Topic 9 (PLANT SCIENCES) IB Biology HL

• 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

Meristems are parts of a plant made up of actively dividing cells. Meristems are responsible for growth of stems and roots. Meristems in dicotyledonous plants include apical meristems (also called primary meristem) and lateral meristems (also called cambium).

Tropism: the growth movement of a plant in response to a stimulus (light, water, gravity)

When the growth movement is in response to light it is called phototropism, in response to water it is hydrotropism, and in response to gravity it is geotropism.

Stem cells grow towards the light so they show positive phototropism. The roots grow away from light so it shows negative phototropism.

Auxin: a group of plant hormones (is responsible for cell division and cell elongation and when these two processes happen, growth occurs)

When there is unidirectional light, the auxin moves to the dark side. The concentration of auxin is high in the dark side compared to the light side. More growth (cell division and cell elongation) occurs on the dark side compared to the light side. The stem bends towards the light as a result.

• Plants take up water and essential minerals via their roots and thus need a maximal surface area in order to optimize 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 maximize 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 membrane, and many mitochondria within the cytoplasm, to aid active transport

1. Diffusion: Movement of minerals along a concentration gradient

2. 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

1. Fungal Hyphae: Absorb minerals from the soil and exchange with sugars from the plant (mutualism)

Require pumps for active transport in cell membrane, ATP and specific carrier proteins

The movement of mineral ions using ATP energy can occur against the concentration gradients, from soil to the roots. Carrier proteins are found in the lipid bilayer which transport the mineral ions into the root. The carrier protein are selective about which ion they take in. Specific proteins are for specific ions. The intake is selective, which means it choses the ions it takes in. The membrane has ATPase (enzyme required to make ATP) which produces ATP. Active uptake of ion increases rate ob absorption because energy is being spent. Due to mass flow ions are transported away so that more ions from outside can be pumped in.

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 vapor from the leaves and stems of plants