plant structures

• adaptations of root hair cells to transport water + mineral ions:

  • large surface area - increases rate of absorption
  • thin cell walls - less distance for substance to travel, cell walls have open structure
  • a lot of mitochondria - release energy needed during respiration for active transport of substances and maintains steep concentration gradient
• in roots, water is transported by diffusion and osmosis

  mineral ions are transported from soil to root by active transport
• xylem: is a tissue which transports water and minerals from the roots up the plant stem and into the leaves.

  • consists of dead cells - no cytoplasm so water can flow through
  • no end cell walls - creates long empty vessels for water to flow
  • thick side walls & rings of lignin - to be able to withstand water pressure + supports plant
  • threads between cells - allow water to pass through
  • hydrogen bonds - cause attractive forces between xylem walls and water molecules helps water travel upwards
  • higher pressure at bottom of plant - forces water up plant, creates pressure gradient
• phloem: transports sucrose + amino acids made from photosynthesis in leaves to other parts of the plant

  • transports up and down stem in sieve tubes
  • sieve tubes have little cytoplasm & no nuclei - allows more room for central channel
  • central channel - where sucrose flows
  • holes in cell walls - allow sucrose to flow between sieve cells + companion cells
  • companion cells - actively pump sucrose in/out sieve cells causing mass flow
  • companion cells have many mitochondria - provides energy from aerobic respiration since pumping sucrose into sieve cells requires active transport + needs energy
• comparison between xylem and phloem:

  xylem:

  • physical process
  • transports water and minerals
  • transports upwards from roots to leaves

  phloem:

  • requires energy
  • transports sucrose + amino acids dissolved in water
  • transports upwards and downwards
• transpiration: the process by which water is carried through plants from the roots to the stomata in leaves, where it diffuses out the plant as water vapour and into the atmosphere (physical process)
• • during transpiration, water moves from root to xylem by osmosis (high to low concentration of water molecules)
  • threads between cells allow water to pass through
  • higher pressure at bottom of plant than top forces water up plant (pressure gradient)
  • hydrogen bonds cause attractive forces between water molecules so unbroken chain of water
  • concentration of water vapour in air spaces (in leaf) is greater than concentration outside the leaf so water diffuses faster out the stomata
  • the water being lost from stomata of leaves causes more water to be pulled up through xylem
• factors affecting rate of transpiration/water uptake:

  • temperature: higher temperature, particles have more kinetic energy, move more, rate increases
  • humidity: lower humidity, reduces concentration of water outside the leaf, steep concentration gradient, rate increases
  • light intensity: high light intensity, pores in stomata open, more water vapour escaping, rate increases
  • air movement: high wind speed, removes water vapour from leaf surfaces, rate increases
• function of stomata:

  • control water loss and gas exchange by opening and closing
  • allow water vapour and oxygen out of the leaf
  • allow carbon dioxide and oxygen into the leaf
• how guard cells work:

  • plants regulate the size of stomata with guard cells
  • in bright light the guard cells take in water by osmosis and become turgid, so stomata open
  • in low light the guard cells lose water and become flaccid, so stomata close
  • they close in the dark when no carbon dioxide is needed for photosynthesis
  • they allow gas exchange and control water loss within the leaf.
  • size of stomata opening controls rate of transpiration
• translocation: the movement of sugars and amino acids from the leaves to the rest of the plant
• process of translocation:

  • glucose is produced by photosynthesis in a green leaf/part of a plant
  • glucose converted to sucrose
  • sucrose transported in phloem vessels
  • companion cells actively pump sucrose into sieve tubes
  • which causes mass flow and REQUIRES ENERGY
  • companion cells contain mitochondria (site of aerobic respiration)
  • mitochondria releases energy required to pump sucrose into sieve cells
  • sucrose converted back to glucose
  • sucrose is transported from source to sink
• source: where glucose is produced
• sink: where glucose is used/stored
• diagram of a leaf:

  
• how leaves are adapted to photosynthesis + gas exchange:

  • have large surface area - maximise amount of gases diffused and light absorbed
  • stomata - allows gases needed to diffuse in & out
  • waxy cuticle - stops water evaporating through epidermis
  • chlorophyll - green pigment that transfers light energy to chemical
  • thin - short distance for gases to diffuse into leaf cell
• • waxy cuticle - reduces rate of water loss
• epidermis (transparent) - allow light to pass through to palisade layer
• • palisade mesophyll - chloroplasts for photosynthesis
• • spongy mesophyll - air spaces to allow gases to diffuse through leaf (increase efficiency of gas exchange)
• • guard cells - regulate transpiration + control gas diffusion by controlling open and close of stomata through turgidity and flaccidity
• • the uptake of water can be measured using a potometer
  • under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration (they are the same)
• • a potometer is a piece of capillary tubing to which a plant has been connected
  • the water uptake is measured by recording the time taken for a bubble in the tube to move a set distance
• method of measuring rate of transpiration:

  • fill reservoir with water
  • cut leafy shoot diagonally underwater so no air bubbles
  • insert shoot into rubber stopper at end of capillary tube
  • set a fan at setting 1
  • open tap for reservoir so one air bubble in capillary tube is formed
  • measure distance air bubble has traveled after 1 minute due to water uptake
  • calculate rate using speed equation
  • reset bubble by opening tap
  • repeat on other fan settings
• potometer experiment variables:

  • Independent: speed of fan -> shows how air movement affects rate of transpiration
  • Dependent: distance travelled by air bubble
  • Control: plant species, number of leaves, temperature, light intensity, humidity, time
• potometer diagram:

  
• mass potometer diagram: