Chapter 4: Cell membranes and transport
carbohydrate part of glycoprotein carbohydrate part of glycoprotein
outside outside
glycolipid glycolipid
protein protein
inside glycoprotein phospholipid channel protein inside glycoprotein cholesterol phospholipid transport protein
Figure 4.4 An artist’s impression of the fluid mosaic model of membrane structure.
length: the longer the tail, the less fluid the membrane. As temperature decreases, membranes become less fluid, but some organisms which cannot regulate their own temperature, such as bacteria and yeasts, respond by increasing the proportion of unsaturated fatty acids in their membranes. Two types of protein are recognised, according to their position in the membrane.
branching carbohydrate attached to a protein to form a glycoprotein
Proteins that are found embedded within the membrane, such as those in Figure 4.5, are called intrinsic proteins (or integral proteins). Intrinsic proteins may be found in the inner layer, the outer layer or, most commonly, spanning the whole membrane, in which case they are known as transmembrane proteins. In transmembrane proteins, the hydrophobic regions which cross the membrane are often made up of one or more α-helical chains. branching carbohydrate attached to a lipid to form a glycolipid
hydrophilic head
transport protein (channel or carrier protein) has hydrophilic interior for ions and hydrophilic molecules
hydrophobic tails outer surface phospholipid bilayer inner surface one phospholipid molecule
cholesterol in both layers
proteins
Figure 4.5 Diagram of the fluid mosaic model of membrane structure.
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