Absorption of Digested Food - Importance, methods, Work

Introduction to Absorption of Digested Food

Alimentary canal is an long duct which converts larges food substances into small molecules that can be easily absorbed by the cells. The digestive tract consists of begins from the mouth, and ends at the anus. This process involves digestion and absorption, where food is broken down into simpler forms and then the nutrients are taken up by the body for use.

Overview of the Digestive System and Absorption

Together, the components of the digestive system allow the extraction and absorption of nutrients from foods, completing the energy and nutrient demands of the body.

• Mouth: Involves in mechanical digestion by churning and since it contains enzymes, it also involves chemical digestion.

• Esophagus: Moves food through the mouth to the stomach with the help of peristalsis.

• Stomach: Swallowed food is digested further by gastric juices in the stomach producing a semi-liquid substance called chyme.

• Small Intestine: The main place where the micronutrients pass and are actively absorbed with the assistance of enzymes and bile. As for the villi, and microvilli, the structure enhances the surface area for better absorption of the nutrients in the bloodstream.

• Large Intestine: Excretes water and electrolytes and synthesizes and eliminates solid waste.

Site of Absorption of Digested Food

The main site of absorption of the digestive food begins in the small intestine. It is the site where absorption of food begins when the digested food molecules that pass through the wall of the small intestine reach into the bloodstream and when the food particles reach the bloodstream they are ready to be transported to the different parts of the body wherever they are needed.

Diagram of the Human Digestive System

The Small Intestine - Primary Site of Absorption

The small intestine is the primary site of absorption

Anatomy

The duodenum is the first part of the small intestine with a length of approximately 25 cm, the second part is the jejunum, which is fairly long, around 252 cm, and the third part is the ileum which has a length of 152 cm.

• Duodenum: As stated, the small intestine comprises the first section in which most of the chemical digestion takes place. The organ also gets chyme from the stomach, bile from the liver, and digestive enzymes from the pancreas.

• Jejunum: The middle part which is the bulb mainly acts as a nutrient sponge.

• Ileum: The last part that acts as the primary absorptive site of vitamin B12 as well as bile salts, and every nutrient that is not absorbed in the jejunum.

Structure of the Small Intestine Wall

On the side of the small intestine, the lining is made of some protrusions known as the villi and microvilli.

• Villi: Pin-like structures which give the walls a larger area on which the nutrients may be absorbed. Every villus has a network of capillaries as well as lacteal which are vessels that transport absorbed nutrients in the bloodstream and lymphatics respectively.

• Microvilli: Much even tinier structures on the epithelial cells of the villi also have a brush border that enhances the surface area and contains enzymes that help in the last steps of digestion and absorption of nutrients.

Mechanism of Absorption

The methods of nutrient uptake within the small intestine include several different means by which substances are allowed to pass through the wall of the small intestine into either the bloodstream or the lymphatic system.

Simple Diffusion

It lets small, non-polar molecules like gases and lipids pass across the cell membrane from a region where there were many to a region of few without using energy.

Passive Transport

As with simple diffusion, this one is also a passive process and transports water and other small molecules across the membrane down their concentration gradient.

It includes two types:

Diffusion: Water and specific ions that do not require any carrier proteins to move across the cell membrane.

Facilitated diffusion: This process involves carrier proteins or channels in the cell membrane through which large or polar molecules such as glucose and amino acid diffuse across the membrane against their concentration gradient without consuming energy.

Active Transport

While in passive transport, molecules move through the selectively permeable membrane naturally, in active transport, molecules need a form of energy, usually ATP, to move from where they are in high concentration to where they are low. This process is useful in transporting nutrients such as glucose and amino acids when their concentration is lesser on the side of the intestinal cells as compared to the concentration in the cells lining the intestine.

Sodium-Potassium Pump

A specific kind of transport in which the molecule ATP is utilised in moving sodium ions out of the cell as well as potassium ions into the cell. Earlier, such a pump was used to maintain the cells’ electrochemical potential essential for the uptake of nutrients and overall cellular processes.

Endocytosis

When large particles or whole amounts of fluids are taken right inside the cell, it is referred to as this type of transport. This mechanism is very important in engulfing large molecules and particles that cannot diffuse through the cell membrane.

Diagram Comparing Active and Passive Transport

Absorption of Digested Food NEET MCQs

Q1.Please select the appropriate combination of the digested products in humans with their corresponding absorption site and mechanism.

1. Xylose and Potassium ions in the Large intestine facilitated diffusion

2. Galactose and Calcium ions in the Small intestine and active transport

3. Sucrose and Magnesium ions Duodenum and endocytosis

4. Lactose and Iron in the Small intestine and passive diffusion

Correct answer: 2) Galactose and Calcium ions in the Small intestine and active transport

Explanation:

Galactose and calcium ions are absorbed in the small intestine through active transport, where carrier proteins require energy to move molecules against their concentration gradient. Xylose and potassium ions are also absorbed in the small intestine, with xylose utilizing facilitated diffusion and potassium ions being absorbed through various mechanisms such as passive diffusion and active transport. Sucrose and magnesium ions are primarily absorbed in the small intestine through secondary active transport and passive diffusion, not in the duodenum. Lactose is absorbed in the small intestine through active transport, while iron is primarily absorbed in the duodenum and proximal jejunum via active transport mechanisms.

Hence, the correct answer is option 2) Galactose and Calcium ions in the Small intestine and active transport

Q2. Assertion: The end products of fat digestion cannot be absorbed into the bloodstream.

Reason: Fat is not classified as a polymer or macromolecule.

1. Both the Assertion and Reason are true, and the Reason provides a valid explanation for the Assertion.

2. Both the Assertion and Reason are true, but the Reason does not provide a valid explanation for the Assertion.

3. The Assertion is false, but the Reason is true.

4. The Assertion is true, but the Reason is false.

Correct answer: 2) Both the Assertion and Reason are true, but the Reason does not provide a valid explanation for the Assertion.

Explanation:

Both the Assertion and Reason are true, but the Reason does not provide a valid explanation for the Assertion. While it is true that fat is not classified as a polymer or macromolecule, it does not directly explain why the end products of fat digestion cannot be absorbed into the bloodstream. The primary reason for this is that fat molecules are insoluble in water, and the bloodstream primarily transports water-soluble substances.

To facilitate fat absorption, fat molecules are first broken down into smaller components (such as fatty acids and glycerol) and then packaged into structures called chylomicrons, which can be absorbed into the lymphatic system before eventually entering the bloodstream.

Hence, the correct answer is option 2) Both the Assertion and Reason are true, but the Reason does not provide a valid explanation for the Assertion.

Q3. Which of the following are absorbed by the mechanism of facilitated transport with the help of the carrier proteins?

1. Glucose and amino acids

2. Fructose and alcohol

3. Fats and glucose

4. Fats and amino acids

Correct answer: 1) Glucose and amino acids

Explanations:

Facilitated Transport -

Small amounts of monosaccharides like glucose, and amino acids and some electrolytes like chloride ions are generally absorbed by simple diffusion. The passage of these substances into the blood depends upon the concentration gradients. Some substances like glucose and amino acids are absorbed with the help of carrier proteins. This mechanism is called facilitated transport.

Hence, the correct answer is option 1) Glucose and amino acids.

Also Read:

• MCQs in Absorption, Assimilation, and Egestion

• Dental Formula

• Disorders of the Digestive System

FAQs on Absorption of Digested Food

What is the main site of absorption of digested food?

The main site of absorption of digested food is the small intestine and particularly the ileum. The inner lining has finger-like projections known as villi and microvilli, which helps to increase the surface area for absorption. Each villus has a collection of capillaries and a lacteal for the transportation of the absorbed nutrients. Simple sugars, amino acids, vitamins and minerals enter the blood stream while fatty acids enter the lacteals.

What is passive transport in absorption?

Passive transport is the movement of molecules across the membrane from high concentration to the lower concentration. It occurs down the concentration gradient and hence there is no use of ATP. For example, the absorption of water, electrolytes like Na+, and some vitamins. This happens through simple diffusion i.e., direct passage through membrane or facilitated diffusion i.e., with the help of carrier proteins.

What is the role of sodium-potassium pump in absorption?

The sodium-potassium pump uses an active transport mechanism to transport ions across the cell membrane. This means it transports against the concentration gradient. It uses ATP to pump 3 sodium ions out of the cell and 2 potassium ions in. By maintaining low sodium levels inside the cell, the pump ensures continuous absorption of nutrients.

What is endocytosis in nutrient absorption?

Endocytosis is a process in which the cell membrane folds inwards to engulf larger molecules and encloses them inside the cell. In the small intestine, endocytosis is used for absorption of macromolecules like immunoglobulins from mother’s milk in infants. Endocytosis is further divided into two types:

• Phagocytosis - for engulfing solids

• Pinocytosis - engulfing liquids

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