Would someone help me and please describe the structure of the chloroplast and the function of each component? Thank you!

science2014 | Student

What is Chloroplast

Chloroplast is a cell organelle and the site of photosynthesis in eukaryotic cells. Hence they are present in plant cells and algae, but not in animal cells or bacteria cells.

Structure & Function of Chloroplast Components

Chloroplast is made up of following components:

1) Envelope

Consists of inner and outer phospholipid bilayers, each 6-8 nm thick, with a 10-20 nm inter-membrane space in between.

Function: Being semi-permeable, the envelope regulates the transportation of various molecules like glucose, ions like Fe2+ and Mg2+, oxygen and carbon dioxide, in and out of the envelope. It is also the site for synthesis of certain lipids and light-harvesting pigments.

2) Stroma (Chloroplast Matrix)

The aqueous matrix encapsulated by the envelope and containing all internal components of the chloroplast.
Function: Dark or light-independent reactions occur at stroma. "The enzymes in the stroma utilize carbon dioxide from the atmosphere, as well as the ATP and NADPH2 molecules released from grana, to synthesize sugar molecules and starch. This process is also known as carbon dioxide fixation, and occurs through a series of reactions collectively called Calvin cycle." [2] Light-dependent oxygen fixation also occurs in stroma.
3) Thylakoids (each thylakoid has a Lumen)

A flattened membrane sac inside the chloroplast, used to convert light energy into chemical energy.

4) Grana (singular: Granum)

Stack of thylakoids in a chloroplast.

Function: Light-dependent reactions occur at grana and thylakoids. It is the place where photosynthetic pigments like chlorophyll a, chlorophyll b, carotenoids, etc. absorb light energy, which in turn breaks down the water molecules, and finally gives rise to ATP, NADPH2 and oxygen.

5) Lamellae  (singular: Lamella)

The tissue that connects the grana to each other.

Function: "Lamellae act as a 'skeleton' of the chloroplast, maintaining efficient distances between the grana, thereby maximizing the overall efficiency of the chloroplast." [1]

6) Circular DNA

7) Starch Granules

Starch exists in chloroplast in the form of tiny little 'grains' or 'granules'.

Function: They are the (insoluble) storage carbohydrate product of photosynthesis.

8) Lipid Globules

9) Ribosomes

Chloroplasts contain the smaller type of ribosomes (70S ribosomes), similar to the ones found in the cytoplasm of prokaryotic cells.

Function: Their main function is to serve as the site of mRNA translation.

10) Chlorophyll

Green pigment in plants that absorbs light energy used to carry out photosynthesis.

11) Peripheral Reticulum

In certain plants, an additional set of membranous tubules budding off from the inner membrane of the envelope.
Function: Fast transport of metabolites and proteins from the inter-membrane space into the chloroplasts.
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rachellopez | Student

Chloroplasts are organelles located in a plant which help plants absorb sunlight and complete the process of photosynthesis. Chloroplasts structures include: membrane envelope, thylakoid membrane, grana, stroma, and chlorophyll.

The membrane envelope has an inner and outer membrane that protect the chloroplasts structures. The thylakoid membrane has flat, sack-like structures called thylakoids, which change light energy to chemical energy. Grana, or the singular granum, is stacks of tylakoid sacks where the conversions take place. The stroma is thick fluid in the chloroplast the converts CO2 to carbohydrates (sugar). Finally, chlorophyll is a green pigment which absorbs light as well as gives plants their green color.

ssarfraz | Student


The word 'Chloroplast' comes from a greek words;

"chloros" which means "green"

"plastes" wich means  "the one who forms"

Chloroplasts are present in all green coloured eukaryotic cells. Chloroplasts are the site where the photosynthesis function is performed. In short Photosynthesis occurs in eukaryotic cell structures called chloroplasts.

A chloroplast is a type of plant cell organelle known as a plastid. Plastids assist in storing and harvesting needed substances for energy production. A chloroplast contains a green pigment called chlorophyll, which absorbs light energy for photosynthesis.

A plastid containing chlorophyll and other pigments, occurring in plants and algae that carry out photosynthesis.

Functions of Chloroplast

  1. Absorption of light energy and converting it into biological energy
  2. Production of NAPDH2 and evolution of oxygen through the process of photosys of water.
  3. Production of ATP by photophosphorylation. NADPH2 and ATP are the assimilatory powers of photosynthesis. Transfer of CO2 obtained from the air to 5 carbon sugar in the stream during dark reaction.
  4. Breaking of 6-carbon atom compound into two molecules of phosphoglyceric acid by the utilization of assimilatory powers.
  5. Conversion of PGA into different sugars and store as stratch. The chloroplast is very important as it is the cooking place for all the green plants. All heterotrophs also depend on plasts for this food.

Structure of Chloroplast

The chloroplast is surrounded by two unit membranes, each about 40 to 60 A° thick. The space between two membranes is called periplastidal space.

A chloroplast is ellipsoidal, about 2.5 microns (1 micron = 0.001 millimetre) thick and 5 microns long. The chloroplast is enclosed in a double membrane, within which are the stroma (a matrix containing dissolved enzymes) and the lamellae (internal membranes folded into closed disks, the thylakoids). These disks are necessary for the formation of adenosine triphosphate (ATP), an energy-rich storage compound. In the chloroplasts of most higher plants are regions called grana (singular granum), in which the thylakoids are tightly stacked.

The chloroplasts have a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner membrane of the chloroplast enclose a semi-gel-like fluid known as the stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma. 
Outer membrane - It is a semi-porous membrane and is permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins.

Intermembrane Space - It is usually a thin intermembrane space about 10-20 nanometers and it is present between the outer and the inner membrane of the chloroplast.

Inner membrane - The inner membrane of the chloroplast forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane. 

Stroma is a alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma. The chloroplast DNA chlroplast ribosomes and the thylakoid sytem, starch granules and many proteins are found floating around the stroma.

Thylakoid System is suspended in the stroma. The thylakoid system is a collection of membranous sacks called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as grana.

Each granum contains around 10-20 thylakoids.
Thylakoids are interconnected small sacks, the membranes of these thylakoids is the site for the light reactions of the photosynthesis to take place. The word 'thylakoid' is derived from the Greek word "thylakos" which means 'sack'. 

Important protein complexes which carry out light reaction of photosynthesis are embedded in the membranes of the thylakoids. The Photosystem I and the Photosystem II are complexes that harvest light with chlorophyll and carotenoids, they absorb the light energy and use it to energize the electrons.
The molecules present in the thylakoid membrane use the electrons that are energized to pump hydrogen ions into the thylakoid space, this decrease the pH and become acidic in nature. A large protein complex known as the ATP synthase controls the concentration gradient of the hydrogen ions in the thylakoid space to generate ATP energy and the hydrogen ions flow back into the stroma. 

Thylakoids are of two types - granal thylakoids and stromal thylakoids. Granal thylakoids are arranged in the grana are pancake shaped circular discs, which are about 300-600 nanometers in diameter. The stromal thylakoids are in contact with the stroma and are in the form of helicoid sheets.

The granal thylakoids contain only photosystem II protein complex, this allows them to stack tightly and form many granal layers wiht granal membrane. This structure increases stability and surface area for the capture of light.

The photosystem I and ATP synthase protein complexes are present in the stroma. These protein complexes acts as spacers between the sheets of stromal thylakoids. 

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