4 CELL MEMBRANES AND TRANSPORT

LO:

• describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins ;

• https://www.youtube.com/watch?v=YPxNQy-K-ns

• All living cells – surrounded by the cell surface membrane

• Controls the exchange of materials• Regulates the transport across the

membranes• Receive messages

• https://www.youtube.com/watch?v=moPJkCbKjBs

Phospholipids

- Made of two fatty acid tails and glycerol head that contains a phosphate group

- Hydrophilic head and hydrophobic tails

- Can form little bags in which chemicals are isolated from the external environment = cells and organelles

• When in water:– Spread as a single layer of molecules on the

surface of water– Forms micelles surrounded by water– Forms bilayer– Bilayers form membrane-bound compartments

Structure of membranes

• The bilayer is seen under the electron microscope at very high magnification

• 1972 – fluid mosaic model– Membrane made of phospholipids and proteins

that can move about by diffusion– Phospholipids move sideways in their own layers– Some proteins move as well– Mosaic – the pattern made by the scattered

protein molecules when viewed from above

• http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane%20fluidity.swf

• Phospholipid bilayer– Molecules move– Tails inwards – form non-polar hydrophobic

interior; the more unsaturated they are, the more fluid the membrane (because the tails are bent and therefore fit more loosely); the longer the tail the less fluid the membrane

– The lower the temperature the less fluid the membranes are (some organisms respond by increasing the number of unsaturated fatty acids)

– Heads – face the aqueous medium that surrounds the membranes

• Proteins• Integral proteins (intrinsic)– In the inner layer, outer layer or spanning the

whole membrane (transmembrane proteins)– Have hydrophilic and hydrophobic regions– Mostly float like icebergs in the phospholipid

layer; some are fixed• Peripheral proteins (extrinsic)– Found on the inner or outer surface of the

membrane

• Cell markers = antigens – allow cell recognition• Enzymes – e.g. digestive enzymes in the

alimentary canal • Transport proteins – form hydrophilic

channels or passageways for ions and polar molecules to pass through the membrane– Channel proteins– Carrier proteins

• Carbohydrates– Short branching chains of carbohydrates are

attached to the proteins and lipids– On the side of the molecule which faces the

outside of the membrane– Glycoproteins– Glycolipid

• Glycoproteins and glycolipids– Molecules on the outer surface of the membrane

that have short carbohydrate chains attached to them

– The chains project like antennae into water fluids where they form hydrogen bonds and so help stabilise the membrane structure

– Can form a sugary coating = glycocalyx

– Glycoproteins and glycolipids act as receptor molecules• Signalling receptors – part of signalling system that

coordinates the activities of animal cells, this receptor recognises messenger molecules like hormones and neurotransmitters • Receptors involved in endocytosis• Receptors involved in binding cells to other cells in

tissues and organs

https://www.wisc-online.com/learn/natural-science/life-science/ap1101/construction-of-the-cell-membrane

TRANSPORT ACROSS THE CELL SURFACE MEMBRANE

•Diffusion• Facilitated diffusion•Osmosis•Active transport•Bulk transport

LO:

• describe and explain the processes of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis

• investigate the effects on plant cells of immersion in solutions of different water potential;

DIFFUSION

• Net movement of a substance from a region of its higher concentration to a region of its lower concentration

• This movement is a result of random motion of its molecules or ions; caused by the natural kinetic energy or the molecules

• The molecules or ions move down the concentration gradient

• Some molecules and ions are able to pass through cell membranes by diffusion

Factors affecting the rate of diffusion

• The steepness of the concentration gradient– Difference in concentration of the substance on the

two sides of the surface– The greater the difference in concentration, the

greater the number of molecules passing in the two directions – the faster the net rate of diffusion

• Temperature– at high temperature, molecules have higher kinetic

energy than at low temperatures; they move around faster = rate of diffusion higher

Temperature

Low temp High tempAt a higher temperature the particles have more kinetic energy and are moving around faster. Therefore in a given time more diffusion will occur.

• The surface area– The greater the surface area the more ions or

molecules can cross it at any moment = faster diffusion (microvilli, cristae)

• The nature of the molecules or ions– Large molecules require more energy to get them

moving than small ones do = small molecules diffuse faster

– Non-polar molecule diffuse much more easily through cell membranes as they are soluble in the non-polar phospholipid tails

Experiment to investigate surface area to volume ratio

• Three cubes of agar are prepared which contain the indicator phenolphthalein.

• These are placed in hydrochloric acid which will diffuse into the cubes.

• As it diffuses in it will turn the indicator colourless.

3cm

3cm

2cm

2cm

1cm1cm

Experiment to investigate surface area to volume ratio

• As the size of the cube increases the surface area to volume ratio decreases.

3cm

3cm

2cm2c

m1cm1c

m

Width of cube (cm)

Surface area (cm2)

Volume (cm3)

Surface area: volume

1 6 1 6

2 24 8 3

3 54 27 2

Experiment to investigate surface area to volume ratio

• The cubes look like this after a few minutes.

• If these were real cells then the bigger cell would not have received what it needs to all parts of the cell.

• Therefore it would need a bigger surface area in order to rely on diffusion.

3cm

3cm

2cm

2cm

1cm1cm

Experiment to investigate surface area to volume ratio

• Watch this video to see the experiment in action.

Surface Area• As the rate of diffusion relies on the surface

area.The parts of organisms that rely on diffusion therefore tend to have a large surface area.

Root hairs Alveoli in the lungs

FACILITATED DIFFUSION

• Diffusion with the help of certain protein molecules

• Large molecules as glucose and amino acids, sodium ions, chloride ions

• Channel proteins– Water filled pores; allow charged substances do

diffuse; have fixed shape– Most of them are gated = part of the protein

molecule on the inside surface of the membrane can move to close or open the pore = control of ion exchange

– Example: nerve cell surface membranes – one type allows sodium ions – production of an action potential; the other allows exit of potassium during the recovery phase (Na-K pump)

• Carrier proteins– Constantly flip between two shapes– The binding site is alternately open to one side of

the membrane, than the other– The rate of diffusion is affected by the number of

opened channel proteins– Example: cystic fibrosis is caused by a defect in a

channel protein which should be present in the membranes of the cells lining the lungs; this protein allows chloride ions to move out of the cells

• https://www.youtube.com/watch?v=mzo_B5F7pk4

OSMOSIS

• Special type of diffusion involving water molecules only

• Solute + solvent = solution• Sugar + water = sugar solution• Partially permeable membrane is present

• Water moves from a dilute solution to a more concentrated one across the partially permeable cell membrane.

Rlawson at en.wikibooks

Water potential and solute potential

• Water potential = tendency of water to move from one place to another

• Symbol for water potential (psi) • Water always moves from a region of higher

water potential to a region of lower water potential (down a water potential gradient)

• Equilibrium is reached when the water potentials are equal

• OSMOSIS – Net movement of water molecules from a region

of higher water potential to a region of lower water potential through a partially permeable membrane

– Pure water has the highest possible water potential;

– Ψpure water= 0; solute potential is always negative

• OSMOSIS IN ANIMAL CELLS

• It is important to maintain a constant water potential inside the bodies of animals

• In animal cells water potential = solute potential

PRESSURE POTENTIAL

– The greater the pressure applied, the greater the tendency for water molecules to be forced back from solution B to solution A

– Increasing the pressure increases the water potential of solution B

– Ψp = pressure potential– Pressure potential makes the water potential less

negative, and is therefore positive

• OSMOSIS IN PLANT CELLS

• Plant cells are surround by cell wall = strong and rigid

• Protoplast = the living part of the cell inside the cell wall

• The cell wall prevents the cell from bursting• When placed to water – volume of the cell

increases – protoplast starts to push against the cell wall and pressure starts to build up – the pressure potential increases the water potential = equilibrium is reached

• water potential is the combination of solute potential and pressure potential:

Ψ = Ψs + Ψp

• Plasmolysis = a plant cell is placed in a solution of low water potential – water leaves the cell by osmosis – protoplast shrinks and the external solution passes through the permeable cell wall = the cell is plasmolysed

• Incipient plasmolysis – the point at which pressure potential has just reached zero and plasmolysis is about to occur

ACTIVE TRANSPORT

• Against the concentration gradient• Achieved by carrier proteins – they are specific

for a particular type of molecule or ion• Energy required – ATP (adenosinetriphosphate);

produced by respiration inside the cell• Energy is used to change the shape of carrier protein

in the process• The energy consuming transport of molecules or ions

across a membrane against a concentration gradient

• SODIUM POTASSIUM PUMP - Na+ – K+ pump

• IMPORTANCE OF ACTIVE TRANSPORT– In kidneys – re-absorbtion into the blood– Absorption of some products of digestion in guts– In plants – to load sugar from the

photosynthesising cells of leaves into the phloem tissue; to load inorganic ions from the soil into root hairs

BULK TRANSPORT

• Transport of large quantities of materials into cells and out of cell

• Endocytosis – transport into the cells• Exocytosis – transport out of the cells

• ENDOCYTOSIS– Involves the engulf of the material by the cell

surface membrane to form a small sack– PHAGOCYTOSIS – cell eating; specialized cells =

phagocytes; engulfing of bacteria by certain white blood cells

– PINOCYTOSIS – cell drinking; uptake of liquid; human egg cell takes up nutrients from cells that surround it

Phagocytosis of a bacterium by a white blood cell

• EXOCYTOSIS– Reverse of endocytosis – materials are removed

from cell– E.g. secretion of digestive enzymes from cells of

the pancreas– Secretory vesicles from the Golgi apparatus carry

the enzymes to the cell surface and release their contents

– Plant cells use exocytosis to get their cell wall building materials to the outside of the cell surface membrane