Biology Today — May 2017

• Their size varies in different species. Chloroplasts of higher plants are 4-10 μm in length and 2-4 μm in breadth.

• Chloroplast has three parts: envelope, matrix and thylakoids.

• The chloroplast is covered by two membranes, separated by intermembrane space.

• Matrix or stroma consists of circular dsDNA, RNA, ribosomes (70S), starch grains and enzymes of light and dark reactions
of photosynthesis.

• Thylakoids are membrane lined flattened structures in the stroma. They are stacked at places to form grana. Each granum
has 2-100 thylakoids.

• Thylakoids present between two grana are called Fret channel or stroma thylakoids.

• Flat membranous tubules that connect the thylakoids of different
grana are called stroma lamellae.

• Thylakoid membrane contains photosynthetic pigments (Chl a,
Chl b, carotenes, xanthophylls) coupling factors (involved in ATP
synthesis) and photosystems I and II.

Functions

• It is the site of photosynthesis as it traps solar energy to convert
inorganic compounds into organic compounds (carbohydrates).

• It liberates oxygen and utilises CO 2 to keep the balance of gases
in the atmosphere.

• It produces ATP and reducing power NADPH for various biochemical reactions.

• It stores starch and lipids.

• It can change into chromoplasts and provide colour to flowers and fruits for attracting various pollinators.

MICROBODIES

• They are small, single membrane bound organelles associated with oxidation reactions (other than those involved in
respiration).

• These are of two types:

Microbodies

Peroxisomes

•    Discovered by De Duve et al. (1965) by fractionation.

•    Found in all eukaryotic cells i.e., both plant and animal cells.

•    Spherical sac-like structure of size 0.5 - 1.0 μm.

•    Contain enzymes for peroxide biosynthesis. E.g., catalase, special docking proteins called peroxins

for obtaining materials from cytosol and ER.

•    In animal cells, they metabolise number of toxic substances and also detoxify alcohol in liver.

•    Breakdown of unusual substances or xenobiotics, which cannot be metabolised by normal enzymes.

•    In plants, they perform photorespiration in association with chloroplast and mitochondria.

•    In root nodules, they convert fixed nitrogen into ureids for transport.

Glyoxysomes

•    These are specialised

peroxisomes.

•    They contain enzymes

for b-oxidation and

glyoxylate pathway

(conversion of fatty acids

into carbohydrates).

•    Found in plants,

germinating oil seeds

and some fungi.

Oleosomes (Sphaerosomes) - They are the small, spherical vesicles (0.5 - 1.0 μmin diameter) bound by single membrane. These

synthesise and store lipids. They arise from endoplasmic reticulum (ER) and occur abundantly in the endosperm cells of oil seeds.

In some tissues, they have hydrolytic enzymes, therefore are considered to have lysosomic activity.

RIBOSOMES

• They were discovered by Robinson and Brown in 1953 in plant cells and by Palade in 1955 in animal cells. Palade also
coined the term ‘ribosome’.

• They are ribonucleoprotein protoplasmic particles (RNP) without membrane, having length of 200 - 340 A° and diameter of
170 - 240 A°.

Fig.: Ultrastructure of chloroplast

Outer membrane

Inner membrane

Stroma

lamella

Ribosome

Starch grains

Stroma

(aqueous space)

Granum

(stack of

thylakoids)

DNA Thylakoid