IB Biology - Curriculum Notes 

​​​​​​​​​​​​​​​​​​​​​​​​​​​​1.1 Introduction to Cells  

∑=  IB Understandings

Cell Theory

The main part of cell theory consists of three main aspects:

1) Cells are the basic unit of structure in all living things (smallest unit of life)
2) All living organisms are composed of cells.
3) New cells are formed from pre-existing cells.

The two scientists given credit to the first two parts of cell theory are Theodor Schwann and Matthias Schlieden (1839).
Rudolf Virchow came up with the 3rd aspect that all cells come from pre-existing cells.

4) More recently, scientists have added another aspect of the cell theory that states “All cells contain hereditary information (DNA) which is passed on from cell to cell during cell division”

∑ According to the cell theory, living organisms are composed of cells.

Over the years many living organisms, both unicellular and multicellular, have been studied under microscopes and all have been found to be composed of cells.

Cells vary extensively in size and shape but contain certain commonalities such as a cell membrane, genetic material, chemical reactions catalysed by enzymes and the production of energy (ATP) through respiration.

Applications and skills: ß

ß Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae, and aseptate fungal hyphae.


Exceptions to General Cell Structure

1) Fungal Cells can have multiple nuclei (multi-nucleated).

Fungi have cell walls made out of chitin surrounding threadlike structures called hyphae
Aseptate hyphae are one long continuous cell that is not separated by dividers called septa and therefore have many nuclei

2) Striated muscle cells are made up of contractile filaments that slide past each other. They have a single surrounding membrane but can contain possibly 100’s of nuclei.

3) Giant algae – Some uncommon algae exist that can actually grow up to approximately 1 cm. An example of this is Acetabularia, which is a genus of green algae. One would expect a cell of this size would consist of many cells, as it would have difficulty getting rid of metabolic waste.

Extra example

Red Blood Cells are biconcave disks that carry oxygen to different tissues.
RBC lack a cell nucleus, cellular organelles and cannot synthesize protein


ß - Skill: Use of a light microscope to investigate the structure of cells and tissues, with drawing of cells. Calculation of the magnification of drawings and the actual size of structures and ultrastructures shown in drawings or micrographs. (Practical 1)

**This lab skill could be completed at the same time as the investigation of the paramecium and euglena.**

Using the formula 

Magnification = size of the image/actual size of the specimen

The size of the image is how large a specimen appears in a photograph or a drawing.
The actual size of the specimen simply means how big the specimen actually is.

Calculate Magnification:

- Measure the actual specimen or a clear part of the specimen under a microscope using a clear ruler next to the specimen, so you can see the ruler under the microscope next to the specimen. This gives the actual size of the specimen value.

- In test questions, the actual specimen size can be given to you in order to calculate the magnification.

- Then measure the same specimen or part of the specimen represented in the drawing or photograph to get the value for the size of the image.

- Make sure to convert the values to the same unit of measurement. For example, if one value is mm and one value is µm, convert both to either µm or mm.
Use the formula above to calculate magnification.

Scale bars are also used on many micrographs (photographs under a microscope), using a line to represent the actual scale or size of the image. 

If a value is given for a scale bar, use that value for theactualand then measure the scale bar on the paper and use that value as the size of the image, to calculate the magnification.

The scale bar to the right shows that half of a red blood cell is about 3 µm. On my computer, the size of the scale bar is 15mm or 15000 µm. Therefore the magnification is 15000/3 = 5000x

The actual image size is therefore based on the scale bar. You can also then measure the size of the image with a ruler, convert the result into µm’s and calculate the actual size of the image based on the magnification.

Note: The size of objects in digital images of microscope fields could be analyzed using graticule baselines and image-processing software.

Work in Class
***Complete introduction to microscope activity on the use of the microscope, drawing cells and cell structure and calculating magnification***

Complete data-based questions on pages 6 and 7 of your textbook.

∑ Organisms consisting of only one cell carry out all functions of life in that cell.

Unicellular organisms carry out all the functions necessary for life including the following:

a) Metabolism – the chemical reactions that occur in organisms in order for them to maintain life, such as the synthesis of ATP during cellular respiration.

b) Response – organisms respond to their environment.

c) Homeostasis – maintaining a stable internal environment within the cell.

d) Growth – increase in size (volume and surface area) until the cell is too large to function efficiently.

e) Reproduction – the majority of prokaryotes reproduce through binary fission while single-cell eukaryotes reproduce generally asexually; however, some can also reproduce sexually through meiosis and then mitosis.

f) Nutrition – creating or synthesizing their own organic molecules or consuming organic molecules.

Unicellular organisms include Prokaryotes (bacteria) which lack a nucleus and membrane-bound organelles and most Protists which are Eukaryotes.  

∑ Surface area to volume ratio is important in the limitation of cell size. 

- Cells need to exchange substances with their surroundings, such as food, waste, heat, and gases.
- In the cytoplasm, chemical reactions take place which is known as metabolic reactions.

- These reactions produce heat, wastes, and also consume resources. The rate of these reactions is proportional to the volume of the cell, while the exchange of these materials and heat energy is a function of the cell’s surface area.
- As the size of an object or a cell increases, its volume increases faster in comparison to the surface area of that object because the volume is x³(cubed), while the surface area of an object or cell is only x² (squared).
- This means as a cell increases in size, its surface area to volume ratio (SA/V) will decrease.
You can clearly see this by looking at cubes of varying sizes.

Surface Area to Volume

Side Length
SA (cm²)
Volume (cm³)
SA/Volume Ratio

- As the SA to volume ratio decreases, the rate or the cell’s ability to exchange materials through diffusion or radiation also decreases.
- If metabolism is to continue at an optimum rate, substances such as oxygen must be absorbed and waste products such as carbon dioxide need to be removed.
- Also if too much heat is produced during metabolism in comparison to the amount the cell is able to remove, the cell might overheat.

- Therefore, the greater the SA/volume ratio is, the faster the cell can remove waste and heat, and absorb oxygen and nutrients essential for the cell to function properly.

ß - Application: Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.

(Paramecium and Euglena can be observed with a light microscope)

1) Have a macronucleus and one or more micronuclei.

  • The macronucleus expresses the genes needed to carry out cell activities. The micronuclei contain the genetic material that is passed on asexually to the next generation through binary fission.

2) Have cilia used for movement
3) Contractile vacuole responsible for expelling water and waste
4) Metabolic reactions catalysed by enzymes take place in the cytoplasm
5) Consume food through an oral groove into the mouth opening ending up in vacuoles. These vacuoles digest the food using enzymes, passing on the nutrients back into the cytoplasm to be used for energy.
6)The outside is composed of a stiff but elastic membrane called a pellicle, which controls what enters and exits the cell.

7) Excretion of waste through diffusion.   

Showing paramecium movement, feeding and other life processes


  • Chlorella is a type of single-celled green algae 
  • It lives in a freshwater habitat and belongs to the plant division called Chlorophyta 
  • ​The cells are small, roughly 5-10 µm in diameter 
  • They contain chloroplasts that carry out photosynthesis using the photosynthetic pigments chlorophyll a and b
  • Chlorella can reproduce quite quickly and it's possible to use as a food source as it contains a high level of protein

Quick Fact: Chlorella is starting to be promoted and used as a superfood!!


∑ Multicellular organisms have properties that emerge from the interaction of their cellular components. 

  • Emergent properties arise from the interaction of component parts, i.e. the whole is greater than the sum of its parts.
  • Emergence in science and system theories is defined as how complex systems and patterns arise out of a multiplicity of relatively simple interactions. Basically, complex life systems involve millions of small simple interactions that work together to allow the complex system to function properly.
  • For example, in the human body, cells make up tissues, tissues make up organs, organs make up organ systems and these organ systems allow humans to live, breathe and function as a whole.
  • Also, ​the cohesive properties of water are an emergent property that allows water to be “sticky” or a natural attraction to itself. Water, therefore, can easily move up trees from the roots to the cells involved in photosynthesis in the leaves. The component parts would be the interactions between the water molecules; more specifically the hydrogen bonding between oxygen and hydrogen molecules.

Interesting Video on Emerging Properties https://www.youtube.com/watch?v=16W7c0mb-rE

∑ Specialized tissues can develop by cell differentiation in multicellular organisms. 

  • Differentiation is basically the process where a less specialized cell becomes a more specialized cell.
  • Large multi-cellular organisms need to have specialized cells in order to live and function efficiently 
  • Differentiation changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals.
  • In development after the zygote divides to form the blastocyst ( around 120-130 cells), and then the gastrula, which is differentiated into several dermal layers of cells (mesoderm, endoderm, ectoderm, and germ cells) that form into specific specialized cells.

∑ Differentiation involves the expression of some genes and not others in a cell’s genome. 

  • Differentiation in cells is controlled by gene expression, which means some genes are turned on or turned off in specific cells that relate to their function.
  • For example, a nerve cell will contain all genetic information that other cells have; however, the genes for the nerve cell are turned on or expressed while the genes for other cells, for example, a pancreatic cell, are shut off.
  • When a gene is turned on, it will produce specific proteins or products that allow that cell to function properly.
  • Some cells when they differentiate lose their ability to reproduce such as nerve cells and muscle cells, while some cells retain this ability throughout their lives, like skin epithelial cells.

∑ The capacity of stem cells to divide and differentiate along different pathways is necessary in embryonic development and also makes stem cells suitable for therapeutic uses.

  • Stem cells are characterized by the ability to divide through mitotic cell division and differentiate along different pathways to become a diverse range of specialized cell types.
  • At early embryonic stages, the stem cells can still divide have the ability to become any type of cell, until they express certain genes and differentiate into a specific type of cell.
  • Two main types of stem cells are adult stem cells which are found in adult tissues such as the bone marrow and embryonic stem cells that are found in the inner cell mass of blastocysts.
  • Another source of stem cells is from the umbilical cord of newly born fetuses (cord blood stem cells)

Can we prolong life using stem cells? Interesting video on why we age. The part on stem cells start at 4:40 https://www.youtube.com/watch?v=MjdpR-TY6QU

Bone Marrow Transplants

  • One of the greatest therapeutic successes for the use of stem cells has been for the treatment of leukemia or lymphomas through bone marrow transplants.
  • This involves using hematopoietic stem (HS) cells (blood stem cells) derived from bone marrow tissue.
  • These cells will divide continually to form new red and white blood cells.
  • Stem cells are removed from the bone marrow of the patient or from a donor person, such as a brother or a sister.
  • The patient undergoes chemotherapy and radiation therapy to kill the cancer cells in the bone marrow; however, normal dividing cells in the blood will also be killed.
  • After chemotherapy and radiation therapy the HS cells will be transplanted directly into the bloodstream through a tube called a central venous catheter.
  • The stem cells find their way into the bone marrow, where they will begin reproducing and making healthy new blood cells.
  • Embryonic stem cells can also be used to regenerate skin tissue for people that have been badly burned or for healing diseases such as type 1 diabetes by replacing the damaged insulin-producing beta cells.

Non-therapeutic uses include creating meat (muscle fibres) for human consumption that has been grown in a lab. 

ß - Application: Use of stem cells to treat Stargardt’s disease and one other named condition.

Stargardt’s Macular Dystrophy –  Is a genetic disease that develops in children that can cause blindness

  • The disease affects a membrane protein in the retina causing the photoreceptor cells in the retina to become degenerative
  • The treatment involves injecting embryonic stem cells that can develop into retina cells into the back of the eyeball
  • The cells attach to the retina and begin to grow, improving an individual’s vision, with limited side effects
  • More human trials are needed


Leukemia (same from above) – Is caused by a mutation in the genes that control cell division, which will create an abnormal amount of white blood cells.

  • These white blood cells are produced in the bone marrow
  • One of the greatest therapeutic successes for the use of stem cells has been for the treatment of leukemia or lymphomas through bone marrow transplants.
  • This involves using hematopoietic stem (HS) cells (blood stem cells) derived from bone marrow tissue.
  • These cells will divide continually to form new red and white blood cells.
  • Using a large needle, stem cells are removed from the bone marrow of the patient or from a donor person, such as a brother or a sister.
  • The patient undergoes chemotherapy and radiation therapy to kill the cancer cells in the bone marrow. However, normal dividing cells in the blood will also be killed.
  • After chemotherapy and radiation therapy the HS cells will be transplanted directly into the bloodstream through a tube called a central venous catheter.
  • The stem cells find their way into the bone marrow, where they will begin reproducing and making healthy new blood cells.

ß - Application: Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult’s own tissues.

The sources of stem cells are as follows:

Embryonic stem cells – fertilize the egg with sperm, fusion forms a zygote, the cell will now divide by mitosis till it is about 12-16 cells. These are all embryonic stem cells.

  • They can differentiate into any cell type but have a higher risk of becoming tumour cells.
  • There is also less chance that the cells have genetic damage as they are very new and don’t have time to accumulate mutations like adult stem cells.

Documentary on understanding embryonic stem cells (long)

Umbilical Cord Stem Cells – stem cells obtained from the cord, can be frozen and used later on in life. These are easily obtained and stored after birth.

These cord blood cells need to be used for others because if you get or get cancer, you would not use your own cells to cure you as your cells turned cancerous. 

You would almost always get the cord blood cells from the public cord blood bank. 
(Good video on cord stem cells)

Adult Stem Cells – obtained from some adult tissue such as bone marrow. They are difficult to obtain and have less growth potential and limited capacity to differentiate when compared to embryonic stem cells; however, they are fully compatible with adult's tissue (no rejection) and there is less chance for a malignant tumour to occur.
(Adult Stem Cells cure blindness video)

Ethical Concerns

  • The therapeutic use of stem cells involves the creation and the death of an embryo that has not yet differentiated in order to supply embryonic stem cell lines for stem cell research and stem cell therapies.
  • The biggest ethical concern involves the creation of a new human embryo. Is it ethically acceptable to create a human embryo for biomedical research even if the research and therapies developed from the research could save human lives? Different people have views of when human life begins.
  • Also once a blastula is created, some people believe this might lead to reproductive cloning (the cloning of an entire human). This means that the embryo would have to be implanted into a surrogate mother.
  • Therapeutic cloning uses human eggs, which can only be obtained from a woman. The most common source of these eggs are eggs that are produced in excess of the clinical need during IVF treatment. This also might create the problem of a woman trying to sell their eggs for stem cell research. A human egg market might develop in the poorer countries in the world.
  • Possible health risks involved in treating women with hormones to induce hyperovulation to provide eggs for research.
  • Even though scientists argue that the embryos that are created by IVF (in vitro fertilization) wouldn’t have existed anyways unless they create them and no human that would have otherwise lived is denied a chance of living, others argue it is unethical to create human life for the sole purpose of harvesting stem cells.
  • The arguments for stem cells is that it allows treatment for diseases that are currently incurable and reduces suffering in many sick and disabled people

​Video on Ethics of Stem Cells https://www.youtube.com/watch?v=dIy-zCaFaWw

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