BIOLOGY Chapter 6: Cell Membranes PDF

Chapter 6 Cell Membranes Functions of Biological Membranes separates the internal environment of the cell or organelle from its external environment selective permeability the ability to choose which molecules can exit or enter based on size, charge, and chemical properties kee...

Chapter 6 Cell Membranes Functions of Biological Membranes separates the internal environment of the cell or organelle from its external environment selective permeability the ability to choose which molecules can exit or enter based on size, charge, and chemical properties keeps damaging compounds out and allows entry of needed compounds compartmentalization organelles sequester appropriate chemicals in an enclosed area decreases interference from outside stressors reactants collide more frequently so the chemical reactions necessary for life occur much more efficiently maintain cell or organelle shape by providing structural support endocytosis and exocytosis Biological Membranes Are Lipid-Protein Bilayers Key Concept #1 Structure of Biological Membranes The general structure of biological membranes is similar among all living organisms. It is described as a fluid mosaic model. “fluid”: lipids and proteins can move relative to each other within the membrane “mosaic”: made up of many discrete components (lipids, proteins and carbohydrates) phospholipids: form the basic framework of a membrane proteins: embedded in the membrane or loosely attached to its surface (carry out key functions) carbohydrates: may be attached to membrane lipids and proteins (carry out key functions) membranes of specific cells and organelles differ in the kinds of proteins and lipids they contain. The framework of a biological membrane is the phospholipid bilayer. Phospholipids are amphipathic molecules. heads (hydrophilic, polar; found on the surface of the membrane) tails (hydrophobic, nonpolar; found in the interior of the membrane) biochemical properties of phospholipids differ; affects the fluidity of the phospholipid bilayer The plasma membrane contains a cytosolic leaflet and an extracellular leaflet. leaflet: half of a phospholipid bilayer; each faces a different region; asymmetrical: different properties = different functions  Structure of Biological Membranes Membranes contain diverse proteins. The number and type varies depending on the membrane’s function. Major Groups of Membrane Proteins integral: penetrate the hydrophobic interior of the lipid bilayer; some extend across the lipid bilayer (transmembrane); others are only partially embedded peripheral: not embedded in the lipid bilayer at all; loosely bound to the surface of the membrane Major Functions of Membrane Proteins transport: hydrophilic channels, carriers, and pumps enzymatic activity: ATP synthase signal transduction: receptor binding sites fit the shape of chemical messengers recognition: glycoproteins serve as identification tags intercellular joining: gap junctions (communication), tight junctions (movement of materials), desmosomes (mechanical stability) attachment to the cytoskeleton and extracellular matrix: movement, maintains cell shape The Cell Membrane Is Important in Cell Adhesion and Recognition Key Concept #2 6.2 The Cell Membrane Is Important in Cell Adhesion and Recognition Plasma membranes adhere to the extracellular matrix. Example: The transmembrane protein integrin binds to the matrix outside epithelial cells and to actin filaments inside the cells. The binding is noncovalent and reversible. Cells can move within a tissue by the binding and reattaching of integrin to the extracellular matrix. This is important for cell movement within developing embryos but also allows for the spread of cancer cells. When integrins no longer mediate this attachment, the cell separates from the matrix structures. Cancer cells may spread throughout the body if they have detached from the extracellular matrix. Structure of Biological Membranes Membranes contain diverse carbohydrates. The number and type varies depending on the membrane’s function. Major Groups of Membrane Carbohydrates glycolipids: (carbohydrate + lipid) facilitate cell recognition (self vs. non-self) glycoproteins: (carbohydrate + protein) serve as receptors for chemical signals Major Functions of Membrane Carbohydrates: Cell-Cell Recognition diversity and location of carbohydrates on the membrane allow them to function as markers that distinguish one cell from another allows for sorting of cells into tissues and organs in an animal embryo basis for the rejection of foreign cells by the immune system (defense) Figure 6.1 Carbohydrates are attached to the outer surface of proteins (forming glycoproteins) or lipids Membrane proteins may consist of multiple polypeptide subunits Cholesterol alters (forming glycolipids). membrane fluidity. Outside of cell Phospholipid bilayer Inside of cell Peripheral membrane Some integral Some integral membrane Anchored membrane proteins are noncovalently membrane proteins proteins are partially proteins are covalently attached to either membrane span the entire embedded in the bilayer. bonded to lipids that surface. membrane. are inserted into the membrane.  Fluidity of Biological Membranes To function properly, membranes must be able to move and change shape. Fluidity allows for this. molecules are held together by hydrophobic interactions; most lipids and proteins can shift laterally Membrane fluidity must remain stable. increased fluidity: membrane can’t hold its shape, or support protein function decreased fluidity: permeability changes, enzymatic proteins are affected Factors Influencing Fluidity Temperature Low, fluidity decreases: decreased energy = decreased movement, so phospholipids are tightly packed High, fluidity increases: increased energy = increased movement, so phospholipids move farther apart Cholesterol (acts as a buffer to maintain stability) At low temps, fluidity increases: cholesterol between phospholipids increases distance between them At high temps, fluidity decreases: cholesterol between phospholipids increases number of molecules in the space Type of fatty acids in phospholipids Saturated, fluidity decreases: single bonds only in fatty acid chains so they are tightly packed Unsaturated, fluidity increases: double bond(s) in fatty acid chains so more distance between them selective permeability: form = function selective permeability: a property of biological membranes that allows them to regulate the passage of substances across them Both the type of substance crossing the membrane and the rate of passage of substances across the membrane is regulated. Nonpolar molecules can cross the lipid bilayer easily without assistance; polar molecules and ions will need assistance from transport proteins. Membrane permeability is crucial: If certain molecules or ions pass through a lipid bilayer more or less readily or rapidly than others, the homeostasis of an organelle or cell can be compromised. Substances Can Cross Membranes by Passive Processes Key Concept #3 6.3 Substances Can Cross Membranes by Passive Processes A membrane is permeable to solutes that can easily cross it and impermeable to those that cannot. passive transport energy for passive transport comes from the concentration gradient; no energy INPUT is required two types of passive transport: simple diffusion (We will discuss simple diffusion in lab.) facilitated diffusion 6.3 Substances Can Cross Membranes by Passive Processes facilitated diffusion requires no INPUT of energy substances diffuse according to their concentration gradients aided by transport proteins channel proteins: transmembrane proteins that form a tunnel ion channels: allow ions to diffuse across the membrane; most are gated so can be closed or opened to ion passage aquaporins: allow water to pass through the membrane during osmosis carrier proteins: membrane proteins that bind substances and speed their diffusion through the bilayer (examples: polar substances such as sugars and amino acids) Active Transport across Membranes Requires Energy Key Concept #4 6.3 Substances Can Cross Membranes by Passive Processes active transport energy-dependent transport of a substance across a biological membrane against: a concentration gradient (a region of ↓ substance [ ] to a region of ↑ substance [ ]) an electrical gradient The energy source is often adenosine triphosphate (ATP). Large Molecules Enter and Leave a Cell through Vesicles Key Concept #5 6.5 Large Molecules Enter and Leave a Cell through Vesicles Macromolecules are too large to cross the membrane. They can be taken in or secreted by membrane vesicles. endocytosis: brings molecules and cells into a eukaryotic cell; the cell membrane folds inward around the material, forming a vesicle 6.5 Large Molecules Enter and Leave a Cell through Vesicles Three Types of Endocytosis phagocytosis: molecules or entire cells are engulfed Some white blood cells engulf foreign substances in this way. A food vacuole or phagosome forms, which fuses with a lysosome (digestion). pinocytosis: a vesicle forms to bring small dissolved substances or fluids into a cell receptor mediated endocytosis (highly specific) macromolecules to be moved bind to receptor proteins: integral membrane proteins located at specific sites on the cell membrane 6.5 Large Molecules Enter and Leave a Cell through Vesicles exocytosis: materials packaged in vesicles are secreted from a cell when it fuses with the cell membrane; a pore may also form without membrane fusion