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Bio Topic 1: Cell Biology Cheat Sheet by

A cheat sheet for the first topic in GCSE biology.
biology     aqa     cells     gcse

Prokar­yotes or Eukaryotes

Prokaryote
a micros­copic single­-celled organism which has neither a distinct nucleus with a membrane nor any other specia­lised organelles
Eukaryote
an organism consisting of a cell or cells in which the genetic material is DNA in the form of chromo­somes contained within a distinct nucleus

Animal Cells

Nucleus
contains genetic material that controls the activities of the cell
Cytoplasm
gel-like substance where most of the chemical reactions happen; it contains enzymes that control these chemical reactions
Cell Membrane
holds the cell together and controls what goes in and out
Mitoch­ondria
these are where most of the reactions for aerobic respir­ation take place; respi­ration transfers energy that the cell needs to work
Ribosomes
these are where proteins are made in the cell

Plant Cells

Cell Wall
a rigid wall made of cellulose; it supports the cell and streng­thens it
Permanent Vacuole
contains cell sap, a weak solution of sugar and salts
Chloro­plasts
these are where photo­syn­thesis occurs, which makes food for the plant; they contain a green substance called chlor­oph­yll, which absorbs the light needed for photos­ynt­hesis
Plants cells usually have all the same bits animal cells have plus the extra bits we just mentioned (see Animal Cells).

The Cell Cycle

1) In a cell that's not dividing, the DNA is all spread out in long strings.
2) Before it divides, the cell has to grow and increase the amount of subcel­lular structures (e.g.: mitoc­hon­dria).
3) It then duplicates its DNA (one copy for each new cell).
4) The chromo­somes line up at the centre of the cell and cell fibres pull them apart (two arms of each chromosome go to opposite ends of the cell).
5) Membranes form around each of the sets of chromo­somes (these become the nuclei of the two new cells).
6) Lastly, the cytop­lasm and cell membrane divide.
The has now produced two daughter cells. Their DNA is exactly the same as their parent's DNA.

Stem Cell Notes

Embryonic stem cells can turn into any type of cell.
Diffe­ren­tia­tion is the process by which a cell changes to become specia­lised.
Adult stem cells can only be found in certain places (e.g.: bone marrow).
Adult stem cells can only turn into certain types of cells (e.g.: blood cells).
Stem cells can be grown in a lab to create clones.
Stem cells in plants are called meris­tems.

Osmosis

Defini­tion: The movement of water molecules across a partially permeable membrane from a region of higher water concen­tration to a region of lower water concen­tra­tion.
-a partially permeable membrane is a membrane with very small holes in it (only tiny molecules, like water, can pass through them)
-osmosis is a type of diffu­sion
 

Bacteria Cells

Has a cell membrane (see Animal Cells).
Has cytoplasm (see Animal Cells).
Has a cell wall (see Plant Cells).
Don't have a 'true' nucleus - instead they have a single circular strand of DNA that floats freely in the cytoplasm.
Don't have chloro­plasts or mitoch­ondria.
May also contain one or more small rings of DNA called plasm­ids.

Micros­copes

Light Micros­copes
uses light and lenses to form an image of a specimen and magnify it; lets us see individual cells and large subcel­lular structures
Electron Micros­copes
uses electrons to form an image and magnify it; lets us see smaller things in more detail (smaller than cell)
The Formula For Magnif­ication
image size = magnif­ication x real size

Specia­lised Cells

Sperm Cells
Reprod­uction
Long tail and stream­lined head (for swimming); A lot of mitoc­hon­dria (to provide energy); Enzymes (to digest through egg cell membrane)
Nerve Cells
Rapid signalling
Long (to cover more distance); Branched connec­tions (to connect to other nerve cells)
Muscle Cells
Contra­ction
Long (so they have space to contract); A lot of mitoc­hon­dria (to provide energy)
Root Hair Cells
Absorbing water and minerals
Long "­hai­rs" (gives the plant a big surface area)
Phloem Cells
Transp­orting substances
Form tubes (to transport substances around plants); Have very few subcel­lular structures (so stuff can flow through them)
Xylem Cells
Transp­orting substances
Form tubes (to transport substances around plants); Hollow in the centre (so stuff can flow through them)

Diffusion

Defini­tion: The spreading out of particles from an area of higher concen­tration to an area of lower concen­tra­tion.
-happens in both solutions and gases
-the bigger the concen­tration gradient (the difference in concen­tra­tion), the faster the diffu­sion rate
-a higher temper­ature will also have a faster diffu­sion rate because the particles have more energy and thus move faster

Active Transport

Defini­tion: The movement of substances against a concen­tration gradie­nt.
-needs energy to be carried out (unlike osmosis and diffu­sion)
-needed to absorb nutrients from food
 

Preparing A Slide (Onion edition)

1) Add a drop of water to the middle of a clean slide.
2) Cut up an onion and separate it out into layers. Use tweezers to peel off some epidermal tissue from the bottom of one of the layers.
3) Using the tweezers, place the epidermal tissue into the water on the slide.
4) Add a drop of iodine solution. Iodine solution is a stain. Stains are used to highlight objects in a cell by adding colour to them.
5) Place a cover slip (a square of thin, transp­arent plastic or glass) on top. To do this, stand the cover slip upright in the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen. Try not to get any air bubbles under there - they'll obstruct your view of the specimen.

How To Use A Light Microscope To View A Slide

1) Clip the slide you've prepared onto the stage.
2) Select the lowest­-po­wered objective lens (i.e. the one that produces the lowest magnif­ica­tion).
3) Use the course adjustment knob to move the stage up to just below the objective lens.
4) Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards until the image is roughly in focus.
5) Adjust the focus with the fine adjustment knob until you get a clear image of what's on the slide.
6) If you need to see the slide with greater magnif­ica­tion, swap to a higher­-po­wered objective lens and refocus.

Notes On Chromo­somes

Most cells in your body have a nucleus. The nucleus contains your genetic material in the form of chromo­somes.
Chromo­somes are coiled up lengths of DNA molecules.
Each chromosome carries a large number of genes. Different genes control the develo­pment of different charac­ter­istics.
Body cells normally have two copies of each chromo­some.
There are 23 pairs of chromo­somes.

Binary Fission

1) The circular DNA and plasm­id(s) replicate.
2) The cell gets bigger and the circular DNA strands move to opposite 'poles' (ends) of the cell.
3) The cytop­lasm begins to divide and new cell walls begin to form.
4) The cytop­lasm divides and two daughter cells are produced (each daughter cell has one copy of the circular DNA, but can have a variable number of plasm­ids).
Bacteria can divide very quickly if given the right conditions (a warm, moist enviro­nment with a lot of nutrie­nts).

Arguments Around Stem Cell Research

Some people feel that human embryos shouldn't be used for experi­ments since each one is a potential human life.
Some people think that curing existing patients who are suffering is more important than the rights of embryos.
One argument in favour of stem cell research is that the embryos used in the research are usually unwanted ones from fertility clinics which would've otherwise been destroyed.
Campai­gners against embryonic stem cells feel that scientists should concen­trate more on finding and developing other sources of stem cells.
In some countries stem cell research is banned, it's legal in the UK as long as it follows strict guidel­ines.

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