Re: How do amoebas eat?

The answer is "through their skin." In the case of amoebas, of course, 
having only one cell, they don't have the kind of skin we do, which is made 
up of many layers of cells. The surface of the ameba, like all individual 
cells, is called a membrane, and it's actually a double layer of fat-like 
molecules that separate the stuff inside the cell from what's outside. But 
embedded in this membrane are many hundreds of specialized molecules made 
of proteins and sugars that cover most of the surface. Some of these go 
right through the membrane to the inside and are connected to a complicated 
bit of machinery that can change the shape of parts of the ameba's surface.

When things the ameba wants to eat - other cells, little food particles, 
etc. -  stick to surface molecules (the term for surface molecules that 
recognize and bind to specific factors - proteins, sugars, hormones - is 
receptors), the shape changing machinery is set in motion, and the membrane 
flows around the particle of lunch. When it's surrounded, the membrane 
comes in contact with itself on the other side of the particle and merges, 
actually capturing the particle within the ameba in a little 
membrane-covered bubble, called a vacuole. This process is called 
"phagocytosis", which basically means cell eating. 

What happens next is that the lunch has to be digested, broken down into 
simpler chemicals that the ameba can use for its own biochemical processes. 
This is accomplished by using other specialized proteins that pump 
digestive enzymes and chemicals into the vacuole. When the ameba has gotten 
what it needs, any remaining undigested material in the bubble is "spit 
out" by reversing the process of phagocytosis. Here, the bubble comes in 
contact with the outer membrane again, and fuses with it. When this 
happens, the bubble "bursts" toward the outside of the ameba. This process 
is called "exocytosis." Amazingly complicated little creatures. They're fun 
to watch in a microscope, too. See if your science teachers can help you do 
that sometime.

Paul Odgren, Ph.D.
Cell Biology
University of Massachusetts Medical School
Worcester  MA