BIOLOGY HL: Topic 9; Plant Science

Monocots:

• parallel venation in leaves
• flowers in multiples of 3
• one cotyledon (e.g. Maize)

Dicots:

• net-like venation in leaves
• flowers in multiples of 4 or 5
• 2 cotlyedons (e.g. hibiscous) 

Upper Epidermis

• Function:  Main function is water conservation (secretes cuticle to create a waxy outer boundary)
• Distribution:  On top of leaves where light intensity and heat are greatest

Palisade Mesophyll

• Function:  Main photosynthetic tissue (cells contains many chloroplasts)
• Distribution:  Upper half of leaf where light intensity is greatest (upper epidermal cells are transparent)

Spongy Mesophyll

• Function:  Main site of gas exchange (made of loosely packed cells with spaces)
• Distribution:  Lower half of leaf, near the stomatal pores (where gases and water are exchanged with the atmosphere) 

Vascular Tissue

• Function:  Transport of water (xylem) and the products of photosynthesis (phloem)
• Distribution:  Found in middle of leaf (allowing all cells optimal access)

Leaf: 

tendrils: coil and support
reproductive leaves: produce tiny plants and fall to the ground
bracts/floral leaves: attract insects for pollination
spines: reduce water loss

Stem:

 Bulbs: food storage
Tubers: horizontal, food store
rhizomes: below surface, spreading and stability
Stolons: asexual production, above the ground

Root:

 prop: thick, brace the plant
storage: specialised cells that store H20 and carbohydr8s 

Apical: 

A root and a shoot tip
The area where growth of plant occurs due to mitosis

Lateral:

Cambium in vascular bundles, eventually grows together forming rings, one new ring every year (just like in trees) 

• A meristem is a tissue in a plant consisting of undifferentiated cells (meristematic tissue) and are found in zones where growth can take place
• Meristematic cells are analogous to stem cells in animals, however have specific regions of growth and development (unlike stem cells)

Apical: 

A root and a shoot tip
The area where growth of plant occurs due to mitosis

Lateral:

Cambium in vascular bundles, eventually grows together forming rings, one new ring every year (just like in trees) 

• Both are composed of totipotent cells (able to divide and differentiate)
• Both are found in dicotyledonous plants

Differences:

Apical:
at root and shoot tips,
increases in length responsible for primary growth develops into primary xylem and phloem produces new leaves and flowers  Lateral occurs at cambium increases in width responsible for secondary grwoth produces secondary xylem and phloem produces the bark on trees

• Phototropism is the growing or turning of an organism in response to a unidirectional light source
• Auxins (e.g. IAA) are plant hormones that are produced by the tip of a shoot and mediate phototropism
• Auxin makes cells enlarge or grow and, in the shoot, are eradicated by light
• The accumulation of auxin on the shaded side of a plant causes this side only to lengthen, resulting in the shoot bending towards the light
• Auxin causes cell elongation by activating proton pumps that expel H⁺ ions from the cytoplasm to the cell wall
• The resultant decrease in pH within the cell wall causes cellulose fibres to loosen (by breaking the bonds that hold them together)
• This makes the cell wall flexible and capable of stretching when water influx promotes cell turgor
• Auxin can also alter gene expression to promote cell growth (via the upregulation of expansins)

Bryophyte: Mosses
Filicinophyte:Ferns
Coniferophyte:Conifers
Angiospermophyte: Flowering plants

can be split into:

monocotyledons and dicotyledons

• Plants take up water and essential minerals via their roots and thus need a maximal surface area in order to optimise this uptake
• The monocotyledon root has a fibrous, highly branching structure which increases surface area for maximal absorption
• The dicotyledon root has a main tap root which can penetrate deeply into the soil to access deeper reservoirs of water and minerals, as well as lateral branches to maximise surface area
• The root epidermis may have extensions called root hairs which further increase surface area for mineral and water absorption
• These root hairs have carrier proteins and ion pumps in their plasma membrance, and many mitochondria within the cytoplasm, to aid active transport

Transport in the Root System

Minerals move into the root system via the following pathways:

• Diffusion:  Movement of minerals along a concentration gradient
• Mass Flow:  Uptake of mineral ions by means of a hydrostatic pressure gradient
  • Water being taken into roots via osmosis creates a negative hydrostatic pressure in the soil
  • Minerals form hydrogen bonds with water molecules and are dragged to the root, concentrating them for absorption
• Fungal Hyphae:  Absorb minerals from the soil and exchange with sugars from the plant (mutualism)

• Minerals that need to be taken up from the soil include K⁺, Na⁺, Ca²⁺, NH₄⁺, PO₄^3- and NO^3-
• Fertile soil invariably contains negatively charged clay particles to which positively charged minerals may attach
• Root cells contain proton pumps that actively pump H⁺ ions into the surrounding soil, which displaces the positively charged minerals allowing for their absorption (the negatively charged minerals may bind to the H⁺ ions and be reabsorbed with the proton)

Three ways by which terrestrial plants may support themselves are:

• Thickened cellulose:  Thickening of the cell wall provides extra structural support
• Cell turgor:  Increased hydrostatic pressure within the cell exerts pressure on the cell wall, making cells turgid
• Lignified xylem:  Xylem vessels run the length of the stem and branches, lignification of these vessels provides extra support

Transpiration is the loss of water vapour from the leaves and stems of plants

• Some of the light energy absorbed by leaves changes into heat, converting water in the spongy mesophyll into vapour
• This vapour diffuses out of the stomata and is evaporated, creating a negative pressure gradient in the leaf 
• New water is drawn from the xylem (mass flow), which is replaced by water from the roots (enters from soil via osmosis)
• The flow of water through the xylem from the roots to the leaf is called the transpiration stream
• Water rises through xylem vessels because of two qualities:
  • Cohesion:  Water molecules are weakly attracted to each other via hydrogen bonds
  • Adhesion:  Water molecules form hydrogen bonds with the xylem cell wall
• These properties create a suction effect (or transpiration pull) in the xylem
• The xylem has a specialised structure to facilitate transpiration:
  • The inner lining is composed of dead cells that have fused to create a continuous tube
  • These cells lack a cell membrane, allowing water to enter the xylem freely
  • The outer layer is perforated (contains pores), allowing water to move out of the xylem into the leaves
  • The outer cell wall contains annular lignin rings which strengthens the xylem against the tension created by the transpiration stream

• The transpiration pull is generated by the negative hydrostatic pressure created by the evaporation of water vapor from the leaf
• Guard cells line stomata and regulate transpiration by controlling how much water vapor can exit the leaf
• When stomata are open the rate of transpiration will be higher than when they are closed

• When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
• Abscisic acid triggers the efflux of potassium from guard cells, decreasing the water pressure within these cells and making them flaccid
• This causes the stomatal pore to close

Light  
Increasing the intensity of light increases the rate of transpiration
Light stimulates the opening of stomata (gas exchange required for photosynthesis to occur) 
Some of the light energy absorbed by leaves is converted into heat, which increases the rate of water evaporation

Temperature  
Increasing the temperature increases the rate of transpiration
Higher temperatures cause an increase in water vaporisation in the spongy mesophyll and an increase in evaporation from the surface of the leaf
This leads to an increase in the diffusion of water vapour out of the leaf (via the stomata) which increases the rate of transpiration

Wind
Greater air flow around the surface of the leaf increases the rate of transpiration
 Wind removes water vapour (lower concentration of vapour on leaf surface), increasing the rate of diffusion from within the spongy mesophyll

Humidity

• Increasing the humidity decreases the rate of transpiration
• Humidity is water vapour in the air, thus a high humidity means there is a high concentration of water vapour in the air
• This reduces the rate of diffusion of water vapour from inside the leaf (concentration gradient is smaller resulting in less net flow)

Xerophytes are plants that can tolerate dry conditions (such as deserts and high altitudes) due to a number of specialised adaptations:

• Reduced leaves:  Reducing the surface area of the leaf will reduce the area for water loss and thus reduce transpiration
• Rolled leaves:  Rolling up leaves (lower epidermis inside) reduces exposure of stomata to air and thus reduces transpiration
• Thick waxy cuticle:  A thickened cuticle prevents water loss from the surface of the leaf and thus reduces transpiration
• Stomata in pits:  Having stomata in pits, surrounded by hairs, concentrates water vapour near the stomata, reducing the rate of transpiration
• Low growth:  Plants located near the ground are less exposed to wind and may be shaded, reducing the rate of transpiration
• C₄ / CAM physiology:  Plants with C₄ or CAM physiology require less amounts of CO₂, meaning stomata can stay closed for longer 

• Organic molecules (sugars, amino acids) move from their source (photosynthetic tissue or storage organs) into a tube system called the phloem
• Sugars are transported as sucrose (because it is soluble but metabolically inert) in the fluid of the phloem (called the sap)
• They are actively loaded into the phloem by companion cells, creating a high concentration which draws water from the xylem via osmosis
• The sap volume and pressure consequently increase to create mass flow which drives the sap along the phloem
• The organic molecules are actively unloaded by companion cells and stored in the sink (fruits, seeds, roots)
• Sucrose is stored as starch (insoluble), while the water in the phloem is released (now that solute concentration is low) and returned to the xylem

Pollination:  The transfer of pollen grains from the anther to the stigma (usually of another plant), often facilitated by animals, wind or water movement

Fertilisation:  Fusion of the male gamete nuclei (in the pollen grain) with the female gamete (in the ovule) to form a zygote

Seed Dispersal:  Fertilised ovules form seeds which move away from the parental plant before germination, reducing competition for resources

• There are a variety of seed dispersal mechanisms, including fruit, wind, water and animals

Germination is the process by which a seed emerges from a period of dormancy and starts to sprout

For germination to occur, a seed requires a combination of:

• Oxygen:  For aerobic respiration (need ATP in order to grow)
• Water:  To metabolically activate the cells
• Temperature:  For the optimal function of enzymes

In addition, particular seed species may require other specialised conditions, such as:

• Fire            • Light or darkness            • Freezing            • Prior animal digestion            • Erosion of the seed coat            • Washing (to remove inhibitors)

• The first step in the germination process is the absorption of water, which causes gibberellin - or gibberellic acid (GA) - to be produced
• Gibberellin causes the synthesis of amylase, which breaks down starch into maltose
• Maltose is transported to the embryo, where it is either hydrolysed to glucose (for energy) or polymerised to cellulose (for cell wall formation)
• Stored proteins and lipids will also be hydrolysed by the addition of water to form enzymes, triglycerides and phospholipids
• Germination uses the food stored in cotyledons as an energy source until the developing shoot reaches the light and can begin to photosynthesise

• Flowering is controlled by phytochrome, which is affected by light (photoperiodicity)
• Phytochrome exists in two forms:
  •  A red (P_r) form absorbs red light (~660 nm) and is converted into a far red form (P_fr)
  • A far red (P_fr) form absorbs far red light (~730 nm) and is converted into a red form (P_r)...

• The P_fr form is the active form of phytochrome, while the P_r form is the inactive form of phytochrome
• Sunlight contains more red light, so the P_fr form is predominant during the day, with the gradual reversion to the P_r form occurring at night
• In long day plants, the active P_fr form is a promoter of flowering and so flowering is induced when the night period is less than a critical length and P_fr levels are high
• In short day plants, the active P_fr form is an inhibitor of flowering and so flowering is induced when the night period is greater than a critical length and P_fr levels are low

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