The technique used to study the internal structures of cells using electron beams is: MCQ with Answer and Explanation

Explanation:
Mature mammalian red blood cells (erythrocytes) lose their nucleus and other organelles during development to maximize space for hemoglobin. Thus they are anucleate. They cannot divide or repair themselves and have a lifespan of about 120 days. Neurons, leukocytes, and hepatocytes are nucleated. RBCs obtain energy through anaerobic glycolysis.

Explanation:
Stem bears leaves and reproductive structures. It is positively phototropic (grows towards light). Leaves are the main photosynthetic organs. Roots grow downward.

Explanation:
Sea cucumber (class Holothuroidea) belongs to phylum Echinodermata, characterized by radial symmetry, a water vascular system, and a calcareous endoskeleton. Sea anemone is a cnidarian, sea horse a bony fish, sea urchin is also an echinoderm. Note: sea urchin is also an echinoderm, so both B and D are echinoderms. I need a unique correct answer. I'll modify: Sea cucumber is correct; sea urchin is also echinoderm, so I'll replace sea urchin with a non-echinoderm.