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Nutrition involves an exchange of matter and energy between organisms and their surrounding environment. To perform this exchange different tracts and systems work together to obtain nutrients and oxygen which are distributed to each cell of the body. They also expel waste substances out of the body. How substances are exchanged
UNICELLULAR ORGANISMS: They exchange substances directly with their surrounding eviroment through-out a plasma membrane.
Nutrition function is performed through the joint action of different systems that act as intermediaries between the outside world and cells.
How substances are exchanged.
MULTICELLULAR ORGANISMS: Most cells don't come into contact with the outside world and cannot exchange substances with it directly.
1. The digestive system: transforms the food we eat into simpler substances (nutrients), which are absorbed and passed into the blood stream. 2. The respiratory system: takes O2 from the air and transfers it to the blood. It also takes CO2 from the blood and expels it out of the body. 3. The circulatory system: uses blood to carry nutrients and O2 to the cells. It also transports waste from the cells to the excretory organs, where they are expelled. 4. The lymphatic system: works with the circulatory system to transport substances. 5. The excretory system: extracts waste substances produced by cellular activity from the blood before expelling them from the body.
● The mouth: Cavity where food is chewed and ground by the teeth. ● The pharynx: (digestive and respiratory tracts). The epiglottis acts as a lid and closes the respiratory tract to prevent food from blocking it. ● The oesophagus: Food travels down the stomach as a result of contractions in its walls (peristaltic movements) ● The stomach: It has strong muscles in its walls and glands that secrete gastric juices. It is connected to the small intestine through an opening called pylorus and to the oesophagus through the cardia. ● The small intestine: long tube with glands that secrete intestinal juices and folds called intestinal villi (nutrients pass into the blood stream through these capillaries) ● The large intestine: 3 parts caecum colon rectum (leads to the anus where faeces are expelled).
ACCESSORY GLANDS: They are located outside the digestive tract although the digestive juices they secrete are passed into it. These juices contain a great variety of digestive enzymes (type of proteins that accelerate the breakdown of complex food into nutrients) ● The salivary glands: 3 pair of glands that secrete saliva into the mouth. ● The liver: Largest gland. It secretes bile, which is stored in the gallbladder. Bile helps us to digest lipids by acting as a detergent. The liver also performs other functions such as storing glucose, iron and vitamins and eliminating toxic substances (alcohol) from the blood. ● The pancreas: Arrowhead-shaped gland found under the stomach. 2 functions:1. Digestive function,2. Endocrine function (pancreatic juices).
The digestive process consists of various different stages: mechanical digestion, chemical digestion, absorption and egestion or defecation.
Processes designed to reduce the size of food particles and make them travel through the digestive system. 3 stages: Mastication: tearing, cutting, chewing and grinding the food. It is carried out by the teeth and the movement of the lower jaw. Insalivation: mixing the ground-up food with saliva by moving the tongue. A bolus is formed as a result. Deglutition: using peristaltic movements to make the bolus travel through the pharynx and oesophagus to the stomach.
In the small intestine: the intestinal juices (secreted by the intestinal walls), the bile (secreted by the liver) and the pancreatic juices (secreted by the pancreas) all act in the small intestine (duodenum) turning the chyme into the chyle.
CHEMICAL DIGESTION: Processes that transform food into nutrients. In the mouth: During the formation of the bolus, the saliva breaks down carbohydrates. In the stomach: the gastric juices act on proteins resulting a half-liquid mush called chyme.
ABSORPTION: Process through which nutrients pass through the intestinal walls into the circulatory system to reach all the cells in the body.
➢ Absorption in the small intestine: relatively quick process because of its large absorption surface (200m2): - very long (7-8 m) - inside walls covered by intestinal folds with thousands of intestinal villi which have many blood capillaries in its inside and are made up of cells with a plasma membrane that has many tiny folds known as microvilli. ➢ Absorption in the large intestine: water and minerals are absorbed here. It has greater diameter, measures 1 m length and has no villi.
In this process undigested products (waste) are eliminated.
In the large intestine, gut flora (a collection of good bacteria that live in the intestine) transforms undigested products into faeces, which are expelled from the body through the anus.
Food contains substances that we cannot digest or, therefore, absorb.
Fibre is one of these substances. None of the digestive enzymes in our bodies can digest it and turn it into glucose. However fibre plays an essential role in the activity of the digestive system as it helps to move substances through the intestine and prevents constipation. 

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