6 INTRODUCTION

Unit 2 Topic 2 Movement of Substances across Membranes—Passive

Have you been through airport security lately? If you have, you’ve probably noticed that it’s carefully designed to let some things in (such as passengers with tickets) and to keep others out (such as weapons, explosives, and bottled water). Flight attendants, captains, and airport personnel travel through quickly via a special channel, while regular passengers pass through more slowly, sometimes with a long wait in line.

In many ways, airport security is a lot like the plasma membrane of a cell. Cell membranes are selectively permeable, regulating which substances can pass through, as well as how much of each substance can enter or exit at a given time. Selective permeability is essential to cells’ ability to obtain nutrients, eliminate wastes, and maintain a stable interior environment different than that of the surroundings (maintain homeostasis).

Some cells require larger amounts of specific substances and must have a way of obtaining these materials from extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms to facilitate transport. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate (ATP), to obtain these materials. Interestingly, most cells spend a lot of their energy (approximately one third) on maintaining an imbalance of sodium and potassium ions between the cell’s interior and exterior.

The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement of substances from an area of higher concentration to an area of lower concentration. A concentration gradient is a just a region of space over which the concentration of a substance changes, and substances will naturally move down their gradients, from an area of higher to an area of lower concentration. In cells, some molecules can move down their concentration gradients by crossing the lipid portion of the membrane directly, while others must pass through membrane proteins in a process called facilitated diffusion (Figure 2-25). In Unit 2 Topic 2, we’ll look in more detail at membrane permeability and different modes of passive transport. In Unit 2 Topics 3 and 4, we will discuss active transport and how it relates to passive transport.

Figure 2-25: Transport of substances across the plasma membrane can be via passive transport (simple and facilitated diffusion) or active transport. By LSumi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=118132547

Learning Objectives

By the end of this topic, you will be able to:

• Describe diffusion and the factors that affect how materials move across the cell membrane
• Explain why and how passive transport occurs
• Describe the process of osmosis and explain how concentration gradient affects osmosis
• Define tonicity and describe its relevance to osmosis in humans and plants
• Describe the need for facilitated diffusion
• Describe the mechanisms of facilitated diffusion and the proteins involved
• Integrate your knowledge from Unit 2 Topic 1 by drawing your own fluid mosaic bilayer and hydropathy plots of membrane transport proteins.

Reading and Activities

Please read the following text and complete the integrated activities as you read. Make your own notes and keep a copy for study purposes. You will find your graded “Check-In Questions” at the end of each Topic.

Instructions

• Read and take notes from the Unit 2 Topic (2.2) readings below.
• Complete Unit 2 Topic 2 Learning Activities 2.2.1 through 2.2.5
• Complete Unit 2 Topic 2 Enrichment Activities 2.2.1 through 2.2.4

 

To Do List

Here is a checklist of the individual learning activities you will be completing in Unit 2, Topic 2. You may find it useful for planning your work.

Key

Code	Meaning
MR	Mandatory Reading + Videos + CQs
MA	Mandatory Activity (Document your work!)
OA	Optional/Extra Practice Activity
RA	Recommended Activity
MEA	Mandatory Enrichment Activity
EA	Enrichment Activity

 

*Note for #6 on the following list: Please document your work (generate reports, save online work, take pictures and videos, use illustrations, etc.), since you will be asked to present evidence of having done at least one of these activities as part of your Unit 2 assignment.

 

Selective Permeability

Recall that plasma membranes are amphiphilic, containing hydrophilic and hydrophobic regions (see Unit 2 Topic 1). This characteristic helps move some materials through the membrane and hinders the movement of others. Non-polar and lipid-soluble material with a low molecular weight can easily slip through the membrane’s hydrophobic lipid core. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs and hormones also gain easy entry into cells and readily transport themselves into the body’s tissues and organs. Oxygen and carbon dioxide molecules have no charge and pass through membranes by simple diffusion.

Polar substances present problems for the membrane. While some polar molecules connect easily with the cell’s outside, they cannot readily pass through the plasma membrane’s hydrophobic lipid core. Water molecules, for instance, cannot cross the membrane rapidly (although thanks to their small size and lack of a full charge, they can cross at a slow rate). Additionally, while small ions could easily slip through the spaces in the membrane’s mosaic, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have special means of moving through plasma membranes. Larger charged and polar molecules, like sugars and amino acids, also need the help of various transmembrane proteins to facilitate transport across plasma membranes.

Please complete Enrichment Activity 2.2.1. Go to the Unit 2 Topic 2 Enrichment Activities tab on your BIOL 2131 Moodle site.

 

Passive Transport: Diffusion, Osmosis, and Facilitated Diffusion

 

Diffusion

Diffusion is a passive process of transport where a single substance moves from a high concentration to a low concentration until the concentration is equal across the space. You are familiar with the diffusion of substances through the air. For example, think about someone opening a bottle of perfume in a room filled with people. The perfume smell is at its highest concentration in the bottle. Its lowest concentration is at the room’s edges. The perfume vapor will diffuse or spread away from the bottle, and gradually, increasingly more people will smell the perfume as it spreads. Materials move within the cell’s cytosol by diffusion, and certain materials move through the plasma membrane by diffusion (Figure 2-23). Diffusion expends no energy.

The left part of Figure 2-26 shows a substance on one side of a membrane only in the extracellular fluid. The middle part shows that, after some time, some of the substance has diffused across the plasma membrane, from the extracellular fluid, and into the cytoplasm. The right part shows that, after more time, an equal amount of the substance is on each side of the membrane.

Figure 2-26: Diffusion. Simple diffusion through a semipermeable membrane moves a substance from a high concentration area (extracellular fluid, in this case) down its concentration gradient (into the cytoplasm). https://alg.manifoldapp.org/read/fundamentals-of-cell-biology/section/a762477a-0cf1-4ff5-ae0c-0a86c45ea15e

Each separate substance in a medium, such as the extracellular fluid, has a unique concentration gradient, independent of other materials’ concentration gradients. Each substance will diffuse, passively, according to that gradient. Thus, over time, the net movement of molecules will be out of the more concentrated area and into the less concentrated one, until the concentrations become equal (at which point, it’s equally likely for a molecule to move in either direction). 

A concentration gradient itself is a form of stored (potential) energy, and this energy is used up as the concentrations equalize. Within a system, there will be different diffusion rates of various substances in the medium. Other factors being equal, a stronger concentration gradient (larger concentration difference between regions) results in faster diffusion. Thus, in a single cell, there can be different rates and directions of diffusion for different molecules. For example, oxygen might move into the cell by diffusion, while at the same time, carbon dioxide might move out according to its own concentration gradient.

Factors that affect diffusion

Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another. This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:

• Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.
• Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules.
• Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion.
• Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, diffusion increases. Because cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm’s density will inhibit the movement of the materials. An example of this is a person experiencing dehydration. As the body’s cells lose water, the rate of diffusion decreases in the cytoplasm, and the cells’ functions deteriorate. Neurons tend to be very sensitive to this effect. Dehydration frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the cells.
• Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.
• Surface area and thickness of the plasma membrane: Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it.
• Distance travelled: The greater the distance that a substance must travel, the slower the rate of diffusion. This places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot reach or leave the center of the cell. Therefore, cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes.
• Please complete the following activities:
• Add H5P 2.2.1

Discuss why the following affect the rate of diffusion: molecular size, temperature, solution density, and the distance that must be traveled.

Unit 2 Topic 2 Activity 1 (2.2.1)

Introduction

View an animation of the diffusion process in action.

Instructions

Go to Cell Membrane Passive Transport at https://www.youtube.com/watch?v=JShwXBWGMyY (4:05 min). Watch the video, make your own notes and keep a copy for study purposes. Answer the following questions:

• What aspects of passive transport are controlled by phospholipids and protein channels?
• What aspects of passive transport are NOT controlled by phospholipids and protein channels?
• What are three types of transport that occur as a result of diffusion?
• What type of integral membrane proteins facilitate the diffusion of molecules across the membrane by water soluble substances?
• What types of water-soluble substances are transported by facilitated diffusion?
• Why is each channel protein lined by specific amino acids?
• What other feature of each channel protein selects which substance can pass?
• How do carrier proteins differ from channel proteins?

 

Osmosis

Osmosis is the movement of water through a semipermeable membrane according to the water’s concentration gradient across the membrane, which is inversely proportional to the solutes’ concentration. While the term diffusion refers to the transport of material (other than water) across membranes and within cells, the term osmosis refers specifically to the transport only of water across a membrane. Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.

Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane, the water level is the same, but there are different concentrations of solutes on each side of the membrane, and the different solutes cannot cross the membrane. In Figure 2-27, there is a container whose contents are separated by a semipermeable membrane. Initially, there is a high concentration of solute on the right side of the membrane and a low concentration of the left. Over time, water diffuses across the membrane toward the side of the container that initially had a higher concentration of solute (lower concentration of water not bound to solute). As a result of osmosis, the water level is higher on this side of the membrane, and the solute concentration is the same on both sides.

Figure 2-27: The movement of water through a semipermeable membrane. In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram, the solute cannot pass through the selectively permeable membrane, but the water can. Note at the beginning, the volume of water is the same, but the concentration of solute-unbound water is greater on the left because there is less solute. There are fewer solute-unbound water molecules on the right because there is so much more solute. Therefore, there is a higher concentration of “free” water molecules on the left than on the right of the membrane in the first beaker. https://alg.manifoldapp.org/read/fundamentals-of-cell-biology/section/a762477a-0cf1-4ff5-ae0c-0a86c45ea15e

 

The beaker has a solute mixture on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the water’s concentration gradient goes to zero or until the water’s hydrostatic pressure balances the osmotic pressure. Hydrostatic pressure is the pressure at a point in a column of fluid. Osmosis constantly proceeds in living systems.

 

Insert H5P 2.2-2

 

Tonicity

Tonicity describes the amount of solute in a solution. The measure of the tonicity of a solution, or the total amount of solutes dissolved in a specific amount of solution, is called its osmolarity. Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells. In a hypotonic solution, such as tap water, the extracellular fluid has a lower concentration of solutes than the fluid inside the cell, and water enters the cell. (In living systems, the point of reference is always the cytoplasm, so the prefix hypo- means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm.) It also means that the extracellular fluid has a higher concentration of water than does the cell. In this situation, water will follow its concentration gradient and enter the cell. This may cause an animal cell to burst or lyse.

In a hypertonic solution (the prefix hyper- refers to the extracellular fluid having a higher concentration of solutes than the cell’s cytoplasm), the fluid contains less water than the cell does, such as seawater. Because the cell has a lower concentration of solutes, the water will leave the cell. In effect, the solute is drawing the water out of the cell. This may cause an animal cell to shrivel, or crenate.

In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the concentration of solutes of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell. Blood cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances (Figure 2-28).

Figure 2-28: Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. (credit: modification of work by Mariana Ruiz Villarreal) https://openstax.org/books/concepts-biology/pages/3-5-passive-transport

 

Insert H5P 2.2-3

Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic compared to the cellular environment, and water will always enter a cell if water is available. This influx of water produces turgor pressure, which stiffens the cell walls of the plant (Figure 2-29). In nonwoody plants, turgor pressure supports the plant. If the plant cells become hypertonic, as occurs in drought or if a plant is not watered adequately, water will leave the cell. Plants lose turgor pressure in this condition and wilt.

Figure 2-29: The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in. (credit: modification of work by Mariana Ruiz Villarreal) https://openstax.org/books/concepts-biology/pages/3-5-passive-transport#fig-ch03_05_03

 

 

 

Facilitated Transport

In facilitated transport or facilitated diffusion, materials that cannot use simple diffusion, are transported passively across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are polar molecules or ions that the cell membrane’s hydrophobic parts repel. Facilitated transport proteins shield these materials from the membrane’s repulsive force, allowing them to diffuse into the cell.

The transported material first attaches to protein or glycoprotein receptors on the plasma membrane’s exterior surface. The substances then pass through specific integral proteins to move into the cell. Some of these integral proteins form a pore or channel through the phospholipid bilayer; others are carrier proteins that contain a binding site for a specific substance to aid its diffusion through the membrane.

Channels

The integral proteins involved in facilitated transport are types of transport proteins, and they function as either channels or carriers/transporters for the material. Channels are specific for the transported substance and have hydrophilic domains exposed to the intracellular and extracellular fluids with hydrophilic channels (or pathways) through their core that provides a hydrated opening through the membrane layers (Figure 2-30A). As such, channels are often described as “pores” in the membrane. Passage through the channel allows polar compounds to avoid the plasma membrane’s nonpolar central layer that would otherwise slow or prevent their entry into the cell. Three types of transmembrane protein channels include ion channels, porins and aquaporins.

Figure 2-30: Channel proteins. (A) Facilitated transport moves substances down their concentration gradients. The substance may cross the plasma membrane with the aid of channel proteins. (B) View of aquaporin channel protein from the top. Aquaporins move water from the extracellular space to the cytoplasm in some cells of the body.

Channel proteins consist of two forms; one form is open at all times allowing substances to move with the gradient (referred to as leakage channels); the second form is “gated,” which controls the channel’s opening and closing. No matter the form, channel proteins will facilitate the passive diffusion of substances with the concentration gradient. Aquaporins are channel proteins that are opened at all times to allow water to pass through the membrane at a very high rate (Figure 2-30B). Alternatively, an example of a gated channel is when a particular ion attaches to the channel protein, and controls the opening, or other mechanisms or substances may be involved (Figure 2-31). In some tissues, sodium and chloride ions pass freely through open channels, whereas, in other tissues, a gate must open to allow passage. An example of this occurs in the kidney, where there are both channel forms in different parts of the renal tubules. Cells involved in transmitting electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes. Opening and closing these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting in facilitating electrical transmission along membranes (in the case of nerve cells) or in muscle contraction (in the case of muscle cells).

Figure 2-31: Gated Ion Channel Proteins. When they are closed, no ions can pass through them. However, when a channel opens, select ions diffuse through the channel. Channel proteins are highly specific, letting only a specific ion or subset of ions pass. Credit: Rao, A., Ryan, K., Tag, A. and Fletcher, S. Department of Biology, Texas A&M University. https://openstax.org/books/biology-2e/pages/5-2-passive-transport

There are the following three types of gated channels, which will be explored in more detail in Unit 2 Topic 4:

• Voltage gated
• Ligand gated
• Mechanically gated

Porins are called beta barrel proteins that cross a cellular membranes and act as a pore, through which molecules can diffuse (Figure 2-32). In protein structures, a beta barrel is a beta-pleated sheet composed of tandem repeats (several adjacent copies) that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrogen bond). They are present in the outer membrane of gram-negative bacteria and some gram-positive mycobacteria (mycolic acid-containing actinomycetes), the outer membrane of mitochondria, and the outer chloroplast membrane. Do you think the presence of porins in bacteria, mitochondria and chloroplasts is a coincidence?

Figure 2-32. An 18-strand β barrel. Bacterial sucrose-specific porin from S. typhimurium. It sits in a membrane and allows sucrose to diffuse through. (PDB: 1A0S​). https://en.wikipedia.org/wiki/Beta_barrel#/media/File:Sucrose_porin_1a0s.png By Opabinia regalis – Self-created from PDB ID 1A0S using PyMol, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1775139

 

Please complete the following activities:

Unit 2 Topic 2 Activity 2 (2.2.2)

Introduction

Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis.

Instructions

• Label the tonicity of the solution represented in each of the three diagrams of a plant cell below.

 

 

Enrichment Activity 2.2.2

Instructions

Go to the Unit 2 Topic 2 Enrichment Activities tab on your BIOL 2131 Moodle site.

*Unit 2 Topic 2 Mandatory Enrichment Activity 3 (2.2.3)

Go to the Unit 2 Topic 3 Enrichment Activities tab on your BIOL 2131 Moodle site.

 

Carrier/Transporter Proteins

Another type of protein embedded in the plasma membrane is a carrier/transporter protein. This aptly named protein binds a substance and triggers a change of its shape, moving the bound molecule from one side of the membrane to another, and can result in movement that can be with (passive) or against (active) the concentration gradient. Carrier proteins are typically specific for a single substance; this selectivity adds to the plasma membrane’s overall selectivity.

Figure 2-33 shows a carrier/transporter protein embedded in the membrane with an opening that initially faces the extracellular surface. After a substance binds the carrier, it changes shape so that the opening faces the cytoplasm, and the substance is released.

Figure 2-33: Carrier/Transporter proteins. Facilitated diffusion through a carrier/transporter protein. Some substances move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane. https://openstax.org/books/biology-2e/pages/5-2-passive-transport

An example of this process occurs in the kidney. In one part, the kidney filters glucose, water, salts, ions, and amino acids that the body requires. This filtrate, which includes glucose, then reabsorbs in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported, and the body excretes this through urine. In a diabetic individual, the term is “spilling glucose into the urine.” A different group of carrier proteins, glucose transport proteins, or GLUTs, are involved in transporting glucose and other hexose sugars through plasma membranes within the body.

The rate of transport by channel and carrier proteins differs because of the way they physically interact with their substrates. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second; whereas, carrier proteins work at a rate of a thousand to a million molecules per second.

 

Please complete the following activities:

Unit 2 Topic 2 Activity 3 (2.2.3)

Introduction

This Khan Academy video will help you to understand how channel proteins and carrier proteins can facilitate diffusion across a cell membrane (passive transport).

Instructions

Watch this Khan Academy video at https://www.khanacademy.org/science/ap-biology/cell-structure-and-function/facilitated-diffusion/v/facilitated-diffusion (6:34 min), make notes and keep a copy for study purposes.

Unit 2 Topic 2 Enrichment Activity 4 (EA 2.2.4)

Go to the Unit 2 Topic 2 Enrichment Activities tab on your BIOL 2131 Moodle site.

Unit 2 Topic 2 Activity 4 (2.2.4)

Introduction

Be able to recognize the difference between integral membrane proteins that facilitate passive transport.

Instructions

Name the type of integral membrane protein labelled as 1.

Name the type of integral membrane protein labelled as 2.

 

By LadyofHats Mariana Ruiz Villarreal – Own work. Image renamed from Image:Facilitated_diffusion_in_cell_membrane.svg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3981034

 

 

Direction of transport: Uniports, Antiports, and Symports

 

A protein involved in moving only one type of molecule across a membrane is called a uniport. Proteins that move two different types of molecules in the same direction across the membrane are called symporters. If two different types of molecules move in opposite directions across the bilayer, the protein is called an antiport (Figure 2-34).

Figure 2-34: Direction of transport. A uniport (yellow), a symport (red), and an antiport (blue).

 

Please complete the following activity:

Unit 2 Topic 2 Activity 5 (2.2.5)

Introduction

A summary of plasma membrane composition and transport.

Instructions

Watch this video at https://www.youtube.com/watch?v=78cjL-o2aoc (4:33 min) and do the following:

• Draw and label your own plasma membrane, including ALL relevant macromolecular components.
• List the three main functions of plasma membranes.
• Include three separate regions showing simple diffusion, facilitated diffusion through a channel and facilitated diffusion through a carrier/transporter.
• Describe how you would test the mobility of your membrane components (integrate with Unit 2 Topic 1).
• Draw a hydropathy plot of a 420 amino acid transmembrane protein that has one transmembrane region beginning at residue 150 and a second at 300. Use the template below.
• Integrate with Unit 2 Topic 1.

300400

 

 

 

 

 

 

 

Here is a hydropathy plot for a water-soluble protein. Values above 1 indicate hydrophobicity of amino acid R groups. Values below 1 indicate hydrophilicity of amino acid R groups. How would you compare this plot to the one in the previous question?

In other words, how would determine that this isn’t an integral membrane protein with multiple transmembrane domains? Where would the hydrophobic amino acids in this protein be found relative to its tertiary conformation?

https://bio.libretexts.org/Bookshelves/Biochemistry/Book%3A_Biochemistry_Online_(Jakubowski)/02%3A_PROTEIN_STRUCTURE/G._Predicting_Protein_Properties_From_Sequences/G3._Prediction_of_Hydrophobicity

 

Check-in Questions

Complete the Unit 2, Topic 2 Check-in Questions.

[Production: Please link to Unit 2, Topic 2 quiz.]

 

Key Terms

 

cell wall

a rigid cell covering made of cellulose in plants, peptidoglycan in bacteria, non-peptidoglycan compounds in Archaea, and chitin in fungi that protects the cell, provides structural support, and gives shape to the cell

channel protein

membrane-spanning protein that has an inner pore which allows the passage of one or more substances

 

concentration gradient

a concentration gradient is present when a membrane separates two different concentrations of molecules

diffusion

movement of a substance from an area of higher concentration to one of lower concentration

facilitated diffusion

the spontaneous passage of molecules or ions down a concentration gradient across a biological membrane passing through specific transmembrane integral proteins.

hypertonic

describes a solution in which extracellular fluid has higher osmolarity than the fluid inside the cell

hypotonic

describes a solution in which extracellular fluid has lower osmolarity than the fluid inside the cell; a cell in this environment causes water to enter the cell, causing it to swell

isotonic

describes a solution in which the extracellular fluid has the same osmolarity as the fluid inside the cell

membrane protein

proteins that are attached to or associated with the membrane of a cell or an organelle.

ligand

molecule that binds with specificity to a specific receptor molecule

osmolarity

the osmotic concentration of a solution, normally expressed as osmoles of solute per litre of solution; the total amount of substances dissolved in a specific amount of solution

osmosis

diffusion of water molecules down their concentration gradient across a selectively permeable membrane

passive transport

a method of transporting material across membranes that does not require cellular energy

peripheral protein

membrane-associated protein that does not span the width of the lipid bilayer, but is attached peripherally to integral proteins, membrane lipids, or other components of the membrane

receptor

protein molecule that contains a binding site for another specific molecule (called a ligand)

selective permeability

feature of any barrier that allows certain substances to cross but excludes others. A selectively permeable membrane is also semipermeable; however, it “chooses” what passes through (size is not the only factor).

semipermeable membrane

a type of biological membrane that will allow certain molecules to pass through it by diffusion

solute

any substance that is dissolved in a liquid solvent to create a solution

tonicity

the amount of solute in a solution

 

 

LICENSES AND ATTRIBUTIONS

 

Wikipedia contributors. (2022, May 18). Membrane transport. In Wikipedia, The Free Encyclopedia. Retrieved 21:05, June 29, 2022, from https://en.wikipedia.org/w/index.php?title=Membrane_transport&oldid=1088585196

This article is a modified derivative of “Passive transport,” by OpenStax College, Biology (CC BY 3.0). Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:24/Biology.

Diffusion and active transport. Khan Academy. Accessed June 29, 2022. https://www.khanacademy.org/science/ap-biology/cell-structure-and-function/facilitated-diffusion/a/diffusion-and-passive-transport

Passive Transport-Diffusion. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/05%3A_Structure_and_Function_of_Plasma_Membranes/5.06%3A_Passive_Transport_-_Diffusion#:~:text=Temperature%3A%20Higher%20temperatures%20increase%20the,decreasing%20the%20rate%20of%20diffusion.

Passive Transport-Facilitated Transport. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/05%3A_Structure_and_Function_of_Plasma_Membranes/5.07%3A_Passive_Transport_-__Facilitated_Transport

Wikipedia contributors. (2022, J). Membrane transport. In Wikipedia, The Free Encyclopedia. Retrieved 21:05, June 29, 2022, from Wikipedia contributors. (2022, June 28). Porin (protein). In Wikipedia, The Free Encyclopedia. Retrieved 18:21, July 4, 2022, from https://en.wikipedia.org/w/index.php?title=Porin_(protein)&oldid=1095439024

Passive Transport-Osmosis. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/05%3A_Structure_and_Function_of_Plasma_Membranes/5.08%3A_Passive_Transport_-_Osmosis

Facilitated Transport. Khan Academy. Accessed June 28, 2022. https://www.khanacademy.org/science/ap-biology/cell-structure-and-function/facilitated-diffusion/e/facilitated-diffusion

 

This article is a modified derivative of “The Cell Membrane,” by OpenStax College, Biology (CC BY 3.0). Betts et al., 2022. Download the original article for free at  https://openstax.org/books/anatomy-and-physiology-2e/pages/3-1-the-cell-membrane

The Canadian Cystic Fibrosis Registry 2013 annual report. Toronto (ON): Cystic Fibrosis Canada; 2015 Jan. https://www.cysticfibrosis.ca/uploads/2016%20Registry%20Annual%20Data%20Report.pdf

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