Seven Characteristics of Living Organisms

Cells, Tissues and Organs of the Digestive System

Living organisms all have various functioning systems within the body that enable life processes to be carried out. Each system uses different organs that coordinate with each other to carry out their functions, and these systems, organs and functions are shown below:

Functioning System
Related Organs
Function
Circulatory
Heart, Arteries, Veins
Transports blood through the body

Digestive
Mouth, Tongue, Pharynx, Esophagus, Liver, Stomach, Gall Bladder, Pancreas, Large Intestine, Small Intestine, Rectum, Anus
Breaks down large molecules into smaller ones in order that they can be absorbed by the body.
Endocrine
Glands, Pancreas, Ovaries, Testicles, Intestines
Hormone producers – hormones are chemicals which control the body’s functions.
Immune & Lymphatic
Tonsils, Adenoids, Lymph Nodes, Appendix, Lymphatic Vessels, Bone Marrow, Spleen, Thymus, Peyer’s Patches
Defends the body against infection and disease.
Muscular
Muscles and Tendons
Enables the body to move
Respiratory
Nasal Cavity, Oral Cavity, Tongue, Trachea, Pharynx, Larynx, Epiglotis, Bronchi, Lungs, Diaphragm, Sinuses
Brings air into the body to enable the chemical reaction of converting glucose into energy within cells.
Reproductive
Female: Fallopian Tubes, Ovaries, Uterus, Vagina, Endometrium, Myometrium
Male: Prostate Gland, Vas Deferens, Penis, Testicles
Enables the body to reproduce and create offspring.
Nervous
Brain, Spinal Cord, Nerves
Enables the body to respond to stimuli by sending, receiving and processing impulses signals.
Skeletal
Bones, Ligaments, Tendons
Shapes the body and protects the organs, and works with the muscular system so that the body can move.
Urinary
Kidneys, Urethra, Bladder, Ureter, Arteries, Veins
Eliminates waste from the body in the form of urine.

Digestive Systems in Different Animals

All living organisms have a digestive system of some kind. These vary from organism to organism, depending on what they are likely to ingest, and how quickly they use up energy. Herbivores have flat teeth so that grass and plant material can be ground down, whereas carnivores have teeth designed to tear flesh. Humans have both kinds of teeth so they can eat meat and vegetation. As animals become more complex, organs with specialized functions develop. Higher order animals require a storage organ, such as the stomach, which allows them to take in large amounts of food in one go, and gradually use it as energy over a long period of time.

The digestive system of a bird is specialized so that it absorbs food very quickly, but this means that they need constant feeding. Birds have a high body temperature, so they use up energy very quickly. Because they have no teeth, they have an area called the crop, into which food passes from the esophagus. In the crop are pebbles and sand that the bird has swallowed, which assist in the breaking up of food before passing it into the stomach.

The Human Digestive System

As shown in the table on page 1, the role of the digestive system is to break down large molecules of food into small ones so that the body can absorb nutrients from them. The different organs that make up the digestive system are:

The first part of the digestive system is made up of the alimentary tract, which consists of the following organs:

Mouth and Tongue - First part of the digestive tract – food is received and chewed up/masticated with saliva. Saliva assists in cleaning the teeth, moistening food so it is easier to swallow, dissolving certain molecules so that food can be tasted and starts off the chemical digestion of starches. Food is pushed to the back of the mouth with the tongue where sensory receptors initiate the involuntary reflex of swallowing

Pharynx - Passageway that connects the nasal and oral cavities to the larynx and the esophagus – is a channel for food and air, and is also used in the respiratory system.

Esophagus - Muscular tube which serves as a passageway between the pharynx and the stomach. Food is propelled into the esophagus by muscles called sphincters, before passing through an opening in the diaphragm called the esophageal hiatus and emptying into the stomach.

Stomach – Food is broken down by gastric juice into different nutrient molecules, before passing into the small intestine upon relaxation of the pyloric sphincter muscle.

Small Intestine - Finishes the digestive process – absorbs nutrients and passes residue into the large intestine. Enzymes (protein molecules) are secreted by the exocrine and endocrine cells, which help to break down the semi-digested food further.

Large Intestine – consisting of the colon, rectum and anal canal – absorbs water and electrolytes from the remaining food, moves the waste to the rectum to be passed through to the anal canal. A smooth thick layer of muscle forms the internal anal sphincter at the end of the anal canal, which is under involuntary control.

Anus – at the end of the anal canal - waste passes out under the voluntary control of the external anal sphincter.

The rest of the digestive system is made up of the liver, the pancreas and the gallbladder. The liver carries out many functions, including detoxification and filtering, the pancreas secretes hormones and enzymes, and the gallbladder stores bile, which acts as an emulsifying agent in the digestion and absorption of fats.
Cells and Tissues in the Stomach

The main role of the stomach is to break down large molecules such as proteins, fats and starch into smaller molecules of amino acids, fatty acids and glucose in order that they can be absorbed into the blood more easily. The smell and thought of food initiates the production of gastric juice in the stomach, which helps to break down food.

The stomach is made up of different layers of tissue:

The food enters the stomach through the esophagus, and is pushed down and mixed by the stomach muscles. When the stomach is full, the folds of muscle (rugae) in the mucosa and submucosa flatten out, and when the stomach is empty, the folds are thrown up. The submucosa is connective tissue which contains blood, lymph vessels and nerves.

Mucosa and the Epithelial Cells

Small holes covering the stomach lining lead to the gastric pits, and these extend into the mucosa as straight and branched tubules forming exocrine gastric glands (glands which secrete substance through a duct opening). These glands are where epithelial cells are found, which are responsible for secreting the thick layer of mucus that is found on the mucosa. Below is a diagram of the branched tubules/gastric glands where the different cell types are located.

Three to seven individual gastric glands empty their secretions into each gastric pit.

The three cell types whose secretions make up gastric juice are:

Mucous Neck Cells - These are foamy in appearance and secrete alkaline mucus which protects the stomach lining (epithelium) from stress and acid, and acts as a defense mechanism so that the stomach is not digested by its own enzymes. It also assists in the lubrication of food so it can move around the stomach.

Parietal Cells – These cells secrete hydrochloric acid, and produce most of the water found in gastric juice. They also produce glycoproteins called intrinsic factor, which are essential to the maturation of red blood cells, vitamin B12 absorption, and the health of certain cells in the central and peripheral nervous systems.

Chief Cells – These cells secrete pepsinogen, from which the protein-digesting enzyme pepsin is formed.

The endocrine cells (i.e. G and D cells) found in the stomach secrete several substances, which are absorbed directly into the blood, including the hormone serotonin.

References:

www.britannica.com www.factmonster.com www.muschealth.com www.organicnutrition.co.uk www.student.loretto.org www.web-books.com

References: www.britannica.com www.factmonster.com www.muschealth.com www.organicnutrition.co.uk www.student.loretto.org www.web-books.com