Chloroplasts

Introduction to Plastids
PLASTIDS

 Plastids- membrane-bound organelles found in cells of plants, algae and some eukaryotes
 There are many plastids types of which most well-known is
CHLOROPLASTS

CHLOROPLASTS

 Predominant plastid type in leavers and other aerial types
 Chloroplast has its own genome and genetic system, stroma also contains special set of ribosomes, RNAs
and chloroplast DNA
 Primary role is PHOTOSYNTHESIS
 Chloroplast are larger than mitochondria

Chloroplasts structure

 Highly permeable outer membrane and much less permeable inner membrane- in which transport
membranes are embedded
o Narrow intermembrane space in between them
o Together- these two membranes form chloroplast envelope
o Chloroplast membranes contain chlorophyll and its associated proteins and are sites of
photosynthesis
 Inner chloroplast membrane surrounds large space- STROMA- contains many metabolic enzymes- where
ATP is made by head of ATP synthase
o Adjacent grana are connected by unstacked membranes- Stroma lamellae
 Thylakoids are unique feature of chloroplasts- contain ETC, Photosynthetic light capturing systems and ATP
synthase
Separate, distinct membrane- forms set of flattened, disc-like sacs- thylakoids
 Thylakoid membrane Is highly folded into numerous local stacks of flattened vesicles- grana-
interconnected by non-stacked thylakoids
o Stacked membranes are grana lamellae
 Lumen of each thylakoid is connected with lumen of other thylakoids- thylakoid space – represents separate
compartment in each chloroplast that isn’t connected to either intermembrane space or stroma

Structure of Photosynthetic membranes

 Thylakoid membrane derives from inner membrane during plastid
development and is pinched off to become discontinuous with it
 Thylakoids from Greek- ‘sac-like’
 Interconnected and closed continuum
 Different parts have specialised functions in
photosynthesis
 Chloroplast thylakoid membrane- site of solar energy conversion in plants and algae

Biogenesis and structure of photosynthetic membranes

, Chloroplast function

Chloroplasts are important in synthesising many different products for rest of cell including:

 Purines and Pyrimidines
 Fatty acids and Lipids
 Amino Acids- Thr, Lys, Ile
 Isoprenoids- plant hormones (gibberellic acid (GA), abscisic acid (ABA), strigolactones)
 Heme

Critical for reduction of nitrate and sulphate- important process on global scale

OTHER TYPES OF PLASTID

Proplastid

 Precursor of all other plastids
 Present in young meristematic regions- where they divide to keep pace with cell
division- ENSURE CONTINUITY
I.E. all cells have a plastid
 Only little larger than mitochondria

Chromoplast

 Non-photosynthetic chromoplasts provide- colour to fruits, flowers and roots
(carrots)
 Contain high concentrations CAROTENOIDS pigments rather than
chlrorophyll- such as Carotenes and Xanthophylls- often with crystalline
appearance
o Causes yellow, orange or red colours of fruits and flowers as well
as autumn leaves

Needle shaped chromoplasts- up to 20μm in length- in
bird of paradise feather
Chromoplasts in Chinese lantern (Physalis algehenki)
Young (L) and older (R) with spiked like stormules

Amyloplast

 Starch- storing plastids in non-green tissues- such as storage organs (e.g. Potato
Tubers)
 Abundant in storage tissues of shoot and root and in seeds
 Specialised amyloplasts in root cap- serve as gravity sensors that direct root growth downward into soil

Leucoplast

 Colourless plastids involved in oil and lipid synthesis (plastoglobuli)- Non pigmentated
 Quite varied in appearance
 Often surrounded by ER Membranes

Elaioplast- type of leucoplast

 Quite rare, contain rounded and angular lipid bodies storing mostly sterol esters
 Found in cells surrounding anther cavity (tapetum)- where pollen develops