2. AS Biology. Foundation. Cell membranes and Transport
AS Biology FOUNDATION Chapter 4 pgs CELL MEMBRANES and TRANSPORT
AS Biology. Foundation. Cell membranes and Transport

3. AS Biology. Foundation. Cell membranes and Transport
The Cell
AS Biology. Foundation. Cell membranes and Transport

4. AS Biology. Foundation. Cell membranes and Transport
Learning Objectives
Describe the fluid mosaic model of membrane structure and explain the underlying reasons for this structure.
Outline the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins in membranes.
Outline the roles of the plasma membrane, and the roles of membranes within cells.
Describe and explain how molecules can get in and out of cells (cross cell membranes) by the processes of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
Describe the effects on animal and plant cells of immersion in solutions of different water potential.
Describe the features of the gaseous exchange surface of mammalian lung.
Describe the features of root hairs that enable the uptake of ions by active transport.
AS Biology. Foundation. Cell membranes and Transport

5. Key words you should know
Phospholipids Solution Pinocytosis
Polar Solute Micropinocytosis
Hydrophilic Solvent Exocytosis
Hydrophobic Partially permeable Gaseous exchange
Micelles Water potential Alveoli
Phospholipid bilayer Solute Potential Root hair
Fluid mosaic model Pressure Potential Surface area
Glycoproteins Turgid Epidermis
Glycolipids Plasmolysis Passive transport
Cholesterol Plasmolysed
Proteins Incipient plasmolysis
Transport proteins Active transport
Enzymes Carrier protein
Receptor molecules Bulk transport
Diffusion Endocytosis
Concentration gradient Phagocytosis
Facilitated diffusion Phagocytes
Osmosis Phagocytic vacuoles
AS Biology. Foundation. Cell membranes and Transport

6. AS Biology. Foundation. Cell membranes and Transport

7. AS Biology. Foundation. Cell membranes and Transport
All living things are surrounded by a membrane.
A cell membrane is also known as plasma membrane.
Controls exchange of materials such as nutrients and waste between cells and their environment.
Has other important functions for example to enable cells to receive hormones.
To understand the function of anything in biology, you must study the structure first!
AS Biology. Foundation. Cell membranes and Transport

8. Cell Membranes from Opposing Neurons (TEM x436,740).
Nerve cell
Gap between cells
Cell membrane {
} cell membrane
7nm wide
Nerve cell
AS Biology. Foundation. Cell membranes and Transport

9. Cell membranes are made of PHOSPHOLIPIDs
HYDROPHILIC heads (water liking) -Attracted to the water
called POLAR
HYDROPHOBIC tails (water fearing) -Not attracted to the water
called NON-POLAR
A Phospholipid
AS Biology. Foundation. Cell membranes and Transport

10. AS Biology. Foundation. Cell membranes and Transport
Phospholipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group (PO4-) replaces one of the three fatty acids normally found on a lipid. The addition of this group makes a polar "head" and two nonpolar "tails".
AS Biology. Foundation. Cell membranes and Transport

11. AS Biology. Foundation. Cell membranes and Transport
A phospholipid
HYDROPHILIC HEAD
At the other end of the phospholipid is a phosphate group and several double bonded oxygens. The atoms at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water. It is POLAR
HYDROPHOBIC TAILS
The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge. They are NON POLAR. Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water.
3D model of a Phospholipid
AS Biology. Foundation. Cell membranes and Transport

12. A Phospholipid Bilayer
Phospholipids can form:
BILAYERS
-2 layers of phospholipids with
hydrophobic tails protected inside by the hydrophilic heads.
The PHOSPHOLIPID
BILAYER is the basic
structure of membranes.
AS Biology. Foundation. Cell membranes and Transport

13. AS Biology. Foundation. Cell membranes and Transport

14. Structure of the cell membrane
Phospholipids
Cell membranes are made mainly of phospholipids. They have:
HYDROPHILIC heads (water liking)
-Attracted to the water POLAR
HYDROPHOBIC tails (water fearing)
-Not attracted to the water NON-POLAR
Phospholipids can form BILAYERS
-2 layers of phospholipids with
hydrophobic tails protected
inside by the hydrophilic
heads.
The PHOSPHOLIPID BILAYER is
the basic structure of membranes.
AS Biology. Foundation. Cell membranes and Transport

15. AS Biology. Foundation. Cell membranes and Transport
Diagram representing the cell membrane Remember the membrane is 7nm wide
AS Biology. Foundation. Cell membranes and Transport

16. AS Biology. Foundation. Cell membranes and Transport
Fluid mosaic model
Cell membranes also contain proteins within the phospholipid bilayer.
This ‘model’ for the structure of the membrane is called the:
FLUID MOSAIC MODEL
FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid.
MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.
AS Biology. Foundation. Cell membranes and Transport

17. Diagram of a cell membrane
AS Biology. Foundation. Cell membranes and Transport

18. TEM of freeze-fractured cell membrane.
The fracture occurs between the two phospholipid layers.
You can clearly see the exposed proteins sticking out of the two layers.
Individual phospholipids are too small to see.
AS Biology. Foundation. Cell membranes and Transport

19. Cell Membranes from Opposing Neurons (TEM x436,740).
} Phospholipid Bilayer
7nm wide
AS Biology. Foundation. Cell membranes and Transport

20. Features of the fluid mosaic model
Double layer – BILAYER of phospholipids which can move about by ………………………… in their own ……………………….
Phospholipid tails point inwards forming a ……. ………. ………………………… interior. The phospholipid heads point outwards facing the aqueous (water containing) medium surrounding the membrane.
Some phospholipids fatty acid tails are ……………………….. – straight so fit together tightly. Some are ………………………… – bent so fit together ……………………. The more unsaturated tails there are the more ……………… the membrane becomes. The lower the temp, the ……………….fluid.
Most protein molecules …………………. like icebergs in the layers, some are fixed to ………………………. inside the cell and don’t float.
Some proteins are embedded in the outer layer, some in the inner layer and some ………………… the two layers. Hydrophobic and Hyrdophilic parts of the protein molecules sit next to the …………………………… and ………………………….. portions of the ……………………………… of the membrane. This ensures the proteins stay in the membrane.
The membrane is ……nm thick on average.
Some phospholipids have carbohydrates attached to them called- ………………………..
Some of the proteins have carbohydrates attached to them called– ……………………
The membrane also contains molecules of ………………………………..
AS Biology. Foundation. Cell membranes and Transport

21. Features of the fluid mosaic model
Double layer – BILAYER of phospholipids which can move about by DIFFUSION in their own MONOLAYER
Phospholipid tails point inwards forming a NON-POLAR HYDROPHOBIC interior. The phospholipid heads point outwards facing the aqueous (water containing) medium surrounding the membrane.
Some phospholipids fatty acid tails are SATURATED – straight so fit together tightly. Some are UNSATURATED – bent so fit together loosely. The more unsaturated tails there are the more ‘fluid’ the membrane becomes. The lower the temp, the less fluid.
Most protein molecules float like icebergs in the layers, some are fixed to structures inside the cell and don’t float.
Some proteins are embedded in the outer layer, some in the inner layer and some span the two layers. Hydrophobic and Hyrdophilic parts of the protein molecules sit next to the Hydrophobic and Hydrophilic portions of the phospholids of the membrane. This ensures the proteins stay in the membrane.
The membrane is 7nm thick on average.
Some phospholipids have carbohydrates attached to them – GLYCOLIPIDS
Some of the proteins have carbohydrates attached to them – GLYCOPROTEINS
The membrane also contains molecules of CHOLESTEROL
AS Biology. Foundation. Cell membranes and Transport

22. Roles of components of cell membranes
Using the following headings produce a table on A4 to summarise roles of t he
different types of molecules found in the cell membrane. Try not to copy, pick
out the relevant information and write it in note form You may want to use
bullet points, different coloured pens etc basically whatever helps you to remember them.
There will be a short test on this next lesson! Use pages
Component Function
Phospholipids
Cholesterol
Proteins
Glycolipids and Glycoproteins
AS Biology. Foundation. Cell membranes and Transport

23. Movement of selected molecules across the cell membrane
AS Biology. Foundation. Cell membranes and Transport

24. AS Biology. Foundation. Cell membranes and Transport
Moving amoeba
AS Biology. Foundation. Cell membranes and Transport

25. AS Biology. Foundation. Cell membranes and Transport
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AS Biology. Foundation. Cell membranes and Transport

26. Review Chapter 4.1

27. Structure of membranes
Bilayer membrane ~7 nm wide
Contains embedded proteins

28. Fluid Mosaic Model
Membrane described as fluid b/c both phospholipids and proteins can move about by diffusion
Bilayer has fluidity we associate with olive oil
Move sideways in their own layers
Proteins move like icebergs in sea

29. Fluid Mosaic Model
Some phospholipid tails are saturated, some are unsaturated
More unsaturated = more fluid
WHY???

30. Factors affecting fluidity
Longer tail = less fluid membrane
Lower temp. = less fluid membrane

31. Phospholipids
Tails of phospholipids are nonpolar, so it is difficult for polar molecules (water soluble) to pass through the membrane
Ex: sugars, amino acids, and proteins cannot leak out of the cell

32. Cholesterol hydrophilic heads + hydrophobic tails
Fits in between phospholipids
Animal cells: amount cholesterol= amount phospholipids in cell membrane
Plant cells: little to no cholesterol; Prokaryotes = none!
Regulates fluidity, stabilize membranes
Hydrophobic regions: prevent ions/polar molecules from passing through membrane
Important in myelin sheath around nerve cells: ion leaks would slow signals

33. Cholesterol

34. Two types of proteins in Cell membranes:
Intrinsic (aka integral): found in inner layer, outer layer, or most commonly spanning the entire membrane (transmembrane proteins- cross both sides of membrane)
Extrinsic (aka peripheral): found on either the inner or outer surface of the membrane

35. Intrinsic Proteins
Stay in membrane due to its hydrophobic and hydrophilic regions
Most float like mobile icebergs although some are fixed like islands to structures inside or outside the cell and do not move about

36. Extrinsic proteins Many are bound to intrinsic proteins
Some are held in place by binding to molecules inside or outside the cell

37. Proteins’ roles in membranes
Transport proteins: hydrophilic channels
Moving Ions, polar molecules
Enzymes: on plasma membrane on small intestine surface hydrolyze disaccharides
Proteins for photosynthesis + cell respiration: E- transport chains in mitochondria membrane and thylakoid membrane of chloroplast
Receptors: bond to antigen and begin communication pathway

38. Glycolipids and Glycoproteins
Many proteins and lipids in membrane have carbohydrate chains attached that face the outside of the membrane
Glycolipids = carb. (polysaccharide) attached to lipid
Glycoproteins = carb. attached to protein

39. Glycolipids and Glycoproteins
Form H-bonds with water to stabilize the membrane
Form sugary coating on membrane called glycocalyx
Act as receptor molecules
for cell-cell recognition
(immune cells)

40. Signaling Receptors Coordinate activities of animal cells
Recognize messenger molecules like hormones and neurotransmitters
When molecule binds with receptor, it triggers a series of chemical reactions in the cell

41. Example: cell Insulin Receptors

42. Endocytosis receptors
Bind to molecules that are parts of the structures that are to be engulfed by cell

43. Cell Marker Receptors Aka antigens; Allow cell-cell recognition
Each cell type has its own specific antigen (similar to how different countries have different flags)
Ex: ABO blood types

44. Transport Proteins
Provide channels or passageways for ions and polar molecules to pass through cell membrane
Two main types:
Channel proteins – Facilitated diffusion (NO ATP!)
Carrier proteins – Active transport (ATP REQUIRED!)

45. Review and practice!

46. AS Biology. Foundation. Cell membranes and Transport
Summary
Cell membranes have a basic structure composed of a PHOSPHOLIPID BILAYER.
Phospholipds have HYDROPHOBIC (non-polar) tails and HYDROPHILIC (polar) heads.
The best model of the cell membrane is called the FLUID MOSAIC MODEL
The average thickness of the membrane is 7nm.
The fatty acid tails of phospholipids can be SATURATED (straight) or UNSATURATED (bent)
Proteins can float or be fixed and also have hydrophobic and hydrophilic portions.
Some proteins and phospholipids have carbohydrates attached to them to form GLYCOPROTEINS AND GLYCOLIPIDS.
Phospholipids form the bilayer, act as barrier to most water soluble substances
Cholesterol regulates the fluidity of the membrane, gives mechanical stability and help to prevent ions from passing through the membrane.
Proteins act as transport proteins to act as channels for substances to move into or out of the cell. Some act as membrane enzymes and some have important roles in membranes of organelles.
Glycolipids and Glycoproteins help to stabilise membrane structure, some act as receptor molecules eg for hormones and neurotransmitters or as antigens for other cells to recognise them.
AS Biology. Foundation. Cell membranes and Transport

47. Transport across the Cell Membrane 4.2

48. Cell Surface Membrane (CSM) transport
Phospholipid bilayer creates effective barrier against water soluble molecules and ions
Prevents aqueous contents from escaping
Some essential transport is achieved through:
Diffusion
Facilitated diffusion
Osmosis
Active transport and bulk transport

49. Diffusion (simple diffusion)
Diffusion: net movement, as a result of random motion of its molecules or ions, of a substance from a region of high concentration to a region of low concentration
Particles move down a concentration gradient

50. Factors that affect Rates of Diffusion
‘steepness’ of concentration gradient: greater difference = higher rate
Temperature: higher temperature = higher rate
Surface area of diffusion: greater surface area = higher rate (more contact space)
Nature of molecules/ions: small molecules = greater rate

52. Diffusion
Respiratory gases cross membrane by diffusion (uncharged and nonpolar)

53. Facilitated diffusion
Diffusion that takes place with the help “assistance” of a certain protein molecule
Channel proteins
Carrier proteins

54. Channel proteins Water-filled pores Fixed shape
Allow charged substances (usually ions) to diffuse through membrane
Gated to allow for selectivity and control of movement

55. Carrier proteins
Flips between shapes to alternately open binding sites on interior/exterior of membrane
Direction of movement depends on concentration gradient inside and outside of cell

57. Osmosis
Type of diffusion involving water molecules only

58. Water potential
Water potential: tendency of water molecules to move from one place to another
Measured by Greek symbol psi Ψ

59. Water Potential Problems
In beaker A, which has a higher water potential, distilled water or the beet core?
2)Where will water flow in the diagram B? Explain why.

60. Water potential
Water potential for pure water (distilled) is 0
Solute potential - amount that solutes lower the water potential
Solutes make water potential less than 0, the more solute, the more negative the water potential!
Pressure potential (symbol Ψp) Increasing pressure increases water potential
Pressure potential makes water potential less negative (more positive)

61. Water potential

62. Water potential in animal cells

63. Osmosis in plant cells

64. Active Transport (ATP needed)
Active transport moves molecules/ions against concentration gradient
Achieved by carrier proteins (specific for structure of particular ion/molecule)
ATP supplied by cellular respiration
Can occur into or out of the cell

65. Sodium-potassium pump
Found in CM of all animal cells
Use ~30% cell energy and ~70% neuron cell energy
Pump 3 Na+ ions out of the cell and 2 K+ ions into the cell
Net result: inside of cell become more negative than the outside

66. Active transport
Important in re-absorption in the kidneys, where certain useful molecules and ions have to be re-absorbed into the blood after filtration into the kidney tubules
Involved in
absorption of
products of
digestion in the
gut

67. Active transport
Used to load sugar from photosynthesizing cells of leaves into phloem tissue for transport around the plant

68. Bulk transport (ATP Required!)
Bulk transport involves the mechanism of moving large quantities of molecules into the cell (endocytosis) or out of the cell (exocytosis)
Large molecules such as proteins or polysaccharides, part of cells, or even whole cells may be transported across the membrane

69. phagocytosis ‘cell eating’ Bulk up take of solid material
Cells specializing in this are called phagocytes and the vacuoles are called phagocytic vacuoles
Ex: engulfing of bacteria by immune cells

70. Pinocytosis ‘cell drinking’ – bulk uptake of liquid into cell
Vacuoles ( vesicles) formed are often extremely small, in which the process is called micro-pinocytosis
Human egg cells take up nutrients from cells that surround it by pinocytosis

71. exocytosis
Ex: how plants transport materials out of CM to make cell wall
Ex: secretion of digestive enzymes of the pancreas
Secretory vesicle from the Golgi carry the enzymes to the CSM and release their contents

72. Study the Study Guide on My Web Site