Topic 5: Movement of Substances in and out of Cells PDF

## Topic 5: Movement of substances in and out of cells ### Introduction - Cells need food (glucose), oxygen, water and salts to survive. - They also need to remove waste products such as ammonia and carbon dioxide. - These substances move in and out of cells through the cell membrane. - Movement...

## Topic 5: Movement of substances in and out of cells ### Introduction - Cells need food (glucose), oxygen, water and salts to survive. - They also need to remove waste products such as ammonia and carbon dioxide. - These substances move in and out of cells through the cell membrane. - Movement can be either through: - **Diffusion:** passive movement of particles - **Osmosis:** passive movement of water molecules - **Active transport:** requires energy. ### Diffusion **Definition:** The passive movement of particles from a high concentration to a low concentration. **Example:** Dropping food colouring in water. - Colour is initially concentrated in one spot, but then spreads out evenly as the molecules diffuse. **Key Points:** - It is a passive process, meaning that no energy is required. - It happens automatically. **Factors affecting diffusion rate:** 1. **Temperature:** Higher temperature means faster diffusion because particles have more kinetic energy. 2. **Size of molecule:** Smaller molecules diffuse faster because they move more easily. 3. **Short diffusion distance:** A thinner cell membrane allows faster diffusion because there is a shorter distance to cross. 4. **High concentration gradient:** A large difference in concentration between the inside and outside of the cell speeds up diffusion. 5. **Larger diffusion surface area:** Having more surface area for diffusion increases the rate. **Examples of Diffusion:** - **Oxygen:* Oxygen diffuses from an area of high concentration (outside the cell) to an area of low concentration (inside the cell) because it is being used for respiration inside the cell. - **Carbon dioxide:** Carbon dioxide diffuses out of the cell because it is being produced as a by-product of respiration and has a higher concentration inside the cell. ### Surface area to volume ratio - **Significance:** The ratio of surface area to volume affects the rate of diffusion. - **Larger cells:** Larger cells have a smaller surface area to volume ratio which makes it harder for diffusion to meet the cell's needs. - **Smaller cells:** Smaller cells have a larger surface area to volume ratio making diffusion easier. ### Diffusion in larger organisms - They have a small surface area to volume ratio. - They rely on specialized structures like lungs to increase surface area and improve diffusion. - Example: Humans rely on lungs for oxygen uptake. ### Osmosis **Definition:** The passive diffusion of water molecules from a dilute solution to a more concentrated solution through a selectively permeable membrane. **Key Points:** - It is a passive process, meaning that no energy is required. - It happens because water molecules are attracted to dissolved substances like salt. **Explanation:** - Imagine a beaker with two compartments separated by a selectively permeable membrane. - One compartment has a dilute solution (lots of water, little salt), the other has a concentrated solution (lots of salt, less water). - Water will move from the dilute solution to the concentrated solution to balance the concentration gradient. ### Cells and osmosis - Cell membranes are selectively permeable like the membrane in the beaker example. - Water moves in and out of cells freely down their concentration gradient. **Key Terms:** - **Solution:** A mixture of a liquid (like water) and a dissolved solid (like salt). - **Concentrated/Strong Solution:** Has a high concentration of dissolved solids. - **Dilute/Weak Solution:** Has a low concentration of dissolved solids. - **Distilled water:** Water without any dissolved solids. **Osmosis Environments:** - **Hypotonic Solution:** Has a lower concentration of dissolved solids compared to the inside of the cell. Water moves into the cell. - **Isotonic Solution:** Has the same concentration of dissolved solids as the inside of the cell. There is no net movement of water. - **Hypertonic Solution:** Has a higher concentration of dissolved solids compared to the inside of the cell. Water moves out of the cell. ### Effects of osmosis on animal and plant cells: - **Animal Cells:** - **Hypotonic:** Cell bursts (lysis) as water enters. - **Isotonic:** Cell stays normal. - **Hypertonic:** Cell shrinks and wrinkles. - **Plant Cells:** - **Hypotonic:** Cell becomes turgid (normal, swollen). - **Isotonic:** Cell becomes flaccid (soft). - **Hypertonic:** Cell becomes plasmolysed (cytoplasm pulls away from the cell wall). ### Active Transport **Definition:** Movement of particles through a cell membrane **requiring energy.** **Key Points:** - **Against concentration gradients:** Active transport moves substances from a low concentration to a high concentration. - **Requires energy:** Active transport uses energy provided by respiration to move substances. - **Transport proteins:** Cells have special membrane proteins to facilitate active transport. **Explanation:** - Imagine a high concentration of ammonia outside a cell, and a low concentration inside. - Ammonia could diffuse into the cell, but because it can be harmful, the cell uses active transport to pump it back out, against the concentration gradient. **Examples of active transport:** - **Animals:** Glucose absorption by villi in the small intestine. - **Plants:** Mineral ions uptake by root hair cells. ### Reverse Osmosis **Definition:** A process of active transport that **reverses the natural direction of osmosis**. **Key Points:** - **Energy needed:** This process requires energy because it works against the concentration gradient. - **Freshwater from saltwater:** This method can be used to desalinate sea water and produce freshwater. ### Summary - **Diffusion and Osmosis:** Passive processes that happen automatically without energy. - **Active transport:** Requires energy and moves substances against the concentration gradient. **Concentration gradient movement:** - **Diffusion:** Molecules move down the concentration gradient (from high to low). - **Osmosis:** Water molecules move down the concentration gradient (from dilute to concentrated). - **Active transport:** Moves molecules up the concentration gradient (from low to high) using energy.