Fifth LECTURE
EUKARYOTIC CELLS
ORIGIN OF EUKARYOTIC CELLS
Prokaryotic cells are simpler and more primitive in their
organization than eukaryotic cells. According to the fossil record,
prokaryotic organisms antedate, by at least 2 billion years, the first
eukaryotes that appeared some 1.5 billion years ago. It seems highly
likely that eukaryotes evolved from prokaryotes, and the most likely
explanation of this process is the endosymbiotic theory. The basis of
this hypothesis is that some eukaryotic organelles originated as freeliving prokaryotes that were engulfed by larger cells in which they
established a mutually beneficial relationship. For example,
mitochondria would have originated as free-living aerobic bacteria
and chloroplasts as cyanobacteria, photosynthetic prokaryotes
formerly known as blue-green algae. The endosymbiotic theory
provides an attractive explanation for the fact that both
mitochondria and chloroplasts contain DNA and ribosomes of the
prokaryotic type
Most biologists are now prepared to accept that the
endosymbiotic theory provides at least a partial explanation for the
evolution of the eukaryotic cell from a prokaryotic ancestor.
Unfortunately, living forms having a cellular organization
intermediate between prokaryotes and eukaryotes are rare. Some
primitive protists possess a nucleus but lack mitochondria and other
typical eukaryotic organelles. They also have the prokaryotic type of
ribosomes. These organisms are all intracellular parasites and they
include Microspora, an organism that infects AIDS patients.
CELL SPECIALIZATION
All the body cells that comprise a single organism share the
same set of genetic instructions in their nuclei. Nevertheless, the cells
are not all identical. Rather, plants and animals are composed of
different tissues, groups of cells that are specialized to carry out a
common function. This specialization occurs because different cell
types read out different parts of the DNAblueprint and therefore
make different proteins, as we will see in Chapter 6. In animals there
are four major tissue types: epithelium, connective tissue, nervous
tissue, and muscle. Epithelia are sheets of cells that cover the surface
of the body and line its internal cavities such as the lungs and
intestine. The cells may be columnar, taller than they are broad, or
squamous, meaning flat. In the intestine, the single layer of columnar
cells lining the inside, or lumen, has an absorptive function that is
increased by the folding of the surface into villi The luminal surfaces
of these cells have microvilli that increase the surface area even
further. The basal surface sits on a supporting layer of extracellular
fibers called the basement membrane. Many of the epithelial cells of
the airways, for instance, those lining the trachea and bronchioles,
have cilia on their surfaces. These are hairlike appendages that
actively beat back and forth, moving a layer of mucus away from the
lungs .Particles and bacteria are trapped in the mucus layer,
preventing them from reaching the delicate air exchange membranes
in the lung. In the case of the skin, the epithelium is said to be
stratified because it is composed of several layers.
The cilia on the epithelial cells that line our airways move a belt
of mucus that carries trapped particles and microorganisms away
from the lungs. These cilia are paralyzed by cigarette smoke. As a
consequence, smokers have severely reduced mucociliary clearance
with the result that they have to cough to expel the mucus that
continuously accumulates in their lungs. Because of this impaired
ciliary function, smokers are much more susceptible to respiratory
diseases such as emphysema and asthma. The cilia of passive
smokers, people who breathe smoky air, are also affected.
Plant cells are also organized into tissues .The basic organization of a
shoot or root is into an outer protective layer, or epidermis, a
vascular tissue that provides support and transport, and a cortex that
fills the space between the two. The epidermis consists of one or more
layers of closely packed cells. Above the ground these cells secrete a
waxy layer, the cuticle, which helps the plant retain water. The
cuticle is perforated by pores called stomata that allow gas exchange
between the air and the photosynthetic cells and also constitute the
major route for water loss by a process called transpiration. Below
ground, the epidermal cells give rise to root hairs that are important
in the absorption of water and minerals. The vascular tissue is
composed of xylem, which transports water and its dissolved solutes
from the roots, and phloem, which conveys the products of
photosynthesis, predominantly sugars, to their site of use or storage.
The cortex consists primarily of parenchyma cells, unspecialized cells
whose cell walls are usually thin and bendable. They are the major
site of metabolic activity and photosynthesis in leaves and green
shoots.
SUMMARY
1. All living organisms are made of cells.
2. Our understanding of cell structure and function has gone hand in
hand with developments in microscopy and its associated techniques.
3. Light microscopy revealed the diversity of cell types and the
existence of the major organelles: nucleus, mitochondrion and, in
plants, the vacuole and chloroplast.
4. The electron microscope revealed the detailed structure of the
larger organelles and resolved the cell ultrastructure, the fine detail
at the nanometer scale.
5. There are two types of cells, prokaryotes and eukaryotes.
6. Prokaryotic cells have very little visible internal organization.
They usually measure 1–2 μm across.
7. Eukaryotic cells usually measure 5–100 μm across. They contain a
variety of specialized internal organelles, the largest of which, the
nucleus, contains the genetic material.
8. The endosymbiotic theory proposes that some eukaryotic
organelles, such as mitochondria and chloroplasts, originated as freeliving prokaryotes.
9. The cells of plants and animals are organized into tissues. In
animals there are four tissue types: epithelium, connective tissue,
nervous tissue, and muscle. Plants are formed of epidermis, cortex,
and vascular tissues.
Cytosol. The plasma membrane is followed by the colloidal organic
fluid called matrix or
cytosol. The cytosol is the aqueous portion of the cytoplasm (the
extranuclear protoplasm) and of the
nucleoplasm (the nuclear protoplasm). It fills all the spaces of the cell
and constitutes its true internal
milieu. Cytosol is particularly rich in differentiating cells and many
fundamental properties of cell are
because of this part of the cytoplasm. The cytosol serves to dissolve or
suspend the great variety of
small molecules concerned with cellular metabolism, e.g., glucose,
amino acids, nucleotides,
vitamins, minerals, oxygen and ions. In all type of cells, cytosol
contains the soluble proteins and
enzymes which form 20 to 25 per cent of the total protein content of
the cell. Among the important
soluble enzymes present in the matrix are those involved in glycolysis
and in the activation of amino
acids for the protein synthesis. In many types of cells, the cytosol is
differentiated into following two
parts : (i) Ectoplasm or cell cortex is the peripheral layer of cytosol
which is relatively non-granular,
viscous, clear and rigid. (ii) Endoplasm is the inner portion of cytosol
which is granular and less
viscous.
B. Cytoplasmic structures. In the cytoplasmic matrix certain nonliving and living structures
remain suspended. The non-living structures are called paraplasm or
inclusions, while the living
structures are membrane bounded and are called organoids or
organelles. Both kinds of cytoplasmic
structures can be studied under the following headings :
(a) Cytoplasmic inclusions. The stored food and secretory substances
of the cell remain
suspended in the cytoplasmic matirx in the form of refractile
granules forming the cytoplasmic
inclusions. The cytoplasmic inclusions include oil drops,
triacylglycerols (e.g., fat cells of adipose
tissue), yolk granules (or deutoplasm, e.g., egg cells), secretory
granules, glycogen granules (e.g.,
muscle cells and hepatocytes of liver) and starch grains (in plant
cells).
(b) Cytoplasmic organelles. Besides the separate fibrous systems,
cytoplasm is coursed by a
multitude of internal membranous structures, the organelles (literally
the word organelle means a tiny
organ). Membranes close off at specific regions of the eukaryotic cells
performing specialized tasks
: oxidative phosphorylation and generation of energy in the form of
ATP molecules in mitochondria;
formation and storage of carbohydrates in plastids; protein synthesis
in rough endoplasmic reticulum;
lipid (and hormone) synthesis in smooth endoplasmic reticulum;
secretion by Golgi apparatus;
degradation of macromolecules in the lysosomes; regulation of all
cellular activities by nucleus;
organization of spindle apparatus by centrosomes and so forth.
Membrane-bound enzymes catalyze
Contents
60 CELL BIOLOGY
reactions that would have occurred with difficulty in an aqueous
environment. The structure and
function of some important organelles are as follows:
1. Endoplasmic reticulum (ER).
2. Golgi apparatus
3. Lysosomes
4. Cytoplasmic vacuoles
5. Peroxisomes
6. Glyoxysomes.
7. Mitochondria
8. Plastids
9. Ribosomes
10.Microtubules and microtubular organelles
Special
Properties of Plant Cells Among eukaryotic cells the most
striking difference is between those of animals and plants
Plants have evolved a sedentary lifestyle and a mode of
nutrition that means
they must support a leaf canopy. Their cells are enclosed within a
rigid cell wall that gives shape to the cell and structural rigidity to the
organism (page 53). This is in contrast to the flexible boundaries of
animal cells. Plant cells frequently contain one or more vacuoles that
can occupy up to 75% of the cell volume. Vacuoles accumulate a high
concentration of sugars and other soluble compounds. Water enters
the vacuole to dilute these sugars, generating hydrostatic pressure
that is counterbalanced by the rigid wall. In this way the cells of the
plant become stiff or turgid, in the same way that when an inner tube
is inflated inside a bicycle tire the combination becomes stiff.
Vacuoles are often pigmented, and the spectacular colors of petals
and fruit reflect the presence of compounds such as the purple
anthocyanins in the vacuole. Cells of photosynthetic plant tissues
contain a special organelle, the chloroplast, that houses the light-
harvesting and carbohydrate-generating systems of photosynthesis.
Plant cells lack centrosomes although these are found in many algae.