Chapter 12: Nutrition and transport in flowering plants

Lamina/ leaf blade

- large surface compared to its volume

> enables it to obtain the maximum amount of light for photosynthesis

- large, thin leaf blade

> carbon dioxide can rapidly reach the inner cells of the leaf

Network of veins (xylem and phloem -> vascular bundle)

- carry water and mineral salts to the cells in the leaf blade

- carry manufactured food from these cells to other parts of the plant

Leaf arrangement

- always organised around the stem in regular pattern

- in general, grow in pairs- opposite one another or singly in alternate arrangement

> ensures that leaves are not blocking one another from light and each leaf receives sufficient sunlight

Leaf stalk

- provides SUPPORT for the leaf and keep it in an UPRIGHT POSITION for maximum absorption of light energy and gaseous exchange

- grasses, maize, leaf stalk ABSENT

> they have long leaf blades

Upper epidermis

- leaf blade has an upper epidermis made up of a single layer of closely packed cells

- covered on the outside by a waxy and transparent cuticle

> waxy cuticle secreted by upper epidermis

Mesophyll

- MAIN SITE of photosynthesis

- between upper and lower epidermis

- two types of tissue palisade mesophyll & spongy mesophyll

Palisade mesophyll

- consists of one or two layers of closely packed, long, and cylindrical cells

> closely packed: further increase the number of photosynthetic cells available

- numerous chloroplasts

- specialised for photosynthesis

Spongy mesophyll

- contains cells with an irregular shape

- numerous large intercellular air spaces among the loosely packed cells

> for rapid diffusion of CO2, O2 and water vapour inside the leaf cells

> for water plants, this can provide buoyancy for the leaf to float

- carries out photosynthesis but contains FEWER chloroplasts than in palisade mesophyll

- cells covered with a thin film of moisture on surface of spongy mesophyll cells to allow CO2 to dissolve first before diffusing into the cells

- contains transport tissue- xylem and phloem which are grouped together to form a vascular bundle

Lower Epidermis

- consists of a single layer of closely packed cells

Cuticle

- cells are covered by an outer layer of cuticle

> reduces water loss through epidermal cells

Stomatal pore (stoma)

- lower epidermis contains many minute openings called stomata

- for carbon dioxide to diffuse into the stoma and oxygen and water vapour to diffuse out of the stoma during the day

Guard cell

- found in lower epidermis

- a pair surrounds each stoma and HELPS TO REGULATE the rate of transpiration by opening and closing them

- contain chlorophyll

Adaptation for photosynthesis

- waxy cuticle on upper and lower epidermis

- stomata present in the epidermal layers

- chloroplasts containing chlorophyll in all mesophyll cells

- more chloroplasts on upper palisade tissue

- interconnecting system of air spaces in the spongy mesophyll

- veins connecting xylem and phloem situated close to mesophyll cells

- leaf stalk

- broad and thin leaf blade

- thin film of moisture on the surface of the spongy mesophyll cells

- presence of guard cells

Waxy cuticle (F)

- reduces water loss through evaporation from leaf

- transparent for light to enter the leaf

Stomata present in epidermal layers (F)

- stomata opens in the presence of light, allowing more carbon dioxide to diffuse in and oxygen to diffuse out of the leaf

Chloroplasts i containing chlorophyll in all mesophyll cells (F)

- chlorophyll absorbs energy from light, light energy absorbed is used to split water and produce oxygen and hydrogen atoms during LIGHT DEPENDENT STAGE OF PHOTOSYNTHESIS

- the high energy hydrogen atoms are then used in the light independent stage of photosynthesis to reduce carbon dioxide to glucose

More chloroplasts in upper palisade tissue (F)

- More light can be absorbed near the upper leaf surface

Interconnecting system of air spaces in the spongy mesophyll (F)

- air spaces allow rapid diffusion of carbon dioxide and oxygen into and out of mesophyll cells

Veins containing xylem and phloem situated close to mesophyll cells

- xylem transports water and mineral salts to mesophyll cells

- phloem transports sucrose and amino acids away from the leaf

Presence of guard cells (F)

- regulates the opening and closing of the stomata for gaseous exchange (CO2, O2, water vapour) between the surrounding and the leaf cells

Thin film of moisture on the surface of the spongy mesophyll cells (F)

- to allow carbon dioxide to dissolve first before diffusing into the spongy mesophyll cells and into the chloroplasts

How do guard cells control the size of stomata

Day

- Photosynthesis takes place

- Glucose formed during photosynthesis and will release chemical energy through aerobic respiration

- this chemical energy is used up to pump potassium ions into the guard cells via active transport

- concentration of potassium ions increases in the guard cells, decreasing water potential of cell sap in guard cells

- net movement of water molecules from adjacent epidermal cells into guard cells by osmosis

- guard cells swell -> more turgid, causing guard cells to become curved and pull the stoma open WIDER

Night

- potassium ions diffuse out of the guard cells

- water potential of cell sap in guard cells increases -> net movement of water molecules out of guard cells via osmosis

- guard cells become flaccid and stoma closes

Hot and Sunny

- leaf cells, eg guard cells, experience excessive water loss via evaporation

- guard cells become flaccid and stoma closes, preventing further water loss through evaporation and diffusion

Entry of carbon dioxide into leaf

Day

- photosynthesis -> CO2 rapidly used up, together with H2O, as raw materials for photosynthesis

- conc. CO2 in leaf lower than that in the atmospheric air, so a diffusion gradient exists

- CO2 diffuses from surrounding air through the stomata into the intercellular air spaces

- CO2 dissolves into the thin film of moisture on the surface of spongy mesophyll cells and diffuses into cells and diffuses into chloroplasts for photosynthesis

Entry of water and mineral salts into leaf

- xylem transports water and dissolved salts to the leaf from the roots

- once out of the veins, the water and mineral salts move from cell to cell right through the mesophyll of the leaf

> net movement of water molecules out by osmosis

Plant vascular tissues

Xylem vessels

- transports water and dissolved mineral salts from roots to stem and leaves (1 directional)

- provides mechanical support for the plant to prevent the plant from collapsing

phloem

- transports sucrose and amino acids from the leaves to other parts of the plant (both directions)

- vascular tissues separated by a layer of cambium

- cambium cells can divide and differentiate to form new xylem and phloem tissues

Structure of Xylem vessel

- made up of many dead cells fused together at the ends to form a long hollow narrow tube

- continuous lumen, with no cross-walls or protoplasm

- lignin deposits in the inner walls of xylem vessels

> inner walls of xylem are strengthened

adaptations of xylem vessel

Long narrow hollow lumen WITHOUT protoplasm and NO cross-walls

- reduces resistance to water and dissolved mineral salts flowing through the xylem vessel

> enables faster rate of transport of water and dissolved mineral salts up to the lumen of xylem vessel

Inner walls are lignified

- provides the plant with mechanical support, and prevent the plant from collapsing

structure of phloem

Sieve tube elements

- sieve tube cells are elongated cells that lack nuclei and have thin layers of cytoplasm

- sieve tube elements are made up of many sieve tube cells that are joined end to end to form a column with sieve plates in between

- sieve plates are the cross-walls within the sieve tube elements, with many small sieve pores

Companion cells

- narrow, thin-walled cell with cytoplasm, nucleus and contains numerous mitochondria

FUNCTION

- to allow active transport of sucrose and amino acids from neighbouring mesophyll cells into sieve tube elements

- carries out metabolic processes needed to keep the sieve tube cell alive

Adaptations of the phloem

sieve tube elements have very little protoplasm and arranged to form a continuous column

- reduces resistance for faster rate of transport of sucrose and amino acids

presence of pores within sieve plates

- allow faster rate of transport of sucrose and amino acids within the phloem

companion cells have numerous mitochondria

- release more energy for active transport of sucrose and amino acids from mesophyll cells into phloem sieve tube cells

Every phloem sieve tube cell has an associated companion cells

- ensure the survival of sieve tube cell

vascular tissues in stems

1. Within a vascular bundle, xylem is located closer inside. Phloem lies outside the xylem with a tissue called cambium between them.

- cambium cells can divide and differentiate to form new xylem and phloem tissues, giving rise to a thickening of the stem

2. Vascular bundle arranged around the pith

3. Stem covered by a layer of cells called epidermis

- epidermal cells are protected by waxy, waterproof cuticle that greatly reduces evaporation of water from stem

4. Region between vascular bundle and epidermis is the cortex

- both cortex and pith are storage tissues

vascular tissues in leaves

- vascular bundles found along the spongy mesophyll

- xylem closer to upper surface of leaf and phloem closer to the lower surface

vascular bundle in roots

1. Xylem and phloem are not bundled together. They alternate with each other

2. Epidermis of the root is the outermost layer of cells, called the root hair cell layer for absorption of water via osmosis

3. Each root hair is a long and narrow extension growing out of an epidermal cell

- increases surface area to volume ratio of the root hair cell

> rate of absorption of water and mineral salts is increased

entry of water into plant

1. Each root hair grows between soil particles

2. A thin film of dilute solution of mineral salts surrounds each soil particles

3. Water moves into the root hair by osmosis

- cell sap in root hair cell is a concentrated solution of sugars and various salts

> cell sap has lower water potential than the soil solution

4. water moves from the root hair cell to the inner cell by osmosis

5. Water moves cell to cell by osmosis until it reaches xylem vessel

entry of ions or mineral salts

Diffusion

- when conc. of ions in soil solution is lower than that in the root hair cell sap

Active transport

- when concentration of ions in soil solution is lower than that in cell sap

Adaptation of root hair cell

long and narrow extension

- increases surface area to volume ratio of -> increases rate of absorption of water and dissolved mineral salts from soil solution to root hair cells

numerous mitochondria

- releases more energy for faster rate of respiration for active transport of dissolved mineral salts from soil solution to root hair cells

cell membrane present

- prevents cell sap from leaking out

Concentrated cell sap with lower WP than soil

- cell sap contains sugars, amino acids and salt

> lower WP than soil -> water entering the root hair by osmosis, down a WP gradient

Photosynthesis

A process in which light energy is absorbed by chlorophyll and converted into chemical energy. Chemical energy will then be used up to synthesise glucose by using CO2 and H2O as raw materials. O2 released.

Reactions in photosynthesis are enzyme-catalysed

fate of glucose

1. used up immediately for cellular respiration to release energy for its cellular activities or to form cellulose cell walls

- EXCESS glucose -> STARCH, stores temporarily in leaves (DAYLIGHT)

- starch converted back to glucose by enzymes (DARK)

2. converted to SUCROSE

> transported to other parts of plant or to storage organs

> converted to other forms of storage compounds at the storage organs, depending on plant

> MAY be converted back to glucose for respiration

> a component of nectar in flowers which attracts insects

3. reacts/combine with nitrates and mineral salts to form amino acids -> combined to form proteins for synthesis of new protoplasm in the leaf

> excess amino acids are transported to other parts of the plant -> synthesis of new protoplasm or storage as proteins

4. Converted to fats for storage, cellular respiration or synthesis of new protoplasm

translocation

Translocation is the transport of manufactured food substances, such as sugars and amino acids in plants

Translocation studies

1. Ringing experiment

- phloem tissue is removed

- concentration of sucrose and amino acids above the ring increases -> swell up

CONCLUSION: shows that sucrose and amino acids are being translocated within the phloem (sieve tube elements)

2. Radioactive isotopes -> technique: autoradiography

- low-level of radioactive carbon isotopes will emit radioactivity which can be measured

- plants absorb the radioactive carbon isotopes through their leaves

3. Using Aphids

- aphids anaesthetized with CO2 while its feeding

- body of aphids cut off, leaving only its proboscis in the phloem sieve tube

- liquid exuded from cut end of the proboscis

> analysis of liquid shows that it contains sucrose and amino acids

- under microscope -> section of stem shows that proboscis is inserted into the phloem sieve tube CONCLUSION: translocation of sucrose and amino acids occurs in the phloem

transpiration

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves

how does water move up the xylem vessel

Root pressure

- large influx of water via osmosis into roots, causing the stream of water and dissolved minerals to be moved up the cell

Capillary action

- tendency of water to move up inside very narrow tubes

- depends on the forces of cohesion and adhesion with the walls

transpiration pull (MAIN FORCE)

- major suction force caused by transpiration which results in water and dissolved mineral salts to move up the xylem

movement of water through leaf

1. Water that moves out of the mesophyll cells form a thin film of moisture around the cells

2. Water from thin film of moisture evaporates to form water vapour in the air spaces

> higher concentration of water vapour accumulates in the air spaces near the stomata

3. Water vapour diffuses out of the stomata into the environment -> transpiration

4. Movement of water out of the cells to replace the thin film of moisture that has evaporated DECREASES THE CELL SAP'S WP

5. Mesophyll cells absorb water via osmosis from the adjacent cells deeper in the leaf

- these cells absorb water from xylem vessels

6. Hence, results in the production of a suction force that pulls the column of water in the xylem vessels up

- suction force is known as transpiration pull

factors affecting rate of transpiration

wind speed/air movement

- wind speed increases, blows away water vapour accumulating outside stomata -> maintains steeper water vapour concentration gradient between the intercellular air spaces in leaf and atmosphere

temp of air

- temperature increases, HIGHER RATE OF EVAPORATION from surface of leaves

humidity

- humidity increases, saturation of water vapour in the air increases, water vapour concentration gradient between inside leaf and atmosphere becomes less steep, lowers rate of transpiration

light intensity

- light intensity increases, rate of photosynthesis INCREASES, stomata open wider, higher rate of transpiration

wilting

- occurs when there is EXCESSIVE TRANSPIRATION

> rate of water vapour LOSS through transpiration is GREATER THAN rate of water absorption by the root hair cells

advantages of wilting

- leaf folds up during wilting, surface area exposed to light is reduced

> reduced surface area of leaf could reduce the exposure of stomata to the atmosphere and reduces rate of water loss through stomata

- excessive loss of water causes the guard cells to become flaccid and stomata closes -> rate of transpiration is REDUCED

- enables cooling of plant

Disadvantages of wilting

- causes stomata to close

> decreasing diffusion of CO2 -> rate of photosynthesis DECREASES

- leaves exposed surface read REDUCED

> decreases absorption of light energy -> rate of photosynthesis DECREASES

importance of transpiration

- transpiration pull, moves water and dissolved mineral salts up the xylem -> water is a raw material required for photosynthesis

- turgidity of plant is maintained as water, which is lost from the aerial portions the plant os replaced

- evaporation of water helps to cool the plant