plant responses OCR (new spec !!!!)

Plant Responses for OCR A-Level Biology new spec (2025)

What are some examples of plant responses?

Responses to abiotic stress, responses to herbivory (chemical defences / response to touch), tropisms

What are some plant responses to abiotic stress?

Increased soil water salinity and presence of heavy metals (lead, copper, zinc)

How do some plants respond to freezing temperatures?

Some plants produce an antifreeze chemical in their cells that decreases the formation of ice crystals

How do plants deal with drought?

Reduce water loss by transpiration, e.g. shutting the stomata or dropping leaves

What are alkaloids?

Bitter-tasting nitrogenous compounds that either deter or kill herbivores

What are pheromones?

Chemical signals to nearby plants of the same species that they are under attack from herbivores, triggering other defences. They act as a signal to attract a herbivorous insect’s natural predators.

What are tropisms?

Directional growth responses, can be positive or negative.

What are nastic responses?

Non-directional responses.

What is phototropism?

Plant response to light; plants usually grow towards light.

What is geotropism?

Plant response to gravity, e.g. roots grow downwards towards gravity.

What is chemotropism?

Plant response to chemicals, e.g. pollen tube grows towards the ovary.

What is thigmonasty?

A non-directional response to touch e.g. Venus flytraps rapidly close in response to touch.

What is the role of plant hormones in responses?

They are chemical messengers that reach target cells, and are specific because they have a specific tertiary structure.

Why do deciduous trees drop their leaves?

They lose their leaves in hot and dry environments to reduce water loss, and in temperate climates during winter when water absorption is difficult.

What is the abscission layer?

Develops at the base of the leaf stalk and is a layer of parenchyma cells with thin walls, which makes them weak and easy to break.

What does ethene contribute to leaf loss in deciduous plants?

Stimulates the breakdown of cell walls in the abscission layer, causing the leaf to drop off.

What role do auxins play in leaf loss?

They inhibit leaf loss and are produced in young leaves, making the leaf stalks insensitive to ethene. Their concentration decreases as the plant ages, leading to increased sensitivity to ethene.

Abscission script

1. Phytochromes detect falling light levels

2. Auxin production decreases, ethene is inhibited

3. Ethene levels increase due to a lack of auxin

4. Ethene activates gene transcription in cells in the abscission zone to produce hydrolases

5. Enzymes digest the cell wall in the separation zone

What happens in times of water stress?

ABA is produced by plants to stimulate the closing of their stomata.

What receptors do guard cells have?

Receptors for ABA on their cell surface membranes.

What does ABA bind with?

The receptors on guard cells. This inhibits proton pumps and stops the active transport of hydrogen ions out of the guard cells.

How is osmosis affected by ABA?

Loss of ions increases water potential of guard cells, so water laves guard cells by osmosis. They become flaccid, causing stomatal closure.

What is the role of calcium ions in stomatal closure?

They move into the cytoplasm of guard cells and act as second messengers. They cause channel proteins to open that allow negatively charged ions to leave guard cells, stimulating the opening of further channel proteins that allow potassium ions to leave. They also stimulate the closing of channel proteins that allow potassium ions to enter guard cells.

Stomatal closure script

1. Root hair cells detect lack of water so produce ABA

2. ABA travels to leaves through xylem and binds to ligand-gated ion channels on guard cells

3. Calcium ions enter the cell, causing negative ions to move outside of the cell

4. ABA binds with these receptors, inhibiting the proton pumps and stopping the active transport of hydrogen ions out of the guard cells

5. This changes the pH of the guard cells, causing potassium ions to move outside of the cell

6. Water potential inside the cell increases

7. Water leaves the cell by osmosis

8. Guard cells become flaccid due to plasmolysis, causing stomatal closure

9. Transpiration is reduced

What is the role of gibberellins in seed germination?

1. Water absorbed

2. Gibberellins synthesised by embryo

3. Gibberellins stimulate aleurone layer cells to synthesise amylase

4. Amylase hydrolyses starch to form maltose

5. Maltose converted to glucose

6. Glucose transported to embryo, providing energy for growth

Auxins and cell elongation script

1. The meristem is at the shoots and roots, in front of the zone of elongation, which is in front of the zone of differentiation

2. Auxins released by the meristem diffuse down to the zone of elongation

3. Cells in the zone of elongation have auxin receptors

4. The receptor is coupled to a hydrogen ion channel which pumps hydrogen ions into the cell

5. The low pH causes the cell wall to become more flexible as it activates enzymes that break down the cell wall

6. This allows pressure from osmosis to cause the cell to expand rather than burst

7. Vacuoles start to form to hold more water

8. Permanent vacuole forms

9. Further down the zone of elongation, enzymes break down the auxin, so the receptor is no longer bound to auxin- hydrogen ions don’t build up and the pH rises so enzymes aren’t activated, meaning the cell wall is no longer flexible

10. Cell size is set in the zone of differentiation

What are some differences between auxins and gibberellins?

1. Auxins are mainly found in higher plants whereas gibberellins are mostly found in fungi

2. Auxins promote growth in shoot segments whereas gibberellins promote growth in intact shoots

3. In auxins, there is little effect on leaf growth, whereas in gibberellins leaf growth is enhanced

4. Auxin transport is polar, whereas gibberellins show channel transport in different directions

Darwin’s experiment (1880)

1. Removing the tip of a coleoptile stopped the phototropic response to a unidirectional light source

2. The tip of the coleoptile was covered with an opaque cap to block out the light

3. This also stopped the phototropic response from occurring, showing that the tip of the coleoptile was responsible for detecting light

Boysen-Jensen’s experiment (1913) (1)

1. The cut tip of the coleoptile was replaced and a gelatin block was inserted as a barrier in between, restoring the phototropic response

2. This showed that the stimulus for growth was a chemical hormone, which was able to travel through the gelatin block

Boysen-Jensen’s experiment (2)

1. A mica barrier was inserted halfway through the coleoptile below the tip, first on the lit side and then on the shaded side

2. Mica barrier on the lit side - phototropic response

3. Mica barrier on the shaded side - no response

4. This confirmed that the stimulus for growth was a hormone, and showed that it was produced at the tip before travelling down on the opposite side to the stimulus

5. It also showed that the stimulus acted by causing growth on the shaded side (not inhibiting growth on the lit side)

Paal’s experiment (1919)

1. The tip of the coleoptile was cut off and then replaced off centre in the dark

2. The side of the coleoptile that the top was placed on grew more than the other side, causing the coleoptile to curve

3. This showed that in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating the growth of the tissue

Went’s experiment (1926)

1. Went placed the cut tip of a coleoptile on a gelatin block, allowing hormones to diffuse in

2. The block was placed off centre and in the dark

3. The side of the coleoptile that the block was placed on grew more than the other side, causing the coleoptile to curve

4. Greater concentration of hormone - more curving

How is growth controlled by elongation?

1. IAA is synthesised in growing tips of roots and shoots

2. IAA coordinates phototropisms by controlling growth by elongation

3. IAA molecules are synthesised in the meristem and pass down the stem to stimulate elongation growth

4. IAA molecules activate expansins, which loosen bonds between cellulose microfibrils, making cell walls more flexible

What is the role of IAA in phototropism?

1. Phototropism affects shoots and the top of stems

2. The concentration of IAA determines the rate of cell elongation within the region of elongation

3. If the concentration of IAA is not uniform on either side of a root / shoot, uneven growth occurs

4. In shoots, higher IAA concentrations result in a greater rate of cell elongation

What is the role of IAA in geotropism?

1. Negative geotropism - gravity modifies the distribution of IAA so it accumulates on the lower side of the shoot, increasing the rate of growth and causing upward growth

2. Positive geotropism - in roots, higher concentrations of IAA result in a lower rate of cell elongation, and the IAA that accumulates at the lower side of the root inhibits cell elongation - causing the lower side to grow at a slower rate

Auxins as selective weed killers

• In high enough concentrations, auxins cause such rapid growth that plant tissues become damaged, causing pathogens to enter

• Synthetic auxins are applied to plants in concentrations 100x greater than natural hormones

• Effective against weeds that occur in cereal crops or grass lawns

Auxins as rooting powders

• At low doses, auxins can be used to stimulate cuttings to grow new roots

• The lower end of the cutting is dipped in the powder before being planted in compost

Ethene in ripening

• Often used for fruits that are delicate and soft when ripe

• These fruits can be harvested when unripe, transported and then ripened artificially

Other commercial uses of plant hormones

• Auxins and gibberellins can be used to make unpollinated flowers develop fruit - this is often used in the production of seedless fruits

• Auxins can also be used to stop trees from dropping their fruit before it has been harvested