The measured distance between genes D and E in a two point test cross is 50 map units. What could this mean in physical terms

Genes that express 50% of recombination frequency or more are not linked genes. They might be in different chromosomes or located in the same one, but too far from each other.    

Explanation:

Genes are physically located in chromosomes. Many genes segregate independently after crossing-over because they are located far away from each other. But some other genes are too close, and they do not segregate independently. These are the linked genes that do not exhibit an independent distribution, and they inherit together more frequently.

We need to know that 1% of recombination frequency = 1 map unit = 1cm. And that the maximum recombination frequency is always 50%.

When the recombination frequency is inferior to 50%, genes are linked. If it is equal or superior, they are not linked. Genes are located far enough from each other to assort independently.

Genes that express 50% of recombination frequency or more are not linked genes. They might be in different chromosomes or located in the same one, but too far from each other.

lysosome is this part of the cell is surrounded by a very thick outer membrane to protect the rest of the cell from its strong enzymes.