Theory:

Unicellular organisms like amoeba are irregular in shape. Based on the functions such as locomotion and capture of prey for nutrients it changes its shape  The cytoplasm and cell membrane of amoeba protrude in and out during this forming projections known as pseudopodia or false feet.
The paramecium is spindle-shaped and bacteria is rod, spherical and spiral in shape.
 
amoeba.PNG
Paramecium and amoeba
 
Shapes_of_bacteria_and_size_comparisons.jpg
Shapes of bacteria
 
In multicellular organisms like humans, different cells have different functions.
The white blood cells of humans, are an example of a single cell that can change its shape similar to the amoeba and are hence capable of independent existence.
Example:
  1. The muscle cells of humans are examples of spindle-shaped cells.
  2. The skins cells are examples of flat cells.
  3. The nerve cells or neurons are long and branched cells that receives and transfers messages. They help in controlling and coordinating the different parts of the body.
shutterstock_196573562.jpg
Different types of human cells
All the various components of the cell are enclosed by a membrane which provides the plants and animal cells, the various above mentioned shapes.
The plant and bacterial cells have an additional covering over the cell membrane known as the cell wall. This gives the plant cells shape and rigidity.
Reference:
https://upload.wikimedia.org/wikipedia/commons/c/ce/Figure_B23_02_02.jpg
https://upload.wikimedia.org/wikipedia/commons/8/8c/Shapes_of_bacteria_and_size_comparisons.jpg