Topic 6 - Receptos

Tropism is the term given to when plants respond, via growth, to stimuli. Tropisms can be positive or negative, growing towards or away from a stimulus. Plant respond to light, gravity and water. Tropisms are controlled by specific growth factors and one key example is indoleacetic acid (IAA).

It is a type of auxin and can control cell elongation in shoots and inhibit growth of cells in the roots. It is made in the tip or the roots and shoots but can diffuse to other cells.

Shoots need light for the LDR in photosynthesis which is why plants grow and then bend towards light. This is controlled by IAA. This is positive phototropism.

Shoot tip cells produced IAA, which causes cell elongation in shoots, and this IAA diffuses to other cells. If there is unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there. The lAA causes the cells on the shaded side to elongate more and this causes the plant to bend towards the light source.

Roots do not photosynthesise and do not require light and are more able to anchor the plant if they are deep in the soil away from light. In roots, a high concentration of IAA inhibits cell elongation, causing roots cells to elongate more on the lighter side and so the root bends away from light. This is negative phototropism.

IAA will diffuse from the upper side to the lower side of a shoot. If a plant is vertical, this causes the plant cells to elongate and the plant grows upwards. If a plant is on its side, it will cause the shoot to bend upwards. This is negative gravitropism.

IAA moves to the lower side of roots so that the upper side elongates elongates and the root bends down towards gravity and anchors the plant in. This is positive gravitropism.

A stimulus is a detectable change in the environment. These changes can be detected by cells, which are called receptors. This triggers a response through the flow chart shown below. Stimulus –> Receptor –>Coordinator–> Effector–>Response

The nervous system is made up of the peripheral and central nervous system. The PNS includes the receptors, sensory and motor neurones, whilst the CNS is the coordination centres such as the brain and spine

Each receptor responds only to specific stimuli and this stimulation of a receptor leads to the establishment of a generator potential which can cause a response.

Pacinian corpuscle Rods Cones

Pacinian corpuscle responds to pressure changes. These receptors occur deep in skin, mainly in fingers and feet. It consists of a single sensory neurone wrapped with layers of tissue separated by gel. The sensory neurone in the Pacinian corpuscle has special channel proteins in its plasma membrane.

Plasma membranes contain channel proteins which allows ion transportation and the membranes surrounding the sensory neurones have stretch-mediated sodium channels. These channels will open and allow Na* to enter the sensory neurone only when they are stretched and deformed.

In the resting state, Na+ channels are too narrow for Na+ to diffuses into the sensory neurone - therefore resting potential is maintained. When pressure is applied it deforms the neurone plasma membrane, stretches and widens the Na+ channels so Na+ diffuses in which leads to the establishment of a generator potential

the retina contains two types of photoreceptors, rods and cones.

Rods, called because of their shape, cannot distinguish different wavelengths of light and processes imagines in black and white. Rods can detect light of very low intensity though, because many rod cells connect to one sensory neurone (retinal convergence).

The pigment of rod cells (rhodopsin) must be broken down by light energy. There is enough energy from low-intensity light to cause the breakdown. Enough pigment has to be broken down for the threshold to be met in the bipolar cell. This threshold can be reached even in low light because so many rod cells are connected to a single bipolar cell, this is in an example of summation.

However, this retinal convergence means that the brain cannot distinguish between the separate sources of light that stimulated it. Two light sources close together cannot be seen as separate- Rod cells give low visual acuity.

There are three types of cone cells that contain different types of iodopsin pigment (red, green and blue) which all absorb different wavelengths of light. Depending on the proportion of each cone cell that is stimulated we perceive colour images.

lodopsin is only broken down if there is a high light intensity, so action potentials can only be generated with enough light.

Usually only one cone cell connects to a bipolar cell. Therefore, no spatial summation occurs and cones can only respond to high light intensity, which is why we can’t see colour when it is dark. As each cone is connected to one bipolar cell, the brain can distinguish between separate sources of light detected- cone can give high visual activity.

The distribution of rods and cones in the retina is uneven. Therefore, most cone cells are located near the fovea as they only respond to high light intensities and rod cells further away as these can respond at lower light intensities.

Light is focused by the lens on the part of the retina opposite the pupil, the fovea, which will receive the highest intensity of light.

Cardiac muscle is myogenic, meaning it contracts on its own accord, but the rate of contraction is controlled by wave of electrical activity