The Dynamic Cell Membrane

5.1 What Is the Structure of a Biological Membrane? The general structure of membranes is known as the fluid mosaic model. Lipids- Responsible for the formation of the membrane (due to hydrophillic and phobic portions); make up the majority of the membrane. Proteins- aid in cell and cell parts moving, receiving chemical signals, cell actions, etc. Carbohydrates- labels on cells like name tags

Fluid Mosiac Model http://www.youtube.com/watch?v=Qqsf_UJcfBc Figure 5.1 The Fluid Mosaic Model Fluid Mosiac Model http://www.youtube.com/watch?v=Qqsf_UJcfBc

Singer and Nicolson Scientists who did experiments that evidenced the materials of the membrane consisted of two layers made up phospholipids and proteins. Materials could change position within the membrane.

Figure 3.20 Phospholipids (A) Repeat Fig 3.20A here

5.1 What Is the Structure of a Biological Membrane? Lipids maintain a bilayer organization spontaneously—helps membranes fuse during phagocytosis, vesicle formation, etc Membranes are dynamic… Phospholipids vary—fatty acid chain length, degree of saturation, phosphate groups Membranes may be up to 25 percent cholesterol Contain different proteins and carbohydrate labels

5.1 What Is the Structure of a Biological Membrane? Membranes contain proteins, the number of proteins varies with cell function Some membrane proteins extend across the lipid bilayer—with hydrophobic and hydrophilic regions or domains.

5.1 What Is the Structure of a Biological Membrane? Two types of membrane proteins: Integral membrane proteins span the bilayer, hydrophilic ends protrude on either side. Permanently attached. Peripheral membrane proteins do not penetrate the bilayer.

5.1 What Is the Structure of a Biological Membrane? Transmembrane proteins may have different domains on either side of the membrane. The two sides of the membrane can have very different properties. Some membrane proteins can move freely within the bilayer, while some are anchored to a specific region. Some can be anchored by cytoskeleton elements, or lipid rafts—lipids in semisolid state.

5.1 What Is the Structure of a Biological Membrane? Membranes have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules. Glycolipids- are lipids with a carbohydrate attached. Their role is to provide energy and also serve as markers for cellular recognition Glycoproteins- are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. (The carbohydrate is attached to the protein)

Figure 5.1 The Fluid Mosaic Model

5.3 What Are the Passive Processes of Membrane Transport? Membranes have selective permeability—some substances can pass through, but not others Passive transport—no outside energy required—diffusion Active transport—energy required

Cell Transport - Review of Solutions Solute - what is being dissolved Solvent - what is doing the dissolving (usually water) Solution - the resulting mixture

5.3 What Are the Passive Processes of Membrane Transport? Diffusion: the process of random movement toward equilibrium from high concentration to low concentration Equilibrium—particles continue to move, but there is no net change in distribution

5.3 What Are the Passive Processes of Membrane Transport? Diffusion rate depends on: Diameter of the molecules or ions Temperature of the solution Electric charges Concentration gradient

5.3 What Are the Passive Processes of Membrane Transport? Simple diffusion: small molecules pass through the lipid bilayer. Lipid soluble molecules can diffuse across the membrane, as can water, oxygen, carbon dioxide and steroids (estrogen and testosterone). Electrically charged and polar molecules can not pass through easily like Ca+, Na+, Cl-.

5.3 What Are the Passive Processes of Membrane Transport? Osmosis: the diffusion of water If two solutions are separated by a membrane that allows water, but not solutes to pass through, water will diffuse from the region of higher water concentration (lower solute concentration) to the region of lower water concentration (higher solute concentration).

5.3 What Are the Passive Processes of Membrane Transport? Isotonic solution: equal solute concentration (and equal water concentration) Hypertonic solution: higher solute concentration Hypotonic solution: lower solute concentration

Figure 5.9 Osmosis Can Modify the Shapes of Cells

5.3 What Are the Passive Processes of Membrane Transport? Water will diffuse (net movement) from a hypotonic solution across a membrane to a hypertonic solution and vice versa. This OSMOTIC PRESSURE causes: Increased H20 pressure: Animal cells to burst when placed in a hypotonic solution (lyse). Plant cells with rigid cell walls build up internal pressure that keeps more water from entering— turgor pressure/cells are turgid

Decreased H20 pressure: Animal cells crenate (cells shrivel) Plant cell membrane pulls away from wall (plasmolysis) and cells become flaccid

5.3 What Are the Passive Processes of Membrane Transport? Facilitated diffusion (passive): Polar molecules can cross the membrane through channel proteins and carrier proteins. Channel proteins have a central pore lined with polar amino acids.

5.3 What Are the Passive Processes of Membrane Transport? Ion channels: important channel proteins Most are gated—can be closed or open to ion passage Gate opens when protein is stimulated to change its shape. Stimulus can be a molecule (ligand-gated) or electrical charge resulting from many ions (voltage-gated).

Figure 5.10 A Gated Channel Protein Opens in Response to a Stimulus

Examples of PASSIVE Cell Transport

Concentration Gradient When there is more of one type of molecule on one side of the membrane than the other.

5.3 What Are the Passive Processes of Membrane Transport? Membrane potential is a charge imbalance across a membrane. Usually with a more negative charge inside and more positive charge outside

Membrane potential function Cell charge imbalance acts as a battery and the charge can activate activities within the cell

From low concentration to high concentration 5.4 How Do Substances Cross Membranes against a Concentration Gradient? Active transport: moves substances against a concentration gradient—requires energy. From low concentration to high concentration

The sodium–potassium pump (Na+–K+) is primary active transport. 5.4 How Do Substances Cross Membranes against a Concentration Gradient? The sodium–potassium pump (Na+–K+) is primary active transport. Found in all animal cells The “pump” is an integral membrane glycoprotein. It is an antiport. https://www.youtube.com/watch?v=v9THf G4ZoN4

Aids in uptake of amino acids and sugars 5.4 How Do Substances Cross Membranes against a Concentration Gradient? Energy can be “regained” by letting ions move across a membrane with the concentration gradient—secondary active transport. Aids in uptake of amino acids and sugars Uses symports and antiports

Figure 5.14 Primary Active Transport: The Sodium–Potassium Pump

Figure 5.15 Secondary Active Transport

Active transport involves three kinds of proteins: • Uniports 5.4 How Do Substances Cross Membranes against a Concentration Gradient? Active transport involves three kinds of proteins: • Uniports • Symports • Antiports

Figure 5.13 Three Types of Proteins for Active Transport

5.5 How Do Large Molecules Enter and Leave a Cell? Macromolecules (proteins, polysaccharides, nucleic acids) are too large to cross the membrane. They can be taken in or excreted by means of vesicles.

5.5 How Do Large Molecules Enter and Leave a Cell? Endocytosis: processes that bring molecules and cells into a eukaryotic cell. The plasma membrane folds in or invaginates around the material, forming a vesicle.

5.5 How Do Large Molecules Enter and Leave a Cell? Phagocytosis: molecules or entire cells are engulfed. Some protists feed in this way. Some white blood cells engulf foreign substances. A food vacuole or a phagosome forms, which fuses with a lysosome.

5.5 How Do Large Molecules Enter and Leave a Cell? Pinocytosis: a vesicle forms to bring small dissolved substances or fluids into a cell. Vesicles are much smaller than in phagocytosis. Pinocytosis is constant in endothelial (capillary) cells.

5.5 How Do Large Molecules Enter and Leave a Cell? Receptor mediated endocytosis: highly specific Depends on receptor proteins—integral membrane proteins—to bind to specific substances. Sites are called coated pits— coated with other proteins such as clathrin

Figure 5.17 Formation of a Coated Vesicle (Part 1)

Figure 5.17 Formation of a Coated Vesicle (Part 2)

5.5 How Do Large Molecules Enter and Leave a Cell? Exocytosis: material in vesicles is expelled from a cell Indigestible materials are expelled. Other materials leave cells such as digestive enzymes and neurotransmitters.

Figure 5.16 Endocytosis and Exocytosis (B)

5.6 What Are Some Other Functions of Membranes? Membranes help transform energy: Inner mitochondrial membranes— energy from fuel molecules is transformed to ATP Thylakoid membranes of chloroplasts transform light energy to chemical bonds.

Figure 5.18 More Membrane Functions (A)

5.6 What Are Some Other Functions of Membranes? Membrane proteins can organize chemical reactions. Many cellular processes involve a series of enzyme-catalyzed reactions—all the molecules must come together for these to occur. Forms an “assembly line” of enzymes.

Figure 5.18 More Membrane Functions (B)

5.6 What Are Some Other Functions of Membranes? Membrane proteins process information. Binding of a specific ligand can initiate, stop, or change cell functions.

Figure 5.18 More Membrane Functions (C)

5.6 What Are Some Other Functions of Membranes? The cholera toxin: One subunit binds to a cell surface receptor—the toxin molecule changes shape and allows the other subunit to enter the cell. The subunit acts as an enzyme to modify a peripheral protein—this opens chloride channels in the membrane. Cl− and Na+ accumulate in the intestines, followed by osmotic loss of water.