Leaf chloroplast
Chloroplast

Chloroplast- Structure and Functions

Where does the photosynthesis happen? As all known in the green part of the plant.

Have you ever wondered what special a green part of the plant have? That it can make food.

The answer is an oval, double membraned structured called Chloroplast.

So, what makes a chloroplast special- 

  • Chlorophyll
  • RuBisCO

These two things are essential for photosynthesis.

What is a Chloroplast

So, as we told above it is oval shape, double-membrane cell organelle found in the leaf of the plant, algae, and Cyanobacteria.

Chloroplast
ChloroplastImg source

It is a semi-autonomous cell organelle that has its own double-stranded circular DNA and can synthesize its own protein (as it has 70 s ribosomes).

But it is dependent on cytoplasm for some kind of protein so-called semi-autonomous cell organelle (Cell within Cell).

It is a photosynthetic plastid that makes food for the plant.

Read about what is a cell and other cell organelles.

Structure of a Chloroplast

Chloroplasts are generally lens-shaped, 3–10 μm in diameter, and 1–3 μm thick. 

The large diversity in shapes of chloroplast exists among the algae, which often contain a single chloroplast that can be shaped like

  • a net (e.g., Oedogonium)
  • a cup (e.g., Chlamydomonas) 
  • a ribbon-like spiral around the edges of the cell (e.g., Spirogyra)
  • slightly twisted bands at the cell edges (e.g., Sirogonium). 

In some algae, the chloroplast takes up most of the cell, with pockets for the nucleus and other organelles, for example, some species of Chlorella have a cup-shaped chloroplast that occupies much of the cell.

It is not true in case of plants cells, since in them the larger space is covered by Vacuole (not chloroplast).

Chloroplast
Chloroplast StructureImg source

Chloroplast contains-

  • The outer and inner membrane.
  • Periplasmic space (space between both membrane).
  • An inner fluid called Stroma.
  • Granum (Stalk of thyllakoid).

Rest is a circular DNA and a 70-s ribosome which will be covered later.

Outer chloroplast membrane

The outer membrane is a semi-porous membrane that small molecules and ions can easily diffuse across.

However, it is impermeable to larger proteins, so polypeptides synthesized in the cell cytoplasm, have to be transported by the TOC complex on the outer membrane.

Intermembrane space 

A thin intermembrane space about 10–20 nm thick exists between the outer and inner membranes of the chloroplast.

Some algae like Glaucophyte algal chloroplasts have a peptidoglycan layer between the membranes. It corresponds to the peptidoglycan cell wall of their cyanobacterial ancestors.

Inner chloroplast membrane

The inner chloroplast membrane borders the stroma and regulates the exchange of materials in and out of the chloroplast. 

After passing through the TOC complex in the outer chloroplast membrane, polypeptides must pass through the TIC complex which is located in the inner chloroplast membrane.

In addition to regulating the passage of materials, the inner chloroplast membrane also synthesizes fatty acids, lipids, and carotenoids.

Stroma

The protein-rich, alkaline, aqueous fluid within the inner chloroplast membrane and outside of the thylakoid space is called the stroma.

It corresponds to the cytosol of the original cyanobacterium. 

Materials found floating around stroma-

  • Nucleoids of chloroplast DNA
  • Ribosomes
  • The thylakoid system (Grana)
  • Starch granules 
  • Proteins 

The stroma contains the enzymes of the Dark reaction (RuBisCO).

The Calvin cycle, which fixes CO2 into Glucose takes place in the stroma.

Grana

Grana is the stacks of thylakoids.

With electron microscopy, it became possible to see the thylakoid system clearly, revealing it to consist of stacks of flat thylakoids which made up the grana.

 A long interconnecting stromal thylakoid links different grana together for faster exchange.

In the transmission electron microscope, thylakoid membranes appear as alternating light-and-dark bands, 8.5 nanometers thick.

Granum structure 

It is a stack of grana thylakoids linked by helical stromal thylakoids that wrap around the grana stacks and form large sheets that connect different grana.

Each granum can contain anywhere from 2 -100 thylakoids, though grana with 10–20 thylakoids are most common. 

Stromal thylakoids, also known as frets or lamellar thylakoids.

Thylakoids

A highly dynamic collection of membranous sacs called thylakoids, where chlorophyll is found and the light reactions of photosynthesis happen.

Thylakoid
3-D Structure of a Thylakoid Img source

Thylakoids are small interconnected sacks, the light reaction of photosynthesis takes place on the membrane of the thylakoid.

Because the thylakoid membranes contains important protein complexes that carry out the light reactions of photosynthesis. 

What are the called the Photo-systems, they are of 2 types namely

  • Photosystem I
  • Photosystem II

Note– The name of the photosystems was given based on their isolation, not based on their order of function.

Photosystem I and photosystem II contain light-harvesting complexes with chlorophyll and carotenoids that absorb light energy and use it to energize electrons. 

Molecules in the thylakoid membrane use the energized electrons to pump hydrogen ions into the thylakoid lumen, decreasing the pH and turning it acidic. 

This H+ concentration gradient is used to synthesize ATP molecules by an enzyme called ATP synthase, as the hydrogen ions flow back out into the stroma their Kinetic energy is used to form Bond energy.

There are two types of thylakoids—

  • Grana thylakoids, that are the part of grana.
    • Grana thylakoids are pancake-shaped circular disks about 300–600 nanometers in diameter. 
  • Stromal thylakoids, which are in contact with the stroma
    • Stromal thylakoids are helicoid sheets that spiral around grana and interlinks the granum.

Number of the thylakoids and the total thylakoid area of a chloroplast is influenced by light exposure. 

Thylakoid concentration can change within minutes of light exposure or removal.

In most vascular plant chloroplasts, the thylakoids are arranged in stacks called grana, though, in certain C4 plant chloroplasts and some algal chloroplasts, the thylakoids are free-floating.

The function of the Chloroplast

The various parts of the chloroplast perform different functions.

In the stroma, fixation of CO2 takes place, because the enzyme required for fixation is present here.

Light reaction of photosynthesis happens in the Grana, more precisely in the Thylakoid Membrane.

  • Photo-oxidation of chlorophyll.
  • Photolysis of water. 
  • Formation of Energy storing molecules (NADPH2 & ATP).
  • Releasing of Oxygen (O2).

Important Point

  • Light-harvesting complex (LHC) is the group of many pigments (chlorophyll b, carotenoids, xanthophyll) surrounding the Chlorophyll a.
  • In stroma lamellae PS II is absent, so only cyclic photo-phosphorylation occurs on stroma lamellae.
  • Photo-phosphorylation is the formation of ATP using light energy with the help of Oxysomes (ATP synthase).

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