（主动运输把物质从低浓度送到高浓度，需要 ATP 和载体蛋白。）

Main Functions

• To maintain ion balance and homeostasis inside and outside the cell
  （维持细胞内外离子浓度平衡与稳态）

• To support nerve impulse transmission and muscle contraction
  （支持神经信号传导与肌肉收缩）

• To allow the cell to absorb necessary substances and remove excess or harmful substances
  （让细胞吸收需要的物质，并排出多余或有害物质）

Primary Active Transport（初级主动运输）

Uses ATP directly to pump ions against the gradient.

（直接使用 ATP 逆浓度梯度运输物质。）

Main ions transported（主要运输离子）： Na⁺、K⁺、Ca²⁺、H⁺

Example（例子）： Na⁺/K⁺ pump

Secondary Active Transport（次级主动运输）

Secondary active transport utilizes the potential energy stored in the electrochemical gradients of ions to drive the transport of another molecule (cotransport). These ion gradients are established and maintained by carrier proteins that use ATP in primary active transport.

（次级主动运输利用离子电化学梯度中储存的势能来推动另一种分子的运输；这些浓度梯度由初级主动运输使用 ATP 的载体蛋白所建立和维持。）

📌Key Features（重点特点）

• Does not use ATP directly, but depends on ATP indirectly
  （不直接使用 ATP，但依赖 ATP 建立的浓度梯度）

• Uses ion gradients (usually Na⁺ gradient) as an energy source
  （使用离子梯度，通常是 Na⁺ 梯度作为能量来源）

• Moves two substances at the same time (cotransport)
  （可一次运输两种物质，即协同运输）

Types of cotransport（两种协同运输方式）

1. Symport (or cotransport) moves two substances across the membrane in the same direction.
   （同向运输会将两种物质沿同一个方向同时穿过细胞膜。）

2. Antiport (or countertransport) moves two substances in opposite directions, exchanging one molecule for another across the membrane.
   （反向运输则让两种物质沿相反方向移动，像交换一样穿过细胞膜。）

Steps of Active Transport（主动运输的运作步骤）

• A molecule or ion first binds to a specific carrier protein on the cell membrane.
  （分子或离子会先结合在细胞膜上的特定载体蛋白。）

• ATP is hydrolyzed to provide energy for the protein to change its shape.
  （ATP 被水解后提供能量，使蛋白质改变形状。）

• The carrier protein pumps the substance across the membrane, moving it to the other side against the gradient.
  （载体蛋白将物质逆浓度梯度运送到膜的另一侧。）

• The protein then returns to its original shape and the cycle repeats.
  （蛋白质恢复原本形状，过程会重复进行。

Main Substances Transported

• Ions such as Na⁺, K⁺, Ca²⁺, and H⁺
  （如钠、钾、钙、氢等离子）

• Nutrients such as glucose and amino acids
  （如葡萄糖和氨基酸等营养分子）

Definition:
Simple diffusion is a type of passive transport where molecules move from an area of high concentration to an area of low concentration, until both sides reach equilibrium.
This process does not require energy (ATP) — molecules move naturally due to their random kinetic motion.

Why It Happens：

Simple diffusion occurs because molecules are always in constant motion.
When there is a concentration difference (called a concentration gradient), particles move from where they are crowded to where they are fewer in number, in order to achieve balance.
This natural movement continues until the concentration becomes equal on both sides of the membrane.
Therefore, simple diffusion is driven by molecular kinetic energy and entropy (the tendency toward balance and disorder), not by any cellular work.

Function and Mechanism：

The main function of simple diffusion is to allow small, nonpolar molecules to move freely across the phospholipid bilayer of the cell membrane.
Because the lipid layer is hydrophobic (nonpolar), only nonpolar or very small molecules can pass easily through it.
It’s a passive, self-regulating process that helps maintain the internal environment of the cell.

Substances Transported:

Molecules that move through simple diffusion include:

• Oxygen (O₂) – diffuses into cells for cellular respiration.

• Carbon dioxide (CO₂) – diffuses out of cells as a waste product.

• Small lipid-soluble substances, such as fatty acids, steroids, and alcohol.

  
  

Role in the Human Body:

In the lungs, oxygen diffuses from alveoli into the bloodstream, while carbon dioxide diffuses from blood into alveoli to be exhaled.
In tissues, oxygen diffuses from blood capillaries into cells to support metabolism.
This process ensures efficient gas exchange, stable internal balance, and continuous energy production within the body.

Summary

Simple diffusion is:

• Passive – requires no ATP.

• Selective – allows only small, nonpolar molecules to pass.

• Essential – maintains homeostasis by allowing gas exchange and waste removal.

It happens naturally because molecules always move randomly and spread out evenly — this is a fundamental law of physics that keeps our cells alive.

• Phagocytosis 吞噬作用– This is the process of engulfing large, solid particles such as bacteria into the cell for immune purposes. Extensions of the cytoplasm, termed pseudopodia (‘false feet’), sense, surround and enclose the target, creating a vacuole or phagosome on the inside of the cell membrane. This allows the process of phagocytosis to be highly specific.

• Pincytosis 饱饮作用 – This describes the non-specific uptake of fluid surrounding the cell, allowing it to take in nutrients such as ions, enzymes and hormones. In this process, the cell membrane invaginates, before budding off to create a vesicle known as a pinosome.

• Receptor-mediated endocytosis 受体介导的内吞作用 – uptake of specific target substances, such as iron, via their receptor. Receptors cluster in regions termed coated pits, as they are coated with proteins such as clathrin. Clathrin causes the coated pit to invaginate and become a vesicle, bringing the desired ligand into the cell. This process can be hijacked to allow for toxins to enter the cell, such as cholera.

What cells take in through Endocytosis?

1. Phagocytosis ("Cell Eating")

· Pathogens (Bacteria & Viruses): Immune cells like macrophages and neutrophils constantly patrol the body, engulfing and destroying invading bacteria and other pathogens.

· Apoptotic Cells (Dead Cells): They also clear away the body's own dead or dying cells, a crucial part of tissue maintenance and remodeling.

· Cellular Debris: They remove debris from damaged tissue, acting as the body's clean-up crew.

2. Pinocytosis ("Cell Drinking")

· Nutrients in Solution: Cells in the lining of the intestine and kidney tubules constantly "sip" the fluid around them to absorb dissolved nutrients, ions, and sugars.

· Electrolytes: Uptake of essential ions from the extracellular fluid.

3. Receptor-Mediated Endocytosis (Highly Specific)

· Cholesterol: This is a classic example. Cholesterol in the blood is packaged into particles called Low-Density Lipoproteins (LDL). Body cells have LDL receptors on their surface that specifically bind to these particles and bring them inside for processing. (Mutations in this receptor cause familial hypercholesterolemia).

· Iron: Iron is transported in the blood bound to a protein called transferrin. Cells that need iron (e.g., for making hemoglobin) have transferrin receptors that bring the iron-transferrin complex into the cell.

· Hormones & Growth Factors: Hormones like insulin and growth factors bind to specific receptors on their target cells, which are often internalized via endocytosis. This not only allows the signal to propagate inside the cell but also helps regulate the sensitivity of the cell by removing the receptors from the surface.

· Viruses Exploiting the System: Some viruses, like the Influenza virus and HIV, have evolved to trick the cell. Their surface proteins bind to specific human cell receptors, triggering receptor-mediated endocytosis and allowing the virus to be unknowingly brought into the cell.

In some cases, the vesicle will only fuse temporarily before reforming on the interior of the cell. Alternatively, the vesicle may fuse completely, becoming a permanent part of the cell membrane.

Exocytosis is used in many areas of the body, including neurotransmitter release at synapses or release of secretions in the sweat glands.  Glands which secrete their products via exocytosis are termed merocrine.

There are two primary functions of exocytosis:

1. Secretion

Cells use exocytosis to secrete products that are needed elsewhere in the body.

· Examples:

· Pancreatic cells secreting insulin into the bloodstream.

· Neurons (nerve cells) releasing neurotransmitters to send signals to other neurons.

· Salivary glands secreting saliva enzymes.

2. Membrane Repair and Growth

By adding the vesicle's membrane to the cell membrane, exocytosis replaces membrane lost during endocytosis and allows the cell to grow. It's also crucial for repairing tears in the cell membrane.

What transports out through Exocytosis?

1. Secretion of Hormones and Enzymes

· Insulin: Beta cells in the pancreas produce and store the hormone insulin. When blood sugar rises, they release insulin into the bloodstream via exocytosis to signal other cells to take up glucose.

· Digestive Enzymes: The pancreas secretes enzymes like amylase and trypsin into the pancreatic duct, which leads to the small intestine, where they break down food.

· Saliva and Mucus: Salivary glands secrete salivary enzymes (e.g., amylase) via exocytosis. Goblet cells in the respiratory and digestive tracts secrete protective mucus the same way.

2. Neurotransmission (Cell Communication)

· Neurotransmitters: This is one of the fastest and most crucial examples. When a nerve signal reaches the end of a neuron, it triggers vesicles filled with neurotransmitters (like dopamine, serotonin, or acetylcholine) to fuse with the cell membrane and release their contents into the synaptic cleft, passing the signal to the next neuron or muscle cell.

3. Secretion of Structural Proteins

· Collagen and Elastin: Fibroblasts in connective tissue secrete these proteins via exocytosis to build and maintain the extracellular matrix, which provides structural support to tissues and organs.

4. Plant-Specific Examples

· Cellulose: Plant cells secrete cellulose and other polysaccharides into their cell walls through exocytosis to build and strengthen the rigid cell wall.

· Wood and Bark Formation: The secretion of lignin and other complex polymers that make up wood and bark occurs through exocytosis.

5. Waste Removal

· Undigested Residues: In some protists, like the paramecium, a vacuole containing undigested waste travels to the cell membrane and fuses with it (exocytosis) to expel the contents.

Introduction

• Different types of transport across the cell membrane

• First one: Simple Diffusion (passive, small molecules)

• Today focus: third type – Facilitated Diffusion (requires proteins, selective)

Diffusion / Review Diffusion

• Diffusion: spontaneous movement of substances from high → low concentration

• Driven by concentration gradient

• Some molecules too large, polar, or charged → cannot pass through lipid bilayer

• Facilitated diffusion helps these molecules cross the membrane

Definition

• Type of passive transport

• Requires carrier or channel proteins embedded in the cell membrane

• Moves substances from high → low concentration without using energy

• Selective transport – only specific molecules are allowed

Key Features

1. Depends on specific carrier or channel proteins

2. Moves substances down concentration gradient (High → Low)

3. Does not require ATP or other energy sources (Passive)

4. Selective – only certain molecules like glucose, ions, water can pass

5. Occurs in many types of cells for essential molecules

Examples

• Ion channels for Na⁺, K⁺, Ca²⁺

• Water channels (Aquaporin) for rapid water movement

• Glucose transporter (GLUT) for glucose uptake

• Important for nutrient uptake, cell signaling, and maintaining homeostasis

Conclusion

• Facilitated diffusion assists molecules that cannot cross the membrane directly

• Essential for proper cell function and survival

Osmosis is the passive movement of water (solvent) across a semipermeable membrane from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration). No metabolic energy (ATP) is required.

渗透是水（溶剂）通过半透膜的被动移动，方向是从溶质浓度较低（水浓度高）的一侧，流向溶质浓度较高（水浓度低）的一侧。此过程不需要细胞的代谢能（不耗ATP）。

Key components:

• Solvent / 溶剂：通常指水（H₂O）。

• Solute / 溶质：溶解在水中的物质（如Na⁺(sodium ion钠离子)、Cl⁻(chloride ion氯离子)、葡萄糖等）。

• Semipermeable membrane / 半透膜：允许水分子通过，但阻挡某些溶质颗粒（按大小或电荷选择通过）。

• Net movement / 净移动：是水分子的总体移动方向；个别分子可能双向移动，但净效果是单向。

Driving force — chemical potential / 驱动力：化学势与浓度梯度

Osmosis is driven by differences in water chemical potential or water concentration across the membrane. Water moves toward the side with lower water potential (often the side with higher solute concentration) until equilibrium (equal chemical potential) or until a counteracting pressure balances the movement.

Water potential 水势（常用于植物学）

Water potential=solute potential+ pressure potential. Water moves from higher to lower

Tonicity（张力，与生理相关）— isotonic / hypotonic / hypertonic

• Isotonic等张: extracellular solute concentration ≈ intracellular → no net water movement → cell size stable.

• Hypotonic低渗: extracellular solute concentration < intracellular → water flows in → cell swells, may lyse (burst).

• Hypertonic高渗: extracellular solute concentration > intracellular → water flows out → cell shrinks (crenation in RBCs).

Osmotic Pressure（渗透压）

Osmotic pressure is the pressure required to stop the movement of water across a semipermeable membrane.
It represents how strongly a solution “pulls” water into it due to the presence of solute particles.
The higher the solute concentration, the greater the osmotic pressure.

Example:

• Pure water has the lowest osmotic pressure (almost zero).

• A solution containing a large amount of salt or sugar has a higher osmotic pressure and will attract water into that region.

• When a cell is placed in a hypertonic solution, the external osmotic pressure is high → water moves out of the cell → the cell shrinks.

• When a cell is placed in a hypotonic solution, the external osmotic pressure is low → water enters the cell → the cell swells.