plasma membranes

fluid-mosaic model

mixture and movement of glycolipids, cholesterol and glycoproteins, arranged in a phospholipid bilayer creating a partially permeable membrane

phospholipid bilayer

hydrophilic heads attracted to the water, hydrophobic tails repelled by water

cholesterol

• restricts lateral movement of fluid in the membrane

• makes membranes less fluid at high temperatures

• prevents water and dissolved ions leaking out of the cell

extrinsic/peripheral proteins

• provide mechanical support

• connected to proteins or lipids to make glycoproteins and glycolipids

• cell recognition as receptors

• don’t extend completely across the membrane

intrinsic/integral proteins

• protein carriers or channels involved in transport of molecules across membrane

• span across one side of bilayer to the other

protein channels and carrier

• channels: form tubes that fill with water to enable water soluble ions to diffuse

• carrier: bind with other ones and larger molecules (eg. glucose and amino acids) and change shape to transport them to other side of membrane

partially permeable

• lipid molecules (eg. some hormones) and very small molecules (eg. co2, o2, h2o) PASS THROUGH

• water soluble, polar substances (eg. sodium ions) and large molecules (eg. glucose) DON’T PASS THROUGH

active transport

movement of ions from an area of low concentration to an area of higher concentration against the concentration gradient using ATP and carrier proteins (act as a pump to move substances across)

selective, only certain molecules bind to the carrier proteins

ATP and carrier proteins

• ATP binds to inside of the membrane and is hydrolysed into ADP and Pi

• protein changes shape and opens inside of membrane

• Pi released from protein and protein reverts to its original shape

transport process

1. carrier proteins spanning the cell membrane

2. molecule binds to a receptor complementary shape on protein

3. ATP binds to carrier proteins from inside of cell and hydrolyses into ADP + Pi

4. carrier proteins change shape and release molecule to other side

5. phosphate ion is released and protein returns to original shape

co-transport required because

to absorb glucose from lumen to gut there must be a higher concentration of of glucose in the lumen compared to the epilelial cell for facilitated diffusion but there’s usually more glucose in spike that cells

sodium ions in the ileum

1. ions actively transported out of epithelial cell into the blood to reduce sodium ion concentration

2. ions diffuse from lumen down conc gradient into epithelial cell

3. they diffuse through co-transported protein, either glucose or amino acids attach and are transported into epithelial cells against conc gradient

4. glucose moves by facilitated diffusion from cell to the blood

conc of glucose

conc of glucose in blood is lower than epithelial cells because blood flows away and carries away absorbed glucose

microvilli on epithelial cells

microvilli on epithelial cells increase the surface area of the for co-transporter protiens