AP Biology - Plants and Human Body Systems
OpenStax Biology for AP — Chapter 23: Plant Form and Physiology — AP BIOLOGY Revision Notes & Topical Questions
5 min read 10 parts Lesson 4 of 15 Unit quiz
Lesson summary
OpenStax Biology for AP — Chapter 23: Plant Form and Physiology — AP BIOLOGY notes: Describe plant tissues and cell types, water and sugar transport, root zones, plant hormones and responses to stimuli.Organ systems — a vascular plant has a shoot system (stems, leaves, flowers, f…
- Dermal tissue
- Vascular tissue
- Ground tissue
- Water potential
- Cohesion-tension
- Taproot and fibrous systems
Part 1 of 8Plant body and tissue systems
Plant body and tissue systems
Organ systems
- Organ systems a vascular plant has a shoot system and a root system.
- (stems, leaves, flowers, fruits)
Dermal tissue
Dermal tissue the protective outer layer, containing epidermal cells, stomata, guard cells and trichomes.
Vascular tissue
Vascular tissue xylem and phloem, which conduct water, minerals and photosynthetic products.
Ground tissue
Ground tissue mainly parenchyma, collenchyma and sclerenchyma, responsible for photosynthesis and support.
Growth
- Growth primary growth at root and shoot tips increases length, and secondary growth in woody plants increases thickness.
Part 2 of 8Plant cell types
| Cell type | Alive at maturity | Function |
|---|---|---|
| Parenchyma | Yes | Thin walls; photosynthesis, storage and repair |
| Collenchyma | Yes | Unevenly thickened primary walls; flexible support of young stems and petioles |
| Sclerenchyma (fibres, sclereids) | Usually no | Thick lignified secondary walls; rigid support |
| Tracheids (xylem) | No | Long, thin cells; water passes through pits, so slower |
| Vessel elements (xylem) | No | Wider, with perforated end walls joined end to end; more efficient conduction |
| Sieve-tube members and companion cells (phloem) | Yes | Carry sugars (translocation); companion cells keep sieve tubes alive |
| Guard cells | Yes | Pair of cells that open and close each stoma |
Part 3 of 8Water transport and stomata
Water potential
Water potential a measure of the potential energy difference between a water sample and pure water, affected by solutes, pressure, gravity and matric potential.
Cohesion-tension
- Cohesion-tension transpiration pulls the water column up the xylem;
- hydrogen bonding makes water cohere and adhesion to the walls supports the column.
Opening
- Opening guard cells take up K+, water follows by osmosis, the cells swell and bow apart.
Closing
- Closing loss of K+ and water closes the stoma.
Stomata are usually open in light, because CO2 is needed, and closed at night, in drought (abscisic acid) and in very hot, dry conditions.
CAM plants reverse this pattern and open their stomata at night.
Transpiration is fastest when it is hot, dry, windy and bright, because the water-vapour gradient is steep and the stomata are open.
Part 4 of 8Sugar transport in phloem
- Sugars (mainly sucrose) move from sources to sinks.
1
Sucrose is actively loaded into the sieve-tube elements at the source.
2
The higher solute concentration draws water in from the xylem by osmosis.
3
The positive pressure pushes water and solutes down the pressure gradient.
4
Sucrose is unloaded at the sink, and the water returns to the xylem.
Part 5 of 8Root growth
Root growth
Taproot and fibrous systems
- Taproot and fibrous systems roots anchor the plant, absorb water and minerals, and store food.
Root cap
- Root cap protects the growing tip and senses gravity.
Zone of cell division (apical meristem)
- Zone of cell division (apical meristem) produces new cells by mitosis.
Zone of elongation
- Zone of elongation cells lengthen, pushing the root through the soil.
Zone of maturation (differentiation)
- Zone of maturation (differentiation) cells specialise and root hairs form to increase absorbing surface area.
Part 6 of 8Plant hormones
| Hormone | Main effects |
|---|---|
| Auxin | Cell elongation, apical dominance, root growth, phototropism and gravitropism |
| Cytokinins | Stimulate cell division and counter apical dominance |
| Gibberellins | Break seed dormancy and promote stem growth |
| Abscisic acid | Induces seed and bud dormancy; closes stomata |
| Ethylene | Speeds fruit ripening and leaf drop |
Part 7 of 8Tropisms and photoperiod
Phototropism
- Phototropism auxin moves to the shaded side, those cells elongate more, and the stem bends toward the light.
- Blue-light receptors (phototropins and cryptochromes) detect the light.
Gravitropism
- Gravitropism shoots grow up and roots grow down; amyloplasts containing starch sense gravity.
Thigmotropism
- Thigmotropism growth in response to touch, such as a vine coiling round a support.
Photoperiodism
- Photoperiodism red light converts phytochrome to its Pfr form, which controls germination and flowering in response to day length.
Part 8 of 8Plant defences
Plant defences
Physical barriers such as bark and spines protect tender tissues.
Chemical defences include toxic secondary metabolites and hormones that trigger further defence responses.
Key terms
| Term | Meaning |
|---|---|
| Transpiration | Evaporation of water from leaves. |
| Water potential | Measure of the potential energy of water relative to pure water. |
| Translocation | Movement of sugars in the phloem from source to sink. |
| Auxin | Hormone causing cell elongation and directional growth. |
| Phytochrome | Light receptor that controls germination and flowering. |
| Thigmotropism | Growth in response to touch. |
Avoid these mistakes
1
Saying sugars are carried in xylem — they travel in phloem
2
Saying auxin accumulates on the lit side in phototropism — it moves to the shaded side
3
Thinking mature sclerenchyma is a living cell — it is usually dead
4
Saying transpiration is greatest in humid, still, dark conditions — it is greatest when hot, dry, windy and bright
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