Transport across cell membranes

Reminder

• Cell walls are non living, slightly elastic and permeable.

• Cell membrane (plasma) is semi-permeable. It is made of fats and lipids. The fats are arranged hydrophilic head out. Embedded in the ‘fluid layer’ are proteins – some are tube like, some act as receptors, some actively pump molecules, some are structural.

PASSIVE TRANSPORT

Diffusion

• Particles are always in motion. The tendency of particles is to move to fill up a space. In biology terms, diffusion is the net movement of molecules from an area of high concentration to an area of low concentration.

• The difference in concentration in the two areas is the concentration gradient. A large difference gives a steep gradient and fast diffusion.

• No energy is used to move the molecules.

Rate varies depending on: size of molecules (smaller = faster) temperature of liquid or gas

(warmer=faster) state being diffused (gas faster than

liquid) concentration of chemical

Facilitated Diffusion

• Some proteins in cell membrane help the diffusion of certain chemicals across the membrane faster than others – glucose is carried faster than many others. The transport proteins are specialized to the type of chemical they can carry.

Osmosis• Net movement of water from hi to low

concentration through a semi-permeable membrane. (only water!)

• A weak solution is hypotonic, a strong solution is hypertonic and if the same they are isotonic.

Osmoregulation• The control of water in cells is very

important as water passes rapidly across cell membranes.

• In plants the vacuole acts as a fluid skeleton – too little and the plant becomes floppy , too much and it becomes crisp.

• Single celled organisms constantly have water flooding them so use a contractile vacuole to pump water out.

• Freshwater fish can drown due to water flooding their cells so produce copious amounts of urine.

• Saltwater fish either keep their bodies at the same saltiness as the sea or drink the seawater and extract salt.

• Humans have water-resistant skin, so don’t need to worry!

Active transport

• Movement against a concentration gradient – from low to high. This process uses energy – we can tell when energy is used if heat is given off, O2 is used, CO2 is made or glucose is used. If cells have a large number of mitochondria then it is likely to have a high energy requirement.

Endocytosis and exocytosis

• Some cells can engulf large particles (endocytosis). If the particle is solid we call it phagocytosis and if liquid pinocytosis.

• Exocytosis is

the reverse process –

getting rid of waste

particles or transporting

materials out of the cell.

Cell size

• Cell size is important in diffusion – the larger a cell is the more difficult it is to diffuse substances in and out. Cells normally grow until they double their size and then divide. To have efficient diffusion you need cells to have a high SA/Vol ratio. This is why cells divide – the volume drops but the surface area increases so the ratio goes up.