The cis and trans faces are interconnected. The concave face of cisternae is called trans and is the maturing face. The convex face of cisternae is called cis and it is the forming face.
The cisternae are stacked parallel to each other and are concentrically arranged near the nucleus. The cisternae are 0.5μm to 1.0μm in diameter. The Golgi Complex consists of many flat, disc-shaped sacs or cisternae. These structures were later named after him. They are extensive and continuous with the outer membrane of the nucleus.Ĭamillo Golgi (1898) was the first to observe densely stained reticular structures near the nucleus.
Rough ER is quite common in those cells which are actively involved in protein synthesis. Rough ER: Ribosomes are present on the surface of rough ER. Smooth ER are the major sites of lipid synthesis. Smooth ER: Ribosomes are absent on the surface of smooth ER. The compartment inside the ER is called luminal compartment, while the compartment in the cytoplasm is called extra-luminal compartment. The ER divides the intracellular space into two distinct compartments, viz. The Endoplasmic Reticulum is a network of tiny tubular structures which are scattered in the cytoplasm. Mitochondria, chloroplast and peroxisomes are not a part of the endomembrane system. Functions of these cell organelles are coordinated with each other and hence they form the endomembrane system. The endomembrane system is composed of endoplasmic reticulum, golgi complex, lysosomes and vacuoles. Plasmodesmata connect the cytoplasm of neighbouring cells. The cell wall and middle lamellae may be traversed by plasmodesmata. This holds or glues the different neighbouring cells together. The middle lamella is a layer mainly composed of calcium pectate. When the cell matures, this capability diminishes and a secondary wall is formed on the inner side of the cell. The cell wall of a young plant cell is called the primary wall and is capable of growth. The cell wall of plants is made up of cellulose, hemicelluloses, pectins and proteins. The cell wall in algae is made up of cellulose, galactans, mannans and minerals like calcium carbonate. The cell wall is a non-living rigid structure and forms an outer covering for the plasma membrane of fungi and plants. This type of transport is dependent on energy and is called active transport. Polar molecules need a carrier protein of the membrane to be transported across against concentration gradient. Passive transport takes place through diffusion and osmosis. The movement without the expense of energy is called passive transport.
Many molecules can move across the membrane without a requirement of energy. Plasma membrane is selectively permeable. Transport of molecules across it is one of the most important functions of plasma membrane. The fluid nature of the membrane is also important from many aspects: like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc. Lateral movement of proteins within the bilayer is possible because of the quasi-fluid nature of the lipid bilayer. It means that cell membrane is like a semi-liquid or semi-solid and hence is quite flexible. This model was proposed by Singer and Nicolson (1972) and is the most widely accepted model of plasma membrane.Īccording to this model, lipids are present as quasi-fluid. The peripheral proteins lie on the surface of the membrane, while the integral proteins lie partially or completely buried in the membrane. The ratio of protein and lipids varies considerable in different types of cells. This ensures protection of the non-polar tail of saturated hydrocarbons from the aqueous environment.Īdditionally, proteins and carbohydrates are also present in plasma membrane. The polar heads of lipids are towards the outer side and the hydrophobic ends are towards the inner side. The cell membrane is composed of lipids which are arranged in a bilayer.
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