5 Cell Membranes and Signaling
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid A membrane s structure and functions are determined by its constituents: lipids, proteins, and carbohydrates. The general structure of membranes is known as the fluid mosaic model. Phospholipids form a bilayer which is like a lake in which a variety of proteins float.
Figure 5.1 Membrane Molecular Structure
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Lipids form the hydrophobic core of the membrane. Most lipid molecules are phospholipids with two regions: Hydrophilic regions electrically charged heads that associate with water molecules Hydrophobic regions nonpolar fatty acid tails that do not dissolve in water
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid A bilayer is formed when the fatty acid tails associate with each other and the polar heads face the aqueous environment. Bilayer organization helps membranes fuse during vesicle formation and phagocytosis.
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Membranes may differ in lipid composition as there are many types of phospholipids. Phospholipids may differ in: Fatty acid chain length Degree of saturation Kinds of polar groups present
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Two important factors in membrane fluidity: Lipid composition types of fatty acids can increase or decrease fluidity Temperature membrane fluidity decreases in colder conditions
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Biological membranes contain proteins, with varying ratios of phospholipids. Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer. Integral membrane proteins are partly embedded in the phospholipid bilayer.
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Anchored membrane proteins have lipid components that anchor them in the bilayer. Proteins are asymmetrically distributed on the inner and outer membrane surfaces. A transmembrane protein extends through the bilayer on both sides, and may have different functions in its external and transmembrane domains.
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Some membrane proteins can move within the phosopholipid bilayer, while others are restricted. Proteins inside the cell can restrict movement of membrane proteins, as can attachments to the cytoskeleton.
Figure 5.2 Rapid Diffusion of Membrane Proteins
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Plasma membrane carbohydrates are located on the outer membrane and can serve as recognition sites. Glycolipid a carbohydrate bonded to a lipid Glycoprotein a carbohydrate bonded to a protein
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Membranes are constantly changing by forming, transforming into other types, fusing, and breaking down. Though membranes appear similar, there are major chemical differences among the membranes of even a single cell.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Active transport requires input of metabolic energy. Biological membranes allow some substances, and not others, to pass. This is known as selective permeability. Two processes of transport: Passive transport does not require metabolic energy.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Passive transport of a substance can occur through two types of diffusion: Simple diffusion through the phospholipid bilayer Facilitated diffusion through channel proteins or aided by carrier proteins
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Diffusion is the process of random movement toward equilibrium. Speed of diffusion depends on three factors: Diameter of the molecules smaller molecules diffuse faster Temperature of the solution higher temperatures lead to faster diffusion
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion The concentration gradient in the system the greater the concentration gradient in a system, the faster a substance will diffuse A higher concentration inside the cell causes the solute to diffuse out, and a higher concentration outside causes the solute to diffuse in, for many molecules.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Simple diffusion takes place through the phospholipid bilayer. A molecule that is hydrophobic and soluble in lipids can pass through the membrane. Polar molecules do not pass through they are not soluble in the hydrophilic interior and form bonds instead in the aqueous environment near the membrane.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Osmosis is the diffusion of water across membranes. It depends on the concentration of solute molecules on either side of the membrane. Water passes through special membrane channels.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion When comparing two solutions separated by a membrane: A hypertonic solution has a higher solute concentration. Isotonic solutions have equal solute concentrations. A hypotonic solution has a lower solute concentration.
Figure 5.3A Osmosis Can Modify the Shapes of Cells
Figure 5.3B Osmosis Can Modify the Shapes of Cells
Figure 5.3C Osmosis Can Modify the Shapes of Cells
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion The concentration of solutes in the environment determines the direction of osmosis in all animal cells. In other organisms, cell walls limit the volume that can be taken up. Turgor pressure is the internal pressure against the cell wall as it builds up, it prevents more water from entering.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Diffusion may be aided by channel proteins. Channel proteins are integral membrane proteins that form channels across the membrane. Substances can also bind to carrier proteins to speed up diffusion. Both are forms of facilitated diffusion.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Ion channels are a type of channel protein most are gated, and can be opened or closed to ion passage. A gated channel opens when a stimulus causes the channel to change shape. The stimulus may be a ligand, a chemical signal.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion A ligand-gated channel responds to its ligand. A voltage-gated channel opens or closes in response to a change in the voltage across the membrane.
Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Water crosses membranes at a faster rate than simple diffusion. It may hitchhike with ions such as Na + as they pass through channels. Aquaporins are specific channels that allow large amounts of water to move along its concentration gradient.
Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)
Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 2)
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Carrier proteins in the membrane facilitate diffusion by binding substances. Glucose transporters are carrier proteins in mammalian cells. Glucose molecules bind to the carrier protein and cause the protein to change shape it releases glucose on the other side of the membrane.
Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)
Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 2)
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Transport by carrier proteins differs from simple diffusion, though both are driven by the concentration gradient. The facilitated diffusion system can become saturated when all of the carrier molecules are bound, the rate of diffusion reaches its maximum.
Concept 5.3 Some Substances Require Energy to Cross the Membrane Active transport requires the input of energy to move substances against their concentration gradients. Active transport is used to overcome concentration imbalances that are maintained by proteins in the membrane.
Table 5.1 Membrane Transport Mechanisms
Concept 5.3 Some Substances Require Energy to Cross the Membrane The energy source for active transport is often ATP. Active transport is directional and moves a substance against its concentration gradient. A substance moves in the direction of the cell s needs, usually by means of a specific carrier protein.
Concept 5.3 Some Substances Require Energy to Cross the Membrane Two types of active transport: Primary active transport involves hydrolysis of ATP for energy. Secondary active transport uses the energy from an ion concentration gradient, or an electrical gradient.
Concept 5.3 Some Substances Require Energy to Cross the Membrane The sodium potassium (Na + K + ) pump is an integral membrane protein that pumps Na + out of a cell and K + in. One molecule of ATP moves two K + and three Na + ions.
Figure 5.7 Primary Active Transport: The Sodium Potassium Pump
Concept 5.3 Some Substances Require Energy to Cross the Membrane Secondary active transport uses energy that is regained, by letting ions move across the membrane with their concentration gradients. Secondary active transport may begin with passive diffusion of a few ions, or may involve a carrier protein that transports both a substance and ions.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles Macromolecules are too large or too charged to pass through biological membranes and instead pass through vesicles. To take up or to secrete macromolecules, cells must use endocytosis or exocytosis.
Figure 5.8 Endocytosis and Exocytosis (Part 1)
Figure 5.8 Endocytosis and Exocytosis (Part 2)
Concept 5.4 Large Molecules Cross the Membrane via Vesicles Three types of endocytosis brings molecules into the cell: phagocytosis, pinocytosis, and receptor mediated endocytosis. In all three, the membrane invaginates, or folds around the molecules and forms a vesicle. The vesicle then separates from the membrane.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles In phagocytosis ( cellular eating ), part of the membrane engulfs a large particle or cell. A food vacuole (phagosome) forms and usually fuses with a lysosome, where contents are digested.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles In pinocytosis ( cellular drinking ), vesicles also form. The vesicles are smaller and bring in fluids and dissolved substances, as in the endothelium near blood vessels.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles Receptor mediated endocytosis depends on receptors to bind to specific molecules (their ligands). The receptors are integral membrane proteins located in regions called coated pits. The cytoplasmic surface is coated by another protein (often clathrin).
Concept 5.4 Large Molecules Cross the Membrane via Vesicles When receptors bind to their ligands, the coated pit invaginates and forms a coated vesicle. The clathrin stabilizes the vesicle as it carries the macromolecules into the cytoplasm. Once inside, the vesicle loses its clathrin coat and the substance is digested.
Figure 5.9 Receptor-Mediated Endocytosis (Part 1)
Figure 5.9 Receptor-Mediated Endocytosis (Part 2)
Concept 5.4 Large Molecules Cross the Membrane via Vesicles Exocytosis moves materials out of the cell in vesicles. The vesicle membrane fuses with the plasma membrane and the contents are released into the cellular environment. Exocytosis is important in the secretion of substances made in the cell.