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Cell Membrane – Definition, Functions, Structure

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  • Last Updated : 03 Jul, 2022
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A cell is a cytoplasmic mass that is outwardly bound by a cell membrane. Cells, which are typically tiny in size, are the smallest structural units of biological matter and comprise all living organisms. Most cells have one or more nuclei as well as other organelles that perform a range of functions. Some single cells, such as bacteria or yeast, are entire creatures. Others are multicellular creatures’ specialized building blocks, such as plants and animals.

Cell Membrane 

A cell membrane is a thin semi-permeable membrane that surrounds and encloses a cell’s cytoplasm. The membrane also provides a cell structure and allows it to join to neighboring cells to form tissues. In certain animals, the cell membrane also acts as a basis of attachment for the cytoskeleton, whereas in others, it functions as the cell wall. As a result, it aids in cell support and shape maintenance. The cell membrane regulates the passage of substances into and out of cells by being selectively permeable to ions and organic molecules. The cell membrane’s primary role is to shield the cell from its environment.

The plasma membrane is another name for the cell membrane. It is the outside layer of animal cells. It is made up of lipids and proteins and is semi-permeable. The cell membrane’s primary roles are as follows:

  • Keeping the inside cell’s integrity intact.
  • Providing support and keeping the cell’s form.
  • Helps to regulate cell development by balancing endocytosis and exocytosis.
  • The cell membrane is also involved in cell signaling and communication.
  • It functions as a selectively permeable membrane, admitting only certain chemicals enter the cell.

Important Functions

  • The cell membrane is a multilayered membrane that surrounds the cytoplasm of a cell. It maintains the cell’s integrity while also supporting and aiding to maintain the cell’s form.
  • The cell membrane is mostly composed of proteins and lipids. The precise mix or ratio of proteins and lipids might differ depending on the function of a particular cell.
  • Phospholipids play a crucial role in cell membranes. They spontaneously organize to produce a semi-permeable lipid bilayer through which only specific molecules may permeate to the cell’s interior.
  • Some cell organelles, like the cell membrane, are surrounded by membranes. Two examples are the nucleus and mitochondria.

Structure of Cell Membrane 

The cell membrane is composed of two phospholipid-based layers. The bilayer is generated by arranging phospholipids such that their head regions (which are hydrophilic) face both the exterior and internal cytosolic environments. These phospholipids’ (hydrophobic) tails are facing each other. Electrostatic, van der Waals, non-covalent interactions, and hydrogen bonding are the forces that drive the development of this bilayer. This unusual configuration of hydrophilic and hydrophobic layers prevents nucleic acids, amino acids, proteins, carbohydrates, and ions from passing through the bilayer. The many components of the cell membrane are listed below.

Integral Membrane Proteins 

Integral Membrane Proteins are structures found on the inside, outside, and all throughout the cell membrane. These proteins may be seen using fluorescence and electron microscopy. These proteins are found on the cell membrane’s entire/complete surface. Cadherins, integrins, clathrinid-coated pits, desmosomes, caveola’s, and other structures are examples of these structures.

Peripheral Membrane Proteins  

These proteins are associated with the membrane’s surface via hydrogen bonds and electrostatic interactions. The hydrophilic phospholipid heads that compose the bilayer create the hydrogen bonds of these peripheral proteins.

Cell Membrane Skeleton 

The cytoskeleton lines the surface of the cell membrane on the cytoplasmic side. The framework, or cytoskeleton, is important in the operations of organelles such as cilia. The cytoskeleton also aids in the attachment of membrane proteins to the cell membrane.

Cell Membrane Composition 

 Proteins and lipids are major components of the cell membrane. Different processes are responsible for incorporating and removing materials into  and out of the membrane. The fusing of the cell membrane with intracellular vesicles leads to the expulsion of the contents of the vesicles.

Cell Membrane

 

Phospholipids: Phospholipids are an important component of cell membranes. Phospholipids naturally arrange their hydrophilic (attracted to water) head portions to face the aqueous cytosol and extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, enabling only specific molecules to pass through.

Carbohydrates: Carbohydrates, or sugars, are occasionally discovered coupled to proteins or lipids on the exterior of a cell membrane. They can only be found on the extracellular side of a cell membrane. These sugars combine to produce the glycocalyx.

Glycolipids: Glycolipids are carbohydrate sugar chains that are linked to cell membrane surfaces. They assist the cell in recognizing other cells in the body.

Cholesterol: Another lipid component of animal cell membranes is cholesterol. Cholesterol molecules are spread preferentially between membrane phospholipids. Preventing phospholipids from being too densely packed together, helps to protect cell membranes from becoming rigid. Cholesterol is not present in plant cell membranes.

Functions of Cell Membrane 

A cell membrane shields the cell’s structures. Cell membranes are semipermeable, which means that only specific items may flow through them. Cell membranes also form and sustain the cell’s structure.

  • To keep the cell’s physical integrity that is, to physically surround the contents of the cell and to govern the passage of particles, such as ions or molecules, into and out of the cell.
  • The cell membrane protects the cell’s physical integrity. In the case of animal cells (which lack cell walls), the cell membrane binds the cell together by encapsulating the cytoplasm and organelles inside it.
  • The cell membrane physically divides internal components (eukaryotic cells’ organelles) from the external environment. The cell membrane shields the cell from some of the hazardous substances found in its surroundings.
  • It also prevents the loss of valuable biological macromolecules retained within the cell by the plasma membrane.
  • A cell membrane is composed of proteins and lipids. A cell membrane contains three types of proteins: structural proteins, transport proteins, and glycoproteins. These maintain cell structure and form, transport substances across the membrane, and send messages between cells.
  • Cell membranes frequently include receptor sites for certain biochemicals such as hormones, neurotransmitters, and immunological proteins. This allows the cell to perceive and interpret signals from the extracellular environment.
  • Cell membranes are selectively permeable and enclose cells (within the cell wall in the case of plant cells and prokaryotic cells). That is, the structure of these membranes allows certain particles, such as molecules, but not others, to flow through the membrane and therefore into or out of the cell.
  • The process by which cells ingest molecules by engulfing them is known as endocytosis. The plasma membrane forms a tiny inward distortion termed an invagination, which captures the material to be delivered. Endocytosis is the process by which solid particles (“cell eating” or phagocytosis), tiny molecules and ions (“cell drinking” or pinocytosis), and macromolecules are internalized. Endocytosis is a type of active transport since it requires energy.

Conceptual Questions 

Question 1: Write any three major functions of a cell wall.

Answer:  

  • Provides form and stiffness.
  • Protects the protoplasm.
  • Involved in the flow of materials in and out.

Question 2: Where do chromosomes reside in a cell? Explain what they do?

Answer : 

They are situated in the cell’s nucleus, and each chromosome contains genes that are responsible for some attribute or characteristic. These genes pass these characteristics down from parents to kids.

Question 3: What is the cell membrane’s function?

Answer:

The cell membrane, unlike the cell wall, is found in all living creatures, including plants. The cell membrane’s primary role is to shield the cell from its environment.

Question 4: Which chemical is found in both the cell membrane and space?

Answer:  

The Ethanolamine molecule, which is found in the cell membranes of all living things on Earth, was discovered in space. In species on Earth, the Ethanolamine molecule (oxygen, nitrogen, hydrogen, carbon, phosphorus, and Sulphur) is important for allowing genetic and metabolic processes to occur.

Question 5: What do you mean by cytoplasm?

Answer:

The cytoplasm is the live component of the protoplasm between the plasma membrane and the nucleus.

Question 6: What is the name of a cell’s boundary?

Answer:

The cell membrane, also known as the plasma membrane, is the border of the cell. The cell membrane shields the cell or its inner components from the outside environment. It gives the cell form and a distinct structure.


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