Who AM I?

1. Who AM I?

2. I work in a factory but I do not have any manufacturing skills. • I’m not management material and so I work in an entry-level position. • I’m nearly always assigned to the warehouse where I’m responsible for shipping and receiving. • I’m quite outgoing and so I act as the factory’s first point of contact with the outside world.

3. Part of my job description is as receptionist- and so all incoming and outgoing phone calls come through me. • I’m also the one responsible for checking ID cards and only admitting those who are welcome. Who am ?? I am the cell membrane- a.k.a plasma membrane. But don’t confuse me with blood plasma………… OK!

4. My Vital Statistics • I surround the cytoplasm of ALL living cells • I am ultra thin, less than 0.01 um (~8nm) (too thin to seen with the magnification and resolution of the light microscope) Neuron 1 Plasma membranes Neuron 2 Viewed with TEM

5. Proteins with various roles are scattered throughout me(like a mosaic). STRUCTURE • I’m made of double layer (bilayer) of special lipids called phospholipids. • In animal cells, cholesterol molecules are scattered amongst the phospholipids within me. They help to keep me together and regulate my fluidity • Proteins with various roles are scattered throughout me (like a mosaic) • I am not rigid, but rather quite flexible.

6. Phospholipid Bilayer with Protein and cholesterol e.g. Tissue fluid Transport protein Outside of cell Cholesterol Phospholipid molecule Phospholipid bilayer Fatty acid tails(hydrophobic) Phosphate head(hydrophilic) Inside of cell e.g. Cytosol

8. Fluid Mosaic Model Vs Sandwich Model •  Current Model is the fluid mosaic model • Fluid: embedded molecules can move sideways within membrane • Mosaic: scattered appearance of embedded protein and cholesterol • (contrast this with the sandwich model on next slide)

9. Previous Model:Sandwich Model It was proposed that the structure of the cell membrane was two layers of protein(like bread of a sandwich) with a layer of phospholipid in between (like sandwich filling) NOT THE CORRECT MODEL

10. INTERESTING FACT: All membranes contain proteins and lipid. However, the proportion of each varies depending on the membrane. For example

11. FUNCTION Holding cell contents: • To contain the cell contents and separate them from the outside environment • In multicellular organisms the immediate environment of their cells is the tissue fluid in which it they bathed.

12. Cells Capillaries and Lymphatic system

13. Some Basic Background • Internal environment • External environment • Extracellular Fluid • Intracellular Fluid

14. FUNCTION To control what substances enter and leave the cell: • A semi (partially /differentially/ selectively) permeable boundary. • It allows some substances to pass across it easily but not others.

15. FUNCTION Cell recognition: • A substance called an ANTIGEN (usually composed of protein and carbohydrate {glycoprotein}) plays a key roll in cell recognition. • An organism's immune system uses the antigens on the cell surface to recognize “SELF" cells from “NON SELF” cells.

16. FUNCTION Cell Signal reception: • Receptor proteins enable specific signalling molecules (hormones) to bind to them • Binding leads to a cellular response

17. INTERESTINGFACTS • Cell membrane of most bacterial cells contain no cholesterol and would not manage to be held together if not surrounded by a cell wall. • The plasma membrane is too thin to be resolved with a light microscope. The boundary of the cell is obvious however, and we label this as the cell membrane’s location

18. FUNCTION To control what substances enter and leave the cell: • A semi (partially /differentially/ selectively) permeable boundary. • It allows some substances to pass across it easily but not others.

20. FUNCTION • The plasma membrane is fully permeable to: • Small polar molecules like water, urea, ether, glycerol and alcohol • Non polar molecules (Carbon dioxide, Oxygen, fat soluble vitamins and steroids) (A cell does not control the passage of these substances across the membrane. They cross the membrane passively, down their individual concentration gradient)

21. continued The plasma membrane is impermeable to: • Large molecules (e.g. glucose, amino acids) and • Charged molecules (e.g. ions) These can only pass across the membrane through transport proteins, either passively or actively. Thus the cell can control or regulate which of these substances pass across the membrane and in what quantities

22. Transport across membranes

23. Diffusion Substance that easily cross the cell membrane by diffusion include: • Non Polar molecules • Carbon dioxide, oxygen, DDT(a poison), steroids and fat soluble vitamins • Tiny polar molecules: • water(osmosis), ether, chloroform, alcohol

24. Diffusion High [ ]n Concentration gradient Low [ ]n

25. Facilitated Diffusion Some material need to cross the membrane via membrane proteins(carrier proteins or channel proteins) This type of transport is passive; movement is along a concentration gradient, and involves specific proteins which change shape to move molecules across

26. Facilitated Diffusion via channel proteins High [ ]n Concentration gradient Low [ ]n

27. Facilitated Diffusion via carrier proteins High [ ]n High [ ]n Concentration gradient Concentration gradient Low [ ]n Low [ ]n

28. Material that Cross the cell membrane facilitated diffusion(via transport proteins) Carrier Protein Protein channel Larger hydrophilic substances: • Amino acids • Glucose • ADP For example: ADP into mitochondria, glucose into red blood cells Ions • Na+ • Cl- In such a case there will be a net movement of the substance down its concentration gradient This process is PASSIVE: NO ENERGY EXPENDED BY THE CELL

29. Active transport (requires the input of energy (ATP)) Transport of IONS • Uptake of ions or large uncharged molecules against a concentration gradient • occurs via specific carrier proteins and requires the input of energy (ATP), which must be produced by the cell. • This allows for a stable intracellular environment to be maintained even in extreme environmental conditions .e.g. bacteria that live in very salty environments.

30. Active Transport via Carrier Proteins Direction of transport Low [ ]n Here ions are moved against their Concentration gradient RequiredATP (energy) High [ ]n

31. Examples • Sodium across membrane of a neuron • Uptake of mineral ions by root hair cells • uptake of glucose by intestinal cells Low [ ]n Concentration gradient High [ ]n

32. Example of active transport of ions: Sodium Potassium Pump From Krogh, “Biology: A Guide to the Natural World, 2nd edition

33. Bulk transport of cell products • Proteins are made at theribosomes • They are transported through the channels of the Endoplasmic reticulum • If they are to be exported they are transported to the golgi body • Here proteins are modified and packaged into vesicles • Vesicles move to the cell membrane where they are released via exocytosis

34. BULK TRANSPORT (I)Exocytosis • Movement of materials out of the cell by fusion of vesicles with the plasma membrane • Requires cell to expend energy(ATP) • Example - export or removal of wastes in single-celled organisms • Example – cells exporting proteins

35. Exocytosis

36. Exocytosis

37. Exocytosis in action

38. Exocytosis

39. Another example of bulk transport In animal cells, and other cells without a cell wall, if too much water enters the cell, the cell membrane will rupture and the cell will be killed. Organisms living in fresh water environments have mechanisms to remove excess water and prevent bursting: ie: contractile vacuole

40. Function of Contractile vacuole in paramecium

41. BULK TRANSPORT (II)Endocytosis • Bringing of large materials into the cell by infolding of the plasma membrane to form vesicles. • Requires cell to expend energy(ATP)

42. Pinocytosis: liquid is bought into a cell in a vesicle Phagocytosis: solid matter is brought into a cell in a vesicle Extension: Receptor mediated endocytosis: Uptake of molecules bound to receptors example- uptake of hormones bound to receptor proteins.

43. 0.5 m Pinocytosis (“drinking” cell) • extra cellular fluid and materials suspended in it (water and solutes) are enclosed in invaginating vesicle • Used in digestive tract

44. Phagocytosis (“eating” cell) • Brings large materials into a cell by wrapping extensions of the plasma membrane around the materials and fusing the extension together to form a vesicle/food vacuole • How the human immune system ingests whole bacteria or one-celled creatures eat • pseudopodia “false feet” – plasma membrane extensions

45. 0.25 m Receptor-Mediated Endocytosis • More specific with receptor binding molecules, bringing them in and concentrating them into a coated pit which forms into a vesicle • The way insulin gets into your cells.

46. Analysing information : A scientist carried out an experiment to determine the time it took for a cell to manufacture proteins from amino acids. The scientist provided the cell with radioactively labelled amino acids and then tracked them through the cell to establish the time at which protein synthesis commenced. He monitored the cell 5 minutes, 20 minutes and 40 minutes after production started in order to track the proteins from the site of synthesis to a point in the cell from which they were discharged from the cell. The scientist made an image of the cell at each of these times but forgot to mark each image with its correct time. The images are given in figure 2.32. Radioactivity is indicated by the green spots. a) Which cell corresponds to each of the particular times of viewing? List the correct order according to time of viewing. b) On what grounds did you make your decision?

47. FUNCTION Cell recognition: • A substance called an ANTIGEN (usually composed of protein and carbohydrate {glycoprotein}) plays a key roll in cell recognition. • An organism's immune system uses the antigens on the cell surface to recognize “SELF" cells from “NON SELF” cells.

48. FUNCTION Cell Signal reception: • Receptor proteins enable specific signalling molecules (hormones) to bind to them • Binding leads to a cellular response

49. INTERESTINGFACTS • Cell membrane of most bacterial cells contain no cholesterol and would not manage to be held together if not surrounded by a cell wall. • The plasma membrane is too thin to be resolved with a light microscope. The boundary of the cell is obvious however, and we label this as the cell membrane’s location