Section 2 j) Specification

2.77 understand that organisms are able to respond to changes in their environment

Organisms have receptors to detect changes in the environment, that sends a signal through the body to create the appropriate response.

2.78 understand that homeostasis is the maintenance of a constant internal environment and that body water content and body temperature are both examples of homeostasis

Humans are homeostatic organisms; their internal conditions don't change. This requires constant maintenance, as we are constantly using and removing water from our bodies, and the external environment is constantly changing in temperature. Thermoregulation is the regulation of body temperature, and osmoregulation is the regulation of water content.

2.79 understand that a coordinated response requires a stimulus, a receptor and an effector

One of the criteria for life is sensitivity. This means that an organism can detect a change in environment, a stimulus, using a receptor, which will then send a signal to an effector which will respond to the change.


Flowering plants:

2.80 understand that plants respond to stimuli

Plants are living organisms, and can therefore respond to stimuli, such as changes in light intensity (phototropism), water (hydrotropism) and gravity (geotropism). The response is usually in plant hormones, auxins being the hormones that are most important.
A positive tropism is when the plant grows towards the stimulus, and a negative tropism is when the plant grows away from the stimulus.
Auxin causes cells in stems to grow more, and cells in roots to grow less:

As depicted above, the auxin gathering on one side of the stem stimulates growth of cells, causing that side of the stem to elongate more than the other side, and the stem to curve. The opposite is true for roots: auxin gathering in the bottom stunts growth, causing that side to grow less than the other side and the root to bend downwards.

2.81 describe the geotropic responses of roots and stems

Roots are positively geotropic, they respond to gravity by growing in the direction of it, whereas stems are negatively geotropic, they respond to gravity by growing in the opposite direction. Auxin collects in the bottom of the plant due to gravity, causing the growth of cells to be uneven (stunted on one side of the root and increased on the same side of the stem), meaning the roots will bend down and the stem will bend up.

2.82 describe positive phototropism of stems

Stems are positively phototropic, auxin collects on the side that receives less light, which causes the cells there to grow more and the plant to then bend towards the light.


Humans:

2.83 describe how responses can be controlled by nervous or by hormonal communication and understand the differences between the two systems

In the human body, there are two systems that respond to stimuli: The endocrine system, which releases hormones, and the nervous system, which releases electrical impulses.
These two systems have a number of similarities and differences.

2.84 understand that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves

The CNS or central nervous system consists of the brain and a bundle of neurones called the spinal cord. It is connected to the rest of the body by neurones, which make up the peripheral nervous system or PNS. The neurones are connected to sensory organs, such as the eyes and skin, which detect stimuli and cause an electrical impulse through the body, to create a response.

2.85 understand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses

When a receptor is stimulated, it sends an electrical impulse to the sensory neurone, which can be identified by the cell body in the middle of the axon. The sensory neurone carries the impulse to the relay neurone, which then carries the impulse to the motor neurone.
The motor neurone carries the impulse from the relay neurone to the effector and causes the response. This all happens in a fraction of a second.

Sensory neurone: it has the cell body poking out, like it's an eye looking around to SENSE what's going on.
Relay neurone: Shorter, can transmit signals both ways because it's versatile like that. symmetrical(ish??) so the cell body is central.
Motor neurone: it causes the effect, so it thinks it's important that's why it has a big head.

2.86 describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object

1. Receptor in the finger detects that the object is hot.
2. Sensory neurone sends an electrical impulse from the finger to a relay neurone in the CNS.
3. Relay neurone transmits the signal to a motor neurone.
4. The impulse is carried along the motor neurone to an effector in the arm muscle, causing it to contract and the finger to be moved away from the hot object.
This prevents the tissue from being damaged in the finger and is an involuntary (reflex) action.
A synapse is the gap between two neurones, and this is where impulses are transmitted from one to another.

2.87 describe the structure and function of the eye as a receptor

The eye is protected by the conjunctiva, which is a membrane at the front of the eye, over the cornea. The cornea is the outer layer of the eye, which can develop cloudiness with age. It contains the aqueous humour, a clear fluid that refracts light. The light is let in through the pupil, which is enlarged or shrunk by the iris to let in more or less light (iris is controlled by circular and radial muscles). The light passes through the pupil to the lens, which is a flexible fluid-filled sac, suspended by suspensory ligaments and ciliary muscles. The ciliary muscles relax to make the lens more disc-like, and they contract to make the lens more round. The lens focuses the light through refraction, allowing it to then pass through the vitreous humour to the retina, focusing specifically on a point known as the fovea, where more rods and cones are found (the actual receptors) and they send an impulse to the optic nerve, which sends electrical impulses through the nervous system to send the information around the body. The whole eye is encased by the choroid (which is black to prevent internal reflection) and the sclera, which is hard to protect the eye.

2.88 understand the function of the eye in focusing near and distant objects, and in responding to changes in light intensity

Accommodation is how the eye focuses on objects that are near and far.
If an object is far away, the ciliary muscles will relax, pulling the suspensory ligaments tight and making the lens more disc-like and flat. This decreases refraction so the light focuses on the fovea.
If an object is closer, the ciliary muscles contract, releasing the suspensory ligaments. This causes the lens to become more round and refraction of light to increase.

Iris reflex is how the eye responds to changes in light intensity.
If the light is brighter, the eye will detect it, sending an impulse to the optic nerve and through the CNS, and then back to the eye and to the radial and circular muscles to cause the radial muscles to relax and the circular muscles to contract, making the pupil smaller so less light is let in to protect the retina from damage from intense light.
If the light is dimmer, the eye will send a message through the nervous system so it reaches the radial and circular muscles, and will cause the radial muscles to contract and the circular muscles to relax, increasing the size of the pupil to let more light in so you can see better.

2.89 describe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation

Upon entering a hot environment, the body will attempt to cool itself to maintain its temperature. This is called thermoregulation. The skin releases heat by sweating. It releases water, which will then evaporate from the skin, taking heat energy with it and thereby cooling the surface of the skin. Blood vessels close to the surface of the skin will become wider to increase the blood flow near the surface so heat can be lost through radiation; this is called vasodilation.
Upon entering a cooler environment, the hairs on the surface of the skin prickle up to trap air and insulate the body (although this is virtually useless now, it would have been more useful before we evolved into fairly hairless humans), and the blood vessels close to the surface constrict to limit blood flow and therefore limit heat loss. This is called vasoconstriction.

2.90 understand the sources, roles and effects of the following hormones: ADH, adrenaline, insulin, testosterone, progesterone and oestrogen.

ADH: Produced in the pituitary gland in the brain, makes the tubules in the nephrons more permeable so more water is reabsorbed.

Adrenaline: Produced in the adrenal glands, creates a fight or flight response (increases heart rate, blood flow and breathing so more glucose and oxygen reaches the muscles for respiration)

Insulin: Produced in the pancreas, controls the blood sugar levels (stimulates the liver to store glucose as glycogen)

Testosterone: Produced in the testes, it is the main male sex hormone. and promotes secondary sex characteristics such as facial hair, body hair and the Adam's apple.

Progesterone: Produced in the ovaries, it maintains the uterine wall lining for pregnancy.

Oestrogen: Produced in the ovaries, it is the main female sex hormone and controls the menstrual cycle and promotes secondary sex characteristics such as widening hips, growth of breasts and body hair.