Biology GCSE Revision


Topic 2

Cells and Control

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The Process


  • Cell replicates organelles and chromosomes
  • Each chromosome is made up of 2 sister chromotids.

Early Prophase

  • Chromosomes condense and become visible
  • Both sister chromotids joint at the centromere.

Late Prophase

  • Nuclear membrane dissipates &  spindle fibres form.
  • These attach to centromere of chromosome at poles of cell.
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Mitosis 2


  • Spindle fibres pull chromosome to line up on equator of cell


  • Spindle fibres contract.
  • Chromotids pulled apart and move to poles of cell.


  • New nuclear membranes form around both groups of chromosomes at each pole


  • Cytoplasm divides.
  • 2 new cells are formed.
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Mitosis 3

Mitosis: The process of cells dividing to produce two diploid daughter cells that are genetically identical to the parents.

Diploid: Have two copies of each type of chromosome. Humans have 23 pairs of chromosomes.

  • Mitosis produces identical cells for:


=>repair of damaged cells & worn out tissue

=>asexual reproduction (in strawberries and aphids).

Asexual Reproduction: Producing new organisms from one parent only. They will be genetically identical to the parent.

  • The offspring are clones & no variation
  • Faster than sexual reproduction
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Mitosis 4

  • Mitosis is the production of two daughter cells
  • Each with identical sets of chromosomes in the nucleus to the parent cell
  • => Formation of two genetically identical diploid body cells.
  • Cancer cells are produced through uncontrollable cell division
  • Produces tumours that damages the body and results in death.
  • Benign: not harmful
  • Malignant: dangerous (when moving through metastatis).
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Growth in Animals

Growth: A permanent increase in the no. of cells or in the mass or size of an organism (normally by mitosis).

  • Nerve Cell=>2m long to carry messages around body.
  • Sperm Cell=> Tail to swim
  • RBC=> No nucleus for more space for Oxygen.

Why is cell differentiation important for specialised cells?

Differentiation: Where an unspecialised cell turns into a very specific cell, with a specific functions and specific adaptations, it has become specialised. 

  • Having specialised cells allows multicellular organisms to work more efficiently.
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Growth in Animals 2

Percentile Charts

  • Used to assess a child's growth over time to overall pattern in development can be seen
  • No. of babies will have that specific mass but be below the percentile line.
  • Draw lines from age and mass and see where they meet.
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Growth in Plants

  • Plants tend to grow at their tips of roots and shoots - meristems
  • These are zones of cell division - mitosis
  • Then the cell goes through elongation (cell walls loosen links between celluslose molecules) to allow water in => expansion of vacoule and cytoplasm.
  • The links reform and walls become rigid again.
  • Cells then differentiate into specialised cells that have different functions.

Types of Plant Cell


  • Undifferentiated cells in plants
  • Can become any type of cell
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Growth in Plants 2 - Specialised Cells 2

Specialised Cells 2

Root Hair Cell

  • Large surface area to take in more water.
  • Thin cell wall to allow easy net movement of water
  • No mesophyll (chloroplast tissue)

Palisade Cell

  • Contain chloroplasts to help plant make food by photosynthesis.
  • Found in the top of the leaf
  • Large surface area to catch as much sunlight as possible
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Growth in Plants 3 - Specialised Cells 3

Specialised Cells 3

Xylem Cell

  • Consist of dead cells
  • Thick strengthened cell wall with hollow middle
  • Involved in movement of water through a plant.

Guard Cells

  • Found either side of stomata pores on underside of leaf
  • Control opening and closing of pores during different times of day and different conditions
  • Able to change shape=> opening and closing of pores
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Stem Cells

Stem Cells: An unspecialised cell that continues to divide by mitosis to produce more stem cells and other cells that differentiate into specialised cells.

  • Stem cells are found in early human embryos and in adult bone marrow (or specific places).

Pluripotent: Can become any cell type

Multipotent: Can only become a limited no. of cells.

Adult Stem Cell

  • Tissues created from these will not be rejected by the body.

Embryonic Stem Cell

  • Found in inner cell mass in the blastocyst.
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Stem Cells 2


  • Can produce any type of specialised cell
  • Shoot => Apical Meristem
  • Root/Stem => Lateral Meristem
  • Middle of the Stem => Intercalary Meristem


  • Can treat many diseases caused by damaged cells like Diabetes I, leukemia and CVD.
  • If stem cells continue to divide in the body => cancer
  • They can be rejected.
  • They can be used to test new drugs on
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The Brain

Brain: The central processing centre of the body

  • If there is no activity in the medula oblongata => brain-dead
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Brain and Spinal Cord Problems

CT Scans

  • X-ray scan
  • Beams move in a circle around the head
  • Detectors measure the absoprtion of the X-ray
  • 3D picture of the brain
  • Diagnose conditions
  • Moniter conditions after diagnosis

PET Scans

  • Positron Emission Tomography
  • Patient is injected with radioactive glucose
  • The more active the cell, the more glucose it will absorb => respiration
  • Radioactive atoms release gamma rays which can be detected by a scanner.
  • Shows changes in real time
  • Useful for studying disorders that change the brain's activity.
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Brain and Spinal Cord Problems 2

Spinal Cord Problems

  • Adult Stem cells do not differentiate into neurones in the spinal cord
  • Wires are used to stimulate nerves and muscles in quadriplegics.

Brain Tumours

  • Brain tumours can squash parts of the brain and stop it working.
  • Cells can be killed by radiotherapy (high energy X-ray beams) and chemotherapy (drugs).
  • These can damage body and brain
  • Chemo might not work due to blood-brain barrier (natural filter that only allows certain substances from the blood to the brain).

Limitations of CNS Surgery

  • Hard to repair damage - no way to repair nervour tissue
  • Not possible to surgically remove tumours growing in certain parts of the brain
  • Surgery could lead to permanent damage
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The Nervous System

  • The nervous system is made up of the brain, spinal cord, nerves and sense organs

Detecting Stimuli

  • A stimulus stimulates a response in the body
  • Detected by receptor cells in sense organs
  • Receptor cells create electrical signals called impulses.
  • Impulses travel from the receptor cells to the brain (voluntary response) or spinal cord (reflex action/ involuntary response) sent along
  • Brain processes this and sends impulses to effectors (muscles & glands) to carry out an action. 
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The Nervous System 2

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The Nervous System 3


  • Sensory Neurone: Long dendrites => quicker impulse than using many.
  • Motor Neurone: Myelin sheath => Electrical Insulator => speeds up impulse
  • Relay Neurone: Long Axon => quicker impulse.


  • An electrical impulse travels along an axon and to the ends of the neuron.
  • End of first neuron releases chemicals =>neurotransmitters
  • Chemicals diffuse across synapse (gap) and bind to receptors in 2nd neuron
  • Receptors in 2nd neurone only bind with specific chemicals from the first.
  • 2nd neurone is stimulated and transmits an electrical impulse down the axon.
  • Slowed down because diffusion takes time
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The Nervous System 4

Reflex Arc

  • If in danger: signals go from sensory to motor via relay
  • Skips CNS but goes through unconscious part of brain.
  • The reflex is from receptor to effector
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The Eye



  • Thick layer to protect the eye


  • Its muscles contract and relax to alter the size of the pupil.

Rod and Cone Cells

  • The photoreceptors are sensitive to black and white and colour. 
  • Cone cells function in bright light and produce detailed vision
  • Rod cells function in low light and movement
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The Eye 2

Common Defects


  • Protein build-up inside the lens and makes it cloudy.
  • Full vision can be restored by using a plastic lens.


  • Cones don't work properly and cannot be corrected.
  • Most common is red-green colour blindness.


  • Distant objects appear blurred
  • Rays focused in front of retina because the eyeball is too long or cornea is too curved.
  • Can be solved using a diverging lens.
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The Eye 3


  • Caused by eyeball being too short or cornea being less curved.
  • Can be fixed by converging lens
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The Eye 4

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