E UKARYOTIC C ELLS The eukaryotes include all the protists, plants, animals and fungi. In eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles. Eukaryotic cells possess an organised nucleus with a nuclear envelope. In addition, eukaryotic cells have a variety of complex locomotory and cytoskeletal structures.
Their genetic material is organised into chromosomes. All eukaryotic cells are not identical. Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles which are absent in almost all plant cells (Figure .
). Rough endoplasmic reticulum Lysosome Smooth endoplasmic reticulum Plasmodesmata Microtubule Nucleus Nucleolus Golgi apparatus Nuclear envelope Vacuole Middle lamella Plasma membrane Cell wall Mitochondrion Ribosomes Chloroplast Cytoplasm Peroxisome Figure . Diagram showing : (a) Plant cell (b) Animal cell Golgi apparatus Smooth endoplasmic reticulum Nuclear envelope Nucleolus Nucleus Microvilli Plasma membrane Centriole Peroxiome Lysosome Ribosomes Mitochondrion Rough endoplasmic reticulum Cytoplasm (a) (b) Cholesterol Sugar Peripheral Protein Phospholipid bilayer Let us now look at individual cell organelles to understand their structure and functions. .
. Cell Membrane The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s. Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane. These studies showed that the cell membrane is mainly composed of lipids and proteins.
The major lipids are phospholipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment (Figure . ). In addition to phospholipids membrane also contains cholesterol.
Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately per