Last Updated 20 Jun 2022

Describe the Fluid-Mosaic Model of a Plasma Membrane

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Describe the fluid-mosaic model of a plasma membrane. Discuss the role of the membrane in the movement of materials through it by each of the following processes:

  • Active transport
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  • Passive transport

The plasma membrane is a semi permeable barrier that separates the inside of the cell from the outside environment. The plasma membrane is made up of carbohydrates, cholesterol, proteins, and a lipid bilayer, or double layer of lipids. The plasma membrane may be known as a fluid mosaic model where the membrane is a fluid structure with various proteins embedded in or attached to the bilayer of phospholipids.

The plasma membrane possesses hydrophilic tails and hydrophobic tails, which may be referred to as amphiphilic. There are various ways that materials may pass through the membrane. Movement across the membrane may be classified into two different categories, passive transport and active transport. Passive transport does not require energy to occur. During passive transport the molecules will move from a place of high concentration to a place of low concentration.

In other words, the molecules are moving down their concentration gradient. A concentration gradient is the increase or decrease in the density of a chemical substance in an area. The three types of passive transport are diffusion, osmosis, and facilitated diffusion. Diffusion is the tendency for molecules of any substance to spread out into the available space. The plasma membrane is semi permeable so diffusion across the plasma membrane may only occur with a few substances such as oxygen, carbon dioxide, and alcohol.

Osmosis is the diffusion of water across a selectively permeable membrane. When osmosis occurs in the plasma membrane the molecules will move from hypotonic to hypertonic. Facilitated Diffusion is polar molecules and ions impeded by the lipid bilayer of the plasma membrane diffusing passively with the help of transport proteins that p the membrane. The transport protein, aquaporins, will allow for water to diffuse passively. There are other transport proteins, such as glucose umps, that aid with the movement of materials. Active transport is movement across the membrane that does require energy to occur. ATP will supply the energy for active transport. During active transport, molecules will move from places of low concentration to places of high concentration. Active transport requires energy because it must transport the molecules against their concentration gradient. There are four different subdivisions of active transport, primary, secondary, endocytosis, and exocytosis.

Primary active will manifest in the form of certain transport proteins that require energy to function, such as the sodium-potassium pump. Secondary active transport is another subdivision of active transport. During secondary active transport molecules may move by symtransport, which is particles moving in the same direction, or antitransport, which is particles moving in the opposite direction. Primary and secondary active transport is utilized for small particles to be transported across the plasma membrane.

Endocytosis is the cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the substance and pinch off to form an intracellular vesicle. Endocytosis is for materials that may be entering the cell. Endocytosis may be completed by phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis is a cell engulfing a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac to be digested. Pinocytosis is a cell “gulping” droplets of extracellular fluid into tiny vesicles.

Receptor-mediated endocytosis is the movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in. Receptor-mediated endocytosis is accomplished with the help of hormones. Exocytosis is the cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane. Exocytosis is for materials that may be exiting the cell. Exocytosis may also be referred to as secretion or excretion. Secretion is the expulsion of digestive enzymes while excretion is the expulsion of waste.

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