4.2.3.2 Plant Organ System Flashcards
What is a plant organ system made up of?
- Roots, stems, and leaves for transport of substances (sugars, water, and minerals) around the plant
- Specialist tissues include root hair cells, phloem and xylem
What is the purpose of water in plants?
- Water is necessary for photosynthesis (how plants use E from Sun to create food)
- To maintain cell turgidity for structure and growth
- Helps in germination of seeds
- Helps in transport of nutrients and minerals from soil to plants
- Water is an essential nutrient for plants and comprises 95% of a plants tissue
- Transpiration from leaves stops plants from overheating
- Warmer temps, wind and dry air increases rate of transpiration
- As water evaporates through the leaves, more water is pulled up through the roots of the plant
Function of root hair cells:
- Efficient uptake of water and mineral ions
- Root hair cells absorb water by osmosis
- Root hair cells absorb minerals (nitrate ions) by active transport
Adaptations of root hair cells:
- Large surface area with a projection/root hair
- Thin cellulose walls
- Many mitochondria
- Adapted for efficient uptake of water by osmosis, and mineral ions by active transport
Function of xylem:
- Xylem tissue transports water and mineral ions from the roots to the stems and leaves.
- They are involved in a process that carries water and mineral ions from the roots to the leaves, called the transpiration stream
- Transport of water in transpiration stream
Adaptations of xylem:
- Mature xylem consists of elongated dead cells arranged end to end to form continuous vessels (tubes)
- Mature xylem vessels contain no cytoplasm
- Have tough walls containing a woody material called lignin that builds up in spirals in cell walls - lignin makes spirals very strong, to withstand the pressure from the water + provide support. - adapted for transport of water in transpiration stream
- The contents and end walls break down to form a hollow centre (lumen)
Function of phloem:
Phloem tissue transports dissolved sugars and amino acids from the leaves to the rest of the plant for immediate use or storage - this movement of food molecules through phloem tissue is called translocation
Adaptations of phloem:
- composed of tubes and elongated cells, made of living cells lined end to end, found in the vascular bundle
- Phloem vessels are made of sieve elements
- The cell walls between phloem cells break down to form perforated sieve plates. These allow water carrying dissolved food (sugars and amino acids) to move freely along the tubes from one phloem cell to the next through pores in the end walls
- Supported by companion cells (which contain nucleus and mitochondria) that keep them alive. The mitochondria of the companion cells transfer the energy needed for translocation to occur
- Cells that have lost most of their internal structure – e.g. no vacuole or nucleus. This helps to reduce any resistance during translocation and allows efficient movement of substances
- Vessels contain cytoplasm
- Two-way flow allows substances to be transported all around the plant
Where are xylem and phloem vessels found in stems?
- The stem has to resist compression (squashing) and bending forces caused by the plants weight and the wind
- The vascular bundles are arranged near the edge of the stem, with the phloem in the outside and the xylem on the inside of the vascular bundle
Where are xylem and phloem vessels found in leaves?
- The vascular bundle are located among the spongey mesophyll
- Phloem nearer lower surface
How does water move through a plant (transpiration stream)?
- Root hair cells have lots of mitochondria for releasing energy for the uptake of mineral ions.
- Root hair cells have a large surface area for rapid water uptake.
- Water enters root hairs by osmosis.
- Water moves across the root one cell at a time.
- Water enters the xylem of the stem.
- Xylem is composed of hollow vessel/lumen strengthened by lignin.
- Water moves up the xylem in the stem.
- Water leaves the xylem and enters the leaf cells by osmosis.
- Water moves across the leaf one cell at a time by osmosis.
- Some water is used in the cells for photosynthesis.
- The remaining water evaporates into the air spaces of the spongy mesophyll.
- In the air spaces the humidity is now high.
- From the air spaces water diffuses out of the stomata and evaporated off the surface of the leaf.
Transpiration stream:
- The flow of water through a plant, from the roots to the leaves, via the xylem vessels
- Water will eventually be lost by evaporation out of the stomata
Transpiration:
The loss of water from leaves by evaporation
What happens in the transpiration stream?
- Water moves through the plant from the roots up tothe leaves.
- When the guard cells are open, water evaporatesout of the leaf via the stomata.
- This creates a shortage of water in the leaf, sowater is drawn up from the roots, via the xylem toreplace it.
- Water molecules inside the xylem cells arestrongly attracted to each other. There is strongcohesion between the molecules becauseofhydrogen bonding. A continuous column ofwater is therefore pulled up the stem.
- More water is drawn into the roots via osmosis toreplace water lost inside the roots.
- This is a constant cycle called thetranspirationstream.
Cohesion:
water molecules are held together by hydrogen bonds and act as a column which is why water molecules ‘stick together’ when travelling up the xylem
What are potometre’s used for?
- Used to measure the uptake of water by plants in different conditions
- The movement of the air bubble in the tube shows how much water is taken up
How is a potometer is used to measure the uptake of water to measure the rate of transpiration?
- Fill a beaker with water.
- Place a capillary tube in the beaker of water and attach it to the bottom of a test tube.
- Place a plant in the test tube sealed with an air tight bung.
- Add a ruler beneath the capillary tube.
- Inside capillary tube there is an air bubble - water from test tube absorbed through xylem and then evaporate from stomata - transpiration.
- Use a ruler to measure the distance moved by air bubble.
- Air bubble in capillary tube and water is going to move up from the beaker.
How accurate is a potometer at measuring transpiration rate?
It measures water uptake, not the water lost, as some of the water will be used in photosynthesis so it isn’t very accurate
Why is water uptake not the same as transpiration uptake?
- It is only a rough gauge as some water taken up by the roots is used inphotosynthesis (5-10%)
- Transpiration will still occur in a water shortage, which will ultimately lead to wilting
Describe how bright light affects the rate of transpiration:
In bright light transpiration increases
Explain how light affects the rate of transpiration:
The stomata (openings in the leaf) open wider to allow more CO2 into the leaf for photosynthesis
Describe how temperature affects the rate of transpiration:
Transpiration is faster in higher temps