Studied by 0 people
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
Phytochromes detect falling light levels
Auxin production decreases, ethene is inhibited
Ethene levels increase due to a lack of auxin
Ethene activates gene transcription in cells in the abscission zone to produce hydrolases
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
Root hair cells detect lack of water so produce ABA
ABA travels to leaves through xylem and binds to ligand-gated ion channels on guard cells
Calcium ions enter the cell, causing negative ions to move outside of the cell
ABA binds with these receptors, inhibiting the proton pumps and stopping the active transport of hydrogen ions out of the guard cells
This changes the pH of the guard cells, causing potassium ions to move outside of the cell
Water potential inside the cell increases
Water leaves the cell by osmosis
Guard cells become flaccid due to plasmolysis, causing stomatal closure
Transpiration is reduced
What is the role of gibberellins in seed germination?
They stimulate production of digestive enzymes, e.g. amylase
Amylase breaks down starch from other food stores to maltose
Simple sugars used in respiration for energy release for seedling growth
Auxins and cell elongation script
The meristem is at the shoots and roots, in front of the zone of elongation, which is in front of the zone of differentiation
Auxins released by the meristem diffuse down to the zone of elongation
Cells in the zone of elongation have auxin receptors
The receptor is coupled to a hydrogen ion channel which pumps hydrogen ions into the cell
The low pH causes the cell wall to become more flexible as it activates enzymes that break down the cell wall
This allows pressure from osmosis to cause the cell to expand rather than burst
Vacuoles start to form to hold more water
Permanent vacuole forms
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
Cell size is set in the zone of differentiation
