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Plasma Membrane

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Plasma Membrane

An important component of a cell's function is the ability to control what can come into and out of the cell, but what separates the inside from the outside? This article will discuss the plasma membrane: its definition, structure, components, and function.

What is the Definition of the Plasma Membrane?

The plasma membrane--also known as the cell membrane--is a selectively permeable membrane that separates the cell's internal contents from its outside environment. Cells of plants, prokaryotes, and some bacteria and fungi, have a cell wall bound to the plasma membrane outside the cell.

Both prokaryotic and eukaryotic cells have a plasma membrane. The structure and components of the cell membrane are shown in Figure 1.

Plasma Membrane Plasma membrane detailed diagram Study Smarter

Figure 1. A detailed diagram of the cell membrane. Source: LadyofHats, Public domain, via Wikimedia Commons.

A plasma membrane is a selectively permeable membrane that separates the cell's internal contents from its outside environment.

Selective permeability: allows some substances to pass through while blocking other substances.

What is the Structure of the Plasma Membrane?

Let's take a closer look at the structure of the plasma membrane.

Plasma Membrane Diagram: Fluid Mosaic Model

The fluid mosaic model is the most widely accepted model describing the structure and behavior of the cell membrane. According to the fluid mosaic model, the cell membrane resembles a mosaic: it has many components, including lipids, proteins, and carbohydrates that make up the membrane plane. These components are fluid, meaning they move freely and constantly slide past one another. Figure 2 is a simple diagram showing the fluid mosaic model.

Plasma membrane: Fluid Mosaic Model | StudySmarter

Figure 2. The fluid mosaic model illustrates the cell membrane as a mosaic of protein molecules embedded and freely moving in a fluid bilayer of phospholipids. Source: Connectivid-D, CC BY-SA 4.0, via Wikimedia Commons.

What are the Components of the Plasma Membrane?

The plasma membrane is mainly composed of lipids (phospholipids and cholesterol), proteins, and carbohydrates. In this section, we will discuss each component.

Lipids (Phospholipids and Cholesterol)

Phospholipids are the most abundant lipids in the plasma membrane. A phospholipid is a lipid molecule made of glycerol, two fatty acid chains, and a phosphate-containing group.

Phospholipids are amphipathic molecules. Amphipathic molecules have both hydrophilic ("water-loving") and hydrophobic ("water-fearing") regions.

  • The phosphate group makes up the hydrophilic head.
  • The fatty acid chains make up the hydrophobic tails.

The cell membrane usually has two layers of phospholipids, with the hydrophobic tails facing inward and the hydrophilic heads facing outward. This arrangement is called a phospholipid bilayer. This arrangement is illustrated in Figure 3.

The phospholipid bilayer acts as a stable boundary between two water-based compartments. The hydrophobic tails attach to one another; they form the interior of the membrane. On the other end, the hydrophilic heads are exposed to aqueous fluids inside and outside the cell.

Plasma Membrane Phospholipid bilayer Study Smarter

Figure 3. This diagram illustrates the phospholipid bilayer. Source: OpenStax, CC BY 4.0, via Wikimedia Commons

Cholesterol is another lipid that is found in the membrane. It is composed of a hydrocarbon tail, four hydrocarbon rings, and a hydroxyl group. Cholesterol is embedded among the phospholipids of the membrane. It helps to maintain the fluidity of the membrane during temperature changes.

Proteins

Phospholipids are the main component of the plasma membrane, but proteins determine most of the membrane's functions. Proteins are not randomly distributed in the membrane; instead, they are often grouped in patches that carry out similar functions.

Two main types of proteins are embedded in the cell membrane:

  1. Integral proteins are integrated into the hydrophobic interior of the phospholipid bilayer. They can either 1) only partially go into the hydrophobic interior or 2) span across the entire membrane, known as transmembrane proteins. Transmembrane proteins are the most abundant proteins in the plasma membrane.
  2. Peripheral membrane proteins are usually attached to integral proteins or phospholipids. They are found on surfaces inside and outside of the membrane. They do not extend into the hydrophobic interior of the membrane; instead, they are usually loosely attached to the surface of the membrane.

Membrane proteins carry out different functions. There are proteins called channel proteins that create a hydrophilic channel for ions or other small molecules to pass through. Some peripheral membranes have roles in cross-membrane transport and cell communication. Other proteins are responsible for multiple functions, including enzymatic activity and signal transduction. Neurotransmitter receptors are an example of proteins involved in signal transduction. These receptors are embedded in the plasma membrane, and once a neurotransmitter, such as glutamate binds a receptor, an intracellular cascade of events leads to neuronal excitation.

Carbohydrates

Carbohydrates (sugars and sugar chains) are attached to proteins or lipids to help cells recognize each other.

  • When carbohydrate groups are attached to proteins, the molecules are called glycoproteins.
  • When carbohydrate groups are attached to lipids, the molecules are called glycolipids.

Glycoproteins and glycolipids are usually found on the extracellular part of the cell membrane. These are different for each species, among individuals of the same species, and even among the various cells of an individual. The uniqueness of the glycoproteins and glycolipids and their position on the surface of the plasma membrane enables them to function as cellular markers that allow cells to recognize each other.

For example, the four human blood types—A, B, AB, and O—are designated based on the carbohydrate part of glycoproteins found on the surface of red blood cells.

Cell-to-cell recognition is the ability of the cell to distinguish one neighboring cell from another. It is crucial to the survival of the organism. For example, cell-to-cell recognition is at work when the immune system rejects foreign cells. It is also at work when cells are sorted into different tissues and organs during the development of an embryo.

What is the Function of the Plasma Membrane?

The plasma membrane serves various functions depending on the type of the cell. These functions include structural support, protection, regulation of movement of substances into and out of the cell, and communication and cell signaling.

Structural Support and Protection

The cell membrane is a physical barrier separating the cytoplasm from the extracellular fluid. This allows activities (such as transcription and translation of genes or production of ATP) to occur inside the cell while minimizing the impact of the external environment. It also provides structural support by binding to the cytoskeleton.

The cytoskeleton is a collection of protein filaments that organize the cell's contents and gives the cell its overall shape.

Regulation of Substances Moving Into and Out of the Cell

The cell membrane controls the movement of molecules into and out of the cytoplasm. The semi-permeability of the cell membrane enables cells to block, allow, and expel different substances in specific amounts: nutrients, organic molecules, ions, water, and oxygen are allowed into the cell, while wastes and toxins are blocked from or expelled out of the cell.

Communication and Cell Signaling

The plasma membrane also facilitates communication between cells. Proteins and carbohydrates in the membrane create a unique cellular marker that allows other cells to recognize it. The plasma membrane also has receptors that molecules bind to carry out specific tasks.

Plasma Membrane - Key takeaways

  • The plasma membrane is a semi-permeable membrane that separates the cell's internal contents from its outside environment. Both prokaryotic and eukaryotic cells have a plasma membrane.
  • The fluid mosaic model is the most widely accepted model describing the structure and behavior of the plasma membrane, describing the plasma membrane as a mosaic of protein molecules embedded and freely moving in a fluid bilayer of phospholipids.
  • The plasma membrane is composed mainly of lipids (phospholipids and cholesterol), proteins, and carbohydrates.
  • The plasma membrane serves various functions depending on the type of the cell. These functions include structural support, protection, regulating substances moving into and out of the cell, and communication and cell signaling.

Frequently Asked Questions about Plasma Membrane

The plasma membrane is a selectively permeable membrane that separates the cell's internal contents from its outside environment.

The plasma membrane separates the cell's internal contents from its outside environment. It also serves various functions depending on the type of the cell including structural support, protection, regulation of substances moving into and out of the cell, and communication and cell signaling.

The plasma membrane serves various functions depending on the type of the cell. These functions include structural support, protection, regulation of movement of substances into and out of the cell, and communication and cell signaling.

The plasma membrane is made of lipids (phospholipids and cholesterol), proteins, and carbohydrates.  

Yes, prokaryotic cells have a plasma membrane.

Final Plasma Membrane Quiz

Question

What is the plasma membrane?

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Answer

The plasma membrane is a semi-permeable membrane that separates the cell's internal contents from its outside environment.

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What does selective permeability mean?

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Selective permeability means some substances are allowed to pass through while other substances are blocked.

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Do prokaryotic cells have a plasma membrane?

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Yes

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What model is the most accepted model that describes the structure of the plasma membrane?

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The fluid mosaic model

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What does the fluid mosaic model say about the structure of the plasma membrane?

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 According to the fluid mosaic model, the plasma membrane resembles a mosaic: it has many components, including lipids, proteins, and carbohydrates that make up the plane of the membrane. These components are fluid, meaning they move freely and constantly slide past one another.

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What are amphipathic molecules?

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Amphipathic molecules have both hydrophilic ("water-loving") and hydrophobic ("water-fearing") regions.  

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What is a phospholipid bilayer?

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A phospholipid bilayer is the arrangement of the phospholipids on the plasma membrane. It is composed of two layers of phospholipids, with the hydrophobic tails facing inward and the hydrophilic heads facing outward.

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What is the importance of the phospholipid bilayer to the function of the plasma membrane?

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The phospholipid bilayer acts as a stable boundary between two water-based compartments. The hydrophobic tails attach to one another; they form the interior of the membrane. On the other hand, the hydrophilic heads are exposed to aqueous fluids inside and outside the cell.

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What is the function of cholesterol in the plasma membrane?

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Cholesterol helps to maintain the fluidity of the membrane during temperature changes.  

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Proteins that are found in the hydrophobic interior of the phospholipid bilayer are called ___.

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Integral proteins

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Proteins that do not extend into the hydrophobic interior of the membrane and are instead loosely attached to the surface of the membrane are called ___.

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Peripheral membrane proteins

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What are channel proteins?

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Channel proteins create a hydrophilic channel in the plasma membrane for ions or other small molecules to pass through.

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What is the function of carbohydrates in the plasma membrane?

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Carbohydrates (sugars and sugar chains) are attached to proteins or lipids to help cells recognize each other.  

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How does the plasma membrane enable cell-to-cell recognition?

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The uniqueness of the carbohydrate molecules and their position on the surface of the plasma membrane enables them to function as cellular markers that allow cells to recognize each other.  

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How does the plasma membrane provide structural support to the cell?

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The plasma membrane provides structural support by binding to the cytoskeleton, a collection of protein filaments that keep parts of the cell in place.

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What is the definition of selective permeability?

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Selective permeability refers to the ability of the plasma membrane to allow some substances to pass through while blocking other substances. 

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What causes the selective permeability of the plasma membrane?

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The plasma membrane has selective permeability because of its composition and structure. The plasma membrane is composed of a phospholipid bilayer, with the hydrophobic tails facing inward and the hydrophilic heads facing outward. It also has proteins embedded that create channels or transport molecules through facilitated diffusion.

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What part of the phospholipid faces outwards and is exposed to aqueous fluid?

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hydrophilic head

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What part of the phospholipid faces inward and forms the interior of the membrane?

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hydrophobic tail

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What proteins create hydrophilic passageways for sodium, calcium, chloride, and potassium ions or other small molecules to pass through?

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channel proteins

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What proteins move molecules across the membrane through facilitated diffusion?

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transport proteins

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What is passive transport?

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Passive transport is a process by which substances are moved across a membrane without the use of energy.

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What is active transport?

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Active transport is a process by which substances are moved across a membrane using energy in the form of adenosine triphosphate (ATP).

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What do you call the process where molecules move in the direction of the concentration gradient without the use of energy?

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Diffusion

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What is homeostasis?

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Homeostasis is the balance in the internal states of living organisms that allow them to survive.

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What parts of the cell are NOT bound by a selectively permeable membrane?

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ribosomes

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Why is the selectively permeable membrane important in organelles?

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Organelles have highly specialized functions, so selectively permeable membranes play an important role in keeping them compartmentalized and maintaining them in optimal condition. 

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What is the difference between a semi-permeable membrane and a selectively permeable membrane?

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A semi-permeable membrane works like a sieve: it allows or prevents molecules from passing through based on their size, solubility, or other chemical or physical properties. It involves passive transport processes like osmosis and diffusion.


On the other hand, a selectively permeable membrane determines which molecules are permitted to cross using specific criteria (for instance, molecular structure and electrical charge). In addition to passive transport, it is able to use active or facilitated transport, which requires energy. 

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What process allows the entry of a molecule into the cell through a vesicle?

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endocytosis

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How does the selectively permeable membrane maintain homeostasis?

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The selective permeability of the plasma membrane enables cells to block, allow, and expel different substances in specific amounts: nutrients, organic molecules, ions, water, and oxygen are allowed into the cell, while wastes and harmful substances are blocked from or expelled out of the cell. 

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What is diffusion?

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Diffusion is a process where molecules distribute themselves to any available space.

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What is a concentration gradient?

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Concentration gradient refers to regions with different concentrations of a substance

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What is dynamic equilibrium?

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Dynamic equilibrium is the lack of concentration gradient and net movement of substances

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How does diffusion take place?

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Molecules distribute themselves to any available space. They move in from a region of higher concentration to a region of lower concentration until the two have reached dynamic equilibrium.  

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Is diffusion a form of passive or active transport?

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Passive Transport

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Is facilitated diffusion a form of passive or active transport?

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Passive Transport

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Does diffusion require energy?

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No, diffusion does not require energy

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Can diffusion transport molecules against the concentration gradient?

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No, diffusion is a form of passive transport where molecules are moved in the direction of the concentration gradient. Molecules require active transport to be moved against the concentration gradient.

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How does the concentration gradient affect diffusion?

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The higher the concentration difference, the faster the diffusion.

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How does solvent density affect diffusion?

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The higher the density of the solvent, the lower the rate of diffusion.

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How does molecular mass affect diffusion?

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Heavier molecules diffuse more slowly, while lighter molecules diffuse faster.

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How does temperature affect diffusion?

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Higher temperatures increase the rate of diffusion.  

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What is the difference between osmosis and diffusion?

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A key distinction is that in diffusion, both solvent and solute molecules move, but in osmosis, only solvent molecules move across the membrane.  

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What is facilitated diffusion?

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Facilitated diffusion is a type of diffusion in which molecules or ions that are blocked by the phospholipid bilayer of the plasma membrane are diffused across the membrane with the help of transport proteins. 

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What is the similarity between osmosis and diffusion?

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Osmosis and diffusion are both passive transport processes that occur when two solutions of different concentrations equalize through the movement of particles from an area of higher to lower concentration.

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What part of the plasma membrane prevents the entry of polar molecules?

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The hydrophobic interior of the phospholipid bilayer prevents the entry of polar molecules.

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What are the similarities between simple diffusion and facilitated diffusion?

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Both simple diffusion and facilitated diffusion are passive transport processes that occur when two solutions of different concentrations equalize through the movement of particles from an area of higher to lower concentration. 

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What is the difference between simple diffusion and facilitated diffusion?

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Facilitated diffusion involves transport proteins while simple diffusion does not.

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How do transport proteins facilitate diffusion?

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Transport proteins allow molecules to move across the plasma membrane without interacting with its hydrophobic interior.  

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What type of particles are moved through facilitated diffusion?

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Facilitated diffusion enables polar and charged molecules including amino acids, carbohydrates, and ions to pass through the plasma membrane.  

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