Cell Membrane Structure Phospholipid Bilayer

A cell is the basic unit of life, and the cell membrane is an important structure present in all cells, irrespective of whether they are plant cells or animal cells. This structure is a vital component of any cell and it has a variety of important functions. Cell membrane functions include maintaining the boundaries of the cells, thus supporting the contents of the cell, maintaining proper cell to cell contact, regulating the entry and exit of molecules in and out of the cell, etc. Thus, to understand how the cell membrane manages to carry out this procedure, one needs to understand the cell membrane structure. Given below are the various components that comprise the structure of the cell membrane according to the Fluid Mosaic model.

The first layer of cell membrane consists of a phosphid bilayer. The phosphate molecules are arranged in such a way that the hydrophilic heads are on the outside, while the hydrophobic fatty acid tails are on the inside, facing each other. The tails of the molecule are said to be hydrophobic and that is why they points inwardly towards each other. This specific arrangement of the lipid bilayer is for the purpose of preventing the entry of polar solutes, like amino acids, proteins, carbohydrates, etc. Thus, the phosphate lipid bilayer is one of the main factors responsible for regulating the entry and exit of molecules in and out of the cell.

Integral Membrane Proteins

Integral membrane proteins are those proteins that are a part of the cell membrane structure. They are present between consecutive molecules of phopholipids. These fibrous proteins present may span the entire length of the cell membrane. These molecules have important functions, as they serve as receptors for the cell. Some of the proteins of the cell membrane may also enter the cell. Sometimes, a part of the protein molecule is inside and some of it is outside. These kind of protein molecules act as carriers for active transport of substances in and out of the cell. Some of these protein molecules form pores and thus, allow fatty acids and other lipid insoluble in water molecules to pass through. Furthermore, other integral proteins serve as channel proteins as well to aid in selective transport of ions in and out of the cell. Such molecules are visible with the help of an electron microscopy.

Other Elements

Certain other elements may also be present along the length of the cell membrane, depending on the location and needs of the cell. These structures include globular proteins, which are peripherally placed and are only at times associated with the cell. These protein molecules may even be enzymes or glycoproteins. In such cases, either the cell will have special functions, or the location of the cell may require it to perform certain specific functions. When speaking of plant cell vs animal cell, there is one important structure that is additionally present most of the time in animal cells. These molecules are cholesterol molecules, which aid the phospholipids in making the membrane impermeable to water soluble substances. These cholesterol molecules also stabilize the membrane and provide the cell with a ‘cushion effect’, which prevents it from suffering any major injuries due to trauma and impact forces.

Cell Membrane Function

Cell membrane is the outer covering of a cell, which keep the ingredients of a cell intact. Apart from that, there are various other functions, that are carried out by this structure. Read on…

It is a common fact that cells are the fundamental building blocks of life. These structures form the basic structural and functional unit of any living thing. While some organisms, like, bacteria are single-celled, most other living things are multicellular. In case of multicellular organisms like humans (an adult human has around 100 trillion cells in the body), there are various types of cells, which are assigned different functions. Each cell is made of intricate structures, which forms an interconnected network, which strives to carry out the function of that cell. As the nature of the function of the cells differ, the functions of various parts of the cells too differ. Let us take a look at the various parts of a cell, especially, the cell membrane and cell membrane function.

Cell Membrane and Other Parts of a Cell

Basically there are two types of cells – eukaryotic and prokaryotic. While plants, animals, fungi, protozoans, etc. possess eukaryotic cells, prokaryotic cells are found in bacteria only. The difference between the two types of cells lie in the fact that prokaryotic cells do not have nucleus (and/or some other organelles) and are comparatively smaller, as compared to eukaryotic ones. As far as eukaryotic cells are concerned, the basic structure includes parts like DNA, ribosomes, vesicle, endoplasmic reticulum (both rough and smooth), Golgi apparatus, cytoskeleton, mitochondria, vacuole, centrioles, lysosome, cytoplasm, plasma membrane and cell wall. While plant cells have a large vacuole and a definite cell wall, animal cells lack cell wall but some may have very small vacuoles. Animal cells do not have chloroplasts too. This article is about cell membrane, which is also known as plasma membrane or plasmalemma. Scroll down for information about cell membrane function.

Read more on:

• Similarities Between Eukaryotic and Prokaryotic Cells
• Plant Cell vs Animal Cell
• Plant Cell Organelles

What is a Cell Membrane?

Cell membrane or plasma membrane is one of the vital parts of a cell that encloses and protects the constituents of a cell. It separates the interior of a cell from outside environment. It is like a covering that encloses the different organelles of the cell and the fluid that harbors these organelles. To be precise, cell membrane physically separates the contents of the cell from the outside environment, but, in plants, fungi and some bacteria, there is a cell wall that surrounds the cell membrane. However, the cell wall acts as a solid mechanical support only. The actual function of cell membrane is the same in both cases and it is not much altered by the mere presence of a cell wall. The cell membrane is made of two layers of phospholipids and each phospholipid molecule has a head and a tail region. The head region is called hydrophilic (attraction towards water molecules) and the tail ends are known as hydrophobic (repels water molecules). Both layers of phospholipids are arranged so that the head regions form the outer and inner surface of the cell membrane and the tail ends come close in the center of the cell membrane. Other than phospholipids, cell membrane contains lots of protein molecules, which are embedded in the phospholipid layer. All these constituents of the cell membrane work jointly to carry out its function. The following paragraph deals with cell membrane function. Read more on cell nucleus: structure and functions and cytoplasm function in a cell.

What is the Function of the Cell Membrane?

As mentioned above, one of the basic functions of a cell membrane is to act like a protective outer covering for the cell. Apart from this, there are many other important cell membrane functions, that are vital for the functioning of the cell. The following are some of the cell membrane functions.

Cell membrane anchors the cytoskeleton (a cellular ‘skeleton’ made of protein and contained in the cytoplasm) and gives shape to the cell.

Cell membrane is responsible for attaching the cell to the extracellular matrix (non living material that is found outside the cells), so that the cells group together to form tissues.

Another important cell membrane function is the transportation of materials needed for the functioning of the cell organelles. Cell membrane is semi permeable and controls the in and out movements of substances. Such movement of substances may be either at the expense of cellular energy or passive, without using cellular energy.

The protein molecules in the cell membrane receive signals from other cells or the outside environment and convert the signals to messages, that are passed to the organelles inside the cell.

In some cells, the protein molecules in the cell membrane group together to form enzymes, which carry out metabolic reactions near the inner surface of the cell membrane. Read more on how do enzymes work.

The proteins in the cell membrane also help very small molecules to get themselves transported through the cell membrane, provided, the molecules are traveling from a region with lots of molecules to a region with less number of molecules.

Biological Membranes and the Cell Surface

http://www.uic.edu/classes/bios/bios100/f06pm/plasmamemb.jpg

Membrane Functions

Form specialized compartments by selective permeability

Unique environment

Creation of concentration gradients

pH and charge (electrical, ionic) differences

Asymmetric protein distribution

Cell-Cell recognition

Site for receptor molecule biding for cell signaling

Receptor binds ligand (such as a hormone)

Induces intracellular reactions

Controls and regulates reaction sequences

Product of one enzyme is the substrate for the next enzyme

Can “line up” the enzymes in the proper sequence

Membrane Structure According to the Fluid Mosaic Model of Singer and Nicolson

http://www.uic.edu/classes/bios/bios100/f06pm/fmm.jpg

The membrane is a fluid mosaic of phospholipids and proteins

Two main categories of membrane proteins – integral and peripheral

• Peripheral proteins – bound to the surface of the membrane
• Integral proteins – permeate the surface of the membrane

Membrane regions differ in protein configuration and concentration

Outside vs. inside – different peripheral proteins

Proteins only exposed to one surface

Proteins extend completely through – exposed to both surfaces

Membrane lipid layer fluid

Proteins move laterally along membrane

Membrane Lipids

Phospholipids most abundant

Phosphate may have additional polar groups such as choline, ethanolamine, serine, inositol

These increase hydrophilicity

Cholesterol – a steroid

Can comprise up to 50% of animal plasma membrane

Hydrophilic OH groups toward surface

Smaller than a phospholipid and less amphipathic (having both polar and non-polar regions of the molecule)

Other molecules include ceramides and sphingolipds – amino alcohols with fatty acid chains

These lipids distributed asymmetrically

Bilayer Formation

Membrane components are Amphipathic (having both polar and non-polar regions of the molecule)

Spontaneously form bilayers

Hydrophilic portions face water sides

Hydrophobic core

Never have a free end due to cohesion

Spontaneously reseal

Fuse

Liposome – Circular bilayer surrounding water compartment

Can form naturally or artificially

Can be used to deliver drugs and DNA to cells

Membrane Fluidity

Membrane is Fluid

Lipids have rapid lateral movement

Lipids flip-flop extremely slowly

Lipids asymmetrically distributed in membrane

Different lipids in each side of bilayer

Fluidity depends on lipid composition

Saturated fatty acids

All C-C bonds are single bonds

Straight chain allows maximum interaction of fatty acid tails

Make membrane less fliuid

Solid at room temperature

“Bad Fats” that clog arteries (animal fats)

Unsaturated fatty acids

Some C=C bond (double bonds)

Bent chain keeping tails apart

Make membrane more fluid

Polyunsaturated fats have multiple double bonds and bends

Liquid at room temperature

“Good Fats” which do not clog arteries (vegetable fats)

Cholesterol

Reduces membrane fluidity by reducing phospholipid movement

Hinders solidification at low (room) temperatures

How Cells Regulate Membrane Fluidity

Desaturate fatty acids

Produce more unsaturated fatty acids

Change tail length (the longer the tail, the less fluid the membrane)

Membrane Carbohydrates – Glycolipids and Glycoproteins

Face away from cytoplasm (on outside of cell)

Attached to protein or lipid

Blood antigens – Determine blood type – bound to lipids (glycolipids)

Glycoproteins – Protein Receptors

Provide specificity for cell-cell or cell-protein interactions (see below)

Membrane Proteins

Peripheral Proteins

completely on membrane surface

ionic and H-bond interactions with hydrophilic lipid and protein groups

can be removed with high salt or alkaline

Integral Proteins

Possess hydrophobic domains which are anchored to hydrophobic lipids

alpha helix

more complex structure

An Example – Asymetry of Intestinal Epithelial Cell Membranes

Apical surface selectively absorbs materials

Contains specific transport proteins

Lateral surface interacts with neighboring cells

Contains junction proteins to allow cellular communication

Basal surface sticks to extracellular matrix and exchanges with blood

Contains proteins for anchoring

The Extracellular Matrix (ECM) and Plant Cell Walls

In animal cells, the ECM is a mish-mash of proteins (usually collagen) and gel-forming polysaccharides

The ECM is connected to the cytoskeletin via Integrins and Fibronectins

Plant Primary Cell Walls for a rigid cross-linked network of cellulose fibers and pectin – a fiber composite

Fiber composites resist tension and compression

Plant Secondary Cell Walls are further strengthened w/ Lignin

Secondary Cell Walls is basically what comprises wood

Cell to Cell Attachments

Tight Junctions and Desmosomes

Tight Junctions are specialized proteins in the plasma membranes of adjacent animal cells

they “stitch together” adjacent cells

form a watertight cell

Desmosomes are specialized connection protein complexes in animal cells

they “rivet” cells together

they are attached to the intermediate fibers of adjacent cells

Cell Gaps

Plasmodesmata & Gap Junctions

In plant cells, Plasmodesmata are gaps in the cell wall create direct connections between adjacent cells

May contain proteins which regulate cell to cell exchange

form a continuous cytoplasmic connection between cells called the symplast

In animal cells, Gap Junctions are holes lined with specialized proteins

allow cell-cell communication (this is what coordinates your heartbeat)

Cell Communication

In multi-cellular organism, cells can communicate via chemical messenger

Three Stages of Cellular Communication

Reception

A chemical message (ligand) binds to a protein on the cell surface 

Transduction

The binding of the signal molecule alters the receptor protein in some way.

The signal usually starts a cascade of reactions known as a signal transduction pathway

Response

The transduction pathway finally triggers a response

The responses can vary from turning on a gene, activating an enzyme, rearranging the cytoskeleton

There is usually an amplification of the signal (one hormone can elicit the response of over 108 molecules

No matter where they are located, signal receptors have several general characteristics

signal receptors are specific to cell types (i.e. you won’t find insulin receptors on bone cells)

receptors are dynamic

the number of receptors on a cell surface is variable

the ability of a molecule to bind to the receptor is not fixed (i.e. it may decline w/ intense stimulation)

receptors can be blocked

Two Methods of Cell-Cell Communication

Steroid Hormones can enter directly into a cell

bind to receptors in the cytosol

hormone-receptor complex binds to DNA, inducing change

testosterone, estrogen, progesterone are examples of steroid hormones

Signal Transduction – conversion of signals from one form to another

Very complicated pathways – all are different!

G Protein receptors

G-proteins are called as such because they have GTP bound to them

Receptors have inactive G-proteins associated with them

When the signal binds to the receptor, the G-protein changes shape and becomes active (into the “on configuration)

The active G-protein binds to an enzyme which produces a secondary message

Frequently, second messengers activate other messengers, creating a cascade…

G-protein signal transduction sequences are extremely common in animal systems

embryonic development

human vision and smell

over 60% of all medications used today exert their effects by influencing G-protein pathways

Tyrosine-Kinase Receptors – Another Example of a Signal Transduction Pathway

Tyrosine-Kinase Receptors often have a structure similar to the diagram below:

http://www.uic.edu/classes/bios/bios100/f06pm/tyro-kin02.jpg

Part of the receptor on the cytoplasmic side serves as an enzyme which catalyzes the transfer of phosphate groups from ATP to the amino acid Tyrosine on a substrate protein

The activation of a Tyrosine-Kinase Receptor occurs as follows:

Two signal molecule binds to two nearby Tyrosine-Kinase Receptors, causing them to aggregate, forming a dimer

The formation of a dimer activated the Tyrosine-Kinase portion of each polypeptide

The activated Tyrosine-Kinases phosphorylate the Tyrosine residues on the protein

The activated receptor protein is now recognized by specific relay proteins

They bind to the phosphorylated tyrosines, which cause, you guessed it, a conformation change.

The activated relay protein can then trigger a cellular response

One activated Tyrosine-Kinase dimer can activate over ten different relay proteins, each which triggers a different response

The ability of one ligand binding event to elicit so many response pathways is a key difference between these receptors and G-protein-linked receptors (that, and the absence of G- proteins of course…)

Abnormal Tyrosine-Kinases that aggregate without the binding of a ligand have been linked with some forms of cancer

Signal Transduction Shutdown

Most signal-transduction/hormone systems are designed to shut down rapidly

Enzymes called phosphatases remove the phosphate groups from secondary messengers in the cascade

This will shut down the signal transduction pathway… at least until another signal is received