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• Structure and Function of Cell membrane
• Chemical composition
• Permeability of cell membrane
• Active transport
• The cell membrane (also known as the plasma membrane or cytoplasmic
membrane, and historically referred to as the plasmalemma) is a biological
membrane that separates the interior of all cells from the outside environment (the
• It consists of a lipid bilayer with embedded proteins. The basic function of the
cell membrane is to protect the cell from its surroundings.
• The cell membrane controls the movement of substances in and out of cells and
organelles. In this way, it is selectively permeable to ions and organic molecules.
• The lipid bilayer hypothesis, proposed in 1925 by Gorter and Grendel, created speculation to the
description of the cell membrane bilayer structure based on crystallographic studies and soap
• In 1925 it was determined by Fricke that the thickness of erythrocyte and yeast cell membranes
ranged between 3.3 and 4 nm, a thickness compatible with a lipid monolayer.
• The instrument could resolve thicknesses that depended on pH measurements and the presence of
membrane proteins that ranged from 8.6 to 23.2 nm, with the lower measurements supporting the
lipid bilayer hypothesis.
Fluid mosaic model
• The fluid mosaic model explains various observations regarding the structure of
functional cell membranes. The model, which was devised by SJ Singer and GL
Nicolson in 1972, describes the cell membrane as a two-dimensional liquid that restricts
the lateral diffusion of membrane components.
• Such domains are defined by the existence of regions within the membrane with special
lipid and protein composition that promote the formation of lipid rafts or protein
and glycoprotein complexes.
• The current model describes important features relevant to many cellular processes,
including: cell-cell signaling, apoptosis, cell division, membrane budding, and cell fusion.
• The cell membrane consists primarily of a thin layer of amphipathic phospholipids that spontaneously
arrange so that the hydrophobic "tail" regions are isolated from the surrounding water while the
hydrophilic "head" regions interact with the intracellular (cytosolic) and extracellular faces of the
• This forms a continuous, spherical lipid bilayer. Hydrophobic interactions (also known as
the hydrophobic effect) are the major driving forces in the formation of lipid bilayers. An increase in
interactions between hydrophobic molecules (causing clustering of hydrophobic regions) allows water
molecules to bond more freely with each other, increasing the entropy of the system.
• This complex interaction can include noncovalent interactions such as van der Waals, electrostatic and
• Cell membranes contain a variety of biological molecules, notably lipids and proteins.
Specifically, the amount of cholesterol in human primary neuron cell membrane changes,
and this change in composition affects fluidity throughout development stages.
• The cell membrane consists of three classes of amphipathic lipids:
phospholipids, glycolipids, and sterols.
• The amount of each depends upon the type of cell, but in the majority of cases
phospholipids are the most abundant, often contributing for over 50% of all lipids in
• The fatty chains in phospholipids and glycolipids usually contain an even number of
carbon atoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are the
most common. Fatty acids may be saturated or unsaturated, with the configuration of the
double bonds nearly always "cis".
• The length and the degree of unsaturation of fatty acid chains have a profound effect on
membrane fluidity as unsaturated lipids which preventing the fatty acids from packing
together as tightly, thus decreasing the melting temperature (increasing the fluidity) of the
Functions of Cell membrane
• The cell membrane surrounds the cytoplasm of living cells, physically separating
the intracellular components from the extracellular environment.
• The cell membrane also plays a role in anchoring the cytoskeleton to provide
shape to the cell, and in attaching to the extracellular matrix and other cells to hold
them together to form tissues.
• The cell membrane is selectively permeable and able to regulate what enters and
exits the cell, thus facilitating the transport of materials needed for survival.
Passive Transport (Diffusion)
• Some substances (small molecules, ions) such as carbon dioxide (CO2) and
oxygen (O2), can move across the plasma membrane by diffusion, which is a
passive transport process.
• Diffusion occurs when small molecules and ions move freely from high
concentration to low concentration in order to equilibrate the membrane.
• It is considered a passive transport process because it does not require energy and
is propelled by the concentration gradient created by each side of the membrane
Passive Transport (Osmosis)
• Osmosis, in biological systems involves a solvent, moving through a semipermeable
membrane similarly to passive diffusion as the solvent still moves with the concentration
gradient and requires no energy.
• It is of three different types, i.e.
• Hypertonic (where the concentration of solutes is greater outside the cell than inside it).
• Hypotonic (where the concentration of solutes is greater inside the cell than outside it).
• Isotonic (where the concentration of solutes is same outside the cell and inside it.)
Active Transport (Endocytosis)
• Endocytosis is the process in which cells absorb molecules by engulfing them. The
plasma membrane creates a small deformation inward, called an invagination, in
which the substance to be transported is captured. This invagination is caused by
proteins on the outside on the cell membrane.
• Endocytosis is a pathway for internalizing solid particles ("cell eating"
or phagocytosis), small molecules and ions ("cell drinking" or pinocytosis).
• Endocytosis requires energy and is thus a form of active transport.
Active transport Sodium-Potassium
• Pumping Na+ (sodium ions) out and K+ (potassium
ions) in against strong concentration gradients, called
• 3 Na+ pumped in for every 2 K+ pumped out; creates a
• Exocytosis occurs in various cells to remove undigested residues of
substances brought in by endocytosis, to secrete substances such as
hormones and enzymes, and to transport a substance completely across a
• In the process of exocytosis, the undigested waste-containing food vacuole
or the secretory vesicle budded from Golgi apparatus, is first moved by
cytoskeleton from the interior of the cell to the surface.
• Mitochondria are thought to have originated from an ancient symbiosis that resulted when a
nucleated cell engulfed an aerobic prokaryote.
• The engulfed cell came to rely on the protective environment of the host cell, and, conversely, the
host cell came to rely on the engulfed prokaryote for energy production.
• It varies from cell to cell and from species to species. Certain cells contain exceptionally large
number of the mitochondria, e.g., the Amoeba, Chaos chaos contain 50,000; eggs of sea urchin
contain 140,000 to 150,000 and oocytes of amphibians contain 300,000 mitochondria.
• The mitochondria may be filamentous or granular in shape and may change
from one form to another depending upon the physiological conditions of
• Thus, they may be of club, racket, vesicular, ring or round-shape.
Mitochondria are granular in primary spermatocyte or rat, or club-shaped in
liver cells size.
• A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and proteins. The
two membranes have different properties. Because of this double-membraned organization, there are five
distinct parts to a mitochondrion. They are:
1. The outer mitochondrial membrane.
2. The intermembrane space (the space between the outer and inner membranes).
3. The inner mitochondrial membrane.
4. The cristae space (formed by enfolding of the inner membrane).
5. The matrix (space within the inner membrane).
• Mitochondria stripped of their outer membrane are called mitoplasts.
• The outer mitochondrial membrane, which encloses the entire organelle, is 60 to
75 angstroms (Å) thick. It has a protein-to-phospholipid ratio similar to that of the
eukaryotic plasma membrane.
• It contains large numbers of integral membrane proteins called porins. These porins form
channels that allow molecules of 5000 Daltons or less in molecular weight to
freely diffuse from one side of the membrane to the other.
• The mitochondrial outer membrane can associate with the endoplasmic reticulum (ER)
membrane, in a structure called MAM (mitochondria-associated ER-membrane).
• The intermembrane space is the space between the outer membrane and the
inner membrane. It is also known as peri-mitochondrial space.
• Because the outer membrane is freely permeable to small molecules, the
concentrations of small molecules, such as ions and sugars, in the
intermembrane space is the same as in the cytosol.
• One protein that is localized to the intermembrane space in this way
is cytochrome c.
• The inner mitochondrial membrane is compartmentalized into
numerous cristae, which expand the surface area of the inner mitochondrial
membrane, enhancing its ability to produce ATP.
• For typical liver mitochondria, the area of the inner membrane is about five
times as large as the outer membrane.
• This ratio is variable and mitochondria from cells that have a greater
demand for ATP, such as muscle cells, contain even more cristae.
• 65-70% protein
• 65-70% in matrix
• 30-35% membranes
• 25-30% lipids
• Phospholipids (Cardiolipin)
• 5% Cholestrol
• 5% free Fatty acids
• 0.5% RNA
• Traces of DNA
• The mitochondrial matrix contains a highly-concentrated mixture of hundreds of enzymes. These include
most of the enzymes that participate in the Krebs Cycle. The TCA Cycle occurs in the mitochondrial matrix.
• The ribosome in mitochondria are of 70S type - as found in prokaryotic cells (bacteria), as opposed to the 80S
type present in many plant and animal cells. They can synthesize proteins.
• Mitochondria have their own genetic material and the facility to produce their own ribonucleic acids (RNAs)
and proteins. All of the mitochondrial DNA is maternal. Mitochondrial DNA carries genes necessary for the
synthesis of many, but not all, mitochondrial proteins.
• The most prominent roles of mitochondria are to produce the energy
currency of the cell, ATP (i.e., phosphorylation of ADP), through
respiration, and to regulate cellular metabolism.
• The central set of reactions involved in ATP production are collectively
known as the citric acid cycle, or the Krebs cycle.
• However, the mitochondrion has many other functions in addition to the
production of ATP.
• A dominant role for the mitochondria is the production of ATP, as reflected by the large number of proteins in
the inner membrane for this task.
• This is done by oxidizing the major products of glucose: pyruvate, and NADH, which are produced in the
cytosol. This type of cellular respiration known as aerobic respiration, is dependent on the presence
• The production of ATP from glucose has an approximately 13-times higher yield during aerobic respiration
compared to fermentation.
• ATP crosses out through the inner membrane with the help of a specific protein, and across the outer
membrane via porins. ADP returns via the same route.
• Mitochondria play a central role in many other metabolic tasks, such as:
• Signaling through mitochondrial reactive oxygen species.
• Regulation of the membrane potential.
• Apoptosis-programmed cell death.
• Calcium signaling (including calcium-evoked apoptosis).
• Regulation of cellular metabolism.
• Certain heme synthesis reactions.
• Steroid synthesis.
• Hormonal signaling
• The Golgi apparatus, also known as the Golgi complex, Golgi body, or simply the Golgi, is
an organelle found in most eukaryotic cells.
• In most eukaryotes, the Golgi apparatus is made up of a series of compartments consisting of two
main networks: the cis Golgi network (CGN) and the trans Golgi network (TGN).
• The CGN is a collection of fused, flattened membrane-enclosed disks known
as cisternae (singular: cisterna), originating from vesicular clusters that bud off the endoplasmic
• Proteins synthesized in the ER are packaged into vesicles, which then fuse with the Golgi
apparatus. The Golgi apparatus is also involved in lipid transport and lysosome formation.
• Removal of mannose residues and addition of N-acetyl glucosamine occur in medial cisternae.
Addition of galactose and sialic acid occurs in the trans cisternae.
• The vesicles that leave the rough endoplasmic reticulum are transported to the cis face of the Golgi
apparatus, where they fuse with the Golgi membrane and empty their contents into the lumen.
Once inside the lumen, the molecules are modified, then sorted for transport to their next
• The lacy membranes of the endoplasmic reticulum were first seen in 1945 by Keith R. Porter and
Albert Claude, using electron microscopy. The word reticulum, which means "network", was
applied to describe this fabric of membranes.
• The endoplasmic reticulum (ER) is a type of organelle found in eukaryotic cells that forms an
interconnected network of flattened, membrane-enclosed sacs or tube-like structures known
• The endoplasmic reticulum occurs in most types of eukaryotic cells, but is absent from red blood
cells and spermatozoa.
Rough and Smooth
• The rough endoplasmic reticulum is studied with protein-manufacturing ribosomes giving it a
"rough" appearance is especially prominent in cells such as hepatocytes.
• The outer (cytosolic) face of the rough endoplasmic reticulum is studded with ribosomes that are
the sites of protein synthesis.
• The smooth endoplasmic reticulum lacks ribosomes and functions in lipid manufacture and
metabolism, the production of steroid hormones, and detoxification.
• It synthesizes lipids, phospholipids, and steroids. Cells which secrete these products, such as those
in the testes, ovaries, and sebaceous glands have an abundance of smooth endoplasmic reticulum.
• A lysosome is a membrane-bound organelle found in nearly all animal cells.
They are spherical vesicles which contain hydrolytic enzymes that can
break down many kinds of biomolecules.
• Lysosomes are known to contain more than 60 different enzymes. Enzymes
of the lysosomes are synthesized in the rough endoplasmic reticulum.
• The enzymes are imported from the Golgi apparatus in small vesicles,
which fuse with larger acidic vesicles.
• Besides degradation of polymers, the lysosome is involved in various cell processes, including
secretion, plasma membrane repair, cell signaling, and energy metabolism.
• Material from outside the cell is taken-up through endocytosis, while material from the inside of
the cell is digested through autophagy. Their sizes can be very different the biggest ones can be
more than 10 times bigger than the smallest ones.
• Synthesis of lysosomal enzymes is controlled by nuclear genes. Mutations in the genes for these
enzymes are responsible for more than 30 different human genetic disorders, which are collectively
known as lysosomal storage diseases.
• The ribosome is a complex molecular machine, found within all living cells, that serves
as the site of biological protein synthesis (translation).
• Ribosomes consist of two major components: the small ribosomal subunit, which reads
the RNA, and the large subunit, which joins amino acids to form a polypeptide chain.
• Prokaryotic ribosomes are around 20 nm (200 Å) in diameter and are composed of 65%
rRNA and 35% ribosomal proteins. Eukaryotic ribosomes are between 25 and
30 nm (250–300 Å) in diameter.
• They assemble amino acids to form specific proteins, proteins are essential to carry out cellular activities.
• The process of production of proteins, the deoxyribonucleic acid produces mRNA by the process of DNA
• The genetic message from the mRNA is translated into proteins during DNA translation.
• The sequences of protein assembly during protein synthesis are specified in the mRNA.
• The mRNA is synthesized in the nucleus and is transported to the cytoplasm for further process of protein
• The proteins that are synthesized by the ribosomes present in the cytoplasm are used in the cytoplasm itself.
• The proteins produced by the bound ribosomes are transported outside the cell.
• It is type of organelle known as a micro body, found in virtually all eukaryotic cells. They are
involved in catabolism of very long chain fatty acids and polyamines, reduction of reactive oxygen
species specifically hydrogen peroxide.
• They also contain approximately 10% of the total activity of two enzymes in the pentose phosphate
pathway, which is important for energy metabolism.
• It is vigorously debated whether peroxisomes are involved in isoprenoid and cholesterol synthesis
• The protein content of peroxisomes varies across species, but the presence of proteins common to
many species has been used to suggest an endosymbiosis origin.
• A major function of the peroxisome is the breakdown of very long chain fatty acids through beta-
oxidation. In animal cells, the long fatty acids are converted to medium chain fatty acids.
• They are subsequently shuttled to mitochondria where they are eventually broken down to carbon
dioxide and water. In yeast and plant cells, this process is carried out exclusively in peroxisomes.
• The peroxisome of plant cells is polarized when fighting fungal penetration. Infection causes
a glucosinolate molecule to play an antifungal role to be made and delivered to the outside of the
cell through the action of the paroxysmal proteins.
• In 1903, Nikolai K. Koltsov proposed that the shape of cells was determined by a network of
tubules that he termed the cytoskeleton.
• A cytoskeleton is present in all cells of all domains of life. It is a complex network of
interlinking filaments and tubules that extend throughout the cytoplasm, from the nucleus to the
• The structure, function and dynamic behavior of the cytoskeleton can be very different, depending
on organism and cell type. Even within one cell the cytoskeleton can change through association
with other proteins and the previous history of the network.
Kinds and functions
• Eukaryotic cells contain three main kinds of cytoskeletal filaments: microfilaments, microtubules,
and intermediate filaments.
• Each cytoskeletal filament type is formed by polymerization of a distinct type of protein subunit and has its
own characteristic shape and intracellular distribution.
• Microfilaments, also called actin filaments, are filaments composed of polymers of actin, usually about
7 nm in diameter, in the cytoplasm of eukaryotic cells.
• Microfilament functions include cytokinesis, amoeboid movement and cell motility in general, changes in cell
shape, endocytosis and exocytosis, cell contractility and mechanical stability.
• Microtubules are a component of the cytoskeleton, found throughout the cytoplasm. These
tubular polymers of tubulin can grow as long as 50 micro meters and are highly dynamic.
• The outer diameter of a microtubule is about 24 nm while the inner diameter is about 12 nm. They
are found in eukaryotic cells, as well as some bacteria.
• They are involved in maintaining the structure of the cell and, together
with microfilaments and intermediate filaments, they form the cytoskeleton.
• They also make up the internal structure of cilia and flagella. They provide platforms
for intracellular transport and are involved in a variety of cellular processes, including the
movement of secretory vesicles and organelles.
• Intermediate filaments (IFs) are cytoskeletal components found in the cells
of vertebrate animal species, and perhaps also in other animals, fungi, plants, and unicellular organisms.
• They are composed of a family of related proteins sharing common structural and sequence features.
Initially designated 'intermediate' because their average diameter (10 nm) is between those of
narrower microfilaments (actin) and wider myosin filaments found in muscle cells.
• The diameter of Intermediate filaments is now commonly compared to actin microfilaments (7 nm)
and microtubules (25 nm). Most types of intermediate filaments are cytoplasmic, but one type,
the lamins, are nuclear.