Key Biology Concepts

Cells are either eukaryotic or prokaryotic cells. 

Eurkaryotes are organisms made up of complex cells (eukaryotic cells). 

Prokaryotes are organisms made up of one prokaryotic cell (single celled organisms). 

Cells have subcellular structures. Animal cells include - 

Nucleus - Contains genetic material that controls activities in the cell. This material is arranged into chromosomes.

Cytoplasm - Gel like substance where most of the chemical reactions happen, containing enzymes to control these reactions. 

Cell membrane - Holds the cell together and controls what goes in and out of the cell. 

Mitochondria - Where most of the reactions for respiration take place. Respiration transfers energy that the cell needs to work. 

Ribosomes - Involved in the translation of genetic material in the synthesis of proteins. 

Plant cells include everything an animal cell has with 3 extra subcellular structures - 

Rigid Cell wall- Made of cellulose, the cell wall supports and strengthens the cell. 

Large Vacuole - Contains cell sap, a weak solution of sugar and salts. It maintains the internal pressure to support the cell. 

Chloroplasts - Where photosynthesis occurs, which provides food for the plant. They contain and green substance called chlorophyll. 

The egg gamete's function is to carry the female DNA and to nourish a developing embryo. It's adaptions include - 

1- It contains nutrients in the cytoplasm to nourish the developing embryo. 

2- It has a haploid nucleus, so when both gametes fuse their nuclei to form a zygote, the zygote has the correct amount of chromosomes that a body cell would normally have. 

3 - After fertilisation, the cell membrane changes structure to prevent more sperm cells from entering the cell and altering the amount of DNA the offspring would end up with. 

The function of a sperm cell is to transport its DNA to a female gamete. It's adaptions include - 

1- It has a tail so it can swim to the egg. 

2- It contains lots of mitochondria in the middle section of the sperm cell, which provides the energy needed to swim to the egg. 

3- In the head of the sperm cell is an acrosome which contains enzymes to digest the egg cell membrane. 

4- Like the egg cell, it also has a haploid nucleus. 

The function of a ciliated epithelial cell is the move materials. 

1- The surface of a ciliated epithelial cell is lined with cilia - halir-like structures which beat. This moves substance in one direction along surface tissue. 

The lining of the airways have lots of ciliated epithelial cells to beat mucus to the throat to prevent the mucus travelling to the lungs. 

Microscopes are used to magnify images, including cells. They also increase the resolution of an image. 

Light Microscopes 

- Invented in 1590s.

- They pass light through a specimen and allow us to nuclei and chloroplasts. These are used to study LIVING cells. 

- Invented in 1930's 

- Rather than light, they use electrons.

- They have a higher magnification and resolution, allowing us to view much smaller subcellular structures. 

- They can only be used on DEAD specimen. 

1- Take a clean slide and use a pipette to add water to the middle of the slide. Use tweezers to apply the thin slice of specimen to the slide. 

2- If your specimen is transparent add a drop of stain to the specimen. 

3- Place a cover slip at one end of the specimen, holding it at an angle with a mounted needle. Then lower it onto the slide. Clip the slide onto the stage. 

4- To start off, use the lowest power objective lens. 

5- Use the adjustment knob to move the stage upwards so the slide is underneath the objective lens. Whilst looking through the eyepiece lens, readjust the stage downwards until the specimen is focused. 

6- Adjust the focus using the fine adjustment knob until there is a clear image. Position a clear ruler on the stage and measure the diameter of the circular area visible to calculate the field of view. 

7- If you need to see your specimen with a greater magnification, swap to a higher powered objective lens, refocus and recalculate your FOV accordingly. (If your FOV was 5mm and the new lens is 10 times more powerful divide the original FOV by 10.) 

You need to know how to draw a scientific drawing of a specimen. 

1) Use a sharp pencil, draw outlines and the main features using clear, unbroken lines. Don't include any colouring or shading. 

2) Make sure your drawing takes up at least half of the space available and remember to keep all the parts in proportion. 

3) Label all the important features of the diagram with straight lines which do not cross over each other. Include magnification used and a scale. 

If you know the power of the lenses used by a microscope to view an image, you can work out the total magnification using this formula : 

Total Magnification = Eyepiece lens magnification X Objective lens magnification.

If the magnification of the lenses are not given, use the size of the specimen that is given to you. This can be found using the formula: 

Magnification = Image Size / Real size. 

This equation can be re-arranged to work out the image or real size. 

Microscopes use standard form because they see tiny objects. You need to be able to convert between units. 

                                                 Millimetre (mm)

                                                 Micrometre (  m) 

                                                 Nanometre (nm) 

                                                 Picometre (pm) 

Enzymes are biological catalysts, speeding up useful chemical reactions within the body without needing extremely high temperatures. 

Enzymes have a high specificity for substrates due to the shape of their active sites. Actives sites are the part of the enzyme which breaks down the substrate, and this is a specific shape to ensure that the substrate can fit. If the substrates shape doesn't fit the active site, then the reaction cannot be catalysed. This is called the 'lock and key' mechanism as the substrate fits into the enzyme like a key into a lock. 

3 things affect the rate of reaction involving enzymes: Temperature, pH and substrate concentration. 

Changing the temperature changes the rate of an enzyme catalysed reaction. Similar to any reaction, a higher temperature increases the rate at first. But once the temperature begins to exceed 37 degrees (normal body temperature), the bonds holding the enzyme together begin to break causing the enzymes to denature. This changes the shape of the active site, so the substrate cannot fit the active site anymore. All enzymes have an optimum temperature that they work at. 

pH - The pH also affects enzymes. If the pH levels are too low or too high, the bonds holding enzymes together are interferred, and therefore the active site changes shape, causing the enzyme to be denatured. Again, all enzymes have an optimum pH. Most enzymes work best at a pH of 7, but enzymes such as pepsin (an enzyme which breaks down proteins in the stomach) works best at a pH of 2 as this suits the acidic conditions of the stomach. 

Finally, Substrate concentration also affects the rate of a reaction, the higher the substrate concentration the quicker the reaction. This is due to the fact that enzymes are more likely to meet up with and react with a substrate. 

This is only true to a certain point. At this point, all enzyme active sites are full and adding more doesn't make a difference. 

Investigate the effect of pH on Enzyme Activity - 

1) Put a drop of iodine solution into every well on a spotting tile. 

2) Place a bunsen burner on a heat-proof mat and a tripod and gauze over the Bunsen burner. Put a beaker of water on top of the tripod and heat the water until it is at 35 degrees. (Use a thermometer to measure the temperature). Try to keep this temperature constant during the experiment. 

3) Use a syringe to add 3cm cubed of amylase solution and 1cm cubed of a buffer solution with a pH of 5 to a boiling tube. Use test tube holders to hold the boiling tube in the beaker of water for 5 minutes. 

4) Use a different syringe to add 3cm cubed of a starch solution to the boiling tube. Immediately mix the contents of the boiling tube and start a stop watch.

5) Using a continuous sampling technique, record how long it takes for the amylase to break down all of the starch. Take a dropping pipette, and take a fresh sample from the boiling tueb every 10 seconds and put a drop into the well. When the iodine solution stays a brown-orange, starch is no longer present. 

6) Repeat this experiment with buffer solutions of different pH values to see how the pH affects the time taken for the starch to be broken down. Control all variables (temperature, concentration and volume of amylase) to ensure its a fair test. 

To find the rate use the equation - Rate = 1000/time taken. 

Enzymes break down large molecules such as Proteins, Lipids and Carbohydrates. This is essential for organisms to be able to breakdown these substrates into their smaller components in order to use them for growth and other like processes: 

Many molecules we eat are too big to pass through our digestive system, so digestive enzymes break them down into smaller soluble molecules that can easily pass through the walls of the digestive system, so they be absorbed back into the bloodstream. These can then be passed into cells to be used around the body.

Plants store energy as starch. When plants need this energy enzymes break down the starch into smaller molecules (sugars) which can then be respired to transfer energy to be used in the cells.

Enzymes known as carbohydrase convert carbohydrates into simple sugars. For example, amylase is broken down into starch. 

Proteases convert proteins into amino acids. 

Lipases convert lipids into glycerol and other fatty acids. 

Synthesis -

Organisms also need to be able to convert the simpler molecules of a substrate back into the original substrate in order to survive. Enzymes are also used in this process. 

Carbohydrates can be synthesised by joining together simple sugars. This is also the same process with proteins and lipids.

Glycogen synthase is an enzyme that joins together lots of chains of glucose molescules to make glycogen (a molecule that stores energy in animals.)

Test for Sugars (Benedict's Reagent) -

1) Add Benedict's reagent (which has a blue colour) to a sample and heat it in a water bath that's set to 75 degrees. If the test if positive it will form a coloured precipitate. 

2) The higher the concentration of reducing sugars, the further the colour change goes, allowing you to compare the amount of reducing sugars in different solutions. 

                          Blue -----> Green -----> Yellow -----> Orange -----> Brick Red. 

Testing for Starch -

1) Add iodine solution to a test sample. If starch is present, the sample will change from the browny-orange of the iodine, to a dark blue/black colour. 

2) If starch is not present, it will stay a browny-orange colour. 

Emulsion Test for Lipids -

1) Shake the test substance with ethanol for about a minute until it dissolves. Then add the solution to some water. 

2) If there are any lipids present, they will precipitate out of the liquid and show up as a milky emulsion. 

3) The more lipid there is, the more noticeable the milky colour will be. 

Biuret Test for Proteins -

1) Add a few drops of potassium hydroxide solution to make the solution alkaline. 

2) Then add some copper (II) sulphate solution (which is bright blue) 

3) If theres no protein present, the solution stays blue. 

4) If there is protein present, the solution will turn purple. 

Food can be burnt to see how much energy the food contains, also known as calorimetry -

Method -

1) Use food that will burn easily, preferably something dry like pasta. 

2) Weigh a small amount of the food and then skewer it onto a mounted needle. 

3) Nest, add a set volume of water to a boiling tube (held with a clamp) - this is used to measure the amount of energy transferred when the food is burnt. 

4) Measure the temperature of the water, then set fire to the food using a Bunsen burner flame. Make sure the Bunsen burner isn't near the test tubes as this could affect your results. 

5) Immediately hold the burning food under the boiling tube until the flame goes out. Re-light the food and keep doing this process until the food won't catch fire again. 

6) Measure the water again. 

- To reduce the energy of the food going to the environment, wrap the boiling tube in an insulator. 

There are 2 equations for calorimetry -

Energy in Food (in J) = Mass of Water (in g) X Temperature change in Water (In degrees) X 4.2

(remember that 1cm cubed of water is equal to 1g of water.) 

Then to work out the amount of Joules in each gram of the food use -

Energy per gram of food (J/g) = Energy in Food / Mass of Food 

Use the second equation to compare two or more foods and which one has more energy. 

Diffusion - The net movement of particles from an area of high concentration to low concentration. (Follows the concentration gradient). 

Diffusion happens in both liquids and gases (Don't let this throw you, check your question to figure out if Diffusion or Osmosis is taking place.) This happens in liquids and gases as particles can move freely and randomly. Very small particles can diffuse through cell membranes, like glucose and oxygen. Molecules like starch are too big to pass through the membrane. 

Active Transport - The movement of particles across a membrane against the concentration gradient (so therefore an area of low concentration to high concentration) using energy transferred during respiration. 

An example of this is - 

When theres a higher concentration of nutrients in the gut than the blood, diffusion occurs normally and naturally into the blood stream. But sometimes there's a lower concentration of nutrients in the gut than the blood, so active transport allows nutrients to be taken into the blood despite going against the concentration gradient. This ensures we don't starve. 

Osmosis - The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to an area of lower water concentration. 

A partially permeable membrane has small holes in it, so small molecules (such as water) can easily pass through, but larger molecules cannot. (such as sucrose)

The water molecules can pass both ways through the membrane during osmosis, as water molecules can move randomly due to their state. 

However, because there are more molecules on one side than the other, there is a steady flow of water into the region with fewer water molecules. 

This means that the solute solution becomes more dilute, and the water acts to try an balance the concentration on both sides of the membrane. 

This experiment involves putting potato cylinders into different concentrations of sucrose solution to see what effect different water concentrations have on them. 

Method -

1) Prepare the sucrose solutions ranging from pure water to a very concentrated sucrose solution (From 0.0M up to 1.0M) 

2) Use a cork borer to cut a potato into the same sized pieces (Around 1cm in diameter and preferably from the same potato.) 

3) Divide the cylinders into groups of three and use a mass balance to measure the mass of each group of potato pieces. Then place one group into each beaker. 

4) Leave the cylinders in the solution for a minimum of 40 minutes and make sure all the potato piece get the same amount of time. 

5) Remove the cylinders and pat them dry with a paper towel. This is to ensure that there is no excess water on the surface of the cylinder so it gives a more accurate measure of the final mass. 

6) Weigh each group again and record the results. 

Graph -

At the points above the x axid the water concentration of the sucrose is higher than in the potato cylinders. 

At the points below the x axis, the water concentration of the sucrose is lower than in the potato cylinders.

At the point where the graph intersects the x axis is where the concentration of water is equal in both the sucrose solution and the potato cylinders. This is also known as isotonic. 

To find the total percentage change in mass use -

Final Mass - Inital Mass / Initial Mass X 100