Biology B2

Mitosis makes new cells for growth and repair

Body cells are diploid- they have two copies of each chromosome, one from each parent. So humans have one two copies of chromosome 1 and two copies of chromosome 2 etc.

There are 23 pairs of chromosomes in a human cell. The 23rd pair controls the baby's sex.

When a body cell divides it needs to make new cells identical to the original cell- with the same number of chromosomes.

This type of cell division is called mitosis. Its used when organisms grow or replace cells that have been damaged.

In a cell thats not dividing, the DNA is spread out in long strings.

If the cells gets a signal to divide, it needs to duplicate its DNA- so theres one copy for each each new cell. The DNA is copied and forms X-shaped chromosomes. Each 'arm' of the chromosome is an exact duplicate of the other.                                                   

The chromosomes then line line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell.       

Membranes form around each of the sets of chromosomes. These beome the nuclei of the two new cells.           

Lastly the cytoplasm divides. 

You now have two new cells containing exactly the same DNA- they're identical.                                                       

Meiosis involves two divisions

Gametes only have half the number of chromosomes as a normal body cell. They are produced by meiosis. In humans it only happens in the reproductive organs. 

Meiosis produces four haploid cells whose chromosomes are NOT identical.

Division 1

AS with mitosis, before the cell starts to divide, it duplicates its DNA- one arm of each chromosome is an exact copy of the other arm. In the first division in meiosis the chromosome pairs line up in the centre of the cell. 

They're then pulled apart, so each of new cell only has one copy of each chromosome. Some of each parents go into each new cell. 

Each new cell will have a mixture of each parents chromosomes. Mixing up the genes in this way creates variation in the offspring. This is a huge advantage of sexual reproduction reproduction over asexual reproduction.                                                                             

In the second division the chromosomes line up again in the centre of the cell. It's a lot like mitosis. The arms of the chromosomes are pulled apart. 

You get our gametes with only a single set of chromosomes in it.

After two gametes join at fertalisation, the cell grows by repeatedly dividing by mitosis. 

Sperm cells are adapted for their function     

The function of a sperm is to transport the male's DNA to the female's egg so their DNA can combine. Sperm are small and have long tails so they can swim to the egg. Sperm have lots of mitochondria to provide the energy needed to swim this distance. Sperm also have an acrosome at the front of the head where they store the enzymes they need to digest their way through the membrane of the egg cell. They're produced in large numbers to increase the chane of fertiisation.                                          

Embryonic stem cells can turn into any type of cell

Most cells in your body are specialised for a particular job. For example, white blood cells are brilliant at fighting invaders but they can't carry oxygen red like blood cells can.

Differentation is the process by whicha cell changes to become specialised for its job. In most animal cells, the ability to differentiate is lost at an early stage, but lots of plant cells don't ever lose this ability. Some cells are undifferentiated. They can develop into different types of cell depending on what instructions they're given. These cells are called STEM CELLS.

Stem cells are found in early human embryos. ALL the different types of cells found in a human being come from the few cells in the early embryo.

These cells are very useful to medical research because they have the potential to turn into any kind of cell at all. This coud be useful in finding cures for diseases and injuries.

Adults also have stem cells but they are only found in certain places like bone marrow. They also aren't as versatile as embryonic stem cells.

Stem cells may be used to cure many diseases

We already use adult stem cells to cure disease. For example, blodd diseases like sickle cell anaemia can be treated using bone marrow transplants. Bone marrow contains stem cells that can turn into new blood cells to replace the faulty old ones.

Scientistscan also extract stem cells from very early human embryos and grow them.

These embryonic stem cells could used to replace faulty heart cells in sick people- you could make beating heart muscle cells for people with heart disease, insulin-producing cells for people with diabetes, nerve cells for people paraysed by spinal injuries, and so on.

To get cultures of one specific type of cell, researchers try to control the differentiation of the stem cells by changing the environment they're growing in.

Some people are against stem cell research

Some people feel that human embryos shouldn't be used for experiments since each one is a potential human life.

Others think that curing patients who already exist and who are suffering is more important than important than the rights of embryos. 

One fairly convincing argumentin favour of this point of view is that the embryos used in the research are usually unwanted ones from fertility clinics which would probably just be destroyed ayway. But campaigners for the rights of embryos usually want this banned too.

These campaigners feel that scientists should concentrate more on finding and developing other sources of stem cells, so people could be helped without having to use embryos.

In some countries stem cell research is banned completely. It's allowed in the UK but must follow strict guidelines.

Auxins are plant growth hormones

• Auxins are plant hormones which control growth near the tips of shoots and roots.
• Auxin is produced in the tips and diffuses backwards to stimulate the cell elongation process which occurs in the cells just behind the tips.
• If the tip of a shoot is removed, no auxin is available and the shoot may stop.
• Auxins are involved in the responses of plants to light, gravity and water.

Plant hormones can be extracted and used by people

Here are five examples of how people use plant hormones:

• Seedless fruits can be made with artificial hormones.
• Selective weedkillers have been developed from plant growht hormones.
• Cuttings can be grown using rooting powder.
• Fruit can be ripened on its way to the shops using a ripening hormone.
• Seeds can be forced to germinate using a hormone called gibberellin.

Sense organs detect stimuli

A stimulus is a change in your environment which you may need to react to. You need to be constantly monitoring what's going on so you can respond if you need to.

• You have five different sense organs- eyes, ears, nose, tongue and skin/
• They all contain different receptors. Receptors are groups of cells which are sensitive to a stimulus. They change stimulus energy into electrical impulses.
• A stimulus can be light, sound, touch, pressure, pain, chemical, or a change in postition or temperature.

The central nervous system coordinates a response to stimuli

• The central nervous is where all the information from the sense organs is sent, and where reflexes and actions are coordinated. The central nervous system consists only of the brain and spinal cord.
• Neurones transmit the information very quickly to and from the CNS.
• Instructions from the CNS are sent to the effectors which respond accordingly.

• Eyes- light receptors
• Ears- sound and balance receptors
• Nose- smell receptors
• Tongue- taste receptors
  • Sensitive to bitter, salt, sweet and sour, plus the taste of savoury things like monosodium glutamate- chemical stimuli
• Skin
  • Sensitive to touch, pressure and temperature change.
• Sensory neurones
  • The nerve cells that carry signals as electrical impulses from the receptors in the sense organs to the central nervous system.
• Motor neurones
  • The nerve cells that carry signals to the effector muscles or glands.
• Effectors
  • Muscles and glands are known as effectors. They respond in different ways- muscles contract in response to a nervous impulse, whereas glands secrete hormones.

Reflexes help prevent injury

• Reflexes are automatic responses to certain stimuli- they can reduce the chances of being injured.
• For example if someone shines a bright light in your eyes, your pupils automatically get smaller so that less light gets into your eye- thus stopping it getting damaged.
• Or if you get a shock, your body releases adrenaline automatically- it doesn't wait for you to decide that you're shocked.
• The route taken by the information in a reflex is called a reflex arc.

The reflex arc goes through the central nervous system

• The neurones in reflex arcs go through the spinal cord or through an unconscious part of the brain.
• When a stimulus is detected by receptors, an impulse is sent along a sensory neurone to the CNS.
• In the CNS the sensory neurone passes on the message to another type of neurone- a relay neurone.
• The relay neurone relays the impulse to a motor neurone.
• The impulse then travels along the motor neurone to the effector.
• The muscle responds by contracting. A gland responds by secreting.
• Because you don't have to think about the response it's quicker than normal responses.

Learn the eye with all its labels:

Focusing on near and distant objects- another reflex

Th eye focuses light by changing the change the shapes of the lens- this is known as accomodation.

To look at distant objects:

• The ciliary muslces relax, which allow the suspensory ligaments to pull tight.
• This makes the lense go thin.

To look at near objects:

• The ciliary musces contract, which slackens the suspensory ligaments.
• The lens becomes fat.

As you get older, your eye's lens loses flexibility, so it can't easily spring back to a round shape. This means light can't be focused wll for near viewing, so older people often have to use reading glasses.

• Long sighted people are unable to focus on near objects. This occurs when the cornea or lens doesn't bend the light enough or the eyeball is too short. The images of near objects are brought into focus behind the retina.
• Short sight people are unable to focus on distant object. This occurs when the cornea or lens bends the light too much or the eyeball is too long. The images of distant objects are brought into focus in front of the retina.

Binocular vision lets you judge depth

Some animals, inluding humans, have two eyes which work together- this is binocular vision. The brain uses small differences between what each eyes sees to judge distances and how fast things are moving. It's handy for catching prey and deciding if it's safer to cross a road.