Tissues and Organs

• multicellular living organisms contain more than one cell and are made up of many cells
• tissues are groups of cells with a similar structure and function.
• an organ is a group of tissues that work together to perform a specific function.
• the leaf is an organ which is highly specialised for photosynthesis but also a number of other functions
• the ileum (alimentary canal) is the part of the digestive system that forms the long hollow tube that runs from mouth to anus
• layers of tissue in the ileum from in to out?

  lumen, mucosa, muscularis mucosa, submucosa, muscularis externa, serosa
• the serosa is the outer layer of connective tissue and provides a very thin protective lining for the alimentary canal from friction from other organs in the abdomen. Mesentery (tissue containing blood vessels, nerves and lacteals) connects to the peritoneum (outer layer) and supports coiled intestine
• the muscularis externa consists of an outer layer of longitudinal muscle and inner layer of circular muscle. Contractions of the longitudinal muscle cause pendular movement and contractions of the circular muscle causing local constrictions - actions which help churn and mix the food. Contractions of the circular muscle help push food along the gut in a series of peristaltic waves.
• the submucosa is a region largely composed of connective tissue and contains many blood vessels and lymphatic vessels - vessels that are crucial in transporting absorbed food products.
• the muscularis mucosa is a thin layer of muscle which lies between the submusoca and the mucosa. It is important in moving the villi that are present in the mucosa, thus increasing contact with digested food in the gut lumen. It is able to do this as small strands of muscle extend from the main muscularis mucosa layer up through the mucosa into each villus, with contractions of these strands helping to produce the characteristic ‘wafting’ movement of the villi
• the mucosa is the layer in contact with the food in the gut lumen. It is highly specialised with a substantially increased surface area due to the presence of villi and microvilli
• the epithelium of the mucosa is made up of simple columnar cells which have long finger-like projections called microvilli on their surfaces. These increase the surface area available for absorption by about 600 times compared to if there were no microvilli present. The microvilli also contain enzymes such as amylase (which breaks down starch) and lipases (which break down fats).
• the columnar epithelial cells of the villus have their surface area extended by the presence of numerous microvilli that give a brush border appearance. Although many digestive enzymes are secreted into the lumen of the gut, many occur within the cell surface membrane of the ileum epidermal cells, where the presence of microvilli further increases the surface area, increasing the number of these enzymes that can make contact with the food.
• the columnar epithelial cells are particularly rich in mitochondria for the active transport of some absorbed food products.
• the absorption of some products involved both active transport and (facilitated) diffusion, involving protein carrier molecules. Glucose and amino acids are taken into the epithelial cells by active transport, and once there, can diffuse into the capillary network due to the concentrations that can build up in the cells
• lipid soluble products such as glycerol and fatty acids can simply diffuse into the cells lining the villi. Lipids are transported out of the villi by the lacteals, which are parts of the lymphatic system
• pinocytosis is also involved in absorption of some substances. For example, in babies, antibodies, which are proteins, can pass from breast milk into their blood system without being digested
• within the columnar epithelium cells are goblet cells, these secrete mucus which provides a slimy protective layer that lubricates the lining of the alimentary canal, facilitating the movement of food and also protects against the actions of digestive enzymes on the epithelial cells
• the capillaries within the villi are important for transporting amino acids and monosaccharides out of the villi. These capillaries eventually combine to form the venules of the hepatic portal vein, which transports these digested products to the liver. Lipids are not absorbed into the capillary network but are instead transported away by lacteals, which are part of the lymphatic system
• The Crypts of Lieberkühn are intestinal glands that lie embedded in the tissue between the villi. The cells at the bottom of the crypts are continually dividing by mitosis to produce new cells that move up the surface of the villi, like a conveyer belt, as new cells are produced below them. By the time they reach the tip of the villus (usually a few days) they are sloughed off and replaced. Paneth cells at the base of the crypts have an antimicrobial function in protecting their neighbouring actively dividing cells. Other cells lining the crypts are involved in mucus secretion.
• the tissue layers in the leaf are clearly organised into discrete layers or zones, these being the upper epidermis, palisade mesophyll, spongy mesophyll, vascular tissue (xylem and phloem) and the lower epidermis (with stomata)
• the leaf is primarily an organ for photosynthesis. It is also well adapted for gas exchange ( to enable photosynthesis and respiration to take place in the cells) and many leaves are highly adapted to reduce water loss by transpiration
• The upper epidermis is the upper protective layer of the leaf and it has no chloroplasts so is not directly involved with photosynthesis. Covered in a waxy cuticle to reduce water loss. The waxy cuticle is thickest in the leaves of those plants best adapted to reduce water loss, but being transparent, it does not significantly impede the passage of light.
• the palisade mesophyll is the primary photosynthesising region in the leaf and to maximise the harvesting of light energy, it occurs immediately beneath the upper epidermis - the surface of the leaf that receives most light. The palisade cells are arranged regularly in order to pack in as many as possible. Each cell has many chloroplasts to maximise photosynthesis. Depending on the species, there may be one or more layers of palisade cells in the palisade mesophyll. The large vacuole in palisade cells helps the light to pass down through the layers with minimum shading
• having palisade and upper epidermis perpendicular, the incident light is maximised, there are other advantages with this arrangement. For example, sugars moving to the phloem have to pass through fewer cells, as do the gases diffusing to and from the intercellular air spaces.
• the spongy mesophyll lies immediately beneath the palisade layer. The spongy mesophyll cells are much more loosely and irregularly arranged. This arrangement leads to the formation of intercellular air spaces that encourages the diffusion of gases for gas exchange.
• While spongy mesophyll do photosynthesise, they have fewer chloroplasts than palisade cells and consequently have a less significant photosynthetic role than the palisade cells diffuses through the spongy mesophyll cells on their way to and from the palisade layer. The loose and irregular arrangement of these cells gives a large gaseous exchange surface in the leaf
• Within the spongy mesophyll lies the vascular tissue. Xylem vessels transport water and inorganic ions (minerals) up through the plant and into the leaves. Phloem sieve tubes transport sugars, usually sucrose, produced in photosynthesis away from the leaf to plant storage regions, such as bulbs or tubers, where it can be converted to starch, or to actively growing regions where it is used in respiration
• In addition to xylem and phloem, the vascular bundle also contains companion cells which supply the phloem with nutrients and hormones needed for its function. There are also parenchymatous cells called bundle sheath cells surrounding the vascular bundles. These cells protect them from damage by insects and fungi and prevent excessive transpiration
• Stomata are small openings found only in the lower epidermis. They allow carbon dioxide to enter the leaf during photosynthesis and oxygen to leave the leaf during respiration. Stomata are surrounded by guard cells which control whether the stomata remain closed or open. When the guard cells become turgid, the stomata opens; when the guard cells lose turgidity, the stomata closes.
• The lower epidermis is similar to the upper epidermis but lacks stomata because the leaf does not need to lose excess moisture via transpiration. Instead, the lower epidermis provides support for the veins within the leaf
• The cuticle is a waxy substance covering the outer surfaces of the epidermal cells. It prevents water loss by reducing evaporation from the leaf surface. In some plants, the cuticle is thickened to form a protective barrier against physical injury and disease-causing organisms
• Stomata are small openings found only on the underside of the leaf. They allow carbon dioxide to enter the leaf during photosynthesis and oxygen to leave the leaf during respiration. Stomata also permit water vapor to escape from the leaf during transpiration
• The opening and closing of stomata is controlled by two factors - the concentration of CO2 inside the leaf and the amount of water available within the leaf. If the concentration of CO2 increases, then the rate of photosynthesis will increase. However, if too many stomata were opened at once, the rate of transpiration would also increase rapidly, leading to rapid loss of water from the leaf. To avoid this, plants use a negative feedback mechanism to regulate the number of stomata that are open at any one time.
• Veins run parallel to one another throughout the leaf. Each vein consists of a central strand of xylem surrounded by phloem. Veins branch out into smaller veins and then into even smaller ones until they reach the spongy mesophyll. Here, the veins divide further into capillaries which penetrate the spongy mesophyll and end at the palisade layer. Capillaries are very narrow so there is a high resistance to flow along them. As a result, most of the water moves between adjacent cells rather than through the capillaries themselves
• Palisade cells contain many chloroplasts packed closely together. This allows light energy to penetrate deeply into the cell so that most of the chloroplasts receive sufficient light for photosynthesis. Palisade cells are tightly packed together to increase the density of chloroplasts per unit area of leaf surface. This increases the rate of photosynthesis
• Guard cells surround the opening of the stomata. The shape of these cells determines whether the stomata remains open or closed. If they become turgid, the stomata will open; if they lose turgidity, the stomata will close
• the lower epidermis is also covered with a waxy cuticle but it is usually much thinner than the cuticle on the upper epidermis, as less transpiration tends to occur through the lower epidermis.
• Stomata are present and their opening and closing is controlled by the presence of the guard cells. Guard cells are unusual for epidermal cells in that they do possess chloroplasts. When turgid, the guard cells expand and open the pore of the stoma, thereby facilitating gas exchange. When less turgid, pore closes and reduces water loss
• an organ is tissues grouped to carry out a task