5-4 – Movement Through the Membrane

1. 5-4 – Movement Through the Membrane All cells exist in a liquid environment. The cell membrane functions as a barrier but must let certain materials in and out of the cell.

3. Structure of the Plasma/Cell Membrane Plasma Membranes consist of 2 types of molecules – phospholipids and proteins A phospholipid has 2 regions – head and tail Polar head region with a phosphorus group Remember, water is polar Non-polar 2 tail region composed of long carbon chains (fatty acids)

4. Structure of a phospholipid

5. Warm Up What are the two main components of a cell membrane? Draw a phospholipid. Label the polar and nonpolar regions. Identify which parts are hydrophobic and hydrophillic. Describe why the cell membrane must form a double layer (lipid bilayer).

6. Diagram of a Phospholipid Polar head attracts water molecules (hydrophilic), nonpolar tails repel water molecules (hyrdrophobic).

7. Phospholipid bilayer Due to the hydrophobic and hydrophilic nature of the phospholipid, a double layer is formed where the heads protect the tails from the water. This is called the phospholipid bilayer. The bilayer is fluid, not a hard shell.

8. Structure of phospholipid bilayer The phospholipid bilayer is called semi-permeable – it allows some materials through and blocks others Polar (head) portion is the true boundary between a cell and its surroundings.

9. Proteins are Embedded Within the Bilayer Proteins act as passageways for nonpolar (hydrophobic) molecules to pass through. Without proteins embedded in the bilayer, essential molecules needed for communication, energy, etc., would not reach the interior of the cell.

10. Proteins are Embedded Within the Bilayer (continued) The model of the lipid bilayer with embedded proteins is called the Fluid Mosaic Model.

11. TWO Types of Transport Through a Cell Membrane Active Transport – the movement of molecules through a cell membrane using energy 2. Passive Transport – the movement of molecules through a cell membrane without the expenditure of energy

12. 3 Types of Passive Transport Diffusion - Movement of molecules from areas of high concentration to low concentration Osmosis - Diffusion of water molecules from higher concentration of water to lower concentration of water Facilitated Diffusion - Diffusion through pores or channel proteins

13. DIFFUSION Molecules tend to move from areas where there is more of them (high concentration) to where there is less (low concentration). This is called moving WITH the concentration gradient. Diffusion continues until equilibrium is reached, or where molecules are equally distributed. If a membrane separates two regions of liquid the solutes within the liquid will move from high to low. Some solutes will be able to move through when others will not.

14. Diffusion (continued) http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html

15. OSMOSIS Water passes through cell membranes rapidly. Osmosis is a specialized form of diffusion that moves water molecules through membranes. Osmotic pressure–when more water molecules accumulate inside a cell, pressure increases. This pressure is called osmotic pressure. Osmotic pressure is the pressure caused by water; when osmotic pressure is high inside the cell, water will want to leave the cell.

16. OSMOSIS

17. Types of Solutions A hypertonic solution is one in which the solute concentration is HIGHER in the solution than in the cell. Water will move out of the cell. A hypotonic solution is one in which the solute concentration is LOWER in the solution than in the cell. Water will move in to the cell. An isotonic solution is one in which there are EQUAL solute concentrations in the solution and in the cell. Water will move in and out of the cell at equal rates. http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html

18. FACILITATED DIFFUSION Molecules like small lipids, alcohol, and water can diffuse directly through a membrane (diffusion & osmosis). Facilitated diffusion occurs because the cell membrane is semi-permeable some molecules cannot diffuse through on their own, and will need assistance (facilitate).

19. FACILITATED DIFFUSION Membranes have pores that are designed to fit specific solutes. When a difference in concentration occurs, solute molecules will diffuse through these pores. Solute must have a specific size, shape, or polarity in order to fit through the pore http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html

20. ACTIVE PROCESSES EXPEND ENERGY Active transport is use of energy to move molecules from LOW concentration to HIGH concentration (against or opposite theconcentration gradient) Some molecules exist in low amounts on one side of a membrane and need to be moved to areas of high amounts Cell membranes have mechanisms to move these molecules against their concentration gradients http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html

21. Types of Active Transport Molecules can be carried through the membrane by membrane-associated molecular pumps Na, K, Ca ions are transported in this manner Cells have an ability to engulf large particles by forming a vesicle around a particle in a process called endocytosis If the particles have been partially broken down into a liquid of tiny dissolved molecules, it is called pinocytosis (cell drinking) If the particles are cell fragments or organic matter, it is called phagocytosis (cell eating)

22. Exocytosis is the process by which wastes or secretions (hormones) are brought to the cell membrane, packaged into vesicles and sent out of the cell It is the reverse of endocytosis