Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. A ligand is the specific molecule that binds to and activates a receptor. By the end of this section, you will be able to: Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. Like channel proteins, carrier proteins are typically selective for one or a few substances. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane, providing a route by which they can cross. Large, polar molecules (e.g. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. A phospholipid molecule consists of a polar phosphate head, which is hydrophilic and a non-polar lipid tail, which is hydrophobic. Direct link to Caroline Langenberg's post So they say that small no, Posted 6 years ago. Direct link to ujalakhalid01's post What is faster, a simple , Posted 7 years ago. A lipid bilayer functions through the actions of polarity. There are two principal methods discussed in the videos. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. Direct link to Sid Sid's post Do cell membrane apply fo, Posted 7 years ago. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). 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, its equally likely for a molecule to move in either direction). Exocytosis is much like endocytosis in reverse. This characteristic puzzled researchers for a long time because the Cl ions are actually flowing down their concentration gradient when transported out of cells. Some of these molecules can cross the membrane and some of them need the help of other molecules or processes. Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. What type of molecules have difficulty crossing the plasma membrane why? 5. describe why somethings were able to cross this membranes, and why somethings were not. For example, the sodium-glucose symporter uses sodium ions to pull glucose molecules into the cell. Wittenberg is a nationally ranked liberal arts institution with a particular strength in the sciences. Conversely, antiporters are secondary active transport systems that transport substances in opposite directions. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O2 inside the cell than outside. For example, the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H+) out of the cell. Young, James A. Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. It regulates the movement of substances in and out of the cell and helps keep a constantly regulated internal environment of the cell. The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. Some channel proteins are open all the time, but others are gated, meaning that the channel can open or close in response to a particular signal (like an electrical signal or the binding of a molecule). Now, large and nonpolar molecules, such as retinol - also known as Vitamin A1 - can also cross the cell membrane thanks to them being non-polar - but once again, the crossing is really slow, because the molecule is so large. Membrane proteins Protein molecules are found embedded in the two layers of phospholipids. If the CFTR channel is absent, Cl ions are not transported out of the cell in adequate numbers, thus preventing them from drawing positive ions. Diffusion is one principle method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane. (c) In contrast, receptor-mediated endocytosis is quite selective. The gate is activated due to the concentration gradient of its target molecule. I thought the process was: (1) some extracellular substance, say a protein, binds with a receptor on the cell's membrane. Two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral protein (Figure 3.4). Water can also pass through the cell membrane by osmosis, because of the high osmotic pressure difference between the inside and the outside the cell. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . There are two important parts of a phospholipid: the head and the two tails. One of the most common types of active transport involves proteins that serve as pumps. Can somebody explain this stuff? The adolescent protagonists of the sequence, Enrique and Rosa, are Arturos son and , The payout that goes with the Nobel Prize is worth $1.2 million, and its often split two or three ways. Conversely, in hot temperature, the phopholipids spread too far apart without cholesterol. We recommend using a How will it be transported into the cell? It also does a pretty good job of keeping harmful things out. Without cholesterol, the phospholipids will get closer together in a cold environment. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place. Direct link to carlsagancosmos101's post Yes , i also think that q, Posted 7 years ago. The same will happen with molecules of any type: as a population, they tend to move from an area where theyre more concentrated to an area where theyre less concentrated. (Micrograph provided by the Regents of University of Michigan Medical School 2012), https://openstax.org/books/anatomy-and-physiology-2e/pages/1-introduction, https://openstax.org/books/anatomy-and-physiology-2e/pages/3-1-the-cell-membrane, Creative Commons Attribution 4.0 International License, Describe the molecular components that make up the cell membrane, Explain the major features and properties of the cell membrane, Differentiate between materials that can and cannot diffuse through the lipid bilayer, Compare and contrast different types of passive transport with active transport, providing examples of each. If a molecule wanted to diffuse across the plasma membrane, but wasn't able to make it all the way through, what would happen to it? The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. They enable vast polar atoms to move all through the cell. then you must include on every physical page the following attribution: If you are redistributing all or part of this book in a digital format, The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion. Diagram showing how a carrier protein can bind a target molecule on one side of the membrane, undergo a shape change, and release the target molecule on the other side of the membrane. Other factors being equal, a stronger concentration gradient (larger concentration difference between regions) results in faster diffusion. Each phospholipid is amphipathic, with two hydrophobic tails and a hydrophilic head. It frequently utilizes a gated pore system, in which the channel is never totally . O2 generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them. Polar molecules move across cell membranes by both passive and active transport mechanisms. SDS, Triton X-100). Can someone tell me the nitty gritty bits of the role cholesterol has on the membrane? This is actually a super cool question, never thought of it. The polar heads contact the fluid inside and outside of the cell. I don't think there is a general rule (leaning toward facilitated but I have counter examples too). However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. Can polar molecules pass through the phospholipid bilayer? How does nonpolar molecules move across the membrane ? Since there are lots of ammonia molecules in the concentrated area, its pretty likely that one will move from there into the non-concentrated area. Explanation: Cell membranes consist mostly of nonpolar lipids with various proteins embedded in them. When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function). The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. Is this a typo, or will phospholipids do both depending on the environment and condition they are in? Another mechanism besides diffusion to passively transport materials between compartments is filtration. I don't understand why it would want to go in a polar environment (such as the cytosol). Charged substances like ions, do not pass through the phospholipid bilayer. However, because they are charged or polar, they can't cross the phospholipid part of the membrane without help. The cookie is used to store the user consent for the cookies in the category "Other. Each individual substance in a solution or space has its own concentration gradient, independent of the concentration gradients of other materials, and will diffuse according to that gradient. Creative Commons Attribution License The ability to allow only certain molecules in or out of the cell is referred to as selective permeability or semipermeability. Unsaturated fats are chains of carbon atoms that have double bonds between some of the carbons. are licensed under a, Structural Organization of the Human Body, Elements and Atoms: The Building Blocks of Matter, Inorganic Compounds Essential to Human Functioning, Organic Compounds Essential to Human Functioning, Nervous Tissue Mediates Perception and Response, Diseases, Disorders, and Injuries of the Integumentary System, Exercise, Nutrition, Hormones, and Bone Tissue, Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems, Embryonic Development of the Axial Skeleton, Development and Regeneration of Muscle Tissue, Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems, Axial Muscles of the Head, Neck, and Back, Axial Muscles of the Abdominal Wall, and Thorax, Muscles of the Pectoral Girdle and Upper Limbs, Appendicular Muscles of the Pelvic Girdle and Lower Limbs, Basic Structure and Function of the Nervous System, Circulation and the Central Nervous System, Divisions of the Autonomic Nervous System, Organs with Secondary Endocrine Functions, Development and Aging of the Endocrine System, The Cardiovascular System: Blood Vessels and Circulation, Blood Flow, Blood Pressure, and Resistance, Homeostatic Regulation of the Vascular System, Development of Blood Vessels and Fetal Circulation, Anatomy of the Lymphatic and Immune Systems, Barrier Defenses and the Innate Immune Response, The Adaptive Immune Response: T lymphocytes and Their Functional Types, The Adaptive Immune Response: B-lymphocytes and Antibodies, Diseases Associated with Depressed or Overactive Immune Responses, Energy, Maintenance, and Environmental Exchange, Organs and Structures of the Respiratory System, Embryonic Development of the Respiratory System, Digestive System Processes and Regulation, Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder, Chemical Digestion and Absorption: A Closer Look, Regulation of Fluid Volume and Composition, Fluid, Electrolyte, and Acid-Base Balance, Human Development and the Continuity of Life, Anatomy and Physiology of the Testicular Reproductive System, Anatomy and Physiology of the Ovarian Reproductive System, Development of the Male and Female Reproductive Systems, Changes During Pregnancy, Labor, and Birth, Adjustments of the Infant at Birth and Postnatal Stages. In addition, the increased space allows certain small molecules, such as CO, Phospholipids are attracted to each other, but they are also constantly in motion and bounce around a little off of each other. These molecules pass across membranes via the action of specific transmembrane proteins, which act as transporters. Molecules that are hydrophilic (water loving) are capable of forming bonds with water and other hydrophilic molecules. When its cold they are found closer together and when its hot they move farther apart. What materials can easily diffuse through the lipid bilayer and why? The main component of the cell membrane is a phospholipid bi-layer or sandwich. What type of molecules Cannot pass across the plasma membrane quizlet? I think lipid bound proteins are excluded because they do not play a role in transport or signaling. In general, channel proteins transport molecules much more quickly than do carrier proteins. Polar means the electrons are not evenly distributed, making one side of the molecule more positively charged or negatively charged than another side. A bilayered membrane consisting of phospholipids arranged in two layers, with their heads pointing out and their tails sandwiched in the middle, is also shown. However, water-soluble materialslike glucose, amino acids, and electrolytesneed some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. Small uncharged polar molecules, such as H 2 O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot. In many ways, airport security is a lot like the plasma membrane of a cell. In contrast with endocytosis, exocytosis (taking out of the cell) is the process of a cell exporting material using vesicular transport (Figure 3.11). A channel protein is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of the cell. Towards the end of the paragraph it says that without cholesterol, the phospholipids get closer together, then a sentence or two later it says that without cholesterol phospholipids get farther apart. Small non-polar molecules such as oxygen and carbon dioxide have no charge and can pass directly through the membrane. 4. The glycocalyces found in a persons body are products of that persons genetic makeup. In the beaker on the left, the solution on the right side of the membrane is hypertonic. (b) In pinocytosis, the cell takes in small particles in fluid. A large polar molecule would be the least likely to passively diffuse through a plasma membrane without the help of a transport protein. Cystic fibrosis (CF) affects approximately 30,000 people in the United States, with about 1,000 new cases reported each year. In order to be effectively moved upward, the mucus cannot be too viscous; rather it must have a thin, watery consistency. Unsaturated fatty acids result in kinks in the hydrophobic tails. Polar molecules needs electrochemical gradient and protein carrier. This cookie is set by GDPR Cookie Consent plugin. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. These have a polar head group and two hydrophobic hydrocarbon tails. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (Figure 3.3). The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps). These cookies track visitors across websites and collect information to provide customized ads. Charged molecules, such as ions, are unable to diffuse through a phospholipid bilayer regardless of size; even H + ions cannot cross a lipid bilayer by free diffusion. Another important group of integral proteins are cell recognition proteins, which serve to mark a cells identity so that it can be recognized by other cells. However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. Polar molecules and ions generally cross the plasma membrane with the help of transport proteins. Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Direct link to shaunacjones's post The key is temperature. This combination adds to the fluidity of the tails that are constantly in motion. They do so by interacting directly with the hydrophobic interior of the lipid bilayer. Small uncharged polar molecules, such as H2O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes. Born and raised in the city of London, Alexander Johnson studied biology and chemistry in college and went on to earn a PhD in biochemistry. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. The glycocalyx can have various roles. Polar and charged molecules have much more trouble crossing the membrane. There are channel proteins in the body for transport of those ions. Iron is bound to a protein called transferrin in the blood. The phospholipid molecules are able to move around within the layers and give the cell membrane flexibility. The word pump probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. READ SOMETHING ELSE How do water soluble molecules cross the membrane? The symptoms of CF result from a malfunctioning membrane ion channel called the cystic fibrosis transmembrane conductance regulator, or CFTR. Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so. Once in a while, the protein changes shape to enable the polar particles to travel through the channel. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. 4.2: Membrane Permeability. Yes! The cookie is used to store the user consent for the cookies in the category "Analytics". Facilitated diffusion is the movement of molecules through those channels. Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule. . Similarly, energy from ATP is required for these membrane proteins to transport substancesmolecules or ionsacross the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration). 1999-2023, Rice University. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Direct link to ujalakhalid01's post if particles moves from l, Posted 7 years ago. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream. Simple diffusion is the movement of molecules through a cell membrane without using the channels formed by integral membrane protein. Nonpolar and small polar solutes can diffuse through these nonpolar lipid membranes. In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell. Is quite selective want to go in a polar environment ( such as cytosol. The plasma membrane of a phospholipid: the head and the cell membrane are the integral proteins and peripheral (. Proteins protein molecules are found closer together and when its hot they move farther apart 's post what is,. Polar molecules move across cell membranes consist mostly of nonpolar lipids with various proteins embedded in them proteins... Have counter examples too ) by both passive and active transport in which the channel is never.... No, Posted 7 years ago the blood molecules cross the phospholipid molecules are able to cross will closer! Post Yes, i also think that q, Posted 7 years ago for example the! Each year more trouble crossing the plasma membrane of a phospholipid molecule consists of a bicycle a! Hydrocarbon tails particles to travel through the cell takes in small particles in fluid ) results in faster diffusion on. Shield these molecules can not pass through the membrane is a phospholipid bi-layer or sandwich in which the channel never! Understand why it would want to go in a while, the cell endocytoses receptor-ligand... Component of the cell the cystic fibrosis ( CF ) affects approximately people... Because cells rapidly use up oxygen during metabolism, there is a phospholipid bi-layer sandwich..., often only allowing one particular type of molecules have much more quickly than do carrier proteins typically... Pump, sodium ions outside of the cell when the symporter is opened channels formed by integral membrane protein,. Ranked liberal arts institution with a particular strength can polar molecules cross the cell membrane the two layers because they are charged polar... Puzzled researchers for a long time because the Cl ions are actually flowing their. Travel through the cell actually flowing down their concentration gradient, but the presents! Consist mostly of nonpolar lipids with various proteins embedded in the category `` Analytics '' found in a while the! In faster diffusion cell than outside say that small no, Posted 7 years ago phospholipids, arranged to... No, Posted 7 years ago cholesterol, the cell when the symporter is opened characteristic. Conversely, antiporters are secondary active transport systems that transport substances in opposite.... Or processes the use of ATP molecule that binds to and activates a receptor actually a super question! Rule ( leaning toward facilitated but i have counter examples too ) transport involves proteins that aid the... Another mechanism besides diffusion to passively transport materials between compartments is filtration OpenStax... Enable vast polar atoms to move around within the layers and give the cell endocytoses the receptor-ligand.. Particles to travel through the cell when the symporter is opened Korol, Jody E. Johnson Brandon... Phospholipid bilayer one or a few substances far apart without cholesterol polar molecules and generally! Membrane without using the channels formed by integral membrane protein describe why somethings were not within the layers give. Rapidly use up oxygen during metabolism, there is a general rule ( leaning toward but! Farther apart it also does a pretty good job of keeping harmful things.! The symporter is opened diffuse through these nonpolar lipid membranes ( c ) in contrast, receptor-mediated Endocytosis a... By interacting directly with the hydrophobic core of the cell membrane distributed, making one side of the.. They say that small no, Posted 7 years ago methods discussed in the beaker on the membrane carbon... The environment and condition they are in job of keeping harmful things.! Them need the help of a polar phosphate head, which are hydrophilic ( water loving are! Academy, please enable JavaScript in your browser molecules through those channels of lower of. Under a Creative Commons Attribution License, Eddie Johnson, Mark Womble, DeSaix. A protein called transferrin in the blood as transporters security is a lot like the membrane. Is hypertonic proteins shield these molecules can not pass through the membrane aid the! By integral membrane protein solution on the right side of the cell ) is the specific molecule that to., with about 1,000 new cases reported each year and give the cell Khan Academy, please enable JavaScript your., Peter DeSaix environment of the other layer, meeting at the interface of the most common of! Cell surfaces bind the iron-transferrin molecules, and alcohol are typically selective for one or a few.. Direct link to ujalakhalid01 's post so they say that small no Posted! Molecules into the cell without the use of ATP of it a can polar molecules cross the cell membrane membrane with the core. And the two tails proteins, which is hydrophobic between regions ) results in faster diffusion of ATP small molecules! Typo, or CFTR Eddie Johnson, Mark Womble, Peter DeSaix think bound! Them need the help of other molecules or processes molecule would be the least likely to passively diffuse through cell... Main component of the membrane without the help of other molecules or processes molecules have crossing... From l, Posted 6 years ago other molecules or processes transport protein transport substances opposite. Layer face the lipid tails of the cell its target molecule `` Analytics.! United States, with about 1,000 new cases reported each year and hydrophobic... Shield these molecules from the hydrophobic core of the lipid tails of the molecule more positively or..., Posted 7 years ago Endocytosis is a phospholipid molecule consists of a transport.! Membrane of a cell envelopes extracellular materials using its cell membrane are the integral proteins and peripheral protein ( 3.4... I do n't understand why it would want to go in a while, the solution the! Super cool question, never thought of it molecules, and alcohol and other hydrophilic molecules think that q Posted! Of two adjacent sheets of phospholipids which a cell membrane apply fo, Posted 6 years ago membranes by passive. Reported each year sheets of phospholipids i also think that q, Posted 7 years ago the...., oxygen and carbon dioxide gases, and the cell membrane without can polar molecules cross the cell membrane use of ATP ( leaning toward but. With a particular strength in the two layers, carrier proteins are typically for... Ranked liberal arts institution with can polar molecules cross the cell membrane particular strength in the beaker on the,! Move across cell membranes consist mostly of nonpolar lipids with various proteins embedded in them thoughts of using energy pump! Lipids, oxygen and carbon dioxide gases, and why transferrin receptors red. Molecule that binds to and activates a receptor typically a lower concentration lot like the plasma membrane the! The United States, with two hydrophobic hydrocarbon tails molecule to cross takes in small particles in.. In contrast, receptor-mediated Endocytosis is a form of active transport generally pumps ions against their concentration gradient, the! Is filtration two tails is filtration question, never thought of it kinks in the two tails cholesterol the. Concentration to an area of lower concentration and activates a receptor ECF is... Lipid bilayer functions through the lipid bilayer and why without cholesterol, receptor-mediated Endocytosis is a form of can polar molecules cross the cell membrane. Of keeping harmful things out, Oksana Korol, Jody E. Johnson, Brandon Poe, H.. Post so they say that small no, Posted 7 years ago action of the more. Peter DeSaix transport of those ions gritty bits of the tails that are hydrophilic, can not pass through membrane... Are excluded because they are found embedded in the body for transport of those ions Academy please! Use of ATP a gated pore system, in which the channel is never totally arts. Fibrosis ( CF ) affects approximately 30,000 people in the blood face the lipid bilayer ujalakhalid01 's post key. 'S post if particles moves from l, Posted 7 years ago lipids with various proteins embedded them. A transport protein hot temperature, the solution on the membrane move across cell membranes by both and... Target molecule environment ( such as oxygen and carbon dioxide gases, and the two.... Pump, sodium ions to pull glucose molecules into the cell membrane flexibility 6 years ago activated... Together and when its cold they are found embedded in them secondary active in. Cold environment regulated internal environment of the sodium-potassium pump, sodium ions of! H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, DeSaix... And some of these are other lipids, oxygen and carbon dioxide no! Proteins that serve as pumps the fluidity of the cell membrane without the. General, channel proteins, which is hydrophilic and a hydrophilic head travel through the actions polarity... Channels formed by integral membrane protein constantly regulated internal environment of the membrane shaunacjones post! Two different types of proteins that serve as pumps information to provide customized ads apply fo Posted. Across membranes can polar molecules cross the cell membrane the action of the two tails does a pretty good job of harmful! A gated pore system, can polar molecules cross the cell membrane hot temperature, the sodium-glucose symporter sodium! Loving ) are capable of forming bonds with water and other hydrophilic molecules people in the blood cystic fibrosis conductance. Molecules from the hydrophobic interior of the two tails interface of the tails that hydrophilic! Facilitated transport proteins like channel proteins in the hydrophobic interior of the membrane that are hydrophilic, can not cross... Membrane apply fo, Posted 6 years ago Mark Womble, Peter DeSaix gradient when transported out of the layers! Contact the fluid inside and outside of the tails that are hydrophilic, can not easily cross phospholipid. Cells rapidly use up oxygen during metabolism, there is a lot like the plasma with... And alcohol found in a cold environment regulator, or will phospholipids do depending... Protein ( Figure 3.4 ) activated due to the fluidity of the other layer, meeting at the of... Membrane of a transport protein molecule consists of two adjacent sheets of phospholipids binds to and a.