本期内容是 实验四：细胞膜与传输 的实验手册，实验模拟请看下一期。本部分内容来自 University of California, Berkeley - UC Berkeley Extension, 虚拟实验的内容来自 Labster. 本部分内容均不会标记为为原创，但由于是UP主购买的课程，因此不接受非授权的转载，谢谢您的理解。

每一个生物基础实验均会分为三部分：第一部分为实验的生物理论；第二部分为实验的指导手册；第三部分为 Labster 的虚拟实验模拟。第一部分的基本信息由 Ying Liu, Ph.D. 提供，第二部分的实验手册来自 Labster, 第三部分的实验模拟过程由UP主操作。

Virtual Lab Manual 4

Cell Membrane and Transport: Learn how transporters keep cells healthy

Synopsis

Join Dr. B.I.O. Hacker in her synthetic biology lab, where she wants to change the world! In this simulation, you will learn about the structure and function of the cell membrane, and discover why membrane transporters are vital for healthy cells and the function of organ systems.

The synthetic biology lab

Your mission begins in the synthetic biology lab. Here you will meet Dr. Hacker, who will introduce you to the concept of selective permeability and the fluid mosaic models of the plasma membrane. Together, you will explore why cells need specialized transporter proteins to transport cargo molecules across their membranes.

Transport molecules into a virtual cell

Next, you will teleport to a virtual cell, where you will explore how different types of molecules can cross the cell membrane. While some molecules are able to diffuse across the cell membrane, most molecules require a transporter protein to enter or leave the cell. Explore the different channels, carriers, and pumps that exist in the membrane and how they ensure that only the right molecules enter under the right conditions.

Apply your knowledge

Return to the lab to test whether inserting a transporter protein in the membrane would help certain molecules to enter the cell. To do so, you will set up a fluorescence microscopy experiment to measure transport in living cells. Finally, discover how some transporter proteins do not only keep the cell healthy but also help organ systems to function. From filtration in the kidneys to the contraction of muscles during exercise, membrane transport contributes to many processes. Can you find out how?

Learning Objectives

At the end of this simulation, you will be able to…

● Describe the plasma membrane structure using the fluid mosaic model

● Recognize the relative permeability of lipid bilayers to different classes of molecule

● Compare active and passive transport of molecules

● Identify the 3 modes of active transport and the different classes of ion channel and carrier molecules

● Relate the expression of specific transport proteins to the cell’s role

Techniques in Lab

● Experimental design

● Microscopy sample preparation

● Fluorescence Microscopy

● Data interpretation

● Interactive learning activities

Theory

Cell membrane

The plasma membrane, which is also called the cell membrane, has many functions. The most basic of which is to define the borders of the cell and maintain cell function. The plasma membrane is selectively permeable. This means that the membrane allows some materials to freely enter or leave the cell, while other materials cannot move freely, but require the use of a specialized structure, and occasionally, even energy investment to pass.

Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors. These proteins act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate signal transduction response cascades when their effectors are bound.

The fluid mosaic model (see Figure below) describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness.

The membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the interior of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell.

Membrane transport

Due to the structure of the cell membrane, only small hydrophobic molecules can easily diffuse across the membrane:

All other types of molecules that need to either enter or leave the cell, such as nutrients or waste products, need specialized proteins to cross the membrane. This transport may be passive (facilitated diffusion - does not require energy) or active (requires energy). When there is a difference in concentration and/or charge between both sides of the membrane, molecules will pass through a channel or carrier protein until the concentration and/or charge is balanced on both sides. The gradient of concentration and charge is known as the electrochemical gradient. Transport of molecules moving down the electrochemical gradient, from high to low concentration, is therefore known as passive transport or facilitated diffusion. Transporters that transport molecules against the electrochemical gradient, however, need energy. This is known as active transport. Primary active transport uses energy directly (from ATP or light), while secondary active transport uses an electrochemical gradient to drive transport. Secondary active transport does not require energy directly but relies on the electrochemical gradient established by primary active transport.

In addition to transporter-mediated transport across the membrane, sections of membrane can form an enclosed structure known as a vesicle. This allows the cell to secrete molecules into the extracellular environment (exocytosis) or take molecules up from the extracellular environment (endocytosis).

本期内容到此结束，感谢阅读！下一期将进行 Labster 实验！