attracted by water on both sides
- Lipid soluble material moves through the membrane via the phospholipid portion
- The functions of phospholipids in the membrane are to:
o Allow lipid-soluble substances to enter and leave the cell
o Prevent water-soluble substances entering and leaving the cell
o Allows small, uncharged molecules to pass through e.g. O 2 and CO2
o Make the membrane flexible and self-sealing
- Phospholipids are fluid which allows proteins to move around
the membrane
o Viscous – more saturated phosphos
o Fluid – more unsaturated phosphos
o Rigid – cholesterol
o Glycolipid – communication between cells (recognition sites), stability of the cell
membrane, allows attachment between cells in tissues
Proteins
- Interspersed throughout the cell surface membrane
- Embedded in the phospholipid bilayer in two main ways:
o Some proteins occur on the surface of the bilayer and never extend completely
across it. Act either to give mechanical support to the membrane or in conjunction
with glycolipids, as cell receptors for molecules such as hormones. Known as
extrinsic proteins
o Intrinsic proteins/ integral proteins go from one side of the membrane to the other
Some have protein channels that let molecules through the membrane that
cannot get through the phospholipids
Others are carrier proteins that bind to ions or molecules like aminos
- Functions of proteins in the membrane:
o Provide structural support
o Act as channels transporting water-soluble substances across membrane
o Allow active transport across the membrane through carrier proteins
o Form cell-surface receptors for identifying cells
o Help cells adhere together
o Act as receptors, for example for hormones
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