rhizoids  Filamentous structures in the plants group known as bryophytes that attach to a substrate and absorb moisture. The term is also applied to similar structures found outside the bryophytes.

rhizome  In ferns, a horizontal stem with upright leaves containing vascular tissue.

rhodopsin  A visual pigment contained in the rods of the retina in the eye..

ribonucleic acid (RNA) Nucleic acid containing ribose sugar and the base Uracil; RNA functions in protein synthesis. The single starnded molecule transcribed from one strand of the DNA. There are three types of RNA, each is involved in protein synthesis. RNA is made up nucleotides containing the sugar ribose, a phosphate group, and one of four nitrogenous bases (adenine, uracil, cytosine or guanine). PICTURE

ribose Sugar found in nucleotides of RNA and in ATP. PICTURE

ribosomal RNA One of the three types of RNA; rRNA is a structural component in ribosomes.

ribosomal subunits Two units that combine with mRNA to form the ribosomal-mRNA complex at which protein synthesis occurs.

ribosomes  Small organelles made of rRNA and protein in the cytoplasm of prokaryotic and eukaryotic cells; aid in the production of proteins on the rough endoplasmic reticulum and ribosome complexes. The site of protein synthesis. The ribosome is composed of two subunits that attach to the mRNA at the beginning of protein synthesis and detach when the polypeptide has been translated. PICTURE

RNA polymerase  During transcription, an enzyme that attaches to the promoter region of the DNA template, joins nucleotides to form the synthesized strand of RNA and detaches from the template when it reaches the terminator region. PICTURE

RNA transcript Term applied to RNA transcribed in the nucleus. PICTURE

Rodinia Name applied to the precambrian supercontinent. PICTURE

rods  Light receptors in primates' eyes that provide vision in dim light.

root cap Structure that covers and protects the apical meristem in plant roots. Cells forming a protective series of layers over the root meristem. PICTURE

root hairs Extensions of the root epidermis that increase the root's ability to absorb water. PICTURE

root-leaf-vascular system axis  Refers to the arrangement in vascular plants in which the roots anchor the plant and absorb water and nutrients, the leaves carry out photosynthesis, and the vascular system connects the roots and leaves, carrying water and n