Download Chapter 4: Cellular Organization

Endoplasmic reticulum
(EM) (x94000 app.)
4.2.6 Endoplasmic reticulum
- an elaborate system of membranes forming a
cytoplasmic skeleton
- an extension of the out nuclear membrane
Cisternae: flattened sacs formed by the membranes
Rough endoplasmic reticulum: membranes are lined with
ribosomes
Rough ER is concerned with protein synthesis and is
abundant with cells which are rapidly growing or
secreting enzymes
Smooth endoplasmic reticulum: membranes lack
ribosomes
Functions of the ER:
1 Provides a large surface area for chemical
reactions
2 Provides a pathway for the transport of
materials through the cell
3 Produces proteins, especially enzymes
4 produces lipids and steroids
5 Collects and storing synthesized materials
6 Provides a structural skeleton to maintain
cellular shape (e.g. smooth ER of a rod cell
of retina)
Golgi apparatus of an olfactory bulb cell
Golgi apparatus (EM) (x30000 app.)
4.2.7 Golgi apparatus (dictyosome)
- similar to smooth ER but is more compact
vesicles: small membranous sacs pinching
off from the cisterna
- normally only one Golgi apparatus in
animal cells but a large number of stacks
known as dictyosomes in plant cells
- well developed in secretory cells and
neurones for secretory functions
Functions:
1 adding carbohydrates to proteins to
form glycoprotiens
2 produce secretory enzymes, e.g.
pancreatic enzymes
3 produce new cell walls by secreting
carbohydrates
4 transport and store lipids
5 form lysosomes
4.2.8 Lysosomes
- bounded by a single
membrane which contains
enzymes (mostly hydrolases)
- abundant in secretary cells &
in phagocytic white blood
cells
1 Digest material which the cell consumes
from the outside by phogocytosis, e.g. white
blood cells digest bacteria, amoeba digests
food
2 Digest part of a cell or worn-out organelles,
autolysis for dead cells
3 Release enzymes outside the cell
(exocytosis) to break down other cells, e.g.
reabsorption of tail of tadpoles during
metamorphosis
Therefore, they are abundant in secretory
cells and phagocytic cells.
4.2.9 Microbodies
- small spherical membrane-bounded bodies; slightly
granular structure
- contains enzymes (mostly catalase) to break down H2O2:
2 H2O2  2 H2O2 + 2O2
- H2O2 is a potentially toxic by-product of many
biochemical reactions in cell, particularly active cells
like the liver cells
nucleus
A: heterochromatin
B: euchrromatin
C: nucleolus
D: nuclear pores
E: microbody
A
B
C
D
E
4.2.10 Vacuoles
- tonoplast: single membrane around the vacuole
- containing a solution of mineral salts, sugars, amino
acids, wastes and sometimes anthocyanins
- large and central in plants ;
numerous but small in animal cells, e.g. food vacuole,
phagocytic vacuole and contractile vacuole
Functions:
1 temporary food reservoir of sugars & amino
acids
2 anthocyanins (coloured pigments) attract
pollinating insects, animals for fruit dispersal
3 temporary stores for wastes, e.g. tannins,
and to be removed through leaf cells when
leaves fall
4 occasionally contain hydrolytic enzymes to
act as lysosomes
5 provide turgidity to herbaceous plants &
seedlings
Non-membranous Structures
4.2.11 Ribosomes
- small cytoplasmic granules of RNA molecules for
protein synthesis
- around 20 um in diameter in eukaryotic cells (80S)
but slightly smaller in prokaryotic cells (70S)
- account for 20% of the mass of a cell
polysomes: ribosomes occur in groups assciated with
rough ER or occur freely within cytoplasm
4.2.12 Storage granules
- soluble form as sugars in vacuoles of plant cells
- insoluble forms:
starch grains - in chloroplasts & cytoplasm;
in specialized leucoplast (amyloplast)
glycogen granules – in animals cells
lipid droplets – both plant and animal cells
Leucoplast are colourless plastids (double membraned
organelles in plants)