Plant Tissues, Organs and Systems

Recall from 4.2.1 that an organ is a group of different tissues working together to perform a specific function. The leaf is a perfect example.

Tissue	Where in the leaf	Job
Upper epidermis	Top surface	Transparent — lets light in; waxy cuticle reduces water loss
Palisade mesophyll	Just below upper epidermis	Main photosynthesis (lots of chloroplasts)
Spongy mesophyll	Middle/lower	Gas exchange (air spaces)
Xylem	Vein	Transport water UP
Phloem	Vein	Transport sucrose (both directions)
Lower epidermis	Bottom surface	Contains stomata + guard cells
Guard cells	Either side of stomata	Open/close stomata to control gas exchange and water loss

Both run through the vein of a leaf and continue through the stem to the roots.

Xylem:

• Dead, hollow cells joined end-to-end to form long tubes.
• Walls strengthened with lignin (a stiff polymer) — this also waterproofs the walls.
• Carries water + dissolved mineral ions from roots UP to the leaves.
• Movement powered by the transpiration stream — evaporation from leaves pulls water up.

Phloem:

• Living cells with end walls perforated to form sieve plates.
• Carries dissolved sugars (sucrose) made in the leaf to other parts of the plant — including down to the roots and up to the fruit.
• The movement is called translocation.
• Powered by active transport — needs energy from respiration.

Feature	Xylem	Phloem
Cells	Dead	Living
What's transported	Water + minerals	Sucrose (dissolved sugars)
Direction	Up only	Both ways
Strengthened with	Lignin	n/a
Process name	Transpiration stream	Translocation

Meristem tissue is found at the tips of roots and shoots and in the cambium between xylem and phloem. The cells are like plant stem cells (4.1.2.3) — they can divide and differentiate into any plant cell type.

This is biologically useful because:

• Plants grow throughout their lives (unlike most animals).
• Meristem cells can be used in cloning by tissue culture to grow many identical plants from one parent.
• Damage (e.g. losing a leaf) can be repaired because meristem cells produce new tissues.

For a 4 or 6 mark application question, examiners want you to link structure → function.

• Broad and flat — large surface area to absorb light.
• Thin — gases diffuse short distances in/out.
• Upper epidermis transparent + waxy cuticle — lets light through, stops water loss.
• Palisade mesophyll near top — chloroplasts close to light source.
• Spongy mesophyll with air spaces — large surface area inside for CO₂ to dissolve and diffuse into cells.
• Stomata on lower epidermis — controlled by guard cells; opens when light/CO₂ needed, closes to save water when hot/dry.
• Vein with xylem + phloem — supplies water for photosynthesis and takes sucrose away.

Together the roots, stem and leaves form the plant transport organ system. Each part is specialised:

Organ	Job
Roots	Anchor plant; absorb water (osmosis) + mineral ions (active transport) through root hair cells
Stem	Holds leaves in the light; contains xylem (water UP) and phloem (sucrose both ways)
Leaves	Photosynthesis; lose water through stomata (transpiration)

Root hair cells are highly adapted:

• Long, thin extensions provide a huge surface area for absorption.
• Thin cell wall = short diffusion distance.
• Many mitochondria → ATP for active transport of mineral ions against the concentration gradient.

Without water from the roots, leaves can't photosynthesise (water is a reactant) and the plant wilts because water also keeps cells turgid.

Stomata are tiny pores in the lower epidermis. Each is bordered by two guard cells which control its opening.

Open stomata (during day, when light is high and the plant needs CO₂ for photosynthesis):

• Guard cells fill with water → become turgid → curve apart → stoma opens.
• CO₂ diffuses in, O₂ diffuses out, water vapour escapes.

Closed stomata (at night, or in hot/dry/windy conditions):

• Guard cells lose water → become flaccid → straighten → stoma closes.
• Reduces water loss (and gas exchange).

This is a trade-off: a plant needs stomata open for CO₂ but closing them is the main way to save water.

Transpiration = the loss of water vapour from leaves through stomata.

The full pathway:

1. Water absorbed by root hair cells (osmosis from soil).
2. Water passes across root cortex and into xylem.
3. Xylem carries water UP the stem to leaves (the transpiration stream).
4. Water moves into mesophyll cells, evaporates into air spaces.
5. Water vapour diffuses out through stomata — transpiration.

The 'pull' on water comes from evaporation in the leaves — it isn't actively pumped. Because water molecules cohere (hydrogen bonds), they're dragged up like a chain.

Why does the plant 'allow' transpiration?

• It's an unavoidable consequence of opening stomata for gas exchange.
• It also helps cool the leaves and pulls minerals up from the roots.

Four key factors — examiners love a graph or table on this.

Factor	Effect on transpiration rate	Why
Light intensity ↑	↑	Stomata open wider for photosynthesis → more evaporation
Temperature ↑	↑	Water molecules have more kinetic energy → evaporate faster
Air movement (wind) ↑	↑	Removes water vapour around the leaf → steep concentration gradient
Humidity ↑	↓	Less concentration gradient between leaf and air → less evaporation

Investigated with a potometer — measures water uptake (a proxy for transpiration). A leafy shoot is sealed in a tube of water; a small air bubble is timed as it moves along a capillary scale.

In hot/dry/windy conditions a plant may lose more water than it can absorb → it wilts. Guard cells then close the stomata to limit loss.

Translocation is the movement of dissolved sucrose (and amino acids) through the phloem from places that make or store food (sources) to places that use or store it (sinks).

• Source = usually leaves where photosynthesis happens.
• Sinks = roots, growing shoots, developing fruit.
• Direction depends on the time of year and the plant's needs (e.g. in spring, stored sugars in roots flow UP to growing shoots).

This is why phloem transports in both directions — unlike xylem (one-way up).

Translocation requires energy from respiration (active transport), so it stops if the plant is starved of oxygen.