How does the small size of cells impact the effectiveness of the cell membrane? Metabolism requires that cells be small  As a cell grows, its volume grows proportionately more than its surface area  Cells need a high surface area to volume ratio to exchange materials with their environment through plasma membrane.

Membrane Structure & Function

Functions of the Cell Membrane Isolates the cytoplasm from the external environment Regulates the flow of materials into and out of the cell Communicates with other cells

The Fluid Mosaic Model Currently accepted model of the cell membrane Proposed by Singer and Nicolson in 1972 Phospholipid bilayer  Hydrophilic “head” – exposed to the outside  Hydrophobic “tail” – hides inside Membrane proteins are randomly dispersed in phospholipid bilayer

Fluid Mosaic Model

Fluidity of the Membrane The lipids and proteins can drift throughout the membrane Membrane is NOT stiff/rigid Cholesterol makes the membrane stronger by limiting the movement of phospholipids

Membrane as a Mosaic Lipid bilayer has membrane proteins “stuck” in it Integral proteins  Go through the membrane (both sides) Peripheral proteins  attached to the surface of the membrane

Selective Permeability of the Cell Membrane The cell membrane can “choose” what enters and exits a cell “Gatekeeper of the Cell”

Passive Transport Definition:  Diffusion of a substance that does NOT require the input of energy by the cell 3 types of passive transport:  Diffusion  Osmosis  Facilitated diffusion

Diffusion Movement of molecules from high to low concentration until equilibrium is reached. Passive Transport= no energy required What substances may diffuse across membrane? Nonpolar (non- charged) molecules; small polar molecules

Diffusion Each substance diffuses down its OWN concentration gradient and is unaffected by concentration gradients of other substances

Diffusion Does all movement stop once equilibrium is reached??  NO!!  Equal rates in all directions

Osmosis Def’n:  The passive transport of water across a selectively permeable membrane  Hyper-, hypo-, iso- tonic RELATIVE TERMS!! Always referring to solute concentration Water moves from areas of lower concentration of solutes (hypotonic) to areas of higher solute concentration (hypertonic)

Osmosis in Plant and Animal Cells Animal Cells:  Plasmolysis Occurs when a cell is in a hypertonic solution Water goes from cell into solution  Cytolysis Occurs when a cell is in a hypotonic solution Water goes from solution into cell Plant Cells:  Turgid  Flaccid

Osmosis in Plant and Animal Cells

Facilitated Diffusion Def’n:  The diffusion of large molecules across the cell membrane using transport proteins  Glucose; ions Does NOT require an input of energy  Solute is still moving down its concentration gradient

Facilitated Diffusion Transport proteins are specific for their solutes Transport proteins can become saturated Some are gated channels:  Chemical or electrical stimulus causes them to open

Example Which direction will sucrose move? Which direction will glucose move? Which direction will fructose move?

Active Transport Def’n:  The pumping of solutes against their gradients  Requires an input of energy by the cell  Used so cells can “stockpile” extra supplies

Electrogenic Pumps Voltage across membranes = stored energy that can be used for cellular work Sodium-Potassium Pump:  3 Na+ OUT of the cell for every 2 K+ pumped in  Net transfer of one positive charge from cytoplasm to ECF Very important for transferring signals between nerve cells

Sodium-Potassium Pump

Cotransport Substance that has been pumped across a membrane can do work as it “leaks” back by diffusion Another substance “hitches a ride”

Endocytosis & Exocytosis Def’n:  The movement of large molecules (polysaccharides, proteins, etc.) across the membrane  Endocytosis = cell takes in macromolecules  Exocytosis = cell secretes macromolecules

Endocytosis Cell takes in macromolecules by forming vesicles made from the plasma membrane Phagocytosis = “cell eating”  Large molecules Pinocytosis = “cell drinking”  Small molecules & liquids Receptor-mediated endocytosis = seeks out specific molecules

Endocytosis

Exocytosis The cell secretes macromolecules by the fusion of vesicles with the plasma membrane Used to release hormones, chemical signals, etc.

Signal Transduction Pathway We’ll discuss this later… But for now:  The cell’s plasma membrane is an important player in a cell’s ability to sense and respond to environmental change

Local (Short-Distance) Signaling Cells may communicate by direct contact  Plasmodesmata in plant cells  Gap junctions in animal cells Animal cells can also use cell-cell recognition  Membrane-bound surface molecules can interact and communicate

Cellular Organelles Extracellular Matrix:  Found in animal cells  Made up of glycoproteins (collagen) & proteoglycans Proteins + carbohydrates  Provides support and anchorage for cells  Differs from one cell type to another

Intercellular Junctions Neighboring cells are connected to one another Plant cells:  Plasmodesmata: Channels in the cell wall through which strands of cytoplasm pass through and connect the living contents of adjacent cells

Intercellular Junctions (Animal Cells) Tight junctions- membrane proteins interlock Desmosomes, (anchoring junction)- intermediate filaments “sew” membranes together Gap junctions- channels align allowing materials to flow between cells

Intercellular Junctions Tight junctions:  Membranes of neighboring cells are fused  Form a continuous “belt” around a cell  Example: junction between epidermis of the skin

Intercellular Junctions Desmosomes  Anchoring junctions  Act as rivets  Muscle cells are held together by desmosomes.  What happens when a muscle is torn?

Intercellular Junctions Gap junctions  Communicating junctions  Cytoplasmic channels between adjacent cells  Salts, sugars, AAs, etc. can pass through