Cells within an organism can be very different from one another. for example, human red blood cells are filled with hemoglobin, are shaped like biconcave discs, lack a cell nucleus, and cannot synthesize proteins. human muscle cells, on the other hand, are cylindrical in shape and have multiple nuclei. cell specialization in the human body is usually due to

Wetlands form where water and land meet. a lake with a wetland fringe can fill in with decayed plant and animal material to become a wet meadow. a pond can fill in to become a bog or fen. sediment can fill in a river marsh to become a swamp.flooding of coastal lowlands from rising sea levels has created broad coastal marshes that are protected from wave action by barrier islands or reefs. coastal wetlands also form when rivers deposit sediment as they reach the ocean. plants then take root and hold the soil firm against the forces of tides and waves.