2.3 Transport across membranes

Mrs Scawn

membranes (cell surface membrane)

every cell is surrounded by a plasma membrane

functions

• partially permeable - so it controls what enters and leaves the cell

• it separates the cells contents from the external environment

general membranes

in general separate components from the cytoplasm

• involved in cell signalling

• hold some of the components of some metabolic pathways in place

plasma membrane strutcure

• cholesterol

• glycoproteins and glycolipids

• carrier proteins

• channel proteins

cholesterol - plasma membrane

• adds strength to membrane

• prevents water loss and dissolved ions

• reduces fluidity of the membrane/ lateral movement - phospholipids

• they consist of a hydrophobic and a hydrophilic region - the hydrophobic regions bind to the phospholipid fatty acid tail making them pack closely together - reducing fluidity.

channel proteins

• water filled tubes that completely span the phospholipid bilayer

• have a hydrophilic tunnel through the centre

• allow water soluble/ions to diffuse across the membrane

• also help adhere cells together and provide structural support

• (aquaporins are channel proteins that allow water molecules to move across the membrane)

carrier proteins

• embedded in the phospholipid bilayer and span the membrane

• having a binding site to a specific ion or molecule

• bind to ions or molecules like glucose and amino acids

• they then change shape in order to move these across the membrane

• allow active transport across the membrane

• provide structural support

glycoproteins and glycolipids

act as cell recognition sites and cell surface receptors for hormones and neurotransmitters

• help maintain stability of the membrane.

• help cells attach to one another so form tissues.

Phospholipids

phospholipid forms two layers

Forms a bilayer because the polar head of the phospholipid is hydrophilic and so is soluble in water and is attracted to it, while the fatty acid tales are hydrophobic and repressed by water and are non polar so they move away from an aqueous solution. this forms a bilayer as the cells cytoplasm is an aqueous solution so the heads point outwards and the tails face inwards

• have hydrophobic core

allow lipid soluble substances to enter/ leave the cell by simple diffusion

prevent polar/charged water soluble substances entering and leaving the cell

makes membrane flexible and self sealing

polar

charged

hydrophilic

water soluble

e.g. H2O, glucose/amino acids, H+, Na+, Cl-

Non polar

hydrophobic

lipid soluble

e.g. O2, CO2

properties pf many phospholipids having an unsaturated fatty acid chain

this makes bilayer fluid - helping prevent membrane breaking

allows membranes to form vesicles and vesicles to fuse with membrane (endocytosis/ exocytosis)

polar hydrophilic head, non polar hydrophobic tails

form bilayers in water (head point towards water, tails point away from water) with a hydrophobic core

• useful for membranes/ compartmentalisation of the cell

• prevents charged/ polar substances entering/ leaving cell/ organelles freely.

Fluid mosaic model

plasma membranes consist of a phospholipid bilayer with embedded proteins, cholesterol, glycoprotein and glycolipids ( mosaic) - embedded unevenly in phospholipid bilayer

everything can move around within this layer (fluid)

what makes it a model?

scientists and researchers have agreed upon the fluid mosaic model based on experimental and chemical data.

L2 - diffusion

simple diffusion

• diffusion across a phospholipid bilayer

• non polar (lipid soluble) molecules can diffuse across the phospholipid bilayer (non polar meaning they can dissolve in the hydrophobic core of the cell membrane)

• small uncharged polar molecules can also diffuse across the bilayer (not as efficently though)

• diffusion is passive - doesn’t require energy from ATP

• described as = the movement of particles of a substance down a concentration gradient from an area of high concentration to an area of low concentration.

Facilitated diffusion

• passive, involves either carrier proteins or channel proteins

• large, charged, polar ions

channel proteins in facilitated diffusion

• provide a specific polar route through the membrane, only open if a specific ion is present, remain closed if not.

• allow charged/ polar substances to diffuse through the membrane

• diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore

• this allows the channel protein to control the exchange of ions

carrier proteins in facilitated diffusion

• provide a specific route through membrane, for complementary molecules only. e.g glucose

• e.g glucose binds to the specific carrier protein

• this causes it to change shape (tertiary structure) in such a way that allows the molecules to be released on the other side of the membrane

• no external energy needed (passive)

• molecules move from a region where they are highly concentrated to a region where they are low in concentration

PPQ: describe how substances move across cell surface membranes by facilitated diffusion? (3 marks)

• either through a channel protein or a carrier protein

• that are protein specific/ complementary to substance

• substance moves down a concentration gradient

simple diffusion molecules

small, uncharged, polar molecules,

• H2O, glycerol, urea, ethanol

hydrophobic molecules

• O2, CO2, N2, steroids

facilitated diffusion molecules

large, uncharged, polar molecules

• Glucose

• sucrose

Ions

• Na+, K+, H-, Ca-, Cl-

Factors affecting rate of the diffusion

1) temperature - at higher temperatures, particles have more kinetic energy and diffuse faster

2) concentration gradient - the steeper the concentration gradient, the faster the rate of diffusion

3) thickness of membrane - particles travel shorter distances through thin exchange surfaces, so diffuse faster

4) surface area - larger surface areas mean more particles can cross the membrane at once, making diffusion faster

5) number of carrier or channel proteins - the more of these proteins, the faster the rate of facilitated diffusion

L3 Osmosis

the net movement of water molecules form a region of high water potential to low water potential across a selectively permeable membrane until water potential is the same on both sides of the membrane

water potential

the pressure created by molecules as they collide with the membrane

pure water has a potential of 0kpa so the more solutes that are dissolved in water the more negative the water potential will become

animal cells in different solutions

hypertonic

hypotonic

isotonic

hypertonic solution

solution outside the cell has a lower water potential than inside the cell, water molecules leave by osmosis and the cell becomes shrunken and shrivelled. Haemoglobin is more concentrated so cell appears darker.

crenation of cel membrane

hypotonic

hydrostatic pressure increases inside.

cell swells and potentially bursts (cytolysis)

contents including haemoglobin are released

solution outside the cell has higher water potential that solution inside the cell

water molecules enter by osmosis

isotonic

solution inside and outside the cell has the same water potential

no net movement of water molecules by osmosis

cell is normal sized

Most molecules can’t freely diffuse across the cell membrane as….

1) they are not lipid soluble so cannot get through the bilayer

they are too large for the channel proteins

they are charged the same as channel proteins so are repelled

they are charged and have difficulty getting through the non polar tails on the phospholipids

L9 - active transport

the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

process of active transport

• molecule binds to receptor sites on carrier protein

• ATP attaches to the membrane of the carrier protein on the inside of the cell

• this causes hydrolysis of ATP into ADP and Pi, the hydrolysis of ATP releases energy

• carrier protein changes shape/ tertiary structure due to phosphate ion binding to membrane

• this results in the carrier protein becoming open to the inside of the cell but closed to the outside and the particle is released

• molecule is released into cell, phosphate molecule is released and carrier protein reverts back to original shape.

diagram of active transport

Sodium -potassium pump

sometimes a molecule or ion is moved into a cell/ organelle at the same time that a different one is been removed from it, e.g. the sodium potassium pump.

explain how hydrogen ion pumps more hydrogen ions into cell

• carrier proteins span the membrane and bind to a hydrogen ion

• hydrogen ion will bind to receptor sites on the carrier protein

• ATP attaches to the inside of the cell on the membrane which is then hydrolysed into ADP and Pi which releases energy

• this changes the tertiary stucture of the carrier proteins which opens the indie of the cell membrane and shuts not he outside - hydrogen ion gets released into cell

• hydrogen ions are moved across the membrane into the cell, against their concentration gradient

• pi is released form the inside of the cell membrane and carrier protein goes back to its original shape

co transport

the movement of an ion/ molecule against its concentration gradient by coupling it to the facilitated diffusion of another ion/molecule down its concentration gradient.

• ‘secondary active transport’

role of ileum?

part of the small intestine

• absorb molecules produced by digestion - sugar glucose

absorption of glucose and cotransport

• concentration of glucose is too low for glucose to diffuse out into the blood

• so its absorbed by active tranposrt

• step 1

  • sodium ions are actively transported out of the epithelial cells in the ileum into the blood, by the sodium potassium pump

  • this creates a concentration gradient - there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cell

• step 2

  • this causes sodium ions to diffuse from the lumen of the ileum to the epithelial cell, down their concentration gradient - do this via sodium-glucose co transporter proteins

  • the co transporter carries glucose into the cell with the sodium - as a result the concentration of glucose inside the cell increases

• step 3

  • glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion

factors affecting the rate of active transport

• the speed of individual carrier proteins - faster they work, the faster the rate of active transport

• the number of carrier proteins present - the more proteins there are the faster the rate of active transport

• the rate of respiration int eh cell and the availability of ATP - if respiration inhibited, active transport can’t take place.