Exchange and Transport in plants

• algae is very small, and has a large surface area to volume ratio so can rely on diffusion for the transport of molecules
• multi cellular plants are large and have a low surface area to volume ratio so cannot rely on diffusion alone for the transport of molecules

  • the green parts of multi cellular plants can carry out photosynthesis producing oxygen and glucose which can then be used in aerobic respiration
  • many parts cannot carry out photosynthesis (e.g. tissues in the roots)
  • these roots absorb mineral ions by active transport so have a high rate of metabolic reactions. due to their high rate sugars must be transported to these tissues.
• mineral ions are transported in the roots to other parts of the plant. (for example nitrate ion in used by plants to make amino acids)
• seeds contains an embryonic leaf called a cotyledon and when a seed germinates the cotyledon unfurls allowing the seed to carry out photosynthesis

  • some plants only have one cotyledon, grass - monocotyledonous
  • some plants have 2 cotyledons, trees - dicotyledonous
• dicotyledonous plants

  • trees are an example of woody dicotyledonous plants
  • geraniums are an example of herbaceous dicotyledonous plants
• herbaceous plants are often fast growing and can be short lived

  • do not have a woody stem
• transport systems in plants

  • xylem
  • phloem
• xylem - carries water and mineral ions from the roots, up the stem and to the leaves

  phloem - transports organic molecules such as sugars produced by photosynthesis in the leaves. these molecules can move down the plant to the roots or up the plant to the flowers
• the xylem vessels and the phloem vessels are grouped together in vascular bundles

  • the arrangement of the vascular bundles are different in the roots the stem and the leaves
• root

  • root hair cells grow from a layer of external tissues called the epidermis
  • then there is a thick layer of cells called the cortex
  • in the centre of the root there is the vascular bundle, which is surrounded by a layer of cells called the endodermis
  • in the vascular bundle there is the xylem vessels in the centre and the phloem vessels around them

  
• xylem vessels are mechanically strong and because they are grouped together in the centre of the root it helps to prevents the root from being pulled out of the soil
• leaf

  • the main vascular bundle is in the centre, called the midrib, as well as transport it also provides support to the leaf
  • the xylem is at the upper part of the vascular bundle and the phloem is at the lower part
  • photosynthesis mainly takes place in the palisade mesophyll which is in the upper half of the leaf

  
• stem

  the vascular bundles are arranged in rings

  • the centre of the plant cell is called the pith
  • around the edge is the epidermis and the cortex
  • the xylem vessels are found closer to the centre and the phloem vessels are found closer to the edge. because the vascular bundles are around the edge it helps the stem withstand bending

  
• xylem consists of 2 main types of tissue

  • xylem vessel
  • xylem fibres
• • xylem vessels start as a series of plant cells running up the stem from the roots to the leaves.
  • at a certain point the carbohydrate lignin forms within the cell walls. lignin is impermeable and prevents substances from passing through the cell wall so the living contents of the cells die and the end walls between the cells break down
  • regions of the cell wall remain free of lignin, called pits, and allow water and dissolved substances to pass between vessels
  • xylem vessels consist of non living, hollow tubes and the function of xylem vessels is to carry water and dissolved minerals
• if a xylem vessel becomes blocked then water can move through the pits to a different xylem vessel. the pits all allow water to move out of the xylem (e.g. to cells in the leaves)
• lignin

  • lignin can be arranged in spirals or rings
  • can be continuous apart from the pits
  • helps to support the structure of the xylem vessel
  • when water is pulled up the xylem it causes the pressure in the vessels to fall slightly and the lignin in the vessel wall helps to prevent the vessels from collapsing
• xylem fibres are formed from long narrow cells and lots of lignin forms in these cells and the interior contents of the cells die

  • xylem fibres are not used to transport water instead they provide mechanical support for the plant
• xylem vessels and xylem fibres are both non living tissues
• the xylem contain parenchyma cells and in the xylem the parenchyma cells act as a store of starch

  also contain tannins which are bitter compounds which deter herbivores from eating the plant
• in the leaves the plant carries out photosynthesis which produces glucose and the glucose is used to form different sugars and amino acids (assimilates)
• the job of the phloem is to transport the assimilates from the leaves to other parts of the plant (e.g. roots or flowers)

  • molecules transported in the phloem can be transported either up or down the phloem
  • the fluid moving in the phloem is called the phloem sap
  • phloem is a living tissue
• phloem consists of 2 different types of tissue

  • sieve tube element
  • companion cells
• sieve tube element

  • consists of a long line of cells arranged end to end and inside these cells almost all organelles have been lost (including the nucleus and vacuole) this leaves the interior entirely free to transport phloem sap.
  • the end walls have been modified to contain large pores (sieve plate). the sieve plates allow the phloem sap to move between the cells.
  • as they have lots most of their organelles they cannot produce large amounts of essential molecules such as ATP
• companion cells

  • contain a nucleus and large amounts of mitochondria
  • microscopic channels link the companion cells to the sieve tube element cells (plasmodesmata) and molecules such as ATP and proteins can move through the plasmodesmata into the sieve tube element cells
  • role of companion cells is to provide essential molecules to the sieve tube element cells
• phloem tubes do not contain lignin in the cell walls but phloem contain two types of tissue which provide support

  • fibres
  • sclereids

  both of these tissues have thickened cell walls containing lignin

  • fibres are long and narrow whereas sclereids have a variety of shapes
• roots are covered with very fine root hairs which grow from cells in the epidermis (outer later)

  • water moves into the root hair cells by osmosis and root hair cells are adapted so that osmosis takes places rapidly
  • the packed root hairs increase the surface area to volume ratio of the root
  • the surface of the root hair consists only of the cell membrane and the cell wall making the surface extremely thin increasing the rate of osmosis
• • soil contains mineral ions and the concentration of mineral ions is lower in the soil than in the root hair so root hair cells use active transport to move these mineral ions into the cell.
  • the root hair cells contains other compounds such as sugars so the water potential inside the root hair cell is lower than in the soil so water moves into the root hair cell by osmosis down the water potential gradient
  • the water must now move from the root hair cells through the cortex to the xylem and water can move through the cortex by 2 pathways (symplast pathway and apoplast pathway)
• symplast pathway

  water moves from the cytoplasm in one cell to the cytoplasm of an adjacent cell. water moves through the plasmodesmata (microscopic channels through the cell wall connecting the cytoplasm of cells)

  the symplast pathway is driven by the water potential gradient between the root hair cells and the xylem.

  • water continuously moves into the root hair cells by osmosis so makes the water potential of the root hair cells greater then the cortex cells
  • in the xylem the water potential is low, so water moves by osmosis across the cortex down a water potential gradient
• symplast pathway is relatively slow because the pathway for water in the cytoplasm is obstructed by the organelles
• apoplast pathway

  water moves through the cell walls and the spaces between the cells. the cellulose cell walls have a relatively open structure allowing water to move easily between the cellulose fibres

  • water molecules are attracted to each other, called cohesion, and this is due water molecules forming hydrogen bonds with each other
  • as water moves into the xylem and is carried away more water moves by the apoplast pathway due to cohesion
  • offers much less resistance too water flow than the symplast pathway
• before the water can pass into the xylem is must pass through a layer of endodermis cells, and a band of waterproof material called suberin runs around the cell wall (casparian strip)

  • because of this water can no longer move trough the apoplast pathway so passes through the cell membrane into the cytoplasm becoming part of the symplast pathway
  • by having all water pass through the cytoplasm this allows the cell membrane to control which substances enter the xylem
• • cells in the endodermis use active transport to pump mineral ions into the xylem lowering the water potential which triggers water to move into the xylem vessels by osmosis (root pressure)
  • root pressure is an active process requiring respiration
  • if we inhibit respiration by using a metabolic poison then root pressure will stop
  • it also stops if we prevent aerobic respiration by excluding oxygen
• • main site of photosynthesis is the palisade mesophyll
  • surface of the leaf is covered in a waterproof layer called the waxy cuticle and the role if this is to reduce water loss from the surface of the leaf by evaporation
  • water is a reactant for photosynthesis and so is carbon dioxide, carbon dioxide diffuses into the leaf from the external air
  • photosynthesis produces the gas oxygen which diffuses out of the leaf
  • thousand of pores on the lower surface (stomata) and when the plant photosynthesises the stomata open and allow carbon dioxide to diffuse into the leaf and oxygen to diffuse out

  
• surface of cells within the leaf are covered with a thin layer of water, water evaporates from the surface of cells so the internal leaf spaces contain a high concentration of water vapour

  • when the stomata are open the water vapour diffuses out of the leaf to the external air
  • evaporation of water followed by diffusion of water vapour is called transpiration
• because of the constant evaporation of water from the cells surface the water potential of the cells in the leaf decreases so this causes water to move by osmosis from adjacent cells which now lowers the water potential of these cells. this eventually reaches the xylem with water passing out of the xylem to the adjacent cells

  • so when transpiration is taking place water is being pulled out of the vessels (tension)
• the movement of water from the roots, up the xylem and out of the leaf is called the transpiration stream
• water molecules form hydrogen bonds to each other (cohesion)

  water can also form hydrogen bonds to molecules in the xylem vessel walls e.g. carbohydrates (adhesion)

  • water can move up thin tubes against the force of gravity (capillary action)
  • so when water moves up the xylem vessels and out to the leaves more water then moves up the xylem vessels by capillary action to take its place (transpiration pull)
• combined effect of the transpiration pull, cohesion and adhesion is that water is drawn into the roots, up the stem and passes out of the leaves - this process is called the cohesion tension theory
• cohesion tension theory evidence

  • if a plant cell is cut air is sucked into the xylem suggesting the xylem vessels are under tension. the air prevents cohesion between the water molecules so water movement stops
  • diameter of the tree trunk, reduces when transpiration is at its maximum supports the idea that transpiration pull generates a negative pressure in the xylem