Page 64 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles
apoblast or through the plasmodesmata of the • The Golgi apparatus (GA) is made up of a stack
symplast pathway. of at least three flattened sacs called cisternae.
(b) • Cytoplasm is aqueous in nature, consisting of • The cisternae are kept in place by microfilaments
the ground substance and cell inclusions. with a thin layer of cytoplasm between one
• The ground substance or cytosol is the soluble cisterna to another.
part of the cytoplasm. It contains gases, mineral • Each cisterna is covered with a layer of
ions and organic substances. lipoprotein menbrane with a colloidal sol
• The cell inclusions include the fine fibrils i.e. in it. 2
microtubules and microfilaments. • The colloidal sol within has enzymes and
• The microtubules are fine unbranched tubules proteins like glycoprotein that are derived from
with diameter of 25 nm, a wall of 5 nm thick the endoplasmic reticulum.
and vary in length. • Besides protein, other substances such as lipids
• The microfilaments are fine filaments made of like steroid hormomes and carbohydrates such
protein with a diameter of 7 nm and a length of as cellulose may be found within the cisterna.
several μm. • Each cisterna is formed with vesicles that keep
• The cytoplasm stores vital chemicals including pinching off from the endoplasmic reticulum at
fats. one side i.e. the cis side.
• It is the site for certain metabolic pathways • On the other side, i.e. the trans side, vesicles
such as glycolysis, synthesis of fatty acids and keep pinching off from the periphery of the
amino acids. cisterna.
• It enables organelles to move about in it. These • Therefore, the outer trans cisterna will finally
organelles include mitochondria, chloroplasts, disappear completely to become vesicles.
ribosomes, lysosomes and vacuoles. • The main function of the GA is to package
• It forms the cytoskeleton that determines the secretion in the form of vesicles.
shape of the cell. • Thus, the GA packages proteins, especially
enzymes, for internal transport or use within
2. (a) • The chromosome of a bacterium lies freely the cell.
in the cytoplasm of the cell whereas that of • The GA can form larger vesicles in the form of
eukaryote is found in the nucleus. lysosomes with hydrolases.
• The chromosome of a bacterium consists of one • The GA also packages secretions like digestive
circular DNA whereas the eukaryote consists of juice from digestive glandular cells of the
linear DNA. stomach, pancreas and small intestine.
• The chromosome of the bacterium is naked, • The GA also packages hormones, both steroid
not bound to histone protein, but the eukaryote and peptide from cells of the endocrine glands
has histone bound to it. e.g. Leydig cells.
• The chromosome of a bacterium has no 3. (a) • One ultra-structure of mature xylem cells is no
centromere and chromatids whereas the protoplasm but hollow lumen.
eukaryote has two chromatids attached at the • The mature vessels join end to end without
centromere during cell division.
• The chromosome of a bacterium has no cross walls to form long continuous vessels.
more than 3500 genes whereas the larger • These hollow vessels offer no resistance to
water flow from the roots into the leaves.
chromosome of eukaryote has more than that • Another ultra-structure of the mature xylem
number.
cells is their thick lignified hard wall enabling
(b) them to support the plant.
Vesicle budding • These thick-walled vessels enable water to
off and moving be transported upwards by high suction and
towards cell
would not collapse.
Cisterna
• Such thick hard wall is made up of layers of
Network of Inter–cisternal cellulose fibres impregnated with lignin.
tubules space (b) • Simple squamous epithelia are found in alveoli
of lungs, endothelium of blood vessels and
Bowman capsule of kidneys.
Sac from ER
added to the • In the alveoli, the thin epithelia enable oxygen
convex face to diffuse into the blood capillaries and carbon
dioxide to diffuse out.

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