Cell Biology: The Cell Membrane

1. Cell Biology:The Cell Membrane Lesson 2 – Transport Across the Cell Membrane (Inquiry into Life pg. 69-80)

2. Today’s Objectives • Analyze the structure and function of the cell membrane, including: • Describe passive transport processes • Explain factors that affect the rate of diffusion across a cell membrane • Predict the effects of hypertonic, isotonic, and hypotonic environments on osmosis in animal cells • Describe active transport processes • Compare specific transport processes • Explain why cells divide when they reach a particular surface area-to-volume ratio, including: • Differentiate between cells that have a high or low surface area-to-volume ratio • Demonstrate an understanding of the significance of surface area-to-volume ratio in cell size

3. Transport across Cell Membranes • 4 main ways: • Passive Transport: • 1) Diffusion • 2) Osmosis • 3) Facilitated Transport • Active Transport: • 4) Active Transport

4. 1) Diffusion (passive transport) • Diffusion is the movement of a solute from an area of high concentration to an area of low concentration until evenly distributed • Solute is some solid particles or molecules suspended in air or liquid • Example: a foul odor in the corner of a room will spread out until it is evenly distributed • Example: cream in coffee will diffuse until concentrations are balanced

5. 1) Diffusion (passive transport) • Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion • Consider two containers of gas A and gas B separated by a partition; the molecules of both gases are in constant motion and make numerous collisions with the partition • If the partition is removed as in the illustration, the gases will mix because of the random velocities of their molecules • In time, a uniform mixture of A and B molecules will be produced in the container

6. 2) Osmosis (passive transport) • Osmosis is a special type of diffusion in which water moves from an area of high water concentration to an area of low water concentration across a membrane

7. 2) Osmosis (passive transport) • Pressure caused by the concentration gradient between two different solutions is called osmotic pressure • Hydrostatic pressure (gravity) offsets osmotic pressure • The solute cannot spread out because it is too big to pass through the membrane. Therefore, water moves across the membrane from [high water] to [low water]. • Water moves through the protein-lined pores of the cell membrane

8. Water will move from side A to side B. Level on A will fall. Level on B will rise. Until gravity stops it = hydrostatic pressure)

9. 3) Facilitated Transport (passive transport) • In facilitated transport, solutes move across the membrane from an area of [high solute] to [low solute] with the help of a carrier protein • Example: molecules needed by the cell such as glucose enter through facilitated transport from the blood • Examples of solutes: gases, molecules

10. 4) Active Transport • In active transport, solutes move from an area of [low solute] to an area of [high solute] across a membrane with the aid of a carrier protein • Example of solute: ions • Since the movement is against the concentration gradient, energy is required (ATP) • Example: Sugar is removed from urine by active transport into the blood. Since there is already a lot of glucose in the blood, it is traveling against the concentration gradient • Example: Na/K Pump. Found in nerve and muscle cells. Carrier protein changes shape to fit Na and K

11. 4) Active Transport

12. Summary • Diffusion • Solute moves from [high solute] to [low solute] • No membrane required • No carrier protein required • No energy required • Osmosis • [high water] to [low water] • Membrane required • No carrier protein required • No energy required

13. Summary • Facilitated Transport • [high solute] to [low solute] • Membrane required • Carrier protein required • No energy required • Gas/molecule movement

14. Summary • Active Transport • [low solute] to [high solute] • Membrane required • Carrier protein required • Energy required (ATP) • Ion movement

17. Factors affecting Diffusion • Surface area vs. Volume

18. Cell Surface AreaVolume S.A. : Volume Ratio LxWx6 LxWxH A 6 units2 1 units3 6:1 B 24 units2 8 units3 3:1 C 96 units2 64 units3 1.5:1 D 384 units2 512 units3 0.75:1 Nutrients (oxygen and glucose) enter a cell while wastes (carbon dioxide and urea) exit a cell across the cell membrane. The amount of cell membrane = surface area

19. Surface Area vs. Volume • Inside the cell, organelles use up nutrients and produce wastes • The larger the cell, the more nutrients needed and the more waste produced • The size of the cell (# of organelles) = volume • Small cells have a high SA : Volume ratio • They can supply the organelles with plenty of nutrients and remove wastes

20. Surface Area vs. Volume • Large cells have a low SA : Volume ratio because volume increases faster than surface area • If a cell gets too big, wastes will increase and nutrients able to enter the cell will decrease • Therefore, cells are limited in size: active cells must be smaller than less active cells because they need more nutrients and produce more waste • Some cells can increase their surface area without increasing volume by producing folds

21. Factors that will Increase Diffusion • A) Concentration Gradient • The difference in concentration between two areas • The greater the [] gradient, the faster the rate of diffusion • B) The size and shape of the molecules • C) Temperature • The higher the temperature, the faster the rate of diffusion • D) Type of Medium • Rate of diffusion is faster in air than in liquid • E) Movement of the Medium • Water or air currents increase the rate of diffusion • Example: stirring while adding cream to coffee

22. Endocytosis and Exocytosis • Endocytosis: • Process in which large materials enter a cell • Vesicles form as a way to transport molecules into a cell • Two types: phagocytosis and pinocytosis

23. Phagocytosis • A type of endocytosis in which whole cells, bacteria, or cell fragments are taken into the cell • Referred to as “cell eating” • Transports very large pieces

24. Pinocytosis • A type of endocytosis in which molecules such as proteins are taken into the cell • Referred to as “cell drinking” • Transports smaller “large” pieces and liquids

25. Exocytosis • A process by which products or wastesexit a cell • Reverse of endocytosis

26. Exocytosis

28. Hypertonic, Hypotonic, Isotonic Solutions • Use your textbook to find the following definitions: • Hypertonic solution: • Solution with a higher concentration of solute than its surroundings • If a cell is placed in a hypertonic solution, water will rush out of the cell in order to balance the concentration of solute causing the cell to shrivel • Hypotonic solution: • Solution with a lower concentration of solute than its surroundings • If a cell is placed in a hypotonic solution, water will rush into the cell in order to balance the concentration of solute causing the cell to swell and possibly burst • Isotonic solution: • Solution in which the osmotic concentration is the same as the solute concentration of its surroundings • If a cell is placed in a isotonic solution, water diffuses into and out of the cell at the same rate

29. Hypertonic, Hypotonic, Isotonic Solutions • Water will move from a hypotonic solution to a hypertonic solution • No net movement of water between isotonic solutions

30. Tonicity • Tonicity refers to concentration of solute in a solution • Below are the effects of tonicity on animal cells

31. RED BLOOD CELLS IN DIFFERENT SOLUTIONS

32. NORMAL RED BLOOD CELLS RBC PLACED IN A HYPERTONIC SOLUTION RBC PLACED IN A HYPOTONIC SOLUTION

33. Tonicity Experiment – Thistle Tube • Inside tube is hypertonic, outside tube is hypotonic • Water moves from [high water] (hypotonic) to [low water] (hypertonic)