B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

Leaves, Chloroplasts and Transport: Plant Systems Working Together

B3.3 B3.5 B2.4 B2.1 A1.5 A1.8 A1.12About 75 minutes

Learning goals

I can label the tissues of a leaf in cross-section and explain how each helps photosynthesis or gas exchange.
I can describe the structure of a chloroplast (grana, thylakoids, stroma) and how it captures light energy.
I can explain how water moves up a plant by transpiration and how sugars are moved from sources to sinks.
I can explain how the root system and shoot system depend on each other, and analyse transpiration data.

Words to know

mesophyll
The ground tissue inside a leaf where most photosynthesis happens: palisade (upper) and spongy (lower).

stoma (plural: stomata)
A tiny pore, usually on the underside of a leaf, that lets gases in and out.

guard cells
A pair of curved epidermal cells that open and close a stoma.

thylakoid
A flattened membrane sac inside a chloroplast that contains chlorophyll and captures light energy.

granum (plural: grana)
A stack of thylakoids in a chloroplast.

stroma
The fluid inside a chloroplast around the grana, where sugars are built.

transpiration
The loss of water vapour from a plant, mainly through the stomata of leaves.

source / sink
A source is where sugar is made or released (a leaf); a sink is where it is used or stored (roots, fruits, growing tips).

A solar-powered sugar factory

A leaf is thin and flat, often only a fraction of a millimetre thick, yet it is a complete organ with at least four tissue types. It captures sunlight, takes in carbon dioxide from the air, receives water pumped up from the roots, makes sugar, and ships that sugar out to the rest of the plant. In Grade 9 you learned the photosynthesis equation. Now you'll see the structures that make it happen.

Inside a leaf

Cuticle: a waxy, waterproof layer on the epidermis that reduces water loss.
Upper epidermis: a single layer of clear cells (no chloroplasts) that protects the leaf and lets light pass through.
Palisade mesophyll: tall, tightly packed cells near the top, crammed with chloroplasts. Most photosynthesis happens here, where light is brightest.
Spongy mesophyll: loosely packed, rounded cells with many air spaces between them, so CO₂ can diffuse to the cells and O₂ can diffuse out.
Veins (vascular bundles): xylem brings water and minerals in; phloem carries sugar out. Veins also stiffen the leaf.
Lower epidermis with stomata: each stoma is a pore between two guard cells. CO₂ enters, and O₂ and water vapour leave, through the stomata.

Cross-section of a typical leaf (simplified). Light enters from the top; most chloroplasts (dots) are in the palisade layer. Gases move through the stoma and the air spaces of the spongy layer. The vein connects the leaf to the rest of the plant.

Guard cells: the leaf's doors

Guard cells control a trade-off. Open stomata let in CO₂ for photosynthesis, but they also let out water vapour.

When guard cells are full of water, they swell and bend apart, so the stoma opens. This usually happens in the light, when water is plentiful.
When the plant is short of water, the guard cells go limp and the stoma closes, saving water but slowing photosynthesis. Many plants also close their stomata at night.

The chloroplast: solar collectors

The curriculum describes the grana and thylakoids of chloroplasts as solar collectors, and that is a good way to picture them:

Each chloroplast has a double membrane.
Inside are thylakoids: flattened membrane sacs containing the green pigment chlorophyll, which absorbs light energy. Thylakoids are stacked like coins into grana, which packs a huge membrane area into a tiny space, just as a solar farm packs in many panels.
Around the grana is the fluid stroma, where the captured energy is used to build sugar from carbon dioxide and water.

So the links run all the way down: chloroplast (organelle) → palisade cell (specialized cell) → mesophyll (tissue) → leaf (organ) → shoot system (system).

A chloroplast (simplified, not to scale). Light energy is captured by chlorophyll in the thylakoid membranes, stacked into grana; sugar is assembled in the surrounding stroma.

Moving water up: transpiration

How does a 30 m maple lift water without a pump?

Water evaporates from the moist surfaces of mesophyll cells into the air spaces and diffuses out through the stomata (transpiration).
Water molecules stick to each other (cohesion) and to the walls of the xylem, forming continuous columns of water from leaf to root.
As water leaves the top of the column, it pulls the whole column upward, a bit like sipping through a very long, very thin straw. This is called the transpiration pull.
In the roots, water enters root hairs by osmosis and moves into the xylem. (Roots can also push water up a little, called root pressure.)

Transpiration is faster when it is warm, bright, windy and dry, and slower when it is cool, dark, still or humid.

Moving sugar: from sources to sinks

Sugar made in leaves is loaded into phloem and carried to where it is needed or stored. A source releases sugar (a mature leaf in summer, or a storage root in spring); a sink uses or stores it (roots, growing shoots, flowers, fruits, seeds). That's why phloem can carry sugar both up and down. The maple sap collected in spring is sugar stored in the roots and trunk last summer, moving up to feed the new buds.

Plant systems working together. Water and minerals absorbed by the root system travel up the xylem to the leaves, where water is lost by transpiration. Sugar made in the shoot system travels in the phloem (dashed arrows) to sinks such as roots and fruits. Neither system could survive alone.

Why the systems need each other

The expectation on interacting systems (B3.5) applies to plants, too:

The shoot system can't make sugar without water and minerals (like nitrogen for proteins and magnesium for chlorophyll) from the roots.
The root system can't grow, absorb or store without sugar from the leaves. Roots have no chloroplasts and live in the dark.
The vascular tissue is the link between them, just as the circulatory system links your organs.

If you cut off a plant's leaves, the roots eventually starve; if you damage its roots, the leaves wilt.

Worked example

Let's calculate stomatal density

A student paints clear nail polish on the underside of a leaf, peels it off with tape and looks at the impression at 400×, where the field of view is 0.45 mm across. She counts 24 stomata in the field. How many stomata are there per square millimetre?

The field of view is a circle: radius r = 0.45 mm ÷ 2 = 0.225 mm.
Area = πr² = 3.14 × 0.225² = 3.14 × 0.0506 ≈ 0.159 mm².
Density = number ÷ area = 24 ÷ 0.159 ≈ 151 stomata per mm².
To make this more reliable, she should count several fields and from several leaves, then average. Counting the upper surface too would let her compare the two sides.

About 150 stomata per mm² (151 to 3 significant figures).

Worked example

Let's analyse transpiration data

Two identical potted plants were watered and their pots sealed in plastic so that water could leave only through the leaves. Plant A sat in a still room; Plant B sat in front of a fan. Masses: A: 312.0 g at the start, 304.8 g after 6 h. B: 309.5 g at the start, 296.3 g after 6 h. Compare the rates and explain.

A lost 312.0 − 304.8 = 7.2 g; rate = 7.2 ÷ 6 = 1.2 g/h.
B lost 309.5 − 296.3 = 13.2 g; rate = 13.2 ÷ 6 = 2.2 g/h.
B lost water about 2.2 ÷ 1.2 ≈ 1.8 times as fast.
Explanation: wind carries away the moist air around the leaves, keeping the air outside the stomata dry, so water vapour diffuses out faster. The mass lost is water, because the pots were sealed (1 g of water ≈ 1 mL).
Limitations: only one plant per condition; the plants may differ in leaf area. Repeating with several plants and measuring leaf area would improve the test.

A: 1.2 g/h; B: 2.2 g/h. Wind increased transpiration by about 1.8 times.

Just for fun

Funny but true: on a hot summer day, a single large tree can move well over 100 L of water from the soil, up its trunk and out through its leaves into the air. That's like silently drinking a bathtub of water every day or two, through a straw as tall as a building.

Hands-on activity

Catch a tree sweating

You need: two clear plastic bags (sandwich size or bigger), two twist ties or elastic bands, a measuring spoon or syringe, and a leafy shrub or tree branch outdoors (or a large houseplant). Optional: clear nail polish, clear tape, a microscope or a strong magnifier. Grown-up note: nail polish is flammable and has strong fumes; use it in a ventilated area, away from flames, and only a thin coat on the leaf.

On a sunny morning, place one bag over a leafy twig and close it gently around the stem. Put the second bag over a similar twig in the shade. Don't break the twig.
After 2 to 4 hours, remove the bags carefully and pour or draw the water into the spoon or syringe. Measure and record the volume collected from each.
Calculate the rate in mL per hour for each. Which was faster? Explain using what you know about transpiration.
Optional stomata peel: paint a thin patch of nail polish on the underside of a leaf, let it dry, press clear tape on it, peel it off and stick the tape to a slide or clear plastic. Look for pairs of bean-shaped guard cells. Compare with the upper surface.

What to look for: droplets condensing inside the bags within an hour or two, with more in the sunny bag (light and warmth increase transpiration). Many leaves show many more stomata on the underside than on the top. Remove the bags so the twigs aren't harmed, and dispose of them responsibly.

Your turn

List the layers of a leaf from top to bottom.

Why are most chloroplasts in the palisade layer?

What is the function of the air spaces in the spongy mesophyll?

How do guard cells control water loss?

What is the role of the thylakoids in a chloroplast?

Explain transpiration pull in two or three sentences.

Give one example of a source and one of a sink.

More practice: warm-up

What are the tiny pores on the underside of a leaf called?

What is a stack of thylakoids called?

In which part of the chloroplast is sugar built?

Which vascular tissue in a vein carries water into the leaf?

Name two conditions that speed up transpiration.

More practice: on your own

Explain why the upper epidermis has no chloroplasts but is clear.

A plant's stomata close on a very hot afternoon. Describe one benefit and one cost.

16 stomata are counted in a field of view of area 0.159 mm². What is the stomatal density?

A potted plant in a sealed pot loses 9.6 g in 8 hours. What is its rate of transpiration in mL/h?

Explain why sugar maple sap flows up in early spring before the leaves open.

Challenge

Water lilies float on ponds. Predict where their stomata are and explain.

A tree's leaves are fine but its roots are damaged by flooding (roots need oxygen for cellular respiration). Explain, using both systems, why the leaves wilt within days.

Compare a leaf's structures with the human respiratory and circulatory systems. Give two matching pairs and explain each.

Check your understanding

Quiz

Which leaf layer contains the most chloroplasts?

What do guard cells do?

Where in the chloroplast is light energy captured?

Which condition would slow transpiration the most?

A potato tuber storing starch in spring that then supplies sugar to new shoots is acting as a...

Why does a plant's root system depend on its shoot system?

For parents and teachers

Big idea: this lesson finishes B3.3 for plants (the expectation's own example is the grana and thylakoids of chloroplasts acting as solar collectors) and applies B3.5 (interacting systems) to the root and shoot systems. It builds on Grade 9's photosynthesis equation.

How to help: The bag-over-a-branch activity works best on a sunny day; you can compare sun and shade together. If you try the nail-polish peel, supervise and ventilate. Ask your teen to explain the "chain" from chloroplast to shoot system out loud.

Common difficulties:

Thinking plants "breathe in" through roots or that water evaporates through the cuticle (it's mainly through stomata).
Mixing up thylakoid, granum and stroma.
Forgetting that open stomata are a trade-off: CO₂ in, water out.