Ileum in mammals
An organ is a structure that consists of several diferent tissues, each performing different functions that contribute to the overall functioning of the organ
- The ileum is the organ in mammals in which the final stages of digestion and most absorption take place.
- The absorptive surface is the columnar epithelium, with microvilli, on the villi of the mucosa layer.
- Monosaccharides and amino acids are absorbed into blood vessels but the products of fat digestion enter the lacteals (lymphatic vessels)
- Crypts of Liberkuhn possess stem cells, which produce new epithelial cells, and Paneth cells, which have an animicrobal function.
- Contraction of the muscularis mucosa imparts a wafting action to the villi ensuring that they are in contact with freshly digested food
- The submucosa contains blood and lymphatic vessels within connective tissue
- The muscularis externa consists of circular and longitudinal muscles, which help churn the food and move it along by peristalsis
- The serosa covers the outside of the intestine and has a protective function
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The leaf in plants
- The leaf is the organ of photosynthesis in higher plants.
- The upper and lower epidermis are colourless and protect the leaf from damage and infection and, with the secretion of a waxy cuticle, from dehydration.
- Stomata are found mostly in the lower epidermis and are open during the day to allow the diffusion of carbon dioxide into the leaf
- The palisade mesophyll consists of columnar cells containing many chloroplasts and represents the main photosynthetic layer.
- The spongy mesophyll contains large air spaces which allow diffusion of gases through the leaf and contain chloroplasts for photosynthesis.
- The veins in the leaf contain the transport tissues xylem and phloem
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