基础生物实验 10 - 实验四理论简介：Cell Membrane and Transport

2021-02-20 07:56 作者:追寻花火の久妹Riku 0人读过 | 我要投稿

本期内容是细胞膜与传输的基础理论部分，实验手册与实验模拟请看后两期。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。

Lab 4 - Cell Membrane and Transport

Lipid Bilayer and Membrane Proteins

The plasma membrane is composed of two layers of phospholipids and is about 5-10 nm thick;

Proteins inserted into the plasma membrane (PM) have important functions in cell communication, import and export of molecules;

Membrane is flexible enough to allow the cell to grow, change shape and move.

Lipid Bilayer

Phospholipids are amphipathic:

- Hydrophilic head including phosphate

- Hydrophobic tails composed of fatty acids

Lipid bilayer is energetically favorable;

Lipid bilayer can self-seal.

Lipid Bilayer

20% of the PM in animal cells are cholesterols;

Cholesterol molecules are amphipathic, short and rigid;

They fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated tails;

Cholesterols tend to stiffen the lipid bilayer at warm temperature, and maintains fluid at low temperature.

Membrane Proteins

- In animals, proteins constitute ~ 50% of the mass of PM;

- Membrane proteins serve many functions;

- Each type of cell membrane contains a different set of proteins, reflecting the specialized functions of the particular membrane.

Integral membrane proteins:

Transmembrane through α-helix or β-barrel.

Peripheral membrane proteins:

Anchored by an amphipathic α helix (cytosolic side);

Anchored by lipid molecules;

Anchored by other membrane proteins.

Multipass Transmembrane Proteins

- Channels are formed by multiple amphipathic α helices;

- The hydrophobic amino acid side chains (green) face the lipid, and the hydrophilic side chains (red) form a water-filled pore;

- Porin proteins in the outer membrane of bacteria form β barrel.

Carbohydrates

- Carbohydrates are always on the exterior surface;

- Attached to proteins (glycoproteins) or lipids (glycolipids);

- 2-60 monosaccharides, unique pattern for cell recognition;

- Glycocalyx: sugar coating, hydrophilic, attracts water; and also involved in cell-cell recognition and adhesion.

Glycoproteins

- Glycoproteins are involved in cell identity, communication, etc.

- HIV gp120 spike recognizes and binds to CD4 receptor (glycoprotein) and CCR5 (coreceptor) on the surface of T cells.

- Coronavirus spike (S) protein binds to ACE-2 (Angiotensin-Converting enzyme 2) receptor on the surface of endothelial cells, epithelial cells, immune cells, etc.

Selective Permeability

- Membranes are asymmetric, the two layers of phospholipids are not identical;

- Permeability of lipid bilayer to different molecules vary.

small, nonpolar molecules can pass through the membrane (fat-soluble vitamins A, D & E, steroid hormones);

Inorganic ions and polar organic molecules (amino acids, glucose) can cross the PM through a transporter or a channel.

Diffusion

- Diffusion: substance moves from high concentration to low concentration;

- Concentration gradient is a form of potential energy;

- Diffusion continues until reaches equilibrium.

Facilitated Transport

- Facilitated transport (uses diffusion): substance moves from high concentration to low concentration with the help of channels or carrier proteins;

- Channels form a pore across the PM, specific inorganic ions can diffuse through;

- Some channels are open all the time;

- Other channels are controlled (gated) external stimuli.

Facilitated Transport

- Carrier proteins: change conformation to transfer small solutes across the PM;

- Carrier proteins are typically specific for a single substance.

Channel Protein vs. Carrier Protein

Osmosis

- Osmosis: movement of water through a semipermeable membrane;

- Uneven distribution of non-penetrating solutes drives osmosis;

- Water diffuses down its concentration gradient;

- .

Tonicity and Animal Cell

- Tonicity: relative solute concentrations;

- Isotonic: equal amounts of solute inside and outside of cell;

- Hypotonic: lower solute concentration outside of the cell, lead to lysis (bursting);

- Hypertonic: higher solute concentration outside of the cell, lead to crenation (shriveling).

Tonicity and Plant Cell

- Cell wall pushes back as water enters the cell;

- Isotonic: plant cell is flaccid;

- Hypotonic: plant cell is turgid (membrane pushed against cell wall, central vacuole is full);

- Hypertonic: plant cell is plasmolyzed (membrane separates from the cell wall).

["Hyper" means high!]

In nonwoody plants, turgor pressure supports the plant;

When plant loses turgor pressure, it wilts.

Active Transport

- Active transport: movement of molecules from low concentration to high concentration;

- Example: Na+ and K+ are transported against their concentration gradients by a sodium-potassium pump.

Na⁺/K⁺-ATPase (sodium–potassium adenosine triphosphatase, also known as the Na⁺/K⁺ pump or sodium–potassium pump)

Electrochemical Gradient

- Membrane potential: voltage difference across the membrane;

- The concentration gradient of the chemical (chemical gradient) and membrane potential (electrical gradient) determine the direction of solute movement.