# The Mammalian Liver: Function ## Overview of Liver Functions - The liver performs three critical functions: - **storage of glycogen** - **formation of urea** - **detoxification** ## Storage of Glycogen - The liver is central to converting **glucose into glycogen** through a process called **glycogenesis** - Think of this like converting loose coins into a neat stack that's easy to store and access later

Glycogenesis is the metabolic pathway that converts glucose molecules into glycogen for storage

- **Insulin** triggers glycogenesis after the pancreas senses elevated blood glucose levels - Imagine insulin as a warehouse manager signaling that incoming glucose needs to be packaged and stored - Glycogen synthesis pulls glucose out of circulation, bringing blood glucose concentration back to normal levels - The glycogen produced is **stored inside hepatocytes** (liver cells) - Hepatocytes act like cellular storage units, keeping glycogen ready for future use - Glycogen is a **compact storage molecule** designed for easy glucose release when the body needs energy - Picture glycogen as a tightly wound ball of yarn—compact when stored, but easily unraveled when needed ## Formation of Urea - Dietary protein breaks down into **amino acids** during digestion - These amino acids absorb into the bloodstream and travel to the liver via the **hepatic portal vein** - Excess amino acids undergo processing inside hepatocytes through a **two-step process**: 1. **Deamination** 2. **The ornithine cycle** ### Deamination

Deamination is the removal of the amino group ($\text{NH}_2$) from an amino acid, producing ammonia and a keto acid

- During deamination: - The **amino group** ($\text{NH}_2$) is stripped from each amino acid - An extra **hydrogen atom** ($\text{H}^+$) also comes off - $\text{NH}_2$ and $\text{H}^+$ combine to form **ammonia** ($\text{NH}_3$) - The remaining portion after deamination is a **keto acid**, which can take three pathways: - Enter the **Krebs cycle** to be respired for energy - Be converted into **glucose** for immediate use - Be converted into **glycogen or fat** for storage - Like recycling old building materials into new structures or storing them for later ### The Ornithine Cycle - **Ammonia** is extremely soluble and **highly toxic**—it can cause serious damage if it accumulates in the bloodstream - Think of ammonia like toxic fumes that must be neutralized quickly

Ammonia buildup in blood is dangerous and must be converted to less toxic urea immediately

- To prevent ammonia toxicity, the liver converts it into **urea**, which is **less toxic** than ammonia - Urea is like a safely packaged version of a hazardous substance - The conversion process is called the **ornithine cycle** - During the ornithine cycle: - Ammonia combines with **carbon dioxide** to form **urea** - One molecule of urea forms from: - One molecule of carbon dioxide - Two amino groups (from two ammonia molecules) $$2\text{NH}_3 + \text{CO}_2 \longrightarrow \text{CO}(\text{NH}_2)_2 + \text{H}_2\text{O}$$

You need to know that ammonia reacts with carbon dioxide during the ornithine cycle to form urea, but detailed cycle steps aren't required

- Urea **diffuses through the phospholipid bilayer** of hepatocytes - It then dissolves in blood plasma and travels to the **kidneys** for excretion - Like waste being flushed through the body's plumbing system to the treatment facility ## Detoxification

Detoxification is the breakdown of substances that are unnecessary or toxic to the body

- Substances the liver detoxifies include: - **Alcohol** (ethanol) - **Hydrogen peroxide** - **Lactate** - **Medicinal drugs** ### Detoxification of Alcohol - Once consumed, **alcohol (ethanol)** absorbs in the stomach and enters the bloodstream - Blood carries ethanol until it reaches the **hepatocytes** in the liver - Inside hepatocytes, the enzyme **alcohol dehydrogenase** converts ethanol into **ethanal** - Think of this enzyme as a molecular cleanup crew breaking down the alcohol - Ethanal then converts into other molecules that feed into **respiration** pathways

Continuous alcohol detoxification can cause severe liver damage including fatty liver and cirrhosis

#### Problems from Continuous Alcohol Detoxification - Ethanol metabolism generates **ATP** as a byproduct - This means hepatocytes don't need to metabolize as much fat for energy - Instead, they **store the fat**, leading to a condition called **fatty liver** - Like a warehouse that stops using its inventory and just keeps piling up more goods - Stored fat **reduces hepatocyte function** - The cells become clogged and can't perform their other vital duties - It's similar to a factory floor becoming cluttered with boxes, making work impossible - Eventually, this can lead to **cirrhosis** - Cirrhosis involves scarring of liver tissue caused by excessive alcohol consumption - The liver becomes permanently damaged, like a machine that's been overworked until it breaks down - This is a severe and potentially fatal condition