MATERIALS MOVE ACROSS THE CELLS MEMBRANE SOME MATERIALS

MATERIALS MOVE ACROSS THE CELL’S MEMBRANE • SOME MATERIALS MOVE BY DIFFUSION – Diffusion= the process by which molecules spread out, or move from areas of high concentration to areas of less concentration – Occurs because molecules are always in constant motion; cells use diffusion to carry out life functions • SOME TRANSPORT REQUIRES ENERGY • CELL SIZE AFFECTS TRANSPORT

DIFFUSION • Occurs naturally as particles move from area of high concentration to area of lower concentration • The greater the difference in concentrations, the quicker diffusion occurs. • Diffusion is how materials move in photosynthesis & cellular respiration. • Diffusion is a type of passive transport (materials move without using cell’s energy. ) • Polar substances (sugar/salts/amino acids) diffuse via protein channel (still passive- no energy)

OSMOSIS • Diffusion of water through a membrane is OSMOSIS. • If you forget to water a plant, the soil dries, the roots dry, and water leaves the plant cells by osmosis, so the cell shrinks and the plant “wilts”.

Some Transport Needs Energy • ACTIVE TRANSPORT – Uses energy to move material through membrane – Marine iguanas that swim & feed in salty ocean which causes lots of salt in blood. – They have special glands using active transport to remove salt from blood & blow it out of their nostrils. – Similar to our kidneys filtering blood for us.

TRANSPORT REQUIRING ENERGY, CONTINUED • ENDOCYTOSIS: large bits of material get captured in pocket of membrane • Pocket breaks off and forms package that moves into cell. • Cells use this to fight bacteria & viruses by absorbing them. • EXOCYTOSIS: a membrane within the cell encloses material needed to be removed • Package moves to cell membrane, joins with it, and expels material • Cells use this to flush out waste or expel proteins & hormones made by the cell.

CELL SIZE AFFECTS TRANSPORT • Most cells are very small; most human cells are about 50 micrometers (. 05 mm), while bacteria are only 3 -5 micrometers. • The amount of cell membrane limits the ability of a cell to transport materials into and out of a cell. • As a cell gets larger, there is a time when its volume grows more than its surface area, so not enough resources can get into the cell to feed it. • Thin, flat cells or long skinny cells (muscle & nerve) give lots of surface area for plenty of food and resources to get into the cell.