Module 3, section 3: Transport in plants.

What is a plant’s ‘vascular’ system made of?

Xylem and phloem vessels. They add structural support and transport substances.

Describe the location of phloem and xylem in root, and the reasoning behind this choice.

located in the centre to provide structure for the root for when it burrows through soil.

Describe the location of phloem and xylem in the stem, and the reasoning behind this choice. Describe the leaves too.

Xylem and phloem vessels surround the perimeter of the stem, to provide a scaffolding structure. This reduces bending.

In leaves xylem and phloem form vein-like structures, which support the leaf.

Describe the structure of xylem, and how this structure allows it to perform its function.

Long tube-like structures made of stacked dead cells. Strengthened by a woody substance called lignin. It also includes pores in its structure, that allow water and minerals in/out.

Describe the adaptations and functions of phloem vessels.

phloem acts as a pure transport tissue. They are comprised of sieve tube elements, companion cells and phloem fibers.

sieve tube elements feature holes to allow solutes to pass through. Due to a lack of nucleus and organelles, they have cells attached to each one called companion cells, which perform the necassary biological processes for both. For example, companion cells provided the energy needed for the active transport of solutes up the stem.

Describe how water enters a plant.

Enters via root hair cells via osmosis, by going down a water potential gradient. First enters the cortex, and then the endodermis. Thus, it enters the xylem.

Why do the leaves having a low water potential cause the transpiration stream?

The soil has a relatively high water potential. Water always moves down a water potential gradient, where there is less water molecules. Thus, it is drawn out of the plant through the leaves.

Describe the two ways that water is transorted through the roots.

1. Symplast pathway. Water moves through gaps in the cell wall called plasmodesmata, via osmosis.

2. Apoplast pathway. water moves instead through cell walls of plants. This is the main way that water moves through the plant.

Describe the casparian strip, and the effect it has on water transport.

The casparian strip is a thin, waxy strip that blocks the apoplast pathway. Instead the symplast pathway must be taken.

Describe the movement of water in the leaves.

Moves through the xylem in the veins of the leaves, and when stomata open, the water moves out of the plant down the water potential gradient. This movement in and out of the plant is called the transpiration stream.

effectively, this evaporation of water out of the leaves is what causes the movement through the plant.

Describe how cohesion and tension contribute to the transpiration stream.

When water evaporates, it creates a suction-like effect, pulling water through the plant. Water molecules are cohesive, meaning they stick together. when one molecule is pulled, other molecules are likely to follow, thus making a column.

Describe adhesion, and why it also contributes to the transpiration stream.

Water molecules are attracted to the xylem, this means it can travel better.

Factors affecting rate of transpiration.

1. light. This increases transpiration rate, because stomata open wide to allow light in for photosynthesis.

2. Temperature. Gives molecules more energy to evaporate, thus increasing the steepness of the water potential gradient

3. Humidity. The amount of water particles outside the plant changes transpiration rate because it alters the water potential gradient. High humidity reduces the rate of transpiration.

4. wind. Due to water molecules being blown away from near the plant, it makes environments more dry. As such, transpiration rate will increase.

What is a xerophyte?

Plants that are adapted to live on sand dunes, and in dry conditions. Plants like cacti and marram grass.

What adaptations might cacti have to reduce water loss?

1. Waterproof thick waxy cuticle layer that prevents evaporation and losing water to environment

2. Thin spines rather than leaves. Allows photosynthesis to occur, but minimises surface area where water could be lost.

3. Stomata close during the hottest points of the day, where transpiration rate is usually highest. Allows little water loss.

What adaptations do marram grass have to prevent water loss?

Sunken stomata, preventing exposure with the outside and thus preventing water loss.

In dry/windy conditions, leaves curl up to prevent water loss.

Hairs on epidermis, that trap moist air round the stomata. Reduces water potential gradient.

What is a hydrophyte? What adaptations might they have?

Plants that live aquatic environments. They feature flexible stems, that are resilient to water pressure, thus minimising damage.

Feature big air pockets, that allow the plant to float on the water surface.

What is translocation?

How dissolved substances are transported around the plant.

What are assimilates in plants.

Dissolved substances that become a part of the plant tissue.

Describe the movement of particles using translocation.

From sources (where the substance concentration is high) to sinks (where concentration is lower.)

How do enzymes maintain concentration gradient?

By breaking down the substance, meaning concentration stays low at the sink.

Describe the action of the mass-flow hypothesis.

Solutes are loaded into the sieve tubes, which decreases water potential and causes water to enter the tube, from xylem and companion cells, causing high pressure.

Substance then moves towards the sink end, where substances are used up by the plant. The removal of solutes increases water potential, causing water to leave the phloem tubes

This creates a pressure gradient from source to sink.

How are hydrogen ions used to pull sucrose ions against the concentration gradient.

ATP is used to push H+ ions out of the cell. This creates a proton gradient.

Outside the cell, a transport protein binds to the hydrogen ion and a sucrose molecule, causing it to be transported into the sieve tube element,