Download New unit: Investigating CELLS - Cells are the basic units of all living

New unit: Investigating CELLS
- Cells are the basic units of all living things.
Activity: Glue diagram of animal and plant cells and label it using your book for help.
The functions (i.e. the jobs) of the different parts of the cell.
Part of cell
Nucleus
Cytoplasm
Cell Membrane
Vacuole
Cell wall
Chloroplasts
Found in all animal
cells?
Found in all plant cells?
Observing cells with a microscope:
- Cell needs to be stained to make the different part easier to see with a
microscope.
- How to do a wet mount:
- How to measure the size of cells:
The length of a cell is 1000m = 200m
5
Investigating cell diffusion
Particles in gases and liquids move from where they are tightly packed (high
concentration) to areas where they are less tightly packed (low concentration).
Diffusion is the name given to the movement of particles from an area of high
concentration to an area of low concentration.
The substance moves down its concentration gradient.
Experiment on the model cell
1- Using a spotting tile, test the starch and sugar mixture for the presence of starch and
sugar.
Testing the starch and sugar mixture
Substance to be
tested
Sugar
Starch
Testing solution
Colour change
Conclusion
Benedict’s reagent
(to be incubated in hot
water)
Iodine
2- Place the mixture into visking tubing, this will be your “model cell”.
3- Place this “model cell” into a test tube half filled with water and leave until the next
lesson.
4- After at least 24h, test the water in the boiling tube to find out whether sugar or/and
starch has diffused through the selectively permeable membrane.
Testing the water in the boiling tube after 24h
Substance to be
tested
Sugar
Starch
Testing solution
Colour change
Conclusion
Benedict’s reagent
(to be incubated in hot
water)
Iodine
5- Write down a conclusion using the words:
high concentration, low concentration, concentration gradient, diffuse, selectively permeable
membrane.
The importance of diffusion to organisms:
Diffusion: allows the movement of gases (i.e. oxygen and carbon dioxide) to move in
and out of cells.
The cell membrane is selectively permeable: size of membrane holes determines
which molecules can go through it.
For small organisms: no specialised organs (e.g. amoeba), no specialised organ needed,
diffusion through cell membrane is enough.
For large organisms; specialised organs needed:
- lungs in mammals
- gills in fish
- leaves in plants.
The special case of diffusion of water
The diffusion of water is called OSMOSIS, from the area of High Water
Concentration (HWC) to Low Water Concentration (LWC
Experiment: osmosis in potatoes
Potato cell directly in contact with water:
- When cells are placed in water, the water moves by osmosis from the area of high
concentration in the water to the area of low concentration in the cell. As a result,
the cell swells up and this is called a turgid cell. It does not burst as the cell wall
helps it keep its shape.
- When the cell is placed in a concentrated salt solution, the water moves by osmosis
from the area of high concentration in the cell to the area of low concentration in the
salt solution. As a result, the cell shrinks or shrivels. For plant cells, this is called a
plasmolysed cell.
- If the cell was placed in a dilute salt solution,, the concentration of water in and out
of the cell are similar and as a result, the cell does not shrink or swell, it stays the
same.
Osmosis in red blood cells
QuickTime™ and a
decompressor
are needed to see this picture.
- When cells are placed in water, the water moves by osmosis from the area of high
concentration in the water to the area of low concentration in the cell. As a result,
the cell swells up and eventually bursts.
- When the cell is placed in a concentrated salt solution, the water moves by osmosis
from the area of high concentration in the cell to the area of low concentration in the
salt solution. As a result, the cell shrinks or shrivels.
- If the cell was placed in a dilute salt solution,, the concentration of water in and out
of the cell are similar and as a result, the cell does not shrink or swell, it stays the
same.
Cell division
Cells divide to increase the number of cells in an organism so that growth and cell
replacement can take place.
Cell division is also called MITOSIS.
A cell divides to produce 2 identical cells called daughter cells.
The hereditary information (also called genetic material) is held in the DNA
contained in the nucleus.
The DNA is sub-divided in chromosomes.
After mitosis, each daughter cell has an identical set of chromosomes that carries
the same genetic information as the original cell.
The number of chromosomes in a nucleus is called the chromosome complement and it
changes depending on species.
To obtain two identical sets of chromosomes, the genetic material contained in a
nucleus is replicated (copied) before mitosis.
If a cell inherits fewer or more chromosomes than the normal number, it will function
abnormally or die.
Cell division in animal cells
1- Chromosomes get shorter and
fatter. The cell is ready for mitosis.
2- Chromosome are attached to spindle
fibres and positioned at the equator
(plane at the centre of the cell). They
can be seen to be made of two
chromatids.
3- Spindle fibres contract separating
the chromatids which are pulled apart
towards opposites poles.
4- Two nuclei form at each end of the
cell
5- The cytoplasm start to divide
6- Two daughter cells are formed
Cell division in plant cells
Same as for animal cells except for 5:
5- New cell wall forms on a plane at
the centre of a cell.
Investigating Enzymes
A chemical reaction is when a substrate is chemically altered into a product.
A catalyst is a substance that speeds up the rate of a chemical reaction without
being changed or used up (i.e. a catalyst is neither a substrate nor a product as it is
unaffected by chemical reactions).
In a cell there is a large number of chemical reactions that take place continuously.
These would not happen fast enough to sustain life without the help of catalysts.
Enzymes are “biological catalysts” produced by the cells themselves.
In biology, it is important for experiments to include a control. A control is a test
where all variables are kept the same except for the variable which is deliberately
changed i.e. the input variable or the dependent variable.
This can be done in two ways:
- using material in which biological activity has been destroyed by boiling, cooking, or
sterilising the biological sample.
- using an equivalent mass or volume of glass beads.
1/ Study of catalase
.Catalase speeds up the following reaction:
H2O2
Hydrogen
Peroxide
H2O + 1/2O2
Water + Oxygen
Catalase is a very common enzyme found in most organisms. In mammals, it is
especially concentrated in the liver.
When doing experiments in order to compare the activity of catalase in various
biological samples, which variables should be carefully controlled?
Class test:
Test
First value
Second value
Mass
Values (g)
Height of
bubbles
(cm)
Surface
Values
(cm2)
area
Height of
bubbles
(cm)
Tempera ture
Value (°C) Height of
bubbles
(cm)
pH
Height of
bubbles
From these experiments, we can conclude that, mass, surface area, temperature and
pH affect catalase activity.
2/ Other enzymes to know:
C-L-A-P-P-P
- Catalase: breaks down hydrogen peroxide into water and oxygen
- Lipase: breaks down fats into fatty acids and glycerol
- Amylase: It breaks down starch into maltose.
- Pepsin: breaks down proteins into polypeptides
- Potato Phosphorylase: in potatoes, joins molecules of Glucose-1-phosphate to form
starch.
3/ Summary of important facts to consider in reaction catalysed by enzymes
- To make a fair experiments:
- mass
- surface area
- volume
- origin of biological material
- temperature
- pH
are variables (or factors) that need to be kept constant.
- Variables affecting the speed of the reaction:
- Temperature
- pH
The effect of temperature on enzyme activity
At low temperatures, enzymes do not work effectively (molecules move too slowly).
Enzymes increase the rate of reaction (i.e. speed up) most effectively at a
temperature called the optimum temperature Beyond that temperature, an enzyme
becomes denaturated, i.e. it is irreversibly damaged.
Examples of enzyme optimal temperatures:
- animal: body temperature
- plant: in our climate, about 20°C
The effect of pH on enzyme activity
Each enzyme has a specific pH, i.e. a pH at which it works most effectively (faster
rate of reaction). Enzyme may work at other pH but the rate of the chemical reaction
that they control is usually not as fast.
Examples of enzyme optimal pH:
- catalase: pH 7
- pepsin: pH 2.8
Conditions (temperature and pH) at which enzymes work most effectively are called
optimum conditions.
The conditions at which the enzymes work is called the working range.
Specificity of enzymes:
Each enzyme only works on one substrate. E.g. Amylase only breaks down starch.
Enzymes and substrates have matching shapes like a “lock and key”.
Enzymes are proteins.