Biology: Topic 3: Cell Membrane

What is diffusion?

• Molecules in high concentration will move to areas of low concentration

• Not by repelling each other, but because they are moving around and eventually will disperse

What makes the cell membrane fluid?

• neither solid or liquid - fluid

  • Fluidity means it can flow around the cell as it changes shape

  • Molecules within the membrane are free to move

• Double bonds in fatty acid tails of phospholipids

• Sterols (cholesterol in animal cells, phytosterols in plant cells) - pack between fatty acid tails

What is the fluid mosaic model?

• describes that the membrane is a fluid phospholipid bilayer with a mosaic of proteins that move

What are the functions of membrane proteins?

1. Enzymes - catalyse reactions, substrates may be on either side of membrane

2. Signal transduction- signalling molecule bind to protein receptor to pass the information along

3. Cell - cell recognition- cells recognise glycoproteins or glycolipids (carbohydrates) to know what other cells are around them (own and foreign)

4. Intercellular joining- binds cells for organisation and strength

5. Attachment to the cytoskeleton (eukaryotic) and extracellular matrix (animal cells)- provide strength to flimsy membrane, and organisation

6. Transport- allows passage of charged and hydrophilic solutes, with or without energy

Why is membrane fluidity important?

• helps small molecules cross (O2)

• Enables proteins to move to where they’re needed

• Helps proteins in the membrane undergo conformational change (for transport and enzymes)

• Helps the entire membrane move around/change shape

What is simple diffusion?

• no energy

• Solute passes directly through the membrane

• Solute diffuses down its OWN conc gradient, from high to low

• Small uncharged solutes and large hydrophobic solutes

• Rate is dependent on OWN concentration gradient (difference in conc)

• Slow

What is facilitated diffusion?

• no energy

• Solute passes through a transport protein (usually specific for a solute)

• Faster than simple diffusion

• Solute diffuses down its OWN conc gradient, from high to low

• Large, charged solutes and hydrophilic solutes

• Rate is dependent on concentration gradient (difference in conc) - until plateau because proteins are saturated and working at their maximum rate

• Transport proteins:

  • Channel proteins - open channel

    • Ion channels- channel protein that transports ions

  • Carrier proteins- changes shape to allow solute to pass

What is active transport?

• needs energy from ATP

• Moves solute from low concentration to high concentration

• Against their concentration gradient

• Proteins used: carrier only

• Primary active transport: uses ATP to directly move ions (Na/K pump, H+ pump)

• Secondary active transport: primary active transporter (e.g. H+ pump) sets up conc gradient that a cotransporter uses to move transport of a second solute against its gradient

What is a cytoplasmic connection between cells?

• Animal cells- Cell cytoplasms are connected by gap junction channels (proteins that span the gap junction) that can be opened and closed to allow small molecules and ions to move from one cell to another

• Plant cells- cell cytoplasms are connected by plasmodesmata (channels lined with cell membrane) to allow passage of small and large solutes (proteins and mRNA) from one cell to another

What is osmosis?

diffusion of free water (water that is not bound to solutes or surfaces) across a membrane

Define moles vs molarity

• mole - 6×10²³ - to compare concentrations - 6×10(23) glucose molecules in 1 L of sol is a 1 mol/L glucose solution

• Molarity - measure of concentration

Define moles vs osmoles

Some solutes dissociate (NaCl) so 1 mol of NaCl becomes 2 mol of solute particles - osmolarity takes this into account

Osmolarity - measure of the total concentration of solute particles in a solution

What is osmosis?

Water flows across a membrane from a region of low solute concentration to a region of high solute concentration

What is an aquaporin?

Protein channel that water passes through to enter the cell - much faster than diffusing directly across the membrane

How does tonicity impact animal cells

• Hypotonic solution - lower solute conc outside the cell- water moves into the cell - it bursts

• Isotonic solution - equal solute conc outside- no net movement of water - normal cell

• Hypertonic - higher solute conc outside the cell - water moves out of the cell - it shrivels

How does tonicity impact plant cells

• hypotonic- lower solute conc outside the cell - water moves into the cell - cell is turgid (preferred)

• Isotonic- equal conc of solutes - no net movement - cells lose turgor - is flaccid

• Hypertonic- higher solute conc outside the cell - water moved out of the cell - cell becomes plasmolysed

What is plasmolysis?

Plant cells - cell shrinks, membrane pulls away from the cell wall

What is water potential?

• the physical property that predicts the direction in which water will flow

• Measured in MPa (megaPascals)

• Water potential = solute potential (osmotic potential) + pressure potential

• In osmosis: water moves from high water potential to low water potential

What water potential does pure water open to the atmosphere have?

• 0 MPa

What is solution potential and how does it impact water potential?

• the effect of dissolved solutes on water potential

• Water flows toward higher solute concentration, and flows from high to low water potential

• Solutes make solute potential negative

What is pressure potential and what effect does it have on water potential?

• the physical pressure that squeezes water

• Makes pressure potential positive

What is membrane potential?

• membrane potential is the voltage across a membrane

• Inside the cell is negative and outside is positive due to the unequal distribution of ions

• Affects the traffic of all charged substances across the membrane - Favours the passive transport of cations into the cell and anions out of the cell

What is the electrochemical gradient?

• the overall force that drives the diffusion of ions across a membrane

• Chemical force: the ions own concentration gradient

• Electrical force: effect of membrane potential on the ions movement

How does electrochemical gradient impact an ions movement across membranes?

• Ions (CHARGED) diffuse down their electrochemical gradient (not just concentration gradient - UNCHARGED)

• If both forces act in the same direction- the ions own concentration can move passively

• If the electrical forces oppose the simple diffusion down the conc gradient - active transport is used

What is an electrogenic pump?

Transport protein that generates voltage across a membrane - like a proton pump (important in cellular respiration for plants, fungi, bacteria)

What is a cotransporter?

A transport protein that can couple the downhill diffusion of the solute to the uphill transport of a second substance against its own concentration gradient (uses energy)