B2.1 Membranes and membrane transport

Diffusion

When particles move from regions of higher concentration to a region of lower concentration

Describe the characteristics of integral proteins

• Partially hydrophobic (amphipathic)

• Embedded in phospholipid bilayer

• Can be embedded across both layers or just one layer
  
  

Describe characteristics of peripheral proteins

• Hydrophilic on their surfaces

• Attached to surface of integral proteins so attachment is reversable

• Attached to surface membrane via hydrocarbon chain

• Found on inner or outer side of membrane

• Do NOT extend into the lipid bilayer

Outline the functions of membrane proteins with examples

• Hormone-Binding- Proteins with specific shapes are exposed to exterior fit the shape of specific hormones. Attachment between protein and hormone changes the proteins shape, causing a message being relayed to the interior of the cell (e.g. Insulin & insulin receptors)

• Enzymatic - Found on the interior or exterior of membrane surface. Are integral proteins with active sites exposed, typically grouped together so metabolic pathway can be catalysed

• Cell adhesion - Allows for temporary or permanent connections between cells (junctions). Can be gap junctions or tight junctions

• Cell-to-cell communication: Proteins with carbohydrate molecules attached. Provide identification label so organism can distinguish between self and non-self

• Channel forming: Proteins that span across the membrane and provide passageways for substances to be transported through

• Pumps for active transport: In active transport, proteins shuttle a substance from one side of the membrane to another by changing shape. Requires ATP

Glycolipids

When cell membrane phospholipids have carbohydrates chains attached to them

Glycoproteins

Cell membrane proteins with chains of carbohydrates attached to them

Glycocalyx

A thin sugar layer made of carbohydrate chains attached to proteins that can cover a cell

Fluid mosaic model

Depicts cell membrane as dynamic, flexible structure made up of different components

• “Fluid” because phospholipid membrane + proteins move laterally

• “Mosaic” due to scattered pattern produced by the proteins of the phospholipid bilayer when viewed above

• 4 main components: Phospholipids, glycoproteins & glycolipids, cholesterol, integral & peripheral proteins

Draw and annotate a 2D diagram of the fluid mosaic model of membrane structure

• The phospholipid bilayer, making it clear which part is the phosphate head and which parts are the hydrocarbon tails Integral proteins, e.g. channel/carrier

• Peripheral proteins that do not extend into the hydrophobic region

• Glycoproteins with a carbohydrate attached

• Cholesterol, with the OH group next to the phosphate heads and the rest positioned next to the tail

Explain how fatty acids can impact membrane fluidity

• Phospholipids are made of fatty acids, which each have a hydrocarbon chain

• Hydrocarbon chains can vary in length which impacts the fluidity of the membrane

• Longer phospholipid tails have more interactions between the tails resulting in a less fluid membrane

• Saturated fatty acids (straight chain) have a denser arrangement allowing the membrane to be stronger, more stable, and less fluid at higher temperature

• Unsaturated fatty acids (have double bonds, kinks) are less dense so have lower melting points, making the membrane more fluid and flexible

Explain how unicellular organisms regulate membrane fluidity

• Bacteria produce fatty acid desaturases which increase the number of double bonds within fatty acids in a membrane

• This maintains membrane fluidity at lower temperatures

Explain how cholesterol regulates membrane fluidity in animal cells

• Cholesterol is a membrane lipid that is with hydrophobic and hydrophilic regions

• Maintains membrane fluidity across low and high temperature

• In low temperature, it disrupts close packing of phospholipids increasing fluidity

• At high temperatures, it holds the fatty acid tails together, increasing membrane stability

• Acts as barrier between phospholipids preventing water-soluble substances from diffusing across membrane

Osmosis

The diffusion of water molecules, from a dilute solutution to a solution with a higher solute concentration, across a partially permeable membrane

Water potential

The tendency of water to move

Simple diffusion

The net movement of, as a result of random motion of molecules or ions, of a substance from a region of higher concentration to a region of lower concentration

Facilitated diffusion

The passive movement of particles across the cell membrane via the aid of a membrane protein

Describe the structure and function of a channel protein

• Integral lipoprotein with pores that opens and closes in response to chemical or mechanical signals

• Allows charged molecules to diffuse through cell membrane

• Some are gated, meaning parts of the channel protein on the inside surface of the membrane can move to open or close the pore

  • Allows them to control exchange of ions

Describe the structure and function of a carrier protein

• Integral proteins that change shape to transport specific substance from one side of the membrane to another

• Binding site it open to one side of a membrane

• Once a solute/substrate binds, the protein undergoes a conformational change

• Carrier protein switches shape and opens to other side of membrane

Active transport

The movement of molecules and ions across a cell membrane, from a region of lower concentration to a region of higher concentration, using energy from respiration

Distinguish between passive and active transport

• Passive transport does not require ATP while active transport does

• In passive transport, substances move from areas of high concentration to areas of low concentration, while in active transport it is the opposite

• In passive transport, movement occurs ALONG the concentration gradient, while in active transport movement occurs AGAINST the concentration gradient

• In passive transport, equilibrium can be reached if left undisturbed, while in active transport equilibrium is not reached

Selective permeability

The ability of the membrane to differentiate between different types of molecules, allowing only some molecules through while blocking others

Describe the structure and function of the Sodium Potassium Pump

• Integral proteins found in cell membranes, using active transport to move sodium and potassium in opposite directions AGAINST concentration gradient

• Are exchange transporters

• 3 binding sites for Sodium ions

• 2 binding sites for potassium ions

• 1 binding site for inorganic phosphate group

Explain the steps of the Sodium Potassium Pump

• 3 sodium ions from inside the axon bind to the pump

• ATP binds to the pump, transferring a phosphate

  • Phosphorylation of pump changes the pumps shape, causing the pump to open outwards towards the outside of the axon

  • Sodium ions released into extracellular fluid

• 2 Potassium ions outside the axon bind to pump protein

• The attached phosphate gets released from the pump, restoring the pumps original shape

• Change in shape causes Potassium to be released into intracellular space

Distinguish between selective permeability in simple diffusion vs active transport and facilitated diffusion

• Facilitated diffusion and active transport make the cell membrane selectively permeable

  • Channel proteins have narrow, hydrophilic cores so only allow ions of specific charge and size to pass

  • Carrier proteins have a unique shape (binding site) corresponding to its specific substrate, which changes shape once bound

  • In active transport, highly selective proteins in the membrane bind to specific substances to be transported

• In simple diffusion, the membrane is not selective against small or non-polar molecules

• In simple diffusion, the membrane is selective against large, charged molecules which need integral proteins to pass through

Factors affecting permeability

• Particle size - Plasma membrane not permeable to large molecules, since hydrophobic core is tightly packed

• Hydrophobic/philic properties of particle

  • Plasma membrane permeable to hydrophobic particles (gases, lipids)

  • Variable permeability to small, hydrophilic molecules (water)

  • Not permeable to hydrophilic molecules with charge (cannot diffuse through hydrophobic core, need transport protein)