Chloroplast | Definition, Function, Structure, Location, & Diagram

Ask the Chatbot Games & Quizzes History & Society Science & Tech Biographies Animals & Nature Geography & Travel Arts & Culture ProCon Money Videos chloroplast

• Introduction & Top Questions
• Characteristics of chloroplasts
• The photosynthetic machinery
• Chloroplast genome and membrane transport

References & Edit History Related Topics Images & Videos For Students Pop Quiz: 13 Things to Know About Photosynthesis chloroplast summary Quizzes Parts of a Cell Quiz Related Questions

• What is a chloroplast?
• Where are chloroplasts found?
• Do chloroplasts have DNA?
• Why is photosynthesis important?
• What is the basic formula for photosynthesis?

Science Biology Cells, Organs & Tissues

chloroplast biology Quick Summary Ask the Chatbot Print print Print Please select which sections you would like to print:

• Table Of Contents

Cite verifiedCite While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. Select Citation Style MLA APA Chicago Manual of Style Copy Citation Share Share Share to social media Facebook X URL https://www.britannica.com/science/chloroplast Feedback External Websites Feedback Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login). Feedback Type Select a type (Required) Factual Correction Spelling/Grammar Correction Link Correction Additional Information Other Your Feedback Submit Feedback Thank you for your feedback

Our editors will review what you’ve submitted and determine whether to revise the article.

External Websites

• National Center for Biotechnology Information - PubMed Central - Function of Chloroplasts in Plant Stress Responses
• Florida State University - Molecular Expressions - Chloroplasts
• Frontiers - Frontiers in Plant Science - Editorial: Structure and Function of Chloroplasts
• Cell Press - Plant Communications - Chloroplast gene expression: Recent advances and perspectives
• Biology LibreTexts - Chloroplast
• Khan Academy - Mitochondria and chloroplasts
• BMC - Genome Biology - Chloroplast genomes: diversity, evolution, and applications in genetic engineering
• American Society for Biochemistry and Molecular Biology - Chloroplasts do the darndest things

Quick Summary Ask the Chatbot a Question Written and fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Dec. 29, 2025 •History Table of Contents Table of Contents Quick Summary Ask the Chatbot a Question Top Questions

What is a chloroplast?

A chloroplast is an organelle within the cells of plants and certain algae that is the site of photosynthesis, which is the process by which energy from the Sun is converted into chemical energy for growth. A chloroplast is a type of plastid (a saclike organelle with a double membrane) that contains chlorophyll to absorb light energy.

Where are chloroplasts found?

Chloroplasts are present in the cells of all green tissues of plants and algae. Chloroplasts are also found in photosynthetic tissues that do not appear green, such as the brown blades of giant kelp or the red leaves of certain plants. In plants, chloroplasts are concentrated particularly in the parenchyma cells of the leaf mesophyll (the internal cell layers of a leaf).

Why are chloroplasts green?

Chloroplasts are green because they contain the pigment chlorophyll, which is vital for photosynthesis. Chlorophyll occurs in several distinct forms. Chlorophylls a and b are the major pigments found in higher plants and green algae.

Do chloroplasts have DNA?

Unlike most other organelles, chloroplasts and mitochondria have small circular chromosomes known as extranuclear DNA. Chloroplast DNA contains genes that are involved with aspects of photosynthesis and other chloroplast activities. It is thought that both chloroplasts and mitochondria are descended from free-living cyanobacteria, which could explain why they possess DNA that is distinct from the rest of the cell.

chloroplast, structure within the cells of plants and green algae that is the site of photosynthesis, the process by which light energy is converted to chemical energy, resulting in the production of oxygen and energy-rich organic compounds. Photosynthetic cyanobacteria are free-living close relatives of chloroplasts; endosymbiotic theory posits that chloroplasts and mitochondria (energy-producing organelles in eukaryotic cells) are descended from such organisms.

Characteristics of chloroplasts

Chloroplasts are a type of plastid—a round, oval, or disk-shaped body that is involved in the synthesis and storage of foodstuffs. Chloroplasts are distinguished from other types of plastids by their green colour, which results from the presence of two pigments, chlorophyll a and chlorophyll b. A function of those pigments is to absorb light energy for the process of photosynthesis. Other pigments, such as carotenoids, are also present in chloroplasts and serve as accessory pigments, trapping solar energy and passing it to chlorophyll. In plants, chloroplasts occur in all green tissues, though they are concentrated particularly in the parenchyma cells of the leaf mesophyll.

Chloroplasts are roughly 1–2 μm (1 μm = 0.001 mm) thick and 5–7 μm in diameter. They are enclosed in a chloroplast envelope, which consists of a double membrane with outer and inner layers, between which is a gap called the intermembrane space. A third, internal membrane, extensively folded and characterized by the presence of closed disks (or thylakoids), is known as the thylakoid membrane. In most higher plants, the thylakoids are arranged in tight stacks called grana (singular granum). Grana are connected by stromal lamellae, extensions that run from one granum, through the stroma, into a neighbouring granum. The thylakoid membrane envelops a central aqueous region known as the thylakoid lumen. The space between the inner membrane and the thylakoid membrane is filled with stroma, a matrix containing dissolved enzymes, starch granules, and copies of the chloroplast genome.

Britannica Quiz Pop Quiz: 13 Things to Know About Photosynthesis

The photosynthetic machinery

The thylakoid membrane houses chlorophylls and different protein complexes, including photosystem I, photosystem II, and ATP (adenosine triphosphate) synthase, which are specialized for light-dependent photosynthesis. When sunlight strikes the thylakoids, the light energy excites chlorophyll pigments, causing them to give up electrons. The electrons then enter the electron transport chain, a series of reactions that ultimately drives the phosphorylation of adenosine diphosphate (ADP) to the energy-rich storage compound ATP. Electron transport also results in the production of the reducing agent nicotinamide adenine dinucleotide phosphate (NADPH).

ATP and NADPH are used in the light-independent reactions (dark reactions) of photosynthesis, in which carbon dioxide and water are assimilated into organic compounds. The light-independent reactions of photosynthesis are carried out in the chloroplast stroma, which contains the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco). Rubisco catalyzes the first step of carbon fixation in the Calvin cycle (also called Calvin-Benson cycle), the primary pathway of carbon transport in plants. Among so-called C4 plants, the initial carbon fixation step and the Calvin cycle are separated spatially—carbon fixation occurs via phosphoenolpyruvate (PEP) carboxylation in chloroplasts located in the mesophyll, while malate, the four-carbon product of that process, is transported to chloroplasts in bundle-sheath cells, where the Calvin cycle is carried out. C4 photosynthesis attempts to minimize the loss of carbon dioxide to photorespiration. In plants that use crassulacean acid metabolism (CAM), PEP carboxylation and the Calvin cycle are separated temporally in chloroplasts, the former taking place at night and the latter during the day. The CAM pathway allows plants to carry out photosynthesis with minimal water loss.

Related Topics: Why Are Plants Green? lamella chloroplastin granum stroma (Show more) On the Web: American Society for Biochemistry and Molecular Biology - Chloroplasts do the darndest things (Dec. 29, 2025) (Show more) See all related content

Chloroplast genome and membrane transport

The chloroplast genome typically is circular (though linear forms have also been observed) and is roughly 120–200 kilobases in length. The modern chloroplast genome, however, is much reduced in size: over the course of evolution, increasing numbers of chloroplast genes have been transferred to the genome in the cell nucleus. As a result, proteins encoded by nuclear DNA have become essential to chloroplast function. Hence, the outer membrane of the chloroplast, which is freely permeable to small molecules, also contains transmembrane channels for the import of larger molecules, including nuclear-encoded proteins. The inner membrane is more restrictive, with transport limited to certain proteins (e.g., nuclear-encoded proteins) that are targeted for passage through transmembrane channels.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Encyclopaedia Britannica.