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By the end of this section, you will be able to:
  • Describe the process of digestion
  • Detail the steps involved in digestion and absorption
  • Define elimination
  • Explain the role of both the small and large intestines in absorption

Obtaining nutrition and energy from food is a multi-step process. For true animals, the first step is ingestion, the act of taking in food. This is followed by digestion, absorption, and elimination. In the following sections, each of these steps will be discussed in detail.

Ingestion

The large molecules found in intact food cannot pass through the cell membranes. Food needs to be broken into smaller particles so that animals can harness the nutrients and organic molecules. The first step in this process is ingestion    . Ingestion is the process of taking in food through the mouth. In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing the food into bolus). While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. The combined action of these processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length of the esophagus.

Digestion and absorption

Digestion is the mechanical and chemical break down of food into small organic fragments. It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across the digestive epithelium. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance. How each of these components is digested is discussed in the following sections.

Carbohydrates

The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.

The next step of carbohydrate digestion takes place in the duodenum. Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases , sucrases , and lactases , which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose. The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body. The steps in carbohydrate digestion are summarized in [link] and [link] .

Questions & Answers

Can i get access to Functional Approach Biology
Bako Reply
How does the synthesis of organic compounds in photosynthetic bacteria differ from that in: green plants ,chemosynthetic bacteria?
Bako
green plants uses photons or light from the sun for synthesis of food but chemosynthetic autotrophs uses chemical subtances for synthesis of organic materials
Ebenezer
what is zygote?
Obialor Reply
a foetus gotten from the fusion of the male and female gamete
Akinpelu
Important of bacteria to human
Ebben Reply
how does meiosis produced
Kauzi Reply
Meiosis happens in sex cells only. And produces 4 nonidentical sex cells.
Eric
what are structure of the cell
wana Reply
what did Darwin say about evolution
Hope Reply
effect of planning beans of using fertilizer
Elizerbeth Reply
what do we mean by transgenic organisms?
FADILAT Reply
what is or are the functions of the Islets of Langarhaans
FADILAT
They are the regions of the pancreas that contains the endocrine cell
Iyadi
is the studly of life
Aisha Reply
what is biology
Asunta Reply
is the study of living organism and their interection with one another and their environment.
Belbon
what is soil
Mukisa Reply
the top layer of the earth in which plant's, tree's
Ahmad
type of soil
Asunta
function of cell wall
Nthati Reply
function of cell wall
Asunta
To protect the cell from bursting
Maurice
to protect the cell from bursting
Deborah
to protect all other internal components of the cell
Olaoye
What is escherichia coli
Tumise Reply
It's an example of gram negative Bacteria
Abdulrasheed
in what type of cells is meiosis taking place?
Rhyeann Reply
sex cells
Eric
hlo
Amit
reproductive system of earthworm plzz describes
Amit
procryotic cell and some eucaryotic cell
Olaoye
Reproductive or sex cells
Abdulrasheed
applications of biology
Namawejje Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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