Differentiate between Phagocytosis and Pinocytosis
Phagocytosis
Phagocytosis, also known as cell eating, is a form of endocytosis where a cell engulfs large particles, such as bacteria, cellular debris or other microorganisms, to internalize and digest them. It plays a crucial role in the immune response, allowing specialized cells like macrophages and neutrophils to remove pathogens and dead or damaged cells from the body. In phagocytosis, the cell membrane extends around the target particle, forming a vesicle called a phagosome. The phagosome then fuses with a lysosome, which contains enzymes that break down the engulfed material. This process makes the material harmless, allowing the cell to recycle useful components and dispose of waste.
Pinocytosis
Pinocytosis, often referred to as cell drinking, is a cellular process by which cells ingest extracellular fluid along with dissolved solutes. This type of endocytosis allows cells to take in small molecules and nutrients that are essential for their metabolic functions. Unlike phagocytosis, pinocytosis is not selective and usually involves the formation of small vesicles through invaginations of the cell membrane, which trap fluid from the environment. These vesicles, once internalized, can fuse with endosomes, where the dissolved nutrients are processed and utilized by the cell. Nearly all cell types use pinocytosis to maintain nutrient balance.
Differences between Phagocytosis and Pinocytosis
Phagocytosis and pinocytosis, both being types of endocytosis, share a common mechanism of engulfing materials into the cell. However, they have distinct functions, mechanisms, and roles in cellular biology.
Here's detailed comparison of phagocytosis and pinocytosis across several factors:
Based on Size of Material Engulfed
Phagocytosis
Phagocytosis is designed to capture large particles, usually bigger than 0.5 micrometers. The particles can be as large as whole cells, like bacteria or as small as cellular debris. Because the materials are so large, the cell forms bigger vesicles, called phagosomes, to hold them.
Pinocytosis:
In contrast, pinocytosis only deals with much smaller particles, which are dissolved in fluid. These particles are generally in the nanometer range, such as ions, simple sugars and amino acids. The vesicles in pinocytosis, called pinosomes, are much smaller than those in phagocytosis, as they only hold fluids and tiny molecules.
Based on Purpose and Function
Phagocytosis:
The primary purpose of phagocytosis is defense. Immune cells, such as macrophages and neutrophils, use phagocytosis to engulf and destroy harmful invaders, like bacteria and viruses. Another purpose is cellular cleanup, where phagocytosis removes dead or dying cells, clearing space for healthy cells to function properly. This process is crucial for maintaining tissue health and for immune defense.
Pinocytosis:
The main function of pinocytosis is nutrient absorption. By continuously taking in extracellular fluid, cells obtain a steady supply of nutrients dissolved in the fluid, such as glucose, amino acids, and ions. Pinocytosis is essential for cells to maintain nutrient levels, stay hydrated, and carry out regular cellular functions.
Based on Selectivity
Phagocytosis:
Phagocytosis is a highly selective process. The cell uses receptors to recognize specific molecules on the surface of harmful particles, like pathogens, or dead cells. This selectivity ensures that only unwanted or harmful materials are engulfed, avoiding damage to healthy cells or tissues.
Pinocytosis:
Pinocytosis is non-selective. The cell takes in fluid and dissolved substances without choosing specific particles. This lack of selectivity allows the cell to constantly absorb whatever nutrients or other small molecules happen to be present in its surroundings, but it also means that some unwanted materials may enter.
Based on Process and Mechanism
Phagocytosis:
The process of phagocytosis begins when a cell detects a target particle, such as a bacterium, through specific receptors on its surface. Once recognized, the cell membrane extends outwards, forming "arms" called pseudopodia. These pseudopodia reach around the particle and enclose it completely, forming a large vesicle called a phagosome. This phagosome then merges with another organelle called a lysosome, which contains enzymes that break down the particle.
Pinocytosis:
In pinocytosis, the process is simpler. The cell membrane forms small inward folds or pockets, which capture some of the fluid outside the cell along with any dissolved nutrients. These pockets pinch off, forming small vesicles called pinosomes inside the cell. Unlike phagocytosis, pinocytosis doesn’t involve pseudopodia or require the cell to recognize specific targets.
Based on Energy Requirements
Phagocytosis:
Phagocytosis is an energy-intensive process. It requires ATP (the cell's energy molecule) to extend the cell membrane, form pseudopodia, and transport the phagosome to fuse with the lysosome. The entire process is complex, as it also involves numerous proteins and cellular structures.
Pinocytosis:
Pinocytosis requires less energy than phagocytosis, as it does not involve the movement of large cell extensions or the recognition of specific particles. However, it still requires ATP for forming vesicles and transporting them within the cell.
Based on Types of Cells Involved
Phagocytosis:
Phagocytosis is primarily performed by specialized immune cells, including macrophages, neutrophils and dendritic cells. These cells are equipped with the necessary receptors and mechanisms for engulfing large particles. Not all cells can perform phagocytosis, as it requires specific adaptations.
Pinocytosis:
Pinocytosis is carried out by almost all cell types, as it is necessary for nutrient uptake and fluid balance. Cells in the intestines, for example, use pinocytosis to absorb nutrients from digested food. Since it does not require special receptors or structures, it is a process that nearly all cells perform regularly.
Based on Role in Immune Response
Phagocytosis:
Phagocytosis is a key part of the immune system. When immune cells like macrophages engulf pathogens, they break down the pathogen and present its fragments (called antigens) on their surface. This “antigen presentation” helps activate other immune cells, which then build a stronger immune response. Phagocytosis not only removes harmful particles but also plays a role in “educating” the immune system to recognize future threats.
Pinocytosis:
Pinocytosis does not directly contribute to the immune response since it’s non-selective. However, certain immune cells may use pinocytosis to sample their surroundings, giving them an idea of what substances or molecules are present in the area, which can help in monitoring the environment.
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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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