A level biology chapter 15: coordination in pants

What are the similarities in electrical communication between mammals and plants?

What are the similarities in electrical communication between mammals and plants?

• have electrochemical gradients

• both have sodium-potassium pumps

• have resting potential

• membrane depolarises — action potential

What are the differences in electrical communication between mammals and plants?

Mammals depolarise due to Na+ entering, plants — due to Cl- moving out

Action potential travels along the neurones in mammals, plants — along the cell membrane and signalling cell-to-cell via plasmodesma

Speed of transmission is faster in mammals, plants — slower

Structure of Venus fly trap in relation to its function

• Nectar glands — to attract insects

• Stiff outer edges — to interlock to trap insect inside

• Lobe — specialised leaf

• Sensory hairs — deflection stimulates action potential and cause leaf to fold

• Digestive glands — to secrete digestive enzymes on lobe surface

• Hinge

How does action potential occurs in Venus fly trap?

Sensory hair cell is receptor and it detects touch.

If 2 hairs are touches Ca2+ ion channels open at cell at base of hair causing Ca2+ to flow in.

Cell membrane is depolarised and action potential occurs.

Depolarisation spreads over lobe to hinge cells.

How does action potential cause the leaf to fold?

Acid growth at hinge cells cause leaf to fold.

1. H+ pumped out of cells into cell walls

2. Cross-links in cell wall are broken

3. Calcium pectate (of middle lamella) dissolves

4. Cell wall loosens

5. Ca2+ enter hinge cells

6. Water enters hinge cells by osmosis

7. Cells become turgid

8. Lobes become concave

9. Trap shuts and elastic tension is released

How are digestive enzymes released after the trap shuts?

As the insect squirms around further defections of sensory hair occur.

This triggers action potentials to seal the trap and stimulates the entry of Ca2+ into gland cells

Ca2+ stimulate exocytosis of vesicles containing digestive enzymes

(Trap stays shut for up to 1 week for digestion)

What happens after digestion (Venus fly trap)?

Cells of upper surface of hinge grow slowly

Leaf reopens and elastic tension builds in the cell walls of hinge

Adaptations of Venus fly trap to conserve energy and avoid closing unnecessarily

Stimulating of single hair does not trigger closure

• At least two hairs must be touched or one hair touched twice within 35 seconds

• Prevent trap from closing when raining or debris falls into trap

Gaps between stiff hairs allow very small insects to crawls out

About chemical communication in plant

Plant hormones are produced in a variety of plant tissues (not in endocrine glands)

Plant hormones interact with receptors inside/outside cell and initiate a signalling cascade

Plant hormones move directly from cell to cell by active transport or diffusion via phloem/xylem vessels

What are auxins?

Auxins are a group of several chemicals

The main auxin?

IAA — the main auxin (indole 3-acetic acid)

Function of auxins

Promotes growth by cell elongation at tips of roots and shoots, inhibits lateral growth/branching at growing tips of shoots via acid growth hypothesis

Where are auxins synthesised?

Synthesised in growing tips of shoots & roots, apical meristems (specialised zones of growth found at the tips of plants) where there is active mitosis

Gibberellin relations to auxins?

It enhances IAA as IAA is not solely responsible for apical dominance

Relations between concentration of auxin and cell elongation

Higher the concentration of auxin the more cell elongation

Explain and describe the acid growth hypothesis

1. Auxin binds to receptors in cell surface membrane

2. Stimulates proton pumps in cell surface membrane

   • H+ is actively transported from cytoplasm into cell wall

   • Cell wall becomes more acidic

3. pH-dependent enzymes — expansis activated to weaken cell wall by breaking hydrogen bonds between cellulose microfibrils

   • Cell wall becomes loose — more elastic, can stretch

4. Ions enter cell and water potential of cell decreases

   • Water enter cell by osmosis

   • Increase in turgor pressure

   • Cell wall expands

   • Causes elongation of cell

What is gibberellins (GA) and where are they synthesised?

They’re plant growth regulators and are synthesised in young leaves, seed and stems.

What are the roles of gibberellin?

Seed germination

Stimulates cell division and cell elongation in stems

Describe and recognise barley seed structure

Aleurone — protein-rich layer

Endosperm — storage of starch

Scutellum — “seed leaf”

Embryo — develops into a new plant

Pericarp and testa — tough, protective layer

Why are seed dormant?

Seed are dormant when there is insufficient water or sunshine.

They remain dormant sue to DELLA proteins acting as inhibitors of cell growth and seed germination. Due to this seed dormancy is maintained.

The process of gibberellin effect on barley seed

1. Seed absorbs water by osmosis and the water stimulates production of gibberellin by embryo

2. GA diffuses into cells of aleurone layer

   • causes breakdown of DELLA proteins

   • switches on gene coding for hydrolytic enzymes (e.g amylase)

   • storage proteins in aleurone are broken down into amino acids which stimulates synthesis of amylase

3. Amylase diffuses into endosperm

   • hydrolyses starch into maltose

   • maltose is converted into glucose

4. Glucose diffuses into embryo

   • provides source of energy for growth of embryo plant