Biology XI Content
Cell membrane is a thin transparent membrane which encloses the cytoplasm of all the living cells. The terms “cell membrane” was given by C. Nageli and C. Cramer (1885). Cell membrane is also called as plasma membrane or plasmalemma.
Sub-cellular Membrane: The similar type of membrane like as cell membrane is found to cover many cells organelles such as mitochondria, chloroplast, endoplasmic, reticulum, vacuoles etc. This membrane is known as sub-cellular membrane.
Biological or Bio- Membrane: The sub cellular membrane and the cell membrane collectively are known as the biological or bio-membrane.
Structure of Cell Membrane:
- Very thin, elastic, permeable, regenerative and living structure.
- Chemical composition = lipids (20-40%), proteins (40-60%) and carbohydrates (about 5%)
To explain the structure of cell membrane, following models are introduced:
- Danielli and Davson Model (Sandwich Model):
- Proposed by James Danielli and Hugh Davson in 1995
- According to this, cell membrane contains four layers: 2 layers of phospholipid and other 2 layers of protein.
- Because of presence of phospholipid molecules between protein layers, this model is known as sandwich model
- Phospholipid molecules are arranged right angle to protein layers.
- Robertson Model (unit membrane concept):
- Proposed by J.D. Robertson (1959) under the observation in electron microscope
- All bio-membranes have ………….
3. Fluid Mosaic Model:
- Proposed by Singer and Nicholson
- According to this model, protein is not found in uniform layer but found in mosaic pattern like iceberg in sea.
- So also called “protein icebergism sea of lipid”
- Fluidly nature of bio-membrane