Cell Boundaries 7 3 How Do Substances Move

Cell Boundaries 7. 3

How Do Substances Move in Cells? A. Cell Membrane B. Cell Walls C. Diffusion Through Cell Boundaries 1. Measuring Concentration 2. Diffusion D. Osmosis 1. How Osmosis Works 2. Osmotic Pressure E. Facilitated Diffusion F. Active Transport 1. Molecular Transport 2. Endocytosis and Exocytosis

Cell Membrane • Selectively permeable – Cell controls the ease with which substances pass in and out of the cell • Fluid mosaic model – Cell membrane acts like a fluid, allowing membrane molecules to move laterally Fluid mosaic model of cell membrane video

Cell Membranes are Selectively Permeable

Cell Membrane Lipid bilayer (phospholipids) Outside of cell Proteins Carbohydrate chains Cell membrane Inside of cell (cytoplasm) Protein channel Lipid bilayer üProtects the contents of the cell üControls what moves into and out of the cell. üHelps maintain homeostasis.

Cell Wall • Found in bacteria, plant, fungi, and some protista • Involved in protection and structure • Carbohydrates – structural component cellulose Cell wall video

Metabolically active cells are small in size • Surface-area-to-volume ratio constrains increases in a cell’s size – – Actively metabolizing cells need to be small Cells that specialize in absorption have modifications to increase the surface-area-to -volume ratio 4 -7

Surface-area-to-volume relationships 4 -8

How Do Substances Move in Cells? • Substances move within cells and across cell membranes through the processes of PASSIVE TRANSPORT and ACTIVE TRANSPORT • CONCENTRATION GRADIENT - the number of molecules or ions in one region is different than the number in another region – In the absence of other forces, a substance will naturally move from a region where it is more concentrated to one where it is less concentrated – A substance naturally moves “down a concentration gradient”

Membrane Crossing Mechanisms § Help supply cells and organelles with raw materials necessary for building and maintenance, and to excrete waste products 1. 2. 3. 4. DIFFUSION OSMOSIS FACILITATED DIFFUSION ACTIVE TRANSPORT using vesicles § § Exocytosis Endocytosis § Pinocytosis and Phagocytosis

Diffusion • Movement from an area of high concentration to an area of low concentration. • Only the kinetic energy of the molecules is necessary; no additional energy is required. • A form of PASSIVE TRANSPORT that does not require energy Diffusion will occur until there is equal amounts of solute spread throughout the system: EQUILIBRIUM

Factors Affecting the Rate of Diffusion • Steepness of concentration gradient – Steeper gradient = faster diffusion • Molecular size – Smaller molecules = faster diffusion • Temperature – Higher temperature = faster diffusion • Electrical or pressure gradients increase the rate of diffusion

Osmosis • The movement of water • • across a selectively permeable membrane Membrane prevents solutes from moving across membrane with water. Water moves to equalize concentration inside and outside of cell. Water solution Glucose solution

Terms • ISOTONIC – Equal: concentration of solutes is equal inside and outside the cell. – No net movement of water • HYPERTONIC – Higher solute concentration outside the cell than inside of the cell. – Net movement of water is outside of the cell. • HYPOTONIC – Higher solute concentration inside the cell than outside the cell. – Net movement of water is into the cell.

2% sucrose solution 1 liter of distilled water a 1 liter of 10% sucrose solution b Hypotonic Solution Hypertonic Solution 1 liter of 2% sucrose solution c Isotonic Solution

Water tends to move from a hypotonic solution to a hypertonic solution HYPOTONIC SOLUTION HYPERTONIC SOLUTION membrane permeable to water but not to solutes fluid volume increases In compartment 2

contractile vacuole full contractile vacuole empty PARAMECIUM Major problem in freshwater environments - hypotonic environment Major problem in saltwater environments - hypertonic environment

Facilitated Diffusion • A protein channel • outside cell allows specific molecules to move across the cell membrane Cell A form of PASSIVE membrane TRANSPORT that does not require energy inside cell Glucose molecules Protein channel

More concentration Active transport Concentration gradient Less concentration • Requires energy • Used to create a concentration gradient • Moving a substance against the concentration gradient - from area of low concentration to high concentration • Pumps: molecules (fig. 7. 19) • Examples: Endocytosis and Exocytosis of larger substances

EXOCYTOSIS (out from cytoplasm) a ENDOCYTOSIS (into cytoplasm) b

Types of Endocytosis • Phagocytosis: to eat (movement of “food”) • Pinocyctosis: to drink (movement of fluids) Hyperlink to endocytosis video

Fig. 4 -19, p. 70

Membrane Crossing Mechanisms No energy required Facilitated diffusion of water-soluble substances through protein channel; No energy required

Endocytosis Exocytosis