1. How does the food change in consistency and form?
The consistency and form of the food begin to change in the mouth. It is first physically broken down (mechanical digestion) by mastication (chewing to tear and grind food) and it turns into a bolus, a flexible and easier-to-swallow lump. It is then swallowed (deglutition) and moved by peristalsis until it reaches the stomach. Once in the stomach, the bolus is churned and mixed with gastric juices until it becomes chyme, which has a semi-liquid consistency. It then travels to the small intestine where most nutrients are absorbed. When it reaches the large intestine (colon), feces begin to form because most of the nutrients have been absorbed. It travels along the colon until it reaches the rectum, which stores feces until they are defecated by the anus.
2. How could the body absorb the nutrients from the foods we eat?
Since the body can only absorb monosaccharides (simple sugars, basic units of carbohydrates), more complex sugars such as disaccharides and polysaccharides must first be chemically broken down with the help of enzymes (chemical digestion). The breakdown of food begins in the mouth with the help of the salivary amylase which breaks down starch into smaller pieces. When the bolus reaches the stomach, it is further broken down by gastric juice which contains pepsin (breaks down proteins, comes from pepsinogen that was converted by hydrochloric acid) and gastric lipase (breaks down triglycerides). Nutrient absorption begins in the stomach, but it can only absorb water due to its impermeable epithelial lining. The pancreas secretes amylase, lipase, and proteases which aid in digesting/breaking down starch, triglyceride, nucleic acid, and protein. The liver also secretes bile (which emulsifies fats) and stores it in the gallbladder. The aforementioned secretions are emptied into the duodenum of the small intestine, where they assist in digestion. The small intestine is also the main site of nutrient absorption because it absorbs monosaccharides, amino acids, lipids, electrolytes, vitamins, and water. The remaining food then goes to the large intestine, where only water and electrolytes can be absorbed (as they are the only remaining absorbable materials).
If we consider how nutrients are absorbed at a cellular level, then we can say that most nutrients are absorbed via active transport (ATP is used as they go against the concentration or electrical gradient). Some lipids are absorbed via diffusion, and water is absorbed through osmosis (both are passive transport). Large molecules are absorbed by cells through pinocytosis, an active transport method where droplets are “drank” by the cell.