Download Structure of Cell and its Functions

Structure of Cell and its Functions
Schleiden and Schwann exchanged their
thoughts and together proposed the Cell theory. It, however, could not explain how new cells
are generated. Rudolf Virchow (1855) first described that cells divide to form similar new cells.
This led to extension of the cell theory. Its postulates are:
All living organisms are composed of cells and their products.
Cells are the basic unit of life.
All living cells arise from pre-existing cells.
Need for Multicellularity
It is well accepted that the earliest organisms were unicellular, some of which gradually evolved
to become multicellular. If a single cell had the ability to survive as a unicellular organism
and carry out all life processes, what led to development of multicellular organisms? Why
unicellular organisms, on attaining a particular size, tend to divide?
The concept of surface area to volume ratio provides the answer to above questions. For
its survival and maintenance, a cell has to perform a variety of functions. The exchange of
gases, absorption of nutrients and elimination of waste products occur through cell membrane
that forms the cell-surface. Cell membrane or cell surface of a particular area can serve for
a particular volume of cell. If a cell increases in volume, a proportionate increase in the cell
surface area is required to meet its increased requirements. This, however, does not happen.
Consider a general shape for a cell; say a cube. Any increase in its length causes its volume to
increase as cube (x3) of the length while its surface area increases as square (x2) of the length.
Therefore, as the volume of the cell increases with time, its surface area becomes insufficient
for diffusion of required amount of material in and out of the cell. If the cell divides instead
of growing beyond a particular size, it attains a more favourable surface area to volume
ratio. This ensures that the cell’s requirements of nutrition, respiration and excretion are
adequately met.
Classification of cells:
Cells can also be classified based on their internal organization. Cells that do not have membrane
bound nucleus and also other membrane bound organelles are called as Prokaryotic cells while
cells that contain membrane bound nucleus and specific organelles are called as Eukaryotic
cells.
Cellular Organization of Prokaryotic Cells
The prokaryotic cells are represented by bacteria, blue green algae, mycoplasma (earlier called
PPLO: pleuropneumonia-like organism), rickettsiae and spirochaetes. Prokaryotic cells are
generally small sized (mostly ranging from 0.5 μm to 10μm). They may occur singly or form
colonies of independent cells. There is remarkable variation in their shapes as well.
Despite variation in size and shape, a general scheme of organization can be observed among
the prokaryotes. A typical prokaryotic cell has a cell wall. Plasma membrane lies on the inner
side of a cell wall. The cytoplasm forms the matrix which is in a fluid form. It fills the cell.
There are no membrane bound organelles inside a prokaryotic cell. Its genetic material is loosely
arranged as it does not have a nuclear membrane. This genetic material (DNA) is double
stranded, helical, circular and localized in a discrete region called nucleoid.
In addition to this genomic DNA, many bacteria have small circular DNA molecules called
plasmids. These plasmids usually encode proteins that provide advantageous features to an
organism such as ability for resistance to antibodies.
Another peculiar characteristic of prokaryotic cells is presence of structures called mesosomes,
formed by infoldings of plasma membrane inside the cell. The Cytoplasm may also contain
reserve material in the form of inclusion bodies or storage granules such as glycogen granules.
Since these are not bound by any membrane system. They lie free in the cytoplasm
Discussion on Important Structural Features of Prokaryotic Cells
The cell envelope of most prokaryotic cells exhibits in addition to cell wall and
plasma membrane an outermost glycocalyx. Glycocalyx is an extracellular layer of
polysaccharide produced by some bacteria. It may be present in condensed form, tightly
attached to the cell wall. Such a thick and tough form of glycocalyx is called as capsule.
If present as a loosely attached, diffused, irregular layer, it is referred as slime layer.
Gram
Positive and Gram Negative Bacteria. This distinction is based on a staining technique
developed by Hans Christian Gram. Named after its discoverer, the staining method is
called as Gram Staining.
Plasma membrane of prokaryotic cells is similar to that of eukaryotic cells (structural
details discussed later). The membrane being selectively permeable allows only certain
ions to pass through it. It, therefore, serves important role in nutrient uptake, waste
removal and protein secretion.
As mentioned earlier, mesosomes occur as extensions of plasma membrane into the
cytoplasm of prokaryotic cells. These may be in the form of vesicles, tubules or lamellae.
These are thought to be involved in many cellular processes like cell wall formation
during cell division, chromosome replication and some enzymatic reactions.
The cytoplasm of prokaryotic cells contains numerous ribosomes. These function as
sites for protein synthesis. Each ribosome is composed of two subunits. The two subunits
30S and 50S combine to form a complete 70S ribosome in prokaryotes Each made up of
rRNA and proteins. There may be 15,000 ribosomes in a single bacterial cell. (Their role
in protein synthesis will be discussed in later units.)
Reserve material in prokaryotic cells is stored in the form of inclusion bodies or storage
granules. Glycogen granules and phosphate granules represent a storage form of glucose
and inorganic phosphates, respectively. Gas vacuoles are present in certain aquatic
bacteria that make them float over the water surface.
Motile bacteria exhibit another important structural feature, the flagellum (Pl. flagella).
Cellular Organization of Eukaryotes
The eukaryotic cells are represented by all the protists, fungal, plant and animal cells. These
differ from prokaryotic cells, primarily, in having membrane bound cell organelles. The genetic
material is located within a membrane bound nucleus. A variety of intracellular filaments are
present which serve for structural support and motility.
Eukaryotic cells exhibit huge diversity. All, however, share a basic structural organization. Being
the most familiar, animal and plant cells would serve as suitable examples for understanding
general organization in eukaryotic cell.
Plasma Membrane
The outermost layer of all eukaryotic cells is made up of a selective barrier. Its presence was
discovered after its functional importance was realized. An observation of selective uptake of
certain dyes by cells led the cytologists to conclude that there exists a semi-permeable covering
outside the cell which later became known as the cell membrane.
Fluid Mosaic Model of Membrane Structure
Based on the various bio chemical evidences various scientists tried to decipher the structure
of the plasma membrane but the most accepted model was the Fluid Mosaic Model conceived
by S.J Singer and G.L Nicholson in the year 1972.
The model explains the physical and chemical properties of the membrane.
According to the model the ultra-structure of the cell membrane is described as follows:
The membrane is made up of a bilayer of amphipathic phospholipid, having polar heads
facing outwards and nonpolar tail inwards. Such an arrangement prevents entry of polar
substances inside the membrane.
The temperature dependent property of fluidity of membrane is because of the composition of
lipids. The lipid bilayer is made up of phosphoglycerides.
It also contains proteins, glycoprotein and glycolipids
The membrane proteins are of three types, some are exposed at one surface (peripheral
proteins), some at both surfaces and some are integral proteins spanning the entire membrane.
The proteins have several functions:- primary amongst them is to act as selective
channels and pumps for the entry and exit of specific molecules (ions and polar
molecules) according to functional requirements of the cell. The regulation of entry
and exit of molecules helps in maintaining the osmolarity of cytosol and prevents
accumulation of waste material. Certain unicellular organisms procure their food
with the help of the plasma membrane.
Functions of Plasma MembranePlamsa membrane is the outermost covering of animal cells which enables the physical
separation of intracellular and extracellular components.
It provides protection to the protoplasm against any external injury
Helps in selective transportation of molecules across the cell. Substances move across the
membrane through various transport mechanisms.
a) Diffusion-Transport of substances along the concentration gradient i.e. passive transport of
substances such as CO2 and O2, etc.
b) Facilitated transport-Transport of substances such as sugars and amino acids across the
membrane is aided by special trans-membrane channel proteins
c) Osmosis-Transportation of water across the membrane
d) Active transport- Movement of substances across the membrane against the concentration
gradient. Such transport is energy dependent, that is, breakdown of ATP molecule is required for
active transport
e) Endocytosis- Process by which cells engulf material by invagination of plasma membrane.
WBCs engulf pathogens in this way.
f) Exocytosis- Process by which cells discard/secrete substances from within the cell to outside.
It has some chemical receptors which helps in passage of nerve impulses. Such receptors for
peptide hormones help in generating second messenger within the cell.
In certain organs like small intestine, plasma membrane have apical finger-like folds, called
microvilli, to increase the surface area for absorption of nutrients.
CELL WALL:
How is the cell wall formed?
As a plant cell divides ,a layer of cell wall precursors is laid at the equatorial plate.These
vesicles are produced by the Golgi apparatus. The vesicles subsequently fuse together to form
cell plate, their membrane forming the cell membrane and the inner content made up of calcium
and pectin forms the middle lamella.
Next a primary cell wall made up of cellulose microfibrils is laid on either sides of the middle
lamellae. Interdispersed between the cellulose microfibrils is a matrix made up of pectates and
hemicelluloses. The matrix acts as a glue to hold the microfibrils together. The primary cell
wall is thin and flexible.
In certain cells after their maturity, many layers of cellulose microfibrils are laid on the inner
side of the primary wall earlier formed. Unlike the microfibrils of primary cell wall, which
are randomly scattered, these are oriented in the direction of elongation of cell. The thickened
layer forms the secondary cell wall. The cells having such additional wall layers are very
rigid and have easy communication with adjacent cells through pits on the walls.
The secondary cell wall in certain cells becomes impregnated with lignin to increase the strength
of the cell wall. In certain other cells it becomes impregnated with waxy suberin.
In addition to primary and secondary cell wall, tertiary cell wall is deposited in some cells. It
was first discovered in tracheids of some gymnosperms, it is considered to be a dried residue of
the protoplasm. It looks like swollen nodules on the inner side of the secondary wall. It is made
up of cellulose, hemicellulose and xylan.
Nucleus - The Control Centre
Is it not a wonder how a cell controls its complex organization of structure and its related in
functions? What is the origin of the various chemical metabolites inside the cell?
The Nucleus is the largest organelle of the cell. First described by Robert Brown in 1831,this
organelle is easily observable under the light microscope. It is a spherical/ovoid (10-20μm
diameter) double membranous structure, the outer membrane being continuous with
Endoplasmic Reticulum (ER).
The space between the two membranes is also in continuation with the lumen of ER.
Such an arrangement is useful coordinating the function of a Nucleus and the ER. During cell
division though nuclear membrane disappears, it could reappear from ER after the division is
over.
The outer and inner nuclear membranes are continuous at certain junctions known as nuclear
pores. The diameter of the pores range from 80-100 nm. They function as gates for the passage
of molecules between cytoplasm and nucleoplasm.
The nucleoplasm (nuclear matrix) contains DNA, RNA and proteins.
Inside the nucleus, the genetic material or DNA is supercoiled with certain basic proteins
(Histone) and non-histone proteins to form a compact entity known as chromatin. The
chromatin fibres further associate with non-histone chromosomal protein to form a condensed
structure known as chromosome.
Chromosomes are visible only during cell division (find out at which stage of cell division
chromosomes are best visible).
CHROMOSOME-STRUCTURE
A Chromosome consists of two sister chromatids which are joined by a primary constriction
in the centre known as CENTROMERE. On either side of the centromere disc shaped
structure known as the KINETOCHORE develops during cell division
TELOMERE is the region of repetitive nucleotide present at the tips of chromosomes.
TYPES OF CHROMOSOME BASED ON THE POSITION OF CENTROMERE
1) Metacentric Chromosome- Centromere is in the middle and both arms
are of equal length.
2) Submetacentric Chromosome– Centromere is not at the centre of the chromosome so
unequal length of the arms.
3) Acrocentric- Centromere is close to one end so one arm is extremely short as compared to the
other.
4) Telocentric- terminal centromere
DNA contains the hereditary information which is replicated prior to cell division. The
hereditary
information is also transcribed to a mRNA and then transported outside the nucleus where it is
translated into proteins.
Nucleolus is a sub organelle of the nucleus where most of the ribosomal RNA and some
ribosomal
proteins are synthesized. The ribosomal sub units are assembled within this sub organelle
Endoplasmic Reticulum - The Cellular Trucks
Within the cells there is a network of interconnected membranes forming sheets, tubes or
flattened sacs. The origin of these structures is the outer membrane of the nucleus.
These organelles are primarily responsible for the synthesis of membrane lipids and proteins.
Based on the presence and absence of ribosomes on their outer surface they are classified as
Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).
These organelles were first observed by Keith R Porter,Albert Claude and Ernest F
Fullam.
RER-The outer surface of RER is granular in appearance due to the presence of ribosomes.
Since ribosomes are involved in the formation of proteins, RER is the site for synthesis of
membrane and secretory proteins. The newly synthesised proteins are stored in the lumen of
RER till it is transported for further modification to the Golgi Apparatus.
RER is present in considerable amount in all cells undergoing active metabolism, especially the
ones which are specialized to synthesize and secrete proteins.
SER- They are specialized for the synthesis of fatty acids and phospholipids.
Functions performed by ER
Various proteins synthesized by ribosomes enter the channels of RER from where they are
transported to the target organelles such as Golgi bodies. They are tagged with signal molecules
for easy recognition.
SER is specialized to synthesize lipids and steroidal hormones
SER helps in detoxification of drugs by the modification of their chemical structure.
Sarcoplasmic reticulum present inside muscle fibres aid in muscle contraction by storing
and releasing calcium ions.
Ribosomes - The Protein Factory
These are minute structures, nearly 20 nm in their diameter, consisting of two subunits. These
are present in both prokaryotic and eukaryotic cells .The only difference lies in the molecular
weight of the two subunits. While in prokaryotes they are of 70S type in eukaryotes they are of
80 S type. The unit of measurement of the density is the Svedberg Unit (A measure of rate of
sedimentation in centrifugation and not size).
The 70S ribosome in prokaryotes consists of a small 30S subunit and large 50S subunit. In
eukaryotes the 80S ribosome is made up of a large 60S sub unit and small 40S sub unit.
They are composed of equal quantities of protein and RNA.
Being the sites of protein synthesis they are considered indispensable for the process of
translation.
When several ribosomes occur along a common strand of m RNA the whole structure is known
as POLYSOME.
Albert Claude, Christian de Duve and George Emil Palade were awarded the Nobel Prize
for Physiology in 1974 for the discovery of Ribosomes.
Golgi Apparatus - The Processing Machines
After synthesis of proteins on RER, most of them exit from the lumen of the organelle via
transport vesicles. These vesicles are formed from RER and they carry the proteins to the Golgi
complex. This organelle consists of a stack of membranous flattened stacks called cisternae
located near the nucleus. In plant cell the sac like structures are referred to as dictyosomes. In
the vicinity of these stacks are a number of spherical vesicles.
Golgi cisternae are having convex and concave faces and these are oriented in a specific way
towards nucleus so that forming cis face is nearer to nuclear membrane and mature or trans face
is away from it. They carry both structural proteins (to maintain the integrity of their flattened
structure) as well as functional enzymes to modify the structure of the incoming primary
polypeptides..
Most molecules from Golgi apparatus and up in lysosome while several other molecules get
packed in secretory vesicles.
Golgi apparatus was discovered in 1898 by Camillo Golgi
Functions of Golgi Apparatus:
It modifies and packages all incoming proteins and lipids into vesicles that are then
exported to their respective destination.
Packaging includes addition of signal molecules such as sugar or phosphate to protein
through glycosylation and phosphorylation.
Helps in formation of cell wall during cytokinesis of plant cell.
The lysosomes, secretory vesicles, acrosome of sperms are all synthesized by Golgi
apparatus.
Lysosome - The waste disposal system of a cell
What happens to the various metabolites/cell organelles once they become redundant and
obsolete for the functioning of the cell?
Most of these compounds are degraded inside lysosomes.
These small spherical vesicles (0.2-0.5 μm in diameter) bounded by a single membrane are
derived from the Golgi apparatus. Till now it was believed that lysosomes are found exclusively
in animal cells but current research supports that structures similar to lysosomes also exists in
certain plant cells.
They contain digestive enzymes collectively known as hydrolases which work optimally at
acidic pH (4.8).
These enzymes have two main functions:
1. They degrade the redundant chemical compounds/non functioning organelles of the cell.
The products are later recycled to be used to form new components of the cell.
2. They also digest complex macromolecules absorbed by cells such as polysaccharides,
proteins and lipids by combining with the food vacuole.
They are popularly known as “Suicidal Bags” because when a cell becomes damaged then its
own lysosome releases hydrolases to digest the remains of the cell.
The acidic pH inside the lysososme is maintained by hydrogen ion pumps and chlorine channel
proteins located on the membrane of lysosomes. The acidic pH is maintained to denature the
proteins which are later degraded by hydrolases.
Vacuoles
Vacuoles are fluid filled cavities bounded by a single membrane. They are formed by pinching
off of part of cell membrane.
Plant cells are characterised by a single large vacuole and animal cells have large number of
smaller vacuoles.
Based on their function vacuoles are classified into four types
a) Sap Vacuole- Found typically in plant cells these are present as numerous small
vesicles in the early stages of plant cell formation. As the plant cells mature these
vesicles fuse to form a single large central sap vacuole.
This vacuole occupies approximately 90% of the cell volume. This pushes the nucleus to the
periphery. The vacuolar membrane known as tonoplast has a number of ion channels and pumps
to selectively permit the entry and exit of ions across the vacuole.
The fluid content is called the cell sap, which contains a number of mineral salts, sugars, amino
acids and waste material.
Certain sap vacuoles contain hydrolytic enzymes and function similar to lysosomes.
a) The characteristic colour of beetroot is because of the water soluble pigment
anthocyanin present in the sap vacuole.
b) Latex of rubber trees is stored in the sap vacuole of latex cells and tubes
b) Contractile vacuole- In unicellular protistans like Amoeba and Paramoecium
there is present a specialized vacuole for excretion and osmoregulation known as
contractile vacuoles. These are specialized to collect extraprotoplasmic fluid from
the cell containing excess water and waste material and dispel it outside.
c) Food vacuole- They are found in protists, coelenterates and phagocytic cells of
higher animals. The food particles ingested by the organism forms an association
with phagosomes together known as the food/digestive vacuole. The hydrolytic
enzymes of lysosomes break down the food material.
Mitochondria - Power house of a cell
Kolliker while observing muscle cells under the microscope noticed some granules (called
them sarcosomes)
These organelles were successfully stained by Janus Green Stain by Michaelis.
These cylindrical/sausage shaped organelle are evenly distributed in cytoplasm of
undifferentiated cells but tend to collect in regions of utilisation like base of cilia and flagella.
The number of mitochondria per cell depends on the metabolic activity of the cell.
Shape and size of this double membrane bound organelle vary widely.
The outer membrane of the mitochondria is smooth and the inner membrane is thrown into
number of infolds called cristae. The space between the two membranes is known as the
perichondrial space.
Cristae has a number of projecting stalked particles called oxysomes having basal FO sub
unit and F1 head connected by a stalk.F1 has ATPase enzyme activity and base acts as proton
tunnel.
ORIGIN OF MITOCHONDRIA AND
PLASTIDS
According to the Endosymbiotic theory proposed by Lynn Margulis based on the work of
Altman and Schimper both Mitochondria and Chloroplast have originated from free living
bacterial cells which over the course of evolution developed a symbiotic relationship
with their host eukarotic cells!
The inner mitochondrial matrix contains protein, lipid, prokaryotic type ribosomes, RNA and a
circular DNA. The various enzymes present in the matrix are involved in the Krebs Cycle.
Mitochondria are the sites for aerobic respiration. The synthesis of ATP by oxidative
phosphorylation takes place during the Electron Transport Pathway within the inner
mitochondrial membrane. ATP is the energy currency of the cell, the breakdown of which is
necessary to carry out a number of energy dependent metabolic reactions.
Apart from the above mentioned functions, the synthesis of certain amino acids and fatty acids
takes place within this semi-autonomous organelles
Plastids - The kitchen of a cell
These are the third type of organelle apart from the nucleus and Mitochondria with double
membrane. Giving members of Kingdom Plantae and some protisans the unique feature of
autotrophism, plastids are exclusively found in these organisms.
Discovered by Haeckel in 1866 these are the organelles which contain pigments to perform the
function of photosynthesis. Plastids originate from proplastids and based on their properties are
distinguished into three types, Leucoplasts, Chromoplasts and Chloroplasts.
Leucoplasts-These are colourless plastids found in non green parts like roots of plants.
Oval/disc shaped plastids contain lamellae which are not organised to form grana.
Photosynthetic pigments are absent hence their major function is storage, for example
Amyloplasts found in tubers of potato store starch.
Chromoplasts- These are non photosynthetic coloured plastids which synthesize and
store carotenoids hence appear in different colours. Transformation of chloroplasts into
chromoplasts is observed during ripening of fruits. In roots of carrots, leucoplasts convert
to chromoplasts.
Chloroplasts- These are the most important plastids which are involved in the process of
photosynthesis. The outer membrane is smooth and continous and the inner gives rise to
lamellae extending through the organelle.
The lamellae are arranged into stacks of flattened sacs called thylakoids.
A stack of such thylakoids is known as granum. The grana are connected by stromal
lamellae.
The interior medium is known as the stroma which contains 70 S ribosomes, circular DNA
and a number of suspended biomolecules including the enzymes for the carbon fixation for
photosynthesis.
WHAT IS COMMON AMONGST MITOCHONDRIA AND PLASTIDS?
Both plastids and mitochondria have their own genetic material and 70 S ribosomes.This has
established that they are of prokaryotic orgin and are semi-autonomous i.e. they are only
partially dependent on the proteins synthesized by nucleus.Rest of the metabolic essentials
are synthesized by their own genome! Moreover they can divide.
Centrioles - The cell centre
Centrosome is found only in animal cells which contain two cylindrical structures called
centrioles, lying perpendicular to each other. The centrosome is called the microtubule
organising
centre from where microtubules are produced.
Edouard van Beneden and Theodor Boveri independently observed this organelle in the year
1883 and 1888 respectively.
Each centriole is made of nine linked triplets of peripheral tubulin proteins .The central portion is
known as the proteinacous hub which is connected to the peripheral triplets by radial spokes.
The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle
apparatus during cell division in animal cells
Flagellaand cilia - Locomotory organelles
These organelles are external appendages which project out from the cell membrane, originating
from the basal body. They aid in the locomotion of the cell.
Flagella occur singly or in pairs on one side or either side of the cell, whereas cilia which are
smaller in size are distributed all over the cell surface.
The ultra-structure of both cilia and flagella is similar.They diifer from prokaryotic cell
appendages in having an outer plasma membrane.
Their core is the axoneme containing microtubules arranged in an outer ring of nine doublets
surrounding one central pair.Such an arrangement is known as 9+2 arrangement.
The central tubules are enclosed in a sheath and connected to peripheral doublets by radial spoke.
The peripheral doublets are also inter-connected by linkers. The microtubules are made up of
tubulin and enzymes which release energy from ATP.The energy released helps in the sliding of
microtubules over each other which aids in the beating of cilia/flagella and thus in movement of
cells.
Peroxisomes
All animal cells with the exception of erythrocytes and most plant cells contain
peroxisomes.These are very small organelles bound by single membrane.
They were first established as organelles by Christian de Duve in 1967.
They contain oxidases which oxidize organic substances such as fatty acids forming hydrogen
peroxide.They also contain another enzyme catalase which degrades hydrogen peroxide into
water and oxygen and acetyl groups.The energy released in the above reaction is converted to
heat and the acetyl groups are used for the synthesis of metabolites.
These organelles are mostly found in liver and kidney cells where the toxic products are
degraded by their enzymes to produce harmless products.
Glyoxisomes
Glyoxisomes are organelles similar to peroxisomes found in plant seeds. During growth and
development. That oxidise stored lipids for release of energy.