Movement in Cells.

Biology AQA new AS level, cells and movement within them.

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  • Microscopes magnify an image of an object.
  • Light microscopes distinguish between objects 0.2 micrometers apart, electrons have a shorter wavelength so 0.1 nanometers apart.

Magnification = size of image/size of object.

  • To get from mm to micrometers x1000. 1nanometer is 1/1000 micrometers.
  • Resolution - is the minimum distance apart that 2 objects can be for them to appear as separate items.
  • Electron microscope- short wavelength so high resolving power, beam is focused by electromagnets, 2000x better than light but require a vacuum.
  • TEM- uses an electron gun, the beam is focused by a condenser electromagnet, some parts absorb electrons so dark, others let them through so bright. No living specimens, complex staining process, black and white, artefacts confuse it, specimen must be very thin and a 2D image is produced.
  • SEM - has same limitations as TEM but specimens don't need to be thin. Electrons come from above not below, the beam is passed back and forth so the electrons are scattered. Pattern depends on specimen's contours. It is 3D but has a lower resolving power than TEM, 10x better than light.
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Cell Fractionation.

This is used to separate organelles from each other. The solution used is:

  • Cold - reduce enzyme activity.
  • Isotonic - stop bursting/shrinking as result of osmosis.
  • Buffered - maintain constant pH.

Cells are broken by an homogeniser (blender) to release organelles The resultant homogenate (fluid) is filtered to remove debris.


  • Fragments in filtered homogenate are separated in an ultracentrifuge.
  • Tube of filtrate spun at slow speed so heaviest organelles fall to the bottom - nuclei.
  • The supernatant (fluid at the top) is removed and transferred to be spun at faster speed.
  • Next heaviest organelle falls - mitochondria.
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Structure of an Epithelial Cell.

  • Ultrastructure - each cell has an internal structure that suits its job (absorb and secrete).
  • Eukaryotic cells - cells with a nucleus (epithelial).
  • Prokaryotic - cells without a nucleus.

Nucleus - has hereditary material and controls activities. 10-20 micrometers diameter. It acts as the control centre, has genetic material and makes RNA and ribosomes.

Nuclear envelope - double membrane surrounding nucleus. Outer membrane continuous with with ER and often has ribosomes on its surface. Controls entry and exit.

Nucleoplasm - granular/jelly-like, makes up bulk of nucleus.

Chromatin - DNAwithin nucleoplasm, the diffuse form of chromosomes when cell not dividing.

Nucleolus- small spherical body in nucleoplasm that manufactures ribosomal DNA.

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Mitochondrion, ER and Golgi Body.

Mitochondrion - 1-10micrometers long, used for respiration. Made up of: double membrane that surrounds organelle, outer controls entry and inner folded into cristae; cristae are shelf-like extensions to provide surface area for enzyme attachment; and matrix makes up remainder, is semi-rigid, contains proteins, lipids and DNA, enzymes are found here.

Endoplasmic Reticulum - 3D system of membrane, forms tubes in cytoplasm of cell wall. Rough - membranes have ribosomes that provide large surface area and pathway. Smooth - has no ribosomes and is more tubular. Synthesis, storage and transport of lipids and carbs.

Golgi Body - stack of flattened, membrane-bound sacs (cisternae). Proteins and lipids passed through from ER, golgi labels them so they can be sorted and sent to correct place, it also adds carbs to them (glycoproteins). Modified proteins then transported in vesicles that are regularly pinched off. Vesicles fuse with cell membrane so contents go out. It produces secretory enzymes, secretes carbs, transports and stores lipids and forms lysosomes.

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Lysosomes, Ribosomes and Microvilli.

Lysosomes - form when vesicles contain enzymes. Up to 1micrometer diameter. They isolate potentially harmful enzymes form cell and release to outside or to phagocytic vesicle. They break down material from phagocytic cells, release enzymes outside (exocytosis) to destroy material around it, digest worm out organelles to re-use and completely break down cells after they have died (autolysis).

Ribosomes - are small cytoplasmic granules and make RNA in protein. 80S type found in eukaryotes (25nm diameter) and 70S type found in prokaryotes (slightly smaller). Have 2 sub-units containing ribosomal RNA and proteins. Important in protein synthesis.

Microvilli - finger-like projections to increase surface area for more efficient absorption.

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  • Contain C, H and O.
  • Insoluble in water, soluble in organic solvents (e.g. alcohol).
  • Main groups: triglycerides, phospholipids and waxes.
  • Found mainly in plasma membranes as they contribute to flexibility and transfer of substances across them.
  • Provide an energy source- when oxidised provide twice the energy of carbs.
  • Used in waterproofing - as insoluble.
  • Provide insulation - fat conducts heat slowly.
  • Give protection - round organs.
  • To test add ethanol and shake, add water and shake - cloudy-white if present.

Saturated and Unsaturated Fatty Acids:

  • Fatty acids are saturated if there are no C=C double bonds.
  • 1 double bond = mono-unsaturated.
  • Many double bonds = polyunsaturated.
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Triglycerides and Phospholipids.


  • 3 fatty acids combined with glycerol (alcohol has 3 C atoms each linked to a hydroxyl group).
  • Each form bond with glycerol in condensation reaction.
  • Hydrolysis = glycerol + fatty acids.
  • Glycerol + 3 fatty acids = triglyceride + 3 water
  • All fatty acids have a carboxyl group with attached hydrocarbon.


  • Different to lipids as 1 fatty acid is replaced by a phosphate molecule.
  • Fatty acid is hydrophobic.
  • Phosphate is hydrophilic.
  • It is polar i.e. 2 ends behave differently.
  • Intracellular and extracellular environment are watery so phospholipids form double layer called phospholipid bilayer.
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The Cell Surface Membrane.

  • All membranes have same basic structure called plasma membrane. Cell surface membrane is the plasma membrane that surrounds cells and forms a boundary between cytoplasm and environment so different conditions can be established in and out of a cell. It controls entry and exit.
  • Phospholipids:
  • 1 layer has hydrophilic head pointing inwards (towards water in cytoplasm).
  • Other has hydrophilic head pointing outwards (towards water surrounding cell).
  • Hydrophobic tails of both point to centre of membrane away from water.
  • Allow lipid-soluble substances in and out but stop water-soluble ones in and out.
  • Make membrane flexible.
  • Proteins:
  • Arranged more randomly than phospholipids and are embedded in the bilayer by: EXTRINSIC PROTEINS - occur onsurface (not across it) and give support or are receptor cells for molecules. INTRINSIC PROTEINS - span bilayer and carry water-soluble material across or are enzymes.
  • Provide structural support, carrier, allow active transport as form channels, make recognition sites, help cells adhere together and act as receptors.
  • Fluid(individual phospholipid molecules move relative to 1 another) Mosaic(proteins embedded in bilayer vary in shape, size and pattern) model.
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Diffusion - movement of molecules from high to low concentration. It is passive.

Factors - surface area, concentration difference, membrane thickness, size of molecule and pores.

All the particles are always in motion. It occurs over short distances down a concentration gradient.

Facilitated Diffusion:

  • Involves protein carrier molecules and is faster.
  • 1. Each carrier binds with specific molecule which changes shape of carrier, then deposited into cytoplasm.
  • 2. Specific points on plasma membrane. Proteins form water-filled channels allowing water-soluble ions and molecules through. The channels are selective.
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Osmosis - movement of water molecules from higher to lower water potential through a partially permeable membrane. It is passive and down the concentration gradient.

Water Potential - Pressure created by water molecules under standard conditions. Pure water = 0kPa. Addition of solute lowers water potential.

Water potential = solute potential of ions in cell + pressure potential of cell wall.

Animal cells:- in pure water they take it in, as their water potential is lower, and burst (lysis). In lower water potentials than their own water leaves the cell and it shrivels.

Plant Cells:- greater external water potential means water moves in to cell, it swells and becomes turgid. Equal water potential means no change which leads to incipient plasmolysis. Lower external water potential means water leaves cell, it shrinks and becomes plasmolysed.

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Active Transport.

Active Transport - movement of molecule in or out of cell from lower to higher concentration using energy/carrier molecules. It is active and against concentration gradient. It is selective and uses ATP in 2 ways: directly to move molecules or in co-transport.

Direct Active Transport:

  • Carrier proteins span cell-surface membrane and accept molecules to be transported. Molecules bind to receptors on channels.
  • In cell, ATP binds to protein, splitting to ADP and phosphate which causes the protein to change shape and open on opposite side.
  • Molecules released to other side. Phosphate molecule is released and recombines with ADP to form ATP. Protein reverts shape.

Sometimes more than 1 molecule may be moved at the same time in same directon. Occasionally they move in different directions e.g. sodium-potassium pump. Sodium ions are removed as potassium ions are taken in.

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Absorption in the Small Intestine.

Villi - the intestinal wall is folded into finger-like projections (1mm) that have a very thin wall, are lined with epithelial cells, have a good blood supply and increase surface area for faster absorption and diffusion. They are able to move to maintain diffusion gradient.

Microvilli - finger-like projections of of the cell-surface membrane that line villi's epithelial cells (0.6 micrometers).

Role of Diffusion - More glucose in intestine than blood so glucose diffuses into blood. Blood is constantly circulated so it is absorbed continuously which maintains the diffusion gardient. The villi contract to mix the contents so diffused glucose is replaced.

Role of Active Transport - Diffusion only results in concentrations on either side of epithelium being equal. Glucose is also absorbed by active transport so none is lost. This is done by co-transport as 2 molecules are involved. Glucose is drawn into cells as sodium is transported out by the sodium-potassium pump.

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Bacteria (prokaryote) has:- no nucleus, circular strand of DNA coiled up, no chromosomes, no membrane-bound organelles, no chloroplasts ER or lysosomes, and 70S ribosomes.

Small (0.1-10micrometers), cell wall made of peptidoglycan (polysaccharides and peptides) (10-80micrometers). Capsule of slime for protection and sticking together. Flagella propels. Plasmid holds genes for survival, circular DNA for replication.

  • Cholera:
  • Muscle/stomach cramps, vomiting, fever and diarrhoea.
  • Circulatory system fails as lots of liquid lost.
  • Caught from water contaminatedwith faecal matter with pathogen.
  • Flagella propel through lining of s. intestineand produce a toxin. 1 part binds to specific carb receptors; the other enters epithelial cells and opens ion channels so chloride ions flood lumen.
  • Loss of Cl from cells raises water potential and lowers lumen's so water moves to lumen. The diffusion gradient means ions move from blood and epithelial cells by osmosis into intestine which is what causes diarrhoea.
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Oral Rehydration Therapy.

Treats diarrhoeal diseases and rehydrates. Water is no good as not absorbed from intestine and contains no electrolytes. It requires minimal training. Includes:

  • Water - rehydrate tissues.
  • Sodium - replace lost ions from epithelium of s. intestine.
  • Glucose - stimulate uptake of sodium ions and for energy.
  • Potassium - replacement and appetite.
  • Other electrolytes - reduce imbalance.

Testing New Drugs:

1. Small number of healthy people for side effects.

2. More people with the condition to check if it works.

3. Large-scale trial, some with placebo.

4. Granted licence, monitored for long-term effects.

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