🏥Chapter 12-Nutrition and Transport in Flowering Plants

• upper and lower epidermis

  • they are made up of a layer of closely packed cells that protect the inner parts of the leaf
  • upper epidermis covered by transparent, waxy cuticle
• cuticle

  • outermost layer of the leaf
  • transparent to allow light to pass through and reach plant cells on the inside of the leaf
  • waterproof to ensure water doesn't leave the leaf too quickly, and gives the leaf its waxy texture
• mesophyll

  • lies between upper and lower epidermis
  • main site of photosynthesis
• palisade mesophyll

  • consists of 1 or 2 layers of closely packed, long and cylindrical cells
  • contains the largest number of chloroplast in a leaf since more light can be absorbed at the upper surface of the leaf
  • specialised for photosynthesis
• spongy mesophyll

  • contains cells of irregular shape
  • has numerous large intercellular air spaces among loosely packed cells to allow oxygen and carbon dioxide to diffuse quickly throughout the leaf
  • carries out photosynthesis but has less chloroplasts than palisade mesophyll
  • covered in a thin film of moisture
  • contains xylem and phloem which bundle together to form vascular bundle
• stomata

  • minute openings in the lower epidermis
  • allows gaseous exchange to occur
  • each stoma surrounded by a pair of guard cells
• guard cells

  • come in pairs and surrounds the stoma to control the opening and closing of stoma
  • contain chloroplast and can carry out photosynthesis
  • open stomata in the day time in the presence of light for photosynthesis to occur, and close them at night
• xylem and phloem

  • xylem transports mineral salts and water to mesophyll cells
  • phloem transports sucrose away from leaf
• chloroplast

  • contain chlorophyll to absorb energy from light and convert it to chemical stores of energy in glucose molecules
• path of carbon dioxide into leaf

  1. in the daytime , during photosynthesis, carbon dioxide is rapidly used up. the carbon dioxide concentration in the leaf is now lower than that in the atmosphere, hence carbon dioxide diffuses from surrounding air through the stomata into the intercellular air spaces in the leaf
  2. the thin film of moisture in spongy mesophyll cells allow carbon dioxide to dissolve into it
  3. dissolved carbon dioxide diffuses into the cells

  
• path of water and mineral salts to the leaf

  1. xylem transports water and dissolved mineral salts from roots to the leaf (elaborated later in chapter)
  2. once out of xylem veins , water and mineral salts move from cell to cell right through the mesophyll of the leaf

  
• xylem

  • made of many dead cells which form a hollow tube stretching from the root to the leaf
  • impermeable to water
  • inner walls of xylem vessels are strengthened by lignin
  • hence xylem provides mechanical support to the plant
  • unlike phloem, xylem does not contain cytoplasm
• phloem


  • consists of sieve tubes
  • each sieve tube consists of a column of elongated, thin-walled living cells called sieve tube elements
  • there are "cross-walls" separating the cell called sieve plates
  • mature sieve tube cell has only a thin layer of cytoplasm that is connected to the cells above and below the holes in the sieve plates, allowing for rapid flow of manufactured food substances

  
• companion cells


  • narrow thin-walled cells with mitochondria, nucleus and cytoplasm
  • mitochondria provides energy for companion cells to load sugar from mesophyll cells into sieve tubes for active transport
  • carries out metabolic activities to keep the sieve tube cell alive
  • provides sieve tube cell with nutrients
• vascular tissues in stems

  • xylem located closer on the inside, and the cambium separates the xylem and the phloem
  • epidermis is protected by waxy, waterproof cuticle
  • acronym "XP" reading from inside to outside

  A) phloem

  B) cambium

  C) xylem

  D) epidermis

  
• vascular tissues in leaves


  • xylem usually found on upper surface, phloem on lower surface of leaf
  • found in spongy mesophyll

  
• vascular tissues in roots


  • xylem closer on the inside
  • each root hair is a long and narrow extension growing out of an epidermal cell. this increases surface area-to-volume ratio of the root hair cell
  • rate of absorption of water and dissolved mineral salts is increased due to root hair cells

  
• autotrophs

  • primary producers who synthesis food by transforming light energy into chemical potential energy via photosynthesis
• photosynthesis equations

  chemical : $6CO_2+$$6H_2O$ $\overrightarrow{light+chlorophyll}$ $6O_2+$$C_6H_{12}O_6$

  word: water+ carbon dioxide $\overrightarrow{light+chlorophyll}$ oxygen + glucose
• conditions essential for photosynthesis

  • light
  • carbon dioxide
  • chlorophyll
  • suitable temperature since photosynthesis depends on enzyme reactions in the chloroplasts
• photosynthesis

  • the process whereby carbohydrates (glucose) are synthesised from water and carbon dioxide. oxygen is released in this process
  • chlorophyll absorbs energy from light and transfers it to chemical stores of energy in carbohydrates that was synthesised by water and carbon dioxide
• limiting factors of photosynthesis

  • light intensity
  • concentration of carbon dioxide
  • temperature
• what happens to glucose formed during photosynthesis (I)

  1. glucose is used immediately for cellular respiration for cellular activities and to form cellulose cell walls
  2. in daylight, rate of photosynthesis is so high that glucose is produced faster than it can be removed. excess glucose is converted to starch
  3. in darkness, photosynthesis stops and starch is converted by enzymes back to glucose
• what happens to glucose formed during photosynthesis (II)

  1. glucose is converted to sucrose and is transported to storage organs , and is converted to other forms of storage compounds at storage organs, depending on the plant. found in nectar in flowers and attracts insects for pollination
  2. glucose is converted to amino acids used to form protein for the synthesis of new protoplasm in the leaves. excess amino acids are transported to other parts of the plant for storage as proteins or for synthesis of new protoplasm
• what happens to glucose formed during photosynthesis (III)


  1. glucose is converted to fat which is used for storage, cellular respiration and synthesis of new protoplasm
• why is photosynthesis important(I)

  
  

  • during photosynthesis, light energy that has been absorbed is transferred to chemical stores of energy in carbohydrate molecules. fat , proteins and other organic compounds can be formed from carbohydrates. All theses substances eventually become food for other organisms thus all organisms directly or indirectly obtain chemical energy from plants
• why is photosynthesis important (II)


  • photosynthesis removes carbon dioxide from the air and produces oxygen. the oxygen is then used for cellular respiration for cellular activities
• translocation


  • the transport of manufactured food substances, such as sucrose and amino acids in the phloem of the plant
• 'ringing' experiment (evidence of translocation)

  • when the phloem of a plant is cut off , the region above the cut part swells up, because food substances from the leaves cannot reach the stem below the cut region, hence accumulates above the cut region

  
• using aphids (evidence of translocation)

  • when aphids feed on plant juices , their stylet penetrates the stem. the aphid can be anaesthetised with carbon dioxide while it is still feeding, and its body can be cut off, leaving the stylet in the plant tissues.
  • a liquid containing sucrose and amino acids with exude from the cut end of the stylet (towards the body)
  • this proves translocation
• using radioactive carbon (evidence of translocation)

  • when a leaf was exposed to carbon dioxide with radioactive carbon ($^{14}CO_2$), the sugars formed contained radioactive carbon. the stem is cut then put under X-ray photographic film and it is found that radioactive material is found in the phloem
• entry of water into a plant(II)


  • entry of water dilutes root hair cell (cell A). Cell A has a higher w.p than cell B, hence water passes from root hair cell to inner cells.
  • similarly, water passes from cell B to C, and this process continues until water reaches xylem vessels and moves up the plant

  
• entry of water into a plant(I)

  • each root hair cells has a narrow extension of an epidermal cell, and it grows between soil particles, coming into close contact with the soil solution surrounding it.
  • the thin film of liquid surrounding each soil particle is a dilute solution of mineral salts
  • the root hair cell's cell sap is a relatively concentrated solution of sugars, hence the root hair cell sap has a lower water potential than the surrounding soil solution. Water enters root hair cell via osmosis

  
• how do root hair cells absorb mineral salts & ions

  • when concentration of ions in soil solution $<$ concentration of ions in root hair cell sap, ions are absorbed via active transport, as ions are being absorbed against a concentration gradient. Energy for this process is from mitochondria found in root hair cell
  • when concentration of ions in soil solution $>$ concentration of ions in root hair cell sap, ions are absorbed via diffusion
• how is root hair cell adapted for absorption

  • has long and narrow extension of root hair cell to increase surface area-to-volume ratio for increased rate of absorption of water and mineral salts
  • has many mitochondria as aerobic respiration of mitochondria release energy for active transport of ions into the cell
  • cell membrane prevents cell sap from leaking out. cell sap contains sucrose, amino acids and mineral salts, thus has a lower water potential than the soil solution, allowing water to enter root hair cell via osmosis
• transpiration

  • loss of water vapour from the aerial parts of the plant (leaves and stem), mainly through the stomata of the leaves
• how does water move up a plant (i.e. against gravity)

  • evaporation of water from the leaves removes water from the xylem, resulting in a suction force which pulls water up the xylem vessels.
  • this suction force is due to transpiration and is called the transpiration pull
• process of transpiration (I)

  • thin film of moisture on spongy mesophyll cells evaporate to form water vapour in intercellular air spaces
  • water vapour diffuses through the stomata to the drier air outside the leaf (transpiration)
  • as water evaporates from the thin film of moisture, more water moves out of mesophyll cells to replenish it
  • as water is lost from mesophyll cells, w.p. of their cell sap decreases and becomes lower than that of neighbouring cells
  • mesophyll cells draw water from cells deeper inside the leaf via osmosis

  
• process of transpiration (II)

  • cells deeper inside the leaf eventually draw water from the xylem via osmosis, which creates a suction force which pulls the whole column of water up the xylem vessels (transpiration pull)
  • as transpiration occurs mainly through stomata, it is linked to gaseous exchange between plant and env. In daylight stomata open to allow carbon dioxide to diffuse into the leaf for photosynthesis, and since oxygen and water vapour are more concentrated in the intercellular air spaces they diffuse out of the leaf through the stomata

  
• importance of transpiration

  • transpiration pull draws water and mineral salts from roots to the stem and the leaves
  • evaporation of water from the surfaces of the leaves cools the plant, preventing it from being scorched by the hot sun
  • water transported to leaves keeps the cells more turgid, to replace water lost by the cells. turgid cells keep the leaves spread out widely to trap light for photosynthesis