Topic 2.1 Cell Theory

Assessment Statements

2.1.1 Outline the cell theory.

2.1.2 Discuss the evidence for the cell theory.

Watch this:

Try this:Cell Theory Jigsaw activity

1. Split into two groups A and B.2. Each group has a different version of a text with

some key words missing.3. Read through your text and write a list of

questions you could ask to identify the missing words.

4. Pair up with a student in the other group and take it in turns to read out the text and ask questions.

2.1.1 The Cell Theory

All living things are made of cells

Cells are the smallest unit of life

All cells come from preexisting cells

Schleiden and Schwann (1838)

2.1.2 Evidence for Cell Theory

All living things are made of cells…

When we look at living things under a microscope they appear to be made of cells

All living things are made of cellsWe can see them when we look down the

microscope:. Can you identify these cells?

Exceptions Muscle cells: have more than one nucleus per cell.

Surrounded by a single cell membrane but are multinucleated (many nuclei).

This does not conform to the standard view of a small single nuclei within a cell

What’s wrong with this?

Fungal Hyphae: very large with many nuclei and a continuous cytoplasm

A tubular system of hyphae form dense networks called mycelium.

MultinucleatedHave cell walls made of

chitinCytoplasm is continuous

along the hyphae with no end cell wall or membrane

What is it?

Amoeba: a single cell capable of all life processes.

In more complex organisms cells are specialised - i.e. one cell per function.

Amoeba are much larger than other cells and some biologist consider them 'acellular' (non-cellular).

The cell is the basic unit of lifeThere are many single celled organisms:

The cell is the smallest unit of organisation that can show all the characteristics of living things.

Organelles often require the cooperation of other organelles for their successful function.

Read this:http://learn.genetics.utah.edu/content/begin/cells/organelles/

to learn about how cells evolved.

All cells come from preexisting cells

We can see this in these processes. Can you name them?

Evidence

Eukaryotic (plant, animal and fungal cells) undergo mitosis

Prokaryotes (bacteria) reproduce by binary fission.

Louis Pasteur carried out a famous experiment to prove that spontaneous generation did not occur.

But when Pasteur broke the swan neck off the bottle microorganisms soon grew in the broth.

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Check this out:

2.1.3 State that unicellular organisms carry out all the functions of life.

ReproductionUsually a form of asexual reproduction called binary fission which produces a clone.

Yeast cells reproduce by budding.

NutritionThe synthesis or absorption of organic matter.

Often in the form of phagocytosis, e.g. this amoeba.

Many bacteria are parasitic whilst others can photosynthesise.

Metabolism includes the process of respiration which produces energy in the form of ATP.In yeast anaerobic respiration produces alcohol as a waste product!

Response to a change in the environment.

This can be seen when bacteria such as this E. coli demonstrate chemotaxis (respond to chemicals in their environment).

Homeostasis is the maintenance of internal cell conditions. Single celled organisms can withstand large changes in environmental conditions as a result of their ability to carry out all of the life processes.

Growth in unicellular organisms is an increase in cell size and volume.

Assessment Statement

2.1.4 Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using the appropriate SI unit.

Relative Sizes of Cells

Assessment Statement:

2.1.5 Calculate the linear magnification of drawings and the actual size of specimens in images of known magnification.

MagnificationThe number of times bigger than the actual size.Biological diagrams and photographs usually

indicate magnification.

As a scale bar

As a number (x500)

Calculating Magnification

magnification = image size actual size

Rules for magnification calculations

1. Convert all units to make them the same (where appropriate).

You will often be working with mm, um and nm.1cm = 10mm = 10,000um1mm = 1000um1um = 1000nm

To convert from mm to um x 1000To convert from um to mm /1000

2. Rearrange the magnification equation as needed to find the value you want.

M = I/A A = I/M I = A x M

3. Convert your answer into appropriate SI units using scientific notation where necessary.

e.g. 0.03mm = 30um = 3 x 10-2um

x 1000

Example calculationMeasure the lengthof the image in mm using a ruler.

Magnification is x600.

Plug the data into theequation: A= I/M

Give answer in SI units.

Paramecium caudatum

Magnification using scale barsScale bars givean indication of the true size of anobject viewed under a microscope.

We need anotherformula to calculatethis:magnification = measured length scale bar label

Assessment Statement

2.1.6 Explain the importance of the surface area to volume ratio as a factor limiting cell size.

Giant Squid and Colossal Squid have nerve cells as long as 12 m . In humans, the longest nerve cells are about 1.5m running from the base of the spine to the toes

The smallest human cell is the sperm cell at 100 to 200um

The smallest cell belongs to a genus of bacteria called Mycoplasma with a size of 0.3 to 0.5 um.

Limit to Cell Size

Factors affected by of cell volume:- rate of heat production- waste production- resource consumption

Factors are affected by surface area:- exchange of materials- exchange of heat

Assessment Statement:

2.1.7 State that multicellular organisms show emergent properties.

Emergent PropertiesThe whole is greater than the sum of its parts.

An analogy - a light bulb.Made of a glass sphere, a tungsten filament and a metal screw cap.Looking at the properties of their individual parts, you couldn’t predict the properties of the light bulb.

Examples•New properties that emerge with each step upward

in the hierarchy of life, owing to the arrangement and interaction of parts as complexity increases.

•Na is a metal, Cl is a poisonous gas – together, they are edible

•Do macromolecules behave like a composite of monomers?

•If you put chlorophyll and all molecules from plant cell into test tube, could they perform photosynthesis?

•If you put functioning nerve cells together, will there be thought?

•Cycling of nutrients in an ecosystem involves complex interaction of all members.

Assessment Statements:

2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their genes but not others.

2.1.9 State that stem cells retain the capacity to divide and have the ability to differentiate along different pathways.

2.1.10 Outline one therapeutic use of stem cells.

Differentiation and Specialisation

Stem CellsUp to the eight-cell

stage, all cells in an embryo are identical.

They are called embryonic stem cells.

Embryonic stem cells have the potential to develop into any other specialised type of cell .

All cells contain the same set of instructions in the form of DNA.

Cells become specialised because the genes that are not required are switched off.

Only the genes needed to make a particular type of cell work are switched on.

Watch these:

Stem Cell DebateIn your groups you will be given a character

profile.

Read the information and decide if your character is for or against the use of stem cells.

Produce a 3 minute speech by your character in response to the task outlined on your brief.

What you need to be able to do:2.1.1 Outline the cell theory.2.1.2 Discuss the evidence for the cell theory.2.1.3 State that unicellular organisms carry out all the functions of life.2.1.4 Compare the relative sizes of molecules, cell membrane thickness,

viruses, bacteria, organelles and cells, using the appropriate SI unit.2.1.5 Calculate the linear magnification of drawings and the actual size of

specimens in images of known magnification.2.1.6 Explain the importance of the surface area to volume ratio as a factor

limiting cell size.2.1.7 State that multicellular organisms show emergent properties.2.1.8 Explain that cells in multicellular organisms differentiate to carry out

specialized functions by expressing some of their genes but not others.2.1.9 State that stem cells retain the capacity to divide and have the ability to

differentiate along different pathways.2.1.10 Outline one therapeutic use of stem cells.