Unit 7 - Gas transport/transpiration in plants

Gas exchange

The exchange of respiratory gases (oxygen and carbon dioxide) between cells/organisms and the environment.

Challenges of gas exchange in larger organisms

More difficult in ______ organisms because the areas where the gas is needed is too far from the surface of the body to recieve enough oxygen by diffusion alone.

Permeability

Allows gases to cross.

thin tissue layer

Makes the shortest distance for diffusion possible.

Moisture

Gases dissolve in the ___________, helping them to pass across the gas-exchange surface.

Large surface area

Allows large quantities of the respiratory gases can cross at the same time.

4 properties of gas exchange surfaces

permeability, thin tissue layer, moisture, and large surface area

Adaptations of leaves

Waxy cuticle, epidermis, air spaces, spongy mesophyll, stomatal guard cells, and veins.

Waxy cuticle

Forms a largely impermeable barrier so gases and vapor are forced to leave through the stomata on the underside of the leaf, allowing gas exchange and water loss to be controlled.

Epidermis

Contains many pores (stomata) that facilitate gas exchange. Most of the stomata are in the lower _________.

Air spaces

These allow gases to circulate around the loosely packed spongy mesophyll cells, maintaining a concentration gradient between air and cells.

Spongy mesophyll

Rounded cells in the leaf that are loosely packed. They create air spaces where air circulates, providing a large surface area for gas exchange.

Stomatal guard cells

Cells that surround the ________. The act as gates for gas exchange. They open and close to regulate gas exchange and water loss.

veins

The vascular bundles in leaves.

Vascular bundles

Transport systems made of xylem and phloem.

Phloem

tissue that moves sugars.

cortex

Stores nutrients and provides support.

Pith

Stores nutrients, such as water and sugars.

Epidermis (2)

Outer layer of tissue that protects the stem.

Cambium

Promotes growth of secondary xylem and phloem.

Apoplast pathway

The pathway through the non-living part of a cell (e.g. the cell walls and spaces between cells).

Symplast pathway

The pathway through the cell membrane and plasmodesmata (living contents of the cells).

Vacuolar pathway

Water movement through the plasma membrane, cytoplasm, and then through the vacuole.

Endodermis

A single layer of cells that surrounds the vascular tissues (xylem and phloem) in the root of the plant.

Casparian strip

A waxy substance impermeable to water found in the endodermal cell walls of all plant roots.

Transpiration

When water evaporates from the spongy mesophyll tissue, and subsequently diffuses through the stomata. This happens because the water loss is an inevitable consequence of gas exchange in the leaf, and as a result, the plant must transport water from the roots to the leaves to replace losses from ________.

Advantages of transpiration

• Has a strong cooling effect when happening in light, which helps to break hydrogen bonds

• Cells receive water by lateral movements of water through the xylem vessels, allowing them to be fully hydrated

• dissolved ions are carried in.a stream of water that is moving up the plant

Cohesion-tension theory

Tension (negative pressure potential) is set up in the water column of the xylem and draws water up. Tension is transmitted down through the roots because water molecules’ cohesion and adhesion, causing for a continuous column of water to rise from the roots to the leaves. With tension, the water column also does not break nor pull away from the sides of the xylem.

Root pressure

The force that drives fluids upwards into the xylem. It is generated by osmotic pressure in the cells.