BIO 308 ~ Exam 1

What is water potential?

• the propensity or potential for water to move from one location to another

  • influenced by various internal variables, including matrix potential, pressure potential, solute potential, etc..

• always negative because of the other solutes that exist within a cell’s cytoplasm

• moves from higher water potential to lower potential (less negative to more negative)

Solute potential

• the amount by which the presence of a solute in pure water reduces the water potential

Pressure potential

• water potential increases when a pressure greater than atmospheric pressure is exerted on the solution

Osmosis

• the movement of water across a selectively permeable membrane from an area of high water potential (low solute concentration) to an area of lower water potential (higher solute concentration) → more positive to more negative

  • hypertonic: water leaves the cell

  • hypotonic: water enters

  • isotonic: equal

Turgor pressure

• the pressure of water pushing the cell membrane against the cell wall

  • increase in pressure = plant cells firm and support the plant

  • decrease in pressure = plants wilt (not firm)

Water movement: Soil-Plant-Atmosphere Continuum (SPAC)

• the continuous movement of water from the soil, through the plant, and into the atmosphere

  • the plant is the pathway that carries water upward

1.) Soil → roots

• water starts in the soil

  • water moves into the roots because of differences in water potential (high → low)

    • happens through osmosis

2.) Roots → xylem

• once water enters the roots, it eventually reaches the xyem

  • xylem = specialized plant tissue that transports water and dissolved minerals (nutrients) from the roots upward through the plant (water pipes)

    • carries water from the roots → stem → leaves

3.) Xylem → leaves

• water continues moving upward through the xylem toward the leaves

  • major reason why this happens: transpiration

    • transpiration = the loss of water vapor from a plant, mainly through the stomata

• when water evaporates from the leaves, it creates a pull that helps draw more water upward through the xylem

Xylem

• plant tissue that transports water and minerals from the roots upward to the leaves

  • system of tiny pipes running through the plant

    • soil → roots → xylem → stem → leaves → atmosphere

Main water-conducting cells: tracheids

• long, narrow cells that transport water through small openings in their walls

Main water-conducting cells: vessel elements

• wider cells that connect end-to-end to form larger tubes called vessels

How does water move through the xylem? - water potential

• water moves through xylem because there’s a water potential gradient

  • high water potential → low water potential

How does water move through the xylem? - transpiration

• (1) water evaporates from the leaves, (2) creating tension (negative pressure) in the xylem, (3) because water molecules stick together (cohesion) water gets pulled upward, (4) water is pulled from the roots and eventually from the soil

  • water evaporates from leaf → pulls water upward → xylem brings more water from the roots

What is cavitation?

• happens when the tension in the xylem becomes so strong that water changes from a continuous liquid column and a gas bubble forms

  • this can happen when the plant becomes very dehydrated

    • water forming inside a water pipe

What is embolism?

• the gas-filled blockage in the xylem that results from cavitation

  • cavitation = gas bubble forming

  • embolism = gas bubble blockage, prevents normal water transport

What is embolism resistance?

• the plant’s ability to prevent or tolerate/block the spread of gas bubbles in its xylem

  • some plants have xylem structures that make them more resistant to embolism → smaller/narrower conduits (water pipes)

    • drought → stronger tension in xylem → cavitation → gas bubble → embolism → reduced water transport

Aquaporin

• tiny water channels in a cell membrane

  • outside cell → aquaporin → inside cell

• allows water to cross the membrane much faster than water could through the membrane by itself

Aquaporin selectivity

• the aquaporin chooses what can pass through

  • mainly selective for water molecules

    • H2O is a yes; ions, large molecules, charged particles are a NO

• extremely narrow, specific chemical properties that help water pass while excluding ions (positively charged regions of the protein channel)

• plant can move water across cell membranes without allowing other ions to freely cross at the same time

Aquaporin gating

• can open/close → drought, water availability, pH, and other cellular signals

Aquaporin gating: PIP (Plasma Membrane Intrinsic Proteins)

• PIP - Plasma Membrane Intrinsic Proteins: located in the plasma membrane (surrounds the cell)

  • outside cell → plasma membrane (PIP) → inside cell

• one cell to another, soil/root area into root cells, through different tissues of the plant

Aquaporin gating: TIP (Tonoplast Intrinsic Proteins)

• TIP - Tonoplast Intrinsic Proteins: membrane that surrounds the vacuole, moves water between the cytoplasm and the vacuole

  • vacuole: large compartment that can store water and other substances

• contributes to turgor pressure

Soil texture

• the relative amounts of sand, silt, and clay in the soil

Soil texture - Sand

• largest particles, large spaces between particles, water drains quickly

Soil texture - Silt

• medium sized particles, holds more water than sand, moderate drainage

Soil texture - Clay

• smallest particles, very tiny spaces between particles, holds water tightly

Soil water availability

• how much of the water in soil is actually accessible to plants

  • not all water in soil can be used by a plant, some water is held too tightly by soil particles for roots to pull it away

Sand vs. clay

• sand = water drains easily (less water remains)

• clay = lots of water remains (some held too right for plants to extract)

  • loam (sand, silt, and clay) = good balance between water retention and availability

Water-holding capacity

• the amount of water soil can retain after excess water drains away

  • sandy soil → water drains quickly = low water-holding capacity

  • clay soil → water is retained = high water-holding capacity

    • DOES NOT automatically mean high water availability

      • soil texture = determines how much water soil can hold, this affects how much water is available to plants

How can water transport?

• 3 different paths for water to take from the soil into the xylem

How can water transport? ~ Apoplastic pathway (around the cells)

• water moves through the cell walls and the spaces between cells

  • does not cross cell membrane

  • fast pathway because it can move through continuous cell-wall spaces

  • the casparian strip eventually blocks this pathway at the endodermis, forcing water to cross a cell membrane

    • soil → cell wall → cell wall → xylem

How can water transport? ~ Symplastic pathway

• water enters a cell and then moves through the cytoplasm of neighboring cells

• cells are connected by tiny channels called plasmodesmata

  • water crosses a cell membrane when it first enters the symplast

  • can move cel to cell through the plasmodesmata

    • soil → cell → cell → cell → xylem

How can water transport? ~ Across membranes

• water repeatedly moves across cell membranes as it travels through the root

  • cell → membrane → cell → membrane → cell

    • aquaporins can help water cross these membranes

    • this pathway gives the plant more control over water movement

Casparian strip

• waterproof barrier in the endodermis of the root

  • its job is to force water and dissolved substances to cross a cell membrane before entering the xylem

    • Why → otherwise water could simply travel through cell wall straight into the xylem without being regulated

    • soil → root → endodermis → xylem

      • controls what enters the xylem

        • YOU CAN’T JUST GO AROUND, YOU HAVE TO PASS THROUGH A CELL MEMBRANE

Roots

• how water gets from the soil into the xylem

Root anatomy - epidermis

• outer layer of the root, directly contacts the soil, water enters the root here

• outside → inside

  • soil → epidermis → cortex → endodermis → pericycle → xylem

Root anatomy - Cortex

• located between the epidermis and endodermis, water travels through the cortex toward the center of the root

Root anatomy - endodermis

• a layer of cells surrounding the vascular tissue, contains the casparian strip, acts as the checkpoint controlling what enters the xylem

Root anatomy - xylem

• located toward the center of the root, transports water upward through the plant

Casparian strip

• a waterproof barrier located in the cell walls of endodermal cells

  • blocks the apoplastic pathway

• makes sure that water move through the membrane, regulates what enters the xylem

• soil → cell walls → casparian strip → xylem

Suberin

• water-resistant substance found in plant cell walls

• the casparian strip contains a lot of suberin

  • hydrophobic, makes the cell wall less permeable to water and other substances

• can also be deposited in other root tissues and can make roots less permeable to water

Hydraulic conductivity

• hydraulic conductivity (k) = how easily water can move through a tissue or structure

• the water permiability of the root

  • high hydraulic conductivity = water moves easily

  • low hydraulic conductivity = water movement is restricted

• depends on things such as: aquaporins, cell membranes, cell-wall properties, suberin, root anatomy

• soil → epidermis (water enters) → cortex (water moves inward) → endodermis (casparian strip and suberin) → water crosses membrane → xylem (water moves upward)

Stomata

• tiny pores on the surface of leaves that control gas exchange

  • stoma = the pore/opening

  • guard cells = two specialized cells surrounding the pore

    • when they swell: stoma opens

    • when they lose water: stoma closes

• allow CO2 in → photosynthesis

• O2 out, water vapor out (transpiration)

How do stomata open/close? - OPEN

• turgor pressure in the guard cells is key

  • when open:

    • guard cells take up K+ and other solutes

    • this makes water potential more negative

    • water enters guard cells through osmosis

    • guard cells become more turgid

    • their shape changes because of the special arrangement of their cell walls

    • the pore opens

      • solutes increase → water enters → guard cells swell → pore opens

How do stomata open/close? - CLOSE

• K+ and other solutes leave guard cells

• water potential becomes less negative

• water leaves guard cells

• guard cells lose turgor

• the pore closes

  • solutes decrease → water leaves → guard cells lose turgor → pore closes

ABA-mediated guard-cell signaling

• ABA = abscisic acid

• ABA is a plant hormone that helps plants respond to drought/water stress when a plant experiences water stress: drought → ABA increase → guard cells lose ions/water → stomata close

ABA-mediated guard-cell signaling - PATHWAY

• drought → ABA increases

  • water = limited

• ABA binds to receptors in guard cells

  • ABA activates signaling pathway inside the guard cell

• ion channels are activated

  • ABA causes anion channels to open, allowing negatively charged ions to leave

  • promotes K+ leaving guard cells

• water leaves

  • with fewer solutes inside, the guard cells’ water potential becomes less negative → water moves out by osmosis

• guard cells lose turgor

• stoma closes

  • ABA = avoid beign arid

    • WE’RE LOSING WATER CLOSE THE FING STOMATA

What are plant drought strategies?

• all plants have different strategies for dealing with drought or periods of limited water

  • major difference: how a plant manages its leaf water status during drought

    • 1.) isohydric: tries to keep leaf water potential stable ~ protect water status

    • 2.) anisohydric: allows lead water potential to decrease more during drought → keep using water even as water status drops

What are plant drought strategies? - Isohydric

• close their stomata relatively early as soil dries

  • soil becomes drier

  • plant detects water stress

  • ABA increases

  • stomata close

  • transpiration decreases

  • leaf water leaves the plant

  • leaf water potential stays relatively stable

• drought → stomata close → less water loss → protect lead water status

• advantage: decreases risk of xylem cavitation and embolism (not too dehydrated)

• disadvantage: less CO2 enters leaf (less photosynthesis)

What are plant drought strategies? - Anisohydric behavior

• allow their leaf water potential to decrease more during drought (keep their stomata open longer)

  • soil becomes drier

  • plant continues allowing some stomatal opening

  • transpiration continues

  • leaf water potential becomes more negative

  • photosynthesis can continue for longer

• advantage: can continue taking in CO2 and carrying out photosynthesis during drought

• disadvantage: lower water potential = risk of xylem cavitation and embolism

What are leaf functional traits?

• measurable characteristics of leaves that affect how they capture resources, grow, survive, and use water and nutrients

  • leaf area → size of the leaf

  • leaf mass per area (LMA) → how much dry mass a leaf has for its area

  • leaf nitrogen content → related to photosynthetic machinery

  • leaf lifespan → how long the leaf stays on the planet

  • photosynthetic rate → how quickly the leaf fixes CO2

  • leaf thickness → can affect durability and resource use

Leaf Economics Spectrum (LES)

• describes the general pattern where leaf traits fall along a spectrum from “fast” leaves → “slow” leaves

Fast return leaves

• leaves are built to capture resources and photosynthesize quickly

  • tend to have:

    • low LMA

    • high photosynthetic rates

    • high nitrogen content

    • shorter leaf lifespan

    • relatively low investment in tough leaf tissues

      • grow fast → photosynthesize fast → replace the lead quickly

• grow fast → photosynthesize fast → replace the leaf quickly

• common when resources like light, water, and nutrients are relatively available

Slow-return leaves

• these leaves are built for durability and long-term survival

  • they tend to have:

    • high LMA

    • lower photosynthetic rates

    • lower nitrogen concentration

    • longer leaf lifespan

    • more investment in tough structural tissues

• invest more → make a durable leaf → keep it for a long time

• these leaves can be useful when resources are limited because the plant gets a longer return on its investment in the lead

Photosynthetic elements

• the structures and substances in a plant that allow it to capture light energy and use it to make sugars through photosynthesis

  • chloroplasts → where photosynthesis occurs

  • chlorophyll → pigment that absorbs light energy

  • thylakoid membranes → where the light-dependent reactions occur

  • photosystems → protein complexes that capture light

  • stomata → allow CO2 to enter the leaf

  • CO2 → carbon source used to make sugar

  • water H2O → provides electrons for the light reactions

Where does photosynthesis happen?

• mainly occurs in the mesophyll cells of leaves

  • inside these cells are chloroplasts:

    • thylakoids

    • grana = stacks of thylakoids

      • thylakoid membrane contains the proteins and pigments needed for the light-dependent reactions

Chlorophyll

• green pigment that captures light energy

• absorbs mainly red and blue light

  • chlorophyll A and B + carotenoids

Stomata and CO2

• stomata regulate gas exchange

  • CO2 → enters through stomata → reaches photosynthetic cells

    • needed for sugars during the calvin cycle

Light-dependent reactions - thylakoid membrane (mesophyll)

• light + H2O → ATP + NADPH + O2

• - light = converted into chemical energy

Calvin cycle - inside the chloroplast (outside grana → stroma of chloroplast)

• CO2 + ATP + NADPH → sugars (glucose or starch)

  • light → energy → CO2 → sugars

What is carbon fixation?

• taking CO2 from the atmosphere and turning it into an organic carbon compound that the plant can use to build sugars and other molecules

  • CO2 (inorganic carbon) → organic carbon

• happens during the calvin cycle in the stroma of the chloroplast

Rubisco - ribulose-1, 5-biphosphate carboxylase/oxygenase

• the main enzyme responsible for carbon fixation in C3 plants

• attaches CO2 to a 5-carbon molecule called RuBP

  • CO2 + RuBP → unstable 6-carbon compound → 2 molecules of 3-PGA

    • grabs CO2 and starts the calvin cycle (in moderate conditions)

• can accidentally bind O2 instead of CO2 (photorespiratation)

  • especially common when temps are high, drought, closed stomata, CO2 inside leaf decreases, O2 becomes more abundant

• drought → stomata close → CO2 decreases → O2 to CO2 ratio increases → rubisco binds to O2

PEP carboxylase

• another enzyme that fixes CO2 (hot and dry conditions, high light)

• key for C4 plants

  • does not bind to O2

• attaches CO2 to a 3-carbon molecule called PEP, producing a 4-carbon compound

• helps C4 plants concentrate CO2 around Rubisco, reducing photorespiration

C3, C4, and CAM photosynthesis

• all of these plants perform photosynthesis and use CO2 to make sugars

• light energy, water, CO2, chloroplasts, light-dependent reactions, calvin cycle, Rubisco can run the calvin cycle

  • main difference is how and when they capture CO2, especially under hot and dry conditions

C3 Photosynthesis

• the most common pathway

• CO2 enters through stomata → Rubisco directly fixes CO2 → calvin cycle

  • the first stable has 3 carbons, which is why it is C3

• advantage: simple and efficient when water and temperatures are moderate, does not require extra CO2

• trade-off: C3 plants are more vulnerable to photorespiration, especially when it’s hot and dry

• when stomata close to conserve water: when CO2 decreases → rubisco more likely to bind to O2 → respiration increases

C4 Photosynthesis

• C4 plants have an extra step that concentrates CO2 around Rubisco

  • PEP carboxylase first fixes CO2 → forms a 4-carbon compound → CO2 is released near Rubisco → calvin cycle

• this helps keep Rubisco surrounded by CO2

• advantages: less photorespiration, better performance in hot/sunny environments, can keep stomata more closed and still capture CO2 effectively, high water use efficiency than C3 plants

• trade-off: C4 phtosynthesis requires extra energy to concentrate CO2

  • plant spends more ATP to get this advantage

CAM Photosynthesis

• CAM plants are especially adapted to very dry environments

• major difference is when CO2 is captured

  • night: temperatures are cooler and the air is more humid

    • stomata open → CO2 enters → PEP carboxylase fixes CO2 → 4-carbon acids are stored

  • day: stomata close to reduce water loss

    • the stored 4-carbon compounds release CO2 → CO2 is given to Rubisco → calvin cycle continues

• advantage: extremely good water conservation

• trade-off: CAM plants generally have slower growth and productivity because CO2 uptake is limited, carbon must be stored and processed over time

Overview on photosynthesis

• C3 → less energy, more photorespiration

• C4 → more energy, less photorespiration and better water efficiency

• CAM → very low transpiration, generally slower carbon gain and growth

  • CO2 come from: atmosphere → stomata → inside the lead → photosynthetic cells

What is water-use efficiency (WUE)?

• describes how much carbon a plant gains through photosynthesis compared with how much water it loses

  • how much photosynthesis a plant gets for the amount of water it loses

  • higher WUE = more carbon gained per amount of water loss

  • especially important in dry environments where water is limited

How do plants lose water?

• most water loss happens through stomata during transpiration

  • stomata open → CO2 enters, water vapor can leave

• open stomata = more CO2, more photosynthesis, more water loss

• closed stomata = less CO2, less photosynthesis, less water loss

How do plants adapt to dry environments? - stomatal control

• plants close their stomata when water is scarce

  • drought → ABA increases → stomata close → water loss decreases

• protects plant from dehydration

How do plants adapt to dry environments? - CAM photosynthesis

• CAM plants open their stomata at night instead of during the day time (hot)

  • night → stomata open → CO2 enters

  • day → stomata close → stored CO2 is used

• night = cooler and less dry, reduces water loss

How do plants adapt to dry environments? - C4 photosynthesis

• C4 plants concentrate CO2 around rubisco, allowing them to maintain carbon fixation while keeping stomata more closed than C3 plants

• higher WUE than C3 plants, especially in hot and sunny environments

How do plants adapt to dry environments? - smaller leaves

• smaller leaves generally have less surface area exposed to the atmosphere, which can reduce water loss

  • thick leaves, waxy cuticles, leaf hairs, rolled leaved (can reduced exposure to dry air)

How do plants adapt to dry environments? - thick/waxy cuticle

• the cuticle is a waxy layer covering the outside of leaves

• a thicker waxy cuticle helps reduce water loss directly through the leaf surface

  • cuticle = waterproof coating

How do plants adapt to dry environments? - deep or extensive roots

• some plants develop roots to reach further underground

• others develop extensive shallow roots to quickly capture water after rainfall

  • depends on their environment

  • plants can use these strategies to reduce water loss and improve water intake