Understanding what passes through and doesn't pass through the plasma membrane

I have difficulty in understanding the reason why certain molecules can pass through phospholipid bilayers.

Firstly, I understand that the outer layer of the lipid bilayer is hydrophilic - my understanding is that they "like water" and can interact better with water.

  1. Bilayers can absorb hydrophobic substance like N₂ and O₂.

Does this mean - because the hydrophilic surface are capable of hydrogen bonding - that's why they are able to absorb hydrophobic substances like F , O and N ?

  1. They can absorb non-polar molecules.

  2. They can absorb small uncharged molecules like H₂O and CO₂.

I'm not sure of the reasons behind 2 and 3…

And of course , vice versa… (what can't pass through the lipid bilayer easily).

Any help would be appreciated!

Great questions! Your 3 questions can be answered in 2 parts:

1. How do small, uncharged gasses (ie: O₂, N₂, CO₂) diffuse through lipid bilayers?

First it's important to clear up some terminology: Absorbing a compound isn't the same as being permeable to that compound. If you say that a compound is absorbed (or partitions) into a bilayer, you're assuming that the absorption of the given compound by the bilayer increases the entropy of the system (and is thus energetically favorable). For example, lipophilic compounds partition into lipid bilayers because of the hydrophobic effect; by partitioning into the bilayer they break a number of hydrogen bonds and thus increase the disorder of the system.

Small, dissolved gasses like O₂, N₂, and CO₂ are not absorbed into lipid bilayers, they simply diffuse through passively because of their small size and because they are non-polar.

Water is also able to passively diffuse through a lipid bilayer even though it is polar. We might think it should be fairly rare for a polar water molecule to pass through the hydrophobic center of a lipid bilayer, but typically living cells are found in aqueous environments, where concentrations of water outside the cell are extremely high, so this does occur at biologically relevant rates. In fact, the passive diffusion of water in and out of cells is so important that many cells have developed special transmembrane proteins to facilitate the passage of water in and out of cells (most notably aquaporins and pressure-gated channels).

2. How do non-polar molecules pass through lipid bilayers?

This is answered in the related question posted in the comments section of your question. Briefly, it has to do with the same electrostatic problem I mentioned above: Though there is an energetic cost for non-polar compounds to pass through the hydropilic layer of a lipid membrane, in most cases the partitioning of the compound into the membrane still raises the overall entropy of the system (due to disruption of the hydrophobic effect), making it an energetically favorable process.

It should be noted that for the most part, polar compounds do not spontaneously cross cell membranes because of the hydrophobic region of the lipid bilayer. When passage of these compounds into or out of the cell is required for life, cells typically produce transmembrane proteins which can act as either active or passive transporters to allow these compounds through.

5.4: Plasma Membrane

  • Contributed by Suzanne Wakim & Mandeep Grewal
  • Professors (Cell Molecular Biology & Plant Science) at Butte College

This simple, cut-away model of an animal cell (Figure (PageIndex<1>)) shows that a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit, the cytoplasm of the cell also contains various structures, such as a nucleus and other organelles. Your body is made up of trillions of cells, but all of them perform the same basic life functions. They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out these basic functions and keep themselves &mdash and you &mdash alive? To answer these questions, you need to know more about the structures that make up cells, starting with the plasma membrane.

Figure (PageIndex<1>): Animal cell model

The plasma membrane is a structure that forms a barrier between the cytoplasm inside the cell and the environment outside the cell. Without the plasma membrane, there would be no cell. The membrane also protects and supports the cell and controls everything that enters and leaves it. It allows only certain substances to pass through while keeping others in or out. To understand how the plasma membrane controls what passes into or out of the cell, you need to know its basic structure.

Simple Diffusion

Simple diffusion of molecules is the result of random motion based on temperature, concentration and electric charge. Molecules diffuse down a gradient, from an area of high concentration to one with a lower concentration of the molecule. If the molecule has an electric charge, it will diffuse to an area of opposite charge, since like charges repel and opposite charges attract. A cell membrane helps maintain the chemical and electrical status of a cell, which means it often acts as a barrier to simple diffusion.

Introduce New Material

Begin the lesson by introducing the vocabulary associated with the lesson: regulate, homeostasis, barrier, phospholipid bilayer, polar, nonpolar, hydrophilic, hydrophobic, selective permeability

Say each word aloud and ask students to repeat the term after you. Clap out the syllables for the terms with 3 or more syllables. This helps students hear the word parts of more complex words so that they can pronounce them correctly.

Instruct students to add the bolded terms to their Vocabulary Map. Remind students that the bolded terms contain prefixes, suffixes, Greek or Latin root words. Provide explicit instruction of each term when it arises during the course of instruction.

Inform students of the learning target for this lesson:

  • I can explain how the structure of the plasma membrane acts as a selectively permeable barrier that maintains homeostasis by regulating the passage of molecules in and out of the cell.
  • I can explain the function of proteins, lipids and carbohydrates in the functioning of the cell membrane.

Display visual information as you instruct and ensure students take notes using guided notes that you have provided or use a note-taking strategy that you have taught. Guided notes provide greater support for the different learning styles of students.

As you teach the characteristics and parts of the cell membrane, use of visual aids will help anchor abstract ideas to increase students’ abilities to conceptualize the learning. For example, a paper towel role can be used to model the protein channel function. Here are a few examples of visual aids to help students conceptualize how the barrier works:

  • Start by explaining that the cell membrane is like a boundary or fence between the cell and its environment. Talks about how a fence acts like a barrier between a yard and the street. This type of real-world comparisons helps students “solidify” the concepts.
  • Next, use old produce bag that is made like a window screen (typically an orange or lemon bag) and tell students to consider the produce bag a cell membrane of a cell.
  • Wave the bag through the air and ask students, what molecules are passing the barrier and entering the bag. Also, ask why. Students should be able to identify air passes through the bag because air molecules are small.
  • Ask students to predict if water can cross the barrier of the cell membrane (the produce bag). Students should be able to correctly identify that water can easily cross the membrane. Pour water through the bag over a trash can to reinforce how easily the water crosses the membrane.
  • Show students a tennis ball. Tell students to consider the tennis ball a large molecule. Ask students to predict if the tennis ball can cross the barrier. Show students how the tennis ball cannot cross through the small openings of the produce bag because it is too big.
  • Show students a marble or similar sized object. Ask students to predict if the marble can fit through the cell membrane. Show students that the marble is too big to fit through the small openings of the produce bag. However, inform students that the marble can cross the cell membrane. Give students 1-2 minutes to talk with a nearby classmate about how this might occur. Allow a few students to share their idea for how the marble might cross into the cell (bag) even though it is too big to go through the small openings of the produce bag. After listening to a few ideas, show students one way that this can occur by taking a paper towel roll and place it into a pre-cut opening in the bag. Explain and model how the paper towel roll acts a channel or hallway that “helps” the marble enter the cell.
  • Compare a protein receptor to a cell phone tower that receives signals and sends messages to the inside of the cell.

Explicitly teach the terms homeostasis, hydrophilic, hydrophobic and phospholipid bilayer. Allow students to identify what each term means without using the knowledge they have about the prefixes or suffixes, and Latin or Greek root words in each term. Provide guidance by helping students first identify the word parts then attach meaning to each of the word parts. For example, for Phospholipid bilayer ask students to identify at least one prefix in the term. Look for students to correctly identify “bi” as a prefix that means two. Point out that this prefix is in many words we use each day. Use the terms bicycle and binary as examples of terms that use this prefix. Ask students to think about what they know about the phospholipid bilayer just by knowing what the prefix, bi- means. Look for students to know that the bilayer is a two-layer structure.

Lastly, show students pictures of different mosaics (mosaic tree, mosaic lily and mosiac fish). Point out how the mosaic is a complete artwork made of different parts. Explain that the cell membrane is called a “fluid mosaic”. Provide students an opportunity to practice verbal fluency. Instruct students to “turn and talk” for 2 minutes with a seatmate about why they think the cell membrane is called a fluid mosaic. At the end of the time, allow a few students to share their ideas with the class. Look for students to identify that the cell membrane is called a fluid mosaic because 1) it moves, and 2) a mosaic is made of a lot of different pieces like the cell membrane is made of a lot of different structures.

Biology Unit 1 2020

All cells are surrounded by a membrane, an envelope that holds the cell's contents in and separates the inside from the outside. In eukaryotic cells, membranes also surround spaces that become organelles and carry out many of the functions that occur in a living cell. The membrane is the boundary where movement of molecules in and out is controlled. The outer membrane that surrounds the entire cell is known as the plasma membrane.

A cell’s plasma membrane defines the boundary of the cell and controls much of the cell's contact with the environment. Plasma membranes enclose the borders of cells, but rather than being like a plastic bag that doesn't change, they are constantly changing and moving. The plasma membrane must be flexible enough to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital in playing a role in the “self” versus “non-self” distinction of the immune response. The video shows an amoeba wrapping itself around a euglena cell so it can digest it. The movement is possible because of the flexible nature of the plasma membrane.

Another key characteristic of the plasma membrane is that it allows some molecules to pass across it from one side to the other. The plasma membrane is selectively permeable.


  1. What are the general functions of the plasma membrane?
  2. Describe the phospholipid bilayer of the plasma membrane.
  3. Identify other molecules in the plasma membrane, and state their functions.
  4. Why do some cells have plasma membrane extensions such as flagella and cilia?
  5. Explain why hydrophilic molecules cannot easily pass through the cell membrane. What type of molecule in the cell membrane might help hydrophilic molecules pass through it?
  6. Which part of a phospholipid molecule in the plasma membrane is made of fatty acid chains? Is this part hydrophobic or hydrophilic?
  7. The two layers of phospholipids in the plasma membrane are called a phospholipid ____________.
  8. True or False. The flagella on your lung cells sweep foreign particles and mucus toward your mouth and nose.
  9. True or False. Small hydrophobic molecules can easily pass through the plasma membrane.
  10. True or False. The side of the cell membrane that faces the cytoplasm is hydrophilic.
  11. Steroid hormones can pass directly through cell membranes. Why do you think this is the case?
  12. Some antibiotics work by making holes in the plasma membrane of bacterial cells. How do you think this kills the cells?
  13. What is the name of the long, whip-like extensions of the plasma membrane that helps some single-celled organisms move?


PURPOSE: The purpose of the experiment was to test the permeability of dialysis tubing to glucose, starch and iodine.

Living cells need to obtain nutrients from their environment and get rid of waste materials to their surroundings. This exchange of materials between the cell and its surroundings is crucial to its existence. Cells have membranes composed of a phospholipid bilayer embedded with proteins.

This cell membrane can distinguish between different substances, slowing or hindering the movement of other substances and allowing others to pass through readily. This property of the cell is known as selective permeability (Ramlingam, 2008).

Selective permeability is a property of a cell membrane that allows it to control which molecules can pass (moving into and out of the cell) through the pores of the membrane. Selective permeable membranes only allows small molecules such as glucose, amino acids to readily pass through, and inhibits larger molecules like protein, starch, from passing through it.

The dialysis tubing is a semi-permeable membrane tubing used in separation techniques and demonstration of diffusion, osmosis, and movement of molecules across a restrictive membrane (Todd, 2012). It separates dissolved substances of different molecular sizes in a solution, and some of the substances may readily pass through the pores of the membrane while others are excluded. The dialysis tubing is made up of cellulose fibers. This is shaped in a flat tube.

In this experiment, the selective permeability of dialysis tubing to glucose, starch and iodine (potassium iodide) will be tested. This experiment consists of two tests the test for starch and the test for reducing sugar. When iodine (potassium iodide) is added to a solution in which starch is present, the solution turns blue-black or purple otherwise it remains yellow-amber.

And when Benedict’s reagent is added to a solution in which reducing sugar is present and it is heated in a water bath, the solution turns green, yellow, orange, red, and then brick red or brown (with high concentration of sugar present). Otherwise, the solution remains blue.

Will glucose, starch and iodine (potassium iodide) readily pass through the pores of the dialysis tubing?


Tight junctions are composed of a branching network of sealing strands, each strand acting independently from the others. Therefore, the efficiency of the junction in preventing ion passage increases exponentially with the number of strands. Each strand is formed from a row of transmembrane proteins embedded in both plasma membranes, with extracellular domains joining one another directly. There are at least 40 different proteins composing the tight junctions. [2] These proteins consist of both transmembrane and cytoplasmic proteins. The three major transmembrane proteins are occludin, claudins, and junction adhesion molecule (JAM) proteins. These associate with different peripheral membrane proteins such as ZO-1 located on the intracellular side of plasma membrane, which anchor the strands to the actin component of the cytoskeleton. [3] Thus, tight junctions join together the cytoskeletons of adjacent cells.

    was the first integral membrane protein to be identified. It has a molecular weight of

They perform vital functions: [11]

  • They hold cells together.
  • Barrier function, which can be further subdivided into protective barriers and functional barriers serving purposes such as material transport and maintenance of osmotic balance:
    • Tight junctions help to maintain the polarity of cells by preventing the lateral diffusion of integral membrane proteins between the apical and lateral/basal surfaces, allowing the specialized functions of each surface (for example receptor-mediated endocytosis at the apical surface and exocytosis at the basolateral surface) to be preserved. This aims to preserve the transcellular transport.
    • Tight junctions prevent the passage of molecules and ions through the space between plasma membranes of adjacent cells, so materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. Investigation using freeze-fracture methods in electron microscopy is ideal for revealing the lateral extent of tight junctions in cell membranes and has been useful in showing how tight junctions are formed. [12] The constrained intracellular pathway exacted by the tight junction barrier system allows precise control over which substances can pass through a particular tissue. (Tight junctions play this role in maintaining the blood–brain barrier.) At the present time, it is still unclear whether the control is active or passive and how these pathways are formed. In one study for paracellular transport across the tight junction in kidney proximal tubule, a dual pathway model is proposed: large slit breaks formed by infrequent discontinuities in the TJ complex and numerous small circular pores. [13]

    In human physiology there are two main types of epithelia using distinct types of barrier mechanism. Epidermal structures such as skin form a barrier from many layers of keratinized squamous cells. Internal epithelia on the other hand more often rely on tight junctions for their barrier function. This kind of barrier is mostly formed by only one or two layers of cells. It was long unclear whether tight cell junctions also play any role in the barrier function of the skin and similar external epithelia but recent research suggests that this is indeed the case. [14]

    Epithelia are classed as "tight" or "leaky", depending on the ability of the tight junctions to prevent water and solute movement: [15]

    • Tight epithelia have tight junctions that prevent most movement between cells. Examples of tight epithelia include the distal convoluted tubule, the collecting duct of the nephron in the kidney, and the bile ducts ramifying through liver tissue. Other examples are the blood-brain barrier and the blood cerebrospinal fluid barrier
    • Leaky epithelia do not have these tight junctions, or have less complex tight junctions. For instance, the tight junction in the kidney proximal tubule, a very leaky epithelium, has only two to three junctional strands, and these strands exhibit infrequent large slit breaks.

    55,000x and 80 kV with Tight junction. Note that the three dark lines of density correspond to the density of the protein complex, and the light lines in between correspond to the paracellular space.

    Membrane transport 1.4

    The membrane controls what enters and leaves the cell. This includes using diffusion and osmosis. Sometimes the membrane uses integral proteins as channels and pumps, sometimes the membrane surrounds something which needs moving into or out of the cell.

    Key concepts

    Learn and test your biological vocabulary for 1.4 membrane transport using these flashcards.

    Essentials - quick revision through the whole topic

    These slides summarise the essential understanding and skills in this topic.
    They contain short explanations in text and images - good revision for all students.

    Read the slides and look up any words or details you find difficult to understand.

    Exam style question about osmosis in plant tissues

    Explaining osmosis is an important skill from this topic.

    Answer the question below, on a piece of paper, then check your answer against the model answer below.

    The images show two views of some red onion cells under the microscope during osmosis.

    Describe how osmosis changes the cells in this experiment [4]

    Click the + icon to see a model answer.

    Model answer

    In a description always try to link a cause to its effect.

    Osmosis is moving water out of the cells.
    Water moves from cytoplasm with a higher water potential to the surrounding solution with a higher solute concentration.
    The water passes through the selectively-permeable plasma membrane.
    The cytoplasm volume gets smaller and separates from the cell wall in places.
    This is called plasmolysis.

    Summary list for 1.4 Membrane transport

    • Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
    • The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
    • Know how the structure helps the function of sodium&ndashpotassium pumps for active transport and potassium channels for facilitated diffusion in axons.
    • Understand why tissues (or organs) waiting to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.
    • Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. (Practical 2)


    These diagram summaries cover the main sections of topic 1.4 about membrane transport.
    Study them and draw your own list or concept map from memory.

    Test yourself - multiple choice questions

    This quiz contains a set of multiple choice questions covering the topic. Explanations of each answer are displayed after you click to check the answer(s).

    1.4 Membrane transport 1 / 1

    Which of the following best describes exocytosis?

    Exocytosis moves molecule out of cells using diffusion.

    Exocytosis moves molecules into cells using diffusion.

    Exocytosis moves molecules out of cells using vesicles.

    Exocytosis moves molecules into cells using vesicles.

    The fluidity of membranes allows materials to be released by exocytosis.
    Vesicles move materials within cells to the membrane, e.g. enzymes in cells of a gland.

    Sodium channels are made from a protein.

    Where in the cell are sodium channel proteins found.

    These proteins span across the plasma membrane.

    They are found in the cytoplasm.

    They are usually only found in mitochondria.

    These proteins are usually found in the nucleus.

    Sodium channel proteins are found spanning the plasma membrane. Their structure helps the function for facilitated diffusion in cells because they allow ions to pass cross the membrane.

    If the protein was not a trans-membrane protein then it would not be able to transport ions across the membrane.

    This box contains a lung waiting for a transplant operation.

    What is special about the solution inside the box which surrounds the tissue?

    The solution has the same osmolarity as the cells in the tissue.

    The solution is made of cytoplasm from cells in the tissue.

    The solution is hypertonic compared to the tissue.

    The solution is hypotonic compared to the tissue.

    Tissues and organs must be kept in a solution with the same osmolarity as the cytoplasm of the cells to prevent osmosis. If they were kept in pure water, osmosis would carry water into the cells and they would burst, causing damage to the cells.

    The four cells shown below have each been surrounded by a solution for 1 hour.

    Which cells have been in a hypertonic solution ?

    Cell A is swollen turgid, it is in a hypotonic solution or an isotonic solution.

    The cells B, C and D show increasing signs of plasmolysis, and so they must be in hypertonic solutions.

    Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. (Practical 2)

    The graph below shows the % change in mass at different concentrations of sucrose.

    Which of the following is the best estimate of the molarity of the cytoplasm of these cells?

    When a sample of cells show no change in mass, then the net movement of water by osmosis must be zero. This shows the concentration of the cytoplasm of the cells. In this graph it would be about 0.3 mol

    Differences between Passive and Active Tranport

    Active Transport vs Passive Transport

    As minute as they are, cells in the body carry some very important processes deep within. These processes are all vital to the overall growth and development of every organism, may it be an animal or a plant. But every internal process must have some unique mechanisms done to make it successful. In this regard, nutrients, chemicals and other substances are flowing to and fro the cells with the use of certain transport systems. These transport mechanisms are classified into two, namely active and passive transport systems.

    In the simplest terms, active transport is termed ‘active’ because of the inclusion of one vital component and that is the use of energy. This energy is being utilized by the cell, in the form of ATP (Adenosine Triphosphate) for it to be able to move most substances in and out of its cellular membranes. On the contrary, passive transport is regarded as such because it is just a plain old ‘passive’ mechanism. It does not use any energy (ATP) from the cell for it to carry out the said processes.

    Another distinct characteristic that separates active from passive transport system is the difference in the concentration gradients. It must be made known that the concentration of substances that are partitioned by cell membranes are relatively different. For example, the inside of the cell has a concentration gradient that is higher (more concentrated) than the outside of the cell (less concentrated) or it can also be the other way around depending on various biological factors. Hence, in active transport, it tries to achieve a more difficult task of opposing the concentration gradient. If the cell wants to transport certain substances towards itself (in this situation, it so happens to be more concentrated) then it needs much energy for its protein or sodium pumps to operate and transfer the said substances.

    In the case of passive transport, it is not against but along the concentration gradient. Because the cell sees that the same ions or molecules can be transferred to the other side immediately due to a ‘favorable’ concentration gradient, it no longer expends any energy. The word ‘favorable’ simply means that it follows the rules of normal diffusion. When the substances from the more concentrated internal environment of the cell are to be transported outside, that is for example the outside happens to be less concentrated, then the substances can easily flow out.

    In brief, active and passive transport differ because:
    1.Active transport makes use of energy in the form of ATP whereas passive transport does not utilize any.
    2.Active transport involves the transfer of molecules or ions against a concentration gradient whereas passive transport is the transfer along a concentration gradient