Primary cell wall
Secondary cell wall
1. It is found in young dividing cells.
2. It lies internal to the middle lamella.
3. It is present in all plant cells.
4. It is elastic and capable of expansion in a growing cell.
5. It is thin.
6. It is composed of mainly cellulose and lacks additional material.
1. It is found in a mature and non dividing cell.
2. It lies internal to the primary wall.
3. It is present in certain cells only.
4. It is rigid and incapable of expansion.
5. It is thick.
6. It has additional materials like hemi-cellulose lignin, suberin etc.
1. Smooth endoplasmic reticulum (agrannular form). The outer surface of the membranes of such endoplasmic reticulum is smooth and does not bear ribosomes. The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.
2. Rough endoplasmic reticulum (grannular from). bears ribosomes on its surface. Due to the presence of ribosomes the outer surface appears to be grannular or rough. Rough endoplasmic is continuos with the outer membrane of the nucleus. It is found in cells actively engaged in protein synthesis e.g. cells of pancreas, plasma cells, liver cells, goblet cells etc.
A. Smooth endoplasmic reticulum - does not have ribosome.
B. Rough endoplasmic reticulum - has ribosomes attached to it.
A. Functions of smooth endoplasmic reticulum :
1. It is especially prominent in cells which are not involved in the synthesis of lipids.
2. It is involved in the synthesis of Golgi complex, lysosmoes and microbodies.
B. Functions of rough endoplasmic reticulum:
1. It is primarily involved in protein synthesis owing to the presence of ribosomes.
2. Packaging. The proteins manufactured in ribosomes are packed in RER. They are processed and packed in vesicles which are pinched off from RER.
3. Formation of glycoproteins. The carbohydrates and proteins are linked in RER to form glycoproteins.
4. Helps in the formation of nuclear envelops.
Occurrence. The Golgi complex is found in all the eukaryotic cells of plants and animals. However, it is absent in prokaryotic cells. In higher animals it is situated between the nucleus and the cell membrane, while in invertebrates and some specialized cells of vertebrates such as nerve cells, liver cells etc., the Golgi complex is found scattered in the cytoplasm.
In the plant cells the Golgi complex consists of a number of unconnected structures known as dictyosomes. Hundreds of such dictyosomes are found scattered in the cytoplasm.
Structure - If observed under electron microscope the Golgi complex consists of the following components :
1. Cisternae. These are the tubular or flattened fluid filled sacs. The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face.
2. Vesicles. These are small drop-like structures associated with convex surface of the Golgi complex.
3. Tubules. These are branched structures which arise from periphery of cisternae.
4. Vacuoles. These are present on the sides and on the maturing surface of the Golgi complex. These are formed by the pinching of the cisternae of the Golgi complex. The vacuoles are filled with amorphous or granular substances.
Location. The ribosomes are the only organelles which are universally found in all cells i.e. prokaryotic and eukaryotic cells. In prokaryotic cells they are found freely floating in the cytoplasm, while in eukaryotic cells they are are usually found attached at the outer surface of endoplasmic reticulum.
Shape and size. They are oblate spheroidal in form and have size of 18nm in bacteria, chloroplasts and mitochondria and 30 nm in eukaryotic cell.
Types. Ribosomes are of two basic types, 70S and 80S ribosomes. The ‘S’ refers to Svedberg units . 80S ribosomes are found in eukaryotes. while the 70S is found in prokaryotes. The 70S ribosome is has 50S and 30S units whereas 80S is made up of 60S and 40S units.