Chapter 4: Membrane Structure & Function “Fluid mosaic model” Chapter 4: Membrane Structure & Function

Cell Membrane Cell membrane controls movement of substances in/out of cell acts as gatekeeper – “selectively permeable” Also… provides structural support recognizes foreign material communicates with other cells

Structure Phospholipid bilayer with imbedded or partially imbedded proteins Only certain substances can pass through the lipid bilayer For example: small, nonpolar substances Ions & polar molecules repelled by nonpolar interior of lipid bilayer

Lipid Bilayer The head is polar and is attracted to water (Note: there is water inside and outside the cell)  Hydrophilic The fatty acid tails are nonpolar and are repelled by water  Hydrophobic

Act as markers identifying the cell Structure continued… Glycoproteins = carbohydrate chains attached to the outside surface of proteins Glycolipids – carbohydrate chains attached to the outside surface of phospholipids Act as markers identifying the cell

Structure continued…. Membrane proteins: Can face inside or outside the cell Can stretch across membrane Remember that proteins are made of amino acids Some of these amino acids are polar and some are nonpolar

Membrane Proteins 5 Types: Channel proteins: move substances in/out freely (Cystic fibrosis is cause by a faulty Cl- channel) Carrier proteins: interacts with certain molecules/ions to move them in/out of cell Cell Recognition: glycoproteins that identify the cell as “self” (issues with transplants) Receptor Proteins: receive signals from surroundings (some types of dwarfism) Enzymatic: biochemical reactions (cholera bacteria releases toxin that interferes with one)

Membrane Proteins

Ways for Substance to Cross the Membrane 1. Passively Requires NO energy Includes: diffusion, osmosis, & faciliated transport 2. Actively Requires ATP energy Includes: active transport, exocytosis & endocytosis

Diffusion

Molecules move from high concentration to low concentration in between phospholipids molecules Moves down its concentration gradient small, nonpolar molecules pass through membrane Examples: O2 in, CO2 out, H2O in/out

Osmosis Water diffusing across the cell membrane Type of facilitated diffusion (i.e. needs a transport protein—channel)

Possible situations with osmosis: 1. Isotonic solutions No net movement of water across membrane. Same concentration of water outside the cell as in the cytoplasm Cells placed in an isotonic solution stay the same size

2. Hypertonic solutions lower concentration of water outside the cell as in the cytoplasm water will leave the cell & the cell will shrivel

3. Hypotonic solutions Higher concentration of water outside the cell as in the cytoplasm water will enter cell & the cell will swell and possibly burst

Summary of what happens to animal cells placed in different solutions:

Summary of what happens to plant cells placed in different solutions:

Facilitated Diffusion movement through a channel protein down a concentration gradient Polar substances, ions, etc.

Active Transport Moves molecules from low concentration to high concentration Requires ATP energy Examples: sodium-potassium pump (important for nerve impulses & muscle contractions)

Endocytosis Cell membrane forms a vesicle to bring in materials 2 types: Pinocytosis = when substance entering cell is small or liquid Phagocytosis = when substance entering cell is large

Exocytosis Internal vesicles fuse with cell membrane releasing substances out of the cell