Topic 9 (PLANT SCIENCES) IB Biology HL

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

• 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

• 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

• Seed absorbs water – hydrates

• Embryo produces hormone called giberellic acid

• This hormone is responsible for protein in the seed to become enzymes

• Enzyme for breaking down food (main enzyme amylase) breaks down starch in the food reserves releasing glucose used for cellular respiration, energy is produced and embryo grows and can then photosynthesize

• Seed absorbs water – hydrates

• Embryo produces hormone called giberellic acid

• This hormone is responsible for protein in the seed to become enzymes

• Enzyme for breaking down food (main enzyme amylase) breaks down starch in the food reserves releasing glucose used for cellular respiration, energy is produced and embryo grows and can then photosynthesize

• Seed absorbs water – hydrates

• Embryo produces hormone called giberellic acid

• This hormone is responsible for protein in the seed to become enzymes

• Enzyme for breaking down food (main enzyme amylase) breaks down starch in the food reserves releasing glucose used for cellular respiration, energy is produced and embryo grows and can then photosynthesize