What are phloem tubes and what do they do?
- made of columns of living cells with perforated end-plates to allow stuff to flow through
- transport food substances (mainly sugar) up and down the stem to growing and storage tissues
- movement of food around a plant is know as translocation
What are xylem vessels and what do they do?
- made of dead cells joined end to end with no end walls between them and a lumen down the middle
- thick side walls are made of cellulose, are strong and stiff, giving plants support
- carry water and minerals from the roots up the shoot to leaves in the transpiration stream
What does a root cross-section look like and why?
- roots have to resist crushing as through the soil
- so the xylem is in the centre to give strength
What does a stem cross-section look like and why?
- stems need to resist bending
- the xylem forms a sort of ‘scaffolding’, the phloem is always around the out side of the stem
- they are together to make a vascular bundle
What does a leaf cross-section look like and why?
- xylem and phloem together to make up a network of veins (vascular bundle)
- these are needed to support the leaveser
How do root hairs take in water?
- by osmosis
- cells on roots grow in long ‘hairs’ which stick out into the soil
- each branch of room is covered in millions of microscopic hairs
- giving the plant a big surface area to absorb water from soil
- usually a higher concentration of water in soil than plant, so water is drawn into root hair cells by osmosis
What is transpiration?
- caused by evaporation and diffusion of water vapour from inside leaves
- creating a shortage of water in leaf, so more water is drawn up from the rest of the plant through the xylem vessels
- meaning more water is drawn up from the roots
- so there is a constant transpiration stream of water through the plant
What are some benefits of the transpiration stream?
- constant stream of water from ground keeps plant cool
- provides constant supply of water for photosynthesis
- creates turgor pressure in plant cells, which helps support the plant and stops it wilting
- minerals needed by plant can be brought from the soil along with the water
What main things increase transpiration rates?
- increase om light intensity
- increase in temperature
- increase in air movement
- decrease in air humidity
How does light intensity affect transpiration rates?
- stomata closes when dark
- photosynthesis can’t happen in the dark, so stomata don’t need to be open to let C02 in
- when stomata are closed water can’t escape
How does temperature affect transpiration rates?
-warm water particles have more energy to evaporate and diffuse out of the stomata
How does air movement affect transpiration rates?
- wind
- if leaf is still water vapour just surrounds it and doesn’t move away
- meaning there is a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn’t happen quickly
- if windy water is swept away, maintaining a low concentration of water outside the leaf
- diffusion then happens quickly from an area of high concentration to an area of low concentration
How does air humidity affect transpiration rates?
-dry air increases transpiration
How are plants adapted to deal with a lack of water?
- waxy cuticle usually covering the upper epidermis of leafs, helps making the upper surface of the leaf waterproof
- most stomata are on the lower surface of a leaf, where it is darker and cooler, helping to slow down diffusion of water out of the leaf
- leaves in hot climates have smaller and fewer stomata on the underside and none on the upper epidermis, so less water is lost
How does the opening and closing of the stomata work?
- closes automatically when water from roots start to dry up
- guard cells have kidney shape which opens and closes the stomata as the guard cells go turgid or flaccid
- thin outer walls and thin inner walls make this opening and closing function work properly
- light sensitive, so open through day and close at night, allowing plant to conserve water without losing out on photosynthesis
How is transpiration just a side effect of photosynthesis?
- leaves have to have stomata in them to exchange gases easily
- as there is more water in a plant than in the air outside, water escapes from the leaves through the stomata
