8. CELL: THE UNIT OF LIFE
EUKARYOTIC CELLS
- They have well organized membrane bound nucleus and organelles.
- Presence of membranes gives clear compartmentalization of cytoplasm.
- Their genetic material is organized into chromosomes.
- They have complex locomotory & cytoskeletal structures.
Plant cell and Animal cell
Cell organelles in eukaryotic cells
1. Cell Membrane
- Chemical studies on human RBCs show that cell membrane is composed of a lipid bilayer, protein & carbohydrate.
- Lipids (mainly phosphoglycerides) have outer polar head and the inner hydrophobic tails. So the non-polar tail of saturated hydrocarbons is protected from the aqueous environment.
- Ratio of protein and lipid varies in different cells. E.g. In human RBC, membrane has 52% protein and 40% lipids.
- Based 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
- 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.
Polar molecules cannot pass through the non-polar lipid bilayer. So they require membrane carrier protein for transport.
- 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. In other plants, it consists of cellulose, hemicellulose, pectins and proteins.
- 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).
- The middle lamella is a layer containing calcium pectate which glues the neighbouring cells together. Cell wall and middle lamellae may be traversed by plasmodesmata. It connects the cytoplasm of neighbouring cells.
- 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 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 extend to 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- 1.0 mm diameter. These are stacked parallelly.
- Cisternae are concentrically arranged with convex cis (forming) face and concave trans (maturing) face. Cis & trans faces are totally different, but interconnected.
- Secretes materials to intra-cellular targets or outside the cell.
- Materials to be packaged as vesicles from the ER fuse with the cis face and move towards the trans face. This is why Golgi apparatus remains in close association with the endoplasmic reticulum.
- Proteins synthesized by ribosomes on the ER are modified in the cisternae of Golgi apparatus before they are released from its trans face.
- 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 acidic pH. They 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 ions and other materials against concentration gradients into the vacuole. Hence their concentration is higher in the vacuole than in the cytoplasm.
- In Amoeba, the contractile vacuole helps for excretion.
- In many cells (e.g. protists), food vacuoles are formed by engulfing the food particles.
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