Following cell organelles are present in eukaryotic cells:
The cell membrane is composed of lipids which are arranged in a bilayer. The polar heads of lipids are towards the outer side and the hydrophobic ends are towards the inner side. This ensures protection of the non-polar tail of saturated hydrocarbons from the aqueous environment. Additionally, proteins and carbohydrates are also present in plasma membrane. The ratio of protein and lipids varies considerable in different types of cells. The peripheral proteins lie on the surface of the membrane, while the integral proteins lie partially or completely buried in the membrane.
Fluid Mosaic Model: This model was proposed by Singer and Nicolson (1972) and is the most widely accepted model of plasma membrane. According to this model, lipids are present as quasi-fluid. Lateral movement of proteins within the bilayer is possible because of the quasi-fluid nature of the lipid bilayer. The fluid nature of the membrane is also important from many aspects; like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.
Functions of Plasma Membrane: Transport of molecules across it is one of the most important functions of plasma membrane. Plasma membrane is selectively permeable. Many molecules can move across the membrane without any requirement of energy. The movement without the expense of energy is called passive transport. Passive transport takes place through diffusion and osmosis. Polar molecules need a carrier protein of the membrane to be transported across against concentration gradient. This type of transport is dependent on energy and is called active transport.
The cell wall is a non-living rigid structure and forms an outer covering for the plasma membrane of fungi and plants. The cell wall in algae is made up of cellulose, galactans, mannans and minerals; like calcium carbonate. The cell wall of plants is made up of cellulose, hemicelluloses, pectins and proteins. The cell wall of a young plant cell is called the primary wall and is capable of growth. When the cell matures, this capability diminishes and a secondary wall is formed on the inner side of the cell.
The middle lamella is a layer mainly composed of calcium pectate. This holds or glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata. Plasmodesmata connect the cytoplasm of neighbouring cells.
The endomembrane system is composed of endoplasmic reticulum, golgi complex, lysosomes and vacuoles. Functions of these cell organelles are coordinated with each other and hence they form the endomembrane system. Mitochondria, chloroplast and peroxisomes are not a part of the endomembrane system.
The Endoplasmic Reticulum (ER)
The Endoplasmic Reticulum is a network of tiny tubular structures which are scattered in the cytoplasm. The ER divides the intracellular space into two distinct compartments, viz. luminal and extra-luminal. The compartment inside the ER is called luminal compartment, while the compartment in the cytoplasm is called extra-luminal compartment.
Smooth ER: Ribosomes are absent on the surface of smooth ER. Smooth ER are the major sites of lipid synthesis.
Rough ER: Ribosomes are present on the surface of rough ER. Rough ER is quite common in those cells which are actively involved in protein synthesis. They are extensive and continuous with the outer membrane of the nucleus.
Camillo Golgi (1898) was the first to observe densely stained reticular structures near the nucleus. These structures were later named after him. The Golgi Complex consists of many flat, disc-shaped sacs or cisternae. The cisternae are 0.5μm to 1.0μm in diameter. The cisternae are stacked parallel to each other and are concentrically arranged near the nucleus. The convex face of cisternae is called cis and it is the forming face. The concave face of cisternae is called trans and is the maturing face. The cis and trans faces are interconnected.
Functions of Golgi Complex: Packaging of materials is the main function of Golgi Complex. The materials are then delivered to the intracellular targets or secreted outside the cell. The materials come from the ER; in the form of vesicles; and fuse with the cis face. Then they move towards the trans face.
The membrane bound vesicles formed by the process of packaging in the Golgi apparatus are called lysosomes. Lysosomes usually contain various hydrolytic enzymes. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.
Membrane bound space found in the cytoplasm is called vacuole. A vacuole contains water, sap, excretory product and other useless materials. The vacuole is bound by a single membrane; called tonoplast. The vacuole can occupy up to 90% of the volume in a plant cell. In plant cells, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradient into the vacuole. Hence, their concentration is significantly higher in the vacuole than in the cytoplasm.
Mitochondrion is sausage-shaped or cylindrical structure. The diameter of mitochondria is 0.2-1.0μm (average 0.5μm) and length is 1.0-4.1μm. A mitochondrion is bound by two membranes. The two membranes divide the lumen of mitochondria into outer and inner compartments. The inner compartment is called the matrix. The inner membrane forms numerous finger-like infoldings; called cristae. The cristae increase the surface area.
Functions of Mitochondria: Aerobic respiration takes place in mitochondria. Energy is produced in the form of ATP and stored in the mitochondria. The matrix of mitochondria also has single circular DNA molecule, a few RNA molecules, ribosomes and the components needed for the synthesis of proteins. Mitochondria can replicate itself by fission.