IB HL Plant Biology

• Photosynthesis converts light energy from the sun into chemical energy stored in molecules of glucose.
• Transpiration is a process of evaporation through the stomata
• oxygen is always needed for seed germination
• How does auxin contribute to photosynthesis ?

  It increases the growth of cells on the shaded side of the stem
• • Photosynthesis: This biochemical process occurs in the chloroplasts of plant cells, where light energy is converted into glucose, a primary organic compound that serves as a source of energy and structural material.
• Auxin exerts its effect on plants by binding to a receptor resulting in expression of genes
• growth towards light source: auxin is concentrated in the side of the shoot without light and promotes cell elongation
• • Source: These are the areas where organic compounds are produced, such as mature leaves during photosynthesis, where sunlight is abundant.
• • Sink: The sinks are areas where these compounds are consumed, modified, or stored, such as roots, fruits, growing tips, and young leaves.
• Active translocation occurs in the phloem
• Sucrose is actively transported in the phloem
• High concentrations of solutes cause water uptake by osmosis
• • Conversion of Glucose to Sucrose: In source tissues, glucose is often converted into sucrose, a more efficient molecule for transport.
• • Active Transport into Phloem: Using energy in the form of ATP, the plant actively loads sucrose into the sieve tube elements against a concentration gradient.
• • Water Follows by Osmosis: As sucrose concentration increases in the sieve tube, water enters by osmosis, generating turgor pressure that assists in pushing the sap along the tube.
• • Multidirectional Flow: The flow of organic compounds in phloem is not restricted in one direction; it can be upward, downward, or lateral, depending on the plant’s requirements.
• • Plasmodesmata: These are microscopic channels that allow communication and transport between different plant cells, contributing to the overall transportation system.
• • Active or Passive Unloading: Organic compounds can be removed from the phloem actively, with energy expenditure, or passively, depending on the sink’s requirements.
• • Plants may face threats like phloem-feeding insects and bacterial infections, and they've developed mechanisms to seal off damaged sections to prevent loss of sap.
• • Seasonal Changes: Different growth or dormant stages require different flows, and plants have adapted to make these adjustments according to the season.
• Water's incompressibility refers to the characteristic that it cannot be compressed into a smaller volume under normal pressures
• • Transport Mechanism: Sugars move from companion cells into sieve tubes.
• the leaves are the main source where photosynthesis occurs.
• Active transport builds a concentration gradient across the membrane.
• • Secondary Active Transport: Makes use of an energy gradient created by primary active transport.
•  Active transport is the process of moving molecules across a membrane against a concentration gradient.
• Water transport in plants

  • Plants transport water from roots to leaves to replace losses from transpiration
  • Cohesive property of water and xylem structure allow transport under tension
  • Adhesive property of water and evaporation generate tension forces in leaf cell walls
  • Active uptake of mineral ions in roots causes absorption of water by osmosis
• Capillary action


  Combination of adhesive forces causing water to bond to a surface and cohesive forces bonding water molecules together
• Measurement of transpiration rates


  Using potometers
• Structural features of xylem


  • Rings of lignin help withstand very low pressure inside the xylem which drives the transpiration pull
• Loss of water from top of xylem vessels due to evaporation


  Lowers pressure inside vessel and pulls more water due to cohesion
• Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates

  Designing an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates
• Plants have adaptations in deserts and saline soils for water conservation
• Active uptake of mineral ions in roots results in a higher concentration of minerals in plants than in the surrounding soil
• Structure and function correlation in the xylem of plants

  Correlated in the xylem of plants
• Adhesion attracts water molecules to the walls of xylem and vice versa
• Surface tension is caused by cohesive hydrogen bonding resisting an object trying to penetrate the surface
• Transpiration


  • Transpiration is the inevitable consequence of gas exchange in the leaf
• Water moving upwards in xylem vessels generates tension
• Models of water transport in xylem


  • Using simple apparatus like blotting or filter paper, porous pots, and capillary tubing