Cell structure IV
Introduction to Metabolism
Friday Dr. Jonaki Sen
Eukaryotic Cell wall
Many single-celled eukaryotic cells have a cell wall that is a structural
component that wraps around the cell membrane.
The cell wall protects as well as provides physical support.
It is porous thus allowing water and solutes to reach the plasma membrane.
Plant cells, fungi and protistans have cell walls.
Primary cell wall: Young plant cells in actively growing regions secrete sticky
polysacchardies like pectin, glycoproteins and cellulose. The cellulose makes rope-like
strands embedded in the gluey matrix. The primary wall makes the cells sticky and are
thin so they allow expansion of the cell surface area.
Secondary cell wall:
As many plant cells
mature they start to
secrete material on
the inner surface of
the primary wall.
make a rigid
secondary wall which
reinforces cell shape.
25% of the secondary
wall in woody plants
is Lignin. This makes
resistant to insects.
Animal cells have no cell wall, however, many are embedded in a matrix of cell
secretions called the ECM or extracellular matrix.
Cartilage cells are embedded in matrix
of collagen or elastin fibers embedded
in a ground substance of modified
In the bone individual bone cells are
widely separated from each other and
there is an extensive matrix in
Neighboring cells of an animal or plant often adhere, interact, and communicate through
special patches of direct physical contact.
The plant cell walls are perforated with channels called plasmodesmata (singular,
plasmodesma; from the Greek desmos, to bind). Cytosol passes through the
plasmodesmata and connects the chemical environments of adjacent cells.
Water and small solutes can pass freely from cell to cell, and recent experiments
have shown that in certain circumstances, specific proteins and RNA molecules
can also do this. The macromolecules transported to neighboring cells seem to
reach the plasmodesmata by moving along fibers of the cytoskeleton.
Animal cell junctions Tight junctions:
Link the cells of epithelial
tissues lining the body’s outer
surface and inner cavities. They
seal adjoining cells such that
water soluble substances
cannot leak between them.
Join cells in tissues subject to
stretching such as skin, heart
Link the cytoplasm of
neighboring cells. They are
open channels for the rapid
flow of signals of and
Prokaryotic cells: Bacteria
Bacteria are the smallest cell only 1
micron in diameter and a few
Bacteria have a cell wall that is
semi rigid and imparts shape to
the cell. The cell wall is permeable
therefore it allows dissolved
substances access to the plasma
membrane. There are sticky
polysaccharides on the cell wall
which help protect the bacteria as
well as help it attach to different
Bacterial cells also have a plasma
membrane that has transporters,
channels and receptors for. Siganls
Bacterial cells are small enough that they do not need to have a cytoskeleton.
Their cytoplasm is continuous with a region of irregularly shaped DNA called
the nucleoid which does not have a membrane surrounding it. There is single
circular molecule of DNA which is the bacterial genome.
They have many ribosomes on which polypeptide chains are assembled.
Extending from the surface of bacterial cells are one or more threadlike
motile structures called “bacterial flagella”. They are not like the eukaryotic
falgella as they do not have the 9+2 arrangement of microtubules. The
function in however similar as it helps a bacterial cell move rapidly in fluid.
There are numerous shorter surface projections called pili (singular pilus).
These are many protein filaments that help bacteria attach to surfaces or to
Differences in eukaryotic and prokaryotic cells
Nuclear envelope Absent Present
Membrane-enclosed organelles Absent Present
Peptidoglycan in cell wall Present Absent
RNA polymerase One kind Several kinds
Histones associated with DNA Absent Present
Circular chromosome Present Absent
Cytoskeleton Absent Present
The endosymbiotic origin of
mitochondria and plastids
The theory of endosymbiosis proposes that mitochondria and plastids
were formerly small prokaryotes living within larger cells. The
proposed ancestors of mitochondria were aerobic heterotrophic
prokaryotes that became endosymbionts; the proposed ancestors of
plastids were photosynthetic prokaryotes that became endosymbionts.
The evidence supporting an endosymbiotic origin of plastids and
mitochondria is overwhelming.
1) The inner membranes of both organelles have enzymes and
transport systems that are homologous to those found in the
plasma membranes of living prokaryotes.
2) Mitochondria and plastids replicate by a splitting process
reminiscent of binary fission in certain prokaryotes.
3) Each organelle contains a single, circular DNA molecule that, like
the chromosomes of bacteria, is not associated with histones or
4) These organelles contain the transfer RNAs, ribosomes, and other
molecules needed to transcribe and translate their DNA into
5) In terms of size, nucleotide sequence, and sensitivity to certain
antibiotics, the ribosomes of mitochondria and plastids are more
similar to prokaryotic ribosomes than they are to the cytoplasmic
ribosomes of eukaryotic cells.
Potential Energy :
The capacity to do work simply owing to the
objects position in space or the arrangements of
Kinetic Energy :
The energy of motion
In the skeletal muscles the ATP gives up some of
its potential energy to move the muscle. The
transfer of energy from ATP also results in another
form of kinetic energy “heat” or thermal energy
The potential energy of molecules = Chemical energy
1 kilocalorie or 1000 calories is the amount
of energy taken to heat 1000 grams of
water from 14.5°C to 15.5°C at standard
What do cells do with Energy?
Any organism whether single celled or multicellular has the capacity to
obtain energy from its environment. Some obtain it from the sun others
extract it from organic and inorganic substances in their surroundings.
Within the cell energy is coupled to multiple energy requiring processes:
1) Chemical work- to store, build, rearrange and break apart substances.
2) Mechanical work- to move the flagella and other cell structures or to move
the whole cell or parts of it.
3) Electrochemical work- to move charged substances into or out of the
How much energy is available?
Energy cannot be created from nothing it must be obtained from somewhere
First law of thermodynamics:
Energy cannot be created or
The total amount of energy in
the universe remains constant
The universe only has a certain
amount of energy distributed in
various forms. One form of energy is
converted to another but energy
cannot vanish or be created out of
One way flow of energy
Energy available for conversion in the cell is
mostly present in covalent bonds.
CO2 and H2O
High chemical energy in
Low chemical energy in
When Glucose is converted to carbon dioxide and
water some of the energy is lost as heat. This is
very “low quality” energy that cells cannot use for
Bad news: The amount of “low quality”
energy is increasing in the universe
No energy transfer process is 100 % efficient so
some energy is always lost as heat. Thus the total
amount of energy in the universe is flowing from
sources rich in energy to those that have less and
less of it (second law of thermodynamics).
Entropy: Measure of disorder in the system
The ultimate destination of
everything in the universe is a state
of maximum entropy. The tendency
of entropy to increase is the second
law of thermodynamics
The second law of thermodynamics
does apply to life on earth.
The primary source of energy for life
on earth is the sun which has been
losing energy since it was formed.
Plants capture energy from sunlight
and convert it to other forms and
then lose it to other organisms that
feed directly or indirectly on plants.
At each step some energy is lost as
heat. Overall energy still flows in
Doing cellular work
When cells convert
one form of energy
to another there is
a change in the
available to them.
The greater the
initial amount of
the larger the
energy change and
more work will be
Energy In , Energy Out
Cells store energy in chemical bonds when bonds are formed.
When the bonds are broken this energy is released.
Both processes change molecules and such processes are called reactions