Write a note on the important functions of the plasma membrane
The plasma membrane, also known as the cell membrane, is a selectively permeable barrier that surrounds the cell, separating the cell's internal environment from its external environment. Composed primarily of a phospholipid bilayer with embedded proteins as well as cholesterol and carbohydrate molecules. It regulates the movement of substances in and out of the cell, provides structural support, and facilitates communication between cells. This dynamic structure enables cells to maintain homeostasis and respond to external signals.
Discovery of the Plasma Membrane
- The concept of the plasma membrane began taking shape in the 19th century. In 1855 Carl Nageli and C. Cramer proposed that cells have an outer boundary, although they did not describe its specific structure.
- The first concrete evidence of a "cell boundary" was provided by Ernst Overton in the 1895, when he found that lipid-soluble substances entered cells more easily than others, suggesting that the boundary had a lipid nature.
- In 1925, Gorter and Grendel proposed the lipid bilayer structure of cell membranes based on surface area measurements from red blood cells, suggesting a double layer of lipids. Later advancements, especially electron microscopy in the 1950s, helped confirm membrane thickness and provided more evidence supporting the bilayer structure hypothesis.
- The detailed fluid mosaic model of the plasma membrane, as we understand it today, was developed in 1972 by S.J. Singer and G.L. Nicolson. This model described the membrane as a fluid structure with proteins embedded within or attached to a bilayer of phospholipids, forming a mosaic-like pattern.
Important Functions of the Plasma Membrane
The plasma membrane is a vital cell structure that acts as a selective barrier, controlling the entry and exit of substances to maintain cellular balance. Composed of a phospholipid bilayer with embedded proteins, it supports communication, protection, and structural integrity, enabling cells to interact with their environment and function efficiently.
Here are the important functions of the plasma membrane:
1. Protection and Compartmentalization
One of the primary roles of the cell membrane is to act as a protective barrier, effectively separating the internal cellular environment from the external surroundings. By encapsulating the contents of the cell, the membrane keeps essential molecules, such as DNA, proteins, and ions, confined within a stable, controlled environment. This compartmentalization is crucial for maintaining homeostasis, as it allows the cell to carry out metabolic reactions and regulate the conditions for these processes independently of external changes. The membrane acts as a "gatekeeper," preventing potentially harmful substances or pathogens from entering while retaining valuable molecules inside the cell.
2. Regulation of Transport
A key function of the cell membrane is its selective permeability, which enables it to regulate the entry and exit of substances. This regulation is achieved through various mechanisms, allowing the cell to balance its internal composition according to its needs and environmental conditions.
- Passive Transport: Passive transport does not require cellular energy (ATP) and involves the movement of molecules down their concentration gradient (from areas of high to low concentration). Examples include:
- Simple Diffusion: Small, nonpolar molecules like oxygen and carbon dioxide can diffuse directly through the phospholipid bilayer.
- Facilitated Diffusion: For larger or polar molecules, such as glucose, specific transport proteins in the membrane act as channels or carriers to facilitate their movement.
- Osmosis: The diffusion of water across the cell membrane is controlled by osmosis. Water moves through special channel proteins called aquaporins, balancing the water concentration on both sides of the membrane.
- Active Transport: In cases where molecules need to be moved against their concentration gradient (from low to high concentration), the cell uses active transport. This process requires energy, usually in the form of ATP, to power transport proteins such as pumps. Examples include:
- Sodium-Potassium Pump: This pump actively moves sodium ions out of the cell and potassium ions in, creating a concentration gradient that is vital for nerve impulse transmission and muscle contraction.
- Endocytosis and Exocytosis: For large molecules or particles, cells use membrane-based vesicular transport mechanisms. In endocytosis, the membrane engulfs external particles, bringing them into the cell. In exocytosis, intracellular vesicles fuse with the membrane to release their contents outside the cell. These processes are essential for nutrient uptake, waste removal, and secretion of signaling molecules.
3. Cell Communication and Signal Transduction
The cell membrane plays a crucial role in communication and signal transduction, enabling the cell to sense, interpret, and respond to signals from its environment. This ability to communicate is fundamental to multicellular organisms, where cells must coordinate their activities to maintain overall function and health.
- Receptors and Signaling Molecules: The membrane contains specialized protein receptors that detect chemical signals, such as hormones, neurotransmitters, or growth factors. When a receptor binds to its specific signaling molecule, it triggers a cascade of reactions inside the cell. This process, known as signal transduction, can result in various cellular responses, including changes in gene expression, metabolic activity, or cell movement.
- Ion Channels and Electrical Signaling: Certain cells, like neurons and muscle cells, rely on the movement of ions across the cell membrane to generate electrical signals. Ion channels, which are protein structures in the membrane, open or close in response to specific signals, allowing ions like sodium, potassium and calcium to flow in or out. These ion movements create voltage changes across the membrane, which are essential for processes like nerve impulse transmission and muscle contraction.
4. Structural Support and Shape Maintenance
The cell membrane provides structural support to maintain the cell's shape, especially in cells that lack a rigid cell wall, such as animal cells. The membrane anchors to the cell's internal cytoskeleton, a network of protein filaments that provide structure, support, and movement within the cell.
- Membrane Cytoskeleton Interaction: Peripheral proteins on the inner side of the membrane connect to cytoskeletal components, helping to stabilize the cell's shape and providing resilience against mechanical stress. This interaction also allows cells to change shape when needed, as seen in cells that can move or engulf particles (like white blood cells during phagocytosis).
- Dynamic Resilience: The flexibility of the cell membrane allows it to adapt to environmental pressures. Cholesterol within the membrane plays a role in adjusting membrane fluidity based on temperature, preventing the membrane from becoming too rigid in cold conditions or too fluid in warm conditions. This adaptability is essential for maintaining the integrity and functionality of the cell across diverse environments.
5. Interaction with Other Cells and the Extracellular Matrix
The cell membrane is instrumental in cell-cell interactions and adhesion, both of which are vital in forming tissues, establishing immune responses, and supporting communication between cells in multicellular organisms.
- Cell-Cell Recognition and Adhesion: Carbohydrate chains on the outer surface of the cell membrane form glycoproteins and glycolipids, collectively called the glycocalyx. This "sugar coating" allows cells to recognize and bind to each other through specific molecular interactions, which is critical for immune system function, as it helps cells distinguish "self" from "non-self" entities. Adhesion molecules on the membrane facilitate stable cell-cell connections, enabling the formation of tissues and cellular layers.
- Communication within Tissues: In multicellular organisms, cells need to work together for the body's overall function. Gap junctions, tight junctions, and desmosomes are membrane structures that connect adjacent cells. For example:
- Gap Junctions: These channels between adjacent cells allow ions and small molecules to pass directly from one cell to another, facilitating rapid communication, especially in heart muscle and nerve tissues.
- Tight Junctions: These seals prevent the passage of materials between cells, maintaining distinct environments on either side of an epithelial layer, such as the lining of the digestive tract.
- Desmosomes: These structures provide strong adhesion between cells, giving tissues mechanical strength, especially in areas subjected to stress, like the skin and muscles.
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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
TERMINAL QUESTIONS
4. Differentiate between:
a) Endocytosis and Exocytosis
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