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2-15 Membranes are also important in their own synthesis and can fold inward for specialized functions

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  • Membranes also contain some of the enzymes that are necessary for membrane synthesis.
  • Some functions that occur in the membrane require large amounts of surface area. Membranes can fold inward, into the cytoplasm to provide the extra needed space.

Membranes also contain specialized enzymes that carry out certain biosynthetic functions. For example, the last few steps of lipid synthesis take place inside the membrane. Another example is cell wall synthesis and assembly. Much of the synthesis of cell wall monomers occurs there and the stitching together of the cell wall polymer takes places while it is anchored to the membrane. In addition, any cellular protein that carries out its function outside the cell membrane (such as outer membrane and extracellular proteins) must pass through that membrane. During their synthesis the ribosome is guided to the cytoplasmic face of the membrane and the growing peptide chain is synthesized directly into the lipid bilayer. Integral membrane proteins then fold up and stay in the membrane while extracellular proteins move through the membrane and take on their final shape on the other side.

Infoldings of the membrane are found in some photosynthetic bacteria. These bacteria use pigments in their membranes to capture light energy. Under low light, they need to increase the surface area to catch more light. They cannot make the membrane thicker, but they can increase the surface area by creating regions where the membrane folds into the cytoplasm. These invaginations are still attached to the cytoplasmic membrane and a picture of such structures, termed the intracytoplasmic membrane in the case of Rhodobacter sphaeroides, is shown in Figure 2-25.

Figure 2-25 The intracytoplasmic membrane of Rhodobacter sphaeroides

The intracytoplasmic membrane of Rhodobacter sphaeroides

This electron micrograph shows the complicated infolding of the cytoplasmic membrane of R. sphaeroides when it is performing photosynthesis. This infolding creates a larger membrane surface area into which light-harvesting complexes can be inserted. Under low-light conditions many light-harvesting complexes are needed to capture the small number of photons striking the microbe. (Source: Samuel Kaplan, University of Texas - Houston Medical School)

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Quick Check 2.11 to 2.15

1. If membranes are composed primarily of lipids (which are symmetrically arranged facing in and out of the cell) why should the inside and outside surfaces of membranes have different properties?


2. Which of the statements are true concerning membrane function:

A. Some non-polar compounds are able to diffuse through the cytoplasmic membrane.
B. Some charged compounds are able to diffuse through the cytoplasmic membrane.
C. Some proteins can span the membrane from the outside of the cell to the inside.
D. The lipids of bacteria can span the membrane from the outside of the cell to the inside.

3. Facilitated diffusion is not really necessary for compounds that do not need to be concentrated against a gradient.

True
False


4. The inside surface of the intracytoplasmic membrane (ICM) is continuous with the inside surface of the cytoplasmic membrane.

True
False


5. In the little pockets formed by the folding of the ICM, those that face the outer surface of the ICM have ion concentrations more like the environment than like the cytoplasm.

True
False


6. Proteins with pigments for capturing light energy are imbedded in the ICM.

True
False


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