R2101 4.3 Movement of Water/Minerals Flashcards
What is the difference between osmosis and diffusion?
Osmosis involves water, and a semi-permeable membrane.
Diffusion involves gases and liquids.
What is a concentration gradient?
(or diffusion gradient)
The difference in concentration between two regions
What is diffusion?
- Diffusion takes place where there is a gradient until all the particles are evenly distributed. = equilibrium.
- Diffusion is a passive process which takes place by random motion. It requires no energy input.
- Osmosis is a form of diffusion.
What is active transport?
- Molecules or ions move against a concentration gradient.
- From a region of low concentration to a region of high concentration.
- This requires energy in the form of ATP.
- Mineral ions are moved into root hairs, where they are in a higher concentration than in the soil.
- Active transport occurs across the root.
How do cells take in water?
- Plant cells generally have a lower water potential than their surroundings. Due to solutes in the fluid within the vacuole (cell sap).
- Water flows through the cell membrane and vacuole membrane into the vacuole by osmosis. As a result the cell will swell.
- The pressure potential reaches maximum where the cell wall is stretched as far as it can go and cannot take in any more water. The cytoplasm is pushed against cell wall. The cell is fully turgid.
- The cellulose cell wall stretches and develops tension, resisting further expansion of the cell.
What is plasmolysis?
When a plant cell is immersed in a solution with relatively lower water potential (hypertonic solution):
- the volume of the cell decreases as water flows out of the vacuole by osmosis
- the cytoplasm will start to pull away from the cell wall, leaving a gap between the cell wall and the cell membrane
- the plant will wilt
How does water move up through plants?
- The gradient of water potential creates a flow of water through the plant.
- Evaporation of water from leaf cells also helps flow of water
How do roots take up water?
Osmosis because of the differential in solute concentration inside and outside cell. Water enters the roots through the cell wall (permeable to both water and minerals) and cell membrane (permeable to water).
Transpirational pull: water evaporated through the leaves of plants causes a ‘sucking’ action of water up through the plant. Causes the majority of the upward movement of water in plants, with root pressure lending a helping hand. (https://study.com/academy/lesson/root-pressure-definition-theory.html)
What are the main components/layers of a root?
Epidermis – Outer layer of protective cells. Also elongate into root hairs to increase absorptive capacity. Each root hair is a thin extension of a epidermal cell.
Cortex – Composed of large, thin-walled parenchyma cells that permit movement of air and water. Makes up the main body of the root. Often have high concentrations of starch stored within them.
Endodermis – A layer of cells that regulates the flow of water across the root and into the xylem.
Pericycle – The outermost layer of the vascular cylinders beneath the endodermis. A layer of parenchyma cells from which lateral roots arise. When a lateral root develops it forces its way through the layers in front of it and out of the side of the root.
Vascular cylinders – Primary xylem (water-carrying tissue) form an x-shape (in dicot) in the centre of the root. Primary phloem (food-conducting tissue) forms between the arms of the X-shaped xylem (if dicot).
The endodermis and the tissues inside it are known as the stele (in dicots this will consist of the phloem, xylem, cambium).
What is transpiration?
- Water in the form of water vapour lost from the plant, mainly from the leaves in the stomata.
- It diffuses into the atmosphere, which has a lower relative humidity than the air immediately surrounding the leaf.
- A passive type of transport.
- Water lost is replaced by water drawn up through the xylem.
- As leaf cells lose water they draw on neighbouring cells.
Why and what is the transpiration stream?
- A continuous flow of water (and dissolved minerals) from roots to leaves
- Water in the spongy and palisade cells of the leaf diffuse out of the stomata, drawing water out of the xylem cells to replace what’s been lost; this draws water through the whole plant
- Some water used for psn; most evaporates from leaves
- Transpiration helps in cooling the leaves, an important effect in hot conditions
- Provides a transport pathway through which mineral salts can be carried
What is adhesion?
The force of attraction between unlike molecules. This would be the attraction of water molecules with the molecules of xylem vessels.
The narrower the container the greater the force of attraction.
What is cohesion?
- The force of attraction between like molecules, in this case the water molecules
- This explains the rise of water in the xylem in all plants including tall trees
- Cohesion holds the columns of water together. As water transpires from the leaves, the column of water moves upwards
How does water leave the leaf?
Each stoma opens into a small area called the sub-stomatal air chamber which is lined with spongy mesophyll cells
Water evaporates from these cells and moves down a water potential gradient from the plant to the atmosphere
Water vapour accumulates in the sub-stomatal chambers from which it escapes via the open stomata by the process of diffusion
In very hot weather the stomata close to prevent water loss through transpiration
Describe stomatal closure and opening.
- Water flows into the guard cells by osmosis, their turgor increases and they expand. But they do not expand uniformly in all directions.
- The inelastic inner wall makes them bend and draw away from each other. So that the pore opens.
- If the guard cells lose water then the opposite happens; turgor decreases and the walls straighten, thus closing the stomata.
