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6.6: Structure of the Cell Membrane - Biology


Learning Objectives

  • Describe the structure of cell membranes
  • Identify components of the cell membrane, including phospholipids, cholesterol, proteins, and carbohydrates

Cell Membranes are Fluid

A cell’s plasma membrane defines the boundary of the cell and determines the nature of its contact with the environment. Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. Plasma membranes enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux. The plasma membrane must be sufficiently flexible to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital as tissues and organs form during early development, and which later plays a role in the “self” versus “non-self” distinction of the immune response.

The plasma membrane also carries receptors, which are attachment sites for specific substances that interact with the cell. Each receptor is structured to bind with a specific substance. For example, surface receptors of the membrane create changes in the interior, such as changes in enzymes of metabolic pathways. These metabolic pathways might be vital for providing the cell with energy, making specific substances for the cell, or breaking down cellular waste or toxins for disposal. Receptors on the plasma membrane’s exterior surface interact with hormones or neurotransmitters, and allow their messages to be transmitted into the cell. Some recognition sites are used by viruses as attachment points. Although they are highly specific, pathogens like viruses may evolve to exploit receptors to gain entry to a cell by mimicking the specific substance that the receptor is meant to bind. This specificity helps to explain why human immunodeficiency virus (HIV) or any of the five types of hepatitis viruses invade only specific cells.

Cell Membranes are Mosaics

In 1972, S. J. Singer and Garth L. Nicolson proposed a new model of the plasma membrane that, compared to earlier understanding, better explained both microscopic observations and the function of the plasma membrane. This was called the fluid mosaic model. The model has evolved somewhat over time, but still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—in which the components are able to flow and change position, while maintaining the basic integrity of the membrane. Both phospholipid molecules and embedded proteins are able to diffuse rapidly and laterally in the membrane. The fluidity of the plasma membrane is necessary for the activities of certain enzymes and transport molecules within the membrane. Plasma membranes range from 5–10 nm thick. As a comparison, human red blood cells, visible via light microscopy, are approximately 8 µm thick, or approximately 1,000 times thicker than a plasma membrane. (Figure 1)

The plasma membrane is made up primarily of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol. The amount of cholesterol in animal plasma membranes regulates the fluidity of the membrane and changes based on the temperature of the cell’s environment. In other words, cholesterol acts as antifreeze in the cell membrane and is more abundant in animals that live in cold climates.

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 1) are in contact with aqueous fluid both inside and outside the cell. Thus, both surfaces of the plasma membrane are hydrophilic. In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic or nonpolar region because of the fatty acid tails. This region has no attraction for water or other polar molecules.

Proteins make up the second major chemical component of plasma membranes. Integral proteins are embedded in the plasma membrane and may span all or part of the membrane. Integral proteins may serve as channels or pumps to move materials into or out of the cell. Peripheral proteins are found on the exterior or interior surfaces of membranes, attached either to integral proteins or to phospholipid molecules. Both integral and peripheral proteins may serve as enzymes, as structural attachments for the fibers of the cytoskeleton, or as part of the cell’s recognition sites.

Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains may consist of 2–60 monosaccharide units and may be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.

Try It

Specific glycoprotein molecules exposed on the surface of the cell membranes of host cells are exploited by many viruses to infect specific organs. For example, HIV is able to penetrate the plasma membranes of specific kinds of white blood cells called T-helper cells and monocytes, as well as some cells of the central nervous system. The hepatitis virus attacks only liver cells.

These viruses are able to invade these cells, because the cells have binding sites on their surfaces that the viruses have exploited with equally specific glycoproteins in their coats. (Figure 2). The cell is tricked by the mimicry of the virus coat molecules, and the virus is able to enter the cell. Other recognition sites on the virus’s surface interact with the human immune system, prompting the body to produce antibodies. Antibodies are made in response to the antigens (or proteins associated with invasive pathogens). These same sites serve as places for antibodies to attach, and either destroy or inhibit the activity of the virus. Unfortunately, these sites on HIV are encoded by genes that change quickly, making the production of an effective vaccine against the virus very difficult. The virus population within an infected individual quickly evolves through mutation into different populations, or variants, distinguished by differences in these recognition sites. This rapid change of viral surface markers decreases the effectiveness of the person’s immune system in attacking the virus, because the antibodies will not recognize the new variations of the surface patterns.

Learning Objectives

The modern understanding of the plasma membrane is referred to as the fluid mosaic model. The plasma membrane is composed of a bilayer of phospholipids, with their hydrophobic, fatty acid tails in contact with each other. The landscape of the membrane is studded with proteins, some of which span the membrane. Some of these proteins serve to transport materials into or out of the cell. Carbohydrates are attached to some of the proteins and lipids on the outward-facing surface of the membrane. These form complexes that function to identify the cell to other cells. The fluid nature of the membrane owes itself to the configuration of the fatty acid tails, the presence of cholesterol embedded in the membrane (in animal cells), and the mosaic nature of the proteins and protein-carbohydrate complexes, which are not firmly fixed in place. Plasma membranes enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux.


GK Questions & Answers Cell its Structure and Functions

All living organisms are made up of cells. It is the basic, structural, and functional unit of life. Also, it is known as "Building blocks of life". Cell biology is the study of cells. It consists of organelles that have different functions and helps the cell to survive. Multicellular organisms are made up of various cells. Let us solve a quiz based on the Cell its structure and functions.

1. Who discovered cell in 1665?

Explanation: In 1665, the cell was first discovered by Robert Hooke. The cell has a rich and interesting history that has ultimately provided the way to various scientific advancements of today.

2. Name an Organelle which serves as a primary packaging area for molecules that will be distributed throughout the cell?

Explanation: Golgi apparatus is also known as Golgi complex or Golgi body. It is an organelle that serves as a primary packaging area for molecules that will be distributed throughout the cell. It is located in the cytoplasm next to the endoplasmic reticulum and near the nucleus of the cell.

3. Name the outermost boundary of the cell?

Explanation: Plasma Membrane is the outermost boundary of the cell. It is also known as the cell membrane. It is the membrane found in all cells that separate the interior of the cell from the outside environment. A cell wall is attached to the plasma membrane on its outside surface in bacterial and plant cells.

4. Name the process in which the ingestion of material by the cells is done through the plasma membrane?

Explanation: Endocytosis is the process in which the ingestion of material by the cells is done through the plasma membrane. Or we can say that it is the process of actively transporting molecules into the cell by engulfing it with its membrane.

5. Which among the following sentence is not correct about the organelles?

a) They are found in all Eukaryotic cells.

b) They are found in multicellular organisms.

c) They coordinate to produce the cell.

d) They are small sized and mostly internal.

Explanation: Organelles are found in all Eukaryotic cells. They coordinate to produce the cell. They are small-sized and mostly internal. Organelle, any of specialised structures inside a cell that perform a specific function (e.g. mitochondria, ribosomes, endoplasmic reticulum). Organelles in unicellular organisms are the equivalent of organs in multicellular organisms.

6. Name the process in which the passage of water goes from a region of higher concentration to a region of lower concentration through a semi permeable membrane?

Explanation: Osmosis is the process in which the passage of water goes from a region of higher concentration to a region of lower concentration through a semi-permeable membrane.

7. Name an organism that contains a single chromosome and cell division occurs through fission or budding?

Explanation: Prokaryotes contain a single chromosome and cell division occurs through fission or budding. The usual method of prokaryote cell division is termed binary fission. The prokaryotic chromosome is a single DNA molecule that first replicates, then attaches each copy to a different part of the cell membrane.

8. Name the process in which the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium?

Explanation: Exocytosis is the process in which the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium.

9. The jelly-like substance present inside the cell is known as:

Explanation: Cytoplasm is the jelly-like substance present inside the cell and contains other organelles. It is a thick solution that fills each cell and is enclosed by the cell membrane.

10. Blue-green Algae are:

Explanation: Blue-green Algae or Cyanobacteria are Prokaryotes. They lack membrane-bound organelles and nuclei.


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In this learning activity you'll study the structure of the cell membrane and construct it using the correct molecules.

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Questions

This interactive is NOT working well.

#1 I can't drag any of the terms to its graphic.

#5 Can't drag fibrous protein to place in the membrane

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Posted by Purviben on 12/4/2014 8:35:50 PM Reply

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This interactive is not working. I used it earlier this week and it was fine. Also, I downloaded it and it works fine as a download, but I would like my students to access this via the internet. Is there someone why can take a look at it. It seems like the start button may just be covered up.

We are not able to view this interactive.

Posted by ayah baydoun on 11/13/2014 8:52:32 PM Reply

Your interactive only works up to page 4 then shuts down. Is there a fix. I really enjoy this interactive and use it with my students in biology class?

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what is the name of joint and the bones that make the joint on flexion

why is the cell membrane considered semi-permeable? explain

Semi-permeable means "allowing certain substances to pass through it but not others, especially allowing the passage of a solvent but not of certain solutes." The cell membrane allows some things to pass through, and not others, which is why it needs transport proteins to let the other things through that the cell needs.

Why does the cell membrane have transport proteins ?

channel proteins help molecules across the membrane via passive transport, a process called facilitated diffusion. These channel proteins are responsible for bringing in ions and other small molecules into the cell. . The other type of transport protein is called a carrier protein.

what type of molecules can easily go through your cell membrane or are PERMEABLE to the cell membrane

how am i able to download this?

Downloading a learning object.

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what is the relationship between lipids and water

What is the cell membrane composed of?

As I go through the interactive component, I can view some of the images to select, but not all. It seems to be only the glycoprotein that is visible.

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This may be a good alternative:

What is the basic fundamental unit of life?

The basic fundament unit is the cell

What are the molecules that make up the majority of the membrane

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Outstanding! I hope more is on the way?

Posted by Ed Johnson on 1/18/2006 12:00:00 AM Reply

Excellent work. My 4th grader is studing cells in school and this will be a great learning tool for her and it was fun also. We need more people like you that care.

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I like the graphics which have been used in it and it is full or information. I would suggest every one to try.

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At first I was very nervous about using a website to learn about the cell membrane. I was very scared and didnt want to use it. Like many of you, I didnt want to receive false information. However my teacher strongly recommended this website to me. Ever since, I use it daily for study purposes. I recently got a hundred on my test! Thank you guys for changing me from a website fearing boy to a website using man! I will use you in the future! I hope to learn about subatomic particles in the future!


Describe the structure of the cell membrane.

The cell membrane is a fluid matrix made of a phospholipid bilayer. Each lipid molecule has a hydrophilic, phosphorylated "head" and a hydrophobic, fatty acid "tail". The hydrophobic parts of the molecules on the bilayer face each other, whereas the hydrophilic parts form the inner and outer surfaces of the cell membrane. Small, hydrophobic molecules can traverse the cell membrane via simple diffusion.

Integrated into the membrane are protein molecules which may span the whole bilayer (and are thus known as transmembrane proteins) or just one layer. Some of these proteins act as channels for transport of ions and other particles that can not simply diffuse across the membrane.

A number of sugar molecules are present on the external surface of the cell membrane- this is known as the glycocalyx. Glucose that is attached to the phospholipid molecules is known as a glyolipid, whereas the glucose attached to proteins is known as a glycoprotein. Also interspersed within the phospholipids are molecules of cholesterol which give the membrane flexibility and fluidity.


Clinical Relevance – Hereditary Spherocytosis

Hereditary spherocytosis is a condition in which spectrin, a peripheral cytoskeletal protein, is depleted by 40-80%. There are both autosomal dominant and recessive forms of the condition, with differing severity. As a result of this lack of spectrin, erythrocytes cannot effectively maintain their biconcave structure, and assume a spherical shape. This decreases their ability to travel through the microvasculature of the body and results in increased erythrocyte lysis. There are 3 other types of spherocytosis that result from defects in ankyrin, band 3 and protein 4.2, however spectrin is the most significant.

The signs and symptoms of the condition include:

  • Mild to moderate anaemia
  • Possible jaundice
  • Possible splenomegaly

Fig 3 – Diagram showing a peripheral blood smear from a patient with hereditary spherocytosis.


6.6: Structure of the Cell Membrane - Biology

The Journal of Membrane Biology is dedicated to publishing high-quality science related to membrane biology, biochemistry and biophysics. In particular, we welcome work that uses modern experimental or computational methods including but not limited to those with microscopy, diffraction, NMR, computer simulations, or biochemistry aimed at membrane associated or membrane embedded proteins or model membrane systems. These methods might be applied to study topics like membrane protein structure and function, membrane mediated or controlled signaling mechanisms, cell-cell communication via gap junctions, the behavior of proteins and lipids based on monolayer or bilayer systems, or genetic and regulatory mechanisms controlling membrane function.

  • Explores the nature, structure, genesis and functions of biological membranes, and the physics and chemistry of artificial membranes
  • Coverage includes transport and secretory functions, including natural and artificial transport carrier systems, membrane channels, diffusion and pinocytosis and more

Interactive resources for schools

Partially permeable

Permeable to some substances but not to others

Fluid mosaic model

Our current model of membrane structure consisting of a fluid phospholipid bilayer with many other molecules (including cholesterol, glycolipids, proteins and glycoproteins) floating or embedded in the lipid sea, all with different functions.

Neurotransmitters

Chemicals which are released in a synapse when an action potential reaches the end of one neurone. They cross the synaptic gap and trigger and impulse in the next neurone.

Active transport

The process which uses energy to move substances against a concentration gradient or across a partially permeable membrane using a special transport protein.

Nuclear membrane

The thin, flexible structure enclosing the contents of the nucleus in a cell.

Phosphate group

A molecule containing phosphorus and oxygen.

Cell membrane

The membrane which forms the boundary between the cytoplasm of a cell and the medium surrounding it and controls the movement of substances into and out of the cell.

Immune system

The body's natural defence mechanism against infectious diseases.

Unit membrane

Bilayer of polar lipid molecules in an aqueous environment - the basis of the structure of the cell membrane.

Glycoproteins

Proteins that have a carbohydrate chain attached to them. The carbohydrate chain sticks out of the outside of the cell and is part of the cell recognition system.

Phospholipid

A lipid molecule with a hydrophilic "head" region around the ionic phosphate group and a long hydrophobic hydrocarbon tail that forms a bilayer in aqueous solutions

Mitochondria

Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse

Carbohydrate

Energy producing organic compounds which are made of carbon, hydrogen and oxygen. Examples of food containing carbohydrate are rice, pasta, bread and potatoes

Cholesterol

A lipid which can be measured in the blood. High levels are linked to an increased risk of cardiovascular disease

Hydrophilic

Molecules that absorb or dissolve in water - usually polar molecules.

Hydrophobic

Insoluble in water, repel water.

Glycolipids

Lipids that have a carbohydrate chain attached to them. The carbohydrate chain is attached to the outside of the cell and is part of the cell recognition system.

Respiration

The biochemical process by which the cells in the body releases energy

Transplant

The process of replacing a damaged or diseased organ with a healthy organ from a dead or living donor.

Cellulose

A complex carbohydrate which makes up plant cell walls

Antigen

A protein on the surface of a pathogenic microorganism, which can stimulate a response from the immune system.

Protein

A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.

Receptors

Protein molecules attached to cells that only bind to specific molecules with a particular structure

Hormone

A chemical messenger produced by a particular gland or cells of the endocrine system. Hormones are transported throughout the body in the blood stream but they produce a response only in specific target cells

Cancer

A mass of abnormal cells which keep multiplying in an uncontrolled way.

Lipid

Molecules which contain a lot of stored energy built up of fatty acids and glycerol. Lipids include oils and fats

Organ

A structure with a particular function which is made up of different tissues.

Nerve

A bundle of neurones - it may be all sensory neurones, all motor neurones or a mixture of both

Messenger RNA

The molecule which transcribes the DNA code and carries it out of the nucleus through the pores in the nuclear membrane to the ribosomes in the cytoplasm which synthesise the required proteins

Adenosine triphosphate

Molecule which acts as the common energy currency in all cells, providing the energy needed to drive chemical reactions in cells.

Cell membranes

Cell membranes are vital to the way cells function. In animal cells they form the outer layer of the cell, the ultimate barrier between the inside of the cell and its surroundings. In plant cells the cell surface membrane is inside a relatively rigid cellulose cell wall but the properties of the membrane still control much of what moves into and out of the cell. Most of the organelles inside a eukaryotic cell are also membrane bound. Understanding the properties of cell membranes is key to understanding how cells work.

The structure of the cell membrane

Our current model of the cell membrane has been built up over many years by a combination of experimental data and electron microscopy

The unit membrane

The basic structure of the cell membrane is a bilayer of phospholipids. Phospholipid molecules have a hydrophilic ‘head’ region around the ionic phosphate group and a long hydrophobic hydrocarbon tail. These polar lipids form a bilayer in aqueous solutions with the hydrophilic heads pointing outwards and the hydrophobic tails forming a hydrophobic layer in the middle. This bilayer is known as a unit membrane.

The phospholipid bilayer in aqueous solution that forms the backbone of the cell membrane

The cell membrane

The cell membrane, however, is more than a simple unit membrane. Our current model is of a fluid phospholipid bilayer with many other molecules associated with it, floating or embedded in the lipid sea. These other molecules include cholesterol, glycolipids, proteins and glycoproteins and they all have different functions in the membrane. This is the fluid mosaic model of membrane structure and it explains many of the properties of membranes that we can observe experimentally.

The fluid mosaic model of the cell membrane.

  • A Phospholipids: lipid molecules with a hydrophilic ‘head’ region around the ionic phosphate group and a long hydrophobic hydrocarbon tail that form a bilayer in aqueous solutions.
  • B Cholesterol: a lipid with a steroid ring structure, and hydrophilic and hydrophobic regions. It makes up part of the membrane structure - there may be up to one cholesterol molecule for every two phospholipids. Cholesterol makes the membrane stiffer and more rigid – so the amount of cholesterol in the structure affects the rigidity of the membrane.
  • C Glycolipids: lipids that have a carbohydrate chain attached to them. The carbohydrate chain is attached to the outside of the cell and is part of the cell recognition system.
  • DProteins: a wide variety of molecules that carry out many of the very specific functions of the cell membrane. There are integral proteins and peripheral proteins. They can form temporary and permanent channels in the membrane, allowing different molecules to pass in and out of the cell. They may be enzymes involved in active transport systems or enzymes linked to biochemical pathways such as photosynthesis or respiration. Proteins also act as receptor molecules for other molecules such as hormones and neurotransmitters
  • E Glycoproteins: proteins that have a carbohydrate chain attached to them. The carbohydrate chain sticks out of the outside of the cell and is part of the cell recognition system

Functions of the cell membrane

Many of the functions of the surface cell membrane and membranes around cell organelles are similar, although there are some which are specific to the outer membrane.

  • Membranes form partially permeable barriers between the cell and its environment, between organelles and the cytoplasm and within organelles. They control the movement of substances both into and out of the cell and into and out of organelles. Permanent and temporary protein pores are involved in this control, as well as temporary and permanent active transport systems. Some channels are gated – they can be opened or closed depending on conditions inside or outside of the cell as described on the next page.
  • Membranes are the site of many chemical reactions because the enzymes involved are embedded in the membrane structure. Reactions take place both on the cell surface membrane and on the membranes in organelles such as mitochondria and chloroplasts.
  • Membranes are important in the development of chemical and electrochemical gradients – for example those involved in nerve impulses and in the production of ATP by chemiosmosis.
  • Membranes are the site of cell identification. The carbohydrate markers attached to glycoproteins and glycolipids along with some membrane proteins act as antigens, identifying one cell to other cells. For example this system enables the cells of the immune system to identify pathogens, cells from other organisms of the same species (eg after an organ transplant), abnormal body cells (eg cancer cells) and toxins produced by pathogens
  • Membranes are the site of cell communication. Cell signalling takes place between cells through the protein receptor molecules in the cell surface membrane and within cells for example in the passing of hormone messages from the body to the nucleus of the cell and the movement of mRNA out of the nucleus through nuclear membrane pores.This process is described in more detail later.

The pores in the nuclear membrane allow chemicals to move into the nucleus and mRNA to move out into the cytoplasm. (Image courtesy of Don W. Fawcett/Hector E. Chemes/Bernard Gilula (CC BY-NC-ND 3.0))

Activity:

Use materials of your choice – anything from plasticine to plastic bottles and beyond – make a three dimensional model of the cell membrane that can be used to explain the structure and functions of this amazing structure.


Cell Membrane Structure and Function

The cell membrane structure and functions covered in this article should provide basic information associated with this cell organelle. Read on to know more.

The cell membrane structure and functions covered in this article should provide basic information associated with this cell organelle. Read on to know more.

Cell membrane is a protective covering that acts as a barrier between the inner and outer environment of a cell (in animals). In plant cells, the membrane encapsulates the protoplasm. This organelle is also referred to as plasma membrane. Images obtained through electron micrography reveal the bilayer structure of cell membranes. The characteristic feature of this organelle is that it allows only certain substances to pass through. Most of the research carried out for the purpose of studying cell membrane structure makes use of red blood cells (RBCs), as the absence of internal membranes and nuclei in RBCs results into the isolation process being carried out quite easily.

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Information pertaining to the function of the cell membrane and its structure is presented in the following paragraphs. This description about cell membrane structure and functions should help in understanding the working in a better manner.

Structure

The cell membrane is made up of two layers that are composed of phospholipids. The bilayer is formed by the arrangement of phospholipids in a manner that their head regions (which are hydrophilic) face external environment as well as the internal cytosolic environment. The (hydrophobic) tails of these phospholipids face each other. Forces underlying the formation of this bilayer are electrostatic, van der Waals, non-covalent interactions, and hydrogen bonds. This peculiar arrangement of hydrophilic and hydrophobic layers doesn’t allow nucleic acids, amino acids, proteins, carbohydrates, and ions to pass through the bilayer. Following are the various parts of the cell membrane.

  • Integral Membrane Proteins:
    These are structures present on the inside, outside, and also throughout the cell membrane. Fluorescence and electron microscopy can be used in viewing these proteins. These proteins are present on the entire/whole surface of the cell membrane. Examples of these structures include the cadherins, integrins, clathrin-coated pits, desmosomes, caveoles, etc.
  • Peripheral Membrane Proteins:
    These proteins are attached/bound to the surface of the membrane by means of hydrogen bonds and electrostatic interactions. The hydrogen bonds of these peripheral proteins are formed of hydrophilic phospholipid heads which form the bilayer.
  • Skeleton of Cell Membrane:
    Surface of the cell membrane on the side of cytoplasm is lined by the cytoskeleton. The framework or cytoskeleton proves to be useful in the processes of organelles like cilia. Cytoskeleton also helps in anchoring the membrane proteins to the cell membrane.
  • Composition of Cell Membrane:
    Proteins and lipids are important components which form the cell membrane. Different mechanisms carry out the function of incorporation and removal of materials into and out of the membrane. The process of the fusion of cell membrane with intracellular vesicles results into excretion of contents present in vesicles.

Function

Demarcating the boundaries of a cell is the primary function of plasma membrane. The contents of a cell are supported by this membrane. Not just supporting the matter present in cells, but also the function of maintaining contact with other cells is carried out by the cell membrane. The plant cell membranes enjoy extra protection in the form of cell walls however, in animals, cell membrane is the only covering/encapsulation. Proteins which compose (or get embedded in) the membrane carry out the diffusion of elements in a selective manner.

The plasma membrane is an important part of a cell, as it provides it with protection and also helps in maintaining a proper shape. The cell membrane structure and functions presented in the article should help in knowing more about this organelle.

Related Posts

The plant cell refers to the structural component of the plant. This BiologyWise article provides you with the structure of plant cells along with the functions of its constituents.

Understanding nuclear membrane function in a cell will help us to become more aware about the crucial role it plays in functioning of our bodies. This BiologyWise article tells you&hellip

The primary function of ribosomes is synthesis of proteins according to the sequence of amino acids as specified in the messenger RNA.


Active Transport

For all the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually against its concentration gradient.

One of the most common types of active transport involves proteins that serve as pumps. The word “pump” probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration).

The sodium-potassium pump, which is also called Na + /K + ATPase, transports sodium out of a cell while moving potassium into the cell. The Na + /K + pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na + /K + pump moves three Na + ions out of the cell and two K + ions into the cell for each ATP molecule that is used (Figure 2.6.8). This process is so important for nerve cells that it accounts for the majority of their ATP usage.

Figure 2.6.8. Sodium-potassium pump. The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.

Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell. In this way, the action of an active transport pump (the sodium-potassium pump) powers the passive transport of sodium ions by creating a concentration gradient. When active transport powers the transport of another substance in this way, it is called secondary active transport.

Symporters are secondary active transporters that move two substances in the same direction. For example, the sodium-glucose symporter uses sodium ions to “pull” glucose molecules into the cell. Because cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside. However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient.

Conversely, antiporters are secondary active transport systems that transport substances in opposite directions. For example, the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H + ) out of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell’s interior.

Other forms of active transport do not involve membrane carriers. Endocytosis (bringing “into the cell”) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane (Figure 2.6.9). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must be broken down or digested. Phagocytosis (“cell eating”) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytose them, and digest them. In contrast to phagocytosis, pinocytosis (“cell drinking”) brings fluid containing dissolved substances into a cell through membrane vesicles.

Figure 2.6.9. Three forms of endocytosis. Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.

Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound enough of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of haemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes.

In contrast with endocytosis, exocytosis (taking “out of the cell”) is the process of a cell exporting material using vesicular transport (Figure 2.6.10). Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases its contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis (Figure 2.6.11). Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.

Figure 2.6.10. Exocytosis. Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.

Figure 2.6.11. Pancreatic cells’ enzyme products. The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will be exported from the cells via exocytosis. LM × 2900. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)


The Glycocalyx

As already discussed, the extracellular portions of plasma membrane proteins are generally glycosylated. Likewise, the carbohydrate portions of glycolipids are exposed on the outer face of the plasma membrane. Consequently, the surface of the cell is covered by a carbohydrate coat, known as the glycocalyx, formed by the oligosaccharides of glycolipids and transmembrane glycoproteins (Figure 12.13).

Figure 12.13

The glycocalyx. An electron micrograph of intestinal epithelium illustrating the glycocalyx (arrows). (Don Fawcett/ Visuals Unlimited.)

Part of the role of the glycocalyx is to protect the cell surface. In addition, the oligosaccharides of the glycocalyx serve as markers for a variety of cell-cell interactions. A well-studied example of these interactions is the adhesion of white blood cells (leukocytes) to the endothelial cells that line blood vessels𠅊 process that allows the leukocytes to leave the circulatory system and mediate the inflammatory response in injured tissues. The initial step in adhesion between leukocytes and endothelial cells is mediated by a family of transmembrane proteins called selectins, which recognize specific carbohydrates on the cell surface (Figure 12.14). Two members of the selectin family (E-selectin and P-selectin), expressed by endothelial cells and platelets, bind to specific oligosaccharides expressed on the surface of leukocytes. A different selectin (L-selectin) is expressed by leukocytes and recognizes an oligosaccharide on the surface of endothelial cells. The oligosaccharides exposed on the cell surface thus provide a set of markers that help identify the distinct cell types of multicellular organisms.

Figure 12.14

Binding of selectins to oligosaccharides. E-selectin is a transmembrane protein expressed by endothelial cells that binds to an oligosaccharide expressed on the surface of leukocytes. The oligosaccharide recognized by E-selectin contains N-acetylglucosamine (more. )


Watch the video: Inside the Cell Membrane (December 2021).