UNIT 5 – Biology of Membrane and Transport of lons (Q&A) | MZO-001 MSCZOO | IGNOU
SAQ 1
Fill in the blanks
a) ............. discovered plasma membrane.
Answer: Karl Nageli and C. Cramer
b) The phospholipid contains .................. charged phosphate group in the hydrophilic part of head.
Answer: Negatively
c) ................... proposed Sandwich (lipid-protein) model of cell membrane.
Answer: Danielli and Davson
d) The protein layer present in cell membrane model proposed by Robertson is .................. thick.
Answer: 20 A°
e) The proteins are aligned properly with the help of ....................... within the lipid bilayer in membrane.
Answer: Transmembrane segments
SAQ 2
i) Answer in one word:
a) Complex integral proteins transmit signals via plasma membrane.
Answer: Receptors
b) The cellular processes such as movement, growth, division etc. are regulated by this property of membrane.
Answer: Fluidity
c) No energy is required for transter of substances from high concentration zone to low concentration zone in this proces.
Answer: Passive
d) Certain temporarily opening passagelways that work only in response to a binding of ligand to cell.
Answer: Gated pores or Gated channels
e) The property of membrane that assists in transfer of some materials through the membrane restricting the entry of others.
Answer: Selective permeability
ii) Match the items in column A with those in column B
Answer: (a) v, (b) vi, (c) i, (d) ii, (e) iii, (f) iv
TERMINAL QUESTIONS
1. Describe plasma membrane structure as given by Singer and Nicolson.
The plasma membrane structure was explained by Singer and Nicolson in 1972 through their famous Fluid Mosaic Model. This model describes the membrane as a dynamic and flexible structure, where proteins and lipids are not rigidly fixed but can move within the layer. This view completely changed the earlier understanding, making it more accurate and relatable to the real biological membranes.
According to Singer and Nicolson, the plasma membrane is mainly made of a phospholipid bilayer in which proteins are embedded like a mosaic. The bilayer acts like a thin, flexible sheet. Each phospholipid molecule has a hydrophilic head (water-loving) that faces outward toward the water and hydrophobic tails (water-fearing) that point inward, away from water. This double arrangement forms a stable barrier between the inside and outside of the cell.
Proteins are interspersed throughout this bilayer and are of two major types, which are integral proteins and peripheral proteins. Integral proteins are embedded deeply into the membrane and may even pass through it completely. These proteins often function as channels, carriers or receptors. On the other hand, peripheral proteins are attached only loosely on the membrane surface and are involved in signaling or providing structural support.
A special feature of the Fluid Mosaic Model is that it shows how the plasma membrane is fluid. The phospholipids and proteins can move sideways within the layer, giving the membrane flexibility. This movement allows membranes to repair themselves, form vesicles and change shape, which is very important for processes like endocytosis and cell division.
Singer and Nicolson also explained the presence of carbohydrate chains attached to proteins (forming glycoproteins) or lipids (forming glycolipids) on the outer surface of the membrane. These carbohydrate groups are important for cell recognition, communication and protection.
In short, the Fluid Mosaic Model by Singer and Nicolson shows the plasma membrane as a dynamic, semi-permeable and highly organized structure made mainly of lipids, proteins, and carbohydrates, which work together to maintain the proper functioning and communication of cells.
2. Write a note on the important functions of the plasma membrane.
The plasma membrane, also known as the cell membrane, is a vital structure that surrounds the cell and separates its internal components from the external environment. It plays several crucial roles in maintaining the integrity and proper functioning of the cell. Some of its most important functions are:
1. Selective Permeability
One of the primary functions of the plasma membrane is its ability to control what enters and leaves the cell. It is selectively permeable, allowing certain molecules to pass while blocking others. This is important for maintaining the cell's internal environment (homeostasis). Small, nonpolar molecules like oxygen and carbon dioxide can freely pass, while larger or charged molecules require specific transport mechanisms (e.g., proteins or channels).
2. Structural Support
The plasma membrane provides structural support to the cell. It is the interface between the cell's interior and the external environment, giving the cell its shape and helping it maintain its integrity. The membrane is anchored to the cytoskeleton inside the cell, which helps the cell maintain its shape and resist deformation.
3. Cell Communication
The plasma membrane is involved in cell signaling. It contains various receptors (usually proteins) that bind to signaling molecules like hormones, growth factors, or neurotransmitters. This binding triggers a series of intracellular events that allow the cell to respond to external signals. This process is crucial for cellular activities such as growth, division and differentiation.
4. Transport of Materials
The plasma membrane regulates the transport of ions, nutrients, and waste products into and out of the cell. It does this through various mechanisms such as passive transport (e.g., diffusion, facilitated diffusion), active transport (e.g., through pumps that require energy) and vesicular transport (e.g., endocytosis and exocytosis). This ensures that the cell maintains the proper concentration of essential substances while removing waste.
5. Cell Recognition
The plasma membrane plays a significant role in cell recognition, especially in multicellular organisms. Carbohydrate chains attached to proteins (glycoproteins) and lipids (glycolipids) on the outer surface of the membrane serve as markers for cell recognition. These markers help the immune system identify foreign cells or pathogens and distinguish self-cells from non-self-cells.
6. Intercellular Connections
In multicellular organisms, the plasma membrane is involved in forming connections between adjacent cells. These connections, known as cell junctions (such as tight junctions, desmosomes and gap junctions), allow cells to communicate and work together. This is particularly important in tissues that require coordinated functions, such as in epithelial layers and muscle tissues.
7. Protection
The plasma membrane also acts as a protective barrier, safeguarding the cell's internal environment from harmful substances, toxins and pathogens. The lipid bilayer, along with proteins embedded in it, prevents unwanted or dangerous materials from entering the cell, while also limiting the loss of essential substances.
3. What do you mean by membrane fluidity? Why it is important?
Membrane fluidity refers to the ability of the lipid molecules and proteins within the plasma membrane to move or flow laterally within the layer. In simple words, it means how freely the lipids and proteins can move sideways inside the membrane. The plasma membrane is not a rigid structure like a solid wall, but rather behaves more like a flexible, oily sheet. The term "fluid mosaic model," proposed by Singer and Nicolson in 1972, describes this property very clearly, where the membrane is seen as a fluid structure with a "mosaic" of proteins embedded in or attached to it.
The fluidity of the membrane is mainly determined by several factors:
- The type of lipids present, especially the ratio of saturated and unsaturated fatty acids
- The presence of cholesterol
- The temperature of the environment
For example, more unsaturated fatty acids (which have double bonds) increase fluidity, while saturated fatty acids (which are straight) make the membrane more rigid. Cholesterol acts like a buffer, reducing fluidity at high temperatures and increasing it at low temperatures.
Why Membrane Fluidity is Important
Membrane fluidity is extremely important for several reasons, because it directly affects how cells live, grow, divide and interact with their surroundings.
Here are some reasons why membrane fluidity is important:
1. Proper Functioning of Membrane Proteins
Membrane proteins like transporters, enzymes and receptors need to move freely within the membrane to function properly. If the membrane is too rigid, these proteins cannot change their shape or position, and their activity would be reduced, affecting transport, signal reception and enzymatic reactions.
2. Cell Movement and Growth
Cell movement processes such as cell migration and cell division require the membrane to be flexible. During division, for example, the membrane must stretch and eventually split into two daughter cells. If the membrane were too stiff, this would not happen properly.
3. Vesicle Formation and Fusion
Processes like endocytosis (taking material into the cell) and exocytosis (expelling material from the cell) require the membrane to bend and form vesicles. A fluid membrane allows this bending and fusion to occur easily.
4. Distribution of Membrane Components
Newly synthesized proteins and lipids are inserted into the plasma membrane at specific points. Fluidity helps them to spread throughout the membrane so that the membrane remains uniform and balanced.
5. Maintaining Cell Integrity
A flexible membrane can adjust to mechanical stress without tearing. This is especially important for blood cells, muscle cells and cells that constantly undergo physical forces.
4. Differentiate between:
a) Endocytosis and Exocytosis
Cells maintain their internal environment and interact with the outside world through various transport processes. Among these, endocytosis and exocytosis are two crucial active transport mechanisms that involve the movement of large molecules or particles. Endocytosis allows cells to intake essential nutrients and substances, while exocytosis enables cells to expel waste materials and secretory products. Although both processes involve the use of vesicles and energy, their direction and purpose are different.
Here we will differentiate between endocytosis and exocytosis based on several important points:
1. On the basis of Direction of Transport:
In endocytosis, substances move from outside the cell into the cytoplasm. The plasma membrane engulfs the material and forms a vesicle that carries it inside. For example, during phagocytosis, a white blood cell engulfs a bacterium into the cell.
In exocytosis, substances are transported from inside the cell to the external environment. Vesicles fuse with the plasma membrane and expel their contents outside. For example, pancreatic cells secrete insulin into the blood through exocytosis.
2. On the basis of Mechanism:
Endocytosis occurs by the inward folding of the plasma membrane, which then pinches off to form an internal vesicle containing the material.
Exocytosis happens when vesicles inside the cytoplasm move towards the plasma membrane, fuse with it and release their contents outside the cell.
3. On the basis of Purpose:
The main purpose of endocytosis is to internalize nutrients, fluids and even harmful organisms. For example, cells use receptor-mediated endocytosis to bring in specific molecules like cholesterol bound to LDL particles.
The purpose of exocytosis is to eliminate waste, release hormones, enzymes, and neurotransmitters for signaling and communication. For example, neurons release neurotransmitters like acetylcholine into the synaptic cleft through exocytosis.
4. On the basis of Energy Requirement:
Endocytosis is an active process that requires ATP for membrane invagination, vesicle formation and movement into the cell.
Exocytosis also requires ATP for vesicle transport along the cytoskeleton and for the fusion of vesicles with the plasma membrane.
5. On the basis of Types:
Endocytosis has three main types: phagocytosis (cell eating, example - engulfment of bacteria), pinocytosis (cell drinking, example - absorption of extracellular fluid), and receptor-mediated endocytosis (example - intake of LDL cholesterol).
Exocytosis has two types: constitutive exocytosis, where substances are continuously released (example - secretion of collagen by fibroblasts) and regulated exocytosis, where release happens in response to a specific trigger (example - insulin secretion after glucose stimulation).
6. On the basis of Role in Cell Physiology:
Endocytosis plays a vital role in nutrient absorption, immune defense by engulfing pathogens, regulation of membrane receptor density and internalization of important signaling molecules.
Exocytosis helps in the secretion of hormones, delivery of membrane proteins, extracellular matrix formation, communication between neurons and removal of waste products from cells.
b) Phagocytosis and Pinocytosis
Phagocytosis and pinocytosis are both forms of endocytosis, where cells take in substances from the outside environment.
Although both processes are similar, they differ on the basis of several factors:
1. On the basis of Type of Material Engulfed:
In phagocytosis, the cell engulfs large solid particles like microorganisms, cell debris, or dust particles. For example, a macrophage engulfing a bacterium during an immune response.
In pinocytosis, the cell engulfs extracellular fluid along with dissolved small molecules and nutrients. For example, cells in the intestine absorb dissolved nutrients through pinocytosis.
2. On the basis of Nature of the Process:
Phagocytosis is often considered as "cell eating" because it involves the intake of solid matter into the cell.
Pinocytosis is termed as "cell drinking" because it involves the intake of fluids along with dissolved substances.
3. On the basis of Vesicle Size Formed:
During phagocytosis, large vesicles called phagosomes are formed that can be clearly seen under a microscope.
In pinocytosis, the vesicles formed are much smaller, and they are usually too tiny to be easily distinguished under a microscope.
4. On the basis of Specificity:
Phagocytosis is usually a selective process. Cells recognize specific particles, such as bacteria, through surface receptors before engulfing them.
Pinocytosis is generally non-specific. The cell engulfs whatever fluid and solutes are present in the extracellular environment without any specific selection.
5. On the basis of Requirement of Surface Receptors:
In phagocytosis, the binding of the particle to specific receptors on the cell surface is essential for the process to begin.
In pinocytosis, surface receptors are not necessarily required, the membrane simply folds inward randomly to engulf extracellular fluid.
6. On the basis of Energy Consumption:
Phagocytosis requires more energy as it involves the engulfment of large particles and the active extension of pseudopodia.
Pinocytosis requires relatively less energy because it involves the passive engulfment of fluid by invagination of the membrane.
7. On the basis of Occurrence:
Phagocytosis is mainly performed by specialized cells like macrophages, neutrophils and amoebas.
Pinocytosis can occur in almost all types of cells where nutrient absorption from extracellular fluid is needed.
8. On the basis of Final Fate of Vesicle:
In phagocytosis, the phagosome usually fuses with a lysosome, where the solid particle is digested by enzymes.
In pinocytosis, the small vesicles may fuse with endosomes or lysosomes where the fluid contents are processed or absorbed.
5. Discuss how the sodium-potassium pump helps in the transport of ions in animal cells.
The sodium-potassium pump (Na+/K+ ATPase) is an essential membrane-bound protein found in the plasma membrane of animal cells. This pump helps maintain the proper concentration of sodium (Na⁺) and potassium (K⁺) ions across the cell membrane, which is vital for many cellular processes. It is an example of an active transport mechanism because it requires energy in the form of ATP to move ions against their concentration gradients. This process is critical for maintaining cellular homeostasis, resting membrane potential, and ensuring proper nerve and muscle cell function.
The sodium-potassium pump actively pumps three sodium ions out of the cell and two potassium ions in, using energy derived from ATP hydrolysis. This transport system not only plays a crucial role in ion balance but also influences other cellular activities by maintaining ionic gradients across the cell membrane.
How the Sodium-Potassium Pump Helps in the Transport of Ions in Animal Cells
The process of ion transport by the sodium-potassium pump occurs in a series of several steps, each playing a crucial role in ensuring the efficient exchange of sodium and potassium ions.
Step 1: Binding of Sodium Ions
The process begins when the pump binds three sodium ions from the cytoplasm. The pump has specific binding sites that selectively recognize sodium ions. This binding induces a slight conformational change in the protein's structure, preparing it for the next action.
Step 2: ATP Hydrolysis
Once the sodium ions are attached, the pump hydrolyzes an ATP molecule into ADP and inorganic phosphate (Pi). The energy released from this reaction causes a major conformational shift in the pump. This energy-dependent change is crucial because it allows the pump to move sodium ions against their concentration gradient.
Step 3: Release of Sodium Ions
Following the conformational change, the pump opens towards the outside of the cell and releases the three sodium ions into the extracellular space. This step lowers the sodium ion concentration inside the cell, helping maintain osmotic balance and proper cell volume.
Step 4: Binding of Potassium Ions
After releasing sodium ions, the pump's new shape allows it to bind two potassium ions from the extracellular fluid. The binding sites are highly specific for potassium ions, ensuring only the correct ions are transported.
Step 5: Release of Potassium Ions Inside the Cell
With potassium ions attached, the pump reverts to its original conformation, facing the cytoplasm. As it does so, the two potassium ions are released inside the cell, increasing the intracellular potassium concentration.
Step 6: Return to Original State
After releasing the potassium ions, the pump returns completely to its original form, ready to begin a new cycle. This continuous operation maintains the essential sodium and potassium gradients required for various physiological activities.
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