9.4 Assimilation
Assimilation is the physiological process that takes place within cells where absorbed nutrients are used to build complex compounds or cellular components. Following digestion and absorption, the circulatory system and the lymphatic system work together to deliver glucose, amino acids, and lipids to the liver and body cells. The liver functions as a central metabolic regulator, directing nutrients for immediate use, storage, or conversion.
Learning Objectives
- State the definition and meaning of assimilation.
- Describe the role of the circulatory system (blood and lymphatic systems) in transporting nutrients for assimilation.
- Explain the functions of the liver in the assimilation of digested food, including metabolism, detoxification, and storage.
- Describe the assimilation pathways of amino acids, glucose, and lipids in the liver and body cells.
Summary of Nutrient Assimilation in the Liver and Body Cells
Glucose, amino acids, and lipids travel from the small intestine through the hepatic portal vein and lymphatic system to reach the liver and body cells for synthesis, respiration, or storage.
Water-soluble nutrients
Glucose and amino acids are absorbed into blood capillaries in the villi. These capillaries combine to form the hepatic portal vein, which transports them directly to the liver.
Lipid-soluble nutrients
Lipids enter the lacteals in the villi. Lymph flows through larger lymph vessels into the thoracic duct and empties into the left subclavian vein, allowing lipids to enter the bloodstream and be distributed throughout the body.
Short Explanation
Definition
Assimilation is the process occurring in cells where absorbed nutrients are used to form complex compounds or structural components of the body.
Role of the circulatory system
The human circulatory system comprises the blood circulation system and the lymphatic system to transport nutrients for assimilation. Blood capillaries in the small intestine combine to form the hepatic portal vein, while lacteals unite to form larger lymph vessels.
Liver as a regulator
The liver acts as a regulator controlling the quantity of nutrients entering the systemic blood circulation. It carries out metabolism of digested food, detoxification, and storage of nutrients.
Metabolism of digested food
Glucose is used for cellular respiration. Amino acids are used to synthesise plasma proteins and enzymes. Excess amino acids cannot be stored; they are converted into urea via deamination to be excreted through urine. If glucose supply is insufficient, the liver converts amino acids into glucose.
Detoxification and storage
Liver cells expel toxic substances from the blood, and these toxic substances are eliminated from the body via urine. Excess glucose is converted to glycogen and stored in the liver. When glycogen storage reaches its maximum capacity, excess glucose is converted into fats. When blood glucose levels decrease and energy is required, liver glycogen is converted back into glucose.
Health context
Liver cirrhosis is a liver disease caused by factors such as alcoholic drinks, toxic substances, and hepatitis. Healthy liver cells are replaced by scarred tissue, leading to liver failure. Hepatitis is an inflammation of the liver caused by viral infections, toxic substances, or autoimmune reactions.
Amino acids in body cells
Amino acids are used to synthesise new protoplasm, repair damaged tissues, and synthesise functional molecules such as enzymes and hormones.
Glucose in body cells
Glucose is oxidised during cellular respiration to release energy, carbon dioxide, and water. Energy released is used for vital cellular activities, including protein synthesis. Excess glucose in circulation is kept as glycogen in muscle tissues.
Lipids in body cells
Phospholipids and cholesterol serve as primary structural components for synthesising plasma membranes. Excess fats are stored in adipose tissue underneath the skin as long-term energy reserves. Fats are oxidised to release energy when the supply of glucose is insufficient.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the meaning of assimilation?
What happens to excess amino acids in the liver?
What are the primary cellular uses of lipids in the human body?
Common Mistakes
- Confusing absorption with assimilation: absorption is the passage of simple digested nutrients into blood capillaries and lacteals, whereas assimilation is the actual uptake and utilisation of those nutrients by cells to produce energy or build structures.
- Assuming excess amino acids are stored as reserve proteins. The human body cannot store amino acids; excess amino acids must undergo deamination in the liver to produce urea.
- Believing lipids enter the hepatic portal vein. Lipids are absorbed into lacteals and transported through lymph vessels and the thoracic duct into the left subclavian vein, not via the hepatic portal vein.
Concept Misunderstandings
Urea is formed by the kidneys during urine production.
Urea is produced exclusively in the liver through the deamination of excess amino acids. The kidneys only filter and excrete urea from the blood into urine.
Lipids only serve as an energy storage substance.
Besides energy storage in adipose tissue, lipids (specifically phospholipids and cholesterol) are essential structural building blocks required to assemble cellular plasma membranes.
Summary
- Assimilation uses absorbed nutrients within cells to synthesise complex cellular components and release energy.
- Water-soluble nutrients (glucose, amino acids) are carried by the hepatic portal vein to the liver; lipid-soluble substances travel through lacteals and the thoracic duct into the left subclavian vein.
- The liver controls nutrient metabolism (glycogen storage, plasma protein synthesis, deamination of excess amino acids to urea) and detoxifies blood.
- In cells, glucose is oxidised for energy and stored as muscle glycogen; amino acids synthesise protoplasm, hormones, and enzymes; lipids construct plasma membranes and store reserve energy in adipose tissue.
Short Activity
Objective: check your understanding of nutrient transport, liver functions, and assimilation in body cells.
A. Objective Quiz
Which blood vessel transports blood containing absorbed glucose and amino acids from the small intestine directly to the liver?
What occurs in the liver when the glycogen storage capacity reaches its maximum level?
B. Fill in the Blanks
The contents of lymph vessels enter the thoracic duct, which flows into the subclavian vein.
Excess amino acids cannot be stored and are broken down in the liver through the process of to form urea.
C. Matching / Drag and Drop
Match each nutrient or lipid component to its primary assimilation role in body cells. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Nutrient / Component to Its Assimilation Role
Write the correct nutrient or component next to each role.
| Assimilation Role in Cells | Nutrient / Component |
|---|---|
| 1. Oxidised through cellular respiration to release energy, water, and carbon dioxide | |
| 2. Used to synthesise new protoplasm, repair damaged tissues, and form hormones and enzymes | |
| 3. Primary components that build the plasma membrane | |
| 4. Stored in adipose tissues underneath the skin as stored energy |
Answer choices: Glucose • Amino acids • Phospholipids and cholesterol • Excess fats
