Prokaryotes & Eukaryotes
Components of cell
Cytoplasm with its
Fluid Mosaic Model
Cytoplasm & its organelles
Structure & Functions
Cell is the universal functional unit of all forms of life.
On the basis of differences in cell structure, all life
forms are divided into two major classes. They are
prokaryotes and eukaryotes.
PROKARYOTES & EUKARYOTES
LECTURE NOTES ON CELL & MEMBRANE TRANSPORT
Dr Vijay Marakala, MBBS. M.D.
Department of Biochemistry,
SIMS & RC, MUKKA - SURATHKAL, MANGALORE.
A COMPARISON OF PROKARYOTES AND EUKARYOTES
ORGANELLE PROKARYOTES EUKARYOTES
Nucleus No definite nucleus; DNA present but not
separate from rest of cell
Cell membrane Present Present
Mitochondria None; enzymes for oxidation reactions
located on plasma membrane
Ribosomes Present Present
COMPONENTS OF THE CELL
The outermost structure of the cell that decides its contour is the cell
It is a lipid bi-layer. It also consist of proteins and small amounts of
Membranes are asymmetric. The outer and inner surfaces have different
components and different enzymatic activities.
Membranes are fluid structures. The unsaturated fatty acids bound to
phospholipids contribute to the fluid state of the membrane. At body
temperature lipids are in a fluid state and this fluidity of the membrane is
essential for the normal functioning to occur.
Cell Membrane (Plasma Membrane)
Cytoplasm with its organelles
FLUID MOSAIC MODEL OF CELL MEMBRANE
In 1972 Singer and Nicolson postulated a theory of membrane structure
called the fluid mosaic model.
A mosaic is a structure made up of many different parts. Likewise, the
plasma membrane is composed of different kind of macromolecules like
phospholipid, integral proteins, peripheral proteins, glycoproteins,
glycolipids and cholesterol.
According to this model the matrix or continuous part of membrane
structure, is a polar lipid bilayer. Phospholipids are arranged in bilayers
with polar head groups oriented towards the extracellular side and
cytoplasmic side with hydrophobic (nonpolar) tails face each other at the
core of the bilayer.
The lipid bilayer is fluid because of more number of unsaturated fatty
acids. The cholesterol content of the membrane alters the fluidity (More
cholesterol less fluid).
Proteins are interspersed in the lipid bilayer, of the plasma membrane,
producing a mosaic effect.
FUNCTIONS OF CELL MEMBRANE
1. It is fluid and dynamic.
2. It is semipermeable; only selected compounds are allowed to pass
through from out-side. The selective permeability is responsible for the
maintenance of internal environment of the cell and for creating
potential difference across the membrane.
3. The modification of the cell membrane results in formation of
specialized structures like axon of nerves, microvilli of intestinal
epithelium and tail of spermatids.
Three major classes – phospholipids, glycolipids and cholesterol
They are all amphipathic molecules, that is, they have both
hydrophobic and hydrophilic ends.
PROTEINS OF THE CELL MEMBRANE
Two major categories
1. Integral or intrinsic or transmembrane proteins: are either partially or
totally (transmembrane proteins) immersed in lipid bilayer. They serve
as channels (pores) and carrier proteins.
2. Peripheral or extrinsic proteins: function almost entirely as enzymes
Most of membrane proteins are glycoproteins. They have short chains of
carbohydrates on the exterior side of the membrane.
MEMBRANE CARBOHYDRATES (THE CELL GLYCOCALAYX)
Membrane carbohydrate is not free. It occurs as glycoproteins or
Many of the carbohydrates act as receptor substances for binding
hormones such as Insulin
CYTOPLASM AND ITS ORGANELLES
The extra nuclear cell content that possess both organelles and other
material constitutes cytoplasm. Material other than subcellular
components in the cytoplasm makes up the cytosol. Cytosol contains
mainly dissolved proteins, electrolytes and glucose.
Five important organelles that are suspended in the cytoplasm are:
MITOCHONDRIA (Power house of the cell)
Mitochondria consist of
outer and inner
The outer membrane is
composed of equal
amount of protein and
The outer membrane is
freely permeable to many
The inner membrane consists of 75% protein and remainder is lipid.
The inner membrane is folded to form number of invaginations known as
cristae extending to matrix. Cristae give large surface area and are the
site of oxidative phosphorylation.
Mitochondrion is the power house of the cell. It is responsible for the
production of energy in the form of ATP.
Mitochondrial matrix contains enzymes of the citric acid cycle and β –
Mitochondria contains some DNA known as mitochondrial DNA
The endoplasmic reticulum (ER) is part of a continuous single-membrane
system throughout the cell; the membrane doubles back on itself to give
the appearance of a double membrane in electron micrographs. The
endoplasmic reticulum is attached to the cell membrane and to the nuclear
membrane. It occurs in two forms, rough and smooth. The rough
endoplasmic reticulum is studded with ribosomes bound to the membrane.
Ribosomes, which can also be found free in the cytosol, are the sites of
protein synthesis in all organisms. The smooth endoplasmic reticulum (SER)
does not have ribosomes bound to it
FUNCTIONS OF ENDOPLSAMIC RETICULUM
Ribosomes and rough endoplasmic reticulum are involved in protein
SER of intestinal cells is involved in formation of triglycerides.
In the adrenal cortex, SER is the site of steroid formation.
Cytochrome P450 dependent monooxygenases are present in liver
Golgi apparatus is separate from the endoplasmic reticulum but is frequently
found close to the smooth endoplasmic reticulum. It is a series of
1. The Golgi apparatus is involved in secretion of proteins from the cell.
Material produced in the cell for export is processed by Golgi body and
is packaged as vesicle and is pinched off. The vesicles fuse with plasma
membrane and their content is released to exterior by the process
known as exocytosis. The digestive enzymes of pancreas and insulin are
produced and released in this way.
2. Golgi apparatus helps in the formation of other subcellular organelles
like lysosomes and peroxisomes.
3. Golgi apparatus is involved in protein targeting.
They are small vesicles present in cytoplasm. They are surrounded by a
membrane enclosed sacs containing hydrolytic enzymes that could cause
considerable damage to the cell if they were not physically separated from
the lipids, proteins, or nucleic acids that they are able to attack. Lysosomes
are called as ‘Suicidal bags’ of the cell.
1. Lysosomes are rich in hydrolytic enzymes, which are active at acidic
pH. The lysosomal enzymes digest the molecules brought into the
cell by phagocytosis.
2. Macrophages are rich in lysosomes.
3. Lack of one or more of lysosomal enzymes cause accumulation of
materials in the cell resulting in lysosomal diseases.
Peroxisomes are similar to lysosomes; their principal characteristic is that
they contain enzymes involved in the metabolism of hydrogen peroxide
(H2O2), which is toxic to the cell. The enzyme catalase, which occurs in
peroxisomes, catalyzes the conversion of H2O2to H2O and O2
Biological membranes are semipermeable membranes. Permeability is
conferred by membrane proteins; these are called channel proteins and
There are two types of transport mechanisms
1. Passive transport or passive diffusion and
2. Active transport
PASSIVE TRANSPORT OR PASSIVE DIFFUSION
Transport of solute molecules from high concentration to low
concentration across membrane is called passive transport. It is a
spontaneous process because it is thermodynamically favourable. It is a
downhill transport and requires no energy.
Two types of passive transport,
Simple diffusion and
Transport of solute molecules from high concentration to low
concentration across membrane is known as simple diffusion. Lipid
soluble (lipophilic) molecules can pass through the cell membrane,
without any interaction with carrier proteins.
Simple diffusion can occur through the cell membrane by ways.
Through the interstices of the lipid bilayer if the diffusing
substance is lipid soluble and
Through the channel proteins.
Example: Transport of O2, CO2, N2, ethanol and urea
Facilitated Diffusion (Carrier mediated diffusion)
Carrier molecules present in membranes mediate transport of many solute
molecules across membrane. Hence, mediated transport involves carrier
molecules. They are known as permeases, translocases, transporters or
pumps. Most of them are proteins.
Facilitated diffusion is faster than simple diffusion. It requires no energy.
Facilitated diffusion by carrier molecule involves conformational change of
carrier molecule. The carrier molecule exists in two conformations. It has
binding site to solute molecule. In the native conformation the carrier is
exposed to high concentration of solute. Then the solute molecules bind to
the sites on carrier molecule. A conformational change in carrier molecule
occurs. It exposes solute molecule to low concentration and solute
molecules are released into the cell. The empty carrier returns to the
native state to transport solute molecules once again.
Example: Transport of glucose and most of amino acids.
Facilitated diffusion displays
saturation behavior while the
rate of transport of a solute in
simple diffusion across a
membrane is directly
proportional to its concentration.
It requires energy in addition to carrier molecules. It moves solute
molecules from low concentration to high concentration or against
Substances that are actively transported through cell membranes
, several different sugars and
most of amino acids.
Active transport is classified into two types according to the source
of energy used as follows:
1. Primary active transport and
2. Secondary active transport.
Primary active transport
Energy is derived directly from hydrolysis of ATP.
Example: , Na+
transport across the membrane.
Primary active transport of Na+ and K+ / sodium-potassium pump
Active transport by Na+
ATPase. Three sodium ions bind
to the transporter protein on
the cytoplasmic side of the
membrane. When ATP is
hydrolyzed to ADP, the carrier
protein is phosphorylated and
undergoes a change in
conformation that causes the
sodium ions to be released into
the extracellular fluid.
Two potassium ions then bind on the extracellular side. Dephosphorylation
of the carrier protein produces another conformational change, and the
potassium ions are released on the inside of the cell membrane. The
transporter protein then resumes its original conformation, ready to bind
more sodium ions.
Importance of Na+-K+ pump
gradient created by this pump in the cells, controls cell
Renders neurons and muscles electrically excitable and
Drives the active transport of sugars and amino acids.
Secondary active transport
Are those active transport systems in which transport of molecules is
indirectly linked to hydrolysis of ATP. The Na+
gradient, which is maintained
by primary active transport, is used to power the transport of glucose,
amino acids, and many other compounds into the cell through secondary
active transport. An example is provided by the transport of glucose into
cells of the intestinal epithelium in conjunction with Na+
transport of glucose
by the Na+
Sodium ion binds to the carrier protein in the luminal membrane, stimulating
the binding of glucose. After a conformational change, the protein releases
and glucose into the cell and returns to its original conformation. Na+
ATPase in the basolateral membrane pumps Na+
against its concentration
gradient into the extracellular fluid. Thus, the Na+
concentration in the cell is
low, and Na+
moves from the lumen down its concentration gradient into the
cell and is pumped against its gradient into the extracellular fluid.
Glucose, consequently, moves
against its concentration
gradient from the lumen into
the cell by traveling on the
same carrier as Na+
then passes down its
concentration gradient into
the extracellular fluid on a
passive transporter protein.
UNIPORT, SYMPORT AND ANTIPORT
UNIPORT: Carries single solute
across the membrane. E.g. glucose
CO-TRANSPORT: If the transport
of one molecule depends on
simultaneous or sequential
transfer of another molecule, it is
called co-transport. The co-
transport system may either be a
symport or an antiport
SYMPORT: The transporter system
carries two solutes in the same
direction across the membrane.
E.g. Sodium dependent glucose
ANTIPORT: Carries two solutes or
ions in opposite direction. E.g.
Sodium pump or chloride-
bicarbonate exchange in RBC.