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5.6Problems

 

Explain the transport mechanism by which certain molecules and ions cross a biological membrane from the inside to the outside against the concentration gradient (assuming that the concentration is higher outside the membrane).

1)

Provide an outline of the transport pathways through which proteins synthesized in the cytoplasm are transported to different organelles.

2)

Are proteins synthesized anywhere other than in the cytoplasm? If so, please specify.

 

It has been suggested that mitochondria and chloroplasts have evolved from symbiotic bacteria that parasitized primitive eukaryotic cells. Explain the basis of this theory.

 

In a biological membrane, it is easy for lipid molecules to move on the surface of the bilayer, but molecules rarely move between the inside and outside lipid surfaces. Outline the reasons for this.

 

Nerve excitation transmission occurs only in one direction. Outline the reasons for this.

 

Although it may be suggested that the material composition of endoplasmic reticula, Golgi apparatuses, the plasma membrane and other organelles should gradually become similar to each other through vesicular transport, they are actually quite different. Explain the mechanism of this phenomenon.

 

Explain the transport mechanism by which certain molecules and ions cross a biological membrane from the inside to the outside against the concentration gradient (assuming that the concentration is higher outside the membrane).

To transport materials against the concentration gradient, active transport by proteins embedded in the plasma membrane is necessary. Three such mechanisms are known:
The first is active transport using ATP energy.
The second is transport against the concentration gradient that occurs in association with transport that follows the concentration gradient.
The third is transport that carries molecules against the concentration gradient through structural changes caused by light energy. (See the text in 5.3 and Figs. 5-4 and 5-5.)
[Commentary]
The diffusion rate of molecules in a biological membrane consisting of a lipid bilayer changes greatly with the nature of the molecules. Large, uncharged molecules (e.g., glucose) and ions (e.g., H+) barely penetrate the membranes, and are therefore transported selectively by protein-based transport mechanisms. This creates a difference in the concentration of ions and molecules between the outside and inside of the membrane.

1)

Provide an outline of the transport pathways through which proteins synthesized in the cytoplasm are transported to different organelles.

The signal peptides in the amino acid sequence of the protein to be transported play an important role. As an example, proteins that have a nuclear localization signal are transported into the nucleus through nuclear pores. Proteins that have an endoplasmic-reticulum localization signal are integrated into endoplasmic reticula upon the translation of the signal sequence. The proteins transferred to the membrane are packed in transport vesicles and transported via endoplasmic reticula and Golgi bodies (See the text in 5.4 and Figs. 5-7 to 5-9).

2)

Are proteins synthesized anywhere other than in the cytoplasm? If so, please specify.

Since mitochondria and chloroplasts have a genome of their own, proteins are synthesized within the organelles.

 

It has been suggested that mitochondria and chloroplasts have evolved from symbiotic bacteria that parasitized primitive eukaryotic cells. Explain the basis of this theory.

The organism with the genome sequence most similar to the genome of chloroplasts is cyanobacteria. Likewise, it has been confirmed that the genome of mitochondria shows high homology to the genome of rickettsiae (a group of bacteria). These examples of DNA sequence homology form the basis of the endosymbiotic theory.

 

In a biological membrane, it is easy for lipid molecules to move on the surface of the bilayer, but molecules rarely move between the inside and outside lipid surfaces. Outline the reasons for this.

Lipid molecules have polarity, meaning that large amounts of energy are not required for in-plane movement as the polar groups of the molecules remain in contact with the aqueous phase. However, movement from one side to the other side of the membrane requires a high level of activation energy, as the polar groups need to penetrate the hydrophobic membrane.

 

Nerve excitation transmission occurs only in one direction. Outline the reasons for this.

This occurs because the first stimulus enters the end of thin neurons and the Na+ channel has a refractory period.

 

Although it may be suggested that the material composition of endoplasmic reticula, Golgi apparatuses, the plasma membrane and other organelles should gradually become similar to each other through vesicular transport, they are actually quite different. Explain the mechanism of this phenomenon.

The material compositions of these organelles are maintained by dynamic equilibrium. One example is that the membrane used in transport between endoplasmic reticula and Golgi bodies is collected during the return transport, and only the proteins transported are passed on.

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