Lipids

$General knowledge$

%%Nutrition and biological structures%%

^^Molecular structure^^

What are lipids?

• Lipids, also known as fats, are a group of $organic molecules$ primarily consisting of ^^carbon, hydrogen, and oxygen^^.
• All lipids are $hydrophobic$.
• %%Most of the fat we consume is triglycerides%%, which are used as an energy source.
• When fat is not burned, it gets deposited into %%fat deposits%%, where the energy is stored and used as isolation. The energy stored can after that be burned whenever the %%body lacks an intake%% of food.

Fatty acids

• Fatty acids are carbon chains with a ^^-COOH (carboxylic acid group)^^.
• ^^The hydrogen atom can split off from the rest of the molecule^^, so the fatty acid can bind with other molecules.
• Fatty acids vary in length, with the shortest being only 2 carbon atoms wide and the longest having above 22 carbon atoms. %%Most natural fatty acids have between 16 and 18 carbon atoms%%.
• The %%more carbon atoms%% a fatty acid has, the %%less likely%% it is %%to dissolve in water%%.

Fig. 1 shows a picture of a ^^straight-chain^^ fatty acid. A straight chain has ^^no side chains^^. The carboxylic acid has been marked red. Heptadecanoic acid is also known as $margaric acid$.

Saturated and unsaturated fats

Saturated and unsaturated fats are two different types of fatty acids. They $differ$ from each other in their $structures and nutrition$. The main way to differentiate the two is that ^^saturated fats have no double bonds^^, whereas ^^unsaturated fats have at least one^^.

Saturated fats

• Saturated fats have ^^no double bonds^^ between their carbon atoms.
• They are $solid and stable$ at room temperature.
• They are %%primarily found in animal products%% like butter and meat but also a few vegetable oils like coconut and palm oil.
• Saturated fats can %%increase LDL%%.

Unsaturated fats

• Unsaturated fats have ^^at least one double bond^^ between their carbon atoms. Depending on the number of double bonds in their molecular structure, they are called something different.
• ^^Monounsaturated^^: one double bond between carbon atoms. (Fig 2.)
• ^^Polyunsaturated^^: two or more double bonds between carbon atoms. (Fig 3.)
• They are $liquid at room temperature$.
• They are %%primarily found in plant-based food%% and vegetable oil like nuts, kernels and olive oil.
• Unsaturated fats can help %%lower LDL and increase HDL%%.

LDL and HDL

• LDL $(Low-Density Lipoprotein)$ and HDL $(High-Density Lipoprotein)$ are two types of lipoproteins that $transport cholesterol$ in the blood.
• LDL is often known as the worst of the two since it $transports cholesterol from the liver to the cells$.
• A too-high LDL level can lead to a %%buildup in the arteries%% that can cause %%cardiovascular disease and stroke%%.
• HDL is often known as the best of the two, as it $transports excess cholesterol from the bloodstream back to the liver$.
• A high HDL level can %%decrease one’s chance of cardiovascular disease%%.

Cis- and trans configuration

• Unsaturated fats either have a $cis- or trans-config.$ (Configuration) around their double bonds.
• In cis config. fats the ^^hydrogen atoms that sit on the double bond point the same way^^. Cis config. fats, therefore, have a crack in their molecular structure.
• The crack makes the unsaturated fats have a %%lower boiling point%% than unsaturated fats with a trans config.
• In trans config. fats ^^the hydrogen atoms on the double bond point in different ways^^. Trans config. fats, unlike cis config. fats $look a lot like saturated fats$ since their structure makes it look like they have straight chains.
• Trans config. fats can %%increase the chance of cardiovascular disease%%.
• When cis config. fats are heated up, their $double bonds can break$, and they turn into trans config. fats. $When cooled down this double bond is restored$.

Lipids are hydrophobic

• Lipids are $hydrophobic$, meaning they are water-repellent.
• They are hydrophobic because of their structure, where the atoms form ^^covalent bonds^^, creating an electronegative difference in the molecule. ^^The electronegative difference makes the molecule electrically non-polar^^.
• This means the lipids ^^neither have any positive nor negative partial charges^^ that can interact with water molecules (^^water molecules are polar^^).
• The ^^non-polar part of^^ the fat molecule is ^^the carbon chain^^.
• Because ^^carbon and hydrogen^^ have ^^similar electronegativity^^ their ^^covalent bonds are non-polar.^^ Therefore the fatty acids carbon chain is hydrophobic and cannot form hydrogen bonds with water molecules.

$*Phospholipids are both polar and non-polar.*$

Phospholipids contain both polar and non-polar groups and can therefore interact with water and lipids.

Structure and function

Triglycerides

• Triglycerides are made of ^^3 fatty acids^^, that are bound together in a ^^glycerol molecule.^^
• Triglycerides are the $most common type of fat$ in the body and are an %%important energy source%%.
• When broken down ^^triglycerides turn into fatty acids and glycerol^^ that can be used as %%fuel for muscle cells%% and other types of tissue.
• Triglycerides have %%high amounts of energy%% %%(37kJ/g)%%, whereas carbohydrates have much lower amounts of energy (17kJ/g).
• Because of their high energy amount and their hydrophobia, $triglycerides are easily packed into adipose tissue and muscle as energy reserves$. Therefore %%when the body needs energy, triglycerides can release from the tissue and break down into glycerol and fatty acids%%, which can be transported through the blood.
• High amounts of triglycerides in one’s diet can %%increase the risk of cardiovascular disease%% and other lifestyle diseases.
• $Triglycerides are often found in saturated- and trans-config$. fats. Examples include %%meat%%(beef, lamb and pig), %%fatty dairy products%% (cheese, butter and cream) %%and deep-fried foods%%.

Phospholipids

• Phospholipids constitute a vital role in the $cell membrane$ of all living organisms.
• A phospholipid consists of a ^^glycerol molecule bound to 2 fatty acid molecules and a phosphate group^^. There is also an ^^organic group bound to the phosphate group^^.
• The phosphate group makes the phospholipid hydrophilic\*(“water-loving”)\*, and the $fatty acid makes the phospholipid hydrophobic$.
• Because the phospholipids build, it makes them ideal as cell membranes. %%In cell membranes, phospholipids form a double layer, where the hydrophilic “heads” points outward and the hydrophobic “tails” points inward.%% This makes the cell membrane selectively permeable, meaning the cell membrane can regulate what substances enter and leave.
• Phospholipids are often found in foods such as %%soybeans, peanuts, eggs and liver%%.

Sterols

• Sterols are a type of lipid that contain ^^4 carbon rings and an -OH group^^ ^^(hydroxyl group)^^. (see fig. 7)
• The $most known sterol is cholesterol$ which plays an important role in eukaryotic cells.
• $Other types of sterols are found in plants and mushrooms where they function as percusses for plant hormones and antifungals$.
• Sterols are often found in foods such as %%meat, cheese, egg, nuts, oils and kernels%%.

Cholesterol

• Cholesterol is a sterol. Cholesterol contains a ^^long carbon chain^^, which makes them $less water-soluble and more fat-soluble$.
• Cholesterol is the $starting point for the production of steroid hormones$. (including testosterone, estrogen and adrenocortical hormone).
• Cholesterol help %%regulate the cell membranes’ fluidity and permeability%%.
• $Cholesterol is formed in the liver$.
• It is %%primarily found in animal products like meat and dairy products%%.