Polysaccharides are found on the surface of cells. Actin filaments are weaker than intermediate filaments. Actin filaments are made of actin monomers held together by non-covalent bonds. Movement or polymerization of actin filaments helps drive extensions of the plasma membrane called pseudopodia. The primary structure of a protein is its unique sequence of amino acids.
All proteins have the secondary structure called an alpha helix.
Alpha helices are held together only by hydrogen bonds. Many second messengers are formed at the cytosolic side of the plasma membrane. Increases in the cytosolic level of the second messenger calcium can only be due to entry of calcium into the cell. Some second messengers are made from phospholipids. Endocytosis is used by one cell to engulf another, smaller cell.