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PG/MEMBRANES - Biology

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PG/MEMBRANES

Practical - Beetroot membranes

All cells are bounded by a plasma membrane, but many internal cell structures are also bounded by membrane. These include the tonoplast, the membrane surrounding a plant cell vacuole, the double membrane (envelope) surrounding the nucleus, mitochondria and chloroplasts, and the membranes of other organelles such as vacuoles, vesicles, endoplasmic reticulum, and Golgi apparatus. All these membranes have basically the same structure.

One of the roles of membranes is to control what passes across them. Membranes are partially permeable, ie. they do not normally allow large molecules to pass across them. Many plant cells contain pigment molecules in the cytoplasm. In beetroot cells these molecules are large and reflect red light, so that a solution of them turns a shade of pink. Many pigment molecules are also affected by pH and change colour at different pH values. Litmus, for example, is a plant pigment and is blue in alkali and red in acid.

Ethanol is an organic solvent.

Proteins are stable at a particular pH and become denatured outside this. The normal pH of cells 7.2.

You are going to find out what happens when you treat beetroot tissue in different ways. Using the above information, plus your knowledge of membranes, you will have to explain your observations.

1. Label 8 McCartney bottles 1 – 8 and measure into them 10cm3 as listed below.

Tube 4 0.1M hydrochloric acid

Tube 5 0.1M sodium hydroxide solution

2. Cut 3 slices from the beetroot chip, each about 2mm thick and place them into bottle 1.

3. In the beaker are a number of similar slices from the same sized chip, but after being cut they have been rinsed to remove any pink colour. If they still show some pink rinse them again. Into each of bottles 2 – 8 place 3 of these slices.

4. Set up a water bath by adding 100cm3 of tap water to a 40 0 cm 3 beaker and place in it bottles 6 – 8. DO NOT PUT THE CAPS ON THE BOTTLES.

5. Heat until it reaches 400C, then remove bottle 6 using the tongs.

6. Continue heating until it reaches 65 0 C, and then remove bottle 7. Take care.

7. Continue heating until it reaches 100 0 C (or near – record the temperature) and then remove bottle 8. Take great care.

8. Allow the hot bottles to cool.

9. Fit the caps to all the bottles and gently shake them.

10. Set up the colorimeter and insert the blue filter slide. Do NOT touch the filter with your fingers. The colorimeter measures the % transmission of light through a solution so it is set to read maximum, 100%, with pure solvent. Thus the darker the colour of the solution the lower the % transmission. The colour filter used is always the opposite end of the spectrum to the colour of the solution under investigation.

11. Handle the cuvettes by the corrugated sides and do NOT touch the clear sides.

12. Use the 5cm 3 syringe to measure 3cm 3 of distilled water into a colorimeter cuvette. Place it in the colorimeter, making sure that the clear sides are facing front/back. Close the rubber cap.

13. Adjust the colorimeter to read as close as possible to 100%. Record this reading. Do not throw away this cuvette as it is the standardising one.

14. Into a second cuvette measure 3cm 3 of the water from bottle 1. Place it in the colorimeter, close the cap and take a reading.

15. Wash AND DRY the second cuvette.

16. Wash the syringe to prevent contamination.

18. Insert the standardising cuvette and record the reading. Re-adjust back to 100%.

19. Repeat steps 15 to 18 for bottles 3, 4, 6, 7 and 8. (You have not done bottle 5 as the colour of the solution requires a different filter and so the results cannot be compared.)

20. Collect all the data from at least 3 other students.

21. The changes in the readings for the standardising cuvette give an indication of the error for the colorimeter. These are not expensive instruments and have a tendency to ‘drift’.

Technician’s notes and equipment:

1. Personal protective equipment (gloves, laboratory gown, mask, goggles)


Phospholipid biosynthesis in eukaryotes

8.2 Enzymes and subcellular location

PG is made in mitochondria and microsomes of animal cells and appears to be primarily converted to DPG. DPG is biosynthesized exclusively on the matrix side of the mitochondrial inner membrane and is found only in this organelle. There is evidence that the rate-limiting step in DPG biosynthesis is the conversion of PA into CDP-DG (G.M. Hatch, 1994). Consistent with this idea, the levels of CTP regulate DPG biosynthesis in cardiac myoblasts (G.M. Hatch, 1996). Using techniques developed by Raetz and co-workers [ 14 ], a temperature-sensitive mutant of PG-P synthase in CHO cells was isolated (M. Nishijima 1993). The mutant had only 1% of wild-type PG-P synthase activity at 40°C and exhibited a temperature-sensitive defect in PG and DPG biosynthesis. This mutant was used to show that DPG is required for the NADH-ubiquinone reductase (complex I) activity of the respiratory chain.

In yeast, DPG synthesis has been genetically interrupted (Schlame, 2000). The yeast grow without DPG at temperatures of between 16 and 30°C but fail to grow at 37°C on fermentable carbon sources such as glucose, a condition for which mitochondria are not required for ATP synthesis. These data support the idea that mitochondria perform a necessary function in yeast survival other than the generation of energy. The fatty acyl chain content of phospholipids can impact mitochondrial function. Incubation of cardiomyocytes with palmitic acid increased the palmitic acid content of PA and PG and decreased DPG levels in mitochondria with a concomitant release of cytochrome c leading to apoptosis (W. Dowhan, 2001).


Membrane Function

Now to the question of what the plasma membrane actually does. First and most obvious is that the plasma membrane is indeed a selective barrier. It allows the chemical activities inside the cell to proceed mostly undisturbed by events outside the cell. The famous cell biologist Gerald Weissmann emphasized the importance of this role:

In the beginning, there must have been a membrane! Whatever flash of lightning there was that organized purines, pyrimidines, and amino acids into macromolecules capable of reproducing themselves it would not have yielded cells [except] for the organizational trick afforded by the design of a membrane wrapping.

The lipid nature of the membrane allows it to serve as a good barrier. Lipids are water-insoluble and repel water, thus they are an ideal medium to separate the watery inside and outside of a cell. Anything that is water-soluble, even tiny single atoms such as H + ions, will not easily pass through a lipid bilayer. However, water-insoluble molecules may pass freely these include small molecules such as oxygen and carbon dioxide, and large water-insoluble hormones such as estrogen, testosterone, cortisol, thyroid hormone, and vitamin D. For these reasons, membranes are said to be semipermeable barriers. They do not let water or water-soluble molecules pass, but they do allow diffusion of water-insoluble (lipid soluble) molecules.

However, membranes are more than passive barriers. This is made clear by the many molecules that cannot pass through simple bilayers very quickly, but can pass into and out of cells. Water is the best example. As the understanding of membranes developed in the scientific community, a conundrum emerged. The phospholipid bilayer structure should not be very permeable to water, but when cells are studied in the laboratory, most are very permeable to water. How could this be? Scientists went so far as to build synthetic membranes using exactly the kinds and quantities of phospholipids found in specific types of cells. These synthetic membranes had very low water permeability, while the cells they modeled had very high water permeability. The hypothesis at the time was that there must be some sort of pore or channel in membranes through which water can pass, but all evidence for this was indirect. Channels for ions had been discovered, but the way that cells move water in and out remained a mystery.

This changed in 1992 when Peter Agre and colleagues reported their accidental discovery of channels called aquaporins (Preston et al., 1992). These channels are embedded in the plasma membrane and allow water to pass into and out of the cell (Figure 7). Agre and colleagues were not in the business of studying water transport. They were studying the Rhesus (Rh) factors that are present on red blood cells and result in blood incompatibility complications. In trying to isolate and purify these Rh factors, they noticed a “contaminant” in their test tubes – a membrane protein that they were not trying to study but which kept getting in the way. When they noticed that this protein is one of the most abundant proteins on the surface of the red blood cell, they decided to take a closer look and eventually realized that this “contaminant” was a protein that scientists had been looking for decades. Over the next few years, a whole family of related aquaporin proteins was discovered, and these proteins have a nearly identical structure in humans, fruit flies, fungi, and plants, indicating an ancient origin and strong conservation throughout more than a billion years of evolution.

Figure 7: Aquaporin proteins in the membrane allow only molecules that are shaped and charged like water molecules to pass freely.

Interestingly, a research group from Romania led by Gheorghe Benga had likely made this discovery at least six years before Agre, but they had not fully isolated nor identified the protein. Nevertheless, controversy has been raised over the issue of proper credit because Benga’s work almost certainly describes the same protein and had been published publically years before, both in a US journal and an international one. Nevertheless, Agre and colleagues did not to cite this work in their publications or Nobel Prize lectures, and most of the scientific community overlooked them as well. It should be noted that, working in an Eastern Bloc country as the collapse of the Soviet Union approached, Benga and his colleagues did not have the prestige or resources that Agre and his colleagues enjoyed at Johns Hopkins University. It is conceivable that, had Benga been working in a more internationally prestigious institution and/or with more financial resources, he may have shared the Nobel Prize in 2003.

The discovery of aquaporins highlights how proteins embedded in the plasma membrane can act as gatekeepers and govern the entry of molecules into and out of the cell. The membrane has many such gatekeepers and, like aquaporin, that are very specific. For example, aquaporin allows water molecules in and out freely, but other molecules much less so. Closely related molecules can pass through, but with much less efficiency (Figure 8). For example, urea, ammonia, and alcohol can each pass through aquaporins and indeed these channels are the main route through which these molecules are absorbed by most cells. However, they pass through more than a million times more slowly than water does. The structure of aquaporins reveals how they achieve this selectivity. Within the tunnel-like chamber through which water molecules pass, there are structural features that fit only a molecule with the size, shape, and partial-charge distribution that water has. Thus, while molecules similar in size and charge to water sometimes can pass through, they pass through at a much lower rate than water itself.

Figure 8: Aquaporins allow molecules like urea, ammonia, and alcohol to pass through at a much slower rate than water molecules.

The examples of aquaporins and CFTR show how the plasma membrane can be selective about what enters and leaves the cell. As cell biologist Daniel Mazia put it:

The cell membrane is not a wall or a skin or a sieve. It is an active and responsive part of the cell it decides what is inside and what is outside, and what the outside does to the inside.

Summary

Cell membranes are much more than passive barriers they are complex and dynamic structures that control what enters and leaves the cell. This module explores how scientists came to understand cell membranes, including the experiments that led to the development of the fluid-mosaic model of membrane structure. The module describes how the components and structure of cell membranes relate to key functions.

Key Concepts

The outer layer of a cell, or a cell membrane, is a complex structure with many different kinds of molecules that are in constant motion, moving fluidly throughout the membrane.

Cell membranes form selective barriers that protect the cell from the watery environment around them while letting water-insoluble molecules like oxygen, carbon dioxide and some hormones pass through.

Most of the cell membrane is formed by phospholipids that have a unique structure that causes them to self-arrange into a double layer that is hydrophobic in the middle and hydrophilic on the outside.


Identification and pathophysiological roles of LTB4 receptors BLT1 and BLT2

Yumiko Ishii , . Takehiko Yokomizo , in Lipid Signaling and Metabolism , 2020

Biosynthesis and metabolism of LTB4

AA is a 20-carbon fatty acid and an important component of the cell membrane. While intracellular free AA is not available at steady state, AA is released intracellularly by activated phospholipase A2 (PLA2), especially cytosolic PLA2 [19–21] , upon appropriate cell stimulation. AA is metabolized to produce lipid mediators via 5-lipoxigenase (5-LO), cyclooxygenase (COX), and cytochrome P450 (CYP450) enzymes to form LTs and lipoxins (LXs), prostaglandins (PGs), and epoxyeicosatrienic acids (EETs), respectively [22] . In resting cells, 5-LO is localized in the cytosol, but when cells are activated, 5-LO is translocated to the nuclear membrane, where 5-LO-activating protein (FLAP), a membrane-spanning protein, transfers AA to 5-LO [23,24] . 5-LO oxygenates AA to form 5-hydroxyeicosatetraenoic acid (5-HpETE) and subsequently leukotriene A4 (LTA4) [25–27] . Interestingly, the localization of 5-LO determines LTB4 synthesis, and inhibition of nuclear localization via mutation of the nuclear localization sequence of 5-LO reduces LTB4 synthesis by more than 60% [28] . Additionally, resolvin D1, a pro-resolving mediator derived from docosahexaenoic acid, translocates 5-LO from the nucleus to the cytosol and decreases the LTB4:lipoxin A4 ratio via calcium-calmodulin-dependent protein kinase II (CaMKII)-p38-mitogen-activated protein kinase-activated protein kinase 2 kinases [29] . Furthermore, phosphorylation of Ser 523 within the nuclear localization sequence and of Ser271 within the nuclear export sequence causes cytosolic localization and inhibition of 5-LO [30,31] . Thus, phosphorylation regulates the localization and activity of 5-LO ( Fig. 12.1 ).

Figure 12.1 . Biosynthesis of leukotrienes and 12-HHT.

Arachidonic acid released from membrane phospholipid by PLA2 is converted to leukotrienes, prostanoids, and 12-HHT. LTB4 binds BLT1, a high-affinity receptor for LTB4, and induces chemotaxis and/or various subsets of leukocytes. BLT2, originally identified as a low-affinity receptor for LTB4, is a high-affinity receptor for 12-HHT, enhances epithelial barrier function and promotes wound healing. Enzyme names are abbreviated as follows: PLA2, phospholipase A2 COX 1/2, cyclooxygenase-1 and cyclooxygenase-2 TxA2S, thromboxane A2 synthase 5-LO, 5-lipoxygenase FLAP, 5-LO-activating protein LTA4H, leukotriene A4 hydrolase LTC4S, leukotriene C4 synthase.

LTA4 is metabolized to LTB4 by leukotriene A4 hydrolase (LTA4H) or cysteinyl leukotrienes (CysLTs) by leukotriene C4 synthase (LTC4S), microsomal glutathione S-transferase 2 (MGST2), and microsomal glutathione S-transferase 3 (MGST3) [32–42] . FLAP is localized on both the inner and outer nuclear membrane, and LTA4H is localized in the cytosol and nucleus. Meanwhile, LTC4S is only present at the outer nuclear membrane. In addition, the nuclear localization of 5-LO contributes to LTB4 synthesis. Thus, LTB4 may be preferentially produced on the inner nuclear membrane [43] .

In addition to its epoxide hydrolase activity, LTA4H also has a zinc-dependent peptidase activity [44,45] . Recently, this aminopeptidase activity of LTA4H was reported to limit inflammation by degrading the neutrophil chemoattractant Pro-Gly-Pro (PGP) [46] . 5-LO is expressed mainly in hematopoietic cells, including PMNs, neutrophils, eosinophils, monocytes/macrophages, DCs, mast cells, and B-lymphocytes [47–52] , while LTA4H is detected in various mammalian cells, even those lacking 5-LO expression [35] . LTA4 is released from human leukocytes, and LTB4 generation is increased by co-culture of activated neutrophils and erythrocytes/alveolar macrophages that do not express 5-LO but do express LTA4H [53–55] . This coordinated mechanism is referred to as transcellular biosynthesis [56] , and transcellular biosynthesis of LTB4 was confirmed in vivo [57,58] . LTC4 is also produced in the same way [57,59] .

There are two major pathways of LTB4 inactivation: omega oxidation and LTB4 12-hydroxydehydrogenase pathways. In granulocytes and hepatocytes, LTB4 is metabolized to a trihydroxy compound by oxidation at C-20 of LTB4 (20-OH-LTB4), followed by transformation to the dicarboxylic acid (20-COOH-LTB4) by several CYP450 enzymes [60–62] . In other cells, the cytosolic enzyme LTB4 12-hydroxydehydrogenase converts LTB4 into 12-keto-LTB4 [63,64] . LTB4 12-hydroxydehydrogenase also inactivates other eicosanoids including PGs and lipoxin A4 by reduction of the 13-14 double bond [65,66] . Another possible pathway of LTB4 inactivation is via peroxidase, H2O2, and halides from eosinophils [67] .


Sending messages across membranes

We have already seen how ion channels and other transport proteins can allow substances to cross the lipid bilayer. Knowledge of how fat-soluble and water-soluble substances cross membranes is important for gaining an understanding of how messages cross membranes and thus how one cell can communicate with another. Cells receive and send messages constantly (e.g. in order to respond to hormone signals, conduct action potentials, and sense external stimuli such as taste and smell).

Messengers: lipid soluble or water soluble?

Substances that send messages are known as messengers, and they vary enormously in their chemical composition, size and hydrophobicity. In order to understand how a cell receives a message, it is important to ascertain first whether the messenger is lipid or water soluble. Hormones are one example of messengers that are released by cells. The human body contains both lipid-soluble and water-soluble hormones. Lipid-soluble hormones are generally transported through the blood, bound to carrier proteins. Steroid hormones such as the androgens and oestrogens are lipid soluble by virtue of their ringed molecular structures, which are derived from cholesterol. This allows these hormones to diffuse freely through the plasma membrane of cells and bind to their receptors, which are located inside cells. In the case of oestrogen, the receptor is located in the cytoplasm and upon ligand binding relocates to the nucleus, where it binds DNA and acts as a transcription factor, altering gene expression. The receptor contains a nuclear localization sequence which is hidden until oestrogen binds, allowing it to be targeted to the nucleus.

Other hormones, such as insulin and adrenaline, are water soluble and therefore cannot pass freely through the membranes of cells. Their receptors are located on the outside of the plasma membrane in order for them to be able to convey a message without entering cells. Insulin binds to the membrane-spanning insulin receptor on the surface of target cells, and initiates a signal cascade that results in an increase in the number of glucose transporters at the cell membrane, and a subsequent increase in glucose uptake.

G proteins and second messengers

Many cell-surface receptors share structural features, including seven membrane-spanning helices. These 7TM receptors bind their ligand (the messenger molecule) on the extracellular side of the membrane, and bind a GTP-binding protein (G protein) on the intracellular side. Due to this interaction with G proteins, these receptors are called G-protein-coupled receptors (GPCRs). When the ligand binds the GPCR, the receptor undergoes conformational changes that are transferred through the membrane-spanning region to the bound G protein. This change in structure allows the G protein to exchange a bound GDP molecule for a GTP molecule, and thereby switch from an inactive state to an active state. G proteins consist of three subunits—α, β and γ. An inactive, GDP-bound G protein consists of all three subunits, with the nucleotide bound in the α subunit. When the GPCR binds the ligand, the G protein is activated and the α subunit, now with GTP bound to it, dissociates from the complex (Figure 12). This activated α subunit now has an exposed face (where the β and γ subunits were bound) and can bind proteins to propagate the signal. An example of this downstream signalling from GPCRs is the activation of adenylate cyclase by the GTP-bound α subunit in the case of the β-adrenergic receptor when it binds its ligand, adrenaline (epinephrine). The effect of this adenylate cyclase activation is an increase in cAMP production from ATP, leading to downstream effects. The dissociated βγ dimer also has downstream effects. The α subunit has GTPase activity so that it can convert the bound GTP back to GDP. The GDP-bound subunit then returns to and binds the β and γ subunits ready for another cycle of signalling.

GPCR and heterotrimeric G-protein signalling.

The ligand bound to the GPCR is shown in red. Binding allows the exchange of GDP for GTP by the associated G protein, and dissociation of the protein into Gα and Gβγ subunits. These then have downstream effects on a range of proteins, thereby propagating the signal from the bound ligand. Yellow arrows indicate either activation (up arrow) or inhibition (down arrow) of the targets. Regulators of G-protein signalling (RGS) proteins aid the GTPase activity of the G protein to turn off the signal. Arrestin can bind the receptor following GPCR phosphorylation by G-protein receptor kinase (GRK), desensitizing the receptor to further signalling. Reproduced from Berridge, M.J. (2012) Cell Signalling Biology doi:10.1042/csb0001002, with permission.

The ligand bound to the GPCR is shown in red. Binding allows the exchange of GDP for GTP by the associated G protein, and dissociation of the protein into Gα and Gβγ subunits. These then have downstream effects on a range of proteins, thereby propagating the signal from the bound ligand. Yellow arrows indicate either activation (up arrow) or inhibition (down arrow) of the targets. Regulators of G-protein signalling (RGS) proteins aid the GTPase activity of the G protein to turn off the signal. Arrestin can bind the receptor following GPCR phosphorylation by G-protein receptor kinase (GRK), desensitizing the receptor to further signalling. Reproduced from Berridge, M.J. (2012) Cell Signalling Biology doi:10.1042/csb0001002, with permission.

After the initial ligand interaction with the GPCR and the G-protein dissociation, the message is then carried by second messengers activated by the signal cascade. In the example that has just been given, the G protein associated with the β-adrenergic receptor activates adenylate cyclase, increasing the production of cAMP, which is a widely used second messenger. Most of the effects of cAMP are due to the activation of protein kinase A (PKA). PKA phosphorylates target enzymes to modify their activities. In the case of adrenaline, PKA activates enzymes involved in the production of glucose from glycogen stores, and inhibits enzymes involved in the production of more glycogen.

Around 25% of drugs are targeted at GPCRs, so an understanding of their structures and functions is crucial in the fight against disease. As explained earlier, membrane proteins are notoriously difficult to crystallize due to their hydrophobic nature, and GPCRs have a very small hydrophilic area. Some techniques, such as the production of an antibody–receptor complex to increase hydrophilicity, have been successful in aiding crystallization. Rhodopsin (Figure 13) was crystallized in 2000, followed by the related β2-adrenergic receptor in 2007. Since then, several more GPCR structures have been solved, providing valuable information that can help computational biologists to work out the detailed mechanisms of GPCR signalling. Molecular dynamics simulations have been performed on the interactions between GPCRs and their partner G proteins using the crystal structures available to inform the modelling process. These studies will play an important part in helping us to understand how the helices in the GPCRs move and twist in order to convey the extracellular signal to the intracellular G protein.

The crystal structure of rhodopsin.

A ribbon representation of the first crystal structure of rhodopsin is shown in the plane of the membrane (a) and from the cytoplasmic side (b). The N- and C-termini are labelled, as are the seven transmembrane helices (I-VII). Adapted from Figure 2 from Palczewski, K., Kumasaka, T., Hori, T., Behnke, C.A., Motoshima, H., Fox, B.A., Le Trong, I., Teller, D.C., Okada, T., Stenkamp, R.E. et al. (2000) Crystal structure of rhodopsin: a G-protein-coupled receptor. Science 289, 739–745.

A ribbon representation of the first crystal structure of rhodopsin is shown in the plane of the membrane (a) and from the cytoplasmic side (b). The N- and C-termini are labelled, as are the seven transmembrane helices (I-VII). Adapted from Figure 2 from Palczewski, K., Kumasaka, T., Hori, T., Behnke, C.A., Motoshima, H., Fox, B.A., Le Trong, I., Teller, D.C., Okada, T., Stenkamp, R.E. et al. (2000) Crystal structure of rhodopsin: a G-protein-coupled receptor. Science 289, 739–745.

Nerve impulses

Nerve impulses are able to occur because biological membranes are impermeable to ions, so a membrane potential can be generated across them, with more of one charge on one side than on the other. These membrane potentials are generated and altered by ion channels. A nerve impulse, or action potential, is generated when a membrane is depolarized upon influx of positively charged ions into the cell. The resting potential in a neuron is around –70 mV, maintained by K + channels and the Na + /K + -ATPase. When an action potential is generated, voltage-dependent Na + channels open once the cell membrane has crossed the threshold potential of around –60 mV. This allows a fast influx of Na + down its electrochemical gradient, increasing the membrane potential (i.e. reducing its negative value). This influx of positive charges enables the inside of the cell to become positively charged compared with the extracellular environment, as the membrane potential exceeds 0 mV. The depolarization itself inhibits the Na + channels, so no more ions enter the cell. To restore the negative resting potential, voltage-dependent K + channels open, allowing K + ions to move out of the cell, thus making the inside of the cell more negative. An after-potential (hyperpolarization) can then occur, whereby the membrane potential decreases below –70 mV before being restored by the action of ion channels and ATPases.


Abstract

The advent of amphiphilic copolymers enables integral membrane proteins to be solubilized into stable 10–30 nm native nanodiscs to resolve their multisubunit structures, post-translational modifications, endogenous lipid bilayers, and small molecule ligands. This breakthrough has positioned biological membrane:protein assemblies (memteins) as fundamental functional units of cellular membranes. Herein, we review copolymer design strategies and methods for the characterization of transmembrane proteins within native nanodiscs by cryo-electron microscopy (cryo-EM), transmission electron microscopy, nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, X-ray diffraction, surface plasmon resonance, and mass spectrometry.


Characteristics of Amniotes

Figure 1. The key features of an amniotic egg are shown.

The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the case with the eggs of many other animals, such as amphibians. However, the eggs of amniotes contain three additional extra-embryonic membranes: the chorion, amnion, and allantois (Figure 1).

Extra-embryonic membranes are membranes present in amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta.

Additional derived characteristics of amniotes include waterproof skin, due to the presence of lipids, and costal (rib) ventilation of the lungs.

Practice Question

Which of the following statements about the parts of an egg are false?

  1. The allantois stores nitrogenous waste and facilitates respiration.
  2. The chorion facilitates gas exchange.
  3. The yolk provides food for the growing embryo.
  4. The amniotic cavity is filled with albumen.

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Membrane

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Membrane, in biology, the thin layer that forms the outer boundary of a living cell or of an internal cell compartment. The outer boundary is the plasma membrane, and the compartments enclosed by internal membranes are called organelles. Biological membranes have three primary functions: (1) they keep toxic substances out of the cell (2) they contain receptors and channels that allow specific molecules, such as ions, nutrients, wastes, and metabolic products, that mediate cellular and extracellular activities to pass between organelles and between the cell and the outside environment and (3) they separate vital but incompatible metabolic processes conducted within organelles.

Membranes consist largely of a lipid bilayer, which is a double layer of phospholipid, cholesterol, and glycolipid molecules that contains chains of fatty acids and determines whether a membrane is formed into long flat sheets or round vesicles. Lipids give cell membranes a fluid character, with a consistency approaching that of a light oil. The fatty-acid chains allow many small, fat-soluble molecules, such as oxygen, to permeate the membrane, but they repel large, water-soluble molecules, such as sugar, and electrically charged ions, such as calcium.

Embedded in the lipid bilayer are large proteins, many of which transport ions and water-soluble molecules across the membrane. Some proteins in the plasma membrane form open pores, called membrane channels, which allow the free diffusion of ions into and out of the cell. Others bind to specific molecules on one side of a membrane and transport the molecules to the other side. Sometimes one protein simultaneously transports two types of molecules in opposite directions. Most plasma membranes are about 50 percent protein by weight, while the membranes of some metabolically active organelles are 75 percent protein. Attached to proteins on the outside of the plasma membrane are long carbohydrate molecules.

Many cellular functions, including the uptake and conversion of nutrients, synthesis of new molecules, production of energy, and regulation of metabolic sequences, take place in the membranous organelles. The nucleus, containing the genetic material of the cell, is surrounded by a double membrane with large pores that permit the exchange of materials between the nucleus and cytoplasm. The outer nuclear membrane is an extension of the membrane of the endoplasmic reticulum, which synthesizes the lipids for all cell membranes. Proteins are synthesized by ribosomes that are either attached to the endoplasmic reticulum or suspended freely in the cell contents. The mitochondria, the oxidizing and energy-storing units of the cell, have an outer membrane readily permeable to many substances, and a less-permeable inner membrane studded with transport proteins and energy-producing enzymes.


Watch the video: P. Bassereau - Physics approaches of biological membranes (August 2022).