Campbell Biology Chapter 36 Review: Resource Acquisition and Transport in Vascular Plants

36.1 Shoots, roots and the resources they gather

pp. 764–767

On the AP exam? Background

Shoot and root architecture isn't in the current course. What carries over: surface area for uptake (Topic 2.2), leaves as the site of photosynthesis (Topic 3.4), mycorrhizae as a mutualism (Topic 8.5) and traits shaped by natural selection (Topic 7.2).

In the course: Topic 2.2 Cell Size, Topic 3.4 Photosynthesis, Topic 8.5 Community Ecology, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

A land plant gathers resources in two places at once: its shoots take in light and CO₂ from the air, and its roots pull water and dissolved minerals out of the soil. No single part gets everything it needs, so materials have to travel.
Algae living in water could soak up what they needed over their whole surface. As plants on land grew taller, leaves and roots ended up far apart, and vascular tissue evolved: xylem carries water and minerals up, and phloem moves sugars to the parts that use or store them.
Almost every trait is a trade-off. Broad, flat leaves catch more light and CO₂ but also lose more water, so big leaves are common in wet, shady places and small leaves in dry or cold ones.
A shoot's shape decides how much light each leaf gets. Leaf arrangement on the stem, leaf angle, plant height and branching all change how much the plant shades itself.
Leaf area index is total leaf area divided by the ground area under it. Past a point, more layers of leaves don't help: the bottom leaves get so little light that they burn more sugar in respiration than they make, and plants often shed them.
Roots adjust too. They branch more in nutrient-rich patches of soil and make more transport proteins there, so one plant's genes can produce different root forms in different soils.
Most land plants team up with mycorrhizal fungi. The fungal threads reach far into the soil and add a huge absorbing surface, especially for phosphate, and the plant pays the fungus with sugar. This mutualism helped early plants survive in thin, poor soils.

Key terms (14)

shoot system: The above-ground part of a plant (stems, leaves and buds), which collects light and CO₂.
root system: All of a plant's roots, which anchor it and draw water and dissolved minerals out of the soil.
vascular tissue: Tissue made of xylem and phloem that moves materials over long distances inside a plant.
xylem: Vascular tissue that carries water and dissolved minerals up from the roots, through hollow dead cells.
phloem: Vascular tissue that carries sugars and other organic molecules around the plant, through living cells.
phyllotaxy: The pattern of leaves around a stem, such as one leaf per node in a spiral or two leaves per node facing each other.
leaf area index: Total leaf area (one side only) divided by the ground area beneath it. A value of 3 means 3 m² of leaf over each 1 m² of ground.
self-shading: When a plant's upper leaves block light from its own lower leaves.
self-pruning: A plant shedding shaded leaves or branches that cost more sugar to keep than they make.
taproot system: A root system with one thick main root and smaller side branches. It anchors tall plants well.
fibrous root system: A mat of many thin roots of about the same size, with no single main root.
mycorrhiza: A partnership between a fungus and a plant's roots. The fungus helps the plant take in water and minerals, and the plant feeds the fungus sugar.
hyphae: The thin threads that make up a fungus's body. Spread through soil, they give a huge surface for absorbing nutrients.
trade-off: When a trait helps in one way but costs something in another, like big leaves catching more light but losing more water.

Check yourself: 36.1 Shoots, roots and the resources they gather

4 questions on 36.1 Shoots, roots and the resources they gather. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A greenhouse bed covers 1.5 m² of ground. It holds 12 pepper plants, each with about 45 leaves, and the average leaf has an upper surface area of 50 cm². What is the leaf area index of the bed? (1 m² = 10,000 cm²)

Question 2 of 4

Two closely related shrub species grow in very different places. Species 1 lives in the shaded understory of a humid tropical forest. Species 2 lives on sunny, rocky slopes that get little rain. Which difference in their leaves would you most expect, and why?

Question 3 of 4

Two rice breeding lines have the same total leaf area. Line E holds its upper leaves steeply upright; line F holds them nearly flat. Grain yield (invented data, tonnes per hectare): dense planting, E 8.1 and F 6.9; sparse planting, E 5.2 and F 5.3. Which explanation best fits the data?

Question 4 of 4

At noon on a bright day, a researcher clamps a gas-measuring chamber onto a leaf deep inside a dense hedge. The leaf gives off a small net amount of CO₂ instead of taking it in. Which explanation fits best?

0 of 4 answered

36.2 Water potential and moving things short and long distances

pp. 767–771

On the AP exam? Yes

Water potential (Ψ = Ψp + Ψs and Ψs = −iCRT) is tested in Topic 2.7, aquaporins in Topic 2.6, and active transport and membrane voltage in Topics 2.5 and 2.8. The words apoplast and symplast won't come up on the exam.

In the course: Topic 2.7 Tonicity and Osmoregulation, Topic 2.6 Facilitated Diffusion, Topic 2.5 Membrane Transport, Topic 2.8 Mechanisms of Transport, Topic 2.4 Membrane Permeability (notes, videos and more questions)

Key points

Plant tissue has two connected compartments. The apoplast is everything outside the living membranes: cell walls, spaces between cells and hollow dead xylem cells. The symplast is the living cytoplasm, linked from cell to cell by plasmodesmata.
Water and solutes can travel through walls (the apoplastic route), through the linked cytoplasm (the symplastic route), or by crossing membrane after membrane (the transmembrane route). Many substances use more than one.
In plants, the main pump moves H⁺, not Na⁺. Proton pumps use ATP to push H⁺ out, which makes the outside acidic and the inside negative. H⁺ flowing back in then powers cotransporters that bring in sucrose, nitrate and other solutes against their gradients.
Water moves from higher water potential (Ψ) to lower Ψ, and Ψ = Ψp + Ψs. Pure water in an open container has Ψ = 0. Solutes make Ψs negative (Ψs = −iCRT), and pressure can make Ψp positive (a push) or negative (a pull). Plant biologists often use megapascals: 1 MPa = 10 bars.
Solutes lower Ψ because they dilute the water and lower its free energy. Some older books say solutes work by tying up water molecules, but that isn't the main reason.
Put a walled cell in a solution with higher Ψ and water flows in until the wall pushes back hard enough (turgor pressure) to make Ψ inside equal Ψ outside. In a solution with lower Ψ, the cell loses water and its protoplast pulls away from the wall (plasmolysis).
Aquaporins are water channels that make osmosis faster without changing its direction. Over long distances diffusion is far too slow, so plants use bulk flow: whole solutions pushed or pulled by pressure differences through hollow xylem cells and nearly empty sieve tubes.

Key terms (15)

apoplast: Everything outside the living cells' membranes: cell walls, the spaces between cells and the hollow insides of dead xylem cells.
symplast: The living cytoplasm of a plant's cells, joined into one connected network by plasmodesmata.
plasmodesmata: Tiny channels through plant cell walls that connect the cytoplasm of neighboring cells.
transmembrane route: A path where a substance leaves one cell across its membrane, then enters the next across another, again and again.
proton pump: A membrane protein that uses ATP to push H⁺ out of a plant cell, making the outside acidic and the inside negative.
membrane potential: The voltage across a membrane, caused by an uneven spread of charges on its two sides.
cotransport: Moving one substance against its gradient by pairing it with another (like H⁺) that flows down its own gradient.
water potential (Ψ): A measure of how strongly water tends to move. Water flows from higher Ψ to lower Ψ, and pure water in an open container is 0.
solute potential (Ψs): The part of water potential caused by dissolved particles. It's 0 for pure water and gets more negative as you add solute.
pressure potential (Ψp): The part of water potential caused by physical pressure. It's positive when water is pushed (turgor) and negative when it's pulled (xylem tension).
turgor pressure: The push of a plant cell's contents against its wall after water moves in. It keeps soft plant parts firm.
flaccid: Limp. Describes a walled cell with no turgor pressure.
plasmolysis: When a walled cell loses so much water that its membrane and cytoplasm shrink away from the wall.
aquaporin: A channel protein that lets water cross a membrane much faster. It changes how fast osmosis happens, not which way.
bulk flow: Movement of a whole liquid together because of a pressure difference, like water through a hose. Over long distances it's far faster than diffusion.

Check yourself: 36.2 Water potential and moving things short and long distances

4 questions on 36.2 Water potential and moving things short and long distances. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A plant cell has a solute potential of −14 bars and a pressure potential of +4 bars. It's placed in an open beaker of 0.25 M CaCl₂ at 20 °C. Assume CaCl₂ splits completely into one Ca²⁺ and two Cl⁻ (i = 3). Using Ψ = Ψp + Ψs and Ψs = −iCRT, with R = 0.0831 L·bar/(mol·K), what is the solution's water potential, and which way does water move overall?

Question 2 of 4

Leaf cells of a pond plant are moved from pond water into a concentrated salt solution and watched under a microscope. Within minutes, the cytoplasm and vacuole of each cell shrink and pull away from the cell wall, while the wall keeps its shape. Which statement best explains this?

Question 3 of 4

Plant cells are kept in a weakly buffered solution. Over 20 minutes, the solution's pH drifts down from 6.5 to 5.6. Sucrose is then added, and as the cells take it up, the pH quickly rises to 5.9. Which explanation fits both observations?

Question 4 of 4

Two batches of plant protoplasts (cells with their walls removed) are identical except that batch 1 makes many more aquaporins. At time 0 both are moved into the same sucrose solution. Relative cell volume (invented data), batch 1 then batch 2: 0 s, 1.00 and 1.00; 30 s, 0.86 and 0.97; 60 s, 0.80 and 0.94; 120 s, 0.78 and 0.89; 300 s, 0.78 and 0.78. Which conclusion is best supported?

0 of 4 answered

36.3 Water's trip from soil to leaf

pp. 772–776

On the AP exam? Background

The exam won't ask about root anatomy or the Casparian strip, but water rising through xylem is the go-to example of cohesion and adhesion (Topic 1.1) and of water moving toward lower Ψ (Topic 2.7). Root hairs are a Topic 2.2 surface-area example.

In the course: Topic 1.1 Structure of Water and Hydrogen Bonding, Topic 2.7 Tonicity and Osmoregulation, Topic 2.2 Cell Size (notes, videos and more questions)

Key points

Most uptake happens close to the root tips, where root hairs add a huge amount of surface. Soil water is usually dilute, so root cells spend ATP to pull in the mineral ions they need and pile them up far above soil levels.
Soil water can seep through the walls of the root cortex without entering any cell. At the endodermis, a waterproof band in the walls (the Casparian strip) blocks that path, so water and minerals must cross a plasma membrane before reaching the xylem. This lets the root screen what gets in, and it stops ions already in the xylem from seeping back to the soil.
Newer research shows the Casparian strip is made mainly of lignin, with suberin added later as a coating on endodermal cells. Older books describe the strip as suberin.
Root pressure is a small push. At night, roots keep pumping ions into the xylem, water follows by osmosis, and sap is pushed up a little. It explains guttation drops, but it's far too weak to lift water up a tall tree.
The main force is a pull, called the cohesion-tension mechanism. Water evaporates from the wet walls of leaf cells into the air spaces and escapes through stomata. That puts the water left behind under tension (negative Ψp), which pulls on the water column below.
The pull reaches all the way to the roots because water molecules hold onto each other by hydrogen bonds (cohesion) and cling to the xylem walls (adhesion). Water moves passively down a Ψ gradient from soil to root to stem to leaf to air. The plant spends no ATP lifting it; the sun's energy drives the evaporation.
Xylem cells are dead and hollow, with thick, stiff walls that don't cave in under tension. Drought or freezing can create air bubbles (cavitation) that break the water chain, and water then has to detour through pits into neighboring cells.

Key terms (15)

root hair: A long, thin outgrowth of a root's surface cell that adds a lot of absorbing surface.
soil solution: The water held in soil together with the mineral ions dissolved in it.
cortex: The layer of root tissue between the root's surface and its central core.
endodermis: The innermost ring of cortex cells. It acts as a checkpoint for everything heading into the root's central core.
Casparian strip: A waterproof band in the walls between endodermal cells, made mostly of lignin. It makes water and minerals go through a cell membrane to get past.
vascular cylinder (stele): The central core of a root, which holds its xylem and phloem.
xylem sap: Water and the mineral ions dissolved in it, carried upward through the xylem.
transpiration: Evaporation of water from a plant's leaves and other above-ground parts, mostly through stomata.
root pressure: A weak upward push on xylem sap. It builds when roots pump ions into the xylem and water follows by osmosis.
guttation: Drops of xylem sap forced out at leaf tips or edges, usually overnight, by root pressure. It isn't dew.
cohesion-tension mechanism: The well-supported explanation that evaporation from leaves pulls water up the xylem, and cohesion keeps the water column from breaking.
cohesion: Water molecules sticking to each other through hydrogen bonds, which lets a water column be pulled as one unbroken chain.
adhesion: Water sticking to other polar surfaces, like the cellulose walls of xylem cells. It helps hold the column against gravity.
cavitation: An air or vapor bubble forming inside a xylem cell, which breaks the water column and blocks flow there.
tracheids and vessel elements: The two kinds of dead, hollow, thick-walled cells that carry water in xylem.

Check yourself: 36.3 Water's trip from soil to leaf

4 questions on 36.3 Water's trip from soil to leaf. Pick an answer to see if you got it, and why.

Question 1 of 4

Which sequence correctly traces water taken in by a root hair on its way to a leaf?

Question 2 of 4

Researchers study a mutant plant whose root endodermis can't build its Casparian strip, so the walls between endodermal cells stay open. Compared with normal plants, which result is most likely?

Question 3 of 4

Water potentials (MPa) were measured in a well-watered sunflower. Before dawn: soil −0.2, root −0.3, stem −0.3, leaf −0.3. At midday: soil −0.2, root −0.5, stem −0.9, leaf −1.4. Which explanation best fits the change from predawn to midday?

Question 4 of 4

Stems from two tree species went through one freeze–thaw cycle, which can release dissolved air as bubbles inside xylem. Species P has narrow water-conducting cells (mean width 15 µm); species Q has wide ones (mean width 200 µm). Loss of water-conducting ability afterward (invented data): species P, 6%; species Q, 72%. Which conclusion is best supported?

0 of 4 answered

36.4 Stomata: trading water for CO₂

pp. 776–778

On the AP exam? Background

Guard cells and stomata are examples for surface area (Topic 2.2) and osmosis (Topic 2.7), stomata closing in a drought is a plant responding to its environment (Topic 8.1), and evaporative cooling is Topic 1.1. You won't need guard-cell wall structure or the names of desert-plant traits.

In the course: Topic 2.2 Cell Size, Topic 2.7 Tonicity and Osmoregulation, Topic 1.1 Structure of Water and Hydrogen Bonding, Topic 3.4 Photosynthesis, Topic 8.1 Responses to the Environment (notes, videos and more questions)

Key points

A leaf's large surface and air-filled inside are built for taking in CO₂, but any opening that lets CO₂ in also lets water vapor out. Stomata make up only a sliver of the leaf surface, yet nearly all of a plant's water loss goes through them, since a waxy cuticle seals the rest.
Two guard cells surround each stoma. When they swell with water, the way their walls are built makes them bow apart, and the pore opens. When they lose water, they relax and the pore closes.
Guard cells change shape through osmosis. To open, proton pumps push H⁺ out, and the resulting voltage drives K⁺ in through channels, along with negative ions like Cl⁻. The extra solute lowers the guard cells' Ψ, so water flows in. To close, K⁺ leaves and water follows it out.
Light (especially blue light), falling CO₂ inside the leaf and an internal circadian clock open stomata in the morning. A water shortage and the hormone abscisic acid (ABA) close them, even in daylight.
Closing stomata saves water but cuts off CO₂, so photosynthesis slows. That's the central trade-off of a plant's life. Transpiration also cools leaves, because evaporating water carries away a lot of heat (water's high heat of vaporization).
Stomatal density depends on genes and on conditions while a leaf develops; many plants make fewer stomata when CO₂ levels are high.
Plants of dry places (xerophytes) have many ways to hold on to water: less leaf surface, better-sealed or sheltered leaf surfaces, tissues that store water, roots that reach deep water, and CAM photosynthesis, which takes in CO₂ only at night.

Key terms (13)

stoma (plural stomata): A pore in the surface of a leaf or stem that lets CO₂ in and lets water vapor and O₂ out.
guard cells: The pair of cells around each stoma. They open and close it by swelling and shrinking.
cuticle: A waxy coat on the outside of leaves and stems that slows water loss.
stomatal density: How many stomata a leaf has per unit of area. Genes and growing conditions both affect it.
potassium ions (K⁺): The main solute guard cells take in to open a stoma and let go of to close it.
abscisic acid (ABA): A plant hormone made when water runs short. It tells guard cells to shut the stomata.
circadian rhythm: A roughly 24-hour cycle run by an internal clock. It keeps going for a while even without day and night cues.
wilting: Leaves and stems drooping because their cells have lost turgor, when water loss outpaces supply.
evaporative cooling: The drop in temperature when water evaporates, since the escaping molecules carry heat away. It keeps sunlit leaves cooler than they'd otherwise be.
xerophyte: A plant adapted to a dry habitat, like a cactus.
CAM photosynthesis: A kind of photosynthesis where the plant opens its stomata at night, stores CO₂ in organic acids, and uses it in the Calvin cycle by day with stomata shut.
trichomes: Hairs on a leaf's surface. They can hold a layer of still, humid air that slows water loss.
photosynthesis–transpiration trade-off: Open stomata let in CO₂ for photosynthesis but also let water out, so a plant can't gain one without risking the other.

Check yourself: 36.4 Stomata: trading water for CO₂

4 questions on 36.4 Stomata: trading water for CO₂. Pick an answer to see if you got it, and why.

Question 1 of 4

Strips of leaf surface are kept in the dark and then lit at time 0. Guard-cell K⁺ (relative units) and stomatal pore width (µm) were measured (invented data): 0 min, 1.0 and 1.5; 15 min, 1.8 and 3.8; 30 min, 2.9 and 6.9; 60 min, 3.4 and 8.0. Which mechanism best explains the data?

Question 2 of 4

Well-watered bean plants in bright light are sprayed with abscisic acid (ABA). Within an hour, which set of changes would you most expect?

Question 3 of 4

In a transpiration experiment, a leafy branch is sealed inside a clear plastic bag, which keeps the air around its leaves very humid. Its water loss drops to about a third of an unbagged control's. Which explanation fits best?

Question 4 of 4

In a desert succulent, the cell sap of the leaves is very acidic at dawn and much less acidic by late afternoon, and the stomata are open mainly at night. Which explanation fits best?

0 of 4 answered

36.5 Phloem: moving sugar from sources to sinks

pp. 779–781

On the AP exam? Background

Phloem transport isn't its own topic in the current course, but it runs on tested ideas: H⁺-driven active transport (Topic 2.8), water potential (Topic 2.7) and the sugars made in photosynthesis (Topic 3.4).

In the course: Topic 2.8 Mechanisms of Transport, Topic 2.7 Tonicity and Osmoregulation, Topic 3.4 Photosynthesis (notes, videos and more questions)

Key points

Translocation is the movement of sugars through phloem. Sugar travels in sieve tubes: living sieve-tube elements joined end to end through porous sieve plates, each kept alive by a companion cell next to it.
Phloem sap is a sugary solution, usually sucrose, with smaller amounts of amino acids, minerals and signaling molecules such as hormones. Unlike xylem sap, it can flow up or down, depending on where it's headed.
A source makes or releases more sugar than it uses, like a mature leaf or a storage organ breaking down its starch. A sink uses or stores more sugar than it makes, like root tips, buds, fruits, seeds and young leaves. The same organ can be a sink in one season and a source in another.
At many sources, sucrose is pushed into companion cells and sieve tubes against its gradient. Proton pumps build an H⁺ gradient with ATP, and H⁺/sucrose cotransporters use it. In other species, sugar moves in through plasmodesmata instead.
Pressure flow drives the sap. Loading sugar at the source end lowers Ψ inside the sieve tube, so water moves in from nearby xylem and pressure rises. At the sink, sugar is unloaded and water leaves, so pressure falls. Sap moves by bulk flow from high pressure to low, and the water returns through the xylem.
Sinks keep sugar coming by using it or turning it into starch, which keeps their own sucrose level low. Sinks also compete with each other, so a plant carrying more fruit than its leaves can feed may shed some of it.

Key terms (12)

translocation: The transport of sugars and other products of photosynthesis through the phloem.
sieve-tube element: A living phloem cell with no nucleus. Joined end to end with others, it forms a tube that carries sugar.
sieve plate: The porous end wall between two sieve-tube elements, which lets sap pass from one to the next.
companion cell: A cell next to a sieve-tube element, linked to it by plasmodesmata. It keeps the sieve-tube element running and often loads sugar into it.
phloem sap: The sugary liquid inside sieve tubes: mainly sucrose in water, with smaller amounts of amino acids, minerals and hormones.
sugar source: A plant part that makes or releases more sugar than it uses, like a mature leaf or a storage root in spring.
sugar sink: A plant part that uses or stores more sugar than it makes, like a growing root tip, a fruit or a young leaf.
phloem loading: Moving sugar into sieve tubes at a source, often by active transport.
phloem unloading: Sugar leaving the sieve tubes at a sink, where it's used or stored.
H⁺/sucrose cotransporter: A membrane protein that lets H⁺ flow back into a cell and uses that energy to carry sucrose in against its gradient.
pressure flow: How phloem sap moves: sugar loaded at a source draws in water and builds pressure, which pushes sap toward sinks, where pressure is lower.
girdling: Removing a full ring of bark, and the phloem with it, around a trunk. It cuts off the sugar supply to everything below the ring.

Check yourself: 36.5 Phloem: moving sugar from sources to sinks

4 questions on 36.5 Phloem: moving sugar from sources to sinks. Pick an answer to see if you got it, and why.

Question 1 of 4

Sugar beet stores large amounts of sucrose in its swollen root during its first summer. In its second spring, the plant quickly sends up a tall flowering stalk, before many new leaves have grown. During that spring growth, what role does the storage root play?

Question 2 of 4

One mature leaf halfway up a bean plant is given ¹⁴CO₂ for 30 minutes. Six hours later, the label that has left that leaf is located (invented data, percent of exported label): young unfolding leaves at the shoot tip, 34%; developing pods, 41%; roots, 25%; other mature leaves, 0%. Which statement best explains why the other mature leaves got none?

Question 3 of 4

A mutant plant lacks the H⁺/sucrose cotransporter that normally loads sucrose into the phloem of its leaves. Which chain of effects is most likely?

Question 4 of 4Calculator allowed

Near a source leaf, the sieve-tube sap's solute potential is −1.6 MPa and the xylem next to it has a water potential of −0.8 MPa. Near a sink root, the sieve-tube sap's solute potential is −0.7 MPa and the xylem next to it has a water potential of −0.4 MPa. Assume each part of the sieve tube reaches the same water potential as the xylem beside it, and use Ψ = Ψp + Ψs. What difference in pressure potential drives sap from source to sink?

0 of 4 answered

36.6 A living network: plasmodesmata and phloem signals

pp. 781–782

On the AP exam? Background

Plasmodesmata and phloem signals aren't named in the current course, but they're good examples for Topic 4.1: cells signaling through direct contact and over long distances, including plant immune responses. Proteins moving between cells to guide development connect to Topic 6.6.

In the course: Topic 4.1 Cell Communication, Topic 6.6 Gene Expression and Cell Specialization (notes, videos and more questions)

Key points

Plant transport isn't just plumbing. Because the symplast is alive, cells keep adjusting what moves where as they develop and as conditions change, for example by changing how many water channels or ion channels their membranes carry.
Plasmodesmata aren't fixed holes. Cells can narrow or seal them within minutes (often by laying down a polysaccharide called callose), build new ones long after a cell divides, and shut old ones down. When a leaf switches from importing sugar to exporting it, its plasmodesmata change too.
Besides small molecules, plasmodesmata can let proteins and RNA through. Some transcription factors move from one cell layer to the next and help decide what those cells become. Tightly linked groups of cells form symplastic domains that develop together.
Plant viruses spread by making movement proteins that widen plasmodesmata. They're hijacking the plant's own system for controlling these channels.
The phloem carries information as well as food. The flowering signal FT (often called florigen) is a protein made in leaves that travels to the shoot tips, and signals from an infected leaf can switch on defenses in leaves far away.
The phloem can also carry electrical signals that reach distant organs and change things there, like gene expression or photosynthesis. They're much slower than animal nerve impulses.
For the AP course, think of these as Topic 4.1 examples: cells can signal through direct contact (plasmodesmata play a role like animal gap junctions) or over long distances.

Key terms (9)

symplastic domain: A group of cells closely linked by plasmodesmata, which lets them share signals and develop as a team.
callose: A polysaccharide that plant cells lay down around plasmodesmata to narrow or seal them.
movement protein: A protein made by a plant virus that widens plasmodesmata so the virus's genetic material can spread to neighboring cells.
florigen (FT protein): The flowering signal: a protein made in leaves that travels in the phloem to shoot tips and switches on flowering.
systemic signal: A signal that spreads through the whole plant body instead of acting only on nearby cells.
systemic acquired resistance: Whole-plant protection that builds up after a local infection, when signals from the infected area prime defenses in distant leaves.
electrical signal: A traveling change in membrane voltage that can move through the phloem and set off responses far away.
gap junction: A channel that links the cytoplasm of neighboring animal cells. Plasmodesmata do a similar job in plants but can also pass proteins and RNA.
mobile RNA: RNA that travels between cells or through the phloem and changes gene expression where it ends up.

Check yourself: 36.6 A living network: plasmodesmata and phloem signals

4 questions on 36.6 A living network: plasmodesmata and phloem signals. Pick an answer to see if you got it, and why.

Question 1 of 4

A mutant plant has a broken copy of a gene that leaves normally switch on when the day length is right, and the mutant never flowers. A shoot from the mutant is grafted onto a normal plant, and both get the same day length. The normal plant's leaves are left on, and the mutant shoot now flowers. Which conclusion is best supported?

Question 2 of 4

A plant virus with one gene deleted still infects and multiplies inside the first leaf cells it enters, but it never spreads to the cells next door. In plants engineered to make that gene's protein themselves, the same mutant virus spreads from cell to cell normally. What does the missing protein most likely do?

Question 3 of 4

Plant cells can narrow or close their plasmodesmata by laying down callose around them. A mutant can't add callose at its plasmodesmata when it detects a pathogen. Compared with normal plants, what would you most expect?

Question 4 of 4

One leaf of a tobacco plant is infected by a pathogen, and a week later the plant's other leaves resist infection better. In a second group of plants, the stalk of the infected leaf is heat-treated right after infection, which kills its phloem but leaves its xylem working. In this group, the other leaves don't become more resistant. Which conclusion is best supported?

0 of 4 answered