diffusion, active transport, osmosis

what is simple diffusion

the net movement of molecules/ions from a region where they are more concentrated to one where their concentration is lower until evenly distributed/reached equilibrium

facilitated diffusion

passive transport process that uses intrinsic proteins to help molecules move across the cell membrane. it occurs down a concentration gradient but occurs only at certain points in the cell surface membrane where there are intrinsic protein molecules.

what are the 2 mechanisms facilitated diffusion uses?

protein channels, carrier protein

facilitated diffusion via protein channels (5)

water-filled, hydrophilic protein channels across membrane. selective and exert control. allow specific water-soluble ions through e.g. na+. if a particular ion isn’t present, the channel remains closed. ions bind to protein, causing it to change shape (close at one side, open at the other)

facilitated diffusion via carrier proteins (3)

carrier proteins span the plasma membrane. molecules that are specific to the protein bind to the protein, e.g. glucose. this causes a change of shape so that the molecule is released onto the inside of the membrane

6 factors that determine the rate of diffusion

concentration gradient, surface area, temperature, length of diffusion path, type of molecule diffusing, number of proteins

How is active transport different from passive transport?

requires atp from respiration in mitochondria, moves against a concentration gradient, uses carrier proteins as pumps, selective and specific

where does active transport get its energy?

mitochondria

where is the energy released from in active transport?

ATP hydrolysis

what is atp?

adenosine triphosphate (3 phosphates)

what is atp hydrolase?

group of enzymes that catalyse hydrolysis

what is the chemical formula for the reaction of atp hydrolysis?

ATP + H20 (atp hydrolase)= ADP + Pi + free energy

what does hydrolysis cause atp to do?

lose a phosphate group- dephosphorylate

what happens in active transport?

substances are moved against a concentration gradient which requires a supply of energy in mitochondria. this energy is released from atp hydrolysis

2 ways to increase the rate of movement across membranes

epithelial cells lining ileum have microvilli- finger-like projections on the cell surface membrane. 0.6 micrometres, often referred to as a ‘brush border’. Provides more SA for insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place. Increase number of protein channels and carrier proteins in any given area of the membrane.

What role does diffusion play in absorption? (long answer)

As carbs and proteins are digested continuously, theres normally a greater concentration of glucose and amino acids within the ileum than the blood. So, there is a concentration gradient down which glucose moves by facilitated diffusion from ileum to blood. As blood is constantly circulated by heart, the glucose absorbed into it is continuously being removed by the cells as they use it up in respiration. This helps to maintain a concentration gradient between the inside of the ileum and the blood. This means the rate of movement by facilitated diffusion across epithelial cell CSMs is increased

Role of active transport in absorption

Diffusion only produces equal concentrations of amino acids/glucose either side of the epithelium. So, remaining glucose and amino acids that haven’t diffused are absorbed by active transport, so all of it is absorbed into the blood.

do sodium and glucose/amino acid ions move into/out of the cell, and up/down a concentration gradient

both move into the cell, but sodium ions move down their concentration gradient and glucose/amino acids move against their concentration gradient

explain the process of co transport (long answer)

Sodium ions actively transported out of epithelial cells by sodium potassium pump, into blood. Takes place in one type of protein carrier molecule found in the CSM of the epithelial cells. This maintains a much higher concentration gradient of sodium ions in the lumen of the intestine than inside the epithelial cells. Sodium ions diffuse into epithelial cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the CSM. As the sodium ions diffuse in through this second carrier protein, they carry either glucose molecules/amino acid molecules into the cell with them. Glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.

What powers the movement of glucose/amino acids into the cells and what does this mean about active transport?

The sodium ion concentration, rather than ATP directly. This makes it an indirect form of active transport.

What is osmosis

The passage of water from a region where it has higher water potential to a region where it has lower water potential through a selectively permeable membrane

What is a solute?

Any substance that is dissolved in a solvent, for example water

What does a solute and a solvent make

a solution

What is the measurement for water potential

kPa

What is water potential

the pressure created by water molecules

Under standard conditions of temperature and pressure (25 degrees 100kPa, what has a water potential of zero?

pure water

What does the addition of a solute do to pure water?

Lowers its water potential

What must the water potential of a solution be?

less than zero

the more solute that is added/the more concentrated solution, what happens to the water potential?

becomes more negative

how to find the water potential of cells or tissues?

place them in a series of different water potentials. where there is no net gain or loss of water from the cells or tissues, the water potential inside the cells or tissues must be the same as the water potential of the externa solution

to prevent an animal cell from bursting in osmosis, what do they have?

animal cells usually live in a liquid that has the same water potential as the cells. in humans, this is blood plasma

what happens if red blood cells are placed in a solution with water potential of its own?

water leaves by osmosis and cell shrinks/becomes shrivelled

in, active transport, ATP is used for 2 things: what are they?

directly move molecules. individually move molecules using a concentration gradient which has already been set up by direct active transport- co transport

4 things: how does active transport differ from passive transport (full stops in between)

metabolic energy in the form of ATP is needed. substances are moved against a concentration gradient, from a lower to higher concentration. carrier protein molecules which act as pumps are involved. process is very selective, only specific substances are transported

similarities and differences between active transport and facilitated diffusion?

both use carrier proteins, but facilitated diffusion occurs down a concentration gradient whereas active transport occurs against a concentration gradient. therefore, facilitated diffusion doesn’t require metabolic energy while active transport does. this is provided as ATP