Chloroplast: Definition, Structure & Function (with Diagram)

••• Dana Chen | Sciencing

Chloroplasts are tiny plant powerhouses that capture light energy to produce the starches and sugars that fuel plant growth.

They are found inside plant cells in plant leaves and in green and red algae as well as in cyanobacteria. Chloroplasts allow plants to produce the complex chemicals necessary for life from simple, inorganic substances such as carbon dioxide, water and minerals.

As food-producing autotrophs, plants form the basis of the food chain, supporting all the higher-level consumers such as insects, fish, birds and mammals right up to humans.

The cell chloroplasts are like little factories that produce fuel. In this way, it's the chloroplasts in green plant cells that make life on Earth possible.

What’s Inside a Chloroplast – the Chloroplast Structure

Although chloroplasts are microscopic pods inside tiny plant cells, they have a complex structure that allows them to capture light energy and use it to assemble carbohydrates at the molecular level.

Major structural components are as follows:

  • An outer and inner layers with an intermembrane space between them.
  • Inside the inner membrane are ribosomes and thylakoids.
  • The inner membrane contains an aqueous jelly called the stroma
  • The stroma fluid contains the chloroplast DNA as well as proteins and starches. It is where the formation of carbohydrates from photosynthesis takes place.

The Function of Chloroplast Ribosomes and Thylkaoids

The ribosomes are clusters of proteins and nucleotides that manufacture enzymes and other complex molecules required by the chloroplast.

They are present in large numbers throughout all living cells and produce complex cell substances such as proteins according to the instructions from RNA genetic code molecules.

The thylakoids are embedded in the stroma. In plants they form closed discs that are arranged into stacks called grana, with a single stack called a granum. They are made up of a thylakoid membrane surrounding the lumen, an aqueous acidic material containing proteins and facilitating the chloroplast's chemical reactions.

Lamellae form links between the grana discs, connecting the lumen of the different stacks.

The light-sensitive part of photosynthesis takes place on the thylakoid membrane where chlorophyll absorbs light energy and turns it into chemical energy used by the plant.

Chlorophyll: The Source of Chloroplast Energy

Chlorophyll is a photoreceptor pigment found in all chloroplasts.

When light strikes the leaf of a plant or the surface of algae, it penetrates into the chloroplasts and reflects off the thylakoid membranes. Struck by light, the chlorophyll in the membrane gives off electrons that the chloroplast uses for further chemical reactions.

Chlorophyll in plants and green algae is mainly the green chlorophyll called chlorophyll a, the most common type. It absorbs violet-blue and reddish orange-red light while reflecting green light, giving plants their characteristic green color.

Other types of chlorophyll are types b through e, which absorb and reflect different colors.

Chlorophyll type b, for example, is found in algae and absorbs some green light in addition to red. This green-light absorption may be the result of organisms evolving near the surface of the ocean because green light can penetrate only a short distance into the water.

Red light can travel farther below the surface.

The Chloroplast Membranes and Intermembrane Space

Chloroplasts produce carbohydrates such as glucose and complex proteins that are needed elsewhere in the plant's cells.

These materials have to be able to exit the chloroplast and support general cell and plant metabolism. At the same time, chloroplasts need substances produced elsewhere in the cells.

The chloroplast membranes regulate the movement of molecules into and out of the chloroplast by allowing small molecules to pass while using special transport mechanisms for large molecules. Both the inner and outer membranes are semi-permeable, permitting the diffusion of small molecules and ions.

These substances cross the intermembrane space and penetrate the semi-permeable membranes.

Large molecules such as complex proteins are blocked by the two membranes. Instead, for such complex substances, special transport mechanisms are available to allow specific substances to cross the two membranes while others are blocked.

The outer membrane has a translocation protein complex to transport certain materials across the membrane, and the inner membrane has a corresponding and similar complex for its specific transitions.

These selective transport mechanisms are especially important because the inner membrane synthesizes lipids, fatty acids and carotenoids that are required for the chloroplast's own metabolism.

The Thylakoid System

The thylakoid membrane is the part of the thylakoid that is active in the first stage of photosynthesis.

In plants, the thylakoid membrane generally forms closed, thin sacks or discs that are stacked in grana and stay in place, surrounded by the stroma fluid.

The arrangement of the thylakoids in helical stacks allows a tight packing of the thylakoids and a complex, high surface-area structure of the thylakoid membrane.

For simpler organisms, the thylakoids may be of an irregular shape and can be free-floating. In each case, light striking the thylakoid membrane initiates the light reaction in the organism.

The chemical energy released by chlorophyll is used to split water molecules into hydrogen and oxygen. The oxygen is used by the organism for respiration or is released to the atmosphere while the hydrogen is used in the formation of carbohydrates.

The carbon for this process comes from carbon dioxide in a process called carbon fixation.

The Stroma and the Origin of Chloroplast DNA

The process of photosynthesis is made up of two parts: the light-dependent reactions that start with light interacting with chlorophyll and the dark reactions (aka light-independent reactions) that fix carbon and produce glucose.

Light reactions only take place during the day when light energy strikes the plant while dark reactions can take place at any time. The light reactions start in the thylakoid membrane while the carbon fixing of the dark reactions takes place in the stroma, the jelly-like liquid surrounding the thylakoids.

In addition to hosting the dark reactions and the thylakoids, the stroma contains the chloroplast DNA and the chloroplast ribosomes.

As a result, the chloroplasts have their own energy source and can multiply on their own, without relying on cell division.

Learn about related cell organelles in eukaryotic cells: cell membrane and cell wall.

This capability can be traced back to the evolution of simple cells and bacteria. A cyanobacterium must have entered an early cell and was allowed to stay because the arrangement became a mutually beneficial one.

In time, the cyanobacterium evolved into the chloroplast organelle.

Carbon Fixing in the Dark Reactions

Carbon fixing in the chloroplast stroma takes place after water is split into hydrogen and oxygen during the light reactions.

The protons from the hydrogen atoms are pumped into the lumen inside the thylakoids, rendering it acidic. In the dark reactions of photosynthesis, the protons diffuse back out of the lumen into the stroma via an enzyme called ATP synthase.

This proton diffusion through ATP synthase produces ATP, an energy storage chemical for cells.

The enzyme RuBisCO is found in the stroma and fixes carbon from CO2 to produce six-carbon carbohydrate molecules that are unstable.

When the unstable molecules break down, ATP is used to convert them into simple sugar molecules. The sugar carbohydrates can be combined to form larger molecules such as glucose, fructose, sucrose and starch, all of which can be used in cell metabolism.

When carbohydrates form at the end of the photosynthesis process, the plant's chloroplasts have removed carbon from the atmosphere and used it to create food for the plant and, eventually, for all other living things.

In addition to forming the basis of the food chain, photosynthesis in plants reduces the amount of the carbon dioxide greenhouse gas in the atmosphere. In this way, plants and algae, through photosynthesis in their chloroplasts, help reduce the effects of climate change and global warming.

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