Section 1Introduction to Cells

Animal Cell

nucleus

cell membran

e

cytoplasm

Plant Cell

cytoplasm

green chloroplastcell wall

nucleus

vacuolecell

membrane

•The nucleus contains the genetic material of an organism.

•It controls all the cell’s chemical reactions.

•It also controls the growth and development of a cell, and so determines the cell’s structure and function.

Nucleus

nucleus

•Cells take in many chemicals from their surroundings, and release other chemicals into their surroundings.

•The cell membrane is a very thin boundary which controls the entry and exit of these materials.

Cell membrane

•There are many chemical reactions happening in all of your cells.

•These reactions keep the cell alive and allow it to carry out its specific function.

Cytoplasm

•The cell wall is a rigid structure made of a tough mesh of cellulose fibres.

•It helps to support a plant cell.

Cell wall

•The vacuole is filled with water and pushes out towards the cell wall.

•This provides support for the plant.

Vacuole

•Plant cells may also contain chloroplasts in the cytoplasm.

•These contain a chemical called chlorophyll which absorbs light energy for photosynthesis.

•This allows plant cells to make food.

•Only the green parts of a plant contain chloroplasts.

Chloroplasts

Structure Feature Function

NucleusLarge, usually round structure

containing genetic materialsControls all cell activities

Cell

Membrane

Very thin layer surrounds the

cytoplasm

Controls the passage of

substances into and out of the cell

Cytoplasm Fluid, jelly-like material Site of all bio-chemical reactions

Cell WallOuter layer made of basket-like

mesh of cellulose fibresProvides plant cells with support

VacuoleFluid-filled sac-like structure in the

cytoplasm

Stores water and minerals and

provides extra support for plant

ChloroplastsDisk-like structure containing green

chlorophyll

Trap light energy for making food

by photosynthesis

Microscopes

• Cells are usually too small to be seen with the naked eye

• Microscopes are used to magnify them• Stains (eg. methylene blue or iodine) can be

applied to highlight certain cell structures• Your teacher will demonstrate how to prepare

slides of onion cells and cheek cells, and you will then prepare your own slides and view them under the microscope.

• You should make labelled drawings of what you see. Include the magnification used.

Examining Onion CellsAim:To observe and draw onion cells using a microscope.

Equipment:

• Glass slide

• Cover slip

• Onion skin

• Iodine stain

• Microscope

• Lamp

• Collect a thin piece of onion skin.• Spread the skin on a slide. The skin must not overlap.• Stain the cells by adding 2 drops of iodine stain.• Place a cover slip over the skin. Use a pencil to lower the

cover slip gently so the air is pushed out.• Examine the cells under low then medium power. You should

be able to see lots of cells arranged like bricks in a wall.• Adjust the microscope to a higher power.• Draw exactly what you see through the “field of view” using a

pencil.• Label as many structures as you can see.• Return the slide and pack your microscope away carefully.

Method:

Onion cells in iodine

cytoplasm

cell wall

nucleus

Examining Cheek CellsAim:To make a slide of cheek cells and draw them.

Equipment:

• Glass slide

• Cover slip

• Cotton bud

• Methylene blue stain

• Microscope and lamp

• Paper towel

• Rub the cotton bud over the inside of your cheek to remove some of the cells.

• Wipe the cotton bud over the surface of a glass slide.

• Place the cotton bud in disinfectant.

• Stain the cells with 1 drop of methylene blue stain.

• Remove some of the stain using paper towel.

• Use a pencil to lower the cover slip so the air is pushed out.

• Draw the cells and label the structures.

• Once you have finished, place the slide and cover slip in disinfectant.

• Pack away your microscope carefully.

Method:

Cheek cells in methylene blue

cytoplasm

cell membrane

nucleus

Plant Cells

•Some plant cells have chloroplasts.

•These disc-like structures contain a

green pigment called chlorophyll that

traps light so that the plant can make its

own food by a process called

photosynthesis

chloroplasts

cell wall

Comparison of Cell Types

Structure Plant Cell Animal Cell

cell wall

cell membrane

nucleus

cytoplasm

chloroplasts

vacuole

Comparison of Cell Types

Structure Plant Cell Animal Cell

cell wall Yes No

cell membrane

Yes Yes

nucleus Yes Yes

cytoplasm Yes Yes

chloroplasts Yes No

vacuole Yes No

Textbook questions

Answer q. 1-4 on page 4 in sentences

Multicellular Organisms• Organisms are usually made up of

millions of cells that work together e.g. oak tree or human

• These are called multicellular organisms

Unicellular Organisms

• But there are also organisms that are made up of just one single cell

• These are called unicellular organisms and are very small

e.g. Amoeba

Different types of cells

Microbes: a word used to describe

a microscopic unicellular organism

such as bacteria and fungi.

Microbes • Some microbes are harmful and can

cause disease.

• Others can be useful e.g. helping to make useful products in biotechnology industries.

• An example of a useful fungus is penicillium, which produces the chemical penicillin, an antibiotic.

Cells & Biotechnology• Yeast cells are important to biotechnology because under

the right conditions they can convert sugars into alcohol

and carbon dioxide – this process is called fermentation

yeast

sugar carbon dioxide and alcohol

Yeast• Is a unicellular fungus.• It cannot photosynthesise,

it has no chloroplasts.• It needs a food source e.g.

sugar.• It can respire anaerobically ( in the absence of oxygen).• It is widely used in the

brewing and baking industries.

• It reproduces by ‘budding’.• Yeast cells can reproduce

rapidly if they have a source of food and a suitable temperature.

30 mins 1 hour

1 hour 30 mins

2 hours

2 hours 30 mins

Yeast dividing calculation

• One yeast cell is placed in a sugar solution. It divides to form 2 cells in 30 minutes. How many yeast cells will there be after 12 hours?

• How to work it out• 12 hours = 24 divisions• Number doubles each division

1. 22. 43. 84. 165. 326. 647. 1288. 2569. 51210. 102411. 204812. 409613. 819214. 1638415. 3276816. 6553617. 13107218. 26214419. 52428820. 104857621. 209715222. 419430423. 838860824. 16777216

Answer = 16,777,216 yeast cells

Looking at microbes

• Yeast is a very useful microbe, it is a fungi.

• Looking at yeast…Collect the following:• Microscope, slide, 1 drop of

yeast, cover slip.• Prepare a yeast cell slide. View the yeast cells at low

and high power. Look for cells that are budding.

Looking at yeast cells

Applications of Fermentation by Yeast

Brewing Industries

Alternative Fuel Industries

e.g. Gasohol (alcohol mixed

with petrol)

Bread-making Industries

Yeast and alcohol

• Yeast can make alcohol when it has a source of sugar.

• This is called alcoholic fermentation and is used in the brewing industries to make wine and beer.

Yeast and alternative fuels

• Yeast can be added to sugar to make alcohol.

• Alcohol is flammable and can be used as fuel. However, it must first be mixed with petrol.

• This forms an alternative fuel called gasohol.

alcohol (made by yeast) + petrol = gasohol

Yeast and breadmaking• Yeast is added to flour,

water and a little sugar (to feed the yeast!).

• The dough is then left for about and hour in a warm place. During this time the yeast produce carbon dioxide and a little alcohol.

• The carbon dioxide gas causes the dough to rise.

• It is then put in an oven to bake. This kills the yeast and evaporates off the alcohol.

Yeast and breadmaking

1. Label two beakers A and B.2. Add 3 spoons of flour and half a spoon of

sugar to each beaker.3. Add yeast suspension to beaker A and mix

with a stirring rod to make a dough.4. Add water to beaker B and stir to form a

dough.5. Transfer the doughs to two measuring

cylinders and transfer the labels A and B onto them.

6. Leave in a warm place for 30 minutes.7. Look at the height of the dough in each

cylinder.

Yeast and breadmaking

ResultsComplete the results table.

Conclusion: What effect does yeast have in

breadmaking?

Antibiotic Production using other fungi

Alexander Fleming

Video clip

Antibiotic Production

• Antibiotics are antibacterial chemicals produced by microbes such as fungi.

• They prevent the growth and may cause the death of other microbes.

• Antibiotics do not work against viruses so cannot be used to treat the cold or the flu.

• Many bacteria are now resistant to antibiotics.Video clip

Penicillium colony

bacterial colony

bacteria cannot grow

near the Penicillium

Antibiotic multidisc

Clear zone around arm shows that the bacteria is killedby the antibiotic. This would be a goodantibiotic to give thepatient.

• A disc with several antibiotics on the ‘arms’ can be used to find out which is the most effective antibiotic to treat an illness.

• This is used in labs where swabs from patients are sent for checking.

Resistant bacteria

In the above example, the bacteria S.Albus is not killed by the antibiotic V.We say that the bacteria is resistant to the antibiotic.

In the above example, the bacteria M.Luteus is killed by antibiotic V. We say that the bacteria is sensitive to the antibiotic.

Resistant bacteria

• If a bacteria is not killed by an antibiotic we say that the bacteria is resistant to the antibiotic.

• If a bacteria is killed by an antibiotic we say that the bacteria is sensitive to the antibiotic.

• An antibiotic multidisc can show which antibiotic is best to treat each bacteria.

The use of bacteria

• Bacteria can be used to produce -Yoghurt -Biogas (another alternative fuel)

Yoghurt Making

• During the souring of milk, bacteria growing in the milk will feed on the

milk sugar (lactose) and break it down to lactic acid. This process is

called lactic acid fermentation

Lactic acid makes milk curdle. The manufacture of

yoghurt depends on the curdling of milk

Lactose

Lactic Acidbacteria

Investigating Microbes…

• True or False…True or False…1. Microbes are all harmful and can cause disease2. Fungi can be used in yoghurt making3. Bacteria can be killed by antibiotics4. Yeast make their own food through the process of

photosynthesis5. Some bacteria are resistant to antibiotics6. Fungi carry out fermentation 7. Fermentation releases oxygen8. Yeast use glucose as an energy source for respiration9. Gasohol is petrol mixed with alcohol10. Antibiotics are produced by fungi

Diffusion

•Diffusion is the movement of

molecules in a liquid or gas from

high to low concentration until they

are evenly spread out

Diffusion clip – BBC learning zone

Cells and diffusion

• The entry and exit of substances in and out of cells is called diffusion.

• This happens across the cell membrane.

• Animal cells take in glucose and oxygen by diffusion.

• Carbon dioxide and waste materials leave animal cells by diffusion.

Cells & Osmosis

• Water also enters and leaves cells by a similar process called osmosis.

• Osmosis is the special diffusion of water from high water concentration to low water concentration through a selectively permeable membrane.

Selectively Permeable Membranes

• Pores in the membrane are small, only small molecules such as glucose, oxygen and carbon dioxide can get through.

• Large molecules such as starch cannot pass through.

• Selectively permeable membranes allow certain molecules to pass through but not others.

nucleus

selectively permeable membrane

cytoplasm

Selectively Permeable Membranes

• Cell membranes are described as selectively permeable.

• This means that they allow small molecules like oxygen and water to pass through them freely.

• This is because the membrane has tiny holes in it called pores that make it permeable.

• Large molecules like starch are unable to pass through.

Visking tubing

• Visking tubing is a selectively permeable material that can be used to show the effect of osmosis on cells.

• The visking tubing behaves like a cell membrane, and we can use it to make model cells.

• Your teacher will show you how to use it.

Osmosis experiment

A B

Visking tubing bag

10% sugar solution

water

Boiling tube

Results

A B

Mass of bag and contents at start (g)

Mass of bag and contents after 20 minutes (g)

Difference in mass (g)

Conclusion

• Bag A increased/decreased in mass. This was because water moved in/out by osmosis.

• Bag B increased/decreased in mass. This was because water moved in/out by osmosis.

• Water always moves from ________ water concentration to ______ water concentration.

Answer the following questions in sentences

1. Why was the visking tubing bag dried in a paper towel before being weighed?

2. Why was visking tubing used in this experiment? What property does it have that makes it a good model cell?

3. What would happen to an onion cell placed in pure water?

4. What would happen to a cheek cell placed in 10% sucrose solution?

Concentration Gradient

high ground

gradient (slope)

low ground

ball ball rolls down gradient

ball stops

Concentration Gradient

• Like the ball on the slope, water always moves down a gradient from high to low

• Solutions are made up of a solute dissolved in a solvent e.g. sugar dissolved in water

• Concentrations of the mass of solute are written in percentages e.g. 1 % sugar solution contain 1% sugar and 99% water.

Concentration Gradient

The difference in concentration of two solutions is called a concentration

gradient

water molecul

e

sugar molecule

Concentration Gradient

• Water moves down a concentration

gradient by osmosis from high to

lower water concentration. The

water will stop moving when the two

concentrations are equal.

Osmotic Effect On Cells

Water concentrations • If we think about solutions in terms of

their water concentrations, it is easier to recognise which direction water molecules will flow in.

• A dilute sugar solution will have a high concentration of water, whereas a concentrated sugar solution will have a lower water concentration

Dilute Sugar Solution

Concentrated Sugar

Solution

Low water concentration

High sugar concentration

High water concentration

Low sugar concentration

Solutions in the body fall into one of three categories:

• Hypotonic – Where the solution has a higher water concentration than the cell.

• Isotonic – Where the solution and the cell have an equal water concentration.

• Hypertonic – Where the solution has a lower water concentration than the cell.

Water Concentrations

• In a hypotonic solution, the water will move from the solution into the cell.

• In an isotonic solution the water concentration will stay the same.

• In a hypertonic solution the water will move from the cell into solution.

Water Concentrations

Hypotonic Solution

H2O concentration

>

A hypotonic solution has a higher water concentration than the water concentration within the cell, so water enters by osmosis.

cell

Direction of water

movement

Water Concentrations

Hypertonic Solution

cellH2O

concentration <

A hypertonic solution has a lower water concentration than the water concentration within the cell, so water leaves the cell by

osmosis.

Water Concentrations

cellH2O concentration =

Isotonic Solution

An isotonic solution has a water concentration that is equal to the water concentration within the cell, so there is

no gain or loss of water by osmosis.

Osmosis in potato tissue

Plant Cells and OsmosisHypotonic solution: more water outside of the cell than inside, therefore water will move into the cell by osmosis. This causes the cell to swell and become turgid

Hypertonic solution: more water inside the cell than outside, therefore water will move out of the cell by osmosis. This causes the cell to become softer or flaccid

Animal Cells & Osmosis

• The effects of osmosis on animals cells are totally different to plant cells because animals cell structures are different

• Animals cells do not have:Cell wallsVacuoles

Animal Cells & Osmosis

• Red blood cells (RBCs) float in a solution called plasma which is isotonic

• RBCs in isotonic plasma do not change size because the water has no concentration gradient to follow

• RBCs in hypotonic and hypertonic plasma will change because there is a concentration gradient for water to follow

Normal RBC

RBC in hypertonic

solution

RBC in isotonic solution

RBC in hypotonic

solution

Cell loses water and

shrinks

No net osmosis, cell stays the same

Cell takes in water,

swells and eventually

bursts