AP Biology 2.5 - Membrane Permeability | Dictionary of Biology (2023)

SUSTAINABLE UNDERSTANDING
IN 2
Cells have membranes that allow them to create and maintain internal environments that differ from their external environment.

LEARNING OBJECTIVE
ENE-2.C
Explain how the structure of biological membranes affects selective permeability.
EN-2.D
Describe the role of the cell wall in maintaining cell structure and function.

ESSENTIAL KNOWLEDGE
EN-2.C.1
The structure of cell membranes leads to selective permeability.
EN-2.C.2
Cell membranes separate the internal environment of the cell from the external environment.
EN-2.C.3
Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model.
EN-2.C.4
Small non-polar molecules, including N₂, THE₂, in con₂, pass freely through the membrane. Hydrophilic substances, such as large polar molecules and ions, move across the membrane via integral channels and transport proteins.
EN-2.C.5
Polar uncharged molecules, including H₂Oh, pass the film in small amounts.
EN-2.D.1
Cell walls form a structural boundary as well as a permeability barrier for certain substances in the internal environment.
EN-2.D.2
The cell walls of plants, prokaryotes and fungi are composed of complex carbohydrates.

2.5 Overview of membrane permeability

Do you know the difference betweensemipermeabilityInselective permeability? These very important concepts describe how the lipid bilayer indiscriminately repels certain molecules, while a living cell membrane contains channel proteins and transporters that actively select which substances can enter and exit a cell. These two forms of permeability allow cells to protect their DNA, create the perfect internal conditions for reactions to take place, and balance their water content.

Permeabilitydescribes how easily a molecule or substance can cross a membrane. You can think of permeability as a coffee filter. A coffee filter allows hot water and dissolved coffee particles to pass through the filter, while the larger coffee grounds are retained. A lipid bilayer looks a lot like a coffee filter.

Oflipid bilayerconsists of phospholipid molecules, each of which has ahydrophilic headattracted by water and ahydrophobic tailwhich is attracted to other non-polar molecules. The head group and the tail group to form the lipid bilayer. On both sides of the membrane is a layer of hydrophilic heads, while the center of a lipid bilayer is a highly hydrophobic core. While a coffee filter is simply a piece of paper with tiny holes, the lipid bilayer can block molecules or allow molecules through based on their chemical nature.

Charged molecules, ions and large molecules are usually blockedthrough the lipid bilayer. However, some doNon-polar molecules and gases such as oxygen and carbon dioxide can pass directly through itthe lipid bilayer.

Many things affect the permeability of a lipid bilayer, starting with the type of phospholipids used. Phospholipids made withsaturated queuesthey tend to be tightly packed, creating a less permeable bilayer. Phospholipids made withunsaturated fatty acidsd tails tend to be more fluid, making the membrane more permeable to all kinds of substances.

Other factors such astemperature and pH also affect permeabilityof a plasma membrane. For example, if we raise the temperature of the water around a cell, the phospholipids move further away and the membrane becomes more permeable. This is why individual organisms use different ratios of saturated and unsaturated fatty acids in their phospholipid membranes to keep their membranes permeable at different temperatures!

Now let's see the difference between asemipermeable membrane versus a selectively permeable membrane.

By itself, a lipid bilayer is a semipermeable membrane. A coffee filter is also a semi-permeable membrane. I willalwaysblock certain substances andalwayslet others through - simply based on the chemical and physical properties of the membrane. Substances can also be partially blockedlargelyallow a substance to pass through the membrane, orlargelyprevents a substance from passing through the membrane.

But a living cell membrane is made up of much more than just phospholipid molecules. Most importantly, the cell membrane has a number of themintegrated protein channels and carrier proteins that select specific molecules to pass through. Because these proteins are produced from the DNA code and incorporated into the cell membrane by internal cellular mechanisms, it is as if the cell itself chooses which substances pass through the membrane and which substances are not allowed to pass!

In summary: lipid bilayers are semipermeable, but cell membranes are selectively permeable!

Consider this... What exactly is the defining characteristic of life on Earth? Some scientists believe that life is defined by the ability to control your internal and external cellular environment.

All organisms on Earth – from tiny nematodes to the giant blue whale – selectively adjust the permeability of their cell membranes by incorporating specific proteins.

This ability allows cells to change their internal and external environment. The only real difference between organisms is variations in their DNA codes that produce different membrane proteins and enzymes. Nematodes produce proteins that allow them to survive and reproduce, and a blue whale does essentially the same thing!

So everything has been done so farHowCell membranes are selectively permeable membranes. Now let's seeWhycellsneedselectively permeable membranes. The final answer is thisCells must establish viable cell conditions.

To better understand this, let's look at its meaningsdistribution(where the particles are uniformly distributed in a solvent) andosmosis(where water moves through a semi-permeable membrane).

If you drop a few sugar molecules into a body of water, those sugar molecules will eventually be distributed evenly throughout the water. Imagine an algae cell that produces sugar.

The algal cell produces glucose in the chloroplast and exports this sugar into the cytoplasm. Here, enzymes can process the sugar and mitochondria can process smaller pieces of the sugar to store energy as ATP. Mitochondria then export this chemical energy to power processes throughout the cell.

Now imagine what would happenwithout cell membrane. First of all, there is no way to hold all these organelles together. So they would just walk away. Furthermore, any glucose produced by the chloroplasts will simply diffuse into the environment. Cell membranes therefore essentially give cells the ability to gather the substances they need to survive.

Now let's look at osmosis. Water molecules are attracted to polar solutes, in the same way that polar solutes are attracted to water. Thus, if you divide a body of water with a semipermeable membrane that blocks larger solutes, the water itself will move through the membrane to equalize the concentration on both sides of the membrane.

Cells need selectively permeable membranes becauseosmosis. Not only do cells need to keep solutes in the cells, but they also need to control the water, pH, and specific concentration of solutes in their cytoplasm. To do this, cells load their cell membranes with proteins responsible for importing or exporting the correct substance. In addition, some cells have acontractile vacuolewhich collects water from the cell and drains the water to the environment.

Basically, cells use many different proteins in their cell membranes to create a selectively permeable membrane and control their internal environment!

Selective permeability is oneimmediate consequenceof the semipermeable membrane and membrane-embedded proteins - also known asliquid mosaic model. There are many components within the cell membrane that affect the permeability of a cell.

The basic semipermeability of the cell membrane comes from phospholipid molecules. Phospholipid molecules stick together through hydrogen bonding and nonpolar interactions to form the lipid bilayer, which naturally repels molecules of a certain size and chemical composition.

The membrane becomes selectively permeable due to the work of proteins. Proteins come in many different types, which have different functions in maintaining the internal cellular environment. Some proteins allow water to move freely. Others are like gated gates that can select when ions can move through the membrane. Some proteins import and export larger molecules to and from the cell. We will discuss all these membrane proteins further in sections 2.6-2.9.

A final component of the cell membrane that drastically affects permeability is cholesterol. Cholesterol is a small lipid steroid molecule found in the plasma membrane. Lipid molecules cluster around cholesterol molecules, constricting the phospholipids and creating a less permeable membrane.

Together, all of these elements allow the cell to control exactly what enters and exits the cells - even in very different environmental conditions!

Now that we understand the basics of how selective permeability is created, let's see what types of molecules cells allow and reject!

Let's start small,non-polar molecules. Molecules such as oxygen can easily pass through the cell membrane. This is important because most eukaryotic cells require oxygen to complete cellular respiration and store energy in ATP. Similarly, small non-polar carbon dioxide molecules can easily diffuse through the cell membrane. This is important for the process of respiration, which allows oxygen to be transported from the blood to the cells and carbon dioxide to be taken out into the bloodstream and out of the lungs.

many smalluncharged polar moleculeslike water are only slightly blocked. These molecules are polar, so it is not as easy for them to work through the hydrophobic core of the lipid bilayer.

The last two teams -large polar molecules and ions– you have a very hard time getting through the lipid bilayer. Larger molecules are usually blocked by their size and polarity, although they can sometimes slip through the bilayer. Ions, on the other hand, may be very small but are completely repelled by their charged nature.

However, cells still need to move these substances. This is truemembrane proteinsGet into the game. For example, glucose molecules can be introduced into cells via specialized glucose carrier proteins. These proteins have a glucose-specific active site, so only glucose molecules can enter. Similarly, hydrogen ions play an important role in ATP production. Proton pumps move hydrogen ions to one side of the membrane, creating a chemical gradient. The proteinATP-synthasecan use the energy stored in this step to assemble ATP molecules.

As we discussed in previous sections, cell walls are produced by many different organisms.

Usually, cell walls are composed of complex carbohydrates such ascellulose, chitin to peptidoglycan. Although these molecules have evolved into very different life forms, all of these molecules are based on 6-carbon rings of glucose! This also means that these fibers store a lot of energy.

Plants and algae use cellulose to make walls, arthropods and fungi use chitin, and prokaryotes such as bacteria use peptidoglycans as the basic molecules for their cell walls. Although we will go into the details of these organisms later in this lesson, the cell walls of each of these organisms affect the permeability of the cell membrane in similar ways.

Generally,cell walls do not affect the permeability of small molecules. Small molecules can easily slip through the fibers of the cell wall. If they are non-polar, they can easily enter the cell.

Secondly,cell walls easily block much larger molecules. Large molecules are often entangled in this extra layer of fibers. So in addition to the structural support that cell walls provide, they also help organisms filter the environment around them and accept only the molecules they need to survive!