AQA GCSE Biology - Paper 2

What is homeostasis?

The regulation of the conditions inside your body and cells to maintain a stable internal environment, in response to changes in both internal and external conditions

Name three coordination centres

The brain, the spinal cord and the pancreas

What is negative feedback?

When your body brings the level of something (e.g. water or temperature) back to normal if it becomes too high or too low

What detects a stimulus?

A receptor, e.g. skin receptors detect if something's too hot

What produces a response?

An effector:
Too hot - a gland - secretes hormones
Too cold - a muscle - contracts to generate heat

What makes up the nervous system?

Receptors, sensory neurones, relay neurones, motor neurones, effectors and the central nervous system (CNS)

What does the CNS do?

In vertebrates, this consists of the brain and spinal cord only
In mammals, this is connected to the body by sensory and motor neurones

What do the sensory neurones do?

They carry information as electrical impulses from the receptors to the CNS

What do the motor neurones do?

They carry electrical impulses from the CNS to effectors

What are effectors?

They are muscles/glands that respond to nervous impulses and bring about a change

What are receptors?

They are cells that detect stimuli
There are many different types e.g. taste and sound receptors

What are synapses?

They are connections between two neurones
The nerve signal is transferred by chemicals which diffuse across the gap - the chemicals then set off a new electrical signal to the next neurone

What are reflexes?

Rapid, automatic responses to certain stimuli that don't involve the conscious part of the brain
They reduce your chance of being injured

What is a reflex arc?

The passage of information in a reflex - from receptor to effector

What is the reaction time?

The time it takes for you to respond to a stimulus

What factors affect your reaction time?

Gender, age and drugs

What are the different areas of the brain?

The cerebral cortex, the medulla & the cerebellum

What does the cerebral cortex do?

This is the outer wrinkly bit
It's responsible for consciousness, memory, intelligence and language

What does the medulla do?

It controls your unconscious activities, like breathing and your heartbeat

What does the cerebellum do?

It's responsible for muscle coordination

How do scientists study the brain?

1) Studying patients with brain damage
2) Electrically stimulating the brain
3) MRI scans

Studying the brain - studying patients with brain damage

The effect of brain damage on a certain area will reveal what this part of the brain does, and the impact it has on the patient's life
E.g. if the patient struggled to move, you could identify that the cerebellum had been damaged

Studying the brain - electrically stimulating the brain

This happens when you push a tiny electrode into the tissue and give it a small zap of electricity
Stimulating a certain part of the brain will reveal what it does
E.g. if the motor area is stimulated, it will cause muscle contraction and movement

Studying the brain - MRI scans

Produces a very detailed picture of the brain's structures
Used to identify which parts of the brain are active when people are doing things e.g. listening to music or trying to remember something

Advantages of studying the brain

1) Has led to the development of treatments for disorders of the nervous system
2) Electrical stimulation can help reduce muscle tremors caused by nervous system disorders e.g. Parkinson's

Disadvantages of studying the brain

1) The brain is incredibly complex and delicate - the investigation of brain function and any treatment of brain damage or disease is difficult
2) It carries risks e.g. physical damage to the brain or increased problems with brain function

Eye - sclera

The tough, supporting wall of the eye

Eye - cornea

The transparent outer layer found at the front of the eye
It refracts light into the eye

Eye - iris

It contains muscles that allow it to control the diameter of the pupil - controls how much light enters the eye

Eye - lens

Focuses light onto the retina

Eye - retina

Contains receptor cells which are sensitive to light intensity and colour

In the eyes, what controls the shape of the lens?

The ciliary muscles and the suspensory ligaments

Eye - optic nerve

Carries impulses from the receptors on the retina to the brain

What happens to the eye in bright light?

The pupil shrinks, the circular muscles in the iris contract and the radial muscles relax

What happens to the eye in dim light?

The pupil widens, the radial muscles contract and the circular muscles relax

Looking at near objects

1) The ciliary muscles contract and the suspensory ligaments slacken
2) The lens becomes fat
3) The rate of refracting light increases

Looking at distant objects

1) The ciliary muscles relax and the suspensory muscles tighten
2) The lens goes thin
3) The rate of refracting light decreases

Long-sighted vision - hyperopia

Happens when the lens is in the wrong shape and doesn't refract the light enough OR the eyeball is too short
The images of near objects are brought into focus behind the retina

Short-sighted vision - myopia

Happens when the lens is in the wrong shape and refracts too much light OR the eyeball is too long
The images of near objects are brought into focus in front of the retina

How do you fix hyperopia?

Convex lens - the lens refracts the light rays so they focus on the retina

How do you fix myopia?

Concave lens - the light rays focus on the retina

Treatments for Vision Defects - contact lenses

Thin lenses that sit on the surface of the eyes and are shaped to compensate for the fault in focusing
They are lightweight and almost invisible
They are more convenient than glasses
Soft lenses are more comfortable, but carry a higher risk of eye infections than hard lenses

Treatments for Vision Defects - laser eye surgery

A laser is used to vaporise tissue, changing the shape of the cornea
Slimming it down makes it less powerful and improves short sight
Changing the shape so that it's more powerful improves long sight
Completely corrects vision
Risk of complications e.g. infection or the eye reacting in a way that makes your vision worse than before

What is the thermoregulatory centre?

It's found in the hypothalamus in the brain and contains receptors that are sensitive to the temperature of the blood flowing into the brain

Temperature is too high

1) Temperature receptors detect that the core body temperature is too high
2) The thermoregulatory centre acts as a coordination centre - it receives the information and triggers the effectors
3) Effectors (glands) produce a response and counteract the change

Temperature is too low

1) Temperature receptors detect that the core body temperature is too low
2) The thermoregulatory centre acts as a coordination centre - it receives the information and triggers the effectors
3) Effectors (muscles) produce a response and counteract the change

Too hot - responses

1) Sweat is produced by sweat glands and evaporates from the skin - transfers energy to the environment
2) The blood vessels supplying the skin undergo vasodilation - more blood flows close to the surface in order to transfer energy to the environment

Too cold - responses

1) Hairs stand up to trap an insulating layer of air
2) Less sweat is produced
3) Blood vessels supplying the skin capillaries undergo vasoconstriction - the skin's blood supply is cut off
4) Your muscles contract to make you shiver - this needs respiration and so transfers energy to warm the body

What are hormones?

Chemical molecules released directly into the blood
They control things in organs and cells that need constant adjustment

What are hormones produced by?

Endocrine glands - these make up the endocrine system

Pituitary gland

Located in the brain
Produces many hormones that regulate body conditions
Known as the 'master gland' - the hormones produced act on other glands and direct them to release hormones to bring about change

Ovaries

Females only
Located in the vagina
Produce oestrogen - involved in the menstrual cycle

Testes

Males only
Located in the testicles
Produce testosterone - controls puberty and sperm production

Thyroid gland

Located in the neck
Produces thyroxine - involved in regulating metabolic rate, heart rate and temperature

Adrenal gland

Located above the kidneys
Produces adrenaline - used to prepare the body for a 'flight or fight' response

Pancreas

Located in front of the kidneys
Produces insulin - regulates the blood glucose levels

Nerves

Very fast actions
Act for a very short time
Act on a very precise area

Hormones

Slower action
Act for a long time
Act in a more general way

How is glucose put into the blood?

Eating foods rich in carbohydrates

How is glucose removed from the blood?

The normal metabolism of cells
Vigorous exercise

What is excess glucose stored as and where?

Glycogen in the liver and muscles

What happens when the blood glucose level is too high?

1) The pancreas secretes insulin into the blood.
2) Glucose moves into the liver and muscle cells.
3) The insulin causes the liver to turn glucose into glycogen and the blood glucose levels decrease

What happens when the blood glucose level is too low?

1) The pancreas secretes glucagon into the blood.
2) Glucagon causes the liver to convert glycogen back into glucose
3) Glucose is released into the blood by the liver and the blood glucose levels increase

Type 1 Diabetes

The pancreas produces little or no insulin - this can cause blood glucose levels to rise to a level that can kill someone
This is identified early on, generally in childhood

What affects the amount of insulin needed for insulin therapy?

The person's diet
The person's PAL

Ways to combat Type 1 Diabetes

1) Insulin therapy - several injections of insulin throughout the day, mostly at mealtimes, ensures glucose is removed from the blood quickly after food has been digested
2) Limiting the intake of foods rich in simple carbohydrates (monosaccharides e.g. glucose and fructose)
3) Regular exercise

Type 2 Diabetes

When a person becomes resistant to their own insulin - insulin is still produced but the cells don't respond properly to it
Obesity can contribute to the development of this - it's a risk factor

Ways to combat Type 2 Diabetes

1) Eating a carbohydrate-controlled diet
2) Regular exercise

What do the kidneys do?

They produce urine by taking waste products out of your blood

What happens during filtration?

Substances are filtered out of the blood as it passes through the kidneys

What happens during selective reabsorption?

Useful substances like glucose, ions and water are absorbed back into the blood

Which substances are removed from the blood?

Urea, ions and water

WP - Urea

Ammonia is converted to urea in the liver as it's toxic and is then transported to the kidneys where it's filtered out of the blood and excreted from the body in urine
Ammonia is produced as a waste product from deamination - a process in the liver which involves converting any excess amino acids which can't be stored in the body as proteins into fats and carbohydrates

WP - Ions

If the ion content of the body is wrong, the balance between ions and water could be disturbed, therefore too much or too little water is taken into the cells via osmosis - this can damage cells or cause them to not function properly
Some ions are lost when we sweat (e.g. sodium) - however, this amount isn't regulated, therefore the kidneys must maintain the balance of ions in the body
During filtration, the right amount of ions is reabsorbed into the blood and the rest is removed in urine

WP - Water

The body is constantly balancing the volume of water coming in and coming out of the body
Water is lost via sweating and exhaling - since we are unable to control how much is lost in these ways, the volume of water is balanced by the amount we consume and the amount removed by the kidneys in urine

What is ADH?

Anti-diuretic hormone - it controls the concentration of urine and is released into the bloodstream by the pituitary gland

What happens when the water content is too high?

1) Osmoreceptors in the hypothalamus detect this
2) The hypothalamus acts as a coordination centre. It receives the information and triggers a response
3) The pituitary gland releases less ADH, therefore less water is reabsorbed from the kidney tubules

What happens when the water content is too low?

1) Osmoreceptors in the hypothalamus detect this
2) The coordination centre in the brain receives the information and triggers a response
3) The pituitary gland releases more ADH, so more water is absorbed from the kidney tubules

What is kidney failure?

When the kidneys don't work properly, so waste substances build up in the blood and you lose the ability to control the levels of ions and water in your body - this will eventually result in death

Dialysis

Has to be done regularly to keep the concentrations of dissolved substances in the blood at normal levels and to remove waste substances - 3 times a week for 3-4 hours
It can cause blood clots or infections
Being on a dialysis machine is not a pleasant experience
Expensive to run - £30,800 per patient per year
Buys a patient valuable time to find a donor organ

What happens during dialysis?

The person's blood flows between partially permeable membranes, surrounded by dialysis fluid - it's permeable to ions and waste substances, but not big molecules like proteins
The dialysis fluid contains the same concentration of dissolved ions and glucose as healthy blood - prevents them being lost from the blood
Only waste substances (e.g. urea) and excess ions and water will diffuse across the barrier

Kidney Transplants

Only cure for kidney failure
Healthy kidneys are usually transplanted from people who have died suddenly - they can be transplanted from people who are alive
Small risk to the person donating the kidney
The donor kidney could be rejected by the patient's immune system - immunosuppressants help to prevent this but it can still happen
Cheaper in the long run than dialysis
Puts an end to long hours on the dialysis machine
Long waiting lists for kidneys

Stage 1 of the Menstrual Cycle

Day 1 - Menstruation starts
The uterus lining breaks down for about four days

Stage 2 of the Menstrual Cycle

The uterus lining builds up again, from Day 4 to Day 14, into a thick, spongy layer full of blood vessels, ready to receive a fertilised egg

Stage 3 of the Menstrual Cycle

An egg develops and is released from the ovary at Day 14 - ovulation

Stage 4 of the Menstrual Cycle

The wall is maintained for about 14 days until day 28 - if no fertilised egg has landed on the uterus wall by day 20, the spongy lining starts to break down and the cycle starts again

FSH - Follicle-Stimulating Hormone

Produced in the pituitary gland
Causes an egg to mature in one of the ovaries, in a follicle
Stimulates the ovaries to produce oestrogen

Oestrogen

Produced in the ovaries
Causes the lining of the uterus to grow
Stimulates the release of LH and inhibits the release of FSH

LH - Lutenising Hormone

Produced by the pituitary gland
Stimulates the release of an egg at day 14

Progesterone

Produced by the ovaries by the remains of the follicle after ovulation
Maintains the lining of the uterus during the second half of the cycle - when the levels of progesterone decreases, the lining breaks down
Inhibits the release of FSH and LH

Hormonal methods of contraception - combined oral contraceptive pill

Contains both oestrogen and progesterone
Over 99% effective at preventing pregnancy
Side effects include headaches and nausea
Doesn't protect against STDs
Progesterone-only pill - fewer side effects and just as effective

Hormonal methods of contraception - contraceptive patch

Contains oestrogen and progesterone
Small patch that's stuck to the skin - each patch lasts one week

Hormonal methods of contraception - contraceptive implant

Inserted under the skin of the arm
Releases a continuous amount of progesterone, which stops the ovaries releasing eggs - makes it hard for sperm to swim to the egg and stops any fertilised egg implanting in the uterus
Each implant lasts three years

Hormonal methods of contraception - contraceptive injection

Contains progesterone
Each dose lasts 2-3 months

Hormonal methods of contraception - intrauterine device (IUD)

T-shaped device that is inserted into the uterus to kill sperm and prevent implantation of a fertilised egg
Plastic IUDs release progesterone and copper IUDs prevent the sperm surviving in the uterus

Non-hormonal methods of contraception - condoms

Worn over the penis during intercourse to prevent sperm from entering the vagina
There are also female condoms that are worn inside the vagina
Only form of contraception that will protect against STDs

Non-hormonal methods of contraception - diaphragm

A shallow plastic cup that fits over the cervix to form a barrier - has to be used with spermicide

Non-hormonal methods of contraception - spermicide

A substance that disables or kills sperm
Only 70-80% effective

Avoiding pregnancy - sterilisation

Cutting or tying the fallopian tubes in a female or the sperm duct in a male
Permanent procedure - there is a very small chance the tubes can rejoin

Avoiding pregnancy - 'natural' methods

Finding out when in the menstrual cycle the woman is most fertile and avoiding sexual intercourse on these days
Popular with people who think that hormonal and barrier methods are unnatural - not very effective