- Explain the fluid mosaic model of cell membranes in terms of membrane structure, composition, and dynamics.
- Identify membrane lipids unique to each of the 3 domains of life
- Predict how variations in membrane lipid composition affect membrane fluidity and integrity
- Predict whether a molecule can diffuse across a cell membrane based on the molecule's size, polarity, and charge.
- Predict the direction of water transport across the membrane under different conditions of salinity and osmolarity.
- Distinguish between modes of transport (simple diffusion, facilitated diffusion, and active transport) based on their kinetic and energetic requirements.
The cell membrane is a fundamental and defining feature of cells. It consists of a phospholipid bilayer, with the hydrophilic phosphate "head" groups on either side facing the aqueous environment and the hydrophobic "tails" in the middle. The two "leaflets", the inner leaflet on the cytoplasmic side of the membrane and the outer leaflet on the extracellular side, have different lipid compositions.
Cell membranes also contain an approximately equal mass fraction of integral membrane proteins; This refers to proteins that are firmly embedded in the hydrophobic lipid bilayer. Some integral membrane proteins have alpha-helical domains that span the membrane (transmembrane domains). Other proteins, termed peripheral membrane proteins, are loosely and reversibly bound to membranes and interact with polar phosphate head groups. Cell membranes also contain carbohydrates in the form of oligosaccharides that are covalently attached to membrane proteins (glycoproteins) or lipids (glycolipids).
Integral membrane proteins and lipids diffuse laterally within the plane of the membrane; hence the "liquid mosaic model" of cell membranes. However, due to the hydrophobic inner core, phospholipids and integral membrane proteins do not spontaneously cross the lipid bilayer or move back and forth.
Membrane lipids differ in the 3 areas of life
The 3 domains of life: Bacteria, Archaea and Eukarya have distinct membrane lipids that pose challenges and provide clues to reconstruct the evolutionary history of life on Earth.
All cells havephospholipid(glycerol phosphate with two hydrocarbon chains) in their membranes. Bacteria and eukaryotes have membrane phospholipids with fatty acid chains ester-linked to D-glycerol. Archaea, however, have completely different membrane phospholipids, with branched chains of isoprene instead of fatty acids, L-glycerol instead of D-glycerol, and ether linkages instead of ester linkages (seehttps://www.ucmp.berkeley.edu/archaea/archaeamm.htmlmihttps://en.wikipedia.org/wiki/Archaea🇧🇷 Ether bonds and isoprene chains make Archaeal membranes more resistant to heat and extreme pH levels.
Current hypotheses suggest that eukaryotic cells arose from a fusion between an archeological cell and a bacterial cell. Based on DNA sequence similarities, eukaryotic information processing genes are derived from archaea (Allers and Mevarech 2005, Cotton and McInerney 2010), while eukaryotic membrane phospholipid synthesis genes and the genes for energy metabolism appear to be derived from bacteria.
Eukaryotes also have lipid membrane innovations not found in archaea or bacteria:sterolsmisphingolipid.sterols(such as cholesterol) and sphingolipids (phospholipids with fatty acid chains attached to the amino acid serine instead of glycerol) are the major components of eukaryotic plasma membranes and together may account for 50% of the lipids of the outer leaflets (Desmond et al. Gribaldo, 2009; sphingolipids are not found on the cytoplasmic side of the lipid bilayer of the plasma membrane). Sterols are essential in all eukaryotic cell membranes. Sterols reduce the fluidity and permeability of the membrane and increase the rigidity and resistance of the membrane. Together with sphingolipids, they help organize regions of the membrane.lipid rafts, more rigid microdomains in the plasma membrane that organize cell signaling proteins into functional complexes (see review by Lingwood and Simons, 2010).
Generic structure of sterols, from Wikipedia
Sterol biosynthesis is a complex metabolic pathway that requires molecular oxygen (O2); It takes 11 oxygen molecules to synthesize a single cholesterol molecule (Desmond and Gribaldo, 2009). Therefore, steroid biosynthesis cannot have evolved earlier.great oxygen event, about 2.4 to 2.5 billion years ago, when free oxygen began to accumulate in Earth's oceans and atmosphere. Significantly, this epoch coincides with the origin of eukaryotes in the fossil record. The evolution of steroid biosynthetic pathways therefore appears to be one of the keys to eukaryotic evolution.
Not to mention, bacteria have their own special membrane adaptations in the form ofHopanoide, the bacterial equivalent of membrane sterols. Hopanoids have 5 rings and do not require oxygen for their biosynthesis.
Hopanoid from Wikipedia
Cells regulate membrane fluidity by adjusting the lipid composition of the membrane
The fluidity of a lipid bilayer varies with temperature. At higher temperatures, lipid bilayers become more liquid (think butter melting on a hot day) and more permeable, or leaky. At lower temperatures, lipid bilayers become stiff (like butter in the fridge). For cell membranes to function properly, they must maintain a balance of fluidity to allow movement of proteins and lipids within the membrane, along with the bending, bending, budding, and fusion of the membrane without compromising the integrity of the membrane and allowing what substances enter or leave the cell.
sterolsB. Cholesterol in mammals, ergosterol in fungi, and phytosterols in plants, fluidity, and permeability of the buffer membrane over a wide temperature range. In mammals, cholesterol increases membrane packing to reduce membrane fluidity and permeability.
ofatty acidsPhospholipid tails also affect membrane fluidity. Fatty acids can vary in length and number of double bonds in the hydrocarbon chain. Fatty acids without double bonds in the hydrocarbon chain are called "run over' because they have the maximum number of hydrogen atoms. 🇧🇷unsaturated“Fatty acids with one or more double bonds have fewer hydrogen atoms (2 fewer per double bond). Natural unsaturated fatty acids are cis-unsaturated, which means that the remaining hydrogens are on the same side of the molecule, creating a curve in the hydrocarbon chain.transunsaturated fatty acids, with the hydrogens on opposite sides, still result in a nearly straight hydrocarbon chain. Trans-unsaturated fatty acids are rare in nature, but are formed during the partial hydrogenation of vegetable oils in food processing.
|Elaidic acid is the most important.transUnsaturated fatty acid commonly found in partially hydrogenated vegetable oils.|
Oleic acid is onecisUnsaturated fatty acid that constitutes 55-80% of olive oil.
|Stearic acid is a saturated fatty acid found in animal fats. Stearic acid is neithercisnotransbecause it does not have carbon-carbon double bonds.|
|3 different 18-carbon fatty acids, images from Wikipedia|
The different structures in different types of fatty acids affect their chemical properties and biological effects:
- Longer fatty acids are stiffer, which reduces fluidity and membrane permeability.
- cis- Unsaturated fatty acids increase fluidity and membrane permeability by disrupting the packing of fatty acid tails.cisPolyunsaturated fatty acids (2 or more double bonds) are even more distorted and disruptive.
- Organisms adapt to low temperatures by increasing the amount ofcis-Unsaturated fatty acids to keep your membranes fluid.
- animal fats, such as butter, with relatively low levels ofcis-Unsaturated fat and high levels of saturated fat, are solid at room temperature
- vegetable oils, with a high content ofcis-Unsaturated fatty acids are liquid at room temperature.
One way to remember how different lipids affect the fluidity or rigidity of membranes is that lipids that can be packed more closely together (such as saturated fats and sterols) make membranes stiffer and stronger, but less fluid. Anything that disrupts lipid compaction, such as higher temperatures or twisted or folded unsaturated fatty acids, makes membranes more fluid.
membrane permeability and osmosis
The lipid bilayer is "semipermeable," meaning that some molecules can diffuse rapidly across the membrane, while others can pass very slowly or not at all. In general, small uncharged molecules such as O2 and CO2 can diffuse freely, whereas charged molecules (Na+, H+) or polar molecules (glucose) cannot. Water molecules also diffuse slowly across cell membranes because water molecules are highly polar.
Osmosisis the diffusion of solvent (water) molecules across a membrane. Diffusion results in the net movement of molecules along their concentration gradient from an area of high concentration to an area of low concentration. In osmosis, water molecules move from the side with the lower solute concentration to the side with the higher solute concentration. When there is a difference in solute concentrations across the membrane, solute molecules try to diffuse across the membrane to equalize the solute concentrations. But when the membrane is impermeable to solute molecules, water moves in to try to match the solute concentrations.
Cells adapt to their aqueous environment in terms of their cytoplasmic concentrations of solutes. Mammalian cells have cytoplasmic concentrations of solutes that balance physiological salt concentrations. In physiological saline solutions, mammalian cells are in aisotonicAmbient, which means that the solute concentrations inside and outside the cell are in equilibrium, so there is no net movement of water across the cell membrane. In onehypotonsolution, the concentration of solute outside the cell is less than that inside the cell, so water enters the cell to try to lower the concentration of solute inside. In onehypertonicIn solution, the concentration of solutes outside the cell is higher than inside the cell, so water moves out of the cell and the cell shrinks.
membrane and transport proteins
How do cells transport molecules such as glucose across the membrane? Membranes have dedicated transport proteins with transmembrane domains. Several transmembrane domains cluster to form channels in the membrane that are specific for various molecules such as glucose, phosphate, Na+, H+, Cl-, and even H2O. Water transport is mediated by highly conserved proteins called aquaporins, which are present in all 3 domains of life.
Click the link to download and view the molecular movie showing the transport of a labeled water molecule through an aquaporin channel. The movie lasts about 12 nanoseconds:
water penetration(same movie as the aquaporin video above)
The movement of molecules through protein channels is calledfacilitated diffusion🇧🇷 Protein channels are highly specific for the molecule and in turn result in net transport along a concentration gradient from an area of high concentration to an area of low concentration.
What if the cell wants to move molecules?contraa concentration gradient? Even when phosphate concentrations outside the cell are very low, cells can transport phosphate into the cell, where the cytoplasmic phosphate concentration can be much higher.Active transportationof molecules against their concentration gradient requires energy expenditure, often in the form of ATP hydrolysis.
Active transport is easy to identify as it requires energy and is transported against the concentration gradient. But is it possible to distinguish facilitated diffusion from simple diffusion?
Because facilitated diffusion is mediated by protein channels and the number of protein channels in a cell membrane is limited, facilitated diffusion exhibits saturation kinetics.
The kinetics of simple diffusion and facilitated diffusion. Transport rate (v) is plotted against solute concentration ([S]) for simple diffusion of a solute (shown in red) and facilitated diffusion of a solute through a carrier protein (shown in green). . Retrieved from https://wikispaces.psu.edu/display/Biol230WFall09/Passive+and+Active+Transport
As the concentration difference across the membrane increases, the rate of diffusion of molecules across the membrane increases, for both facilitated and simple diffusion. However, facilitated diffusion through protein channels reaches a limit when molecules pass through all available protein channels as quickly as possible. A further increase in molecular concentration cannot cause faster diffusion since all channels are busy and molecules have to wait for them to open. Active transport will also exhibit saturation kinetics for the same reason. Simple diffusion will rarely reach this limit since the entire area of the membrane is available.
To summarize membranes and transport, watch this animation:http://www.johnkyrk.com/cellmembrane.html– An error is that the animation incorrectly shows the sphingolipids on both sides (leaflets) of the plasma membrane, while the sphingolipids are only on the outer leaflet (extracellular side).
To test your understanding of transportation processes, check out this factcell membrane permeability.
put it all together
case study oncystic fibrosis
Molecular Fossils - Lipid Biomarkers Worksheet
The slides of the video clips of the lectures on this page are in this file:B1510_module3_2_membranes_transport_f2012
Allers, T, Mevarech M 2005. Archaeal Genetics - The Third Way,Nature Reviews Genetik 6:58-73doi:10.1038/nrg1504
Cotton, JA, JO McInerney 2010. Eukaryotic genes of archaeabacterial origin, regardless of their function, are more important than the more numerous eubacterial genes.Proc. Acad. National. Sciences United States published online ahead of printdoi: 10.1073/pnas.1000265107
Desmond, E, S Gribaldo 2009. Phylgenomics of sterol synthesis: insights into the origin, evolution and diversity of a key eukaryotic trait,Biol Evol 1-Through: 364-381doi: 10.1093/live/evp036
Lingwood, D, K Simons 2010. Lipid rafts as a principle of membrane organization,science 327: 46-50
Primary active transport moves ions across a membrane and creates a difference in charge across that membrane. The primary active transport system uses ATP to move a substance, such as an ion, into the cell, and often at the same time, a second substance is moved out of the cell.What are the 4 types of membrane transport? ›
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.What are the 3 types of membrane transport? ›
Basic types of membrane transport, simple passive diffusion, facilitated diffusion (by channels and carriers), and active transport .What is cell membranes and transport? ›
Membrane transport refers to the movement of particles (solute) across or through a membranous barrier. 2. These membranous barriers, in the case of the cell for example, consist of a phospholipid bilayer.What are the 3 main principles of cell theory? ›
1) All organisms are made of cells. 2) All existing cells are produced by other living cells. 2) All existing cells are produced by other living cells. 3) The cell is the most basic unit of life.What are the three cell principles? ›
The three tenets of the cell theory are: All living organisms are composed of one or more cells. The cell is the basic unit of structure and organization in organisms. Cells arise from pre-existing cells.What are the 5 types of membranes? ›
- Epithelial Membranes. Epithelial membranes consist of epithelial tissue and the connective tissue to which it is attached. ...
- Mucous Membranes. ...
- Serous Membranes. ...
- Connective Tissue Membranes. ...
- Synovial Membranes. ...
The four main functions of the plasma membrane include identification, communication, regulation of solute exchange through the membrane, and isolation of the cytoplasm from the external environment.What is the importance of membrane transport? ›
Membrane transport proteins fulfill an essential function in every living cell by catalyzing the translocation of solutes, including ions, nutrients, neurotransmitters, and numerous drugs, across biological membranes.What is the process of membrane transport? ›
Pinocytosis is a process by which extracellular fluid is engulfed by invaginating cell membranes, forming a vesicle that then separates from the membrane. This vesicle may move through the cell cytoplasm and release its contents on the other side of the cell layer by means of exocytosis.
The plasma membrane, or the cell membrane, provides protection for a cell. It also provides a fixed environment inside the cell, and that membrane has several different functions. One is to transport nutrients into the cell and also to transport toxic substances out of the cell.What are the two types of transport membrane? ›
There are two classes of membrane transport proteins—carriers and channels. Both form continuous protein pathways across the lipid bilayer. Whereas transport by carriers can be either active or passive, solute flow through channel proteins is always passive.What are 3 functions of the cell membrane? ›
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 ...What are the factors affecting cell membrane transport? ›
Cell permeability and cell fluidity are two essential properties, having roles in transporting molecules across the membrane. These properties are affected by physiological factors like temperature, pH, and the membrane's composition.What are the 6 types of transport? ›
Therefore; an essential part of transportation management lies in building an efficient supply chain from the six main modes of transportation: road, maritime, air, rail, intermodal, and pipeline. Understanding the strengths and weaknesses of each mode is paramount to building an effective supply chain.Who made the 3 principles of cell theory? ›
Key Points. The cell theory describes the basic properties of all cells. The three scientists that contributed to the development of cell theory are Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. A component of the cell theory is that all living things are composed of one or more cells.What is the first principle of cell theory? ›
The first part states that all organisms are made of cells. The second part states that cells are the basic units of life. These parts were based on a conclusion made by Schwann and Matthias Schleiden in 1838, after comparing their observations of plant and animal cells.What are the two important principles in cell theory? ›
The cell theory has three principles: All organisms are made of cells. All existing cells are produced by other living cells. The cell is the most basic unit of life.What are the parts of a membrane? ›
The principal components of the plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrate groups that are attached to some of the lipids and proteins. A phospholipid is a lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group.What are 5 functions of membranes? ›
- protects the cell by acting as a barrier.
- regulates the transport of substances in and out of the cell.
- receives chemical messengers from other cell.
- acts as a receptor.
- cell mobility, secretions, and absorptions of substances.
(1) Cell membranes are thin enclosures that form closed boundaries. (2) Cell membranes are made up of lipids, proteins and carbohydrates. (3) Cell membranes consists of a phospholipid bilayer. (4) Cell membranes are held together by non-covalent interactions (5) Membranes are fluid-like structure.What are the 4 types of membrane proteins? ›
Integral proteins come in different types, such as monotopic, bitopic, polytopic, lipid-anchored proteins, or transmembrane proteins.What are the 7 functions of the cell membrane? ›
- Mechanical Structure. Defines/encloses the Cell. ...
- Selective Permeability. ...
- Active Transport. ...
- Bulk Transport: Exocytosis & Endocytosis. ...
- Markers & Signalling. ...
- Metabolic Activities.
Based on their structure, there are main three types of membrane proteins: the first one is integral membrane protein that is permanently anchored or part of the membrane, the second type is peripheral membrane protein that is only temporarily attached to the lipid bilayer or to other integral proteins, and the third ...How are membranes formed? ›
The formation of biological membranes is based on the properties of lipids, and all cell membranes share a common structural organization: bilayers of phospholipids with associated proteins.What are the 3 types of transport proteins? ›
Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion. A channel protein, a type of transport protein, acts like a pore in the membrane that lets water molecules or small ions through quickly.What means membrane? ›
membrane. noun [ C ] us. /ˈmem·breɪn/ a thin, soft layer of tissue that covers organs or connects parts of living things, or the outer covering of a cell.What are the 3 proteins in the cell membrane? ›
The three main types of proteins embedded in the cell membrane are receptors, channels and markers.What is cell membrane made of? ›
Cell membranes are composed of proteins and lipids. Since they are made up of mostly lipids, only certain substances can move through. Phospholipids are the most abundant type of lipid found in the membrane. Phospholipids are made up of two layers, the outer and inner layers.
Like all other cellular membranes, the plasma membrane consists of both lipids and proteins. The fundamental structure of the membrane is the phospholipid bilayer, which forms a stable barrier between two aqueous compartments.What are the 4 factors that contribute to membrane fluidity? ›
- Factor #1: The length of the fatty acid tail. The length of the fatty acid tail impacts the fluidity of the membrane. ...
- Factor #2: Temperature. ...
- Factor #3: Cholesterol content of the bilayer. ...
- Factor #4: The degree of saturation of fatty acids tails.
Now, a lot of different factors can affect membrane fluidity. But the three most important ones that we're going to focus on today are, number one, temperature, number two, cholesterol, and number three, which is whether we have unsaturated or saturated fatty acids.What factors affect transport? ›
The transport of people or goods from one place to another depends on many factors: distance, physiography, routes and means, market, climate, products, time, cost and duration.What is transport process? ›
The process by which mass, energy, or momentum are transported from one region of a material to another under the influence of composition, temperature, or velocity gradients.What are the 4 transport mechanisms? ›
Types of transport mechanism
There are four types of transport mechanisms in a cell. These are simple diffusion, facilitated diffusion, primary active transport and secondary active transport.
Four Common Parts of a Cell. Although cells are diverse, all cells have certain parts in common. The parts include a plasma membrane, cytoplasm, ribosomes, and DNA.What are the principles of cell division? ›
In eukaryotes, there are two distinct types of cell division: a vegetative division (mitosis), producing daughter cells genetically identical to the parent cell, and a cell division that produces haploid gametes for sexual reproduction (meiosis), reducing the number of chromosomes from two of each type in the diploid ...What are the four principle statements of the cell theory? ›
Example 1: Recalling the Principles of Cell Theory
Cells are the smallest structures found within an organism. All living organisms are made of one or more cells. All cells are made from preexisting cells. The basic functional unit of all living organisms is the cell.
Written for the undergraduate Cell Biology course, Principles of Cell Biology provides students with an accessible approach to the fundamental concepts of cell biology. The text focuses on the underlying principles that illustrate both how cells function as well as how we study them.
The plasma membrane, or the cell membrane, provides protection for a cell. It also provides a fixed environment inside the cell, and that membrane has several different functions. One is to transport nutrients into the cell and also to transport toxic substances out of the cell.What are the two key principles of the cell theory? ›
The cell theory has three principles: All organisms are made of cells. All existing cells are produced by other living cells. The cell is the most basic unit of life.What is the principle of mitosis? ›
Mitosis is a process of cell duplication, in which one cell divides into two genetically identical daughter cells. In the various stages of mitosis, the cell's chromosomes are copied and then distributed equally between the two new nuclei of the daughter cells.What are the 4 types of cell division? ›
- Mitosis: The process cells use to make exact replicas of themselves. ...
- Meiosis: In this type of cell division, sperm or egg cells are produced instead of identical daughter cells as in mitosis.
- Binary Fission: Single-celled organisms like bacteria replicate themselves for reproduction.
Which of the following is NOT a principle of cell theory? Not all cells are alike.What are the 6 biological principles? ›
Six biology principles: Organization and function, adaptation, response to the environment, growth and development, reproduction, and Homeostasis.