Chloroplasts Structure and Function | MANISH MEVADA

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CELL : STRUCTURE AND FUNCTIONS

 Chloroplasts (Gk . chloros - grass green , plastos - moulded)

• They are greenish plastids which possess photosynthetic pigments, chlorophylls and carotenoids, and take part in the synthesis of food from inorganic raw materials in the presence of radiation energy.
• Chloroplasts of algae other than green ones are called chromatophores (e.g. , rhodoplasts of red algae, phaeoplasts of brown algae).

Number

• The number of chloroplasts per cell of algae is usually fixed for a species.
• The minimum number of one chloroplast per cell is found in green alga Ulothrix and several species of Chlamydomonas.
• However,different species of a genus may have different number of chloroplasts,e.g. , 1 in Spirogyra indica and 16 in S. rectospora.
• A photosynthetic leaf chlorenchyma cell has 20-40 chloroplasts.
• An internodal cell of Chara (an alga ) has several hundred chloroplasts.

Shape

• In algae the chloroplasts have various shapes.
• They may be plate like (e.g. ,Ulothrix ),cup - shaped (e.g.Chlamydomonas), ribbon - like (e.g.,Spirogyra), polygonal or stellate (e.g. ,Zygnema) and reticulate (e.g. ,Oedogonium).
• The chloroplasts of higher plants are generally disc - shaped with oval or circular outline.
• Rarely, they may be lens - shaped, rounded or club - shaped.

Size

• Like shape, the size of the chloroplasts is different in different species.
• The discoid chloroplasts of higher plants are 4-10 um in length and 2-4 um in breadth.
• The size is generally larger in case of polyploid cells as compared to diploid and haploid cells.
• Normally it is much smaller than that of the cell.
• However,in many algae the chloroplast may occupy almost the whole length of the cell, e.g., Spirogyra,
• The chloroplast of Spirogyra may reach a length of 1 mm.

Chemical Composition

• Protein - 50-60 %
• Lipids , 25–30 %
• Chlorophyll - 5-10 %
• Carotenoids (carotenes and xanthophylls) - 1-2 %.
• DNA- upto 0.5 %.
• RNA - 2-3 %
• Vitamins K and E
• quinones
• Mg , Fe , Co , Mn , P , etc.- in traces.

Ultrastructure 

• A Chloroplast  has three parts- envelope, matrix and thylakoids.
• Pyrenoid and stigma are The attira scructures present in the chloroplasts of some algae.

Chloroplast Envelope

• A chloroplast is covered by an envelope made up of two month membranes.
• Each membrane is about 90-100 Å thick.
• It has trilaminar lipoprotein structure.
• The two membranes are separated by an intermembrane space of 100-200 A width.
• The outer membrane may be attached to endoplasmic reticulum.
• At places the names is connected to thylakoids.
• As in mitochondria, the outer membrane is more permeable than the inner membrane.
• The inner membrane has more of proteins including camer proteins.

Matrix

• The ground substance of a chloroplast is known as matrix or stroma.
• It is semifluid colloidal complex that is made of 50 % soluble proteins.
• The remaining is DNA, RNA, ribosomes, plasto - globuli and enzymes.
• Chloroplast or cpt DNA is naked, circular or occa sionally linear.
• A chloroplast may have several copies of it.
• DNA makes the chloroplast genetically autonomous because it can both replicate and transcribe to form RNA.
• Chloroplast ribosomes are 70 S.
• They resemble the ribosomes of prokaryotes.
• With the help of ribosomes the chloroplast is able to synthesize most of the enzymes required by it.
• The important enzymes present in chloroplast are those that take part in synthesis of photosyn thetic pigments, photolysis of water, photophosphorylation, dark assimilation of CO2, synthesis and degradation of starch, synthesis of lipids, etc.
• Plastoglobuli are lipid droplets of 10–500 nm diameter.
• They may contain some enzymes, vitamin K and quinones.
• The chloroplast matrix of higher plants may store starch temporarily, as starch grains.
• It is known as assimilation starch.
• In green algae (e.g. , Spirogyra, Ulothrix ), the chloroplasts possess special starch storing structures called pyrenoids.

Thylakoids ( Menke , 1961 )

• They are membrane lined flattened sacs which run through out the stroma or matrix of the chloroplast.Since, they take part in photosynthesis, they are also called photosynthetic thylakoids.
• Thylakoids are thus the structural elements of the chloroplast.
• They generally run parallel but may show interconnections.
• Thylakoids may also be attached to the inner membrane of chloroplast envelope.
• In the chloroplasts of higher plants, thylakoids are stacked at places to form grana. 40 60 grana may occur in a chloroplast.
• Each granum has 2-100 thylakoids.
• Grana are absent in bundle sheath and algal chloroplasts.
• The latter are, therefore, agranal.
• Because of the presence of grana, thylakoids are differentiated into two granal thylakoids and stroma or intergranal thylakoids.
• A granum is attached to only a few stroma or nongranal thylakoids, though it is made up of upto 100 thylakoids.
• It is, therefore, believed that the thylakoids get folded and bifurcated in the region of grana.
• Thylakoid membranes possess photosynthetic pigments and coupling factors.
• Coupling factors are involved in ATP synthesis.
• Photosynthetic pigments include chlorophyll a, chlo rophyll b, carotenes and xanthophylls.
• They occur in specific groups called photosystems (previously quantasomes).
• There are two photosystems, I and II.
• Photosystem II occurs in appressed parts of granal thylakoids while photosystem I is found in stromal thylakoids and nonappressed parts of granal thylakoids.

Functions

• Photosynthesis
• Chloroplasts are the centres of photosynthesis or formation of organic compounds from inorganic raw materials.
• The organic substances, thus synthesised, not only provide body building material to autotrophic plants themselves but also to all heterotrophic plants as well as animals.
• Energy Transduction
• Chloroplasts are able to trap sun energy and change it into chemical energy The chemical energy is used by all living organisms to perform their life activities.
• Consumption of Carbon Dioxide
• Chloroplasts pick up carbon dioxide and use the same in photosynthesis.
• This keeps the percentage of this gas balanced in the atmosphere as carbon dioxide is being constantly added to it through combustion and respiration.
• Liberation of Oxygen
• Chloroplasts liberate oxygen which is passed into the atmo sphere.
• This keeps the balance of oxygen constant in the atmosphere , as oxygen is being consumed in respiration and combustion.
• Storage of Starch
• They store starch either temporarily (in higher plants) or perma nently in several algae ).
• Photosensitivity
• Chloroplasts of some algae provide photosensitivity because of the presence of stigma or eye spot
• Reducing Power
• The reducing power produced during light reaction (NADPH) is used in the reduction of nitrate and synthesis of amino acids.
• Synthesis of Fatty Acids.
• Murphy and Leech ( 1978 ) have reported the synthesis of fatty acids in Spinach chloroplasts.
• Storage of Lipids
• Chloroplasts store fat in the form plastoglobuli.
• Formation of Chromoplasts.
• They can be changed into the chromoplasts to pro vide colour to many flowers and fruits for attracting animals.

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