Topic 1 - key concepts in biology

what are the two types of cells?

prokaryotic or eukaryotic

what type of cell are animal and plant cells and what do they have?

eukaryotic, they have a:

• cell membrane cytoplasm

• nucleus containing DNA

what are the two types of cells?

prokaryotic or eukaryotic

what type of cell are animal and plant cells and what do they have?

eukaryotic, they have a:

• cell membrane cytoplasm

• nucleus containing DNA

what type of cell are bacterial cells and what do they have?

prokaryotic, they have a:

• cell wall

• cell membrane

• cytoplasm

• single circular strand of DNA and plasmids (small rings of DNA found in the cytoplasm)

what is the nucleus function?

• contains DNA coding for a particular protein needed to build new cells

• enclosed in a nuclear membrane

what is cytoplasms function?

• liquid substance in which chemical reactions occur

• contains enzymes (biological catalysts, ie. proteins that speed up the rate of reaction)

• organelles are found in it

cell membrane function?

• controls what enters and leaves the cell

mitochondria function?

• where aerobic respiration reactions occur, providing energy for the cell in the form of ATP

ribosomes function

• where protein synthesis occurs

• found on a structure called the rough endoplasmic reticulum

what structures are only found in plant cells?

• chloroplasts

• permanent vacuole

• cell wall (also present in algal cells)

chloroplasts function

• where photosynthesis takes place, providing food for the plant

• contains chlorophyll pigment (which makes it green) which harvests the light needed for photosynthesis

permanent vacuole function

• contains cell sap

• found within the cytoplasm

• improves cell’s rigidity

cell wall (also present in algal cells) function

• made from cellulose

• provides strength to the cell

what structures are in bacterial cells?

• cytoplasm

• cell membrane

• cell wall

• chromosomal DNA (circular)

• plasmids

• flagella

cell wall function

made of a different compound (peptidogylcan)

chromosomal DNA (circular) function

as bacterial cells have no nucleus, this floats in the cytoplasm

plasmids function

small rings of DNA - code for extra genes to those provided by chromosomal DNA

flagella function

long, thin ‘whip like’ tails attached to bacteria that allow them to move

how do cells specialise?

cells specialise by undergoing differentiation: a process that involves the cell gaining new sub-cellular structures in order for it to be suited to its role. cells can either differentiate once early on or have the ability to differentiate their whole life (stem cells). in animals, most cells only differentiate once, but in plants many cells retain the ability

animal specialised cell, sperm cells

specialised to carry the male’s DNA to the egg (ovum) for successful reproduction

• streamlines head and long tail to aid swimming

• many mitochondria (where respiration happens) which supply the energy to allow the cell to move

• the acrosome (top of the head) has digestive enzymes which break down the outer layers of the membrane of the egg cell

• haploid nucleus - haploid simply means that it has 23 chromosomes, rather than 46 that most other body cells have

animal specialised cells, egg cells

specialised to accept a single sperm cell and develop into an embryo

• surrounded by a special cell membrane which can only accept one sperm cell (during fertilisation) and becomes impermeable following this

• lots of mitochondria to provide energy source for the developing embryo

• large size and cytoplasm to allow quick, repeated division as the embryo grows

specialised animal cells, ciliated epithelial cells

specialised to waft bacteria (trapped by mucus) to the stomach

• long, hair like processes called cilia waft bacteria trapped by sticky mucus (produced by nearby goblet cells) down to the stomach, where they are killed by the stomach acid. this is one of the ways our body protects against illness

specialised plant cells, root hair cells

specialised to take up water by osmosis and mineral ions by active transport from the soil as they are found in the tips of roots

• have a large surface area due to root hairs, meaning more water can move in

• the large permanent vacuole affects the speed of movement of water from the soil to the cell

• mitochondria to provide energy from respiration for the active transport of mineral ions into the root hair cell

specialised plant cells, xylem cells

specialised to transport water and mineral ions up the plant from the roots to the shoots

• upon formation, a chemical called lignin is deposited which causes the cells to die. they become hollow and are joined end to end to form a continuous tube so water and mineral ions can move through

• lignin is deposited in spirals which helps the cells withstand the pressure from the movement of water

specialised plant cells, phloem cells

specialised to carry the products of photosynthesis (food) to all parts of the plants

• cell walls of each cell form structures called sieve plates when they break down, allowing the movement of substances from cell to cell

• despite losing man sub-cellular structures, the energy these cells need to be alive is supplied by the mitochondria of the companion cells

what do microscopes do

help to see extremely small structures such as cells to enlarge the image

light microscope

the first cells of a cork were observed by Robert Hooke in 1665 using a light microscope

• it has two lenses

• it is usually illuminated from underneath

• they have, approximately, a maximum magnification of 200x and a resolving power (this affects resolution: the ability to distinguish between two points) of 200nm. the lower the RP, the more detail is seen

• used to view tissues, cells and large sub-cellular structures

electron microscope

in the 1930s the electron microscope was developed, enabling scientists to view deep inside sub-cellular structures, such as mitochondria, ribosomes, chloroplasts and plasmids

• electrons, as opposed to light, are used to form an image because the electrons have a much smaller wavelength than light waves

• there are two types: a scanning electron microscope that creates 3D images (at a slightly lower magnification) and a transmission electron microscope which creates 2D images detailing organelles

• they have a magnification of up to 2,000,000x and resolving power of 10nm (SEM) and 0.2nm (TEM)

what has the discovering of the electron microscope done?

allowed us to view many organelles more clearly - especially very small structures such as ribosomes. transmission electron microscopes (TEMs) in particular, have been used to discover viruses such as poliovirus, smallpox and ebola - and are still used for this function today. this is useful as viruses are much smaller than bacteria, and are very hard to identify using a standard light microscope. electron microscopes are also used to examine proteins in much greater detail than can be achieved with a light microscope, which has lead to many important scientific discoveries.

common calculations in microscopy

1. magnification of a light microscope: magnification of the eyepiece lens x magnification of the objective lens

2. size of an object: size of image/magnification = size of object (this formula can be rearranged to obtain the other values, make sure you are in the same units!)

name the parts of a light microscope and their functions

eyepiece - this is the part of the microscope that we can look through to view specimens

barrel - the upper part of the microscope that can be moved up or down to focus the image

turret - the part of the microscope that is rotated to change the magnification lens in use

lens - the lens increases the magnification of the specimen

stage - the flat surface on which we place the specimen

how should you use a light microscope

1. place the slide on the stage and look through the eyepiece lens

2. turn the focus wheel to obtain a clear image

3. start with the lowest objective lens magnification

4. increase the magnification of the objective lens and refocus

how do you prepare a slide in order to use specimens with a light microscope

1. take a thin layer of cells from your sample by either peeling them off or using a cotton bud

2. add a small amount of the correct chemical stain (you will be told by your teacher which stain to use). chemical stains are used to make some parts of the specimen more visible when you look at them through the microscope

3. apply the cells to your glass slide by placing them on or wiping the cotton bud against it

4. carefully lower a coverslip onto your slide, taking care to avoid air bubbles

   You should know how to perform magnification calculations, remember:

   • magnification = measured size/actual size

   • actual size = measured size/magnification

   • total magnification = objective lens magnification x eyepiece lens magnification

what are enzymes?

enzymes are biological catalysts (substance that increase rate of reaction without being used up)

• enzymes are present in many reactions - allowing them to be controlled

• they can both break up large molecules and join small ones

• they are protein molecules and the shape of the enzyme is vital to its function

• this is because each enzyme has its own uniquely shaped active site where the substrate binds

lock and key hypothesis

1. the shape of the substrate is complementary to the shape of the active site (matches the shape of the active site), so when they bond it forms an enzyme-substrate complex

2. once bound, the reaction takes place and the products are released from the surface of the enzyme

what is enzyme specificity

enzymes can only catalyse (speed up) reactions when they bind to a substrate that has a complementary shape, as this is the only way that the substrate will fit into the active site.

what do enzymes require?

optimum pH and temperature, because they are proteins. they also need an optimum substrate concentration

what is the optimum temperature in humans

the optimum temperature in humans is a range around 37 degrees celsius (body temperature). this temperature is different in other organisms

• the rate of reaction increase with an increase in temperature up to this optimum, but above this temperature it rapidly decreases and eventually the reaction stops

• when the temperature becomes too hot, the bonds that hold the enzyme together will begin to break

• this changes the shape of the active site, so the substrate can no longer ‘fit into’ the enzyme

• the enzyme is said to be denatured and can no longer work

what is the optimum pH for most enzymes?

the optimum pH for most enzymes is 7 (neutral), but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH

• if the pH is too high or too low, the forces that hold the amino acid chains that make up the protein will be affected

• this will change the shape of the active site, so the substrate can no longer fit in

• the enzyme is again said to be denatured, and can no longer work

what happens as the substrate concentration

as the substance concentration (concentration of the substance binding to the enzyme) increases, the rate of reaction will increase - up to a point

• this is because, as substrate concentration increases, the rate at which enzyme-substrate complexes can be formed increases

• this only occurs up to a point, however - this is called the saturation point, and increasing the substrate concentration above this will have no effect on the rate of reaction. the saturation point is different for every enzyme

core practical - effect of pH on enzyme activity

looks at how pH affects the rate of activity of a particular enzyme. the enzyme being used is called amylase - which breaks down carbohydrates such as starch into simple sugars such as maltose. we can use iodine (dark orange colour) to check for the presence of starch in the solution at any time. when starch is present, the iodine solution will turn to a blue-black colour. amylase has an optimal pH, and we can use this experiment to estimate what it might be

what materials are required?

1% amylase solution, 1% starch solution, iodine solution, labelled buffer solution of different pH

method

1. place single drops of iodine solution on each well of a tray

2. label a test tube with the pH to be tested. place it in a water beaker with 50ml cold water and place this above a Bunsen Burner for 3 minutes

3. place 2cm³ of amylase solution, 2cm³ of starch solution and 1cm³ of the buffer pH solution in a test tube and start a stopwatch

4. after 10 seconds, use a pipette to place a drop of the solution into one of the wells containing iodine solution. the mixture should turn blue-black to indicate that starch is still present and has not yet been broken down

5. repeat step 4 after another 10 seconds. continue repeating until the solution remains orange, and record the time taken

6. repeat steps 1-5 with a buffer solution of different pH

7. record your results on a graph of pH (on the x-axis) and time taken to complete reaction (on the y-axis)

why do we use a Bunsen Burner and water beaker?

we use this equipment to keep the solution at a relatively constant temperature throughout the reaction (temperature is a control variable in this experiment)

what results do we expect to see?

the optimum pH of amylase will be at whichever pH the reaction completes in the shortest time. this should be somewhere around pH 7.0

rate calculation formula

rate = change/time

change refers to the change in the substance being measured and time refers to the time taken for that change to occur

what are proteases?

a type of enzyme used to break down proteins

what do carbohydrase’s do?

carbohydrases convert carbohydrates into simple sugars

• example: amylase breaks down starch into maltose

  • it is produced in your salivary glands, pancreas and small intestine (most of the starch you eat if digested here)

what do proteases do?

proteases convert proteins into amino acids

• example: pepsin, which is produced in the stomach, other forms can be found in pancreas and small intestine

what do lipases do?

lipases convert lipids (fats) into fatty acids and glycerol

• produced in the pancreas and small intestine

what do these things do?

soluble glucose, amino acids, fatty acids and glycerol pass into the bloodstream to be carried to all the cells around the body.

they are used to build new carbohydrates, lipids and proteins, with some glucose being used in respiration. building these new carbohydrates, lipids and proteins requires some different, more complex enzymes to increase the rate of reaction

food test for starch

iodine solution

• add iodine solution to the food sample. if starch is present, the colour will change from orange to blue-black

food test for reducing sugars

Benedict’s solution

• add 2cm³ of the sample solution and 2cm³ of blue Benedict’s solution to a test tube

• place in a boiling water bath for 5 minutes, or until there is no further change in colour

• presence of reducing sugar is indicated by a colour change to reddish-brown

test for protein

biuret test (potassium hydroxide and copper sulfate)

• add 1cm³ of 40% potassium hydroxide to the food sample, and then add the same amount of 1% copper sulfate

• shake well and observe colour change if protein is present (blue —> violet)

test for lipids

emulsion test

• add 2cm³ ethanol to food sample and shake thoroughly

• add 2cm³ deionised water and shake thoroughly

• if lipids are present, this will be indicated by the formation of a white emulsion layer at the top of the sample

how could we improve these experiments?

we should use a control in each experiment to ensure we know what a positive and negative result looks like. eg. a positive control for the Biuret test would be anything containing protein (eg. egg white) whereas a negative control would be a solution that does not contain protein (eg. distilled water)

how do we measure the amount of ‘energy’ (calories) in food

calorimetry is a way to measure the energy taken in and given out during a chemical reaction

method for calorimetry

1. take a tube of 50ml cold water

2. record the starting temperature of the water

3. place the test tube at 45 degrees and hold a burning food sample just beneath it

4. when the food is burned up, record the final temperature of the water

   we can work out the energy transferred to the water using the equation: energy transferred (joules, J) = mass of water (grams, g) x 4.2 (J/g) x temperature increase (‘c)

transport in and out of cells

substances like oxygen, glucose and waste products need to be transported in and out of cells constantly to support life processes. this transport generally occurs in one of 3 ways: diffusion, osmosis or active transport

what is diffusion

a form of passive transport (does not require energy). it is important to remember that molecules move in every direction and collide with each other, but the net (or resultant) movement is from an area of high concentration to one of low concentration

what is osmosis

osmosis is also a form of passive transport (does not require energy) but it only applies to water. the same rules as diffusion apply - however there is no such thing as ‘concentration of water’, so we say that movement is from a dilute solution to a more concentrated solution, across a semi-permeable membrane.

what is active transport

active transport is a form of transport that does require energy. this energy comes from ATP, which is the molecule produced in respiration. active transport is used to move molecules against a concentration gradient (ie from an area of low concentration to an area of high concentration)

method for osmosis in potatoes practical - percentage gain and loss of mass

1. cut potato into small discs of equal size

2. blot the potato discs gently with tissue paper to remove excess water

3. measure the initial mass of each disc

4. place the discs in sucrose solutions of different concentrations (1%, 2% etc)

5. blot with tissue paper again and record new mass

6. find difference in mass (end mass - start mass) and use the percentage change equation to calculate percentage gain or loss of mass

   percentage change = (change in mass/start mass) x 100

what are the independent, dependent and control variables in this experiment?

• sucrose solution is changing, so is independent variable

• change in mass of the potato discs is being measured, so is dependent variable

• we are controlling the diameter of the discs, so is a control variable

what is happening in this experiment?

water is moving by osmosis from a more dilute solution (in the potato) to a more concentrated solution (the sucrose solution) across a semi-permeable membrane (the cell membranes of all the potato cells holding water)