Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the fluid nature of the membrane. It is also the precursor of steroid hormones such as testosterone.

Examples of steroid hormones include estradiol, which is an estrogen, or female sex hormone, and testosterone, which is an androgen, or male sex hormone.

Within the body, lipids function as an energy reserve, regulate hormones, transmit nerve impulses, cushion vital organs, and transport fat-soluble nutrients.

The lipids of physiological importance for humans serve as structural components of biological membranes; provide energy reserves, predominantly in the form of triglycerides, serve as biologically active molecules exerting a wide range of regulatory functions, and the lipophilic bile acids aid in lipid emulsification …

Fat Functions Triglycerides, cholesterol and other essential fatty acids–the scientific term for fats the body can’t make on its own–store energy, insulate us and protect our vital organs. They act as messengers, helping proteins do their jobs.

They maintain a selective permeability for the import and export of water-soluble compounds, enabling the living cell to maintain a stable chemical environment for biological processes.

There are three major classes of membrane lipid molecules—phospholipids, cholesterol, and glycolipids.

A nuclear membrane is a double membrane that encloses the cell nucleus. It serves to separate the chromosomes from the rest of the cell. The nuclear membrane includes an array of small holes or pores that permit the passage of certain materials, such as nucleic acids and proteins, between the nucleus and cytoplasm.

The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins.

The nucleus (plural, nuclei) houses the cell’s genetic material, or DNA, and is also the site of synthesis for ribosomes, the cellular machines that assemble proteins. Inside the nucleus, chromatin (DNA wrapped around proteins, described further below) is stored in a gel-like substance called nucleoplasm.

The membranes create two compartments. The intermembrane space, as implied, is the region between the inner and outer membranes. It has an important role in the primary function of mitochondria, which is oxidative phosphorylation. The matrix contains the enzymes that are responsible for the citric acid cycle reactions.

The mitochondrion has a primary role in energy metabolism, a role that is intimately connected with its double-membrane structure (outer and inner, each comprising a lipid bilayer). Formation of mitochondria (mitochondrial biogenesis) is under the dual control of the nuclear and mitochondrial genetic systems.

As previously mentioned, mitochondria contain two major membranes. In contrast, the inner membrane has much more restricted permeability, much like the plasma membrane of a cell. The inner membrane is also loaded with proteins involved in electron transport and ATP synthesis.