Biological Membranes #2

Biological Membranes

Plasma membranes are partially permeable as they allow some but not all substances to pass through them.

  • Very small molecules diffuse through the plasma membrane
  • Some substances dissolve in the lipid layer to pass through
  • Larger substances pass through protein channels or are carried by carrier proteins

Roles of the plasma membrane

  • Separates the cell’s contents from the external environment
  • Regulates transport of materials into and out of the cell
  • May contain specific enzymes involved in metabolic pathways
  • Contains antigens so that the immune system can recognise the cell as being self and not attack it
  • May release chemical signals to other cells and contains receptors for cell communication and signalling (hormone bind to membrane bound receptors)
  • May be the site of chemical reactions

Roles of membranes within cells include:

  • The cristae of mitochondria which provides a large surface area for aerobic respiration
  • The thylakoid of chloroplasts which house chlorophyll and are the site of photosynthesis
  • The plasma membrane of the epithelial cells of the small intestine which contain digestive enzymes that breakdown certain sugars

Fluid mosaic model – theory of cell membrane structure with proteins embedded in a sea of phospholipids

  • Channel proteins – allow ions to mass through
  • Carrier proteins – allow specific molecules across the membrane
  • Glycolipid – lipid/phospholipid with a carbohydrate chain
  • Glycoprotein – protein with a carbohydrate chain
  • Others include: Enzymes, antigens & receptor sites for hormones
  • Cholesterol – regulates fluidity and gives mechanical stability and resists the effect of temperature changes on the membrane
  • Glycocalyx – the hydrophilic area just outside the cell consisting of carbohydrate chains attached to both lipids and proteins


Neuron cell membranes

  • Protein channels and carriers covering the long axon allow the transport of ions to bring the conduction of electrical impulses along their length
  • They have a myelin sheath of flattened cells around them several times to give more membrane layers and to insulate the electrical impulses

Root hair cell membranes

They have many carrier proteins which transport nitrate ions from the soil into the cell as part of the nitrogen cycle.




Cristae of Mitochondria

These contain many electron carriers made of protein and hydrogen ion channels which are associated with ATP synthesis

White Blood Cell Membranes

These contain protein receptors for detection of antigens on foreign cells and pathogens

Diffusion across membranes

Diffusion – movement of molecules from an area of high concentration of that molecule to an area of low concentration across a partially permeable membrane along a concentration gradient. It is passive and does not involve metabolic energy (ATP)

Facilitated diffusion – the movement of molecules from an area of high concentration of that molecule to an area of low concentration across a partially permeable membrane via protein channels or carriers. This still does not require metabolic energy (ATP)

Passive processes only use the kinetic energy of the molecules, not ATP.

When molecules move down their concentration gradient they are still moving randomly but remain evenly dispersed which is called net diffusion. They have reach equilibrium.

Concentration gradient is maintained by the respiring cells using the O2 in animals and the carbon dioxide diffusing into the palisade cell to be used in chloroplasts for photosynthesis and the constant use of these molecules inside the cell maintains a concentration gradient as there is always a higher concentration in the external environment.

Factors the affect the rate of simple diffusion

  • Temperature – as this increases, kinetic energy increases so rate of diffusion increases
  • Diffusion Distance – the thicker the membrane/diffusion distance, the slower the rate of diffusion
  • Surface area – more diffusion can take place of a larger surface area
  • Size of diffusing molecule – smaller molecules/ions diffuse more quickly
  • Concentration gradient – steeper the gradient the faster the diffusion

Neurons have many ion channels at synapses to aid the electrical conductivity between cells.

Epithelial cell membranes always have chloride ion channels as these play a part in regulating mucus composition.


This is the net passage of water molecules down their water potential gradient, across a partially permeable membrane.

Water potential – measure of the tendency of water molecules to diffuse from one region to another

In a solution the solute is dissolved in the solvent. Water molecules can pass directly through the phospholipid bilayer.

If solute molecules dissociate into charged ions water will be attracted to them, as it is a polar molecule.


Water potential

  • Pure water has the highest water potential of 0kPa
  • Solute molecules lower the water potential
  • Water molecules move from a high water potential to a low water potential
  • When water potential is equal on both sides there will be no net movement of osmosis
  • Water with solutes has negative water potential values

Active transport

Active transport – the movement of substances against their concentration gradient across a cell membrane requiring ATP

Endocytosis  – bulk transport of molecules too large to pass through a cell membranes into a cell

Exocytosis – bulk transport of molecules too large to pass through a cell membrane out of a cell

Sodium Potassium pumps

3x Na+ ions are transported in one direction while 2x K+ ions are transported in the opposite direction.

Carrier proteins

ATP allows some carrier proteins to change their conformation to carry the molecule from one side of a gradient to another

Bulk transport


Pinocytosis – cells ingesting liquids

Phagocytosis – cells ingesting solid matter (e.g. WBC ingesting a bacteria)


  1. A membrane bound vesicle, containing substance to be secreted, is moved towards the cell surface
  2. The vesicle fuses to the cell membrane
  3. The fused site splits, releasing the contents of the vesicle to the external environment

Factors affecting membranes


Increased temperature

  • Molecules have more kinetic energy and therefore move faster
  • Membrane fluidity increases as phospholipids have more kinetic energy
  • Permeability increases
  • The tertiary structure of the proteins may be compromised both on the plasma membrane and the cytoskeleton threads beneath the membrane surface which may impede their functions (e.g. enzymes unable to catalyse reactions when they are denatured)
  • Membrane fluidity may affect the infolding of the membrane during phagocytosis
  • It may also change the ability of the cell to signal other cells by exocytosis
  • The presence of cholesterol acts as a temperature buffer as it provides more stability even through the phospholipids move more fluidly in high temperatures

Decreased temperature

  • Saturated fatty acids become compressed
  • Unsaturated fatty acids maintain fluidity as the kinks in their tails push the other phospholipids away
  • Cholesterol also buffers this as it prevents phospholipid molecules packing in too tightly
  • Molecules have lowered kinetic energy and therefore move slower


  • Organic solvents such as acetone and ethanol will damage cell membranes as they dissolve lipids