Section 9: Transport In Plants Flashcards
Why does transpiration happen?
Transpiration is the evaporation of water from a plant’s surface, especially the leaves. This happens as a result of gas exchange.
A plant needs to open its stomata to let in carbon dioxide so that it can produce glucose (by photosynthesis). But this also lets water out - there’s a higher concentration of water inside the leaf than in the air outside, so water moves out of the leaf down the water potential gradient when the stomata open. So transpiration’s really a side effect of the gas exchange needed for photosynthesis.
What are the factors affecting transpiration rate?
Light intensity, temperature, humidity and wind.
How does the light intensity affect the transpiration rate?
The lighter it is the faster the transpiration rate. This is because the stomata open when it gets light (the lighter it gets, the wider they open). When it’s dark the stomata are usually closed, so there’s little transpiration.
How does the temperature affect the transpiration rate?
The higher the temperature the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out the leaf faster.
How does the humidity affect the transpiration rate?
The lower the humidity, the faster the transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration rate.
How does wind affect the transpiration rate?
The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases the rate of transpiration.
Why do plants need transport systems?
Plants need substances like water, minerals and sugars to live. They also need to get rid of waste substances. Like animals, plants are multicellular so have a small surface area to volume ratio. They are also relatively big so have a high metabolic rate. Exchanging substances by direct diffusion (from the outer surface to the cells) would be too slow to meet their metabolic needs. So plants need transport systems to move substances to and from individual cells quickly.
Where can you find xylem and phloem tissues?
There are two types of tissue involved in transport in plants. Xylem tissue transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves. Phloem tissue mainly transports sugars (also in solution) both up and down the plant.
Xylem and phloem make up a plant’s vascular system. They are found throughout a plant and they transport materials to all parts. Where they’re found in each part is connected to the xylem’s other function, which is support. The position of the xylem and phloem in the root, stem and leaf are shown in the transverse cross-sections (the sections are cut through each structure at a right angle to its length.
Where are the xylem and phloem located in the roots?
In a root, the xylem and phloem are in the centre to provide support for the root as it pushes through the soil.
Where are the xylem and phloem located in the stem?
In the stems, the xylem and phloem are near the outside to provide a sort of ‘scaffolding’ that reduces bending.
Where are the xylem and phloem located in the leaves?
In a leaf, xylem and phloem make up a network of veins which support the thin leaves.
What is translocation?
Translocation is the movement of dissolved substances (e.g. sugars like sucrose, amino acids) to where theyre needed in a plant. Dissolved substances are sometimes called assimilates. Translocation is an energy-requiring process that happens in the phloem.
Translocation moves substances from ‘sources’ to ‘sinks’. The source of a substance is where it’s made (so it’s at a high concentration there). The sink is the area where it’s used up (so it’s at a lower concentration there).
Some parts of a plant can be both a sink and a source.
Enzymes maintain a concentration gradient from the source to the sink by changing the dissolved substances at the sink (e.g. by breaking them down or making them into something else). This makes sure there’s always a lower concentration at the sink than at the source.
Give examples of translocation.
The source for sucrose is usually the leaves (where it’s made following photosynthesis), and the sinks are the other parts of the plant, e.g. food storage organs ans meristems (areas of growth) in the roots, stems and leaves.
Another example is that sucrose can br stored in the roots. During the growing season, sucrose is transported from the roots to the leaves to provide the leaves with energy for growth. In the case, the roots are the source and the leaves are a sink.
Another example is that in potatoes, sucrose is converted to starch in the sink areas, so there’s always a lower concentration of sucrose at the sink than inside the phloem. In other sinks, enzymes such as invertase break down sucrose into glucose (and fructose) for use by the plant - again this makes sure there’s a lower concentration of sucrose at the sink.
What is the mass flow hypothesis?
Scientists still aren’t certain exactly how the dissolved substances (solutes) are transported from source to sink by translocation.
1. Source.
Active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) into the sieve tubes of the phloem at the source (e.g. the leaves). This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells. This creates a high pressure inside the sieve tubes at the source end of the phloem.
2. Sink
At the sink end, solutes are removed from the phloem to be used up. This usually happens by diffusion (a passive process) because the solutes are at a higher concentration in the phloem than they are in the surrounding tissue at the sink. The removal of solutes increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes.
3. Flow
The result is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes towards the sink. When they reach the sink the solutes will be used (e.g. in respiration) or stored (e.g. as starch). The higher the concentration of sucrose at the source, the higher the rate of translocation.
What is active loading?
Active loading is used at the source to move substances into the companion cells from surrounding tissues, and from the companion cells into the sieve tubes, against a concentration gradient. The concentration of sucrose Is usually higher in the companion cells than in the surrounding tissue cells, and higher in the sieve tube cells than in the companion cells.
Sucrose is moved to where it needs to go using active transport and co-transport proteins. Co-transport proteins are a type of carrier protein that bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient. In active loading, h+ ions are used to move sucrose against its concentration gradient.
