Page 37 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

(b) Food vacuoles or phagosomes in phagocytes are small. They
contain bacteria, organic particles and dissolved proteins.
(c) Food vacuole is a place for food digestion. Any undigested food
is egested through the plasma membrane.
5. Contractile vacuole
(a) Contractile vacuole is found in freshwater protozoans.
2 (b) Contractile vacuole is spherical in shape, able to absorb water
and contract, forcing water out through the membrane. Those
found in Euglena and Paramecium have feeding smaller sacs or
tubules to empty water into the main vacuole.
(c) Contractile vacuole acts as an osmoregulatory mechanism to get
rid of excess water in freshwater protozoans. If not, the cell may
burst due to excessive water absorbed through osmosis.

Quick Check 3

1. Mitochondria and chloroplasts seem to have evolved from endosymbiosis of prokaryotic cells.
Suggest reasons for this.
2. Export of substances from cells requires concerted actions of nucleus, ER, the Golgi apparatus and
lysosomes. Why?

Differential Centrifugation
1. Differential centrifugation is a technique of separating cell components,
including macromolecules using a centrifuge. Centrifuge uses
centrifuging force equivalent to many times that of gravitational force
(g) to spin down cell components of different S values (sedimentation
units) step by step. 2017

2. The procedure to fractionate cellular components is as follows:
(a) Tissue is first homogenated with a homogeniser, which can be
very sophisticated using ultrasound to break up cells to the level
required.
(b) Normally the tissue has to be chilled and added with buffer to
keep the enzymes functional and added with isotonic solution to
prevent the organelles from breaking.
(c) The homogenate is centrifuged at 600 times gravity for 10
minutes for animal tissues. Nuclei and unbroken cells are spun
down.
(d) Then, the supernatant is centrifuged at 10,000 times gravity for
20 minutes. Mitochondria, cisternae of endoplasmic reticulum
and Golgi apparatus are spun down.
(e) Further centrifuge at 100,000 times gravity for 60 minutes will
spin down ribosomes, microtubules and microfilaments. The
supernatant will then contain macromolecules such as nucleic
acids and proteins.

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