Name: facilitated transport across cell membranes
Uploaded: 2023-04-30T09:15:55.000Z
Duration: 1 min 30 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Biology

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

9-12.B.1.4.a

Membrane Proteins

Membrane proteins fall into two major categories: peripheral and integral.
Peripheral membrane proteins associate with phospholipid heads or hydrophilic domains of integral proteins by non-covalent interactions.
In contrast, integral proteins are amphipathic molecules with their hydrophilic region facing the cytoplasm or extracellular fluid and the hydrophobic domain interacting with the phospholipid tails.
Integral membrane proteins can be further classified as monotopic, bitopic, and polytopic proteins.
Monotopic proteins are embedded into a single face of the membrane, whereas bitopic proteins with an alpha-helix structure span the entire lipid bilayer only once.
Polytopic proteins extend across the membrane multiple times and consist of multiple alpha-helices or a cylindrical beta-sheet, known as a beta-barrel. Bitopic and polytopic proteins are also known as transmembrane proteins.
Integral proteins perform diverse functions such as transferring molecules and signals across the cell membrane.
Many peripheral proteins also participate in cell signaling cascades as they can easily detach from the membrane. Other peripheral proteins link the membrane with the cytoskeleton, providing structural support.

membrane proteins

Plasma membranes have integral transmembrane proteins involved in facilitated transport. These proteins are collectively referred to as transport proteins, and they function as either channels for the material or as carriers themselves. Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids and a hydrophilic channel through their core that provides a hydrated opening for solutes to pass through the membrane layers. Passage through the channel allows polar compounds to avoid the nonpolar central layer of the plasma membrane that would otherwise slow or prevent their entry into the cell.
Aquaporins are channel proteins that allow water to pass through the membrane at a high rate. Channel proteins are either open at all times or &#34;gated,&#34; with the opening of the channel being regulated. The attachment of a particular ion to the channel protein may control the opening, or other mechanisms or substances may be involved. In some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues, a gate must be opened to allow the passage. An example of this occurs in the kidney, where both forms of channels are found in different parts of the renal tubules. Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes.
Another type of protein embedded in the plasma membrane is a carrier protein. This protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior; depending on the gradient, the material may move in the opposite direction. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. For example, glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney. This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins carriers can handle, the excess is excreted from the body in the urine. In a diabetic individual, this is described as &#34;spilling glucose into the urine.&#34; A different group of carrier proteins called glucose transport proteins, or GLUTs, are involved in transporting glucose and other hexose sugars through plasma membranes within the body. Channel proteins transport much more quickly than carrier proteins.
This text is adapted from <a href="https://openstax.org/books/biology-2e/pages/5-2-passive-transport">Openstax Biology 2e, Section 5.2</a>

Plasma membranes have integral transmembrane proteins involved in facilitated transport. These proteins are collectively referred to as transport ...

Education

JoVE Core

Cell Biology

Unit 1

Membrane Structure and Components

KEY TERMS AND CONCEPTS

facilitated transport across cell membranes

Facilitated transport uses membrane proteins to help materials cross the plasma membrane. These integral transmembrane proteins act as channels or carriers. They move selected substances through the membrane more easily than the lipid layer alone would allow.

Channel proteins provide a hydrophilic, or water-friendly, path through the membrane. Their core forms a hydrated opening that lets solutes pass between the inside and outside fluids of the cell. This helps polar compounds avoid the nonpolar middle of the plasma membrane, which would otherwise slow or block entry. Aquaporins are channel proteins that move water at a very high rate.

Some channels are open all the time, while others are gated. In gated channels, the opening is controlled by a signal such as the binding of a specific ion, and other mechanisms or substances may also regulate them. In some tissues, sodium and chloride ions pass freely through open channels. In other tissues, a gate must open first.

The kidney shows both channel types in different parts of the renal tubules. Cells that transmit electrical impulses, such as nerve and muscle cells, also use gated channels for sodium, potassium, and calcium. These channels help control movement across the membrane in specialized tissues.

Carrier proteins work differently from channels. A carrier binds a substance and changes shape to move the bound molecule from one side of the membrane to the other. Depending on the gradient, the material may move in the opposite direction. Each carrier is specific for one substance, and there are only a limited number in any membrane.

Glucose handling in the kidney shows how carrier limits matter. Glucose, water, salts, ions, and amino acids are filtered in one part of the kidney, and the filtrate is reabsorbed in another part. If more glucose is present than the carrier proteins can move, the extra glucose is excreted in urine. In diabetes, this is often described as spilling glucose into the urine. Glucose transport proteins, or GLUTs, are another carrier group that move glucose and other hexose sugars through plasma membranes in the body. Channel proteins move substances much faster than carrier proteins.

Facilitated Transport Across Cell Membranes

facilitated-transport-across-cell-membranes

what are membranes?

Membrane Receptors and Cell Signals

The plasma membrane sets the outer boundary of a cell and helps control how the cell interacts with its environment. It lets some substances enter, keeps others out, and releases some materials in controlled amounts. The membrane also needs to stay flexible so cells such as red and white blood cells can change shape as they move through narrow capillaries. The membrane surface also carries markers that help cells recognize one another. This recognition is important during early development,...

membrane fluidity

How Cell Membranes Stay Flexible

Cell membranes stay flexible because of the fluid mosaic model. The plasma membrane is a mosaic of phospholipids, cholesterol, proteins, and carbohydrates. These parts are separate but loosely attached, so the membrane keeps a fluid character. The membrane is still fairly rigid, but it can flow and self-seal after a very fine needle passes through it. Phospholipid tails also affect membrane fluidity. Saturated fatty acids in phospholipids have straight tails because they are packed with...

fluid mosaic model

Fluid Mosaic Model of the Membrane

The fluid mosaic model explains the structure and function of the plasma membrane. Scientists first identified the plasma membrane in the 1890s, and by 1915 they had identified its main chemical parts: lipids and proteins. Early electron micrographs showed a railroad track pattern, which helped lead to the first widely accepted membrane model in 1935.

membrane lipids

Membrane Lipids in Cells and Tissues

Membrane lipids give biological membranes their structure and help set their properties. In mammalian cells, lipids make up about 50% of the membrane on average. The amount can be much lower in the inner mitochondrial membrane or much higher in the myelin sheath around nerve cells. Several phospholipids are common in mammalian membranes. These include phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin. At physiological pH, phosphatidylserine carries a negative...

asymmetric lipid bilayer

Membrane Lipid Unevenness

Biological membranes have an uneven mix of lipids between the inner and outer layers. This membrane asymmetry gives each side a different chemical makeup and supports different cell functions. Evidence for this uneven layout came from treating an erythrocyte membrane with phospholipase. Phospholipase is an enzyme that breaks lipids into fatty acids and hydrophilic groups. It acted only on the outer layer, while the inner layer stayed intact. The treatment degraded about 80% of total membrane...

membrane asymmetry regulating transporters

membrane-asymmetry-regulating-transporters

Membrane Lipid Flipping and Transport

Membrane lipid flipping and transport help cells keep an uneven, or asymmetric, distribution of phospholipids across the membrane. This balance is controlled by flippase, floppase, and scramblase enzymes, which move lipids between the two membrane leaflets. Flippases move specific phospholipids from the outer leaflet to the inner leaflet. In eukaryotes, these enzymes are type-IV P-type ATPases, also called P4-ATPases. They are membrane pumps that use ATP and contain one transmembrane domain...

membrane carbohydrates

Membrane Sugars and Cell Identity

Membrane carbohydrates help give cells their surface identity and support plasma membrane function. The plasma membrane is a dynamic barrier made of lipids, proteins, and carbohydrates. These sugar chains have unique structural and chemical traits that help the membrane work properly for cell growth and survival. Membrane carbohydrates do not have a hydrophobic region, so they are found only on the outer surface of the cell. Sugar addition, or glycosylation, of proteins occurs in the lumens of...

Facilitated transport uses membrane proteins to help materials cross the plasma membrane. These integral transmembrane proteins act as channels or carriers. They move selected substances through the membrane more easily than the lipid layer alone would allow. Channel proteins provide a hydrophilic, or water-friendly, path through the membrane. Their core forms a hydrated opening that lets solutes pass between the inside and outside fluids of the cell. This helps polar compounds avoid the...

lipids as anchors

Lipid Anchors in Membrane Proteins

Lipid anchors help attach proteins to the plasma membrane. In eukaryotic cells, the main lipid anchors are prenyl groups, fatty acyl groups, and glycosylphosphatidylinositol, or GPI, groups. Prenyl and fatty acyl groups anchor proteins on the cytosolic side of the membrane. GPI anchors proteins on the extracellular side. Prenylated proteins often have a CaaX motif at the carboxy terminus. In this sequence, C means cysteine, a means any aliphatic amino acid, and X means any other amino acid...

single-pass transmembrane proteins

How Membrane Proteins Span Cell Membranes

Membrane proteins help cells communicate, send signals, stick to other cells, and move molecules across the membrane. Integral membrane proteins are tightly linked to the cell membrane and carry out these jobs. Some integral membrane proteins sit only in one membrane layer. Fatty acid amide hydrolase is one example, and it is found on the cytoplasmic side of the membrane monolayer. Other integral membrane proteins are transmembrane proteins, which span across the membrane. These proteins have...

multi-pass transmembrane proteins and &#946;-barrels

multi-pass-transmembrane-proteins-and-barrels

Membrane-Spanning Helices and β-Barrels

Membrane-spanning proteins can cross the cell membrane more than once. Their transmembrane chain usually folds into either an alpha-helix or a beta-strand. Alpha-helix multi-pass proteins are common, while beta-strand proteins are found mainly in gram-negative bacteria, mitochondria, and chloroplasts. Some multi-pass alpha-helix proteins are G-protein-linked receptors, or GPCRs, and bacteriorhodopsin. GPCRs have seven transmembrane alpha-helices. Each receptor also has its own extracellular...

detergent purification of membrane proteins

detergent-purification-of-membrane-proteins

How Detergents Solubilize Membrane Proteins

Detergents help solubilize membrane proteins so they can be purified from the lipid bilayer. Their hydrophobic region can replace membrane phospholipids and keep integral membrane proteins in solution. A detergent works well only when its monomers reach the critical micelle concentration, or CMC. At that point, the molecules assemble into micelles, which are small clusters of detergent. Above the CMC, free detergent monomers stay in equilibrium with the micelles. The CMC varies with the...

protein diffusion in the membrane

How Membrane Proteins Move Side to Side

Membrane proteins can move across the cell surface by lateral diffusion, and they can also rotate in place. Scientists confirmed lateral movement with a cell fusion experiment. Mouse and human cells were fused to make hybrid cells. The membrane proteins on the two cells were tagged with red and green fluorescent markers. At first, each color stayed on its own side of the fused cell. After 40 minutes, the colors mixed across the whole cell surface, showing that the proteins had moved laterally. <br...

membrane domains

Lipid Rafts and Sperm Membrane Regions

Membrane domains organize specific lipids and proteins in defined parts of a cell membrane. This arrangement supports cell signaling, adhesion, and other important cellular processes. These domains can vary in size, composition, function, and how long they last. Some membrane domains are built from proteins. The membrane can contain distinct protein groups that carry out a cell’s specific functions. A clear example is the plasma membrane of a human sperm cell, which has different proteins in...

mechanisms of membrane domain formation

How Cell Membranes Form Domains

Cell membranes form domains when lipids and proteins gather into distinct regions. These regions, or membrane domains, are shaped by different physical properties that help some molecules stay together while others move apart. Some domains form through protein-protein interactions. Others depend on specific lipids, such as sphingolipids and sterols. Large proteins, including bacteriorhodopsin, can also cluster and create visible membrane domains. Membrane proteins can be held in place by...

mechanisms of membrane-bending

How Proteins Shape Cell Membranes

Cell membranes can bend into many shapes because they are flexible and made of a fluid mosaic. That bending can be temporary, as in vesicles, or long-lasting, as in microvilli. Cells control the size, place, and time of membrane curvature. Membrane bending can start inside the membrane itself or from proteins attaching to it. Changes in lipid makeup can create curvature, and proteins can help by making the membrane asymmetric or by clustering certain lipids. The shape of integral proteins also...

key terms and concepts

Learn how channel proteins, aquaporins, gated channels, and carrier proteins help substances cross the plasma membrane in kidney, nerve, and muscle cells.

Facilitated Transport Across Cell Membranes | JoVE K12

cell biology

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9-12.B.1.2.a

Facilitated Transport Across Cell Membranes

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