Membranes can control the composition of cells by regulating the passage of materials

• Transport can be regulated because membranes are both semi-permeable and selective

Semi-permeability:

• The non-polar lipid bilayer restricts the movement of hydrophilic molecules

• It effectively acts as a barrier to all large polar or charged substances (like ions)

Selectivity:

• The integral membrane proteins can enable the transport of hydrophilic molecules

• These proteins can adopt ‘open’ or ‘closed’ conformations to regulate molecular transport

Membrane Permeability

The ability of molecules to move across a membrane is

determined by their comparative size and properties

• Hint:  Large or charge = no voyage!

In terms of material transport across the bilayer:

     Yes,  Lipophilic, uncharged molecules can freely cross

    Yes,    Very small polar molecules can also freely cross

   No,     Large polar molecules cannot freely cross

   No,     Ions (charged molecules) cannot freely cross

Types of Transport

Membrane transport occurs by one of two processes:

Passive Transport

• Involves movement along a concentration gradient

• Does not involve the expenditure of energy (ATP)

Active Transport

• Involves movement against a concentration gradient

• Involves the expenditure of energy (ATP hydrolysis)

• Requires the involvement of transport proteins

Passive Transport:

There are three main types of passive transport mechanisms employed by living organisms:

Simple Diffusion:

• Small or lipophilic molecules can freely cross the membrane (O₂ , CO₂ , steroid hormones)

Facilitated Diffusion:

• Large or charged molecules require the assistance of membrane proteins (ions, glucose)

Osmosis:

• Water movement is determined by relative internal and external solute concentrations

Simple Diffusion:

Simple diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration (along a concentration gradient) until equilibrium is reached (High—>low concentration)

Facilitated Diffusion:

Certain substances cannot freely cross a plasma membrane
(examples include ions or large and polar macromolecules)

• These materials need proteins to facilitate their transport
  

Protein Channels:

• Have hydrophilic internal pores to allow movement of ions

• May be gated with a selectivity filter to regulate transport

Carrier Proteins:

• Undergo a conformational change to translocate material

Osmosis

Osmosis is the net movement of free water molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration (Low solute levels  (high free water) —→ High solute levels  (low free water)

Solute Concentration:

Solutions can be classified according to solute concentration:

Hypertonic:  Have a relatively higher solute concentration

 _🢧  Hypertonic solutions will gain water via osmosis

Hypotonic:  Have a relatively lower solute concentration 

 _🢧  Hypotonic solutions will lose water via osmosis

Isotonic:  Solutions have the same solute concentration

 _🢧  Isotonic solutions will maintain a constant water level

Osmotic Effects:

When a cell is placed in a hypertonic solution, the loss of water causes the cell to shrivel

• The loss of cytoplasmic volume causes the membrane to form a ruffled shape (crenation)

When a cell is placed in a hypotonic solution, the gain of water causes the cell to swell

• As the plasma membrane is fluid, it will stretch until the cell eventually bursts (cell lysis)

Hypertonic = Crenation Isotonic = Normal Shape Hypotonic = Swelling

If a cell has an outer cell wall, the osmotic consequences for the cell will be different

• Cell walls are found in plants (cellulose), fungi (chitin) and bacteria (peptidoglycan)

The cell wall is a rigid external boundary and will not change shape due to water transport

• In a hypertonic solution, the membrane will recede away from the cell wall (plasmolysis)

• In a hypotonic solution, the cell membrane will be pressed against the cell wall (turgor)

For organisms to survive, the fluid surrounding cells must remain isotonic to prevent damage

• There are several natural and artificial ways of maintaining this homeostatic requirement

Isotinicity

Living Adaptations:

• Unicellular organisms may contain contractile vacuoles to regulate their water levels

• Multicellular organisms may use aquaporins to control water intake and release

Medical Applications:

• Organs used in medical procedures must be bathed in isotonic solutions until transplant

• Isotonic solutions can also be delivered intravenously to restore fluid levels in patients

Contractile Vacuoles

Unicellular organisms (such as protists) may use contractile vacuoles to expel excess water

• Excess water is absorbed into the contractile vesicle, causing it to swell (diastole)

• The vacuole then fuses to the plasma membrane and contracts, expelling water (systole)

1. Water enters a cell by osmosis

2. Water then fills the vacuole

3. Vacuole fuses to membrane

4. Contraction expels water

Aquaporins

Multicellular organisms may contain integral proteins called aquaporins within their plasma membranes

• The aquaporins will act as selective water channels

Aquaporins facilitate a faster rate of water transport in response to any changes in solute concentration

• The quantities of aquaporins can be regulated via gene expression to help control and maintain the osmotic conditions within a multicellular organism

Active Transport

Active transport uses energy (ATP) to pump molecules against the concentration gradient (from low → high)

• Protein pumps are required for the translocation

Active transport involves the binding of a molecule to a specific protein pump – which is hydrolysed by ATP

• ATP hydrolysis causes a conformational change in the pump, causing the molecule to be translocated

• The pump then returns to its original conformation

The pumps change conformation when hydrolysed by ATP and move the molecule against a gradient

Vesicular Transport

Materials can also cross the membrane by having the bilayer break and reform around them

• Membranes are held together by weak hydrophobic associations, making them very fluid

The internalisation of material (endocytosis) results in the formation of a surrounding vesicle

• Material to be exported (exocytosis) must be packaged into a vesicle by the Golgi complex

Topic connections:

Water

•  Osmosis is a consequence of the solvent properties of water

Systems Integration

•  Neurons use active transport to establish membrane potentials

Excretory System (AHL)

•  Osmoregulation in the kidneys involves the use of aquaporins 

Translocation (AHL)

•  Changes in water potential impact translocation rates in plants