Biology Test 253

Compound epithelium is many layer thick. This epithelium covers the surfaces subjected to heavy wear and tear. It functions to provide better protection to the underlying tissues. It is found present in lining of the pharynx and buccal cavity and ducts of salivary glands. The lining of stomach is lined by simple epithelium.

Erythropoiesis refers to the process of RBC production. Erythropoiesis occurs in liver (in 3rd-4th month) then moves to the bone marrow, which in adult life becomes the only source of RBC production. Other sites of erythropoiesis are spleen, lymph nodes, etc.

Glial cells are known as the supporting cells of the nervous system. The four main functions of these cells are as follows

(i) To surround neurons and hold them in position for manufacturing myelin.

(ii) To insulate one neuron from another.

(iii) To supply nutrients and oxygen to neurons, acting as blood-brain barrier.

(iv) To destroy and remove the dead neurons.

Sapwood refers to the outer light coloured living layers of the secondary wood of trees. This part is engaged in the transportation of sap, i.e. water and minerals. Unlike the heartwood, the vessel and tracheid are not plugged, no tyloses formation is observed.

Tight junctions are closely associated areas of two cells whose membranes join together to form a virtually impermeable barrier to anybody fluid, e.g. blood-brain barrier in CNS that keeps the circulating substances out of the brain, while selectively permitting the entry of essential substances like oxygen.

Schizocarpic or spitting fruits on ripening divide into one seeded segments or mericarp, but the mericarp remains unruptured. These include lomentum, cremocarp, regma, carcerulus and double samara.

Phylloclade is a stem modified to perform the functions of leaf in certain xerophytic (desert) plants. The phylloclade of Opuntia is flattened and contains chlorophyll to perform photosynthesis.

Although many plant’s stems do store the reserve food material, it cannot be considered their primary function. This is because such plants usually undergo stem modification in order to perform storage function.

The cytoplasmic pH is slightly basic at a value of 7.2. The cytoplasm tends to acidify because of various metabolic pathways occurring such as ATP production in cytoplasm by glycolysis.

Desmosomes are a type of anchoring junction. They connect adjacent cells in an animal tissue by forming strong cell to cell adhesion. They are present in tissues that experience intense mechanical stresses.