ما هي معادلة البناء الضوئي الكيميائية الموزونة؟ What is the balanced equation for photosynthesis?

المعادلة الموزونة هي: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂ (ستة جزيئات ثاني أكسيد الكربون + ستة جزيئات ماء في وجود طاقة الضوء وصبغة الكلوروفيل تعطي جزيء جلوكوز وستة جزيئات أكسجين). وتتطلب العملية تفاعلات ضوئية لإنتاج ATP و NADPH يعقبها تفاعلات لاضوئية لتثبيت الكربون. [الصيغة الرمزية الموزونة: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂]

3.1.2 Generating a Nervous Response · 3. Organism Level Systems | Ocr Gateway GCSE Combined Science A J250: Biology

Section 1 of 9From stimulus to impulse

Each sense organ has receptor cells sensitive to one type of stimulus.

From stimulus to impulse

When stimulated, the receptor produces electrical impulses in sensory neurones, which travel to the brain.

The brain interprets the impulses by where they arrive and how many arrive per second.

This explains why light striking the eye is seen as vision, not sound: the impulses go to the visual area of the brain.

Section 2 of 9The eye

The eye contains light receptors (rods and cones) in the retina. Rods work in dim light and detect black and white; cones detect colour and work in bright light. Key parts:

The eye

Cornea

Cornea transparent front surface; refracts (bends) light, doing most of the focusing.

Iris

Iris ring of muscle controlling the size of the pupil.

Lens

Lens flexible and transparent; refracts light further to focus an image on the retina.

Ciliary muscles and suspensory ligaments

Ciliary muscles and suspensory ligaments change the shape of the lens.

Optic nerve

Optic nerve carries impulses to the brain.

Section 3 of 9Adapting to light and distance

Bright Light — Interactive Biology Diagram

Pupil reflex

in bright light, circular muscles in the iris contract and radial muscles relax, so the pupil gets smaller, which protects the retina from damage.
In dim light, radial muscles contract and circular muscles relax, so the pupil gets larger and lets in more light.

Accommodation to focus on a near object, the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes fatter (more curved) and refracts the light more.

Adapting to light and distance

To see a distant object the ciliary muscles relax, the ligaments are pulled tight and the lens becomes thinner (less curved).

Common defects: short sight (myopia) is when distant objects are blurred because the image forms in front of the retina, corrected with concave lenses; long sight (hyperopia) is when near objects are blurred because the image would form behind the retina, corrected with convex lenses.

Section 4 of 9The ear and skin

In the ear, sound waves make the eardrum vibrate.

1

The vibrations are passed by small bones to the fluid-filled cochlea, where sound receptors turn the movement into impulses,

which travel in the auditory nerve.

2

Other parts of the ear contain receptors for balance.

3

The skin contains receptors for touch, pressure, temperature and pain.

4

These are more numerous in sensitive areas such as the fingertips, which allows detailed information to be gathered.

Section 5 of 9Reaction time practical

Reaction time is the time between a stimulus and a response, such as catching a ruler.

1

Method: one person holds a metre rule vertically, with the zero level with the second person's thumb and finger.

2

The ruler is dropped without warning and the second person catches it.

3

Record the distance fallen, repeat several times, find the mean and convert it to a time with a conversion table.

4

To investigate the effect of a factor such as caffeine or practice, change only that factor and control others: use the same hand, the same person,

and the same dropper.

5

Reaction times usually improve with practice because the neural pathways become more efficient.

6

Results are only a rough estimate because the dropper may release the ruler inconsistently and the person may anticipate the drop.

Section 6 of 9Colour vision, night vision and eye problems

Pupil size in dim light compared with bright light

The retina contains about 120 million rods but far fewer cones.

1

Rods are very sensitive and many share a single neurone, which is why vision is good in dim light but poorly detailed and without colour.

2

Cones are concentrated at a central spot called the fovea, where vision is sharpest.

3

There are three types of cone, sensitive to red, green or blue light; the brain interprets the mixture of signals as a colour.

4

Colour blindness occurs when one type of cone is missing or weak.

5

Cones need brighter light, so at night coloured objects appear grey.

As people age the lens becomes less elastic, so it can no longer become fat enough to focus on close objects, and many people need reading glasses.

1

Cataracts, where the lens becomes cloudy, can be treated by replacing the lens with a plastic one.

2

Laser surgery reshapes the cornea to correct short or long sight.

3

When describing eye defects, always state where the image is focused relative to the retina and which kind of lens is used to correct it.

4

Bright light and sudden darkness both illustrate that the nervous system adjusts the eye automatically,

and the pupil reflex is a good example of a reflex that you do not control consciously.

Section 7 of 9Exam-style questions

Reflex arc: cat withdraws from painful stimulus (cactus)

Rod Cell — Dark vs Light (Side-by-Side Simulation)

Explain how the eye changes shape of the lens to focus on a near object. [3]3 marks
Describe and explain what happens to the pupils when a person walks from a dark room into bright sunlight. [3]3 marks
A student's reaction times in a ruler test are 0.21 s, 0.19 s, 0.23 s and 0.17 s. Calculate the mean. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Retina	The light-sensitive layer at the back of the eye.
Cornea	Transparent front part of the eye that refracts light.
Iris	Muscle that controls pupil size.
Accommodation	Change in lens shape to focus on near or distant objects.
Cochlea	Part of the ear containing sound receptors.
Reaction time	The time taken to respond to a stimulus.
Optic nerve	Carries impulses from the retina to the brain.

Section 9 of 9Exam technique

1

Saying that the lens gets thinner to see near objects; it gets thicker.

2

Saying that rods detect colour; cones do.

3

Describing the pupil as a muscle; the iris is the muscle and the pupil is the hole.

4

Claiming the ear detects sound with the eardrum; the receptors are in the cochlea.

5

Forgetting to control variables in the reaction time practical.