F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.1
CYTOLOGY - CELLS AND THEIR STUDIES
The Cell theory
Every living thing is composed of one or more cells. Robert Hooke, a British scientist, was the first
to use the term cell in describing certain structures in a piece of c____, which he observed using a
microscope. This occurred in 1665; but it was over 150 years later that Dutrochet, a French scientist,
realised the significance of the cell as a basic building block and proposed that all living things were
made up of cells. In 1838, Schleiden and Schwann postulated that cells were capable of independent
existence, and in 1855 Virchow stated that cells could only arise from pre-existing cells.
Q.
State the three principles of cell theory.
One reason why scientists took so long to recognise the common nature of cells must have been the
great v_______ of shapes and functions which cells exhibit. (syllabus requirements: leaf epidermis,
parenchyma, collenchyma, sclerenchyma, phloem, xylem, epithelia-squamous, ciliated, stratified,
blood cells and neurones). Different cells have developed particular sizes, shapes and chemistry
which is suited to the specific function which each performs. Such cells are said to be sp_______ and
an organism which is composed of groups of specialised cells is described as mult________. This is in
contrast to those organisms which consist of only one cell capable of carrying out all the necessary
functions which are termed uni_______, such as an Amoeba.
I)
Cell Ultrastructure as Revealed by the Electron Microscope
When electron
microscope
techniques were
used for the first time
(since 1950s) both
plant and animal cells
were shown to have a
detailed structure
previously
undreamed of.
Not only were some
components
(subcellular
components called
organelles)
discovered for the
very first time, others
already well-known
were found to be
incredibly complex
in structure.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.2
A)
Comparison of Light and Electron Microscopes
Whereas the light microscope uses visible light, the electron microscope uses a beam of e________.
The radiation is focused on to the specimen by large electromagnets. The light rays or electrons,
after passing through the specimen, is focused by an obj______ lens. An eye_____ lens in the
light microscope further enlarges the image.
The final image is projected on to a viewing screen coated with a fluorescent compound. When
irradiated with electrons the fluorescent substance emits light visible to the human eye.
With a high quality compound microscope a magnification of about 1500 times is possible; but an
electron microscope can magnify up to 500, 000 times.
However, it is the R___________ POWER of
the electron microscope rather than its
magnifying power that enables it to produce
images containing so much more de____.
Resolving power or resolution is the ability to
make out as separate entities bodies which lie
close to one another.
The power of resolution (R) of a microscope depends mainly on the wavelength of the radiation used
and on the numerical aperture (n sin ) of the objective lens.
Resolution (R) = 0.5 
n. sin 
. = wavelength of radiation
n = refractive index of medium between
specimen and objective lens
 = angle of aperture
The . of e- beam is 0.05 nm which is 10,000 times sh_____ than the average wavelength of white
light. This makes much improvement in resolution. (Present-day EMs have R of about 0.5 nm.)
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.3
Comparison of advantages and disadvantages of the light and electron microscopes
Light microscope
Electron microscope
Advantages
Cheap to purchase & operate (_ 100-500)
Disadvantages
Unaffected by magnetic fields
Ex______ to purchase & operate, requires up to 100, 000
volts to produce the electron beam
Affected by m______ fields
Preparation of material is relatively quick and
simple, requiring only a little expertise
Pre______ of material is lengthy and requires
considerable expertise and sometimes complex equipment
Material rarely distorted by preparation
Preparation of material may dis______ it (creates artifacts)
Living as well as dead material may be viewed
A high vac______ is required and liv______ material
cannot be observed
Natural colour of the material can be observed
All images are in b______ and w______
Disadvantages
Advantages
Magnifies objects up to1500 x
Mag______
Can resolve objects up to 200 nm apart
Has a r______
1 nm
B)
objects over 500 000 x
power for biological specimens of around
Cell fractionation
How then is so much known about the functions of the various components of cells? One way is to
separate or fractionate the organelles. The activities of the organelles can then be studied without
interference from other reactions that take place in whole cells.
The tissue e.g. liver is first chopped up
in a c___ iso____ buf___ solution. The
isotonic solution prevents distortion
of the organelles.
The chopped tissue is then ground up
in an homogeniser which develops
shearing forces just sufficient to rupture
the cells. Cells can also be ruptured
using ultrasonic waves.
The homogenate is then transferred to a
cen______ in which the mixture is spun
at known speeds whereby the
organelles are sed_______ separately.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
II)
P.4
Structures common to Animal and Plant cells
A)
Cell membranes
Cell membrane serve important functions both inside and outside the cell, match the Letters on the
diagram to the specific function of the membrane on the right
Differential permeability.
They separate the contents
of cells from their external environments. The membrane to
act as a bar____ to 'undesirable' molecules and as a 'gate' to
allow en___ of molecules necessary for the cell's metabolic
activities. i.e., regulating movement of substances
Compartmentation. They enable separate
com________ to be formed inside cells in which specialised
metabolic pathways can take place inde_________. eg.
Hy_______ enzymes are kept within lyzosomes.
Increase surface area. Membrane info_______ and
pro_______ increase surface area for absorption or for
housing enzymes.
House and organize enzyme systems. Biochemical
reactions in chloroplasts and mitochondria take place through
enz______ systems neatly org_______ on the membranes.
Transfer of information. Re_______ sites are located
on surface of membrane for recognising external stimuli such
as hormones and other chemicals.
Intracellular transport. They make up chan____ and aid
in Intracellular transport and secretion.
Cell recognition. Anti___ such as glycoproteins often
present on cell membrane
a)
Properties of Plasma Membrane
The chemical and physical properties of plasma membrane tell something about its nature and
structure:

Differential permeability, it is the ability to be sel_______ in allowing certain molecules to pass
easily whilst restricting the passage of others. For instance, li___ soluble molecules can pass
rapidly through the membrane. eg. alcohol, ether, chloroform; P____ and c______ ions such as
glucose, amino acids, fatty acids, glycerol and ions can only diffuse slowly through them.

Chemical analysis confirmed that membranes are comprised almost entirely of p______ and
l_____. The lipids are mainly phospholipids.

The membranes are about 7.5 nm wide as measured under the electron microscope (EM).

Characteristic 3-layered trilaminar appearance when viewed with the electron microscope.

F.6 notes Cytology --Cell ultrastructures W.K.Leung
b)
P.5
The Structure of the Plasma Membrane--Evolution of Membrane models
i)
Evidence for Lipid bilayer
Phospholipids are amphipathic substances, that is they are
polar molecules with heads which are soluble in water
(hydrophilic) and long, insoluble (hydrophobic) tails.
Q.If a collection of phospholipid molecules were dropped into a
beaker of water, how would they become arranged?
Two Dutch scientists, Gorter and Grendel conducted an investigation into the amount of lipids available in red
blood cells for their cell membranes. They extracted the lipids from the membranes of red blood cells with a
lipid solvent, acetone. The lipids were then carefully added to the surface of a trough of water where they
arranged themselves with their head regions dissolved in the water and tail regions 'sticking up' into the air
The water trough was designed so that a
moveable piston would be drawn across the
surface, pushing together the lipid molecules,
As soon as the molecules became tightly
packed there was an increased resistance to
the movement of the piston. The molecules
now formed a single layer and the surface
area covered by them was measured. They
then compared this surface area with the total
surface area estimated for the red blood cells
used to supply the lipids
Q1 a) Suggest three reasons why red blood cells
were used rather than any other cells in this
investigation
b) What sort of relationship would you expect to find between the surface area of lipids and that of red blood cells if
the cell membrane was made up of one layer of phospholipid molecules ?
Q2 a)
Gorter and Grendel obtained the following results:
1. number of cells per cm3 of human blood = 4.74 x 109
2. surface area of one cell assuming structure to be a disc: 99.4 m2
3. estimated surface area of lipids from 1 cm3 of blood = 0.92 m2
What is the approximate ratio of lipid to cell surface area?
b) State a hypothesis, consistent with Gorter and Grendel's original hypothesis, to account for this ratio.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
ii)
P.6
The 'unit membrane' hypothesis (reference)
This and other evidences led DanielIi
and Davson in 1934 to suggest that :
a layer of protein was adsorbed onto
both sides of the lipid bilayer, giving
a tri________ structure as observed
in electron micrographs.
In 1959 Robertson combined the available evidence and put forward the 'unit membrane' hypothesis which
proposed that all biological membranes shared the same basic structure:
a) they are about 7.5 nm wide;
b) they have a characteristic trilaminar appearance when viewed with the electron microscope;
c) the three layers are a result of the same arrangement of proteins and polar lipids as proposed by Davson
and DanielIi and represent two protein layers surrounding a central lipid layer.
iii)
Freeze fracturing technique
The unit membrane hypothesis has since been criticized in the light of evidence from a variety of
sources, notably freeze fracturing.
The technique allows membranes to be split and the surfaces inside to be examined. It reveals the
presence of particles (proteins) which penetrate into, and sometimes right through, the lipid bilayer.
Electron micrographs were revealing that the plasma membrane was not such a uniform structure as
predicted by the unit membrane model and, in particular, it appeared that particles were embedded
in the membrane at irregular intervals.
In general, the more met________ active the membrane, the more p_____ particles that are found;
chloroplast membranes (75% protein) have many particles, whereas the metabolically inert myelin
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.7
sheath (18% protein) has none. The inner and outer faces of membranes also differ in their particle
distribution.
Singer and Nicolson criticised the model because it did not fit the thermodynamic requirements for
the best arrangement of molecules present in it.
Both phospholipids and proteins are amp_______ and ideally their hydrophilic parts should be in
contact with w____ and their hydrophobic parts away from water. Singer and Nicolson on reviewing
all the available evidence in 1972 constructed what is now the most widely accepted model of the
plasma membrane which they have named the FLUID MOSAIC MODEL :
iii)
The fluid mosaic model
In this model the lipid bilayer remains unchallenged as in the unit membrane, but it is regarded as a
dynamic structure in which proteins can float in the lipid like islands, some moving about freely
while others are fixed in position. Lipids also move about.
Both the hydrophobic portions of proteins and phospholipids are positioned away from water
molecules whilst the hydrophilic groups are at the membrane surfaces, in contact with water
molecules.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.8
Proteins
Some proteins penetrate only part of the way into the membrane while others pen______ all the
way through. Usually they have hydrophobic portions which interact with the lipids, with
hydrophilic portions facing the aq_____ contents of the cell at the membrane surface.
In all there are thousands of different proteins that can occur in cell membranes. They may be:
 Structural proteins
adhesion molecules for holding cells to extracellular matrix.
 Carrrier
pumps for transporting specific materials across the membrane
molecules
 Hydrophilic channel
(protein-lined pore)
occur within a protein, or between adjacent protein molecules. The pore
spans the membrane, allowing the passage through the membrane of
p____ molecules that would otherwise be excluded by the lipid region.
 Enzyme
membrane-bound enzymes
 Recognition sites
glycoproteins with their sugar residues act as antigens
 Receptor molecules
for hormones,
 Electron carriers
as energy transducers in respiration and photosynthesis
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.9
Lipid
 Variations in lipid composition affect such properties as fluidity
and permeability.
 Fluidity affects membrane activity, such as the ease with which
membranes fuse with each other, and the mob______ and
act_____ of mem____-bound enzymes and transport proteins.
 Cholesterol (another lipid) stabilizes cell membranes; fits
between phospholipid tails
Carbohydrates
 Carbohydrates may be attached to the Outer Membrane Surface -- found only on outer (extracellular)
side of cell membrane - none exposed to cytosol
 Carbohydrate chains (oligosaccharides) attach to some amino acid membrane proteins. e.g. Human
ABO blood group antigens are oligosaccharides
 Proteins with sugar groups attached are called glycoproteins
 Some sugars are also attached to lipids, forming glycolipids
Q. Why is the fluid mosaic model a better thermodynamic arrangement of molecules than that of the unit membrane
model?
Q Explain why Singer and Nicolson called their membrane model the 'fluid mosaic model'.
Why membrane is ‘fluid’?
The bilayer is like a 2-dimensional fluid- there is not much movement across the membrane, but there is a
lot of sideways (lateral) movement
Fluidity depends upon types of fatty acids in phospholipids: unsaturated fatty acids have kinks
don't pack as well -- more fluid Some organisms increase unsaturated fatty acids in membranes when
exposed to cold weather
What role does the physical state of the lipid bilayer have for the biological properties of the membrane?
Membrane fluidity seems to provide the perfect compromise that provides both mobility and interactions
between membrane components and organization and mechanical support in the membrane.

Most importantly, fluidity allows for interactions of membrane components( e.g. between two
membrane proteins) within the plane of the membrane. In any case, the interacting molecules can
come together, carry out the necessary reaction, and either remain together or move apart
depending on the conditions.
For example, in the role of membranes in information transfer. There are agents, such as hormones, which combine
with the membrane by external receptors and cause a change in activity of an enzyme, adenylate cyclase,
present on the inner side. The evidence suggests that the receptor and this enzyme are not always linked to one
another, but that they interact as a result of their movements within the membrane. The interaction occurs of only
when the receptor and hormone are combined.
 Fluidity is also important in membrane assembly. Membranes arise only from preexisting
membranes, and growth is accomplished by the insertion of lipid and protein components into the fluid
matrix of the membranous sheet.
 Many of the most basic cellular processes, including cell movement, cell growth, cell division,
formation of intercellular junctions, secretion, and endocytosis, depend on the movement of
membrane components and would probably not be possible if membranes had a rigid, non-fluid
organization.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
iv)








c)
Summary of structure of cell membrane
Different types of membranes differ in thickness but most fall within the range 5-10 nm, for
example plasma membranes are 7.5 nm wide.
Membranes are mainly composed of lipid and protein , with carbohydrate portions attached to
the external surfaces of some lipid and protein molecules.
The lipids spontaneously form a bilayer owing to their polar heads and non-polar tails.
The proteins are variable in function.
The sugars are involved in recognition mechanisms.
The two sides of a membrane may differ in composition and properties—Asym_________.
Both lipids and proteins show rapid lateral diffusion in the plane of the membrane unless
anchored or restricted in some way.
The inner surface is supported by the cyto________; some membrane proteins attach to the
cytoskeleton
Transport across the Plasma Membrane
Transport across membranes is vital for a number of reasons:





P.10
to maintain a suitable p__ and i____ concentration within the cell for enzyme activity,
to obtain f___ supplies for energy and raw materials,
to excrete t____ substances or
to se_____ useful substances and
to generate ionic gra______ essential for nervous and muscular activity.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.11
There are four basic methods of entry
into, or exit from, cells, namely
diffusion, osmosis, active transport
and endocytosis or exocytosis.
The first two processes are passive, that
is they do not require the expenditure of
energy by the cell; the latter two are
active, e______ consuming processes.
i) Diffusion
Diffusion is the process by which a substance moves from a region of h_____ concentration of that
substance to a region of l_____ concentration of the same substance. Diffusion occurs because the
molecules of which substances are made are in ran___ motion (kinetic theory).
Gases, like the resp_____ gases oxygen and carbon dioxide,
diffuse rapidly in solution through membranes down
diffusion g_______.
Uncharged and fat soluble
molecules pass through membranes readily.
I___ and small p___ molecules such as glucose, amino acids,
fatty acids and glycerol normally diffuse slowly through
membranes.
The rate of diffusion depends upon:
1. The concentration gradient - The greater the difference in concentration between two regions of a
substance the greater the rate of diffusion. Organisms must therefore maintain a fr___ supply of a
substance to be absorbed by creating a stream over the diffusion surface. Equally, the substance,
once absorbed, must be rapidly tran____ away.
2. The distance over which diffusion takes place - The shorter the distance between two regions of
different concentration the greater the rate of diffusion. The rate is proportional to the reciprocal of
the square of the distance (inverse square law). Any structure in an organism across which
diffusion takes place must therefore be thin. Cell membranes for example are only 7.5 nm thick.
3. The area over which diffusion takes place - The larger the surface area the greater the rate of
diffusion. Diffusion surfaces frequently have structures for increasing their surface area and
hence the rate at which they exchange materials. These structures include villi and microvilli.
4. The nature of any structure across which diffusion occurs - Variations in the structure of
epithelial layers or cell membranes may affect diffusion. For example, the greater the number
and size of pores in cell membranes the greater the rate of diffusion.
5. The size and nature of the diffusing molecule - Smaller molecules diffuse faster than large ones.
Fat-soluble ones diffuse more rapidly through cell membranes than water-soluble ones.
Facilitated diffusion is a special form of diffusion which allows more rapid exchange. It appears to
involve ch_____ within a membrane which make diffusion of spe____ substances easier. Car___
molecules may also be involved. The process is pas___, not involving any energy expenditure. eg.
movement of glucose into RBC
F.6 notes Cytology --Cell ultrastructures W.K.Leung
A hypothesis for facilitated diffusion of glucose :
P.12
Facilitated diffusion is
thought to be mediated by
membrane-spanning
protein.
The binding of the solute on
the outer surface would
trigger a conformational
change in the protein,
exposing the solute to the
inner surface of the
membrane, from which it can
diffuse into the cytoplasm
down its concentration
gradient
ii) Osmosis
Water diffuses through selectively permeable membranes in a process called osmosis.
iii) Active transport
Active transport is the en_____-consuming transport of molecules or ions across a membrane al____
or ag______ the concentration gradient.
Energy is required because the substance might be moved against its natural tendency to diffuse in
the opposite direction.
Movement is usually unid________, unlike diffusion which is reversible.
Specific carr____ or ch_______ exist in the membrane that allow the subatance to be transported
across the membrane.
Examples of Active Transport :

Active transport in the intestine.

Active transport in nerve cells and muscle cells.

Active transport in the kidney.
When the products of digestion are absorbed in the small
intestine they must pass through the epithelial cells lining the gut wail. Glucose, amino acids
absorption is partly a result of diffusion. However, this is very slow and must be supplemented by
active transport.
In nerve cells and muscle cells a
sodium-potassium pump is responsible for the development of a potential difference, called the
resting petential, across the plasma membrane.
Active transport of glucose occurs from the renal fluid in the
proximal convoluted tubules of the kidney.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.13
iv) Endocytosis and exocytosis
Endocytosis and exocytosis are active
processes involving the bulk transport of
materials through membranes, either into
cells (endocytosis) or out of cells
(exocytosis).
Endocytosis occurs by an infolding or
extension of the plasma membrane to form
a vesicle* or vacuole.* It is of two types.

Phagocytosis ('cell eating') - material taken up is in solid form. Cells specialising in the process
are called phagocytes and are said to be phagocytic; for example some white blood cells. The sac
formed during uptake is called a phagocytic vacuole.

Pinocytosis ('cell drinking') - material taken up is in liquid form (a solution, colloid or fine
suspension). Vesicles formed are often extremely small, in which case the process is known as
micropinocytosis and the vesicles as micropinocytotic. eg. amoeboid protozoans, liver cells, and
certain kidney cells
Exocytosis is the reverse process of endocytosis by which materials are removed from cells, such as
solid, undigested remains from food vacuoles or reverse pinocytosis in secretion.
STUDY ITEM
The uptake of mineral ions by plant tissue
A common technique for investigating the passage of mineral ions across plant cell membranes uses small
discs of root vegetables such as carrot or red beet. Discs some 8 mm in diameter and 1 mm thick are cut from
fresh tissue and immersed in a solution containing a known concentration of the ion under investigation.
Sometimes the root tips of plants such as maize are used in place of the discs. After a given period of time, the
discs are removed and the solution is analysed to determine the amount of ion remaining. Thus the rate of
absorption of the ion can be calculated.
a Suggest a reason why the carrot or red beet tissue is cut into thin discs for such an investigation.
b Why is it unnecessary to cut the maize roots in a similar way?
c What practical steps would you take, after removing the discs from the solution, before determining the amount
of ion remaining in the solution?
d How might the amount of ion remaining in the solution be measured?
3--
Table below shows the rate of absorption of bromide ions (Br - ) by carrot discs and of phosphate ions (P04 ) by maize root
tips when (1) air and (2) nitrogen were bubbled through the solution.
Ion absorption in a 24-hour period from 5-mmol solutions at room temperature.
Method of
aeration of plant
material and
solution
Air
Nitrogen
Ion absorption, in mole g-1
fresh mass, in 24 hours
Ion absorption, in mole g-1
fresh mass, in 24 hours
Br- by carrot discs
29
3
PO43- by maize tips
33
4
e
What do the differences in the rates of absorption under these two forms of aeration suggest about the
mechanism of absorption of Br- and P043-
f
Suggest a mechanism for the absorption of these ions when nitrogen only is bubbled through the solution.
The rate of absorption of an ion from a mixed salt solution may be different from that from a single salt solution of the same
concentration. Table below summarizes the results from an experiment in which red beet discs were placed in a mixed
solution of sodium chloride and potassium chloride. These are compared with the results of immersing the discs in solutions
of each of these salts on its own.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
Initial external concentration
10-mmol KCI solution
10-mmol NaCI solution
10-mmol KCI + 10-mmol NaCI solution
20-mmol KCI solution
20-mmol NaCI solution
Uptake, in moles g-1 fresh
mass in 4 days
K+
62
28
84
P.14
Uptake, in moles g-1
fresh mass in 4 days
Na+
69
63
88
The uptake of potassium and sodium ions from mixed and single solutions by red beet discs at room temperature.
g
h
i
B)
+
What effect does increasing the concentration ofcations (Na and/or K+) have on the rate of uptake of cations?
What is the effect on the rate of absorption of each of these ions when they are in mixed solution ?
Suggest a mechanism that will explain your answer to question h.
The Nucleus
Nuclei are found in all euk______ cells, the only common exceptions being mature phloem sieve tube
elements and mature red blood cells of mammals.
The nucleus is vitally important because it control
the cell's activities. This is because it contains the
g_______ (hereditary) information in the form of
DNA.
The nuclear membrane is actually a nuclear
envelope composed of two membranes. The outer
membrane is continuous with the endoplasmic
reticulum (ER)
The nuclear envelope is perforated by nuclear p___. Nuclear pores allow ex______ of substances
between the nucleus and the cytoplasm, for example the exit of messenger RNA (mRNA) and of
ribosomal subunits and the entry of ribosomal proteins, nucleotides and molecules that regulate the
activity of DNA.
Within the nucleus is a gel-like matrix called nudeoplasm (or nuclear sap) which contains chromatin
and one or more nucleoli. Nucleoplasm contains a variety of chemical substances such as ions,
proteins (including enzymes) and nucleotides, either in true or colloidal solution.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.15
Chromatin is composed mainly of coils of
DNA bound to basic proteins called histones.
It is easily stained for viewing.
During nuclear division Chromatin condenses
into more tightly coiled threads called
chromosomes. During interphase (the period
between nuclear divisions) it becomes more
dispersed.
The nucleolus has a role in the manufacture of
ribosomal RNA. One or more nucleoli may be
present. It stains intensely because of the large
amounts of DNA and RNA it contains.
C)
Cytoplasm
Cytoplasm consists of an aqueous ground substance containing a variety of cell organelles and
other inclusions such as insoluble waste or storage products.
a)
The Cytosol or ground substance
The cytosol is the soluble part of the cytoplasm. It is about 90% water and forms a solution which
contains salts, sugars, amino acids, fatty acids, nucleotides, vitamins and dissolved gases. Others are
large molecules which form colloidal solutions, notably proteins and to a lesser extent RNA.
Apart from acting as a store of vital chemicals, the ground substance is the site of certain metabolic
pathways, an important example being glycolysis. Synthesis of fatty acids, nucleotides and some
amino acids also takes place.
Some Living cytoplasm might exhibit'cytoplasmic streaming', this is an active mass movement of
cytoplasm and the cell organelles that it contains. eg young sieve tube elements.
b)
Endoplasmic Reticulum (ER)
A complex network of membranes running through
the cytoplasm of all eukaryotic cells.
The ER consists of flattened, membrane-bound sacs
called cisternae.
These may be covered with ribosomes, forming rough
ER, or ribosomes may be absent, forming smooth ER,
which is usually more tubular. Both types are concerned
with the synthesis and transport of substances.
i)
Rough ER
It is concerned with the transport of proteins which are made by ribosomes on its surface.
The growing polypeptide chain, is bound to the ribosome until its synthesis is complete. The protein
than pass into the ER cisternae, protein folds up into its tertiary structure, thus trapping it inside the
ER.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.16
The protein is now transported
through the cisternae, usually
being extensively modified en
route. For example, it may be
phosphoryIated or converted into
a glycoprotein.
A common route for the protein is
via smooth ER to the Golgi
apparatus from whence it can be
secreted from the cell or passed on
to other organelles in the same cell,
such as storage bodies or
lysosomes.
ii)
Smooth ER
One of the chief functions of smooth ER is lip__ synthesis. For example, in the epithelium of the
in______ the smooth ER makes lipids from fatty acids and glycerol absorbed from the gut and passes
them on to the Golgi apparatus for export.
Steroids are a type of lipid and smooth ER is extensive
in cells which secrete steroid hor______, such as the adrenal cortex and the interstitial cells of the
testis. In mu____ cells a specialised form of smooth ER, called sarcoplasmic reticulum, is present.
d)
Ribosomes
Ribosomes are minute organelles, about 20 nm in diameter, found in
large numbers throughout the cytoplasm of living cells, both
prokaryotic and eukaryotic. They are the sites of protein synthesis.
There are two basic types of ribosome, called 70S and 80S ribosomes.
The 70S(smaller) ribosomes are found in prokaryotes such as
bacteria.
Ribosomes are made of roughly equal amounts by mass of RNA and
protein. The RNA is termed ribosomal RNA (rRNA) and is made in
nucleoli.
Two populations of ribosomes can be seen in eukaryotic cells, namely
free and ER-bound ribosomes. Proteins made by ER-bound
ribosomes are usually for secretion.
During protein synthesis, the ribosome moves along the thread-like
mRNA molecule; the process is carried out more efficiently by a
number of ribosomes moving simultaneously along the mRNA, like
beads on a string. The resulting chains of ribosomes are called
polyribosomes or polysomes.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
e)
P.17
Golgi apparatus
It is found in virtually all eukaryotic cells and consists of a stack of flattened, membrane-bound sacs
called cisternae, together with a system of associated vesicles called Golgi vesicles.
At one end of the stack new cisternae are constantly being formed by fusion of vesicles which are
probably derived from buds of the smooth ER.
This 'outer' or 'forming' face is convex, whilst the other end is the concave 'inner' or 'maturing' face
where the cisternae break up into vesicles once more.
The whole stack consists of a number of cisternae thought to be moving from the outer to the inner
face. The function of the Golgi apparatus is to transport and chemically modify the materials
contained within it.
They are particularly important and prominent in secretory cells, a good example being provided by
the secretary cells of the pancreas.
Details of the pathway have been confirmed by using radioactively labelled amino
acids and following their incorporation into protein and subsequent passage through
different cell organelles.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.18
After concentration in the
Golgi apparatus, the protein
is carried in Golgi vesicles to
the plasma membrane. The
inactive enzyme is then
secreted by exocytosis.
In general, proteins received
by the Golgi apparatus from
the ER have had short
c__________ chains added
to become gly_________.
An important glycoprotein
secreted by the Golgi
apparatus is mucin, which
forms mucus in solution. It is
secreted by goblet cells of
the respiratory and intestinal
epithelia.
The root cap cells of plants contain Golgi apparatus which secretes a mucous polysaccharide, helping
to lubricate the tip of the root as it penetrates the soil.
The Golgi apparatus / body is also sometimes involved in the secretion of carbohydrates, an example
being provided by the synthesis of new cell walls by plants.
The Golgi body is also sometimes involved in
lipid transport. When digested, lipids are
absorbed as fatty acids and glycerol in the small
in______.They are resynthesised to lipids in the
smooth ER. coated in protein and then transported
through the G____ apparatus to the plasma
membrane where they leave the cell, mainly to
enter the lymphatic system.
A second important function of the Golgi
apparatus. in addition to the secretion of proteins,
glycoproteins, carbohydrates and lipids, is the
formation of lysosomes, described below.
f)
Lysosomes
Lysosomes (lysis, splitting; soma, body) are found in most eukaryotic cells, but are particularly
abundant in animal cells exhibiting phag_____ activity.
They are bounded by a single membrane and are simply sacs that contain hyd______ (digestive)
enzymes, such as proteases, nucleases, lipases and acid phosphatases.
The contents of the lysosome have to be kept apart from the rest of the cell or they would des____ it.
Incidentally, bodies similar to the lysosomes are sometimes seen in dying cells.
The enzymes contained within lysosomes are synthesised on r____ ER and transported to the G___
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.19
apparatus. Golgi vesicles containing the processed enzymes later bud off and are called primary
lysosomes. These have a number of functions, mostly involving digestive processes within the cell. but
sometimes involving secretion of digestive enzymes. Their functions are summarised below:
i)
Digestion
of
material
taken
in by endocytosis
Primary lysosomes may f____ with the vesicles or vacuoles formed by endocytosis to form
secondary lysosomes in which the material is dig____. This material might be taken in for f___, as in
some protozoans such as Amoeba, or for def_____ purposes, as is the case when phagocytic white
blood cells. The secondary lysosome may also be called a food vacuole. The products of digestion are
ab_____ and ass______ by the cytoplasm of the cell leaving undigested remains. These usually
migrate to the plasma membrane and egest their contents (exo_______).
ii)
Unwanted structures within the cell are removed (autophagy)
They are first enclosed by a single membrane, usually derived from smooth ER, and then fuses with a
primary lysosome to form a secondary lysosome in which the unwanted material is digested.
This is part of the normal turnover of cytoplasmic organelles, o__ones being replaced by n___ ones.
It becomes more frequent in cells undergoing reorganisation during differentiation.
iii)
Release of enzymes outside the cell (exocytosis)
Sometimes the enzymes of primary lysosomes are released from the cell.
iv)
Autolysis
Autolysis is the self-destruction of a cell by release of the contents of lysosomes within the cell.
--'suicide bags'. Autolysis is a normal event in some differentiation processes, as when a tadpole
tail is resorbed during metamorphosis. It also occurs after cells die.
g)
Peroxisomes or Microbodies (reference only)
They are spherical (slightly smaller on average than mitochondria) and bounded by a single membrane. Their
contents are finely granular, sometimes with a distinctive crystalline core which is a crystallised protein
(enzyme) and they are derived from the ER, with which they often remain in close association.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.20
Their most distinctive feature is the presence of the enzyme cat_____, which catalyses the decomposition of
hyd_____ per______ to water and oxygen (hence the name peroxisome). H2O2 is a by-product of certain cell
oxidations and is also very toxic, so must be eliminated immediately.
h)
Microtubules
Nearly all eukaryotic cells contain unbranched, helical cylindrical organelles called microtubules.
They are very fine tubes, having an external diameter of about 24 nm and with walls about 5 nm
thick.
Each tubule is made up of helically arranged
globular subunits of a protein called tubulin.
Growth of microtubules occurs at one end by
addition of tub____ subunits. It is inhibited by a
number of chemicals, such as colchicine, which
have been used to investigate the functions of
microtubules.
Growth apparently requires a template to start and certain very small ring-like structures that have
been isolated from cells, and which consist of tubulin subunits, appear to serve this function. In intact
animal cells, centrioles probably also serve this function and are therefore sometimes known as
microtubule-organising centres, or MTOCs.
Functions of microtubules:

Intracellular transport. Microtubules have also been implicated in the movements of other cell
organelles such as Golgi vesicles. Regular movements of larger organelles, such as lysosomes and
mitochondria, in many cells.

Cytoskeleton. Microtubules also have a passive architectural role in cells, their long, fairly rigid, tube-like
structure acting in a skeletal fashion to form a 'cytoskeleton'. They help to determine the shape of cells
during development and to maintain the sh___ of differentiated cells. Animal cells in which
microtubules are disrupted revert to a spherical shape. In plant cells the alignment of microtubules
corresponds with the alignment of cellulose fibres during deposition of the cell wall, thus indirectly establishing cell shape.

Formation of Centrioles, basal bodies, cilia and flagella.
i)
1)
Centrioles, basal bodies, cilia and flagella
Centrioles
Centrioles are small hollow cylinders that
occur in pairs in most animal and lower
plant cells.
Each contains nine triplets of microtubules (__x__)
At the beginning of nuclear division, the
centrioles rep______ themselves and the
two new pairs migrate to opposite poles of
the sp_____, the structure on which the
chromosomes become aligned.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.21
The spindle itself is made of micro_____, presumably synthesised using cen______ as MTOCs. The
microtubules control separation of chromatids or chromosomes.
Cells of higher plants lack centrioles, although they do produce spindles during nuclear division. The
cells may contain smaller MTOCs that are not easily visible even with the electron microscope.
2)
Cilia and flagella
They are hair-like projections on the cell’s surface. They are
encased in membrane continuous with the plasma membrane.
Flagella and cilia have i________ internal structure; if the
structures are f___ and relatively l___, they are called fl_____
(eg. tail of sperm) ; if s____ and num_____, they are considered
c____ (eg. on ciliated epithelial cell).
Internally, flagella and cilia contain a characteristic __ pairs
peripheral and __ pairs central pattern of microtubules.
At the base of the cilium is a basal body which is composed of a
ring of microtubules continuous with those in the cilium itself.
However, the two central microtubules are absent, and the
peripheral ones are in threes (triplets). microtubules
arrangement.
Many uni_______ organisms move by means of cilia or flagella. The surface of the freshwater
protozoan Paramecium, for example, is covered with cilia that beat in a co-________ fashion,
driving the organism through the water.
Flagella are nearly always associated with loco_____, but cilia, which are found more widely,
perform other functions as well. For example, they are often found lining d____ and tubules and
other specialised surfaces, along which ma______ are wafted by means of their rapid and
rhythmical b_______.
Little arm-like processes project from the peripheral doublets. These
are thought to be the site of ____ hydrolysis where energy is
transferred for bending of the flagellum or cilium.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
3)
P.22
Basal bodies
Identical in structure to centrioles are basal bodies. They are usually found at the b____ of cilia and
flagella and probably originate from replication of centrioles. They also seem to serve as MTOCs
because cilia and flagella contain a characteristic '__ + __' arrangement of microtubules.
j)
Microvilli
Microvilli are finger-like extensions of the plasma membrane of some animal cells. They increase
the s______ area for absorption, and are particularly numerous in cells specialised for absorption,
such as in intestinal ep________ and kidney tubule epithelium.
The fringe of the microvilli can
just be seen with a light
microscope and is called a brush
border.
Microvilli can contract, probably
by a sliding movement of
contractile protein fibres (actin
and myosin). Alternate shortening
and elongation of the microvilli
probably aids the absorption
process.
Plant cells lack microvilli because their rigid cell walls impose restrictions on extensions of the
plasma membrane. However, comparable increases in surface area achieved by transfer cells. Here
the cell walls develop irregular thickening, increasing their surface area and hence the surface area
of the plasma membrane.
k)
Mitochondria
A typical cell contains about a thousand mitochondria, though some have many more than this. Their
shape and size vary, but are generally round or sausage shaped.
The wall of the mitochondrion
consists of t___ thin membranes
separated by an extremely narrow
fluid-filled space.
The inner membrane is highly
f_____, giving rise to an irregular
series of partitions, or cristae,
which project into the interior.
The interior contains an organic
matrix containing numerous
enzymes and chemical
compounds.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
P.23
The chemical reactions of a_______
respiration take place in the
mitochondria.
During respiration chemical energy in
food is transferred to A___, and this
energy is then available for a variety of
cellular functions.
The cristae have the effect of increasing
the surface area so that more ATP can
be produced.
Cells whose function requires them to expend particularly large amounts of energy contain
unusually large numbers of mitochondria. These are often packed close together in the part of the cell
where the energy is required. This is seen in spermatozoa where the mitochondria are tightly packed
at the base of the motile tail. Mitochondria are also found alongside the contractile fibrils in m_____,
and at the surface of cells where active transport occurs.
III)
Structures Characteristic of Plant Cells
The cells of higher plants contain all the organelles found in animal cells (except centrioles). In
addition, plant cells also possess structures that cannot be found in animal cells.
a)
Cell walls
Plant cells are surrounded by a relatively ri___ w which is secreted by the living cell (the protoplast)
within. The wall laid down during cell div____ is called the pri____ wall. This may later be thickened
to become a secondary wall.
1)
Structure of the cell wall
The primary wall consists of cellulose micro______ running through a matrix of complex
polysaccharides (eg. pectins and hemicelluloses). Individual molecules of cellulose are long chains
cross-linked by hyd____ bonds to other molecules to form strong bundles called microfibrils.
The middle lam____ that holds neighbouring cell walls together is composed of sticky, gel-like
magnesium and cal____ pectates.
In some cells, such as leaf mesophyll cells, the primary wall remains the only wall. In most, however,
ext__ layers of cellulose are laid down on the inside surface of the primary wall (the outside surface
of the plasma membrane), thus building up a sec_____ wall. This usually occurs after the cell has
reached a maximum s___.
The cellulose fibres of a given layer of secondary thickening are usually orientated at the same angle,
but different layers are orientated at different a____, forming an even stronger cross-ply structure.
Some cells such as xy___ vessel elements and sclerenchyma undergo extensive lignification whereby
lig___, a complex polymer (not a polysaccharide), is deposited in all the cellulose layers. Lignin
cements and anchors cellulose fibres together. It acts as a very hard and rigid matrix giving the cell
wall extra tensile and particularly compressional strength. In others the deposition is complete, apart
from p___ which represent areas of the primary wall which remain unthickened—plasmo_______
might be present linking adjacent cells.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
2)
P.24
Functions of the cell wall

Mechanical strength and skeletal support is provided for individual cells and for the plant as a
whole. Extensive lign________ increases strength.

Allow development of turgidity when water enters the cell by osmosis is the main source of
support in herbaceous plants and organs such as leaves which do not undergo secondary growth.
The cell wall prevents the cell from bursting when exposed to a hypotonic solution.

Orientation of cellulose microfibrils limits and helps to control cell growth and shape because the cell's ability to stretch is determined by their arrangement.

Allows major pathways for movement for water and dissolved mineral salts.
1) Apoplast pathway--the system of interconnected c__ w___ and intercellular spaces.
2) The cell walls possess minute pores through which structures called plasmodesmata can
pass, forming living connections between cells, and allowing all the protoplasts to be linked
in a system called the symplast pathway.

Coated with waxy cut___ or impregnated with suberin to reduce water loss. eg exposed
epidermal surfaces / Cork cell.

The cell walls of root endodermal cells are impregnated with suberin forming a barrier to
water movement.

The cell walls of tran___ cells develop an increased surface area and the consequent increase in
surface area of the plasma membrane increases the efficiency of transfer by active transport.
b)
Plasmodesmata
Plasmodesmata are living conn_______ that pass between neighbouring plant cells through very fine
p____ in adjacent walls. They are sometimes found in groups known as primary pit fields. Sieve plate
pores of phloem sieve tubes are derived from plasmodesmata.
c)
Vacuoles
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P.25
A vacuole is a fluid-filled sac bounded by a
single membrane. Animal cells contain relatively
small vacuoles, such as phagocytic vacuoles, food
vacuoles, autophagic vacuoles and contractile
vacuoles. However, plant cells, have a large
central vacuole surrounded by a membrane
called the tono____.
The fluid they contain is called cell s__. It is a
concentrated solution of mineral salts, sugars,
organic acids, oxygen, carbon dioxide,
pigments and some waste and 'secondary' products of metabolism. The functions of vacuoles
are summarised below.

Water generally enters the concentrated cell sap by osmosis through the differentially permeable
tonoplast. As a result tur___ pressure builds up within the cell and the cytoplasm is pushed
against the cell wall. Osmotic uptake of water is important in cell expansion during cell gr____,
as well as in the normal water relations of plants.

W____ products of plant metabolism may accumulate in vacuoles. eg. tannins (which are
astringent to the taste), may offer protection from consumption by herbivores.

Some of the dissolved substances act as f____ reserves, which can be utilised by the cytoplasm
when necessary, for example sucrose and mineral salts.
d)
Plastids
Plastids are organelles found only in plant cells and in higher plants. They are surrounded by two
membranes (the envelope).
Chromoplasts. These are non-photosynthetic coloured plastids containing mainly red, orange or
yellow pigments (carotenoids). They are particularly associated with fruits (such as the tomato and red
pepper) and flowers in which their bright colours serve to attract insects, birds and other animals for
pollination and seed dispersal. The orange pigment of carrot roots is also contained in chromoplasts.
Leucoplasts. These are colourless plastids lacking pigments. They are usually modified for food
storage, and are particularly abundant in storage organs such as roots, seeds and young leaves.
Chloroplasts. These are plastids that contain chlorophyll and carotenoid pigments and carry out
photosynthesis. They are found mainly in leaves.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
IV)
P.26
Prokaryotic and Eukaryotic cells
There are two levels of cellular structure.
Prokaryotic cells are smaller and simpler in
organization than the eukaryotic cells. The evolution of prokaryotic cells preceded that of eukaryotic
cells by 2 billion years. They are probably closely related to the first kind of cells that appeared on
Earth.
The major similarities between the two types of cells (prokaryote and eukaryote) are:
1.They both have DNA as their genetic material.
2.They are both membrane bound.
3.They both have ribosomes .
4.They have similar basic metabolism .
5.They are both amazingly diverse in forms.
The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotes
have a nucleus and membrane-bound organelles, while prokaryotes do not. The DNA of
prokaryotes floats freely around the cell; the DNA of eukaryotes is held within its nucleus. The
organelles of eukaryotes allow them to exhibit much higher levels of intracellular division of labor
than is possible in prokaryotic cells.
PROKARYOTIC CELLS
EUKARYOTIC CELLS
No distinct nucleus; only diffuse area(s) of nucleoplasm
with no nuclear membrane
A distinct, membrane- bounded nucleus
No chromosomes - circular strands of DNA
Chromosomes present on which DNA is located
No membrane-bounded organelles such as chloroplasts
and mitochondria
Chloroplasts and mitochondria may be present
Ribosomes are smaller
Ribosomes are larger
Flagella (if present) lack internal 9 + 2 fibril arrangement
Flagella have 9 + 2 internal fibril arrangement
No mitosis or meiosis occurs
Mitosis and/or meiosis occurs
Cell wall composed of peptidoglycan, a single large
polymer of amino acids and sugar .
Many types of eukaryotic cells also have cell
walls, but none made of peptidoglycan.
Much smaller in size
Eukaryotic cells are, on average, ten times the
size of prokaryotic cells.
F.6 notes Cytology --Cell ultrastructures W.K.Leung
Examples of Prokaryotes include
bacteria and blue-green algae.
END
P.27