Biology I for Non-Majors

1. Biology I for Non-Majors Cell Membranes

2. Cell Membranes are a Fluid Mosaic • Defines the boundary of the cell • Determines the nature of its contact with the environment • Dynamic and constantly in flux

3. Major Components of Cell Membranes • Made up primarily of a bilayer of phospholipids with: • Embedded proteins • Carbohydrates • Glycolipids • Glycoproteins • Cholesterol (in animal cells)

4. Passive Transport • The most direct forms of membrane transport are passive • Naturally occurring phenomenon • Does not require the cell to exert any of its energy to accomplish the movement • Substances move from an area of higher concentration to an area of lower concentration • In other words, down a concentration gradient • Remember membranes have hydrophilic and hydrophobic regions • This helps the movement of some materials through the membrane and hinders the movement of others • Lipid-soluble material with a low molecular weight can easily slip through the hydrophobic lipid core of the membrane

5. Types of Passive Transport • Diffusion: single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space

6. Types of Passive Transport • Facilitated transport: materials diffuse across the plasma membrane with the help of membrane proteins • Still requires a concentration gradient • Transport proteins are integral proteins involved in facilitated transport • Channels are specific for the substance that is being transported • Carrier proteins binds a substance and move the bound molecule from the outside of the cell to its interior

7. Types of Passive Transport • Osmosis: a special case of diffusion • Only material capable of moving through a membrane will diffuse through it • If a solute is too large to fit, water will move instead down a concentration gradient

8. Types of Passive Transport • Osmosis: a special case of diffusion • Only material capable of moving through a membrane will diffuse through it • If a solute is too large to fit, water will move instead down a concentration gradient

9. Tonicity • Describes how an extracellular solution can change the volume of a cell by affecting osmosis • Hypotonic means that the extracellular fluid has a lower concentration of solutes • Hypertonic refers to the extracellular fluid having a higher osmolarity than the cell’s cytoplasm • Isotonic means the extracellular fluid has the same osmolarity as the cell

10. Active Transport • Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP) • Usually when a substance is moving against its concentration gradient (from an area of lower concentration into an area of higher concentration) • Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane • The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient

11. Active Transport Requires Energy and Pumps • To move substances against a concentration or electrochemical gradient, the cell must use energy • Active transport mechanisms, collectively called pumps, work against electrochemical gradients • There are 3 types of pumps

12. Types of Active Transport • Primary Active Transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP

13. Types of Active Transport • Secondary Active Transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP

14. Active Transport for Large Molecules • Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell • There are variations of endocytosis, but all share a common characteristic: • Plasma membrane of the cell invaginates, forming a pocket around the target particle • The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane

15. Phagocytosis • The process by which large particles, such as cells or relatively large particles, are taken in by a cell • “Cell eating”

16. Pinocytosis • A process that takes in molecules, including water, which the cell needs from the extracellular fluid • Results in a much smaller vesicle than does phagocytosis • “Cell drinking”

17. Receptor-Mediated Endocytosis • A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances

18. Exocytosis • The reverse process of moving material into a cell is the process of exocytosis • Purpose is to expel material from the cell into the extracellular fluid

19. Cellular Transport

20. Quick Review • Why are cell membranes said to be fluid mosaics? • List the major components of membranes and their roles • What are the types of passive transport? • Can you describe which direction water will move in solutions of different tonicity? • Define the three types of pumps used in active transport. • What are the types of active transport? • Define three types of endocytosis • What are the steps of exocytosis?