1. A group of ribosomes attached to a messenger RNA is called a polysome. 

2. Oxygen diffuses across alveoli in the lungs.

3. The tympanic membrane is found in the ear.

4. Amino acids are linked by peptide bonds.

5. The pacemaker of the heart is the sinoatrial node.

6. The blood-brain barrier is formed by astrocytes.

7. Auxin stimulates stem elongation, development of fruit and root growth