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

Distant object
Ciliary muscles contract, suspensory ligaments relax so the lens becomes fat

Near object
Ciliary muscles relax, suspensory ligaments contract so the lens becomes thin

Source: BBC Bitesize
Figure 1

Low light
Circular muscles in the iris relax and radial muscles contract

Bright light
Circular muscles contract and radial muscles relax

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

Structure of the eye is shown above.

Each part and its function is below:

• Sclera: the tough outer coat of the eye which protects it from mechanical damage
• Cornea: a transparent 'window' that refracts light directly into the eye and helps focus the image on the retina
• Iris: the front of the lens that controls the amount of light entering the eye
• Pupil: an opening that lets light through
• Retina: Inside a choroid layer, it's a light sensitive membrane with receptors called cones and rods (see below for more info on those)
• Optic nerve: a nerve that transmits electrical impulses with information to the brain to be interpreted
• Vitreous humor: A jelly-like substance that helps the eye keep its shape and helps refract light onto the retina
• Conjunctiva: a transparent membrane that keeps the eye moist by secreting mucus
• Lens: A biconvex structure that can change shape according to whether the ciliary muscle and suspensory ligaments relax/contract
• Fovea: Where the majority of rods and cones are found (not sure about this one)
• Suspensory ligaments: keep the lens in place and help control its shape so light can be refracted onto the retina appropriately
• Ciliary muscles: contract and relax to help control the lens shape

The eye has cones and rods in the retina. Cones detect COLORS and have high sensitivity to detail. There are red, green and blue cones. Rods are responsible for vision with low light levels and detect black and white only.

When objects are far away, ciliary muscles relax and the suspensory ligaments contract so the lens is thin. When objects are near, ciliary muscles contract and suspensory ligaments relax so the lens is fat.

Distant
Is
Thin

Near
Is
Fat

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

DEFINITION
Reflex: an automatic reaction - an action performed without conscious thought in response to a stimulus

DOWN TO BUSINESS NOW

1. Sense organ detects stimulus (let's say a hot object)
2. Sensory neurone carries electrical impulse to CNS (central nervous system)
3. Relay neurone carries impulse through CNS where a response is decided to the motor neurone
4. Motor neurone carries impulse to effector (eg muscle)
5. Response is executed (eg you pull your hand away from a hot object)

HERE'S A LINK TO THE NERVE SONG 

http://scicast.org.uk/films/2011/06/the-nerve-song.html

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

Electrical impulses are sent from receptors in sense organs when there is a stimulus to the CNS. They are then sent straight back (after a response has been decided) through nerves to the effector. This has a sort of reflex-arc effect. 

(I wrote this in purple because I felt like it)

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

The central nervous system, known as the CNS, consists of the brain and spinal cord, linked by nerves which send electrical impulses. The CNS is responsible for deciding a response to a stimulus. The receptors in sense organs (like the tongue, eyes, mouth, etc.) send electrical impulses to the CNS through nerves to your brain/spinal cord which will send electrical impulses back to the effector.

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

To carry out a response, 3 main things are needed:

• A stimulus: A change in environment
• A receptor: Something that detects the change 
• An effector: To carry out a response

Example:

• Stimulus: you touch a hot object
• Receptor: nerves in your finger
• Effector: muscles that pull your hand away from the potentially dangerous hot object

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

So yes, we know that organisms can respond to changes to their environment. Changes are known as stimuli. They can be light, pressure, chemicals, temperature, etc. Organisms must have receptors and, in order to respond, must have effectors (such as muscles or glands). The response determines whether an organism is able to survive the change in environment. 

Plant examples: they grow towards the light, roots grow towards water and towards gravitational pull.
Animal example: a gazelle will run away from a lion