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Biology Revision, chromosomes and mitosis, Exchanging Substances, Osmosis,…
Biology Revision
B1: Cell Biology
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Microscopy
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Electron microscope
let us see smaller things, e.g. internal structure of mitochondria and chloroplasts
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PRACTICAL
Preparing the slide
- using tweezers, place the epidermal tissue into the water on the slide
- add a drop of iodine solution (a stain), to highlight objects in a cell by adding colour to them
- cut and separate an onion into layers. Use tweezers to peel off some epidermal tissue from the bottom of one of the layers
- place a cover slip on top. Stand the cover slip upright next to the water droplet and slowly tilt and lower it to cover the specimen, trying to avoid air bubbles as they obstruct your view.
- add a drop of water to the middle of a clean slide
Looking at the slide
- use the coarse adjustment knob to move the stage up to just below the objective lens
- look down the eyepiece, use the coarse adjustment knob to move the stage downwards until the image is roughly in focus
- adjust the focus with the fine adjustment knob until you get a clear image
- select the lowest powered objective lens
- clip the prepared slide onto the stage
- use a higher-powered objective lens to see the slide with a greater magnification and refocus
Drawing observations
- it should not any colouring or shading
- sub-cellular structures should be drawn in proportion
- make sure it is drawn with clear, unbroken lines
- include a title, as well as writing the magnification it was observed under
- draw what you see using a sharp pencil
- label the features of the drawing using straight, uncrossed lines
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chromosomes and mitosis
Cell cycle
- multicellular organisms use mitosis to grow and develop or repair cells that have been damaged
- the end of the cell cycle results in two new cells identical to the original cell, with the same number of chromosomes
- the stage of the cell cycle where the cell divides is called mitosis
- body cells in multicellular organisms divide to produce new cells as part of a series of stages called the cell cycle
most cells in the body have a nucleus. The nucleus contains genetic information in the form of chromosomes.
each chromosome carries a large number of genes. Different genes control the development of different characteristics, e.g. hair colour
body cells normally have two copies of each chromosome - one from the mother and one from the father
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Mitosis
Lastly, the cytoplasm and cell membrane divide
The cell has now produced two new daughter cells. the daughter cells contain exactly the same DNA - they're identical. Their DNA is also identical to the parent cell.
Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells - the nucleus has divided
The chromosomes line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell
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Exchanging Substances
with the environment
how easy it is for an organism to exchange substances with its environment depends on the organisms surface area to volume ratio. The larger an organism, the smaller its surface area is compared to volume.
an example of this is: a mouse can be represented by a 1cm x 1cm x 1cm block. It's surface area is (1x1) x 6 = 6cm^2. Its volume is 1 x 1 x1 = 1cm^3. So the surface area to volume ratio of the mouse is 6:1. This is a large ratio because the organism is small. If it was a hippo represented by a 2 x 4 x 4 block, the surface area to volume ratio would be 2:1. Therefore, the larger an organism, the smaller its surface area to volume ratio.
cells can use diffusion to take in substances they need and get rid of waste products. For example, oxygen and carbon dioxide are transferred between cells and the environment during gas exchange. In humans, urea ( a waste product produced from the breakdown of proteins) diffuses from cells into the blood plasma for removal from the body by the kidneys.
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in organs and leaves
in the small intestine
they increase the surface area in a big way so that digested food is absorbed much more quickly into the blood.
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in leaves
the walls of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase the area of this surface so there's more chance for carbon dioxide to get into the cells.
the water vapour evaporates from the cells inside the leaf. Then it escapes by diffusion because there's a lot of it inside the leaf and less of it in the air outside.
the flattened shape of the leaf increases the area of this exchange surface so that it's more effective.
the size of the stomata is controlled by guard cells. These close the stomata if the plant is losing water faster than it is being replaced by the roots. Without these guard cells, the plant would wilt.
oxygen (produced in photosynthesis) and water vapour also diffuse out through the stomata. Water vapour is actually lost from all over the leaf surface, but most of is lost through the stomata.
the underneath of the leaf is an exchange surface. It's covered in little holes called stomata which the carbon dioxide diffuses through.
carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leafs structure is adapted so that this can happen easily
in the lungs
to do this the lungs contain millions of little air sacs called alveoli where gas exchange an take place
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in fish gills
the gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more
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each gill is made of lots of thin plates called gill filaments, which give a big surface area for exchange of gases
they also have a thin surface layer of cells to minimise the distance that the gases have to diffuse
water ( containing oxygen) enters the fish through its mouth and passes through the gills. As this happens, oxygen diffuses from the water into the blood in the gills and carbon dioxide diffuses from the blood into the water
blood flows through the lamellae in one direction and water flows over in the opposite direction. This maintains a large concentration gradient between the water and the blood.
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the concentration of oxygen in the water is always higher than in the blood, so as much oxygen as possible diffuses from the water into the blood.
Osmosis, diffusion and active transport
diffusion
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the simplest type is when different gases diffuse through each other. This is what's happening when the smell of perfume diffuses through a room
diffusion is the spreading out of particles from an area of high concentration to an area of low concentration
the bigger the concentration gradient, the faster the diffusion rate. A higher temperature will also give a faster diffusion rate because the particles have more energy so move around faster
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osmosis
a partially permeable membrane is just one with very small holes in it. So small that only tiny molecules like water can pass through them, and bigger molecules such as sucrose can't
the water molecules actually pass through both ways of the membrane during osmosis. This happens because the water molecules move about randomly all the time
osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration
because there are more water molecules on one side of the membrane than the other, there is a steady net flow of water to the region with fewer water molecules, i.e into the stronger sugar solution
this means the stronger sugar solution gets more dilute. The water acts like it's trying to 'even up' the concentration either side of the membrane
osmosis is a type of diffusion - passive movement of water particles from an area of high concentration to an area of lower water concentration
Osmosis practical
- take the cylinders out, dry them with a paper towel and measure their masses again.
- if the cylinders have drawn in water by osmosis, they'll have increased in mass. If water has been drawn out, they'll have decreased in mass. You can calculate the percentage change in mass, then plot a graph
- measure the mass of the cylinders, then leave one cylinder in each beaker for around 24 hours
- the dependent variable is the chip mass and the independent variable is the concentration of the sugar solution. all the other variables such as time and temperature must be kept the same to ensure a fair test
- cut up a potato into identical cylinders, and get some beakers with different sugar solutions in them. One should be pure water and another should be very concentrated sugar solution. Then, you have a few others with concentrations in between.
- some errors could occur, for example if some of the cylinders were not fully dried, or if water evaporated from the beakers. This can be reduced by repeating the experiment and calculating a mean percentage change at each each concentration
active transport
root hair cells
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the concentration of minerals is usually higher in the root hair cells than in the soil around them.
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root hair cells are covered in microscopic hairs that cover each branch. There are millions on each branch
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