Transport across membranes

what are the different types of transport membranes

• simple diffusion

• facilitated diffusion

• osmosis

• active transport

• co transport

simple diffusion

the net movement of molecules from an area of high concentration to an area of low concentration

kinetic energy of molecules in simple diffusion

molecules have kinetic energy that they possess which allows them to constantly move in fluids

facilitated diffusion

the passive movement of molecules or ions across a cell membrane, from a high concentration to a low concentration, using specific transport proteins (carrier or channel proteins)

what type of substances does facilitated diffusion enable to diffuse across membranes

ions, polar molecules, large molecules

what do protein channels form

tubes filled with water which allows water soluble ions to pass through the membrane

are protein channels selective

yes as only open in presence of certain ions when they bind to a protein

carrier proteins in facilitated diffusion

• molecule complementary in shape binds to protein

• causing it to change shape

• releasing the molecule to the other side of the membrane

what is osmosis

the movement of water from an area of high water potential to an area of lower water potential across a partially permeable membrane

hypotonic meaning

water potential of a solution more positive (closer to 0) than the cell

hypertonic meaning

water potential of a solution more negative than the cell

what is active transport

movement of molecules and ions from an area of lower concentration to an area of higher concentration against the concentration gradient, using ATP and carrier proteins

how does active transport work

• molecules bind to receptor sites on carrier proteins

• ATP binds to inside of protein membrane

• this causes it to be hydrolysed to ADP + Pi

• this causes the protein molecule to change shape

• releasing molecule to other side of the membrane

• Pi released from the protein, causing it to revert back to its original shape

what is co-transport

• the coupled movement of two substances across a membrane via a carrier protein

• one moves down its concentration gradient, allowing the other to move against its gradient

why is co-transport and active transport used for glucose and sodium ions in the ileum

• to absorb glucose from the lumen to the gut must have a higher concentration of glucose in the lumen compared to the epithelial cell (for facilitated diffusion)

• however there is usually more glucose in the epithelial cells

• needs active and co transport

process of co transport of glucose and sodium ions in the ileum

• sodium ions are actively transported out of the epithelial cell into the blood

• this reduces the sodium ion concentration in the epithelial cell

• sodium ions then diffuse from the lumen down their concentration gradient into the epithelial cell

• sodium ions diffuse through a co-transporter protein, so either glucose or amino acids also attach and are transported into epithelial cells against their concentration gradient

• glucose then moves by facilitated diffusion from the epithelial cell to the blood

why doesn’t blood have a build up of glucose molecules as they are being absorbed

concentration of glucose in the blood is lower than the epithelial cells as blood flows and carries away absorbed glucose

function of microvilli on epithelial cell

increase surface area for co-transporter proteins giving maximum absorption of glucose

what is the sodium potassium pump

an active transport carrier protein that uses ATP to move ions across the cell membrane

what ions are moved per pump cycle

• 3 Na+ out of the cell

• 2 K+ into the cell

• using 1 ATP

what is the role of the sodium potassium pump in neurons

maintains the resting potential by keeping the inside of the cell negatively charged