Animal Cells I

Cells and their contents

                  The Cell is the smallest unit that retains the properties of life. It is the basic structural and functional unit of life. It is often called the building block of life. Its structure is highly organized for metabolism. It senses and responds to its environment. There are two different types of cells and they form the sharpest and clearest division in the living world, rather than between plants and animals. These are the Eukaryotic cells with true nuclei and prokaryotic cells without organized nuclei. prokaryotes were the first life form on earth. You have already learnt about them in the General Biology. The basic difference in the structural organization of these two cell types are listed below for you to see whether you can remember them.

• Prokaryotic cells are smaller and simpler than Eukaryotic cells. They are less than 5um in diameter and on average 10 times smaller than Eukaryotic cells. 

• Prokaryotic cells have cell walls composed of peptidoglycan (amino acid and sugar). Some eukaryotic cells also have cells walls, but none are made of peptidoglycan.

• Prokaryotic cells do not have cytoskeletons while all eukaryotic cells have them.

• Membrane bound organelles such as mitochondria, chloroplasts, lysosomes, nucleus and membranous tubular canals such as endoplasmic reticulum, Golgi bodies are absent in prokaryotes while they are present in eukaryotes.

• Ribosomes of prokaryotes are of 70S smaller type. Eukaryotes have larger ribosomes of 80S type.

• Prokaryotes have a single loop of DNA (circular DNA ) not associated with any protein to form chromatin. DNA is linear in Eukaryotes and associated with protein to form chromatin.

• In Prokaryotes, the cytoplasm has a region that contains DNA and it is know as the nucleoid region, but there is no nucleus. In eukaryotes, the DNA is surrounded by a nuclear envelop, which is a double membrane with nuclear pores. Also, a nucleolus is present in the nucleus , which is involved in making ribosomes and processing mRNA.

• A flagellum of Prokaryotes is a rigid rotating one with the motor embedded in the cell membrane. A flagellum/cilium in Eukaryotic cells is composed of microtubules and motor proteins surrounded by the cell membrane.

Eukaryotic cells are found in the in the organisms of domain Eukarya, in its entire four kingdoms; protista, fungi, plantae, and Animalia.

All the Eukaryotic cells are surrounded by a plasma membrane/cell membrane. Inside the plasma membrane is the cytoplasm consisting of a semi fluid matrix. A variety of organelles such as, nucleus, endoplasmic reticulum, golgi apparatus, lysosomes, ribosomes, mitochondria, plastids, and vacuoles are embedded in this fluid matrix. Organelles are intracellular structures that carry out specific functions. Many are membrane enclosed and are regulated compartment in which a specific chemical environment can be maintained. 

Now, we will look in to the structure and function of animal cell components briefly. We will be dealing with the plant cell components in the next section, when we look at the differences between the plant cell and the animal cell.

Being Eukaryotic cells all animal cells possess nuclei and other membranous organelles characteristic of complex cells. Unlike plant cells their shapes very very much. Like plant cell most animal cells are between 1 and 100 um and therefore are visible only under the microscope.