Cell Biology

Cell Structure

Cells=building blocks (smallest unit) of every living thing.

Cells can be prokaryotic or eukaryoticEukaryotic=organisms, complex + inlcude all animal and plant cell. Prokaryotic=smaller, simpler + is single-celled organism.  Both eukaryotic + prokaryotic contain subcellular structures.

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Animal and Plant Cells

Animal Cells contain following subcellular structures:

  • Nucleus - contains genetic material that controls activities of cell
  • Cytoplasm - gel-like substance where most chemical reactions happen + contains enzymes that control reactions
  • Cell membrane - holds cell together + controls what goes in&out
  • Mitochondria - where most aerobic respiration reactions take place (Respiration transfers energy that cells need to work)
  • Ribosomes - are where proteins made in cell

Plant cells have everything animal cells have plus a few more..

  • Cell wall - rigid, made of cellulose, supports&strengthens cell
  • Vacuole - contains cell sap (weak solution of sugar+salts)
  • Chloroplasts - where photosynthesis occurs=makes food for plant + contain green pigment=chlorophyll=absorbs light for photosynthesis
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Bacteria Cells

Bacteria=prokaryotes. Has cytoplasm, cell membrane + cell wall. But doesn't have 'true' nucleus - instead has circular strand of DNA that floats in cytoplasm. May also contain 1 or more small rings of DNA called plasmids.


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Light Microscopes - use light and lenses to form image of specimen + magnify it. Let us see individual cells + subcellular structures e.g nuclei

Electron Microscopes - use electrons to form image, have higher magnification + resolution, lets you see smaller things in more detail (plasmids, ribosomes).

Resolution=the ability to distinguish between two points (higher resolution=sharper image)

Using Light Microscope: preparing slide - e.g to view onion cells =

  • add drop water to middle of clean slide
  • cut onion + separate into layer & use tweezers to peel off some epidermal tissue from one of bottom layers
  • using tweezers place epidermal tissue into water on slide
  • Add drop of iodine solution. Iodine=stain=to highlight objects by adding colour to them
  • place cover slip on top - try not to get any air bubbles as will obstruct view of specimen
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Observing Specimen: how to set up light microscope

  • start by clipping prepared slide on stage
  • select lowest objective lens  (lowest magnification)
  • use coarse adjustment knob to move stage below lens
  • look down eyepiece. use coarse adjustment knob to move stage downwards until image roughly in focus
  • adjust focus with fine adjustment knob until you get clear image of specimen
  • if need to see slide with greater magnification swap to higher objective lens + refocus
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Cell differentiation & specialisation

Specialised cell=cell that performs specific function.

Cell differentiation=process by which cell changes to become specialsed for its job. Most differentiation occurs as organism develops. in most animal cells ability to differentiate lost after become specialised. Plant cells don't ever lose this ability. cells that differentiate in mature animals=mainly for repairing/replacing cells (blood cells)Undifferentiated cells=stem cells.

Examples of specialised cells:

  • Sperm cells - specialised for reproduction. function of sperm=to get male dna to female dna. has long tail+streamlined head to help it swim to egg. also=lots of mitochondria in cell to provide energy needs to do this. also carries enzymes in head to digest through egg cell membrane
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Examples of specialised cells...

  • Nerve cells - specialised for rapid signalling. function to carry electrical signals from one part of body to another. these cells long to cover more distance & have branched connections at their ends to connect other nerve cells + form network throughout body
  • Muscle cells - specialised for contraction. function to contract quickly. these cells long so have space to contract & contain lots of mitochondria to transfer energy needed for connection
  • Root hair cells - specialised for absorbing water+minerals. grow into long 'hairs' that stick out into soil. gives plant big surface area for absorbing water+mineral ions
  • Red blood cell - function to carry oxygen. has large surface area (has no nucleus) for oxygen to pass through. contains haemoglobin that joins with oxygen
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most cells in body=nucleus. nucleus=contains genetic material. this material stored in form of chromosomes. Body cells normally have 2 copies of each chromosome - one from mother & one from father. So humans have 2 copies of chromosome 1, 2 etc.. Chromosomes=long lengths of molecule=DNA. DNA coiled up to form arms of chromosome.


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Stage of cell cycle when cell divides=mitosis. Mitosis (asexual) is when cell reproduces itself by splitting to form 2 identical offspringInterphase (cell cycle):

  • in cell thats not dividing=dna spread out into long strings
  • before divides cell has to grow&increase amount of subcellular structures (mitochondria, ribosomes..)
  • then replicates dna.

Then Mitosis happens:

  • chromosomes line up at centre of cell + spindle fibres pull apart
  • membranes form around each of sets of chromosomes=become nuclei of 2 new cells=nucleus divided
  • cytoplasm+cell membrane divide=2 new identical daughter cells produced
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Mitosis Diagram


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Stem Cells

Stem cells=undifferentiated cells. Stem cells can divide to produce lots more undifferentiated cells + can also develop into different types of cell depending on instructions given.

Stem cells found in early human embryos. Adults also have stem cells but only found in certain places (bone marrow..). Not as versatile as embryonic stem cells=can't turn into any cell type only certain e.g. blood cells.

Stem cells from embryos & bone marrow can be grown in lab to produce clones + made to differentiate into specialised cells to use in medicine/research.

Adult stem cells in medicine - already used to cure disease.. people with blood disease (anaemia) can be treated by bone marrow transplants. Bone marrow=tissue found inside bone=contains stem cells that can turn into new blood cells to replace faulty old ones.

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Embryonic stem cells in medicine

Embryonic stem cells could also be used to replace faulty cells in sick people in future. In type of cloning=therapeutic cloning (embryo could be made to have same genetic info as patient)=stem cells produced from it contain same genes as patient & wouldn't be rejected if used to replace faulty cells.

However=risks involved. e.g stem cells grown in lab may become contaminated with virus=patient=even sicker + are also ethical issues related to use of embryonic stem cells.

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Issues involved in stem cell research


  • can save lives=replace faulty cells
  • could make insulin-producing cells for diabetics + nerve cells for paralysed (spinal injuries)
  • therapeutic cloning
  • all embryos used from fertility (abortion) clinic=unwanted


  • some fell human embryos=potential life
  • religious views
  • obtaining stem cells from embryo destroys the embryo

In some countries stem cell research banned but allowed in UK as long as follows strict guidelines.

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Stem Cells in Plants

In plants stem cells=meristems (parts of plant where growth occurs). throughout plant's entire life cells in meristem tissues can differentiate into any type of cell.

These stem cells can be used to produce clones of whole plants quickly & cheaply.

This is useful for:

  • growing crops of identical plants that have desired features for farmers
  • growing more plants of rare species to prevent them from being wiped out

e.g. Dutch elm disease=fungal tree disease that's destroyed millions of elm trees worldwide. One way to protect elm trees from being wiped out by this disease could be to create and grow clones of naturally-resistant individuals.

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