Answered Jan 26
Pseudopodia are sort of like fingers that form and sort of blob out of the surface of the cell that is making them.
Inside the cell, there is always a sort of girder-like supporting structure/network called the cytoskelton. This gives the cell physical support and shape, a bit like a person’s skeleton.
However, the cytoskeleton can grow or shrink very quickly, and can be made to grow in a certain place and certain direction, as needed.
The cytoskeleton is partly made of a building block called actin. It is a protein that can be stacked into long strings. Stacking it in a certain way causes the pseudopod “fingers” to poke outwards. Kind of like growing a new arm, just because you needed one at that moment.
Later, the actin filament gets taken apart, and the pseudopod shrinks back.
In your muscle tissues (where the components are more fixed in place), actin is partnered with another protein called myosin, to do contraction (pulling). There, the strings sit there parallel to each other, and contraction is a sliding motion (the chemical and mechanical actions are somewhat complicated).
In the phagocytes, there is a further process called (strangely enough) phagocytocis, in which your cell pulls in the pathogen, and puts it into a vesicle (like a bag) where the pathogen is dissolved by enzymes. This involves proteins on the cell surface pulling a small area inwards, to form the vesicle.
Some other cells, like amoeba stages of some single-cell protozoa organisms can also use pseudopodia to reach out and pull themselves along (similar to crawling).