Plant Science

                Stem Tissue                                           Leaf Tissue

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 

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) 

Vascular Tissue

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

• A storage organ is a part of a plant specifically modified to store energy (e.g. carbohydrates) or water
• They are usually found underground (better protection from herbivores) and may result from modifications to roots, stems or leaves:
  • Storage roots:  Modified roots that store water or food (e.g. carrots)
  • Stem tubers:  Horizontal underground stems that store carbohydrates (e.g. potato)
  • Bulbs:  Modified leaf bases (may be found as underground vertical shoots) that contain layers called scales (e.g. onion)
• Some plants (called succulents) have modified leaves or stems (thickened, fleshy and wax-covered) to enable water storage (e.g. cacti)
• Other plants (e.g. vines) have modifications to their leaf or stem to enable climbing support and attachment - these are called tendrils

Similarities:

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

Differences:

• Phototropism is the growing or turning of an organism in response to a unidirectional light source
• Auxins are plant hormones  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 

• 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

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)

• Fertile soil invariably contains - charged particles to which + 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)

• This mode of absorption is called indirect active transport - it uses energy (and proton pumps) to establish an electrochemical gradient by which mineral ions may be absorbed via diffusion
• Alternatively, the root cells may absorb mineral ions via direct active transport - using protein pumps to actively translocate ions against their concentration gradient

• Some of the light energy absorbed by leaves changes into heat, converting water in the spongy mesophyll into vapour
• Vapour diffuses out of the stomata, evaporates, creates 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 cohesion and adhesion 
  
• 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, 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

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.