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5: Structure & Function of Plasma Membranes - Biology

5: Structure & Function of Plasma Membranes - Biology


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The plasma membrane, which is also called the cell membrane, has many functions, but the most basic one is to define the borders of the cell and keep the cell functional. The plasma membrane is selectively permeable. This means that the membrane allows some materials to freely enter or leave the cell, while other materials cannot move freely, but require the use of a specialized structure, and occasionally, even energy investment for crossing.

  • 5.0: Prelude to Structure and Function of Plasma Membranes
    Despite its seeming hustle and bustle, Grand Central Station functions with a high level of organization: People and objects move from one location to another, they cross or are contained within certain boundaries, and they provide a constant flow as part of larger activity. Analogously, a plasma membrane’s functions involve movement within the cell and across boundaries in the process of intracellular and intercellular activities.
  • 5.1: Components and Structure
    Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors. These proteins act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound. Occasionally, receptors are hijacked by vir
  • 5.2: Passive Transport
    Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable—they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells.
  • 5.3: Active Transport
    Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane.
  • 5.4: Bulk Transport
    In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles (see Table 5.4.1 for examples). Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell.
  • 5.E: Structure and Function of Plasma Membranes (Exercises)

Osmosis

Osmosis is the movement of water through a semipermeable membrane according to the water's concentration gradient across the membrane, which is inversely proportional to the solutes' concentration. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the solutes' diffusion in the water. Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.


Related Biology Terms

  • Cell wall – A structure that surrounds the plasma membrane of plant and fungus cells and provides additional support to those cells.
  • Phospholipid – a molecule that forms the characteristic double layer of the plasma membrane.
  • Semi-permeable – allowing only certain molecules to pass through due to the chemical properties of the membrane.
  • Fluid Mosaic Model – a model that describes the composition of the plasma membrane and how phospholipids, proteins, and carbohydrates freely move within it.

1. What type of molecule forms the double layer of the plasma membrane?
A. Phospholipids
B. Ion Channels
C. Ribosomes
D. Deoxyribonucleic acid

2. Which sentence best describes the Fluid Mosaic Model?
A. The plasma membrane allows fluid to pass between the extracellular fluid and the cytoplasm.
B. Too much fluid will cause animal cells to burst.
C. The components of the membrane fit in place like the tiles in a mosaic.
D. The lipids, proteins, and carbohydrates of the plasma membrane travel freely across its surface.

3. Which is NOT a function of the plasma membrane?
A. To generate the energy to power cell activities
B. To protect the cell from the surrounding environment
C. To facilitate cell-cell communication
D. To control the rate of certain molecules entering and leaving the cell


The main fabric of the membrane is composed of two layers of phospholipid molecules, and the polar ends of these molecules (which look like a collection of balls in an artist’s rendition of the model) (Figure 2) are in contact with aqueous fluid both inside and outside the cell. Thus, both surfaces of the plasma membrane are hydrophilic (“water loving”). In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic (“water fearing”) or nonpolar region because of the fatty acid tails. This region has no attraction for water or other polar molecules.

Figure 2 Phospholipid bilayer. “Extracellular” = outside the cell “Intracellular” = inside the cell. Photo credit: OpenStax Anatomy and Physiology.

A phospholipid molecule (Figure 3) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot.

Figure 3 This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.


Plasma Membrane: Structure and Functions

All types of cells are bounded by a thin, porous selectively permeable membrane which is known as the plasma membrane, cell membrane, plasmalemma or cytoplasmic membrane. It distinct the content of cell from the outside environment.

Carl Nägeli and Carl Cramer first discovered the term cell membrane in 1855 while Janet Quentin Plowe given the term plasmalemma in 1931. According to some Scientists, cell membrane originated from the endoplasmic reticulum. Plasma membrane lies between the cell wall and cytoplasm in the bacteria and plant cells and it is the outer limiting membrane of most animal cells.

Structure of Plasma Membrane

The plasma membrane is invisible and sometimes it contains brush border or sac-like structure, known as pinocytic vacuoles. If you observe it under an electron microscope, finger-like brush borders are available. They are known as microvilli. In between two adjacent cells, the plasma membranes become thicker in certain regions. From these areas, many fine filaments are seen, known as tonofilaments radiate towards the interior of the cell. Such thickened areas of the plasma membrane are known as desmosomes.

According to Dannielli and Davson (1935), the plasma membrane is about 75-80 Å in thick. The plasma membrane is composed of triple-layered structure. If you observe under a high magnification electron microscope, you will find double layer of lipid molecules 35 Å thick. Triple-layered structure of plasma membrane was discovered by J.D. Roberson in 1959. Two densely protein layers are also found in plasma membrane. In this case, thickness of each protein layer is 20 Å.

The lipid layers consist of most of the phospholipids. Its top end contains phosphate group and the tail end bears lipid group. In this case, phosphate group is positively charged while lipid group is negatively charged.

The Unit Membrane Model of Robertson

The Fluid Mosaic Model of Plasma Membrane

The lipid-globular protein mosaic model suggests, as the name implies, that instead of a continuous layer of protein on the surface of the membrane there is discontinuous mosaic globular protein. They remain partially embedded in and partially protruding from the phospholipid bilayer. There are also some discontinuous peripheral globular proteins arranged just outside and along the surface of the phospholipid bilayer.

This model was observed by English Scientists S. J. Singer and Garth Nicolson in 1972. This model is also known as Singer – Nicolson’s fluid mosaic model. According to this model, the plasma membrane looks like a mosaic which contains some components like phospholipids, cholesterol, proteins, and carbohydrates, etc. which gives the membrane a fluid character. Generally, the percentages of proteins, carbohydrates, and lipids in the plasma membrane vary with cell type. In myelin, the proportion of proteins and lipid are 18% and 76% respectively while the inner membrane of mitochondrial contains 76% protein and 24% lipid.

According to this theory, the main component of the cell membrane is a bimolecular lipid layer which actually consists of two rows of amphiphilic phospholipids molecules. Each phospholipid molecule contains three-carbon glycerol backbone with two fatty acid molecules which are attached to carbons 1 and 2 and a phosphate-containing group that is attached to the third carbon.

Fluid Mosaic Model of Plasma Membrane

Each phospholipid molecule has a water-soluble polar head and two fat-soluble non-polar tails. Top head of phospholipids is hydrophilic while tail end is hydrophobic. The phospholipid layer also contains protein and cholesterol. They make the plasma membrane look like a mosaic.

Chemically, the second major component of plasma membranes is proteins. Some protein molecules exist outside the lipid layer called peripheral protein molecule and some are partially or entirely pass across the lipid layer, called integral protein molecules. Integral protein molecules create an ion channel through the cell membrane for passing water-soluble molecules. A single integral protein usually consists of 20–25 amino acids.

The third major component of plasma membranes is oligosaccharide molecules (carbohydrates). These oligosaccharide molecules attached to some protein and lipid molecules of the outer side of the cell membrane to form glycoprotein and glycolipid respectively. Generally, these carbohydrate chains contain 2–60 monosaccharide units which can be either branched or straight.


Trans Fats

You can divide unsaturated fats into two more categories: cis-unsaturated fats and trans-unsaturated fats. Cis-unsaturated fats have two hydrogens on the same side of a double bond.

However, trans-unsaturated fats have two hydrogens on opposite sides of a double bond. This has a big impact on the shape of the molecule. Cis-unsaturated fats and saturated fats occur naturally, but trans-unsaturated fats are created in the lab.

You may have heard about health concerns related to eating trans fats in recent years. Also called trans-unsaturated fats, food manufacturers create trans fats through partial hydrogenation. Research has not shown that people have the enzymes necessary to metabolize trans fats, so eating them can increase the risk of developing cardiovascular diseases and diabetes.


Functions of the Plasma Membrane

Apart from holding the contents of a cell, the plasma membrane serves various important functions in cell regulation. This BiologyWise article explains what a plasma membrane is along with its functions.

Apart from holding the contents of a cell, the plasma membrane serves various important functions in cell regulation. This BiologyWise article explains what a plasma membrane is along with its functions.

Cells are the most basic entities that are responsible for life on this Earth. There have been numerous researches about the structure and functioning of cells, and scientists are still trying to unravel the mysteries of these life-sustaining cells. On an average, there are nearly trillions of cells in the body and all work together for the proper functioning of the body.

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What is a Plasma Membrane?

It a biological membrane of the cell that forms the external covering of both types of cells, the prokaryotic and eukaryotic. It acts as an outer boundary, preventing the cell from the invasion of external germs. The plasma consists of various biological molecules, lipids, and proteins that help in the regulation of body functions. There have been various theories concerning their structure that have been given by various scientists after years of research. The Fluid Mosaic Model and lipid bilayer are two theories that give explanations regarding their formation.

Functions

In order to understand the functioning of these membranes in cells, we’ve to understand basic functions of phospholipids, that are an integral part of this membrane. Phospholipids are the lipids that have two opposite functioning parts. While one end of the phospholipid is hydrophilic (water-loving), the other end is hydrophobic (water-repelling). These two ends help in proper functioning of the cells when they are mixed with water molecules. In the phospholipid bilayer, there are various proteins like peripheral, maker, transport, and receptor proteins that are the most important workers of the cell. With the help of these proteins, the cell membrane transfers the required materials in and out of the cell.

Its another function is to act as an attachment to the non-living matter that is found outside the cell membrane. This matter, known as extracellular matrix, helps in grouping the cells so that they can form tissues. Enzymes are another important part of cells, and protein molecules in the cells combine together to form enzymes that are involved in carrying out metabolic processes near the surface of the plasma membrane.

It also helps in the transportation of materials, that is crucial for the proper functioning of various cell organelles. This semipermeable membrane of the cells helps in the transferring those nutrients and chemicals that are required for the functioning of the cell. The other foreign materials are obstructed on their path, thereby preventing the invasion of the membrane.

It maintains a suitable ‘cell potential’. Just like electric signals can be transferred by creating some potential difference between two points, the cell maintains a cell potential that helps in the exchange of signals with the parts outside the cell. There are certain proteins in this membrane that act as molecular signals for this communication process. It contains carbohydrates, and the materials that pass through them are carefully regulated by these cellular molecules. While important requirements like carbon dioxide, oxygen, and water are allowed to be exchanged, the passage of molecules like amino acids and sugar is maintained effectively by these membranes.

As we can see, every function is significant in its own way and helps in the regulation of the health and maintenance of the body. These were some of the functions that help us in our body functions.

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2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.

After proteins have been synthesized by ribosomes they are transported to the rough endoplasmic reticulum where they can be modified. Vesicles carrying the protein then bud off the rough endoplasmic reticulum and are transported to the Golgi apparatus to be further modified. After this the vesicles carrying the protein bud off the Golgi apparatus and carry the protein to the plasma membrane. Here the vesicles fuse with the membrane expelling their content (the modified proteins) outside the cell. The membrane then goes back to its original state. This is a process called exocytosis. Endocytosis is a similar process which involves the pulling of the plasma membrane inwards so that the pinching off of a vesicle from the plasma membrane occurs and then this vesicle can carry its content anywhere in the cell.


Top 5 Functions of Plasma Membrane | Cytology

The following points highlight the top five functions of plasma membrane. The functions are: 1. Providing a Selectively Permeable Bar­rier 2. Transporting Solutes 3. Transporting Macromolecules 4. Responding to External Signals 5. Intercellular Interaction 6. Energy Transduction.

Plasma Membrane: Function # 1.

Providing a Selectively Permeable Bar­rier:

Plasma membrane prevents the free interchange of materials from one side to the other, and it ensures that the appro­priate substances are allowed into the cytoplasm from the external space and the inappropriate substances are kept out.

Plasma Membrane: Function # 2.

In all types of cells, there exists a difference in ionic concentration with extracellular medi­um. The plasma membrane contains the machinery for physically transporting substances from one side of the mem­brane to another. Transport across the membrane may be active or passive and occur via the phospholipid bilayer or by the help of specific integral membrane proteins, called permeases or transport proteins.

It is a type of diffu­sion in which an ion or molecule crossing a membrane moves down its electrochemical or concentration gra­dient.

It is of following types:

The process by which the water molecules pass through a semipermeable membrane from a region of higher water concentration to the region of lower water concen­tration is known as osmosis.

In simple diffu­sion, transport across the membrane takes place without the help of any permease and occurs only in the direction of concentration gradient. During simple diffusion, a small molecule in aqueous solution dis­solves into the phospholipid bilayer, crosses it and then dissolves into the aqueous solution on the opposite side. There is little specificity to the process.

(c) Facilitated Diffusion:

In this special type of passive transport, ions or molecules cross the membrane rapidly with the help of permeases in the membrane. It occurs only accor­ding to concentration gradient and is very specific, i.e., each facilitated diffusion protein transports only a single type of ion or molecule.

Currently, it is believed that trans­port proteins form the channels through the membrane that permit certain ions or molecules to pass across the membrane. For example, Ca ++ channels occur in axonal mem­branes for the entrance of Ca ++ in the cell and glucose permease in the mammalian RBC facilitates the diffu­sion of glucose into the cell.

In this process, ions or molecules move across the mem­brane against the concentration gradi­ent using metabolic energy. It is done with specific transport proteins, called pumps. For example, Na + -K + -ATPase – it is an ion pump or cation exchange pump which is driven by energy of one ATP molecule to export three Na + outside the cell in exchange of the import of two K + inside the cell.

Plasma Membrane: Function # 3.

Transporting Macromolecules:

Cells routinely import and export large molecules across the plasma membrane through different processes like:

Macromolecules such as proteins, lipids or carbohydrates are secreted out from the cell by exocytosis.

The process of inges­tion of large-sized solid substances (e.g., bacteria, food, parts of broken cells etc.) by the cell is known as phagocytosis.

In endocytosis, small regions of the plasma membrane fold inwards or invaginates, until it has formed new intracellular membrane limited vesicles.

In eukaryotes two types of endocytosis can occur:

(a) Pinocytosis which is the non­specific uptake of small droplets of extra-cellular fluid by endocytosis vesicles or

(b) Receptor-mediated endocytosis which is a specific endocytosis, in which a receptor on the sur­face of the plasma membrane “recog­nizes” an extracellular macromolecule and binds with it.

Unlike pinocytosis, which is a constitu­tive process that occurs continuously, the phagocytosis is a triggered process. However, both phagocytosis and pinocytosis are active mechanisms in the sense that the cell requires energy for their operation.

Plasma Membrane: Function # 4.

Responding to External Signals:

The plasma membrane plays a critical role in the response of a cell to external stimuli, a process known as signal transduction. Membrane possesses receptors that com­bine with specific molecules or ligands having a complimentary structure.

The interaction of a membrane receptor with the ligand may cause the membrane to generate a new signal that stimulates or inhibits internal activities. For example, signals generated at the plasma mem­brane may tell a cell to manufacture more glycogen, to prepare for cell division etc.

Plasma Membrane: Function # 5.

Intercellular Interaction:

Being situated at the outer edge of living cell, the plas­ma membrane mediates the interactions that occur between the cells of a multi­cellular organism. The plasma mem­brane allows cells to recognize one another, to exchange materials and infor­mation.

Plasma Membrane: Function # 6.

Membranes are intimately involved in energy transduc­tion, a process, by which one type of energy is converted to another type. For example, during photosynthesis energy in sunlight is absorbed by membrane bound pigments and converted into chemical energy contained in carbohy­drates.


Organelle Membranes

Some cell organelles are also surrounded by protective membranes. The nucleus, endoplasmic reticulum, vacuoles, lysosomes, and Golgi apparatus are examples of membrane-bound organelles. Mitochondria and chloroplasts are bound by a double membrane. The membranes of the different organelles vary in molecular composition and are well suited for the functions they perform. Organelle membranes are important to several vital cell functions including protein synthesis, lipid production, and cellular respiration.


Watch the video: HIO 01: Eukaryontní buňka organely a jejich základní funkce (February 2023).