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 (.05mm), 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.