Chapter 3 Movement of Substances

3.1 Diffusion Diffusion is the net movement of particles from a higher concentration region to a region of lower concentration. After diffusion, overall concentration remains the same Only happens in substance in gaseous or liquid state

3.1.1 Concentration Gradient Concentration Gradient is the difference in concentration between two regions The steeper the gradient, the faster rate of diffusion it will occur.

3.1.2 Diffusion across a membrane Cell Surface Membrane Partially permeable surface Allow some substances to pass through but not others Only small particles can pass through and not others. Unicellular organism like amoeba get oxygen and removes carbon dioxide by diffusion through the cell surface membrane.

Osmosis Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Allow some substance to pass through but not other.

Water Potential Hypertonic: Lesser water molecules Hypotonic : More water molecules Isotonic: Equal water molecules Water potential is a measure of the tendancy of water to move from one place to another. Dilute solution > Concentrated solution When partially permeable membrane separates 2 solutions of different water potential, a water potential gradient is established.

Hypotonic – High Water Potential Plant Cell Animal Cell Water enters cell by osmosis. Plant cell expands and become swollen/turgid causing the cell to be turgor (filled with water) Pressure exerted by water on cell wall is call turgor pressure - Cell expands and will eventually burst.

Hypertonic – Low Water Potential Plant Cell Animal Cell As cell loses water, vacuole decrease in size Shrinkage of cytoplasm and cell membrane from cell wall is known as plasmolysis Tissues of plant become flaccid Plasmolysed cell can be restored to its original state. Shrink in size and become spiky  Crenation Cell become dehydrated and eventually die.

Turgor Pressure in Plants Maintaining shape of soft tissues in plants To keep the plants firm and remains upright Without turgor, plants will wilt Opening of flowers, moving of leaves and opening/closing of guard cells are results of changes in turgor pressure Plant requires turgor to keep them upright. Cutting stem of plant exposes inner cortex cells.

Turgor Pressure in Plants Exertion of cortex cells prevented by epidermal cells which are prevented by cuticle layer Cutting releases this restraint exerted by epidermis, thus cortex expands & cause strip to curl outwards.

3.3 Surface Area to Volume Ratio Rate of movement of a substance across the surface of a cell depends on how big the area of cell membrane is. The greater the area of cell surface membrane per unit volume, the faster the rate of diffusion of a substance for a given concentration gradient. Increase rate of diffusion, there must be presence of many small cells instead of one big cell.

3.4 Active Transport Movement of molecule from a region of low concentration to a region of high concentration against a concentration gradient. Energy is required. Only occurs in living cells as living cell respire. For example: Absorption of dissolved minerals by root hairs Absorption of glucose and amino acids by cells in small intestine.

Movement of Substances Diffusion Refer to any substances Membrane not required. Osmosis Refers to only water Partially permeable required Active Transport Energy Required Movement of substances is against concentration gradient