OCR biology

Revision cards with the key points about each topis, great for revision on the move :) hope this helps and good luck x

I was trying to get these done quickly so there are probably quite a few spelling mistakes, if you find one let me know and i'll sort it

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  • Created by: sammy
  • Created on: 06-01-13 14:48

Magnification and resolution

Magnification- the degree to which the size of an image is larger than the object itself.

Resolution- The degree to which it is possible to destinguish between two points that are very close together and the clarity they can be seen with







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Different types of microscopes

There are 2 types of microscopes, Light microscopes and electron microscopes

Light microscopes

  • Maximum magnification = X1500
  • Resolution = 200nm
  • Preparation can involove staining, so specific structures can be seen and Sectioning, where a specemin is embeded in wax so it can be cut without distorting the structure

Electron microscopes

  • there are two types of electron microscopes, Transmission (TEM) and Scanning (SEM)
  • Both produce black and white images but the image taken with a TEM is 2D whereas the image taken with a SEM is 3D
  • Resolution = 0.2nm
  • Magnification = X100 000-500 000
  • no live samples as they have to be examined in a vacum


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Cell size and magnification

Actual size = Image size/Magnification

mm------------->um    (x1000)

mm------------->nm    (x1000,000)

mm------------->cm     (divide by 10)

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Cytoskeleton- Keep the cells shape stable and are responsible for movement of the cell itself and substances in the cell, eg proteins

Flagellla and cillia- Hair like projections for movement

The nucleus- Largest organelle, contains the genetic information. nucleolus surrounded by the nuclear envelope

Rough endoplasmic reticulum- Studded with ribosomes, Transporst proteins made in the ribosomes

Smooth endoplasmic reticulum- Same as Rough ER but without ribosomes. Involved in making the ipids the cell needs

Gogi apparatus- Recieves proteins from the rough ER and modifies them.

Mitochondria- Two membranes, the iner one is highly folded forming cristae. produces most of the ATP during respiration

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Organelles (cont.)

Chloroplasts- Found only in plant cells. Two membranes, the inner one is highly folded forming thylokoids. They are the site of photosynthesis.

Lysosomes- Contain powerful digestive enzymes so they can breat down materials.

Ribosomes- Some are free in the cytoplasm, others are attatched to ER. they are the site of protein synthesis

Centrioles- small tubes of protein fibres. form the spindle fibres during nuclear division

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Division of labour

There are 9 steps in the production and excretion of proteins...

1. mRNA copy of gene for protein is made

2. mRNA leaves the nucleus through a nuclear pore

3. mRNA attatches to a ribosome on the ER

4. protein molecule is pinched off in a vesicle and heads towards the Golgi apparatus

5. Vesicle fuses with golgi apparatus

6. Golgi appparatus modifies protein and packages it for release

7. Protein molcules are pinched off in vesicles

8. Vesicle fusses with cell surface membrane

9. Cell surface membrane opens to release the protein

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Animal and plant cells are Eukaryotes. The key features of Prokaryotes are...

  • Only have one membrane, the cell surface membrane
  • Cell wall made of peptidoglycan rather than cellulose (plants)
  • often have a slippery protective layer outside the cell wall called the capsule
  • Contain ribosomes which are much smaller than Eukaryotic ribosomes
  • Contain a single loop of DNA in the cytoplasm rather than chromosomes. Many also contain small loops of DNA called plasmids
  • No nucleus
  • ATP production takes place in mesosomes within the cell surface membrane
  • Some have a flagella but they have a different internal structure to those on animal cells
  • Have hair-like projections called pili

Remember! Prokaryotes have no nucleus or membrane bound organelles.

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Biological membranes

The roles of membranes are...

  • Seperating cell contents from outside environmnet
  • Seperating ccell components form cytoplasm
  • cell recognition and  signalling
  • Regulating what materials enter and leave the cell

The main structure of a membrane is the phospholipid bilayer. Phosolipids have a Hydrophillic (water loving) head and a Hydrophobic (water hating) tail.


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The fluid mosaic model

The main features of the fluid mosaic model are...

  • phospolipid bilayer forming the main structure
  • Glycoproteins and glycolipids used for cell signalling, either for hormoes or to act as antigens so the cell can be recognised by the immune system
  • Cholesterol give the membrane mechanical stbility and make the barrier more complete
  • Carrier proteins allow the movement of large molecules, eg glucose, through the membrane
  • Channel proteins allow the movement of small polar molecules through the membrane
  • Some contain enzymes for metabolic proceses eg in the cristae in mitochondria

Higher temperatures affect membranes as the phospholipids get more kinnetic energy. This causes them to move making the membranes leaky and more permeable 

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Crossing membranes- Simple diffusion

Diffusion- the movement of molecules from an area of higher concentration to an area of lower concentration down a concentration gradient

Diffusion is a passive process, no energy is required.

The rate of diffusion is affected by...

  • Temperature- more tmperature, more kinnetic energy
  • concentration gradient- steeper concentration gradient increases rate of diffusion
  • Stirring- increases movement of molecules
  • Surface area- greater surface area means more diffusion
  • Distance/thickness- diiffusion is slowed down by thick membranes
  • size of molecules- small molecules diffuse faster

In cells small, non-polar molecules cross membranes by simple diffusion as they can fit through gaps in the bilayer. Examples of these substances include oxygen, carbon dioxide and steroid hormones 

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Crossing membranes- Facilitated diffusion

large or charged molecules cannot simply diffuse in and out of cells, special proteins are required.

Channel proteins- form pores in the membrane to allow small charged particles across, eg sodium and calcium ions

Carrier proteins- Have a complementary shape to that of the molecule they transport to allow larger molecules to cross, eg glucose and amico acids

Both these methods are examples of facilitated diffusion so are passive processes  

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Crossing membranes- active processes

Some carrier proteins act as pumps to transport substances up the concentration gradient. This requires ATP. They can carry molecules much faster than by diffusion alone. examples of this are...

  • ions/salts into root hair cells
  • Hydrogen ions out of companion cells
  • sucrose out of seive tube element at sink
  • calcium ions into muscle cells

If large quantities of substances need to be transported Bulk transport is used. substances are packaged into vesicles which can easily fuse with membranes. ATP is required to transport the vesicles. some examples of bulk transport are hormones such as insulin and when white blood cells engulf microorganisms they form a vesicle around it.

Different names are used for the movement of substances by bulk transport...

Endo = inwards,  Exo=outwards,  Phago=solid material,  Pino= Liquid material

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Osmosis- the movement of water from an area of higher water potential to an area of lower water potential through a partially permeable membrane

water potential is always negative. distilled water has the highest water potential at 0kPA

If you put a cell into distilled water water will move into the cell. If it's an animal cell it will eventually burst. We say it has been Haemolysed. Plant cells cannot burst as they have a cell wall. The membrane pushes against the cell wall making the cell stif. it is said to be Turgid.

If you put a cell into a concentrated sugar solution water will move out of the cell. if it's an animal cell it will shrivel up, it's crenated. If it's a plant cell the cell membrane stops puching on the cell wall and pulls away completely. We say it is plasmolysed.

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The cell cycle

The cell cycle is divided into 3 main stages...

  • Interpahse- the S phase when DNA replicates and the G phase when proteins and organelles are made. DNA is checked for mutations
  • mitosis- The nucleus divides
  • cytokinesis- the cytoplasm divides

Mitosis only takes up a very short part of the cell cycle

Making new cells is important for...

  •  Asexual reproduction
  • growth
  • repair
  • replacement

Mitosis forms two daughter cells which are gennetically identical to the parent cell

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Stages of mitosis

Prophase- replicated chromosomes supercoil, nuclear envelope breaks down and disapears, centriole divides and moves to opposite ends

Metaphase- Chromosomes line up down the centre of the cell and spindle fibres form and attatch to the centromeres

Anaphase- spindle fibres shorted pulling the chromosomes apart towards opposite poles

Telophase- New nuclear envelopes form around each group of chromosomes and the spindle fibers break down and disapear.

Remember! IPMAT (Interphase, prophase, metaphase, telophase)

In animals most cells are capable of mitosis, in plants only meristem cells can divide this way

plants don't have centrioles, spindle fibres are formed in the cytoplasm

In animal cells cytokinesis starts from the outside, in plants a cell plate forms at the equator

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Stem cells

Stem cells are unspecialised cells. They have the ability to perform mitosis, grow and become specialised (differentiate)

They can be found in young embryos, umbilical cord blood and bone marrow in humans. Stem cells from young embryos and umbilical cord blood can differentiate into any type of cell, it is said to be Totipotent. Stem cells from bone marrow can only turn into certain types of cells, they are said to be pluripotent.

In plants unspecialised cells are in meristems and cambium.

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Meiosis produces 4 cells which are not gennetically identical and only contain half the gennetiac information of the parent celll. this type of division is responsible for gametes (sex cells)

Meiosis begins the same way as mitosis but the replicated chromosomes paired up before they are pulled apart. The replicated chromosomes are then seperated in the sames way.

Meiosis is important to ensure that children have the correct amount of gennetic material and that there is variation rather than children being clones (gennetically identical) of their parents 

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Specialised cells

Erythrocytes- no nucleus, mitochondria, golgi apparatus or rough ER to make room for haemoglobin. Biconcave disc shape increases their surface area and makes them flexible.

Neutrophils- many lysosomes for digesting microbes; lobed nucleus for flexibility; Very large so they can engulf microbes; small golgi apparatus, few ribosomes and no rough ER as they don't produce many proteins.

Sperm cells- Many mitochondria to provide energy for movement; lysosomes in sperm head help penertrate the egg; small, long and thin to aid movement, single, long undulipodium proppels the cell; contains only half the number of chromosomes in normal cells

Palisade cell- Packed with chloroplasts for photosynthesis

Root hair cell- hair like projections increase surface area able to absorb water

Guard cells- spiral thickenings of cellulose in cell walls for strength.

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