Cell Structure and Membranes


Cell Organelles

Plasma Membrane - Regulates the movement of substances in and out of the cell. It has receptor molecules on it which allow it to respond to chemicals like hormones.
Nucleus - Surrounded by a nuclear envelope, contains pores. These pores allow substances to move between the nucleus and cytoplasm.
Lysosome - Contains digestive enzymes. Can be used to digest invading cells or to break down worn out components of the cell.
Ribosome - The site where proteins are made.
Endoplasmic Reticulum - Smooth, Synthesises and processes lipids. Rough, folds and processes proteins that have been made by the ribosomes.
Golgi Apparatus - Processes and packages new lipids and proteins. It also makes lysosomes.
Microvilli - Found in cells involved in absorption, such as epithelial cells. They increase the surface area.
Mitochondrian - Double membrane, cristae is the inner and matrix is the outer which contains enzymes involved in respiration. Site of aerobic respiration, found in active cells which require a lot of energy.

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Analysis of Cell Components - Microscopes

Magnification = Image size/Actual size
Light Microscopes - Lower resolution than electron, maximum resolution of 0.2 micrometres, magnification is about x1500.
Electron Microscopes - Higher resolution so more detailed, maximum resolution of 0.0001 micrometres (about 2000 more than light), magnification is about x1500000.
Electron split up into transmission and scanning.
Transmission - Use electromagnets to focus a beam of electrons which is transmitted through the specimen. Denser parts of the specimen absorb more electrons, which makes them look darker on the image you end up with. They give high resolution images but can only be used on thin specimens.
Scanning - Scan a beam of electrons across the specimen, this knocks off the electrons from the specimen which are gathered in a cathode ray tube to form an image. The image you end up with show the surface of the specimen and they can be 3D. They are good as they can be used for thick specimens but they give a lower resolution image than transmission.

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Analysis of Cell Components - Cell Fractionation

Homogenisation - Can be done in several ways. For example, vibrating the cells or by grinding the cells up in a blender. This breaks up the plasma membrane and releases the organelles into solution.
Filtration - The homogenised cell solution is filtered through a gauze to seperate any large debris or tissue debris, like connective tissue from the organelles. The organelles are much smaller than the debris so can pass through the gauze.
Ultracentrifugation - The centrifuge is spun at a low speed. The heaviest organelles, like nuclei, get flung to the bottom of the tube. They form a thick sediment at the bottom - the pellet. The rest of the organelles stay suspended in the fluid above the sediment - the supernatant. The supernatant is drained off and poured into another tube and spun at a higher speed. This time the heavier, like mitochondria, form a pellet at the bottom of the tube. The supernatant is drained off and spun around again at a higher speed. This is repeated until all the organelles are seperated out. Each time, the pellet at the bottom of the tube is made up of the lighter organelles.

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Plasma Membranes

Triglycerides - glycerol with three fatty acids. The fatty acids are hydrophobic which makes the lipids insoluble. They are formed by condensation reactions and have ester bonds. Glycerol is made of three carbons, attached to five carbons and three OH which is replaced by three hydrogens when bonded together. A fatty acid consists of a carbon attached to a double bonded oxygen, a variable and an OH.
Saturated fatty acids don't have any double bonds between their carbon atoms in the 'tail'. Unsaturated do have double bonds between carbon atoms in the 'tail'.
Phospholipids are just a swap of a fatty acid for a phosphate group. This is hydrophylic and the fatty acid tails are hydrophobic.
Emulsion test for lipids - shake the test substance with ethanol for about a minute, then pour the solution in water. Any lipid will show a milky emulsion. The more lipid there is, the more noticeable the milky colour will be.

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Exchange Across Plasma Membranes 1

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. It continues until the particles are evenly distributed. Diffusion is a passive process as no energy is needed.
Rate of diffusion depends on the concentration gradient (higher = faster), thickness of exchange surface (thinner = faster), surface area (larger SA = faster).
Osmosis is the diffusion of water molecules from a high water potential to a low water potential. If two solutions have the same water potential, they are isotonic.
Faciliated diffusion occurs when large molecules can't diffuse directly through the phospholipid bilayer and so have to diffuse through carrier proteins or protein channels (faciliated diffusion). It moves down a concentration gradient and is a passive process. Carrier proteins move large molecules into or out of the cell, different carrier proteins faciliate the diffusion of different molecules. A large molecule attaches to a carrier protein in the membrane, causes it to change shape so it is released on the opposite side. Protein channels form pores in the membrane for charged particles to diffuse through. Different protein channels faciliate different charged particles.

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Exchange Across Plasma Membranes 2

Active transport uses energy to move molecules against the concentration gradient.
Carrier proteins are also involved in active transport. A molecule attaches to the carrier protein, protein changes shape and moves molecule to the other side. Energy is used from ATP.
Co-transporters are a type of carrier protein. They bind two molecules at a time. The concentration of one of the molecules is used to move the other molecule against its gradient.
Fluid Mosaic Model
1. Membrane is a good barrier against most water soluble molecules. The hydrophyllic tails of phospholipids make it difficult for water-soluble molecules to get through.
2. Membrane controls what enters and leaves, protein channels and carrier proteins allow this to happen.
3. Membrane allows cell communication, contain receptor molecules which detect chemicals from other cells.
4. Membrane allows cell recognition. Some proteins and lipids have short carbohydrate chains called glycoproteins and glycolipids. They tell white blood cells that they are the bodies own.
5. Membrane is fluid, phospholipids are constantly moving around. The more unsaturated fatty acids in the phospholipid bilayer, the more fluid. Cholesterol fits in between the bilayer which makes less fluid in the membrane. It is good as it is rigid and prevents the cell breaking up.

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Cholera bacteria is a prokaryotic cell. They have a flagellum, plasma membrane, cell wall, slime capsule (helps to protect from attack), plasmids (small loops of DNA), circular DNA floating free in the cytoplasm.
Cholera bacteria produces a toxin that causes chloride ion protein channels to open more than usual in the plasma membranes of the small intestine. Chloride ions move into the small intestine lumen.This lowers the water potential. Meaning water moves out of the blood, across the epithelial cells and into the small intestine lumen by osmosis. This massive increase in water secretion into the intestine lumen leads to watery faceces, causing dehydration.
Oral Rehydration Solution (ORS). Contains large amounts of salts and sugars dissolved in water. Sodium ions are included to increase glucose absorption, so the sodium-glucose co-transporter.
Ethical Issues - Mostly affects children but parents decide whether children take part in trial, so they don't make their own decision. Blind trials used, but the children should have a right to know what they are going to have. However, these are needed to eliminate any bias and skew the data, if they know what they are getting. When a new ORS is first trialled don't know if this or old ORS is more effective, so there is a risk of the person dying.

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