The document summarizes the structure and functions of cell membranes. It describes the fluid mosaic model, which states that membranes are made of phospholipids and proteins that can move about freely. Phospholipids form a bilayer with hydrophobic tails facing inward and hydrophilic heads outward. Membrane proteins are either integral proteins spanning the membrane or peripheral proteins attached to one surface. The document also explains various transport mechanisms like diffusion, facilitated diffusion, osmosis, active transport, and bulk transport across membranes.

1. CELL MEMBRANES
AND TRANSPORT

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

3. • All living cells – surrounded by the cell surface
membrane
• Controls the exchange of materials
• Regulates the transport across the
membranes
• Receive messages

4. 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

5. • 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

8. 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

10. • 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

11. • 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

12. • 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

13. • 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

14. • 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

15. – 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

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

17. TRANSPORT ACROSS THE CELL
SURFACE MEMBRANE
• Diffusion
• Facilitated diffusion
• Osmosis
• Active transport
• Bulk transport

18. 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;

19. 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

20. • The molecules or ions move down the
concentration gradient
• Some molecules and ions are able to pass
through cell membranes by diffusion

21. 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

22. At a higher temperature the particles have more kinetic energy and are
moving around faster. Therefore in a given time more diffusion will occur.

23. • 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

24. • 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
1cm
1cm

25. • As the size of the cube increases the surface
area to volume ratio decreases.
3cm
3cm
2cm
2cm
1cm
1cm
Width of cube
(cm)
Surface area
(cm2)
Volume
(cm3)
Surface area: volume
1 6 1 6
2 24 8 3
3 54 27 2

26. • 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
1cm
1cm

27. • Watch this video to see the experiment in
action.

28. • 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.

29. FACILITATED DIFFUSION
• Diffusion with the help of certain protein
molecules
• Large molecules as glucose and amino acids,
sodium ions, chloride ions

30. • Channel proteins
– Water filled pores; allow charged substances to
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)

31. • 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

33. OSMOSIS
• Special type of diffusion involving water
molecules only
• Solute + solvent = solution
• Sugar + water = sugar solution
• Partially permeable membrane is present

34. • Water moves from a dilute solution to a more
concentrated one across the partially
permeable cell membrane.
Rlawson at en.wikibooks

35. 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

36. • 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

37. • OSMOSIS IN ANIMAL CELLS

38. • It is important to maintain a constant water
potential inside the bodies of animals
• In animal cells water potential = solute
potential

39. PRESSURE POTENTIAL

40. – 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

41. 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

42. • SODIUM POTASSIUM PUMP - Na+ – K+ pump

45. • IMPORTANCE OF ACTIVE TRANSPORT
– In kidneys – re-absorption 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

46. 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

47. • 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

48. Phagocytosis of a bacterium by a white blood cell

49. • 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