What is Protein?

• Protein is a substance made of amino acids.

• It is found in all parts of the body — muscles, skin, hair, and blood.

• Protein helps the body grow, repair, and stay healthy.

• It also helps in chemical reactions, transporting oxygen, and fighting diseases.

What is an Amino Acid?

• An amino acid is the basic unit of protein.

• Many amino acids join together by peptide bonds to make a protein chain.

• So, amino acids are like small building blocks that make up proteins.

Functions of Proteins :

• Structural function: Provide support and shape to the body (e.g., collagen, elastin).

• Enzymatic function: Act as enzymes to speed up chemical reactions (e.g., amylase, pepsin).

• Transport function: Carry substances through the body (e.g., hemoglobin transports oxygen).

• Regulatory function: Act as hormones to control body processes (e.g., insulin, growth hormone).

• Defense function: Protect the body against diseases (e.g., antibodies).

• Movement function: Help muscles contract and enable body movement (e.g., actin, myosin).

• Storage function: Store nutrients or minerals for later use (e.g., ferritin stores iron, ovalbumin stores amino acids).

Protein Structures

1. Primary structure:
   The order of amino acids in a chain. It decides the final shape of the protein.

2. Secondary structure:
   The chain folds or twists into shapes like alpha helix or beta sheet using hydrogen bonds.

3. Tertiary structure:
   The whole chain folds into a 3D shape. This shape gives the protein its function.

4. Quaternary structure:
   Two or more folded chains join together to make one big protein (e.g., hemoglobin).

Relationship Between the Four Levels of Protein Structure

1. Primary structure is the amino acid sequence.
   → It decides how the protein will fold into the next levels.

2. Secondary structure forms when the chain folds or coils (α-helix or β-sheet).
   → It is based on the primary structure and gives local stability.

3. Tertiary structure is the overall 3D shape made from many secondary folds.
   → The function of the protein depends on this 3D shape.

4. Quaternary structure forms when two or more tertiary chains join together.
   → It makes the protein more stable and allows teamwork between subunits.

Conclusion

Each level builds on the one before it:

Primary → Secondary → Tertiary → Quaternary.

If the lower level changes, the higher levels — and the protein’s function — will also change.

Lipids脂类 are a diverse group of organic compounds mainly composed of carbon碳(C), hydrogen氢(H), and oxygen氧(O) — sometimes also phosphorus磷(P). Because most lipids are made up of long nonpolar hydrocarbon chains非极性的碳氢链, they are hydrophobic疏水性(repel water不易与水混合).

• Component elements: C, H, O, P.

Physical Properties物理特性

Lipids form emulsions乳化液 in water — mixtures of substances that do not blend well. They can be obtained from both plant-based and animal-based foods.

Functions in the Human Body在人体内的功能

• Serve as a major energy source能量来源.

• Form structural components细胞膜的结构成分 of cell membranes.

• Act as precursors for hormones激素和信号分子 and other signalling molecules.

  
  

Main Types of Lipids(脂类的主要种类)

1. Triglycerides(三酰甘油)

2. Phospholipids(磷脂)

3. Steroids(类固醇)

   
   

1. Triglycerides三酰甘油
Triglycerides are formed from one glycerol molecule一个甘油分子 and three fatty acid chains三条脂肪酸链. They are the main form of stored fat主要储存脂肪形式 in the body and a major source of long-term energy长期能量的主要来源. When broken down, they release large amounts of ATP.

2.Phospholipids磷脂

Structure结构:
A phospholipid molecule consists of 1 glycerol一甘油分子, 2 fatty acid chains两条脂肪酸链, and 1 phosphate group with a polar head带极性的磷酸基团.

Behaviour in Water(水中的特性)
Phospholipids have amphipathic两性特征 properties:

• Their hydrophilic含磷酸基与氮原子 (water-loving heads亲水头部)are attracted to water.

• Their hydrophobic脂肪酸链 (water-fearing tails疏水尾部)avoid water.
  This unique structure allows them to form cell membranes细胞膜双层结构 — with the hydrophobic tails facing inward and the hydrophilic heads facing outward.

Example:
Phosphatidylcholine卵磷脂 — the major component of cell membranes细胞膜的主要成分.

3.Steroids（类固醇）

Structure结构:
Steroids have a carbon skeleton碳环骨架 made up of four fused rings四个相连, with various functional groups attached连接不同的官能团.

Functions:

• Important components of cell membranes that help regulate membrane fluidity细胞膜的重要组成部分.

• Act as signaling molecules调节膜的流动性(such as hormones如激素).

  Example:

  Cholesterol胆固醇
  Cholesterol is a vital steroid that胆固醇是一种重要的类固醇:

  • Forms part of animal cell membranes动物细胞膜.

  • Helps control the passage of substances调节物质进出细胞的通道 in and out of cells.

  • Serves as a precursor for bile acids, vitamin D, and steroid hormones胆汁酸、维生素D和类固醇激素的前体物质.

碳水化合物是人体的“燃料”，

像车的汽油一样，

为身体各器官提供能量，

并在细胞结构和代谢调节中起支撑作用。

From the human body perspective, carbohydrates are the body’s main and most immediate source of energy. After eating foods like rice, bread, and fruits, carbohydrates are broken down into glucose, which enters the bloodstream as blood sugar and fuels all body cells — especially the brain, nerves, and red blood cells, which rely almost entirely on glucose.

碳水化合物是人体最主要、最直接的能量来源。

当我们吃下米饭、面包、水果等食物后，

碳水化合物会在消化系统中被分解成葡萄糖（glucose）。

这些葡萄糖进入血液，成为血糖，供给全身细胞使用。

尤其是大脑、神经系统和红细胞，

几乎只依赖葡萄糖来获取能量。

Excess glucose is stored in the liver and muscles as glycogen, which can be broken down later to maintain blood sugar levels. Besides energy, carbohydrates also play structural and signaling roles in cell membranes (e.g., glycoproteins and glycolipids).

当身体有多余的葡萄糖时，

会被储存在肝脏和肌肉中，形成糖原（glycogen）；

当能量不足或禁食时，糖原会被分解以维持血糖稳定。

除了提供能量外，

碳水化合物还参与细胞结构的组成，

例如细胞膜上的糖蛋白、糖脂，以及免疫识别等功能。

_{单糖（Monosaccharides）}
单糖是最基本、最直接的糖类形式，

主要包括葡萄糖（glucose）、

果糖（fructose）和半乳糖（galactose）。
它们来自水果、蜂蜜、乳制品及体内其他糖类的分解。
单糖可直接被血液吸收进入细胞，

是人体最迅速的能量来源，

尤其为大脑与神经系统提供主要燃料。

For example, when you eat honey, it contains a large amount of fructose and glucose, both of which are monosaccharides.
These sugars do not require digestion and are directly absorbed into the bloodstream,
providing immediate energy for the brain, heart, and muscles.

例如吃蜂蜜。
蜂蜜中含有大量的果糖（fructose）

和葡萄糖（glucose），这两种都是单糖。
当我们食用蜂蜜时，这些单糖不需要再经过消化分解，

可以直接被小肠吸收进入血液，迅速转化为能量，

供大脑、心脏和肌肉细胞使用。

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Disaccharides like sucrose, lactose, and maltose come from sweets, milk, and grains. They must be broken down into monosaccharides before absorption and help maintain steady blood glucose levels.

双糖（Disaccharides）
双糖由两个单糖结合而成，

如蔗糖（sucrose）、乳糖（lactose）、

麦芽糖（maltose）。
它们主要来自甜食、牛奶、谷物发酵产物。
双糖需先在小肠内被分解成单糖后才能吸收，

用于维持血糖稳定、供应能量。
若体内缺乏相关分解酶（如乳糖酶），

会导致乳糖不耐症。

For example, when you drink milk, it contains lactose, a disaccharide made of glucose and galactose.
In the small intestine, the enzyme lactase breaks lactose down into monosaccharides,
which are absorbed into the bloodstream to supply energy and stabilize blood sugar.
If a person lacks lactase, lactose remains undigested, leading to lactose intolerance, causing bloating or diarrhea.

例如喝牛奶。
牛奶中含有大量的乳糖（lactose），

它是一种由葡萄糖（glucose）

和半乳糖（galactose）组成的双糖。
当我们饮用牛奶后，

乳糖会在小肠中由乳糖酶（lactase）分解成单糖，
然后被吸收到血液中，

用于提供能量并维持血糖稳定。
如果体内缺乏乳糖酶，

乳糖无法被分解，

就会引起乳糖不耐症，

出现腹胀、放气或腹泻等症状。

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• Polysaccharides such as starch, glycogen, and cellulose come mainly from plant foods like grains and tubers. Starch stores energy, glycogen provides backup energy in the liver and muscles, and cellulose supports digestion by promoting bowel movement.

多糖（Polysaccharides）

多糖由许多单糖组成，

如淀粉（starch）、糖原（glycogen）、

纤维素（cellulose）。
淀粉主要来自谷物、薯类等植物食物，

是人体主要的能量储备来源。
糖原储存在肝脏与肌肉，

当血糖下降时分解供能。
纤维素不能被人体消化，

但能促进肠道蠕动、维持消化健康。

For example, when you eat rice, it contains plenty of starch, a polysaccharide made up of many glucose units.After digestion by amylase enzymes in the mouth and small intestine, starch is broken down into glucose.Glucose enters the bloodstream to provide energy, while the excess is stored as glycogen in the liver and muscles,ready to be released when blood sugar drops or during physical activity.

例如吃米饭。
米饭中含有丰富的淀粉（starch），

它是一种由许多葡萄糖分子组成的多糖。
吃下米饭后，

淀粉会在口腔和小肠中被淀粉酶分解成单糖

——葡萄糖。
这些葡萄糖被吸收到血液中，

为身体各组织提供能量；
多余的葡萄糖则会在肝脏和肌肉中储存为糖原（glycogen），
当血糖下降或运动时再分解出来，

持续为身体供能。

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Carbohydrates are the body’s fuel, powering daily life and supporting cellular structure and communication.

单糖提供即时能量，

双糖维持稳定能量，

多糖负责能量储备与肠道健康。
它们共同维持人体能量供给的平衡与生命活动的正常运作。

Each nucleotide is made up of three parts: a nitrogenous base, a five-carbon sugar (either ribose or deoxyribose), and one or more phosphate groups.

1. ATP – Adenosine Triphosphate

ATP is known as the energy currency of the cell.

It consists of adenine, ribose, and three phosphate groups.

The covalent bonds between the phosphate groups store a large amount of potential energy.

When one phosphate group is removed, energy is released to fuel the body’s activities such as muscle contraction, active transport, and chemical reactions.

ATP can also be reformed from ADP and phosphate in a reverse reaction.

2. DNA – Deoxyribonucleic Acid

DNA is a double-stranded molecule that carries genetic information.

Each strand is made up of nucleotides containing deoxyribose sugar and one of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

The two strands form a double helix structure where the bases pair specifically — A pairs with T and C pairs with G.

DNA provides the instructions for protein synthesis and determines the traits of living organisms.

3. RNA – Ribonucleic Acid

RNA is single-stranded and contains ribose sugar instead of deoxyribose.

It also has uracil (U) instead of thymine.

RNA’s main role is to carry and translate genetic information from DNA into proteins.

There are three main types of RNA:

• mRNA (messenger RNA) – carries genetic code from DNA,

• tRNA (transfer RNA) – brings amino acids during protein synthesis,

• rRNA (ribosomal RNA) – forms part of the ribosome where proteins are made.

Nucleotides are the basic building blocks of nucleic acids such as DNA and RNA, and they also form important energy-carrying molecules like ATP.

Each nucleotide is made up of three parts: a nitrogenous base, a five-carbon sugar (either ribose or deoxyribose), and one or more phosphate groups.

1. ATP – Adenosine Triphosphate

ATP is known as the energy currency of the cell.

It consists of adenine, ribose, and three phosphate groups.

The covalent bonds between the phosphate groups store a large amount of potential energy.

When one phosphate group is removed, energy is released to fuel the body’s activities such as muscle contraction, active transport, and chemical reactions.

ATP can also be reformed from ADP and phosphate in a reverse reaction.

2. DNA – Deoxyribonucleic Acid

DNA is a double-stranded molecule that carries genetic information.

Each strand is made up of nucleotides containing deoxyribose sugar and one of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

The two strands form a double helix structure where the bases pair specifically — A pairs with T and C pairs with G.

DNA provides the instructions for protein synthesis and determines the traits of living organisms.

3. RNA – Ribonucleic Acid

RNA is single-stranded and contains ribose sugar instead of deoxyribose.

It also has uracil (U) instead of thymine.

RNA’s main role is to carry and translate genetic information from DNA into proteins.

There are three main types of RNA:

• mRNA (messenger RNA) – carries genetic code from DNA,

• tRNA (transfer RNA) – brings amino acids during protein synthesis,

• rRNA (ribosomal RNA) – forms part of the ribosome where proteins are made.