Cells and Control

  • Created by: kati
  • Created on: 12-05-19 12:43

Mitosis

When a cell divides by mitosis, it makes two cells identical to the original cell - the nucleus of each new cell contains the same number of chromosomes as the original cell.

Multicellular organisms use mitosis to grow or to replace cells that have been damaged. Some organsisms use mitosis to reproduce - asexual reproduction.

Interphase: before a cell divides, it has to grow and increase the number of sub-cellular structures such as mitochondria and ribosomes. It then duplicates its DNA and is copied and forms X-shaped chromosomes. Each chromatid is an exact duplicate of the other. Prophase: the chromosomes condense, the membrane around the nuclues breaks down and the chromosomes lie free in the cytoplasm. Metaphase: the chromosomes line up at the centre of the cell. Anaphase: Spindle fibres pull the chomosome apart, then the chromatids are pulled to opposite ends of the cell. Telophase: membranes form around each set of chromosmes, these become the nuclei of the two new cells.

Before telophase ends, cytokinesis occurs - the cytoplasm and cell membrane divide to form two seperate cells.

At the end of mitosis, the cell has produced two new daughter cells, they contain the exact same set of chromosomes in the nucleus as the other daughter cell, this means they are genetically identical diploid cells, they are also genetically identical to the parent cell. Number of cells (after multiple divisions by mitosis) = 2^n.

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Cell Division and Growth

Plants and animals grow and develop due to: cell differentation (the process by which a cell changes to become specialised for its job, this allows multicellular organisms to work more effciently) and cell division. Plants also grow by cell elongation, this is where a plant cell expads making the cell bigger and so the plant grows. 

Growth in animals happens by cell division. Animals tend to grow while they are young, and they reach full growth and stop growing, this means that when they are young, cells divide at a fast rate but as an adult, most cell division is for repair. In most animals, cell differentation is lost at an early stage. 

In plants, the stages of growth happen in three different regions: zone of cell division - this zone is near the tip of a root and shoots (areas called meristems), cells divide by mitosis and new cells are created. Zone of elonagation - this is further up the root, next to the zone of cell division, here, the new cells grow in size. Zone of differentation - this is even further up the root, next to the zone of elongation, this is where the new cells specialise into different types of cells. 

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Cell division and Growth, 2

Whenever a baby is born, their growth is compared to that of millions of other babies to make sure that they are growing as they should be. This is represented by percentile charts. The baby's age (x-axis) and weight (y-axis) can be compared to growth curves for different percentiles of the population. If a baby is in the 1st percentile, 99% of all other babies at that age are heavier. This could mean that it is developing too slowly, and doctors can then investigate this.

Cancer is caused be uncontrolled cell division, the rate at which cells divide is controlled by chemical instructions/genes in an orgainsms DNA. If thre is a change in this, cells may begin to divide uncontrollable which can result into a mass of abnormal cells, a tumour. If the tumour invades and destroys surrounding tissue, it is called cancer.

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Stem Cells

Undifferented cells are called stem cells, these cells are found in the embryo and bone marrow in humans, and in plant meristems in plants. These cells create more stem cells or differentiate into a cell to do a specific function, ie a heart cell. 

Embryos: cell differentiation happens during an organism’s development. Organisms start as one cell. These cells divide to form embryos that differentiate (specialise) to produce cells that can perform all of the body's functions. The stem cells in embryos can differentiate into most cell types, to produce all of the cell types that will make up the organism.

Plants: many plant cells keep their ability to differentiate throughout their life. Because of this, plants are always able to create new tissues (matter that animals and plants are made from).

Adults: cell differentiation is rare in mature (adult) animals. Their cells mostly divide (one cell splits to create two cells) in order to replace cells and repair tissues. New tissues are rarely created. 

Bone marrow: in human adults, stem cells can be found in bone marrow (as well as other tissues and organs). Adult stem cells differentiate into fewer cell types than stem cells in embryos. They are used to replace dying cells and damaged tissues.

Plant meristems: plant stem cells are found in the meristem tissue.  Plant stem cells can differentiate into all types of plant cell throughout the life of the plant. This allows plants to grow for their whole lives.

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Stem Cell Uses

Stem cells can be used in medicine. Currently adult stem cells are used to cure some diseases like sickle cell anaemia. Under certain conditions the stem cells can be stimulated to differentiate into specialised cells. It might be possible to use stem cells to create specialised cells to replace those which have been damaged by disease or injury. 

Uses of stem cells: stem cell treatments - Stem cells may be able to replace damaged cells in the body. For example, stem cells may replace the damaged cells that cause diabetes or paralysis. They can also be used to treat patients with burn injuries, arthritis and Parkinson's disease. Theraputic cloning - Therapeutic cloning is a process that produces an embryo with the same genes as the patient. Stem cells taken from this embryo will have the same DNA as the patient. This means that the patient's body will not reject the stem cells or body cells made from the embryo's stem cells. This is ideal for use in stem cell medical treatments.

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Stem Cell Uses, 2

However there are disadvantages in stem cells in medical use: viral infections - virsuses live inside cells, if donor cells are infected with a virus and this isn't picked up, the virus could be passed on to the recipient and make them sicker. Tumour development - stem cells divivde very quickly, if scientists are unable to control the rate at which the transplated celss divide in a patient, a tumour may develop. Rejection - as with other organ transplants, introducing stem cells into a patient could cause the patient's immune system to reject and destroy the new cells, the recognition of the cells as foreign can trigger an immune response to attempt to get rid of them. A patient can take immunosuppresants but this makes them susceptible to disease. Ethical reasons - some people disagree with the use of stem cells on ethical or religious grounds: they believe that life begins at conception, which means that the embryo is alive. They, therefore, view the use of embryonic stem cells as 'killing' an embryo. However, others think that the aims of curing patients who are suffering should be more important than the potential life if th embryo.

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The Brain

The spinal cord is a long column of neurones that run from thr base of the brain down the spine. At several places down, neurones branch off and connect to other parts of the body, the spinal cord relays information between the brain and the rest of the body.

The brain is made up of billions interconnected neurones, different parts of the brain have different fuctions: Cerebrum: largest part of the brain and is made up of two halves (cerebral hemispheres), the right side controls the left side of the body and vice versa. It is responisible for functions such as movement, intelligence, memory, language and vision. Cerebellum: responsible for muscle coordination, balance, posture and speech. Medualla oblongata: controls unconscious activities like breathing and your heart rate. 

Many things that can go wrong in the CNS can be difficult to treat: it's hard to repair damage to the nervous system - neurones in the CNS don't readily repair themselves and as of yet, scientists haven't developed a way to repair nervous tissue in the CNS. If a problem occurs in a part of the brain that is not easy to access, it can be hard to treat - it's not surgically possible to remove tumours in certain parts of the brain. Treatment for problems in the nervous system may lead to permanent damage, for example someone might need surgery on their spine which could cause further damage during the operation leading to permanent damage.

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The Brain, 2

Many areas of the brain cannot be accessed safely through surgery while people are alive. But neuroscientists have other ways to understand how the brain works.

PET scans: these scans are used to look at how parts of the brain are functioning. Using radioactive glucose, the amount of glucose different parts of the brain are using up can be detected. An image of brain activity is then made. PET scans are very detailed and can be used to investigate both the structure and the function of the brain in real time. PET scans can show if areas of the brain are unusually inactive or active, so they are useful for studying disorders that change the brain's activity. CT scanners: lots of x-rays are taken of the brain, and a computer then builds a 3D image of the brain's structure. Different types of cells absorb different amounts of the x-rays. Brain tumours show up as white blotches. It shows that main structures but nont the functions of them. However if a CT scan shows a diseased or damaged brain structure and patient has lost some function, the function of that part can be worked out. For example, if an area of the brain is damaged and the patient can't see, then that area is involved in vision. Studying brain damage: by studying patients with brain damage, where part of their brain doesn’t function, neuroscientists have been able to link particular regions of the brain to particular functions.

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The Brain, 3

Brain tumours: brain tumours are lumps of dividing cancer cells. They can block blood flow to other parts of the brain. These cancers can be treated with chemotherapy (using drugs), radiotherapy (using radiation) or brain surgery, but these also cause lots of damage to the body. 

Spinal injuries: if the spinal cord is severed (cut), electrical impulses can no longer travel to the brain. Wires can partly re-connect the spinal cord, but this isn't very effective. 

Neurone damage: neurones cannot be replaced like other cells in the body, so damage to these cells is often permanent and irreversible. Researchers are trying to find a way to make new neurones using stem cells.

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Neurones

The body has sensory receptors. Different receptors detect different stimuli. When a stimulus is detected, the information is converted into an electrical impulse and sent along the sensory neurones to the CNS. The CNS coordinates the response, impulses travel through the CNS along relay neurones. The CNS sends information to an effector along a motor neurone, this allows it to act accordingly. 

All neurones have a cell body with a nucleus. The cell body has extensions that connect to other neurones - dendrites and dendrons carry nerve impulses towards the cell body and axons carry nerve impulses away from the cell body. Axons are covered in a layer called a myelin sheath. This insulates the neurone, so electrical impulses travel down it more quickly. Neurones can be very long, which also speeds up the impulse.

Sensory neurone structure: one long dendron carries nerve impulses from receptor cells to the cell body which is located in the middle of the neurone. One short axon carries the nerve impulses from the cell body to the CNS.

Relay neurone structure: many short dendrites carry nerve impulses from the sensory neurone to the cell body. An axon carries nerve impulses from the cell body to the motor neurone.

Motor neurone structure: many short dendrites carry nerve impulses from the CNS to the cell body. One long axon carries nerve impulses from the cell body to the effector cells.

 

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Reflexes

Synapses: at each junction of the reflex arc, there is a synapse. Synapses are gaps between neurones. Nerve impulses must travel across these gaps: the electrical impulse reaches the end of the neurone before the synapse. This triggers the release of chemicals called neurotransmitters. The neurotransmitters diffuse (move down a concentration gradient) across the synapse. The neurotransmitters bind to receptors on the next neurone. The presence of the neurotransmitter causes the production of an electrical impulse in the next neurone.

Reflex actions allow us to respond to dangerous situations rapidly and automatically. Reflex actions do not involve conscious thought. The nervous system responds to stimuli via a reflex arc. The neurones in refelx arcs go through the spinal cord or through an unconscious part of the brain. When a stimulus is detected by receptors, impulses are sent along a sensory neurone to a relay neurone in the CNS. When the impulses reach a synapse between the sensory neurone and the relay neurone, they trigger neurotransmitters to be released. These cause impulses to be sent along the relay neurone. This happens again between the relau neurone and the motor neurone. The impulses then travel along the motor neurone to the effector. The muscle contracts causing you to move involuntarily. Because you don't have time to think about the response, it's quicker than normal responses. An exmple of this is the eye and protecting it: very bright light can damage the eye so there is a reflex to protect it. Light receptors in the eye detect very bright light and send a message along a sensory neurone to the brain. The message then travels along a relay neurone to a motor neurone, which tells circular muscles in the iris to contract, making the pupil smaller. In dark light, the pupil becomes bigger in the same to allow more light in.

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The Anatomy of the Eye

The cornea refracts light into the eye. The iris contols how much light eneters the pupil. The lens also refracts light, focusing it onto the retina. The retina is the light sensitive part and it's covered in receptor cells called rods and cones, which detect light. Rods are more sensitive in dim light but cn't sense colour. Cones are sensitive to different coloursbut are not so good in dim light. The information from light is converted into electrical impulses. The optic nerve carries these impulses from the receptors to the brain. The lens is elastic, so the eye can focus light onto the retina by changing the shape of the lens: distant objects - the ciliary muscle relaxes which allows the suspensory ligaments to pull tight, this pulls the lens into a less rounded shape so light is refracted less; close objects - the ciliary muscles contracts, which slackens the suspensory ligaments. The lens becomes a more rounded shape, so light is refracted more.

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Problems with the Eye

Long-sighted people are unable to focus on near objects: this occurs when the lens is the worng shape and doesn't bend the light enough or the eyeball is too short. Light from near objects is brought into focus behind the retina. You can use glasses or contact lenses with a convex lens to correct it.

Short-sighted people are unable to focus on distant objects: this occurs when the lens is the wrong shape and bends the light too much or the eyeball is too long. Light from distant objects is brought into focus in front of the retina. You can use glasses or contact lenses with a concave lens to correct it.

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Problems with the Eye, 2

Colour blind poeple can't tell the difference between certain colours. The most common form of the disorder is red-green colour blindess - it's caused when red or green cones in the retina are not working properly. There is no cure at the moment as the cone cells cannot be replaced.

Cataracts are a cloudy patch on the lens, which stops light from being able to enter the eye normally. People with cataracts are likely to have blurred vision. They may also experience colours looking less vivid and have difficulty seeing in bright light. A cataract can be treated by replasing the faulty lens with an artificial one (cataract surgery).

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