The document discusses different methods of transport across the cell membrane, including passive transport mechanisms like diffusion, facilitated diffusion, and osmosis that do not require energy, as well as active transport mechanisms like endocytosis, exocytosis, and phagocytosis that use energy. It also describes the differences between isotonic, hypotonic, and hypertonic solutions and their effects on cell volume. Key examples of transport across the cell membrane are given.

1. Electron micrograph of a section of the
surface of an epithelial cell, showing the unit
membrane with its two dark lines enclosing
a clear band. x100,000.
Transmission electron micrograph of freeze-fractured plasma membranes of
two adjacent cells. The 'E Face,' (Ectoplasmic) is on the right half of the image.
The 'P Face' (Protoplasmic) is on the left. The bumps were impression of
membrane proteins on the platinium plate replica.
From left - red blood cell (erythrocyte),
platelet (thrombocyte), white blood cell
(T-lymphocyte).

2. 2.2.1.2
Functions
of the cell
membrane

3. Objectives
• Understand the functions of cell membrane
• Understand how the structure of cell membrane contribute to those
functions
• Differentiate active and passive transport
• Differentiate types of passive transport
• Understanding how tonicity affects osmosis

4. Functions of the cell membrane
• The cell membrane physically separates the intracellular components (e.g.
organelles in eukaryotic cells) from the extracellular environment.
• The cell membrane protects the cell from some harmful chemicals and
pathogen 病原 in its external environment, and from the loss of useful
biological macromolecules held within the cell.
• Surface protein markers identify the cell.
• Receptors for molecules such as hormones allows the cell can recognize
and process some signals received from the extracellular environment.
• Plasma membranes include as part of their structures certain proteins and
enzymes that are involved in some of the metabolic processes of the cell.
• Secretions 分泌 and excretions 排泄 are regulated by the cell membrane.

5. Selective barrier
• Cell membrane is a
semipermeable
membrane 半滲膜.
• A semi-permeable
membrane allows some
particles to pass
through (by size) by
diffusion.
• Other molecules
required special
channels to pass
through the membrane.

6. Quiz
• Oxygen enters the blood from the alveoli in the lungs by the process
of ______.
a. phagocytosis
b. active transport
c. osmosis
d. diffusion
e. facilitated diffusion

7. 2.1.3
Transportation
across the
cell membrane

8. Methods of transportation
• Passive transport 被动运输 – no energy required
• Diffusion 扩散作用
• Free diffusion 自由扩散
• Facilitated diffusion 协助扩散
• Osmosis 渗透作用
• Active transport 主动运输 – required energy
• Active transport
• Endocytosis 内吞作用
• Exocytosis 外排作用

9. 2.1.3.1
Passive
transport

10. Diffusion
• Movement of a substance down a concentration gradient.
• Move from a region of high concentration to a region of low
concentration until the substance is distributed uniformly.
• A.k.a going down a concentration gradient
• No energy required.
• Two types of diffusion:
• Free diffusion
• Facilitated diffusion
When particles are allowed to move around
randomly (diffuse), they would eventually spread
uniformly in the solution.

11. Quiz
• A dye is dissolved in water. The ______ is the solute and the solvent is
the ______ molecules.
• water, dye
• dye, water
• dye, dye
• water, water

12. “going down the concentration gradient”

13. Free diffusion/Simple diffussion
• A substance pass through the cell membrane directly
• Examples: gasses (oxygen, nitrogen), small molecules (glycerol,
ethanol), hydrophobic molecules (lipids, benzene)
Given enough time, solutes in a close system
will spread uniformly across the cell
membrane.

14. Quiz
• In passive diffusion, particles move _____.
a. From higher to lower areas of concentration in a solution or gas
b. From lower to higher areas of concentration in a solution or gas
c. Only with energy provided in a solution or gas
d. In both lower and higher levels of concentration in a solution or gas

15. Quiz
• Diffusion of particles in a solution or gas along the concentration
gradient will occur until _____.
a. All the particles are dissolved
b. Random motion starts
c. There are equal number of particles in the areas of solution or gas
d. There are more particles in the solution or gas then when diffusion
started

16. Facilitated diffusion
• Use a protein carrier or channel to move in and out of the cell
• No energy required
• Examples: hydrophilic molecules (amino acids) and ions (sodium ion,
potassium ion), large molecules (glucose)

17. Osmosis
• Water molecules pass through a selective permeable membrane.
• Moving from a region of low solute concentration to a region of high
solute concentration.
• The direction of movement of the water molecules across the cell
membrane is determine by the osmotic condition of the cell.

18. Quiz
• Osmosis is best defined as the movement of
a. molecules from an area of high concentration to an area of lower
concentration.
b. molecules from an area of low concentration to an area of higher
concentration.
c. water molecules across a membrane from an area of low water to
an area of higher concentration.
d. water molecules across a membrane from an area of high water
concentration to an area of lower concentration.
e. water molecules inside a container.

19. Tonicity 张力
• Tonicity is a measure of the effective osmotic pressure gradient.
• The osmotic condition of a cell is determine by differences of the
solute concentration between the internal and the external of the cell
membrane.
• Three tonicity:
• Isotonic
• Hypotonic
• Hypertonic

20. Quiz
• Tonicity refers to the ______.
a. air pressure in an experiment
b. strength of a muscle contraction
c. strength of a solution in relationship to pinocytosis
d. strength of a solution in relationship to osmosis
e. strength of a solution in relationship to active transport

21. Hypotonic solution 低张溶液
• The concentration of the solute outside
the cell is the lower than that of inside
the cell.
• A net movement of water into the cell.
• A cell will swell and may lyse without a
cell wall.
Animal Cell
Plant Cell
膨胀

22. Isotonic solution 等张溶液
• The concentration of the solute outside the
cell is the same as that of inside the cell.
• No net water movement across the cell
membrane.
• Cell maintains its shape.
Animal Cell
Plant Cell
萎缩

23. Hypertonic solution 高张溶液
• The concentration of the solute outside
the cell is the higher than that of inside the
cell.
• A net movement of water leaving the cell.
• A cell may be shriveled or plasmolyzed.
Animal Cell
Plant Cell
乾癟
质壁分离

24. Isotonic solutionHypotonic solution Hypertonic solution

25. Quiz
• Which type of solution will cause cells to swell, or even to burst?
a. isotonic solution
b. hypotonic solution
c. hypertonic solution

26. Quiz
• The diffusion of water across a differentially permeable membrane is
called ______.
a. simple diffusion
b. facilitated diffusion
c. osmosis
d. exocytosis
e. endocytosis

27. Summary: type of passive transport
No energy required.

28. Summary: tonicity

29. Quiz
• Suppose a thistle tube containing a 10% sugar solution is covered at
one end by a membrane and is placed in a beaker containing a 5%
sugar solution. Which of the following conditions must NOT exist for
water to enter the tube by osmosis?
a. A differentially permeable membrane must separate the two
solutions.
b. The beaker must have less water per unit volume than in the tube.
c. The membrane must not permit passage of the solute.
d. The membrane must permit the passage of water.
e. An osmotic pressure must be present.

30. A cell engulfing a yeast under TEM. Vesicles formation during endocytosis.
Macrophage consuming bacteria. Drawing depicting endocytosis and exocytosis.

31. 2.1.3.2
Active
transport

32. Active transport
• Some molecules, such as glucose, waste products etc. may need to
move against the concentration gradient into the cell.
• Movement of a solute from a region of low concentration to a region
of high concentration requires energy such as ATP.
• Use a transport protein or form vesicles (endocytosis and exocytosis).
In this example, the ATP (adenosine triphosphate) is
used by the transport protein. ATP is broken down into a
ADP (adenosine diphosphate) and a phosphate group.

33. Quiz
• ______ is the net movement of any type of molecule from a
region of higher concentration to a region of lower concentration.
a. Osmosis
b. Diffusion
c. Facilitated diffusion
d. Active transport
e. Pinocytosis

34. Endocytosis 内吞作用
• endo- (within) + -cyte (cell) + -osis (denote a noun of process)
• Cell transports large molecules into the cell by engulfing them.
• The cell membrane is invaginated 套疊 to form a vesicle 囊泡.
• Two examples: phagocytosis and pinocytosis.
The cell membrane is
said to be pinched off to
form a vesicle.
vesicle

35. Phagocytosis 吞噬作用
• Phago- = eat/devour
• engulfs a solid particle (not liquid)
• Examples:
• Acquisition of nutrients in unicellular organism
such as amoeba.
• Protection against pathogens such as dendritic
cells engulf bacteria and disease-causing cells.

36. Pinocytosis 胞饮作用
• Pin- = drink
• Non-specific intake.
• Cell takes in surrounding fluids, including
all solutes present.
• Example:
• Consumption of digested food, e.g. fat
absorption by Villus cells in small intestines
• Absorption of nutrient, e.g. bacteria and
other unicellular organisms.

37. Exocytosis 外排作用
• Exo- (external/outside) + -cyte (cell) + -osis (denote a noun of process)
• Cell transports molecules out of the cell by fusing a vesicle with the
cell membrane, releasing the content of the vesicle into the
environment.
• Example: secretion of hormones, disposal of waste products

38. Summary
• Required energy
• Going against the
concentration gradient
ADP+ Pi
Phagocytosis Pinocytosis
Endocytosis Exocytosis
vesicles vesicles

39. Quiz
• Which of the following is NOT an active method where molecules
pass across the plasma membrane?
a. simple diffusion
b. active transport
c. endocytosis
d. exocytosis

40. Quiz
• Lipid-soluble molecules and gases enter the cell by ______.
a. diffusion through the channel proteins
b. osmosis through the channel proteins
c. diffusion through the lipid bilayer
d. osmosis through the lipid bilayer
e. active transport through the lipid bilayer

41. Quiz
Below are descriptions of cells, which one correct?
I. Cell membrane primary constructed by lipid, protein and little bit of
saccharide
II. Saccharide primary exist on surface of cell membrane and attach to lipid
III. All small substances can move through cell membrane by diffusion.
IV. Some big substances can enter the cell by pinocytosis.
A. I, IV
B. I, II, III
C. II, III
D. II, III, IV

42. Major examples of movements
across the cell membrane
Note: Many particles can be uptake by both passive and active transport.

43. Free Diffusion
• Gas exchange at the alveoli — oxygen from air to blood, carbon
dioxide from blood to air.
• Gas exchange for photosynthesis — carbon dioxide from air to leaf,
oxygen from leaf to air.
• Gas exchange for respiration — oxygen from blood to tissue cells,
carbon dioxide in opposite direction.
• Gas exchange in unicellular organism – such as Ameoba sp.

44. Facilitated diffusion
• Accumulation of ethanol by aquatic alga (also free diffusion)
• transport of glucose in all body cells essentially, but especially in the
brain.
• Absorption of calcium in the intestinal cells

45. Osmosis
• Absorption of water by plant roots.
• Transport of water in the cortex of plant stems.
• Re-absorption of water by the proximal and distal convoluted tubules
of the nephron.
• Re-absorption of tissue fluid into the venule ends of the blood
capillaries.
• Absorption of water by the alimentary canal — stomach, small
intestine and the colon.

46. Active transport
• Re-absorption of glucose, amino acids and salts by the proximal convoluted
tubule of the nephron in the kidney
• Absorption of sodium ions in the small intestine
• Sodium/potassium pump in cell membranes (especially nerve cells)
• Endocytosis
• Phagocytosis
• Acquisition of nutrients in unicellular organism such as amoeba.
• Protection against pathogens such as dendritic cells engulf bacteria and disease-causing cells.
• Pinocytosis
• Consumption of digested food, e.g. fat absorption by Villus cells in small intestines
• Absorption of nutrient, e.g. bacteria and other unicellular organisms.
• Exocytosis – secretion of hormones, disposal of waste products