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BOTANY LECTURE FOUR
SCIENCE SOCIETY TEAM
They are proteins specialized to catalyse biological reactions.
Catalysts is a substance that increase the speed of chemical reactions without itself being
changed as it decrease the activation energy needed to initiate the chemical reaction. This
process is called catalysis and the reaction is called catalysed reaction.
For living cells, enzymes are important as they accelerate a very large number of essential
biochemical reactions under certain physiological conditions such as pH value,
temperature..etc.
NATURE OF ENZYMES
Enzymes, which are organic biological catalysts are very similar to the man-made inorganic
catalysts and characterized as follows:1. They are active in extremely small amounts. I.e. Small amount of enzymes is
necessary to convert a large amount of substrate to product.
Substrate is the substance on which enzyme acts and convert it into product
through catalysed reaction.
2. Each enzyme has a significant turnover number which is called turnover number of
enzyme. Turnover number of enzyme is the number of moles of substrate converted
per minute by 1 mole of enzyme.
3. All enzymes are proteins or simple polypeptides formed of amino acids. Since they
have large molecular weight, they do not dialyse. ( can’t be separated by
semipermeable membrane)
4. They are denatured by various agents as changing temperature or pH value, this
change the shape of active site, thus enzymes do not function anymore when they’re
denatured
5. Enzymatic reactions are reversible, they can work in either direction, in other words.
They can convert substrate to products, or products to substrate, the direction
depends on the amount of substrate and products in the cell, and what the cell
needs.
A+B
C
If for some reason, a large amount of C is present in the cell, the reverse reaction will
occur, turning C into A and B until equilibrium is established. The enzyme only
accelerate the speed of reaction and doesn’t effect on the equilibrium point.
6. Enzymes are very specific for the substrates they act upon. Unlike inorganic manmade catalysts that can catalyse several quite different reactions.
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BOTANY LECTURE FOUR
SCIENCE SOCIETY TEAM
NOMENCLATURE OF ENZYMES
Enzymes are often named from the substrate they act upon adding the suffix –ase.
if an enzyme is used to catalyse lipids it’s called Lipase..
Some enzymes are named according to the type of reaction as succinate dehydrogenase,
nitrate reductase..
While few of them have trivial names such as pepsin, trypsin, chymotrypsin and thrombin.
THE CHEMICAL CHARACTERISTSICS OF ENZYMES
Some enzymes are simple proteins, others are conjugate having a non-protein group more
or less associated with the protein group. The protein group is called apoenzyme the total
complex (non-protein group + apoenzyme) is called holoenzyme
when the non-protein part is firmly bonded to the apoenzyme it’s termed prosthetic group,
and when it is loosely bounded it can be considered as a separate entity and termed
coenzyme
PLANT CELL WALL
The presence of a cell wall in plant cells distinguishes them from animal cells, cell wall may
be referred as a non-protoplasmic component of the cell, because after it is doesn’t perform
any metabolic activities
Cell wall has many functions as
supporting, protecting, and play an
important as absorption, transpiration
and secretion of cell substances, also
the cell wall determines the shape of
the cell and texture of tissues.
Cell wall is composed of three layers
A. The middle lamella
B. Primary cell wall
C. Secondary cell wall
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BOTANY LECTURE FOUR
SCIENCE SOCIETY TEAM
1. MIDDLE LAMELLA
It is the cement that holds the individual cells together.
It is found between the primary cell walls of neighbouring cells and mainly consists
of pectic substance and may be lignified in woody tissues
In supporting tissues it may fill the intercellular spaces
2. PRIMARY CELL WALL
It is the first true cell wall which develops on the new cell. It contains cellulose,
hemicellulose and some pectin
It initiates before the cell enlarges, so it passes through a period of growth in surface
area, and usually associated with the living protoplasts.
It is present between middle lamella and plasma membrane.
The cellulose constituent of cell wall occurs in the form of long chain molecules that
are combined together forming micelles, which in turn form bundle called
microfibril. Microfibrils are combined together forming cellulose fibers.
Cellulose micelles microfibril  cellulose fibers.
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BOTANY LECTURE FOUR
3.
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SECONDARY CELL WALL
It is formed on the inner surface of primary cell wall facing the cytoplasm. Develops
in the cells that have stopped growing
It is much more rigid than primary cell wall. And layering can be observed in it
It is composed of cellulose, hemi-cellulose. Depositions of lignin, suberin, cutin and
various other substances that strengthen and support the cell.
Figure 1 Layering in secondary cell wall
NOTE
Lignification is the increase of the lignin substance in the wall. It is important
to form woody cells. It begins in middle lamella, spreads to primary wall and
finally to secondary wall.
Cutinization is the deposition of cutin. It starts in a direction opposite to that of lignification
to form a surface layer called cuticle.
PITS
Are areas through which substances pass from cell to cell.
The primary cell walls have thin areas called primary pit fields over which the secondary cell
wall does not deposit and thus a pit is formed.
A characteristic feature of the primary pits of living cells is the presence of a very thin
cytoplasmic strands called plasmodesmata.
Plasmodesmata play an important role in the transport of materials and relay of stimuli.
The middle lamella and two primary walls of two adjacent cells form a pit membrane. This
pit membrane with two opposite pits form the pit-pair
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BOTANY LECTURE FOUR
SCIENCE SOCIETY TEAM
If the pit has no complementary pit in the adjacent cell or it is facing an intercellular space it
is termed a blind pit.
In some bordered pits, the central portion of the pit is thickened to form a disc-shaped torus
Torus consists of impermeable substance, thus it regulates the passage of water from one
cell to its adjacent ones.
SCIENCE SOCIETY TEAM 5