Plant Transport of Water and Nutrients Osmosis Osmosis

Plant Transport of Water and Nutrients

Osmosis �Osmosis drives the absorption of water and minerals from the soil by the root tips. �Osmosis=movement of water from higher concentration to lower concentration across a semi-permeable membrane. �Water then moves deeper into the roots until it reaches the endodermis. Microscopic root hairs absorb water and nutrients by osmosis.

Water Movement Across the Casparian Strip �Once inside the root itself, water and nutrients must get past the Casparian strip—they can only travel through the selective endodermal cells that choose which nutrients and minerals they let through. �If they pass the “test”, they move into the vascular cylinder. The Endodermis is shown in bright red in the root cross section above. Water and nutrients must move into the vascular cylinder across the Casparian Strip next to the Endodermis.

Osmosis—How does it drive water movement? �Osmosis is the driving force for moving water from the soil into xylem cells. �How does the plant keep the concentration gradient going that is necessary for osmosis to occur?

What maintains the concentration gradient of water in the root? �Two Contributing Factors: � 1)Water is constantly moving away from the root tips creating more space for water to enter � 2)There is a higher mineral concentration inside the vascular cylinder, which drives water into the xylem by a force known as root pressure.

Capillary Action �Capillary action is the force of adhesion between water and a passageway that puls water up along the sides. �Along with osmosis, this mechanism is a minor contributor to the movement of water up the xylem.

Cohesion-Tension Theory and Transpiration �This process is the major mover of water in the xylem. �Transpiration creates a negative pressure in the leaves and xylem tissues due to water evaporation from the leaves. �Transpiration is evaporation of water from the surface of leaves.

Water Molecules & Hydrogen Bonds �Water molecules are attracted to other water molecules. Why? �The molecular attraction of water to water is called cohesion. �This attraction essentially creates a single united column of water that runs the length of the plant

How does Cohesion pull water molecules up the plant? �Imagine that you tie a bunch of soda cans to a rope. If you are standing in a tree, and pull up on the cans at the top of the rope, the cans at the bottom will follow—not really because they are loyal to the other cans, but because they are connected (bonded).

Transpiration �The movement of water through the xylem is due mostly to cohesion between the water molecules. �When water evaporates off the surface of the leaf, the water is pulled up through the xylem toward the leaves. �http: //www. phschool. com /science/biology_place/lab bench/lab 9/transpull. html

Regulating Stomate Activity �Stomata are the structures that regulate the flow of O 2, CO 2, and H 2 O in/out of the leaf �When closed, photosynthesis is halted. Why? �When open, mesophyll cells have access to water and CO 2.

Opening and Closing Stomata �With every reward, there is a risk. �When the stomata are open, the plant could dry out as a result of excessive transpiration. �The process of opening and closing the stomata, then, must be very carefully controlled. Guard cells such as these shown above control the opening and closing of the stomata.

Guard Cells �Guard cells surround and tightly regulate the actions of the stomata. �When water flows into neighboring guard cells (leading to an increase of turgor pressure), a structural change occurs that causes the opening of the stomata. �When water flows out, (a decrease in turgor pressure), the stomata will close. http: //www. phschool. com/science/ biology_place/labbench/lab 9/stoma mov. html

Transpiration Lab—AP Lab 9 �http: //www. phschool. co m/science/biology_place /labbench/lab 9/design. h tml

Carbohydrate Transport Through Phloem �The transport of carbohydrates through the phloem is called translocation. �After they are produced, they are moved into the phloem (the sugar superhighway), near the site of their creation. �They will be distributed throughout the plant.

Translocation Through Phloem �The movement of sugar into the phloem creates a driving force because it establishes a concentration gradient. �Because there are more sugars in the phloem, the gradient causes water to move into the phloem by osmosis. This increases the pressure inside these cells.

Pressure Inside Phloem cells �The pressure of these cells is what drives the movement of sugars and water through the phloem. �As the sugars arrive at various destination sites, the sugar is consumed by plant cells, causing a reversal in the driving force for water. �This pushes water out of the phloem. As water exits the phloem, the increased pressure disappears.

Translocation occurs in a series of steps: � 1. Sugars enter sieve-tube members by active transport. (now there is a greater concentration of sugars inside these cells) � 2. Water enters sieve-tube members by diffusion. Why? � 3. Pressure in sieve-tube members at the source (leaves)moves water and sugars to sieve-tube members at the sink (where the sugar will be used)through sieve tubes. The water and sugars move by bulk flow. � 4. Pressure is reduced in sieve-tube members at the sink as sugars are removed and used. Water then leaves also.