DAIRY CHEMISTRY AND BIOCHEMISTRY be maintained active and secreting milk for several hours; substrates may readily be added to the blood supply for study. 1. 6.4 Tissue slices The use of tissue slices is a standard technique in all aspects of metabolic biochemistry. The tissue is cut into slices, sufficiently thin to allow adequate rates of difusion in and out of the tissue. The slices are submerged in physiological saline to which substrates or other compounds may be added Changes in the composition of the slices and or incubation medium give some indication of metabolic activity, but extensive damage may be caused to the cells on slicing: the system is so artificial that data obtained by the tissue slice technique may not pertain to the physiological situation. How ever, the technique is widely used at least for introductory, exploratory experiments 1.6.5 Cell homogenates Cell homogenates are an extension of the tissue slice technique, in which the tissue is homogenized. As the tissue is completely disorganized, only individual biosynthetic reactions may be studied in such systems; usefu preliminary work may be done with homogenates 16.6 Tissue culture Tissue cultures are useful for preliminary or specific work but are in- complete In general, the specific constituents of milk are synthesized from molecules absorbed from the blood. These precursors are absorbed across the basal membrane but very little is known about the mechanism by which they are transported across the membrane. Since the membrane is rich in lipids, and precursors are mostly polar with poor solubility in lipid, it is unlikely that the precursors enter the cell by simple diffusion. It is likely, ommon with other tissues, that there are specialized carrier systems to transport small molecules across the membrane; such carriers are probably proteins The mammary gland of the mature lactating female of many species is by far the most metabolically active organ of the body. For many small mammals, the energy input required for the milk secreted in a single day may exceed that required to develop a whole litter in utero. a cow at peak lactation yielding 45 kg milk day- secretes approximately 2 kg lactose and 1.5 kg each of fat and protein per day. This compares with the daily weight gain for a beef animal of 1-1.5 kg day, 60-70% of which is water. In large10 DAIRY CHEMISTRY AND BIOCHEMISTRY be maintained active and secreting milk for several hours; substrates may readily be added to the blood supply for study. 1.6.4 Tissue slices The use of tissue slices is a standard technique in all aspects of metabolic biochemistry. The tissue is cut into slices, sufficiently thin to allow adequate rates of diffusion in and out of the tissue. The slices are submerged in physiological saline to which substrates or other compounds may be added. Changes in the composition of the slices and/or incubation medium give some indication of metabolic activity, but extensive damage may be caused to the cells on slicing; the system is so artificial that data obtained by the tissue slice technique may not pertain to the physiological situation. How￾ever, the technique is widely used at least for introductory, exploratory experiments. 1.6.5 Cell homogenates Cell homogenates are an extension of the tissue slice technique, in which the tissue is homogenized. As the tissue is completely disorganized, only individual biosynthetic reactions may be studied in such systems; useful preliminary work may be done with homogenates. 1.6.6 Tissue culture Tissue cultures are useful for preliminary or specific work but are in￾complete. In general, the specific constituents of milk are synthesized from small molecules absorbed from the blood. These precursors are absorbed across the basal membrane but very little is known about the mechanism by which they are transported across the membrane. Since the membrane is rich in lipids, and precursors are mostly polar with poor solubility in lipid, it is unlikely that the precursors enter the cell by simple diffusion. It is likely, in common with other tissues, that there are specialized carrier systems to transport small molecules across the membrane; such carriers are probably proteins. The mammary gland of the mature lactating female of many species is by far the most metabolically active organ of the body. For many small mammals, the energy input required for the milk secreted in a single day may exceed that required to develop a whole litter in utero. A cow at peak lactation yielding 45 kg milk day-' secretes approximately 2 kg lactose and 1.5 kg each of fat and protein per day. This compares with the daily weight gain for a beef animal of 1-1.5 kgday-', 60-70% of which is water. In large