The document discusses the structure, composition, and functions of the cell membrane. It begins by introducing the cell membrane and its key components - a lipid bilayer and membrane proteins. It then describes the fluid mosaic model of the cell membrane's structure, which depicts proteins floating within the lipid bilayer. The document outlines the chemical composition of the cell membrane, including phospholipids, cholesterol, and carbohydrates. It concludes by explaining several important functions of the cell membrane, such as controlling movement of particles into and out of the cell and maintaining the physical integrity of the cell.
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MIBCC 101(CELL Membrane - Structure,composition and Functions).pdf
1. CELL MEMBRANE - STRUCTURE,
COMPOSITION AND FUNCTIONS
Prepared by-
Name : AMARNATH UPADHYAY
Class Roll No. : 21
Paper code : MIBCC 101
Dept. : MICROBIOLOGY
University : Assam University
3. INTRODUCTION
The first cell was discovered by Robert Hooke in 1665 using a microscope. His discovery of cells led
to the proposal of Cell Theory. The cell theory was proposed by Theodor Schwann and Matthias
Jakob Schleiden 1830s. According to them :
All living organisms are composed of one or more cells.
The cell is the most basic unit of life.
All cells arise only from pre-existing cells.
The cell membrane, also called the plasma membrane,
is found in all cells and separates the interior of the cell
from the outside enviroment. The cell membrane, consists
of a lipid bilayer that is semipermeable. The cell membrane regulates the transport of materials
entering and exiting a cell.
The membrane also gives a cell its shape and enables the cell to attach to other cells.
It is involved in a variety of cellular process such as Cell adhesion, Ion conductivity and cell signaling.
Provides mechanical strength to the cell.
4. WHAT IS A CELL MEMBRANE ?
A cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically
referred to as plasmalemma), is a biological membrane that separates the interior of all cells from the
outside enviroment.
The cell membrane consists of a lipid bilayer, including cholesterols( a lipid component) that sit
between phospholipids to maintain their fluidity at various temperatures.
The membrane also contains membrane proteins, including integral proteins that go across the
membrane serving as membrane transporters, and peripheral proteins that loosely attach to the outer
side of the cell membrane, acting as
enzymes shaping the cell.
The cell membrane controls the movement of
substances in and out of cells and organelles.
In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion
conductivity and serve as the attachment surface for several extracellular structures, including the cell
wall, the carbohydrates layer and the intercellular network of protein fibres.
5. STRUCTURE OF CELL MEMBRANE
The cell membranes are typically composed of lipids, proteins and carbohydrates. They contain about
50% lipids, 40% proteins and 10% carbohydrates of various types. The most commonly accepted model
for the structure of cell membrane(the Fluid Mosaic Model) was developed by Seymour T. Singer and
Garth L. Nicolson in 1972 and shows proteins “floating like icebergs in a sea of lipid”.
The lipid bilayer portion of a bacterial cell membrane is
composed primarily of phospholipids. These are
amphipathic/amphiphilic molecules ,i.e., they have both
polar and non-polar portions arranged with their
hydrophobic tails toward the inside of the membrane and
their hydrophilic heads towards the watery enviroment either inside or outside the cell.
The proteins associated are usually globular in form. They are divided into two namely integral proteins
and peripheral proteins. The orientation of these molecules is influenced by their solubility in either lipid
or water, and is a major factor maintaining the structural integrity of the membrane itself.
6. FLUID MOSAIC MODEL
In 1972, SJ Singer and GL Nicolson proposed fluid mosaic model.
Fluid – Individual phospholipids and some proteins can move sideways(laterally) in each
layer.
Mosaic – Range of different proteins resting on the surface on through the phospholipid
layer gives it a mosaic appearance.
Fig. Fluid Mosaic Model
7. CHEMICAL COMPOSITION
Cell membranes contain a variety of biological molecules. The principal components are as follows :
Lipids ( phospholipids and cholesterol)
Proteins
Carbohydrates
1. Lipids
Phospholipids :
It is a lipid made of glycerol, two-fatty acid tails and a
phosphate linked head group. They usually involve two-layers
of phospholipids with their tails pointing inward, an arrangement
called a phospholipid bilayer. They are well suited for this role because
they are amphipathic, meaning that they have both hydrophilic and
hydrophobic regions.
8. CONT.….
Cholesterol : It is another lipid component of animal cell. Cholesterol molecules are
selectively dispersed between membrane phospholipids. This helps to keep cell membranes
from becoming stiff by preventing phospholipids from being too closely packed.
2. Proteins
Integral proteins : They are integrated into the membrane and they have at least one
hydrophobic region that anchors them to the hydrophobic core of the phospholipid bilayer.
Peripheral proteins : They are found on the outside and inside surfaces of the membranes,
attached either to integral proteins or to phospholipids.
3. Carbohydrates :
They are found on the outside 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 can be either straight or branched.
9. FUNCTIONS OF THE CELL MEMBRANE
To maintain the physical integrity of the cell, that is to mechanically enclose the contents of the
cell.
To control the movement of particles, e.g. Ions or molecules, into and out of the cell.
The cell membrane forms a barrier between the inside of the cell and the enviroment outside the
cell-enclosing cytoplasm and other organelles within the cell, and enabling different chemical
enviroments to exist on each side of the cell membrane.
Proteins and lipids make up the composition of a cell membrane. There are three different types
of proteins found within a cell membrane namely structural protein, transparent protein and
glycoprotein. These support cell structure and shape, move molecules through the membrane
and transmit signals between cells.
Cell membranes often include receptor sites for introduction with specific biochemicals such as
hormones, neurotransmitters and immune proteins. In this way, the cell can recognize and
process some signals received from the extracellular enviroment.
Endocytosis is the process in which cells absorb molecules by engulfing them. It is a pathway for
internalizing solid particles( cell eating), small molecules and ions( cell drinking), and
macromolecules.
10. REFERENCES :
Structure of the cell membrane – The cell – NCBI Bookshelf ( retrieved on 27-01-2022)
https://www.ncbi.nlm.nih.gov
Google chrome, Cell membrane – Wikipedia
https://en.m.wikipedia.org ( retrieved on 27-01-2022)
Dr. Renu Negi, Associate Professor, Department of Botany, PDBH Govt. College, Kotdwar, General
Microbiology book, ( 2009 edition) [data retrieved on 29-01-2022]
Studies in Botany, by J.N.Mitra, D.Mitra and S.V.Chowdhuri, ninth revised edition
Kimball’s Biology pages, Cell membranes ( retrieved on 29-01-2022)
11. CONCLUSION :
While Robert Hooke’s discovery of cells in 1665 led to the proposal of cell theory, Hooke mislead the cell
membrane theory, that all cells contained a hard cell wall, since only plant cells could be observed at that
time. In the early 19th century, cell were recognized as being separate entities. This theory extended to
include animal cells also. Microscope was still not advanced enough to make a distinction between cell
membranes and cell walls. However, some microscopists correctly identified at this time that while
invisible, it could be inferred that cell membranes existed in animal cells due to intracellular
movements of components internally but not externally and
that membranes were not the equivalent of a cell wall to plant cell.
Many refuted the existence of a cell membrane still towards the
end of 19th century. Despite the numerous models of cell
membranes proposed prior to the “Fluid Mosaic Model”, it
remains the primary archetype for the cell membrane. Although
the fluid mosaic model has been modernized to detail
contemporary discoveries the basics have remained constant.
