Chapter 36

By Tom Tessitore, Hannah Turk, and Allie Duca

-36.1: Land plants acquire resources both above and below ground

• Live above ground and below ground• Algae absorbs CO2, water, and minerals from water• Competition for light, water, and nutrients• Taller plants have advantage• Natural selection favored the production of multicellular,

branching roots• Natural selection favors plants capable of transporting

nutrients, water, and minerals over a long distance• Plant success depends on photosynthesis• Many mechanisms have been developed to acquire light,

CO2, and water

• Arrangement and size of leaves on a stem = phyllotaxy• Determined by the shoot apical meristem and specific

species• Most angiosperms have alternate phyllotaxy (spiral

around stem)• Each leaf max light and reduces shading by other leaves

(lower)• Features reduce self-shading increase light absorbed• Leaf area index: the ratio of the total upper leaf surface

of a single plant divided by the surface area of the land it’s growing on

• Higher than 7 is not helpful Add more leaves means respire not photosynthesize Programmed cell death, leaves and branches shed =

self-pruning Leaf orientation affects light capture

• Variation in shoot architecture because plants have finite amount of energy for it

• Allows to grow tall and prevent shading by other plants• Natural selection shoots to optimize light absorption• Vary in stem thickness• The stronger the plant, the stronger the root system• Taproot = anchored tall plant• Fibrous root systems do not anchor tall as strongly• Mycorrhizae = specialized mutualistic assosiations

between roots and fungi• The elongation, branching, and mycorrhizae help roots

obtain water and minerals

-36.2: Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow

• Diffusion and active transport of solutes• Diffusion is the spontaneous movement down

concentration gradients• Diffusion if facilitated by aquaporins• Transport proteins help diffusion across membranes• Proton pumps generate an H+ gradient used to transport

solutes• Diffusion of water (osmosis)• Osmosis = spontaneous movement of free water down

its concentration gradient• Water flows across membranes from regions with higher

water potential to regions with lower water potential

• Solutes decrease• When plants lose water the become flaccid• Greater solute becomes turgid• Wilts with loss of water• Protoplast shrinks and pulls away from cell wall =

plasmolysis• External solution has lower water potential and water

diffuses out of cell

• 3 major pathways of transport:• the apoplastic-moving in the spaces between cells• Symplastic-moving through the cells themselves• transmembrane routes-a combination of both apoplastic

and symplastic routes cytoplasmic channels = plasmodesmata bulk flow due to pressure differences at opposite ends

36.3- Water and minerals transported from roots to shoots

• Absorption of water and minerals by root cells• most of the water and mineral absorption occurs at the

roots• epidermal cells are permeable to water• root hairs are a type of epidermal cells that account for

much of the absorption of water by roots• the soil solution the root hairs absorb consists of water

and mineral ions• although soil solution usually has a low mineral

concentration, active transport helps roots obtain essential minerals

• Transport of water and minerals into the Xylem• Water cannot be transported to the rest of the plant until

they enter the xylem of the stele, or vascular cylinder.• Endodermis- the innermost layer of cells in the root

cortex, surrounds the stele and functions as the last checkpoint for the passage of minerals into the vascular tissue.

• Casparian Strip- a “dead-end” that blocks minerals from reaching the stele. It forces water and minerals that are passing moving to cross the plasma membrane.

• Bulk Flow Driven by Negative Pressure in the Xylem• Transpiration lowers water potential in the leaf by

producing negative pressure (tension)• Low water potential draws from the xylem• Xylem Sap- the water and dissolved minerals in the

xylem• At night- root cells continue pumping mineral ions into

the xylem of the stele.• Root pressure- a push of xylem sap.• Guttation- water pushed out of leaves; happens when

more water enter the leaves than is transpired.

• Material can be moved:• Upward by positive pressure from below• Downward by Negative Pressure above.• Transpirational pull depends on • Adhesion• Cohesion• Surface Tension• Cohesion and adhesion help transportation by bulk flow• Cohesion is due to hydrogen bonding- can pull xylem

sap without the water breaking• Adhesion is also due to hydrogen bonds- bonds to

hydrophilic cell walls to slow the force of gravity.

• Xylem sap ascent by bulk flow: a review• Transpiration maintains movement of xylem sap against

gravity• The water potential gradient is essentially a pressure

gradient.• Plants do not use energy to lift xylem by bulk flow.

36.4- Stomata help regulate the rate of transpiration.

• Plants control loss of water from their leaves using stomata• Stomata open and close using guard cells• Potassium ions are transported through the plasma and vascular

membranes and causes the guard cells to become more turgid when the water enters

• when it becomes more turgid, it opens• when the potassium ions leave, it closes• generally stomata open during the night and close during the day• Plants know that it is dawn because:• 1. Light• 2. Carbon Dioxide• 3. an internal clock• some plants called xerophytes are adapted to regions of little

moisture. • CAM plants photosynthesize differently than other plants.

36.5-Transportation of Sugars

– Transpiration only pulls water from the roots up• Instead, sugar moves from a sugar source to a

sugar sink– Sugar source-a plant organ that makes sugar

(Ex:a fully developed leaf)– Sugar sink-organ that consumes sugar

(Ex:growing roots, fruit)• Sugar sinks use nearest sugar source• Sugar moves both apoplasticly and symplasticly into

the phloem• Building up of sap at a sugar source gives it the

pressure to move quickly to sugar sinks

36.6-The Dynamic Symplast

– Plasmodesmata were once thought to be rigid– In fact, they are actually fluid and changing

• Plants are able to dilate plasmodesmata to allow proteins to pass through

• Viruses make proteins that cause the plasmodesmata to open, allowing the viruses to move from cell to cell

– The symplast allows proteins and RNA molecules to coordianate development between cells

• It also allows electric signals to pass through it, triggering different systems

– These include change in gene transcription, respiration, photosynthesis, and hormone levels

– This allows the phloem to act as a pseudo-nerve