Membrane Structure and Function

Membrane Function Membranes organize the chemical activities of cells. The outer plasma membrane forms a boundary between a living cell and its surroundings Exhibits selective permeability Controls traffic of molecules in and out

Membrane Function Internal membranes provide structural order for metabolism Form the cell's organelles Compartmentalize chemical reactions

Fluid Mosaic Model of the PM A membrane is a mosaic Proteins and other molecules are embedded in a framework of phospholipids A membrane is fluid Most protein and phospholipid molecules can move laterally

Membrane Structure Phospholipid Phospholipids are the major structural component of membranes.

Membrane Structure All membranes are phospholipid bilayers with embedded proteins. Phospholipid Bilayer Label the: Hydrophilic heads Hydrophobic tails

Embedded in the bilayer are proteins Most of the membrane’s functions are accomplished by the embedded proteins. Integral proteins span the membrane Peripheral proteins are on one side or the other of the membrane

Plasma Membrane Components Glycoproteins and glycolipids are proteins/lipids with short chain carbohydrates attached on the extracellular side of the membrane.

Carbohydrate of glycoprotein Glycoprotein Glycolipid Integrin Fig. 5-1a Carbohydrate of glycoprotein Glycoprotein Glycolipid Integrin Phospholipid Microfilaments of cytoskeleton Cholesterol

Types of Membrane Proteins Cell-cell recognition proteins Integrins Intercellular junction proteins Enzymes Signal transduction proteins Aka - Receptor proteins Transport proteins Passive and active

Cell-cell recognition proteins - identify type of cell and identify a cell as “self” versus foreign Most are glycoproteins Carbohydrate chains vary between species, individuals, and even between cell types in a given individual. Glycolipids also play a role in cell recognition

Integrins are a type of integral protein The cytoskeleton attaches to integrins on the cytoplasmic side of the membrane Integrins strengthen the membrane Intercellular junction proteins - help like cells stick together to form tissues

Many membrane proteins are enzymes This is especially important on the membranes of organelles.

Signal transduction (receptor) proteins bind hormones and other substances on the outside of the cell. Binding triggers a change inside the cell. Called signal transduction Example: The binding of insulin to insulin receptors causes the cell to put glucose transport proteins into the membrane.

Fig. 5-1c Messenger molecule Receptor Activated molecule

Transport Proteins Passive Transport Proteins allow water soluble substances (small polar molecules and ions) to pass through the membrane without any energy cost Active Transport Proteins The cell expends energy to transport water soluble substances against their concentration gradient

Fig. 5-1d

Transport of Substances Across the Plasma Membrane (PM) Passive Transport (Simple) Diffusion (5.3) Facilitated diffusion (5.6) Osmosis (5.4, 5.5) Active Transport (5.8) Bulk Flow (5.9) Endocytosis Exocytosis

Passive Transport In passive transport substances cross the membrane by diffusion Diffusion - net movement of substances from an area of high concentration to low concentration no energy required

Factors Affecting Diffusion Rate Steepness of concentration gradient Steeper gradient, faster diffusion Molecular size Smaller molecules, faster diffusion Temperature Higher temperature, faster diffusion

Simple Diffusion Nonpolar, hydrophobic molecules diffuse directly through the lipid bilayer Simple diffusion does not require the use of transport proteins. Examples: O2, CO2, steroids Polar, hydrophilic substances cannot pass directly through the lipid bilayer Examples: water, ions, carbohydrates

Simple Diffusion Polar molecules ions small, nonpolar molecules (ex. Glucose, water) ions (ex. H+, Na+, K+) small, nonpolar molecules (ex. O2, CO2) LIPID-SOLUBLE WATER-SOLUBLE LIPID-SOLUBLE

Facilitated Diffusion In facilitated diffusion small polar molecules and ions diffuse through passive transport proteins. No energy needed Most passive transport proteins are solute specific Example: glucose enter/leaves cells through facilitated diffusion

Facilitated Diffusion Higher concentration of Passive transport protein Lower concentration

Osmosis Osmosis – diffusion of water across a selectively permeable membrane Water moves from an area of _______ water concentration to an area of _____ water conc. Is energy required ? Water travels in/out of the cell through aquaporins

Consider two solutions separated Osmosis Terms Consider two solutions separated by a plasma membrane. Hypertonic solution with a relatively high concentration of solute Hypotonic solution with a relatively low concentration of solute Isotonic solutions with the same solute concentration

cluster of water molecules Lower concentration of solute Higher concentration of solute Equal concentration of solute H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water

Osmosis and Animal Cells

Osmosis and Plant Cells

Osmosis When a Cell is Placed in a Hypotonic Solution Water concentration is _________ the cell. Water flows ___________ the cell.

Osmosis When a Cell is Placed in a Hypertonic Solution Water concentration is _________ the cell. Water flows ___________ the cell.

See page 83 Isotonic solution Hypotonic solution Hypertonic solution H2O H2O H2O H2O Animal cell (1) Normal (2) Lysed (3) Shriveled H2O H2O H2O Plasma membrane H2O Plant cell (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed) See page 83

Osmosis Summary When a cell is placed in a Hypotonic solution: Cell gains water through osmosis Animal cell lyses; plant cell becomes turgid (firm) When a cell is placed a Hypertonic solution: Cell loses water through osmosis Animal cell shrivels; plant cell plasmolyzes

Active Transport Active transport proteins move substances across the PM against their concentration gradient. Requires energy (ATP) Active transport proteins are highly selective Active transport is needed for proper functioning of nerves and muscles

Active Transport of “X” Active transport proteins span the plasma membrane They have openings for “X” on only one side of the membrane “X” enters the channel and binds to functional groups inside the transport protein. Cytoplasmic ATP binds to the transport protein

Active Transport of “X” A phosphate group is transferred from ATP to the transport protein protein is energized by the added –P. The energized transport protein changes shape and releases “X” on the other side of the cell. The phosphate group is released from the transport protein and it resumes its original shape. Process repeats.

Fig. 5-8-1 Transport protein Solute 1 Solute binding

Fig. 5-8-2 Transport protein Solute 1 Solute binding 2 Phosphorylation

Transport protein Protein changes shape Solute 1 Solute binding 2 Fig. 5-8-3 Transport protein Protein changes shape Solute 1 Solute binding 2 Phosphorylation 3 Transport

Transport protein Protein changes shape Phosphate detaches Solute 1 Fig. 5-8-4 Transport protein Protein changes shape Phosphate detaches Solute 1 Solute binding 2 Phosphorylation 3 Transport 4 Protein reversion

Active Transport tell the story… ATP P ADP

Bulk Flow Vesicles are used to transport large particles across the PM. Requires energy Types: Exocytosis Endocytosis Phagocytosis, pinocytosis, receptor-mediated

Exocytosis Fluid outside cell Vesicle Protein Cytoplasm

Bulk Flow Exocytosis Cytoplasmic vesicle merges with the PM and releases its contents Example: Golgi body vesicles merge with the PM an release their contents How nerve cells release neurotransmittors

Endocytosis Endocytosis can occur in three ways Vesicle forming Endocytosis can occur in three ways Phagocytosis ("cell eating") Pinocytosis ("cell drinking") Receptor-mediated endocytosis

Endocytosis Endocytosis PM sinks inward, pinches off and forms a vesicle Vesicle often merges with Golgi for processing and sorting of its contents

Endocytosis - terms Phagocytosis – cell eating Membrane sinks in and captures solid particles for transport into the cell Examples: Solid particles often include: bacteria, cell debris, or food Pinocytosis – cell drinking Cell brings in a liquid

Endocytosis - comments Phagocytosis and pinocytosis are not selective Membrane sinks inward and captures whatever particles/fluid present. Vesicle forms and merges with the Golgi body…

Receptor Mediated Endocytosis Receptor Mediated Endocytosis is a highly specific form of endocytosis. Receptor proteins on the outside of the cell bind specific substances and bring them into the cell by endocytosis

Receptor Mediated Endocytosis Receptor proteins on PM bind specific substances (vitamins, hormones..) Membrane sinks in and forms a pit Called a coated pit Pit pinches closed to form a vesicle around bound substances Cytoskeleton aids in pulling in the membrane and vesicle formation

Receptor-mediated endocytosis Fig. 5-9c Plasma membrane Receptor-mediated endocytosis Coat protein Receptor Coated vesicle Coated pit Coated pit Specific molecule Material bound to receptor proteins

Fig. 5-9 Phagocytosis EXTRACELLULAR CYTOPLASM Food FLUID being ingested Pseudopodium “Food” or other particle Food vacuole Pinocytosis Plasma membrane Vesicle Receptor-mediated endocytosis Plasma membrane Coat protein Receptor Coated vesicle Coated pit Coated pit Specific molecule Material bound to receptor proteins