- Name all cellular organelles with one function for each.The function of different cellular organelles is given in Table 1.1.
- Compare and contrast prokaryotic cell with eukaryotic cell.The comparison between prokaryotic cell and eukaryotic cell is given in Table 1.2.
Table 1.2 Comparison of prokaryotic and eukaryotic cells PropertyProkaryotic cellEukaryotic cellSizeSmallLargeCell membraneRigidFlexibleNucleusNot well-definedWell-defined with nucleolusSubcellular organellesAbsentPresentCytoplasmOrganelles and cytoskeleton absentOrganelles and cytoskeleton presentCell divisionBinary fissionMitosis and meiosisTransport systemAbsentPresent - Draw a neat and labeled diagram of eukaryotic cell.The structure of eukaryotic cell is shown in Figure 1.1.
- Write short notes on fluid mosaic model of membrane.As proposed by Singer and Nicolson in 1972, membrane is made up of lipid bilayer with embedded proteins (enzymes, transporters and receptors). Membrane lipids are amphipathic in nature, so they spontaneously form a bilayer in aqueous medium, by arranging their hydrophilic ends exposed to water and hydrophobic tails away from water (Fig. 1.2). Membrane lipids are mainly phospholipids, glycolipids and cholesterol.
- Phospholipids: Glycerophospholipids and sphingomyelin
- Glycolipids: Cerebrosides and gangliosides, present on the outer surface of the membrane
- Cholesterol: Provides fluidity to membrane.
Membrane lipids show lateral movements and flip-flop movements. Hence, membrane is fluid in nature. Hydrophobic interaction between the hydrocarbon tails in the phospholipids keeps the bilayer intact.
Factors Affecting Membrane Fluidity
- Amount of unsaturated fatty acids: More the unsaturated fatty acids, more will be the fluidity
- Saturated fatty acids: Decreases the membrane fluidity
- Cholesterol: Increases the membrane fluidity at low temperatures.
Membrane Proteins
Peripheral membrane proteins: Are attached loosely to the surface of membrane bilayer.
Integral membrane proteins: Are deeply embedded in bilayer structure (proteins that extend all along the membrane bilayer are called transmembrane proteins).
Functions of Membrane Proteins
- Transport of molecules across the membrane
- Act as receptors
- Function as enzymes
- Components of respiratory chain.
Asymmetry in Membranes
The protein to lipid ratio varies in different membranes to suit their functions. For example, inner mitochondrial membrane, which has electron transport chain, is rich in proteins with protein and lipid ratio of 3.2, whereas in myelin sheath, which is designed to insulate the nerve fibers, this ratio is 0.23. Also, there is asymmetry with respect to distribution of phospholipids. For example, phosphatidylcholine, sphingomyelin are predominantly on the outer leaflet and phosphatidylserine, phosphatidylinositol, phosphatidylethanolamine are predominantly on the inner leaflet.
- Write the functions of plasma membrane.Plasma membrane is a barrier with selective permeability. It is made up of lipids and proteins. It separates the cell from external environment and divides the interior of cell into different compartments. Fluid outside the membrane is called extracellular fluid and inside the cell is intracellular fluid.
Functions of Plasma Membrane
- Protects cytoplasm and organelles
- Maintains shape and size of the cell
- Selective barrier—permits transport of required substances in either direction
- Cell-cell interaction
- Signal transmission.
- Describe the characteristics of facilitated diffusion. Mention two examples of transport by facilitated diffusion.Definition: Movement of particles along the concentration gradient with the help of transport proteins. Facilitated diffusion does not require energy, e.g. transport of glucose, galactose, leucine and other amino acids.
Mechanism: Ping-pong Model
Carrier protein has two conformations—ping and pong conformation: Pong conformation of the carrier protein exposes it to higher concentration of molecules (solute) to be transported. Binding of molecules induces conformational change in the carrier protein to ping state, which exposes it to lower concentration of the molecules resulting in their release. Once the molecules are released, the conformation of the carrier protein reverts back to pong form (Fig. 1.3).
- Explain active transport with suitable examples.Definition: Carrier-mediated transfer of molecules against the concentration gradient (from lower concentration to higher concentration) with the help of energy [adenosine triphosphate (ATP)]. Substances that are actively transported through cell membranes include Na+, K+, Ca2+, Fe2+, H+, Cl−, I−. Active transport is susceptible to inhibition and this property is used for designing of drugs in some diseases.
Classification
- Primary active transport: Transport of substrate against its concentration gradient with utilization of energy directly. For example, Na+-K+ ATPase, Ca2+-pump, H+-pump.
- Secondary active transport: ATP is used indirectly for transport.For example,Symport: Glucose-sodium cotransport, amino acid-sodium cotransport; two different substances are carried across the membrane in the same direction.Antiport: Sodium-calcium cotransport, sodium-hydrogen pump; two different substances are carried across the membrane in the opposite direction.
Primary Active Transport
Na+-K+ ATPase: It pumps 3 Na+ from inside to outside of the cell and brings in 2 K+ from outside to inside of the cell against their concentration gradient, using energy provided by hydrolysis of one ATP molecule (Fig. 1.4).
Inhibitor of Na+-K+ ATPase and its significance:
Digoxin: Used in the treatment of congestive cardiac failure (CCF).
Fig. 1.4: Na+-K+ antiport (ECF, extracellular fluid; Na+, sodium ion; K+, potassium ion; ADP, adenosine diphosphate; ICF, intracellular fluid)
Secondary Active Transport
For example, Na+-glucose cotransport (Fig. 1.5). The Na+- K+ ATPase in the basolateral membrane of the cell transports Na+ out of the cell with the help of energy (ATP hydrolysis) creating a Na+ gradient. This Na+ gradient is used by sodium-glucose cotransporter—sodium moves 6along its concentration gradient into the cell pulling glucose along with it against its gradient. Hence, energy is utilized indirectly.
- Describe transport processes across the membrane.Membrane is a selectively permeable barrier. Non-polar substances gain easy access because of solubility in lipid bilayer, but polar substances cross the membrane selectively.Selectivity of membrane transport depends upon:
- Size of molecules: Small solutes pass through easily than larger ones.
- Charge of the molecule: Molecules with less charge pass through the membrane easily than one with more charges.
- Transport proteins: Specific proteins transport specific molecules.
- Type of molecules: Water readily traverses through the membrane.
Classification of transport mechanisms across the membrane:- Passive transport.
- Active transport.
- Endocytosis/exocytosis.
- Ionophores.Passive transport: Simple diffusion, facilitated diffusion and transport through ion channels.Simple diffusionDefinition: Movement of the particles across the membrane, along the concentration gradient, without any involvement of carrier proteins. Energy is not required for simple diffusion.
For example, small and lipophilic molecules like O2, CO2, N2 and H2O are transported by this process.
Facilitated diffusion (Refer question number 6)
Definition: Movement of the particles with the help of transport proteins along the concentration gradient. Facilitated diffusion does not require energy and is carried out by Ping-Pong mechanism (refer Fig. 1.3), e.g. glucose, galactose, leucine and other amino acids.
Ion channels
Ions pass through the ion channels, which open or close in response to a signal. Ion channels are:
- Voltage gated: Open due to changes in membrane potential, e.g. Ca2+, Na+ and K+ channels.
- Ligand gated: Binding of ligand to receptor site results in opening and closing of the channel, e.g. acetylcholine receptor.
Active transport (Refer question number 7)
Endocytosis and exocytosis
Endocytosis
Uptake of macromolecules into the cells. For example, uptake of low-density lipoproteins (LDL), polysaccharides, proteins and polynucleotides.
Two types:
- Pinocytosis: Uptake of fluid and fluid contents by cell (cellular drinking).
- Phagocytosis: Ingestion of larger particles like bacterial cells and tissue debris by macrophages, which are further hydrolyzed by lysosome.
Exocytosis
Release of macromolecules from the cell to outside. For example, calcium-dependent secretion from vesicles (secretion of hormones).
Ionophores
Ionophores are the molecules that facilitate transport of ions across membranes.
Two types:
- Carrier ionophores: They increase permeability for a particular ion, e.g. valinomycin transports K+ and inhibits oxidative phosphorylation.
- Channel-forming ionophores: They facilitate passage of ions by forming channels, e.g. gramicidin A inhibits oxidative phosphorylation by facilitating movement of Na+ and K+ across the membrane.