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Thursday, 23 February 2017

Cell Structure and Function III


EUKARYOTIC CELLS
  • They have well organized membrane bound nucleus and organelles (endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies etc).
  • There is a clear compartmentalization of cytoplasm due to the membrane bound organelles.
  • They have complex locomotory & cytoskeletal structures. Their genetic material is organized into chromosomes. Eukaryotes include protists, plants, animals and fungi.
Differences between Plant and animal cells
Cell organelles in eukaryotic cells
1. Cell Membrane
  • Chemical studies on the human RBCs showed that the cell membrane is composed of a lipid bilayer
  • The lipids are arranged within the membrane with the outer polar head and the inner hydrophobic tails. This ensures that the non-polar tail of saturated hydrocarbons is protected from the aqueous environment
  • The lipid component of the membrane mainly consists of phosphoglycerides
  • Cell membranes also possess protein and carbohydrate
  • Ratio of protein and lipid varies in different cells. E.g. In human RBC, membrane has 52% protein and 40% lipids
Depending on the ease of extraction, membrane proteins are 2 types:
Integral proteins: Partially or totally buried in membrane.
Peripheral proteins: Lie on the surface of membrane.

Fluid mosaic model of cell membrane: Proposed by Singer & Nicolson (1972). According to this, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity

Functions:

• Transport of the molecules. The membrane is selectively permeable to some molecules present on either side of it.
• Due to the fluid nature, the plasma membrane can help in cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.

Types of Transport
i. Passive transport: 
It is the movement of molecules across the membrane along the concentration gradient (i.e., from higher concentration to the lower) without the expenditure of energy.
It is 2 types: 
Simple diffusion: It is the movement of neutral solutes across the membrane.
Osmosis: It is the movement of water by diffusion across the membrane. As the polar molecules cannot pass through the non-polar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport.

ii. Active transport: 
It is the movement of molecules across the membrane against the concentration gradient (i.e. from lower to the higher concentration) with the expenditure of energy (ATP is utilized). E.g. Na+ /K+ pump.

2. Cell Wall
  • It is a non-living rigid structure found outer to the plasma membrane of fungi and plants
  • Cell wall of Algae is made of cellulose, galactans, mannans and minerals like CaCO3.
  • Cell wall of a young plant cell (primary wall) is capable of growth. It gradually diminishes as the cell matures and the secondary wall is formed on the inner side (towards membrane) of the cell
  • The middle lamella is a layer mainly of calcium pectate which glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells
Functions:
  • It gives shape to the cell
  • It protects the cell from mechanical damage & infection
  • It helps in cell-to-cell interaction
  • It acts as barrier to undesirable macromolecules
3. Endomembrane System
  • It is a group of membranous organelles having coordinated functions.
  • They include endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.
Endoplasmic Reticulum (ER):
  • These are a network of tiny tubular structures scattered in the cytoplasm.
  • ER divides the intracellular space into 2 compartments: luminal (inside ER) & extra luminal (cytoplasm).
Endoplasmic reticulum is of 2 types:
  • Rough endoplasmic reticulum (RER): Bear ribosomes on their surface. RER is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus
  • Smooth endoplasmic reticulum (SER): Ribosomes are absent. SER is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesized in SER.
Golgi apparatus
  • Densely stained reticular structures near the nucleus.
  • First observed by Camillo Golgi (1898).
  • They consist of flat, disc-shaped sacs (cisternae) of 0.5 - 1.0m diameter. These are stacked parallel to each other.
  • Cisternae are concentrically arranged with convex cis (forming) face and concave trans (maturing) face. Cis & trans faces are totally different, but interconnected.
Function of Golgi apparatus:
  • Packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face and move towards the maturing face. This is why the Golgi apparatus remains in close association with the endoplasmic reticulum.
  • Proteins synthesized by ribosomes on the ER are modified in the cisternae of the Golgi apparatus before they are released from its trans face.
  • Golgi apparatus is the important site of formation of glycoproteins and glycolipids.
Lysosomes
  • These are membrane bound vesicular structures formed by the process of packaging in the Golgi apparatus
  • Lysosomal vesicles contain almost all types of hydrolytic enzymes (hydrolases– lipases, proteases, carbohydrases). They are active at the acidic pH. These enzymes can digest carbohydrates, proteins, lipids and nucleic acids.
Vacuoles
  • These are the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell.
  • Vacuole is bound by a single membrane called tonoplast.
  • In plant cells, the vacuoles can occupy up to 90% of the volume of the cell.
  • In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole; hence their concentration is significantly higher in the vacuole than in the cytoplasm.
  • In Amoeba, the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.
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