Membrane Proteins and Functions

• Cell membranes separate the internal environment of a cell or organelle from its environment.
• Selective permeability is the ability to choose which molecules can enter or exit based on size, charge, and chemical properties.
• Damaging compounds can be kept out and entry of needed compounds is allowed by membranes.
• Compartmentalization occurs when organelles sequester appropriate chemicals in an enclosed area.
• Compartmentalization decreases interference and allows reactants to collide more frequently.
• Cell membranes maintain cell or organelle shape by providing structural support.
• Endocytosis and exocytosis are key functions of biological membranes.
• Biological membranes are described as a fluid mosaic model.
• Lipids and proteins can move fluidly relative to each other within the membrane.
• Biological membranes are a mosaic made up of many discrete components, including lipids, proteins, and carbohydrates.
• Phospholipids form the basic framework of a membrane.
• Proteins are embedded in the membrane or loosely attached to its surface and carry out key functions.
• Carbohydrates may be attached to membrane lipids and proteins and also carry out key functions.
• Membranes of specific cells and organelles differ in the kinds of proteins and lipids.
• Phospholipids are amphipathic molecules.
• Hydrophilic, polar heads are found on the surface of the membrane.
• Hydrophobic, nonpolar tails are found in the interior of the membrane.
• Biochemical properties of phospholipids differ and this affects the fluidity of the phospholipid bilayer.
• The plasma membrane contains a cytosolic leaflet and an extracellular leaflet
• The leaflet is half of a phospholipid bilayer, each side faces a different region with different properties corresponding to different functions and asymmetry.
• Membranes contain diverse proteins based on the membrane's function.

Major Groups of Membrane Proteins

• Integral proteins penetrate the hydrophobic interior of the lipid bilayer.
• Some integral proteins extend across the lipid bilayer, called transmembrane proteins, others are only partially embedded.
• Peripheral proteins are not embedded in the lipid bilayer at all, and are loosely bound to the surface of the membrane.

Major Functions of Membrane Proteins

• Transport occurs through hydrophilic channels, carriers, and pumps
• Enzymatic activity is produced by ATP synthase
• Signal transduction occurs when receptor binding sites fit the shape of chemical messengers
• Glycoproteins serve as recognition identification tags.
• Intercellular joining: gap junctions communicate, tight junctions control movement of materials and desmosomes provide mechanical stability.
• Attachment to the cytoskeleton and extracellular matrix allows for movement and maintains cell shape.
• Plasma membranes adhere to the extracellular matrix.
• Transmembrane integrin binds to the matrix outside epithelial cells and to actin filaments inside cells.
• Integrin binding is noncovalent and reversible.
• Cells can move within a tissue by the binding and reattaching of integrin to the extracellular matrix.
• Integrin attachment is important for cell movement within developing embryos and can enable the spread of cancer cells.
• When integrins can no longer mediate attachment, the cell separates from the matrix structures.
• Cancer cells may spread throughout the body if they have detached from the extracellular matrix.
• Membranes have diverse carbohydrates and the amount varies based on the membrane's function.

Major Groups of Membrane Carbohydrates

• Glycolipids consist of carbohydrate + lipid and facilitate cell recognition (self vs. non-self).
• Glycoproteins consist of carbohydrate + protein and 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 different cells.
• Carbohydrates allow sorting of cells into tissues and organs in animal embryos.
• The presence of carbohydrates on the outside surface of cells is the basis for the rejection of foreign cells by the immune system (defense).
• Membranes must be able to move and change shape in order to function properly.
• Fluidity results from molecules held together by hydrophobic interactions where lipids and proteins can shift laterally.
• Membrane fluidity must remain stable to hold its shape, allow for protein support, avoid permeability changes, and prevent effects to enzymatic proteins.

Factors Influencing Fluidity

• Low temperature decreases fluidity because it decreases energy and increases the concentration of phospholipids.
• High temperature increases fluidity because it increases energy and decreases the concentration of phospholipids.
• Cholesterol acts as a buffer to maintain stability.
• At low temperatures, cholesterol increases fluidity by increasing the distance between phospholipids.
• At high temperatures, cholesterol decreases fluidity by increasing the number of molecules in the space
• Saturated bonds decrease fluidity (single bonds in fatty acid chains pack tightly)
• Unsaturated bonds increase fluidity (double bond(s) in fatty acid chains provide more distance between them).
• Selective permeability is 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 is regulated.
• Nonpolar molecules can cross the lipid bilayer easily without assistance.
• Polar molecules and ions will need assistance from transport proteins to cross the cell membrane.
• Membrane permeability is crucial for homeostasis.
• Membranes are permeable to solutes that can easily cross and impermeable to those that cannot.
• Passive transport energy comes from the concentration gradient and no energy input is required.
• Two types of passive diffusion: simple and facilitated.
• Facilitated diffusion requires no energy input.
• Substances diffuse according to their concentration gradients, aided by transport proteins.
• Channel proteins are transmembrane proteins that form a tunnel:
• Ion channels allow ions to diffuse across the membrane and are typically gated.
• Aquaporins allow water to pass through the membrane during osmosis.
• Carrier proteins are membrane proteins that bind substances and speed their diffusion across the membrane.
• Active transport is energy-dependent transport of a substance across a biological membrane against a concentration gradient or an electrical gradient.
• The energy source is often adenosine triphosphate (ATP).
• Macromolecules are too large to cross the membrane, so they are taken in or secreted by membrane vesicles.
• Endocytosis brings molecules and cells into a eukaryotic cell as the cell membrane folds inward to form a vesicle.

Three Types of Endocytosis

• Phagocytosis occurs when molecules or entire cells are engulfed.
• Some white blood cells engulf foreign substances by phagocytosis.
• A food vacuole or phagosome forms, which then fuses with a lysosome for digestion.
• Pinocytosis occurs when a vesicle forms to bring small dissolved substances or fluids into a cell.
• Receptor mediated endocytosis is highly specific because macromolecules bind to receptor proteins, which are integral membrane proteins at specific sites.
• Exocytosis occurs when materials packaged in vesicles are secreted from a cell when the vesicle fuses with the cell membrane, sometimes without a pore.

Explore the functions of membrane proteins, cell junctions, and membrane carbohydrates. Understand how these components contribute to cell-cell recognition, mechanical stability, and selective permeability of biological membranes. Match membrane components with their functions.