**Cell Organelles Without Membranes**

There are two cell organelles that do not have membranes: ribosomes and centrioles. These organelles play important roles in the cell's functioning and are composed of distinct structures.

**1. Ribosomes:**
Ribosomes are tiny structures found in both prokaryotic and eukaryotic cells. They are responsible for protein synthesis, a crucial cellular process that occurs in all living organisms. Ribosomes are composed of two subunits - a small subunit and a large subunit. These subunits are made up of ribosomal RNA (rRNA) and proteins.

**Structure:**
Ribosomes consist of a complex arrangement of molecules rather than a membrane. They are formed by the assembly of ribosomal RNA (rRNA) molecules and various proteins. The small subunit of the ribosome binds to the messenger RNA (mRNA), while the large subunit catalyzes the formation of peptide bonds during protein synthesis.

**Function:**
Ribosomes are responsible for translating the genetic information encoded in mRNA into proteins. They read the mRNA sequence and link amino acids together to form a polypeptide chain, which eventually folds into a functional protein. Ribosomes can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum (ER).

**2. Centrioles:**
Centrioles are cylindrical structures found in animal cells, usually located near the nucleus in a region called the centrosome. They play a crucial role in cell division and the organization of microtubules within the cytoskeleton.

**Structure:**
Centrioles are composed of nine sets of microtubule triplets arranged in a cylindrical shape. Each triplet consists of three microtubules arranged in a circular pattern. The centriole structure lacks a membrane and is formed by the polymerization of tubulin proteins.

**Function:**
Centrioles are primarily involved in organizing microtubules during cell division. They form the spindle fibers that separate chromosomes during mitosis and meiosis. Additionally, centrioles are responsible for the formation of cilia and flagella in some cells. These hair-like projections aid in cell movement and the movement of substances across the cell surface.

In conclusion, ribosomes and centrioles are two cell organelles that lack membranes. Ribosomes are involved in protein synthesis, while centrioles play a crucial role in cell division and microtubule organization. Despite not having a membrane, these organelles are essential for various cellular processes, highlighting the diversity of structures within cells.