Cell Boundaries

Essential Questions What are the main functions of the cell membrane and the cell wall? What happens during diffusion? What is osmosis?

The CELL MEMBRANE is a thin, flexible barrier around a cell that regulates materials entering and leaving the cell; also provides protection and support Nearly all cell membranes are composed of a PHOSPHOLIPID BILAYER-two layers of lipids with phosphate heads pointing away from the membrane

Fluid-Mosiac Model Outside of cell Inside (cytoplasm) Carbohydrate membrane Proteins Protein channel Lipid bilayer Carbohydrate chains

Phospholipid bilayer is made of a hydrophilic (“water loving”) head and a hydrophobic (“water fearing”) tail Cells are surrounded by extracellular fluid and have cytoplasm inside (both watery solutions) Heads are outside and tails are inside

Phospholipid Bilayer

CELL WALLS, present in plants, algae, fungi, bacteria, surround the cell membrane and provide support and protection for the cell; in plants, typically composed of CELLULOSE

Recall that a SOLUTION is a mixture of two or more substances; two parts-SOLUTE and SOLVENT. The CONCENTRATION of a solution is the amount of solute in a given amount of solvent Example: salt water; salt is the solute, water is the solvent

Because particles in a solution are in constant, random motion, a solute will undergo DIFFUSION-movement of solute particles from area of high concentration to an area of lower concentration; diffusion continues until it reaches EQUILIBRIUM-concentrations are the same throughout

If a solute is at different concentrations on either side of a cell membrane, the solute will diffuse from the higher to lower concentration side until equilibrium is achieved

High Concentration Cell Membrane Low Concentration Glucose molecules Protein channel

Because diffusion relies on random particle motion, diffusion across a membrane does not require energy  Note that even after equilibrium is reached, particles continue to move back and forth across the membrane, but at equal rates

A PERMEABLE MEMBRANE allows materials to pass through; IMPERMEABLE MEMBRANES do not SEMIPERMEABLE (SELECTIVELY PERMEABLE) MEMBRANES allow some materials to pass, others not Paper towel demo

Osmosis Water usually passes through most membranes, while many solute particles cannot; OSMOSIS is the diffusion of water through a selectively permeable membrane If a membrane is water-permeable, but impermeable to a solute at different concentrations on either side of the membrane, WATER DIFFUSES TO THE HIGHER CONCENTRATION SIDE UNTIL EQUILIBRIUM IS ACHIEVED

OSMOSIS

Water will move across a membrane until equilibrium is reached – isotonic “same strength” When comparing 2 solutions, the solution with the greater concentration of solutes –Hypertonic “above strength” When comparing 2 solutions, the solution with the lesser concentration of solutes –Hypotonic “below strength” Draw Chart p. 186

Homework Questions Describe the functions of the cell membrane and cell wall. What happens during diffusion? Describe how water moves during osmosis. What is the basic structure of a cell membrane?

Osmotic Pressure As water diffuses toward the hypertonic side of a membrane, this produces OSMOTIC PRESSURE acting on the hypertonic side of the membrane Cells have salts, sugars, proteins, and other solutes dissolved in the cytoplasm, making the inside of cells hypertonic to fresh/distilled water; if not checked, water entering the cell can make it swell to the point of bursting

Some molecules (glucose, proteins) appear to be too large to diffuse through the cell membrane, but still move across it through FACILITATED DIFFUSION-diffusion of materials through a PROTEIN CHANNEL imbedded in the cell membrane Facilitated diffusion does not require cellular energy

A CONCENTRATION GRADIENT occurs when there is an area at high concentration near an area of lower concentration. Solutes diffuse WITH THE CONCENTRATION GRADIENT-from high to low concentration

Sometimes a cell must move solute against the concentration gradient-from lower to higher concentration; this is done through ACTIVE TRANSPORT-movement of solute against conc. gradient via transport proteins, which require energy

Molecule being carried Molecule to be carried Energy Molecule being carried

Larger molecules and solid clumps of material may be transported by movements of the cell membrane 1. ENDOCYTOSIS-taking material into cell by cell membrane folding around material, pinching off inside cell to form vacuole Ex. PHAGOCYTOSIS-cell membrane engulfs particle, pinches off into a vacuole; PINOCYTOSIS-cell membrane forms pockets that fill with liquid and pinch off into vacuoles 2. EXOCYTOSIS-membrane of vacuole merges w/cell membrane, opens to outside, releasing contents Ex. contractile vacuole