Tissues and Organs

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Animals and plants are multicellular - made up of large numbers of cells. Cells become specialised according to their function.

Tissues - composed of groups of cells that carry out the same functions. Often the cells are of the same type

Organs - structures made of several different tissues that work together to carry out a number of functions. E.g. the leaf is an organ which contains palisade tissue, vascular tissue, epidermal and spongy mesophyll tissue. 

Organ systems - groups of organs working together. E.g. the digestive system which consists of the mouth, oesophagus, stomach, small and large intestines, liver, gall bladder and pancreas and they work together to digest and absorb food and eliminate undigested material.

The ileum

The region of the small intestine where digestion is completed and where most absorption of the products of digestion occurs.

There is a vast surface area for digestion and absorption provided by: folds in the inner surface of the intestinal wall; projections called villi that are present on the folded surface of the wall; and microscopic projections called microvilli on the cell-surface membranes of columnar epithelial cells that line the villi.

Structurally thw ileum consists of tissues in distinct layers: 

  • serosa - outer layer of connective tissue provides very thin protective and supportive lining for the alimentary canal
  • muscularis externa - outer layer of longtitudinal muscle (runs along the length of the alimentary canal) that contract to cause pendular movement and an inner layer of circular muscle (runs around the alimentary canal) that contracts to cause local constrictions, which help push food along the gut in a series of peristaltic waves. These actions help to churn and mix the food
  • submucosa - largely composed of connective tissue…

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