Nutrition and Digestion

Human nutrition involves the following requirements and resources:

• Energy is required, and the primary sources are carbohydrates and fats.
  • Lipids include fats, but also phospholipids and cholesterol which can have structural roles.
    脂类包括脂肪，也包括磷脂和胆固醇，后者可能具有结构功能。
  • Carbohydrates include sugars and starches, which are sources for quick energy.
    碳水化合物包括糖和淀粉，它们是快速提供能量的来源。
  需要能量，主要来源是碳水化合物和脂肪。
• Proteins are required as sources for amino acids.
  • Dietary proteins are broken down into amino acids by digestion.
    膳食蛋白质通过消化分解为氨基酸
  • Amino acids are used in the body to build up the new proteins needed.
    氨基酸被用于身体中构建所需的新型蛋白质。
  • Body proteins play a vast number of roles, involved in accomplishing most of the body's tasks.
    蛋白质在身体中扮演着众多角色，参与完成身体的大部分任务。
  蛋白质是氨基酸的来源
• Minerals are required for specific molecular roles in the body.
  矿物对于身体中的特定分子功能是必需的
• Vitamins are required in small amounts for many specific roles in metabolism.
  维生素在许多特定代谢过程中需要少量参与。

Providing the elements of nutrition in a usable form for the body's purposes requires digestion.

• Chewing and swallowing are the first mechanical phase of digestion, but the brain is involved with the smell, taste and sight of food to stimulate secretioin of saliva and initiate the secretion of acid in the stomach and the hormone gastrin which stimulates further acid secretion.
  • Food is broken up mechanically
    食物被机械地破碎
  • Saliva contains the digestive enzyme amylase, which begins the breakdown of starches into sugar.
    唾液中含有消化酶唾液淀粉酶，它开始将淀粉分解为糖。
  咀嚼和吞咽是消化的第一个机械阶段，但大脑会通过食物的气味、味道和视觉来刺激唾液分泌，并启动胃酸分泌，而胃酸分泌刺激素胃泌素进一步促进胃酸分泌。
• The stomach holds the food, begins chemical digestion, and controls forward movement.
  • Glands in the lining of the stomach secrete enzymes to facilitate chemical breakdown of the food.
    胃黏膜中的腺体分泌酶以促进食物的化学分解。
  • The hormone gastrin stimulates the secretion of hydrochloric acid by specialized stomach cells. It is limited by a negative feedback loop which inhibits the gastrin production at a certain level of acidity.
    胃泌素刺激胃部特殊细胞分泌盐酸。其分泌受到负反馈调节，当酸度达到一定电平时，会抑制胃泌素的产生。
  • Other cells release pepsinogen, which serves as a precursor for the next step
    其他细胞释放胃蛋白酶原，它作为下一步的前体
  • In the acidic interior of the stomach, pepsinogen is converted to pepsin, an active protease that begins the breakdown of proteins into shorter chains of amino acids.
    在胃内的酸性环境中，胃蛋白酶原转化为胃蛋白酶，这是一种具有活性的蛋白酶，开始将蛋白质分解为较短的氨基酸链。
  • The multiple processes in the stomach convert the food and digestive fluids into an acidic fluid called chyme.
    胃内的多种过程将食物和消化液转化为一种称为食糜的酸性液体。
  • Under the control of the muscular pyloric sphincter at lower opening of the stomach, the fluid is allowed to move in small (~ teaspoon) quantities about every 20 seconds into the small intestine.
    在胃的下部开口处受肌层幽门括约肌的控制下，流体以小量（约茶匙量）每隔20秒进入小肠。
  胃部容纳食物，开始化学消化，并控制向前移动。
• The small intestine accomplishes the main part of the food digestion
  • The liver produces bile, a critical component in the digestive process
    • Bile is a complex mixture of bile salts, other salts, water, and cholesterol
      胆汁是由胆盐、其他盐类、水和胆固醇组成的复杂混合物
    • The bile is transported to and stored in the gallbladder, and released into the small intestine through the small bile duct.
      胆汁被运输到胆囊中储存，并通过小胆管释放到小肠。
    • The bile salts act as detergents to disperse the fats into microscopic particles.
      胆汁酸起表面活性剂作用，将脂肪分散成微小颗粒。
    肝脏产生胆汁，这是消化过程中的关键成分
  • The pancreas provides a supply of lipases, lipid-digesting enzymes.
    • The microscopic lipid particles produced by the bile are quickly digested by the lipases
      胆汁产生的微小脂质颗粒很快被脂酶消化
    胰腺提供一种脂肪酶，用于分解脂质。
  • The pancreas injects a sizable volume of "pancreatic juice" into the small intestine.
    • The injection of perhaps a liter per day of water, sodium bicarbonate and several enzymes acts to neutralize the acidic chyme to a slightly basic pH.
      注入每天约一升的水、碳酸氢钠和几种酶，可将酸性食糜中和至微碱性pH值。
    • The additional enzymes are amylase, peptidases, trypsin and chymotrypsin, which need to work in a basic environment.
      这些额外的酶包括淀粉酶、肽酶、胰蛋白酶和胰凝乳蛋白酶，它们需要在碱性环境中发挥作用。
    • Amylase breaks down carbohydrates to constituent sugars
      淀粉酶将碳水化合物分解为构成糖
    • The pancreatic proteases trypsin and chymotrypsin break proteins and peptides in to shorter peptide chains.
      胰腺蛋白酶胰蛋白酶和胰凝乳蛋白酶将蛋白质和肽分解为更短的肽链。
    • Carboxypeptidase completes protein digestion by separating the individual amino acids from the ends of the peptide chains.
      羧肽酶通过将氨基酸从肽链的末端分离出来，完成蛋白质的消化过程。
    胰腺将大量“胰液”注入小肠。
  • The wall of the small intestine completes the digestive process.
    • Specialized cells form the lining of the wall of the small intestine to complete the digestive process
      特殊细胞构成了小肠壁的内层，以完成消化过程
    • Proteases complete the breakdown of the remaining peptides into amino acids.
      蛋白酶将剩余的肽完全分解为氨基酸
    • Sucrase, lactase and maltase break down any disaccharides into simple sugars.
      蔗糖酶、乳糖酶和麦芽糖酶能分解任何二糖为单糖
    • Small amounts of lipase complete the digestion of remaining lipids.
      少量的脂酶完成剩余脂质的消化
    • These process occur while these resulting small molecules are being absorbed into the cells of the intestinal wall, which is protected by a thick mucous from being digested itself.
      这些过程发生在这些生成的小分子被吸收到小肠壁细胞的过程中，小肠壁被厚厚的黏液保护，免受自身消化。
    小肠壁完成消化过程
  • The wall of the small intestine completes the absorption process.
    • Numerous foldings and projections of the small intestine wall give it an absorbing area some 600 times that of a smooth tube.
      小肠壁的众多折叠和投影使其表面积达到光滑管状结构的600倍。
    • The total absorbing area of the small intestine including the villi is about 250 square meters!
      小肠的总吸收面积，包括绒毛，约为250平方米！
    • Minute villi cover the wall and move back and forth to efficiently absorb the small molecules and place them into the blood supply.
      微绒毛覆盖在壁上，并来回移动，以高效地吸收小分子并将它们送入血液供应中。
    小肠壁完成吸收过程
  
  
  

  This material is part of a brief overview of the topics studied in biology with the intent to highlight the connections to basic ideas in physics and physical science. 这部分内容是对生物学中研究主题的简要概述，旨在突出其与物理学和物理科学基本概念的联系。

  小肠完成食物消化的主要部分