Diffusion and Osmosis

Functions of Cell Membrane 1. Protect cell Control incoming and outgoing substances Selectively permeable - allows some molecules in, others are kept out

Phospholipid Bilayer

Plasma membrane components Phospholipid bilayer polar head P – cytosol hydrophobic molecules hydrophilic molecules nonpolar tails

Membrane Permeability Selective permeability The ability of a cell membrane to control which substances and how much of them enter or leave the cell Allows the cell to maintain a difference between its internal environment and extracellular fluid Supplies the cell with nutrients, removes wastes, and maintains volume and pH

The Selectively Permeable Nature of Cell Membranes

Types of Membrane Proteins

Methods of Transport Across Membranes 1. Diffusion -passive transport – no energy expended 2. Osmosis - Passive transport of water across membrane 3. Facilitated Diffusion - Use of proteins to carry polar molecules or ions across 4. Active Transport- requires energy to transport molecules against a concentration gradient – energy is in the form of ATP

Concentration and Gradients The number of molecules (or ions) of substance per unit volume of fluid Concentration gradient The difference in concentration between two adjacent regions Molecules always move from a region of higher concentration to one of lower concentration

Diffusion Solute molecules moving from an area of high concentration to an area of low concentration Random motion drives diffusion Equilibrium is reached when there is an even distribution of solute molecules (water)

Diffusion

Osmosis Diffusion of water through a semi- permeable membrane Semi-permeable: permeable to solvents (WATER), but not to large molecules High [water] to low [water] Dissolved molecules (i.e. glucose, starch) are called solutes REMEMBER: Water = solvent Glucose, Starch = solutes

Osmosis

Effect of Water on Cells Hypertonic Environment High [solute], low [water] Hypotonic Environment High [water], low [solute] Isotonic Environment [water] = [solute] Hypertonic Hypotonic Isotonic Part 3 pg. 85

Osmosis in Living Cells Cellulose in cell wall

Osmosis in Red Blood Cells Isotonic Hypotonic Hypertonic Observe sheep RBCs via a wet mount of the sample Aliquot one drop the following solutions with a ½ drop of RBC to a slide 0.9% saline 10% NaCl Distilled water Record observation in the table on page 85 Crenation Predictions?

Effect of Water on RBC

Osmosis in Plant Cells Plasmolysis Hypertonic Plasmolysis Observe Elodea leaves via a wet mount of the sample Aliquot two drops the following solutions with a new Elodea leaf to a slide. Incubate for 10 minutes @ room temp. 10% NaCl Distilled water Record observation in the table on page 85 Hypotonic Predictions?

Hydrostatic Pressure in Plants Normal tomato on left – salt water gives effect (r) in 30min Iris petal cells Iris petal cells in wilted stage

Types of Transport

Transportation of Molecules Passive Transport Movement of molecules across a semi-permeable membrane no energy required Active Transport Movement of molecules across a semi-permeable membrane against a concentration gradient ENERGY – ATP Facilitated Diffusion Movement of molecules across a semi-permeable membrane with a protein

Active transport in two solutes across a membrane FLUID OUTSIDE CELL Phosphorylated transport protein Active transport in two solutes across a membrane Na+/K+ pump Protein shape change Transport protein First solute 1 First solute, inside cell, binds to protein 2 ATP transfers phosphate to protein 3 Protein releases solute outside cell Second solute 4 Second solute binds to protein 5 Phosphate detaches from protein 6 Protein releases second solute into cell Figure 5.18

Exocytosis and endocytosis transport large molecules exocytosis = vesicle fuses with the membrane and expels its contents FLUID OUTSIDE CELL CYTOPLASM Figure 5.19A

or the membrane may fold inward, trapping material from the outside (endocytosis) Figure 5.19B

Receptor-mediated endocytosis

Cholesterol can accumulate in the blood if membranes lack cholesterol receptors Phospholipid outer layer LDL PARTICLE Receptor protein Protein Cholesterol Plasma membrane Vesicle CYTOPLASM Figure 5.20

Membrane-Crossing Mechanisms