脂肪酸合成是指利用乙酰辅酶A以及丙二酰辅酶A经过脂肪酸合酶的催化，反应合成脂肪酸的过程。这对细胞和生物体内的脂肪生成作用与糖解作用是相当重要的一项流程。

直链脂肪酸具有两种型态：饱和直链脂肪酸以及不饱和直链脂肪酸

饱和直链脂肪酸

细胞通过不断重复进行下表六个类似β氧化的反应可以获得具有十六个碳的棕榈酸。 左图描绘了大肠杆菌中的脂肪酸合成酶II(缩写为FASII，一个由多种酵素结合成的复合体)如何产生饱和直链脂肪酸，这种酵素复合体能够在原核生物、真菌、植物、寄生虫以及粒线体内找到。

大多数真菌、酵母菌以及动物体内的饱和直链脂肪酸是利用脂肪酸合成酶I进行合成。脂肪酸合成酶I是一种巨大而多功能的二聚体蛋白质酵素，具有催化脂肪酸合成过程所有步骤的能力。脂肪酸合成酶I不仅能够合成直链脂肪酸也能够合成具有支链的脂肪酸。

Fatty acid synthesis is the creation of fatty acids from acetyl-CoA, via malonyl-CoA, through the action of enzymes called fatty acid synthases. This process takes place in the cytoplasm of the cell. Most of the acetyl-CoA which is converted into fatty acids is derived from carbohydrates via the glycolytic pathway. The glycolytic pathway also provides the glycerol with which three fatty acids can combine (by means of ester bonds) to form triglycerides (also known as "triacylglycerols", "neutral fats" - to distinguish them from fatty "acids" - or simply as "fat"), the final product of the lipogenic process. When only two fatty acids combine with glycerol and the third alcohol group is phosphorylated with a group such as phosphatidylcholine, a phospholipid is formed. Phospholipids form the bulk of the lipid bilayers that make up cell membranes and surround the organelles within the cells (e.g. the cell nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus etc.)