K325 / 2027

Lesson 1 of 12 / Nutrition and transport in plants

Leaf structure and gas exchange

How does a leaf bring carbon dioxide close to its chloroplasts?

I can identify tissues in a dicot leaf section and explain their roles in photosynthesis and gas exchange.

  • I can identify dicot leaf tissues and relate chloroplasts, gas exchange and vascular transport to structure.Syllabus

    Syllabus K325, Topic 10(a). Identify dicot leaf tissues and relate chloroplasts, gas exchange and vascular transport to structure.

  • I can trace carbon dioxide to mesophyll cells.Syllabus

    Syllabus K325, Topic 10(g). Trace carbon dioxide to mesophyll cells.

About 6 min

Make a guess. You are not marked.

A leaf needs carbon dioxide for photosynthesis. How does most of it reach the cells inside the leaf?

  1. Up from the soil through the roots and xylem.
  2. Straight through the waxy cuticle.
  3. Through stomata, then through air spaces inside the leaf.
Show the answer

Through stomata, then through air spaces inside the leaf.

Carbon dioxide diffuses in through open stomata and spreads through air spaces to the mesophyll cells.

The key idea

Stomata and mesophyll air spaces shorten the diffusion route to photosynthesising cells; chloroplast-rich palisade tissue captures light.

How does leaf structure shorten the route to a chloroplast?
Lower sideUpper side

Brown ring: selected feature. Dark green ovals: chloroplasts. Blue/gold in the vein: xylem/phloem. Original schematic of a typical dicot section, not a microscope photograph; sizes and cell numbers are simplified.

Read a real microscope section

Find the upper epidermis, palisade tissue and spongy tissue. Which visible pattern suggests a route for gases within the leaf?

Light micrograph of a privet leaf cross-section: a row of upper epidermal cells, elongated palisade cells below, loosely arranged spongy cells and a central vein.
Tap the image to see it full size. Image: Berkshire Community College Bioscience Image Library. CC0 1.0. Used without further changes.

Image size changes with your screen. Use a stated scale bar for measurements; do not calculate from the display size alone. Practise slide preparation and focusing in your laboratory.

Explanation

A typical dicot leaf has an upper epidermis, palisade mesophyll, spongy mesophyll and lower epidermis. A transparent epidermis lets light reach the mesophyll. A waxy cuticle reduces water loss. Palisade cells lie near the illuminated upper surface and contain many chloroplasts.

Spongy mesophyll has irregularly arranged cells with large air spaces. These allow gases to spread through the leaf and provide a large moist cell surface for gas exchange. Spongy cells also contain chloroplasts, though usually fewer than palisade cells. Most epidermal cells lack chloroplasts; guard cells are an exception.

Stomata are pores controlled by pairs of guard cells, often concentrated on the lower epidermis in a typical terrestrial dicot leaf. During net photosynthesis, carbon dioxide diffuses through open stomata into air spaces, dissolves in the moist surfaces of mesophyll cell walls and diffuses into the cells, reaching chloroplasts where it is used.

A vascular bundle contains xylem supplying water and mineral ions and phloem transporting food. In a leaf section, connect each structure to a process: chloroplast distribution to photosynthesis, stomata and air spaces to gas exchange, and vascular tissue to transport. Compare the original schematic with the real privet leaf micrograph. Use the arrangement of cells and air spaces as evidence; actual sections vary in shape and detail.

Step by step
  1. 1

    Orient the section

    Find the upper epidermis and the palisade layer below it.

  2. 2

    Trace the gas route

    Follow carbon dioxide from a stoma through air spaces into mesophyll cells.

  3. 3

    Link structure to function

    Explain how the arrangement supports light absorption, diffusion or transport.

Worked example

An irregular tissue with air spaces

A light-microscope section shows irregular cells separated by large spaces below the palisade layer. Identify the tissue and explain a function.

One way to explain it

It is spongy mesophyll. Its interconnected air spaces allow gases such as carbon dioxide to diffuse through the leaf towards mesophyll cells, where carbon dioxide is used in photosynthesis.

Why this answer works
  • Use position and arrangement together.
  • Name the tissue rather than just "air holes".
  • Link the spaces to diffusion and photosynthesis.

Watch out for this

A student says: "Carbon dioxide enters the xylem and is carried to each chloroplast." What is wrong with this?

Show the answer

Carbon dioxide reaches mesophyll cells mainly by diffusion through stomata and air spaces. Xylem transports water and mineral ions.

Check your understanding

Which route correctly describes carbon dioxide entering a photosynthesising leaf?

  1. Root hair -> xylem -> phloem -> mesophyll cell -> chloroplast in the leaf
  2. Cuticle -> sieve tube -> nucleus only
  3. Stoma -> air space -> moist mesophyll cell wall -> cell -> chloroplast
Show the answer

Stoma -> air space -> moist mesophyll cell wall -> cell -> chloroplast

Carbon dioxide diffuses down its concentration gradient and dissolves at the moist cell surface before entering cells.

Quick recall

Card 1 of 3. Answer in your head, then check.

How does a leaf bring carbon dioxide close to its chloroplasts?

You can return to this lesson any time.