Describe plasma membrane structure as given by Singer and Nicolson

The plasma membrane or cell membrane is a flexible protective layer surrounding cells that controls the movement of substances in and out and supports cell signaling as well as maintains cellular homeostasis. It is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol and carbohydrates.

The concept of the plasma membrane was first recognized in the late 19th century, with its structural understanding advancing in 1972 when S.J. Singer and G.L. Nicolson proposed the Fluid Mosaic Model and illustrating the membrane's dynamic and fluid nature.

Fluid Mosaic Model

The structure of the plasma membrane is best described by the fluid mosaic model. The fluid mosaic model of the plasma membrane proposed by S. J. Singer and G. L. Nicolson in 1972, describes cell membranes as a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol and carbohydrates. The bilayer's hydrophilic heads face outward, while hydrophobic tails are shielded inward, allowing lateral movement of lipids and proteins. Integral proteins span the membrane for transport and signaling, while peripheral proteins are attached to the surface. Carbohydrates on glycoproteins and glycolipids facilitate cell recognition. Cholesterol stabilizes membrane fluidity. This model highlights the membrane's flexibility and functionality, enabling essential processes like communication, transport, and response to environmental changes.

Structure of the Plasma Membrane

Phospholipid Bilayer

At the core of the plasma membrane is the phospholipid bilayer, which consists of two layers of phospholipid molecules. Each hospholipid consist of a hydrophilic head (water-attracting) and two hydrophobic tails (water-repelling):
  1. Hydrophilic Head: This part of the molecule is attracted to water. It contains a phosphate group that interacts with the aqueous environments inside and outside the cell.
  2. Hydrophobic Tails: These are long fatty acid chains that repel water. There are usually two tails per phospholipid molecule.
When phospholipids are exposed to water, they arrange themselves in such a way that the hydrophilic heads face outward, towards the water, while the hydrophobic tails face inward, shielded from the water. This orientation forms a bilayer, creating a semi-permeable barrier that controls what substances can pass in and out of the cell.

Proteins

The plasma membrane is embedded with a variety of proteins, which play critical roles in its function. These proteins can be classified into two main categories:
  1. Integral Proteins: These proteins span across the phospholipid bilayer and can either fully traverse the membrane (transmembrane proteins) or partially embed within one layer. Integral proteins often function as:
    • Transport Proteins: These proteins facilitate the movement of ions and molecules across the membrane. They can function as channels, allowing specific molecules to pass through, or as carriers that change shape to transport substances.
    • Receptor Proteins: These proteins bind to signaling molecules, such as hormones or neurotransmitters, and trigger responses inside the cell. This is essential for cell communication and coordination of activities.
  2. Peripheral Proteins: These proteins are located on the inner or outer surface of the membrane. They are not embedded within the lipid bilayer. Peripheral proteins serve various functions, including:
    • Enzymatic Activity: Some peripheral proteins act as enzymes, facilitating biochemical reactions.
    • Structural Support: They help maintain the cell's shape and are connected to the cytoskeleton (a network of fibers within the cell).

Carbohydrates

Carbohydrates are also an important component of the plasma membrane, often attached to proteins and lipids:
  • Glycoproteins: These are proteins with carbohydrate chains attached. They are involved in cell recognition and communication, helping the immune system distinguish between self and non-self cells.
  • Glycolipids: These are lipids with carbohydrate groups. They also play a role in cell recognition and stability.
The carbohydrate portions of these molecules extend into the extracellular space, forming a protective layer known as the glycocalyx. This layer is important for cell signaling, protection, and interaction with other cells.

Cholesterol

Cholesterol molecules are interspersed within the phospholipid bilayer. Cholesterol is essential for maintaining the fluidity and stability of the plasma membrane:
  • Fluidity: Cholesterol helps to prevent the fatty acid chains of the phospholipids from packing too closely together, which maintains the membrane's flexibility. This is particularly important in maintaining membrane integrity at varying temperatures.
  • Permeability: By filling gaps between phospholipids, cholesterol reduces the permeability of the membrane to small water-soluble molecules, thus enhancing the cell's selective barrier function.



Read More:

SAQ 1

Fill in the blanks 
a) ............. discovered plasma membrane.

b) The phospholipid contains .................. charged phosphate group in the hydrophilic part of head.

c) ................... proposed Sandwich (lipid-protein) model of cell membrane.

d) The protein layer present in cell membrane model proposed by Robertson is .................. thick.

e) The proteins are aligned properly with the help of ....................... within the lipid bilayer in membrane.

Answers: (a) Karl Nageli and C. Cramer, (b) Negatively, (c) Danielli and Davson, (d) 20 A°, (e) Transmembrane segments

SAQ 2

i) Answer in one word:
a) Complex integral proteins transmit signals via plasma membrane.

b) The cellular processes such as movement, growth, division etc. are regulated by this property of membrane.

c) No energy is required for transter of substances from high concentration zone to low concentration zone in this proces.

d) Certain temporarily opening passagelways that work only in response to a binding of ligand to cell.

e) The property of membrane that assists in transfer of some materials through the membrane restricting the entry of others.

Answers: (a) Receptors, (b) Fluidity, (c) Passive, (d) Gated pores or gated channels. Gated pores open in response, (e) Amphipathic.

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




4. Differentiate between:
     a) Endocytosis and Exocytosis




Comments

Post a Comment

Popular posts from this blog

UNIT 16 – Cell Death and Renewal (Q&A) | MZO-001 MSCZOO | IGNOU

Image
SAQ i) Match the items in column I with column II: Answer: a) → iv;    b) → iii;    c) → vii;    d) → viii;    e) → vi;    f) → i;    g) → ii;    h) → x;    i) → v;    j) → xi;    k) → ix ii) Fill in the blanks: a) Autophagy includes the formation of three different vesicles including ....................., which circularizes to form ......................, which then fuses with a lysosome to form the ..................... . Answer: phagophore, autophagosome, autolysosome b) Apoptosis can be mediated by two modes or pathways, ....................... or ....................... . Answer: intrinsic, extrinsic c) Pharmacological inhibitors such as ....................... block the lysosomal proton transport and thus autophagy. Answer: Bafilomycin A1 d) ...................... is released from the mitochondria upon MOMP and binds to the adapter protein Apaf1 to form the complex known as ..........
Read more

UNIT 1 – Mendel's Laws (Q&A) | MZO-002 MSCZOO | IGNOU

Image
SAQ 1 Fill in the blanks a) The title of the paper presented by Mendel in 1866 is ......................... . Answer: Experiments in Plant Hybridization b) The term 'genetics' was coined by ........................ . Answer: William Bateson c) Mendel proposed the concept of ...................... units. Answer: hereditary   d) Mendel's work was rediscovered by ........................., ........................... and .......................... . Answer: Carl Correns, Hugo de Vries and Erich von Tschermak SAQ 2 Answer in one word. a) In Mendelian crosses, progeny is produced from the mating of individuals in the parental generation. Answer: first filial generation or F1 b) The factors that regulate the inheritance of traits and act like discrete particles that remain separate. Answer: hereditary factors (genes) c) A genetic cross involving a single (mono) trait. Answer: monohybrid cross d) A trait/factor that remains hidden and not fully expressed in the progeny. Answer: re...
Read more

What are the events involved in the S-phase of Interphase?

Image
The S-phase (Synthesis phase) is a critical part of interphase in the cell cycle, occurring between the G1 (gap 1) phase and the G2 (gap 2) phase. During this phase, the cell duplicates its DNA to prepare for cell division, ensuring that the genetic material is accurately passed on to the daughter cells. This phase is essential for maintaining genomic integrity and stability. Key Events in the S-phase 1. DNA Replication: The main event of the S-phase is DNA replication. This process ensures that the entire genome is copied so that each daughter cell will have an identical set of chromosomes. The helicase enzyme unwinds the DNA double helix, creating two single strands. These single strands act as templates for the synthesis of new complementary strands. 2. Activation of DNA Polymerases: DNA polymerases are key enzymes that catalyze the addition of new nucleotides to the growing strand. On the  leading strand,  DNA polymerase synthesizes continuously in the 5' to 3' direction. ...
Read more

UNIT 2 – Gene Action and Interactions (Q&A) | MZO-002 MSCZOO | IGNOU

Image
SAQ 1 Fill in the blanks with appropriate word: a) A condition in which a gene or gene pair suppresses or hinders the expression of another non-allelic gene is called ...................... . Answer: epistasis b) The phenotype effect gets enhanced because of the effect of ....................... genes. Answer: cumulative or additive  c) Red eye colour in Drosophila melanogaster is due to the deposition of pigment ...................... . Answer: drosopterins d) Progeny shows a ratio of ........................ in case of duplicate dominant epistasis.  Answer: 15 : 1 e) Epistasis in which a dominant allele at one locus can mask the expression of both (dominant and recessive) alleles at the second locus occurs due to .......................... gene interaction. Answer: inhibitory SAQ 2 State whether these statements are 'True' or 'False.' a) Pleiotropy occurs when a gene shows a complementary gene interaction with another gene. Answer: False b) Pleiotropic effects of albi...
Read more

Give a brief note on the control of the Cell Cycle

The cell cycle is a highly regulated process that ensures cells grow, replicate their DNA and divide accurately. Its control is crucial for normal development, tissue repair and prevention of diseases like cancer. This control is achieved mainly through a combination of regulatory proteins, checkpoints, inhibitory pathways and external signaling factors, all of which coordinate to monitor and regulate the progression of the cycle at every stage. 1. Cyclins and CDKs – The Core Regulators Cyclin-dependent kinases (CDKs) are special enzymes that become  active only when  they bind to a protein called  cyclin,  to form a  cyclin-CDK complex.  Different cyclins appear and disappear at specific times in the cell cycle, and this timing controls CDK activity. Each cyclin-CDK complex triggers important events of a particular phase of cell cycle. In the G1 phase,  Cyclin D binds to CDK4 or CDK6 to push the cell toward the S phase. During the S phase,  Cycli...
Read more

What is intracellular protein turnover?

Intracellular protein turnover is the continuous and regulated process by which proteins inside a cell are  synthesized  (built or formed) and  degraded  (broken down). This process plays a vital role in maintaining the cell's internal balance, known as  proteostasis.  It ensures that old, misfolded, damaged, and excess proteins are efficiently removed and replaced with newly synthesized, functional proteins. This dynamic balance between synthesis and degradation helps the cell adapt to changes, control growth, respond to stress and perform normal physiological functions. Each protein inside the cell has a specific half-life or lifespan. Some proteins are very short-lived and degrade within minutes, while others may last for hours or days. When a protein becomes non-functional, either due to damage or mutation, it must be removed. If this removal does not happen properly, faulty or misfolded proteins may accumulate and cause various diseases, including canc...
Read more

What is Cyclin-CDKs kinases? Write a brief note on the relation of cyclin with CDKs

Image
Cyclin-CDK kinases are enzyme complexes that control the progression of the eukaryotic cell cycle. These complexes consist of two main components: a  cyclin protein  and a  cyclin-dependent kinase (CDK).  CDKs are serine/threonine protein kinases that are present in the cell in an  inactive form.  They require the binding of a regulatory protein, called a  cyclin,  to become  active.  Once a cyclin binds to a CDK, the complex becomes enzymatically active and can phosphorylate various target proteins involved in controlling key steps of the cell cycle, such as DNA replication, chromosome condensation and mitotic spindle formation. The activity of cyclin-CDK complexes is regulated at multiple levels, including cyclin synthesis and degradation, phosphorylation and dephosphorylation of CDKs, and the presence of CDK inhibitors (CKIs). This regulation ensures that each phase of the cell cycle occurs only once and in the proper order, preventin...
Read more

What is the role of the p53 tumour suppressor gene in preventing cancer?

The p53 gene is a very important tumour suppressor gene found in almost every cell of the body. It produces a protein called  p53 protein,  which is also known as the  "guardian of the genome".  Its main job is to keep the cell healthy by checking the condition of the DNA. If any damage or mutation is found, p53 takes action to stop that cell from becoming a cancer cell. How p53 Prevents Cancer p53 protects the body from cancer in many ways. Here are its main roles: 1. DNA Damage Detection p53 is always monitoring the DNA. When it finds any damage or mutation, it quickly becomes active. This is the first step. If such damage is ignored, the cell may continue dividing with mistakes, which can lead to cancer. So, p53 first detects problems and prepares the cell to respond. 2. Halting the Cell Cycle Once damage is found, p53 stops the cell cycle to give the cell enough time to repair the DNA. It does this by increasing the production of a protein called  p21, ...
Read more

UNIT 3 – Gene Structure and Function (Q&A) | MZO-002 MSCZOO | IGNOU

Image
SAQ 1 i) Define Recon, Muton and Cistron. The terms Recon, Muton and Cistron were introduced by Seymour Benzer during the 1950s to study the detailed structure and function of genes at the molecular level. He worked on the rII region of T4 bacteriophage and used bacteriophage genetics to analyze how small changes in DNA affect phenotypes. At that time, the gene was considered as a single indivisible unit. But Benzer showed that a gene has a finer internal structure and can be divided into smaller functional units. Based on this, he proposed three molecular units: Recon, Muton and Cistron, each having a specific role related to recombination, mutation and expression. Recon Recon is defined as the smallest unit of recombination. It refers to the smallest segment of DNA within which crossing over cannot occur, but recombination can occur between two such units. According to modern molecular understanding, recombination between two genes or within a gene occurs at the level of nucleotid...
Read more

How is the p53 protein activated?

The p53 protein is a tumour suppressor that remains inactive under normal cell conditions. Its activation is carefully regulated and occurs only when the cell experiences  stress,  especially DNA damage. This activation helps the cell decide whether to pause the cycle for repair or undergo apoptosis. The process of activation mainly depends on blocking its negative regulator and allowing p53 to become stable and active. Mechanism of p53 Activation The activation of p53 happens mainly in response to DNA damage, oxidative stress, hypoxia, or oncogene activation. The steps involved are: Step 1: Detection of DNA Damage When DNA is damaged due to UV rays, radiation, chemicals, or errors during replication, special sensor proteins such as  ATM (Ataxia Telangiectasia Mutated)  and  ATR (ATM and Rad3-related)  are activated. These proteins sense the damage and initiate a signalling cascade. Step 2: Activation of Checkpoint Kinases ATM and ATR then activate downstre...
Read more