Pōʻakahi 09/09/12 • Review Quest • Cell Membrane (chapter 5)
Nāʻana • What kind of lipid makes a cell membrane? • Draw a cross section of what you think the cell membrane looks like.
The Working cell 5.1-5.5
Membrane structure • Membrane called fluid (like oil) mosaic (proteins). • Proteins in membrane play a key role in cell function • Membrane is selectively permeable: some material easily cross membrane, some don’t.
Membrane protein function • Attachment to cytoskeleton – maintain cell shape • Signal transduction– receptors for chemical messengers (cause change in protein -> message inside cell ->activate molecules) • Enzymatic activity – perform steps in a metabolic pathway • Cell-cell Recognition – attached carbs recognized by proteins of other cells (recognize foreign bacteria) • Intercellular junctions – attach to other cells • Transport – some molecules require proteins to enter or exit cell
5.2 Membranes form spontaneously! • Phospholipid bilayer forms when mixture w/ water is shaken • No genes required! • Probably a critical step in evolution of the first cells • Allows for enclosed solution to differ from surroundings • Allows for regulation of chemical exchanges
Movement across the membrane • Divided into two categories: • Passive transport and active transport • Main difference is use of energy (active needs it)
Passive transport - diffusion • Diffusion is the movement of molecules from an area of high concentration to low concentration • Results is equilibrium (concentration equal on both sides) • Materials that move easily are O2 and CO2. • Ions and polar molecules cannot move easily. Why?
Osmosis - Passive transport/diffusion of h2o • Osmosis: diffusion of water across a selectively permeable membrane.
5.4 osmosis – diffusion of h2o Tonicity: ability of surrounding solution to cause cell to lose/gain water. ISOTONIC – solute concentration inside and outside cell same HYPOTONIC – solution with solute concentration lower than that of the cell HYPERTONIC – solution with solute concentration higher than that of the cell
Osmosis Hypotonic Solution Water enters cell Cell will swell (get larger) Isotonic Solution Water enters and leaves cell at same rate Cell stays same size Hypertonic Solution Water leaves cell Cell will shrink
Effects of tonicity • Water balance critical because if not regulated, organism cannot survive. • Plasmolysis: membrane of plant cell pulls away from wall.
Homework • 5.1-5.5 • Cell Membrane Worksheet