9.1 Xylem transport

Photosynthesis

Synthesis of carbohydrates using light energy

What sustains photosynthesis?

The exchange of carbon dioxide (raw material) and oxygen (waste product)

What is the primary organ for photosynthesis?

The leaf

Describe the structure of the leaf

Leaf is covered by a waxy cuticle, which causes a very low permeability. Hence, the epidermis has pores in it called stomata

What is the role of the stomata?

Allows carbon dioxide to be absorbed, but in the process, allows water vapour to escape

What is transpiration?

The loss of water vapour from the leaves and stems of plants

Guard cells

Cells found in pairs on either side of a stomata

Role of guard cells

Control the aperture of the stomata, adjusting it from wide open to fully closed. Hence, it can minimize water loss from transpiration.

How do guard cells perform their function?

When guard cells take in water, the turgor pressure increases, causing the cells to swell. The swelling pushes the guard cells outwards, opening the stomata

In what plants are stomata found?

Almost all groups of land plants for at least part of the plant's life cycle

What factors affect rate of transpiration?

Air movement, temperature, light intensity and humidity

Air movement

As air movement increases, transpiration increases. Air currents carry water molecules away from the leaf surface, increasing the concentration gradient (higher concentration inside) and causing more water to diffuse out

Temperature

As temperature increases, transpiration increases up to a certain point, then decreases. An increase in temperature results in an increase in kinetic energy of molecules. Hence, water molecules evaporate out f the leaf at a faster rate. However, if the temperature gets too high, the stomata close to prevent excess water loss, reducing the rate of transpiration.

Light intensity

As the light intensity increases, transpiration increases until it plateaus. Stomata close in the ark to minimize water loss. As light intensity increases, the stomata will open to enable gas exchange for photosynthesis, increasing the rate of transpiration. Once fully open, any increase in light intensity will not affect the rate of transpiration.

Humidity

As humidity increases, transpiration increases. High humidity means that the air surrounding the leaf surface is saturated with water vapor. Hence, transpiration will decrease as the concentration gradient between the inside and the outside of the leaf gets smaller. At a certain level, equilibrium is reached, and there is no net loss of water vapor from the leaves.

Xylem vessels

Structures that transport water and nutrients from roots to stem and leaves

Arrangement of cells in xylem tissue

Formed from long lines of cells that are connected at each end

Mature xylem vessels

Long, continuous, hollow tubes made of non-living cells

Formation of mature xylem vessels

As the xylem vessels develop in a flowering plant, the cell walls between the connected cells degrade an the cell contents are broken down

Contents of xylem wall + their purpose

Thickened with cellulose and strengthened with a polymer called lignin. These make xylem vessels extremely tough so they can withstand very low internal pressures without collapsing

Intermolecular attractions in the xylem

Cohesion and adhesion

Cohesion

Attraction between water molecules

What causes cohesion?

Within water molecules, the oxygen atom has a partial negative charge and the hydrogen atoms have a partial positive charge. As a result, hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules.

Adhesion

Attraction between molecules of a different type

Adhesion in plants

Water molecules are attracted to the hydrophilic surface of the cell walls on the interior of xylem vessels

How is the water lost from transpiration replaced?

1. When water evaporates from the surface of the leaf during transpiration, more water is drawn from the nearest xylem vessels to replace the water lost by evaporation
2. Since water molecules adhere to the cell walls of plant cells in the leaf, water moves through the cell walls of the xylem cells into the cells of the leaf
3. The loss of water from the xylem vessels causes low pressure within the xylem

Transpiration stream

The continuous upwards flow of water in the xylem vessels

Describe the transpiration stream

1. When water evaporates from the surface of the leaf during transpiration, more water is drawn from the nearest xylem vessel to replace the water lost by evaporation. This occurs because of the adhesion between water molecules and the cell walls of the leaf, whose force draws water through the cell walls
2. The loss of water creates low pressure within the xylem
3. The low pressure creates a pulling force throughout the xylem vessel
4. Adhesion between water and cell walls in the leaf is strong enough to suck water out of the xylem, further reducing its pressure
5. The column of water created by cohesion and adhesion transmits this pulling force all he way down the stem and to the ends of the xylem in the roots.
6. This is called transpiration-pull
7. This allows water to be moved upwards through the plant, against the force of gravity

Energy used in the transpiration stream

The energy needed for the transpiration stream comes from the thermal energy that causes transpiration. Therefore, the plant oes not use any energy for the transpiration pull, which makes it a passive process.

Plant roots

1. Responsible for the uptake of both water and mineral ions 2. Root hair cells increase the available surface area for these uptakes

Mineral uptake

1. The concentration of mineral ions is higher in the root cells than in the soil surrounding them (100x or more), creating a concentration gradient
2. Mineral ions are actively transported into the root cells via protein pumps in their plasma membrane, maintaining the concentration gradient

What does the active transport of mineral ions depend on?

Each mineral ion has a specific protein pump, and active transport can only occur if the ion makes contact with it.

What brings mineral ions in contact with their pumps?

1. Diffusion: As minerals are absorbed into the roots, a small concentration gradient is created. Mineral ions diffuse slowly towards the roots
2. Mass flow: As water carrying the ions rains through the soil, it brings the ions in contact with the pump proteins

Fungi role in mineral uptake

Some ions bind to the surface of soil particles, causing them to move very slowly. Some plants have developed a mutualistic relationship with fungi to overcome this problem.
1. Fungus grows on the surface of the roots, an even into the cells of the root.
2. The hyphae of the fungus grows out into the soil and absorbs mineral ions from the surface of soil particles
3. These ions are then supplied to the roots, allowing them to grow n mineral-deficient soil
4. In return, most (but not all) plants supply sugars and other nutrients to the fungus

Examples of plants with this mutualistic relationship

Many trees, members of the heather family and in orchids

Water uptake

Occurs by osmosis. However, osmosis is only possible because of the active transport of mineral ions into root cells.

Describe how mineral uptake makes water uptake possible

Mineral ion uptake raises the solute concentration (osmolarity) of the root cells, causing water to move from the soil (lower osmolarity) to the root cells (high osmolarity)

Root cortex

Central region of the root

Two pathways in the root cortex

Apoplastic and symplastic pathways

Apoplastic pathway

Water moves through the spaces in the cellulose cell walls of the root cells
1. Not osmosis since water is not diffusing through any membranes
2. Occurs rapidly

Casparian strip + function

A waterproof and that infiltrates the cell wall of the roots cells near the xylem. It forces the water out of the cell walls and into the cytoplasm of the cell, moving from the apoplastic pathway to the symplast pathway

Symplastic pathway

Water moves through the cytoplasm of the root cells
1. Occurs by osmosis in and out of each cell
2. Slower tan apoplastic pathway
3. Volume of water that takes this pathway is lower

Xerophytes

Plants with adaptations to conserve water

What is the purpose of xerophytes adaptations? Give examples of adaptations.

Increase the rate of water uptake from the soil and reduce the rate of water loss by transpiration. Some adaptations include: very few stomata, sunken stomata, hairs surrounding stomata. needle-shaped or small leaves, thickened waxy cuticle

Types of xerophytes

Ephemeral and perennial

Ephemeral xerophytes

Plants with a very short life cycle that is completed in the brief period when water is available after rainfall. They remain dormant as embryos inside a seed until the next rainy period.

Perennial xerophytes

Plants that rely on storage of water in specialized leaves, stems or roots

Examples of xerophytes

Cacti and marram grass

Adaptations of cacti

1. Leaves reduced in size, consisting of spines that cannot photosynthesize
2. Epidermis of cactus stems have a thick cuticle and contain stomata, although they are more sparsely distributed
3. Epidermis contains water storage tissue
4. Crassulacean acid metabolism (CAM)
5. Shallow and deep penetrating roots

Function of spines

Reduces leaf surface area, which also reduces water loss

Function of water storage tissue

Allows the stem to expand and contract in volume rapidly, becoming swollen with water after rainfall

Crassulacean acid metabolism (CAM)

A specialized form of photosynthesis, that enables plants to keep their stomata closed during the day.
1. CO2 is absorbed at night and stored as C4 (malic acid)
2. CO2 is released from C4 during the day, allowing for photosynthesis even when stomata are closed

Function of CAM

Reduces water loss by evaporation in the heat of the day

Function of shallow and deep penetrating roots

Allow access to all available water

Adaptations of marram grass

1. Leaves can be rolled up
2. Stomata are sunken in small pits within the curls of the structure
3. Has hairs on the inner surface of the leaf, called trichomes
4. Thick waxy cuticle on the exposed surface

Function of rolled up leaves

Creates a localized environment of water vapour, which reduces the exposure of surfaces to the wind, thus minimizing water loss by evaporation

Function of sunken stomata

Makes them less likely to open and lose water

Function of trichomes

Shield the stomata, slowing or stopping air movement, which reduces water loss

Function of thick cuticle

Reduces evaporation

Halophytes

Plants with adaptations to saline, or salty, conditions

Saline soils + examples

Soils with a high concentration of salts, e.g. coastal salt marshes and land with tides

Adaptations of halophytes

1. Ability to sequester, or store away, salts within their cell wall or vacuoles
2. Some can concentrate the salts they absorb in certain leaves, which then fall off the plant
3. Shed leaves when water is scarce to reduce water loss. The stem becomes green and takes over photosynthesis.
4. Thick cuticle and multi-layered epidermis
5. Reduced leaf surface area and sunken stomata
6. Long, deep roots, which go in search of water
7. Salt glands excrete salt to prevent build-up

Primary xylem

Visible in cross sections of young stems

Structure of primary xylem

1. Thin primary wall that is unlignified and freely permeable
2. Secondary wall is strengthened with lignin, which allows xylem vessel to continue growing in length as the plant grows taller. Lignin structures stretch apart from each other as the xylem grows.

Type of secondary walls

1. Annular thickening: lignin form rings
2. Helical thickening: lignin forms spirals

Secondary xylem

Formed when the stem has stopped growing in length

Structure of secondary xylem

Less need for lignin to stretch, so its walls are much more extensively lignified. This makes the secondary xylem stronger but less flexible than the primary xylem.

Drawings of secondary walls