Movement of substances Flashcards
(15 cards)
Define concentration gradient
The difference in concentration between two regions
The steeper the concentration gradient, the faster the rate of diffusion of particles
Define diffusion
Diffusion is the net movement of particles from a region where they are of higher concentration to a region of lower concentration down a concentration gradient.
How is diffusion across a membrane like
Dissolved particles will diffuse across different sides of the membrane until there are equal concentrations of all particles on both sides of the membrane (no net movement.)
What are the factors that affect the rate of diffusion?
- Diffusion distance - The shorter the diffusion distance of a particle from one region to another, the lesser the time needeed for the particle travel, hence the higher the rate of diffusion
- Concentration gradient - The steeper the concentration gradient for a particle from one region to another, the higher the rate of diffusion of a particle
- Surface area to volume ratio - The larger the SA:V ratio, the faster the rate of diffusion of substances (smaller cell has larger SA:V ratio then larger cell)
- Molecular mass
- Temperature
Examples of diffusion distance as a factor
1. Aleveoli are air-sacs in the lungs with one-cell thick alveolar walls. The short distance between the air and bloodstream allows gaseous exchange by diffusion to occur rapidly (See chapter 6)
2. Walls of blood capillaries are one-cell thick. This increases the rate of diffusion of oxygen from the blood to the tissue cells, and carbon dioxide from tissue cells to the blood (See chapter 6)
Examples of SA:V ratio as a factor
1. The extension of the root hair cell increases the surface area-to-volume ratio so that water can move into the cell by osmosis quickly. (See Chapter 13 Transport in Flowering Plants)
2. The microvilli on the epithelial cells in the small intestine increases the surface area for greater rate of diffusion of digested food molecules into the epithelial cells. (See Chapter5 Nutrition in Humans)
3. There are also microvilli in the proximal convoluted tubule in the kidney. The microvilli increase the surface area for reabsorption of useful substances such as glucose, amino acids and mineral ions from the filtrate. (See Chapter 8
Excretion in Humans)
Define osmosis
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Effect of osmosis on plant cells
What happens to a plant cell in a solution with lower water potential? (concentrated solution)
- When a plant cell is plased in a solution with lower water potential, the cell sap has a higher water potential than the solution outside the cell
- By osmosis, water molecules move out of the plant cell through the partially permeable cell membrane
- The cell decreases in size and becomes flaccid or limp
- As the cell loses water, the vacuole decreases in size and the cytoplasm shinks away from the cell wall. The cell is said to be plasmolysed.
A plasmolysed cell can be retored to its original state by placing it in water or a solution with higher water potential.
Effect of osmosis on plant cells
What happens to a plant cell in a solution with higher water potential? (Dilute solution or pure water)
- When a plant cell is placed in a solution with higher water potential, the cell sap has lower water potential than that of the solution outside the cell
- By osmosis, water molecules from the solution outside cell enter the cell through the partially permeable cell membrane
- The cell swells and becomes turgid. (hard)
As the water molecules enter the cell, the vacuole increases in szie and pushes the cytoplasm agains the cell wall. However, the cell does not burst as it is protected by the cell wall.
Effect of osmosis on plant and animal cells
What happens to a plant cell/ animal cell in a solution with the same water potential?
- When a plant cell is placed in a solution with the same water potential, the cell sap has the same water potential as the solution outside the cell
- The movement of water molecules is the same in both directions, and there are no net movement of water molecules in or out of the cell
- Size and shape will not change.
Effect of osmosis on animal cells
What happens to an animal cell in a solution with lower water potential? (concentrated solution)
- When an animal cell is placed in a solution with lower water potential, the animal cell has a higher water potential than the solution outside
- By osmosis, wtaer molecules move out of the animal cell through a partially permeable membrane into the solution outside
- The animal cell crenates (shrink in size and have spikes formed on their cell surface membrane/ appear on the cell)
It will then become dehydrated and eventually die.
Effect of osmosis on animal cells
What happens to an animal cell in a solution with higher water potential? (dilute solution)
- When an animal cell is placed in a solution with higher water potential, the animal cell has a lower water potential than the solution outside
- By osmosis, wtaer molecules move into the animal cell through a partially permeable membrane from the solution outside
- The animal cell expands and eventually bursts when too much water molecules enter it.
This occurs when the pressure becomes too great for the cell membrane of animal cell to withstand, as there is no cell wall to protect it.
Why is turgor important to plants
Tugor pressure ( pressure exerted by water in the vacuole ) - helps keep plant firm and upright
Loss of turgor (due to excessive water loss) may cause a plant to wilt
Define active transport
Active transport is the process which energy from respiration is used to move particles of a substance across a membrane against its concentration gradient, that is from a region where the particles are of a lower concentration to a region of higher concentration.
energy from aerobic respiration of mitochondria
Differences between diffusion, osmosis and active transport
- Both diffusion and osmosis do not require energy from respiration while active transport requires energy from respiration.
- Osmosis involves only the net movement of water molecules while diffusion and active transport involve the net movement of any particle.
- Both diffusion and osmosis involve the net movement of particles down a concentration gradient while active transport involves the net movement of particles against its concentration gradient.
- Diffusion does not require a cell membrane while osmosis and and active transport requires a cell membrane.
Where does active transport occur?
Active transport only occurs in living cells.
This is because living cells respire. It is during respiration that energy is released and part of this energy is used in active transport.
Why do vegetables become soft after soaking in a concentrated solution for too long?
The solution has a lower water potential than the cell sap of the plant cells in the vegetable. Water molecules from vacuole and cytoplasm of the plant cells of the vegetable move into the solution via osmosis through a partially permeable cell membrane. The plant cell decreases in size and becomes flaccid or limp. As the cell continues to lose water, the vacuole of the plant cells decreases in size and the cytoplasm shrinks away from the cell wall. The cell is said to be plasmolysed. The loss of turgor pressure makes the vegetable soft.
