Biology IB HL - 1.4 Membrane Transport Part 1

they are

semi-permeable

selective

They are semi-permeable (only certain materials may freely cross – large and charged substances are typically blocked)

They are selective (membrane proteins may regulate the passage of material that cannot freely cross)

Movement of materials across a biological membrane may occur either ACTIVELY or PASSIVELY

Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)

NO

Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)

simple diffusion

facilitated diffusion

osmosis

movement of small or lipophilic molecules (e.g. O2, CO2, etc.)

movement of water molecules (dependent on solute concentrations)

movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)

Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)

Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)

primary (direct)

secondary (indirect)

Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport

Involves coupling the molecule with another moving along an electrochemical gradient

Diffusion is the (random) net movement of molecules from a region of high concentration to a region of low concentration

This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium)

temperature

molecular size

steepness of gradient

affects kinetic energy of particles in solution

larger particles are subjected to greater resistance within a fluid medium

rate of diffusion will be greater with a higher concentration gradient

Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)

Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes)

Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions

At a higher solute concentration there are less free water molecules in solution as water is associated with the solute

Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration (not associated with other molecules)

Osmolarity is a measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)

hypertonic

hypotonic

isotonic

Solutions with a relatively higher osmolarity are categorised as hypertonic (high solute concentration ⇒ gains water)

Solutions with a relatively lower osmolarity are categorised as hypotonic (low solute concentration ⇒ loses water)

Solutions that have the same osmolarity are categorised as isotonic (same solute concentration ⇒ no net water flow)

by bathing the sample in solutions with known osmolarities

The tissue will lose water when placed in hypertonic solutions

gain water when placed in hypotonic solutions

Water loss or gain may be determined by weighing the sample before and after bathing in solution

Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)

Tissues or organs to be used in medical procedures must be kept in solution to prevent cellular dessication

This solution must share the same osmolarity as the tissue / organ (i.e. isotonic) in order to prevent osmosis from occurring

Uncontrolled osmosis will have negative effects with regards to cell viability:

In hypertonic solutions, water will leave the cell causing it to shrivel (crenation)

In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis)

In plant tissues, the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall

In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape

In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)

Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein

It is utilised by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)

This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins

Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane