How do plants make fat and oil?

December 26, 2025 •

3 min read

Over 200 million tons of vegetable oil are produced annually, and all of it originates from the biological pathways that explain how do plants make fat. Unlike animals that store fat in specialized adipose tissue, plants produce lipids primarily for energy reserves in seeds, membrane components, and protective waxes. This intricate process relies on the conversion of photosynthetic energy into high-energy fatty acid chains, which are then assembled into various lipid forms.

Quick Summary

The synthesis of fats and oils in plants is a multi-step process beginning with photosynthesis. Carbon is converted into acetyl-CoA within plastids, serving as the fundamental building block. From here, a series of enzymatic reactions create fatty acid chains, which are then modified and exported to the endoplasmic reticulum for final assembly into storage lipids like triacylglycerols, found predominantly in seeds.

Key Points

Location: De novo fatty acid synthesis occurs in the plastids, such as chloroplasts, not the cytoplasm as in animals.
Precursors: The carbon for fatty acid chains comes from acetyl-CoA, a product of carbohydrate metabolism derived from photosynthesis.
Enzymes: The Fatty Acid Synthase (FAS) complex, along with acetyl-CoA carboxylase (ACCase), are key enzymes for elongating fatty acid chains.
Storage: Storage lipids, mainly triacylglycerols (TAGs), are assembled in the endoplasmic reticulum and stored in oil bodies, especially in seeds.
Regulation: Transcription factors like WRINKLED1 (WRI1) play a master regulatory role in controlling the metabolic flow toward oil synthesis.
Purpose: Plant fats serve as a compact energy source for germinating seeds, provide essential membrane components, and form protective waxes.

From Sunlight to Lipids: The Cellular Journey

The fundamental building blocks for plant lipids originate from the energy captured during photosynthesis. This process converts sunlight, water, and carbon dioxide into carbohydrates, which are then broken down to produce acetyl-CoA. Unlike animal cells, where fatty acid synthesis occurs in the cytoplasm, plants conduct the primary synthesis within their plastids—organelles like chloroplasts.

The Fatty Acid Synthesis Pathway

The synthesis of fatty acids, the core component of fat, is carried out by a multi-enzyme complex called Fatty Acid Synthase (FAS) located in the plastid stroma. The process starts with the carboxylation of acetyl-CoA to form malonyl-CoA, a reaction catalyzed by acetyl-CoA carboxylase (ACCase). Malonyl-CoA, attached to an acyl carrier protein (ACP), then serves as the donor for successive two-carbon units that build the fatty acid chain. This cycle continues until a saturated fatty acid, typically palmitic acid (C16:0) or stearic acid (C18:0), is produced.

Initiation: Acetyl-CoA is converted to malonyl-CoA by ACCase.
Elongation: The FAS complex extends the fatty acid chain by adding two-carbon units from malonyl-ACP.
Desaturation: Specific enzymes, known as desaturases, introduce double bonds into the fatty acid chains. The soluble stearoyl-ACP desaturase (SAD) converts stearate (C18:0) to oleate (C18:1), a monounsaturated fatty acid.
Termination: Fatty acyl-ACP thioesterases (FAT) release the completed fatty acid chains from the ACP carrier.

The Role of Cellular Compartments

The final destination for these newly synthesized fatty acids depends on their function. Many are exported from the plastids to the endoplasmic reticulum (ER), which plays a crucial role in assembling complex lipids. This movement involves distinct metabolic pathways:

Prokaryotic Pathway: Fatty acids are used within the plastids to create galactolipids, which are the major membrane lipids of chloroplasts.
Eukaryotic Pathway: Exported fatty acids are used in the ER to synthesize phospholipids for cellular membranes and, critically, for creating triacylglycerols (TAGs).

Synthesis of Storage Lipids

Triacylglycerols (TAGs), the primary form of energy storage in plants, are synthesized in the endoplasmic reticulum. This involves sequentially adding three fatty acid molecules to a glycerol backbone, a process catalyzed by enzymes like Diacylglycerol Acyltransferase (DGAT). In oilseed crops, this process is particularly active, leading to the accumulation of oil in specialized organelles called oil bodies. Transcription factors such as WRINKLED1 (WRI1) are key regulators, controlling the flow of carbon from sugars to oil accumulation.

Comparison of Storage and Structural Lipids


Feature	Storage Lipids (e.g., TAGs in seeds)	Structural Lipids (e.g., galactolipids in leaves)
Function	Long-term energy reserve for seed germination and plant growth.	Form the basis of cellular membranes, especially in chloroplasts.
Location	Synthesized and stored in oil bodies within the ER, primarily in seeds and fruits.	Synthesized in the plastids (chloroplasts) and integral to thylakoid membranes.
Composition	Glycerol backbone with three fatty acids. Often contains unusual fatty acids.	Glycerol backbone with fatty acids and a sugar group, like galactose.
Fatty Acid Profile	Varies widely by plant species. Can be saturated, monounsaturated, or polyunsaturated.	Typically characterized by high levels of polyunsaturated fatty acids.
Regulation	Heavily influenced by transcription factors like WRI1 during seed development.	Coordinated with photosynthetic activity and membrane turnover.

Industrial Relevance and Bioengineering

The intricate lipid metabolism of plants is of significant interest for agriculture and industry. Genetic engineering efforts focus on manipulating these pathways to increase oil yield or modify fatty acid composition for specific applications. For example, by controlling the activity of enzymes like FAD2 (fatty acid desaturase), scientists have successfully altered the oleic acid content in crops like rapeseed. These modifications are crucial for producing biofuels, specialty lubricants, and healthier cooking oils.

Conclusion

In conclusion, the question of how do plants make fat involves a sophisticated metabolic choreography spanning multiple cellular compartments, primarily the plastids and the endoplasmic reticulum. Starting with the captured energy from sunlight, plants synthesize fatty acid chains through the FAS complex, modify them with desaturases, and ultimately assemble them into both essential structural components and high-energy storage reserves. This remarkable biological process ensures the plant's long-term survival, provides the energy required for germination, and gives rise to the diverse oils that play a critical role in human nutrition and industrial applications. IntechOpen offers additional detailed information on plant lipid metabolism.

Frequently Asked Questions

The main source of carbon for plants to make fat is the carbohydrates produced during photosynthesis. These sugars are then converted into acetyl-CoA, which is the precursor molecule for fatty acid synthesis.

The amount of oil a plant stores is determined by its species and genetics. Oilseed crops, for example, have evolved to accumulate large amounts of triacylglycerols in their seeds to provide energy for germination, with this process being heavily regulated by specific genes and transcription factors.

Fatty acids are primarily synthesized within the plastids, such as the chloroplasts of leaves and other green tissues. This is a key difference from animals, where fatty acid synthesis occurs in the cytoplasm.

The ER is where fatty acids exported from the plastids are assembled into more complex lipids. These include phospholipids for cellular membranes and triacylglycerols (TAGs), which are stored as oil.

Unsaturated fatty acids are created by desaturase enzymes, which introduce double bonds into the fatty acid chains. Some of these enzymes, like stearoyl-ACP desaturase (SAD), operate in the plastids, while others are found in the ER.

Oil bodies are specialized organelles derived from the ER that store triacylglycerols (TAGs). They consist of a lipid core surrounded by a membrane of phospholipids and proteins, and serve as a crucial energy source during seed germination.

Yes, plants can control the type of fatty acids they produce through specific enzymes called acyl-ACP thioesterases (FAT). These enzymes determine the length and saturation of the fatty acid chains released from the synthesis complex, leading to a diverse array of plant oils.