Chapter 7: Membrane Structure & Function

membrane a fluid mosaic of lipids, proteins, and carbohydrates. held together by hydrophobic interactions membrane lipids / proteins can drift laterally Membranes must be fluid to work properly.

Integral proteins are embedded in the membrane – Unilateral – – Transmembrane – Peripheral proteins attached to the membrane’s surface (cytoplasmic side )

Protein functions 1.Cell to cell recognition 2. enzymatic activity 3- signal transduction (hormones) 4- intercellular joining 5- attachment to ECM 6-Cell Transport:

Solubility characteristics: Nonpolar and hydrophobic molecules dissolve in the membrane and cross it with ease (e.g. hydrocarbons and oxygen) Polar (hydrophilic) molecules are dependent on size and charge  Small molecules (e.g. H 2 O, CO 2 ) may be small enough to pass between membrane lipids  Larger molecules (e.g. glucose) will not easily pass IONS- even small ones (e.g. Na +, H + ) have difficulty penetrating the hydrophobic layer.

Transport proteins integral transmembrane proteins that transport specific molecules or ions across biological membranes.  Are highly specific

Passive Transport diffusion of a substance across a biological membrane does not require the cell to expend energy.

Osmosis – diffusion of water across a selectively permeable membrane

Hypertonic solution – a solution with a greater solute concentration Hypotonic solution – a solution with a lower solute concentration Isotonic solution – solutions with an equal solute concentration

Water potential – measure of the tendency for a solution to take up water by a selectively permeable membrane.  Water potential of pure water is zero. -Solutes lower the water potential (ex -2) -Water flows from HIGH to LOW (water potential) -Water flows from hypotonic to hypertonic

Water Balance of Cells Without Walls  In a hypertonic environment, an animal cell will shrivel.  In a hypotonic environment, an animal cell will swell, and perhaps lyse (burst).

Organisms without cell walls prevent excessive loss or uptake of water by:  Living in an isotonic environment  Osmoregulating

Water Balance of Cells With Walls prokaryotes, some protists, fungi, and plants In a hypertonic environment, walled cells will plasmolyze (Plasmolysis)

a hypotonic environment  Creates turgid cells = ideal for support Turgidity is the firmness or tension found in walled cells.

In an isotonic environment, the plant cells become flaccid (limp).

Facilitated diffusion – diffusion of solutes across a membrane with the help of transport proteins.

Active Transport ATP requiring process during which a transport protein pumps a molecule across a membrane against its concentration gradient.

Electrochemical gradient – diffusion gradient resulting from the combined effects of membrane potential and concentration gradient

Electrogenic pump – a transport protein that generates voltage across a membrane Na + /K + pump is the major electrogenic pump in animals

Exocytosis and Endocytosis Exocytosis – process of exporting macromolecules from a cell by fusion of vesicles with the plasma membrane Endocytosis – process of importing macromolecules into a cell by forming vesicles derived from the plasma membrane

three types of endocytosis: Phagocytosis – endocytosis of solid particles Pinocytosis – endocytosis of fluid droplets taken in as small vesicles.

Receptor-mediated endocytosis – importing specific macromolecules into the cells by the binding of specific ligands to receptors

Stages of Receptor-Mediated Endocytosis: Extracellular ligand binds to receptors in a coated pit  Causes inward budding of the coated pit  Forms a coated vesicle  Ingested material is released from the vesicle  Protein receptors are recycled to the plasma membrane