IB Categories Archives: Topic 1: Cell biology

1.5 – Cell Division

1.5 – Cell Division

1.5.1 – Outline the stages in the cell cycle, including interphase (G₁, S, G₂), mitosis and cytokinesis
Interphase is when the DNA replicates. The cell will also replicate its
centrosome, which is important for movement of chromosomes. This is split into three stages:

G₁ is the first stage, and stands for
Gap 1. During this time, the
cytoplasm is still active, and the cell
continues with its normal
functions, such as protein synthesis, mitochondria replication or chloroplast
replication. There is all the activity of a growing cell.
S is the synthesis phase when the DNA is replicated. The mass of the DNA in the cell
doubles. All the chromosomes are copied and form chromatids. These remain
attached until they divide in mitosis.
G₂ is the third stage, standing for Gap 2, when there is more growth of the cell, then
preparation takes place for cell division.


Mitosis then happens. This consists of four stages: prophase, metaphase, anaphase and telophase. The chromosomes are separated and distributed.
Cytokinesis is the division of the cytoplasm to form two daughter cells. The cell cycle is then repeated.


1.5.2 – State that tumors (cancers) are the result of uncontrolled cell division and that these can occur in any organic tissue

Tumors, or cancers, are cell mass formed as a result of uncontrolled cell division. They as a result of uncontrolled cell division. They can occur in any tissue.

In a tumor, the normal repressed state of mitosis is disrupted by mutation to the proto-oncogene. As a result, the cells begin to divide uncontrollably. The proto-oncogene mutates into the oncogene, resulting in the loss of control of cell division.

The cells form an irregular mass of cells; the tumour. Some cells may break away and form a secondary tumour elsewhere. Eventually they take over the surrounding, healthy cells, which leads to malfunction and death. It is caused by damage to DNA chromosomes. The accumulation of mistakes in DNA causes cancer, which is why it is more common in older people. Another cause is damage to the gene that codes for p53, the protein which stops the copying of damaged DNA.
The damage to the DNA can result from ionising radiation (X-rays, gamma rays…), some chemicals (tar in tobacco smoke) as well as virus infections. Some factors are also inherited. The development of cancer requires at least two mutations; one of the proto-oncogene;
two of the tumour suppressor.
Cancer exerts its deleterious effect on the body by destroying the adjacent tissues (such as compressing nerves, eroding blood vessels), replacing normal functioning cells (such as replacing blood forming cells in the bone marrow or the heart muscles so that the heart

1.5.3 – State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts

This is always the longest part of the cell cycle. During interphase, the nucleus undergoes many changes. The chromosomes disperse as chromatin and become actively involved in protein synthesis. Copies of the information in particular genes or groups of genes are taken from the chromosomes for use in the cytoplasm. Proteins are assembled in the ribosomes by combining amino acids in sequences dictated by the information from the gene.

The synthesis of new organelles takes place in the cytoplasm during interphase. There is intense biochemical activity in the cytoplasm and the organelles, and there is an accumulation of stored energy before nuclear division occurs again. Also, each chromosome replicates into the two identical structures called chromatids. These remain attached until they divide in mitosis.

In this time, the cell itself will continue to carry out its specialized function. The length of interphase varies between cell types. After cytokinesis, G1 occurs: when various proteins are synthesised to allow the cell to specialise. Then, in the S stage, the DNA is replicated. G2 is
the preparation stage for mitosis. Mitochondria (and chloroplasts in plants) are replicated.
1.5.4 – Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase)

During mitosis, the chromatids are separated and distributed to two daughter nuclei. Mitosis is a continuous process with no breaks in it, although we divide it into four stages.

The chromosomes become visible as long, thin threads. They shorten and thicken through the process of supercoiling. In supercoiling, DNA is combined with histone proteins and non-histone proteins to form the readily stainable chromatin. The genes must be left in predictable positions and a distinctive overall chromosome shape. The supercoiling makes the structure so dense that it can be seen with a light microscope during mitosis.

At the end of prophase, it is possible to see that the two chromatids are held together at the centromere. During this time, the nucleolus disappears and the nuclear membrane breaks down.





The centrioles move to opposite ends of the cell. Microtubules in the cytoplasm start to form a spindle, radiating out from the centrioles. These attach to the centromeres and are arranged at the equator of the spindle. In plant cells, the same structure is formed, but without the presence of centrioles.




The centromeres divide, the spindle fibres shorten, and the chromatids are pulled by their centromere to opposite poles. Once separated, they are referred to as chromosomes.




The nuclear membrane reforms around both groups of chromosomes at opposite ends of the cell. They begin to decondense and become chromatin again. The nucleolus reforms. Interphase then follows the division of the cytoplasm.



1.5.5 – Explain how mitosis produces two genetically identical nuclei

Cell division produces genetically identical daughter cells. They have a set of chromosomes identical to each other and to the parent cell from which they were formed.

An exact copy of each chromosome is made by accurate replication during interphase. There is conservation of the chromosome number, so the chromosome numbers of the daughter cells are exactly the same as each other and the parent cell.

The chromatids remain attached by their centromeres during metaphase, when each becomes attached to a spindle fibre at the equator of the spindle.

Centromeres then divide during anaphase, and one copy of each chromosome moves to each pole of the spindle. The chromosomes then form two new nuclei when the cell splits into two, each with an exact copy of the original nucleus.

Hence, the two nuclei from each new cell are genetically identical.


1.5.6 – State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis

In the growth and development of an embryo, it is very important that all cells carry the same genetic information as the existing cells from which they are formed, and with which they share surrounding cells or tissues. Multicellular organisms increase their size through growth, which involves increasing the number of cells through mitosis. These cells can then differentiate and specialize their function. In the same way, when repair of damaged or worn out cells occurs, they are exact copies of the ones they replace. If this was not the case, different body part would begin working to conflicting blueprints, resulting in chaos.

This form of cell division is also the basis of asexual reproduction, in which the offspring produced are identical to the parent. In other words, they are clones. This is very common in nature.

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1.1 – Cell Theory

1.1 – Cell Theory

1.1.1 – Outline the cell theory

The cell theory is that cells are the basic unit structure and function of every living thing. This contains three main ideas:

 1. Cells are the building blocks of structure in living things

 2. They are the smallest unit of life

 3. Cells are formed from other, pre-existing cells by division

Two additional concepts have been added today:

 1. Cells store all the information they require for growth, development and behaviour

 2. Cells are the location for all the chemical reaction needed for life, metabolism.

1.1.2 – Discuss the evidence for the cell theory

Cells are the Building Blocks of Structure in Living Things

All living things observed through microscopes have been found to be made up of cells, proving that living things are made up of cells.

Cells are the Smallest Unit of Life

Viruses: These are non-cellular, crystalline structures. They can only reproduce at the expense of the cell’s metabolic machinery. They cannot obtain life from any other source.

Cell Substructures: The lifespan of organelles has been found to be extremely short when they attempt to function outside the cell. This was shown through biochemical investigations of these organelles.

All Cells are Formed from Pre-existing Cells

Pasteur’s observations: He proved that the apparent ‘spontaneous’ generation of microorganisms was in fact due to the presence of unnoticed cells.

Resistant Spore Phase: This is a stage in the life cycle of many organisms. A related discovery was that spores of many microorganisms are found everywhere, but will only develop in favourable conditions.

Behavior of Chromosomes: Observing chromosome during cell division (mitosis and meiosis) and during reproduction has shown that new cells contain information from old cells.

Function of Genes: Each cell contains the blueprint needed for growth, development and behaviour in DNA. Research has also established the nature and roles of genes in the day-today control of cells and in the process of heredity. Experimental evidence through genetic engineering of the effects on cells of the deliberate transfer of genes between organisms also supports this.

Cells are the site of the necessary chemicals needed for life: The discovery of enzymes and
their machinery, which are used in the chemical processes with a cell. This includes aerobic respiration and fermentation. The discovery of biochemical events with cells also proved this. Examples are the formation of proteins from amino acids

Cell Ultrastructure: the presence of discrete organelles and the biochemical events located in particular organelles shows that cells are designed to function independently. Scientists have shown that, because every cell contains organelles which are the locations of specific
chemical reactions, they are the place where all these necessary processes of life take place.

1.1.3 – State that unicellular organisms carry out all the functions of life
These organisms are capable of carrying out all the necessary processes needed in living things.

  • Metabolism – this includes the respiration and synthesis of ATP
  • Response – to any change in the environment
  • Homeostasis – the maintenance and regulation of internal cell conditions
  • Growth – when the cell increases in size and volume
  • Reproduction – unicellular organisms are asexual, therefore they reproduce through division to form a clone
  • Nutrition – this is either the synthesis of organic molecules or the absorption of organic matter


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


Relative Sizes

1 nm – Molecule

10 nm – Cell membrane

100 nm – Virus

1000 nm – Bacteria (1μm)

10000 nm – Organelles (10μm)

100000 nm – Cells (100μm)


1.1.5 – Calculate the linear magnification of drawings and the actual size of specimens in images of know magnification

1.1.6 – Explain the importance of the surface area to volume ratio as a factor limiting cell size
As the size of a structure increases, the surface area to volume ratio decreases. For example, using a cube:


As organisms get bigger, their volume and surface area both get bigger, but not by the same amount. As a result, larger organisms have a slower rate of exchange (diffusion/radiation) with their outside surroundings.
This is true for organelles, cells, tissues, organs and organisms. All organisms need to exchange substances such as food, waste, gases and heat with their surroundings. The rate of exchange of substances depends on the surface area of the organism which is in contact
with its surroundings.

For this reason, cells are very small so that they are able to exchange substances efficiently.


1.1.7 – State that multicellular organisms show emergent properties

Emergence is the unexpected occurrence of characteristics or properties in a complex system, which could not be expected based on the properties of the constituents. Multicellular organisms have properties that exceed the sum of the properties of its constituents. Therefore, only when these parts are combined that we can determine the properties, as this cannot be done by individually analysing the properties of the parts. The properties emerge as a result of the interaction of the parts.

For example, a mitochondrion makes excess energy in the form of ATP, but for no purpose. It needs other things to give this to.


1.1.8 – Explain that cells in multicellular organisms differentiate to carry out specialised functions by expressing some of their genes but not others

Multicellular organisms are large and need to have specialised parts to their structure so that all the necessary functions of life can be performed.

Differentiation – The cells can become specialised to perform their function. These cells switch on, or express, particular genes that correlate with these specific functions. The expression of these genes will influence the shapes, functions and adaptations with that cell.
For example, a muscle cell will only express muscle genes, but not nerve cell genes.

Specialization in multicellular organisms is more efficient for organisms competing for a specific resource. Movement of nutrients, water, etc, can happen faster and more effectively than passing between cells through diffusion.


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

Stem cells can divide, however they have not yet expressed any of their genes so that they might specialise in a particular function. They will express particular genes under the right conditions and differentiate into a particular type of cell. They can be obtained from a variety of places including blastocyte, or even the placenta. Children possess more stem cells than adults.

Stem cells used for research are usually human embryonic stem cells, which come from embryos only a few days old. These are more flexible, and can grow into any type of mature cell. The techniques are controversial, thus there are international principles regarding this


1.1.10 – Outline one therapeutic use of stem cells

Stem cells could be used to treat and perhaps cure many diseases. One of these possibilities is with cystic fibrosis. Patients would be treated with their own stem cells Cells are removed and genetically engineered, then planted back into the patient, which might lead to the healthy formation of cells in the airway of the lungs. This therapy would also eliminate the problem of tissue rejection in transplant surgery, as the cells are from the patient Stem cells also hold promise for the

Stem cells also hold promise for the testing of new drugs. This would lead to safer testing, but would be more difficult because the conditions must be carefully controlled. Other cells are already used in this way. However, current knowledge does not allow us to be able to precisely control these conditions to generate pure populations of cells for testing.

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1.2 – Prokaryotic Cells

1.2 – Prokaryotic Cells
Prokaryotes are usually unicellular organisms like bacteria. They do not have a nucleus, but have their DNA located in a nuclear area. They are smaller than eukaryotic cells.

1.2.1 – Draw and label a diagram of the ultrastructure of Escherichia coli (E. coli) as an
example of a prokaryote

1.2.2 – Annotate the diagram from 2.2.1 with the functions of each named structure 

Nucleoid – Storage of genetic material/ information, the site of DNA replication. It consists of a circular chromosome of about 4000 genes.

Ribosomes – The site of protein synthesis, the translation of RNA.

Flagellum – These bring about movement of the bacterium in external medium, and may play a role in sexual conjugation.

Pili – Enable adhesion to surfaces and other bacteria, as well as assisting in sexual conjugation

Cytoplasm – The region where metabolic reaction occur which are essential for life.

Mesosome – Permeable boundary that allows for entry and exit of nutrients and waste, and may play a role in DNA replication.

Cell/ Plasma Membrane – This is a barrier across which all nutrients and waste products must pass

Cell Wall – Protects against mechanical and hypertonic stress, rupture caused by osmosis and possible harm from other organisms.

Plasmid – Aid DNA exchange. These are DNA molecule capable of replicating.
1.2.3 – Identify structures from 2.2.1 in electron micrographs of E. Coli


1.2.4 – State that prokaryotic cells divide by binary fission

Prokaryotic cells, in the right conditions, can multiply rapidly by binary fission. The cell will divide into two cells, which then grow to full size and divide again. It is used by the cells as asexual reproduction. In this process, the cell is replicated to form two identical daughter cells.

  • The DNA is first replicated, then it attaches itself to the plasma membrane.
  • The cell then elongates to separate the chromosomes.
  • The membrane then invaginates, pulling itself together in the middle.
  • The cell then splits into the two daughter cells.


This form of reproduction allows the organism to multiply very quickly. In the right conditions, one organism can multiply into billions in a short space of time.

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1.3 – Eukaryotic Cells

1.3 – Eukaryotic Cells
Eukaryotic cells will often join with other cells to form multicellular organisms.

1.3.1 – Draw and label a diagram of the ultrastructure of a liver cell as an example of an animal cell

1.3.2 – Annotate the diagram from 2.3.1 with the functions of each named structure

Nucleus – Contains the DNA of the cell, with pores in the nuclear membrane to allow movement of mRNA.

Nucleolus – The location of synthesis of ribosomes for use in the cell.

Rough endoplasmic reticulum – Ribosomes sit on the surface, synthesising proteins for use outside the cell.

Smooth endoplasmic reticulum – Synthesises lipids and steroid hormones, as well as breaking down lipid-soluble toxins

Golgi apparatus – Modifies, processes and packages macromolecules (especially proteins) into vesicles for transport within the cell

Mitochondrion – The location of the reactions of aerobic respiration, providing energy forthe cell in the form of ATP.

Ribosomes – The free-floating ribosomes synthesise proteins that are used within the cell

Cell membrane – A lipid bilayer that acts as a protective barrier for the cell. It contains chemical receptors and pores for the movement of ions and other molecules.

Cytoplasm – Where the chemical reactions of life, including respiration, occur. This is mostly made up of water, but also some proteins (i.e. enzymes for metabolic reactions).

Lysosomes – Membrane-bound vesicles that contain enzymes for intracellular digestion. It is important for cell defence, digesting harmful organisms and chemicals.

Vacuoles – Store water to increase cell turgor.


1.3.3 – Identify structures from 2.3.1 in electron micrographs of a liver cell

1.3.4 – Compare prokaryotic and eukaryotic cells
Both have:

  • Cell membrane
  • Metabolism
  • DNA
  • Require energy
  • Ribosomes
  • Cytoplasm


The differences between them are:


1.3.5 – State the three differences between plant and animal cells

1.3.6 – Outline two roles of extracellular components

Cell Wall – This is found around all plant cells, and is composed of cellulose. It maintains the shapes of the cell and provides structural support. It also prevents the excessive uptake of water.
Animal Extracellular Matrix – this is a secretion, sometimes of glycoproteins. It sits between cells, and can perform many additional functions such as support, adhesion, filtering, as well as a basis for the formation of tissue.

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1.4 – Membranes

1.4 – Membranes
1.4.1 – Draw and label a diagram to show the structure of membranes

Phospholipid Bilayer – This is arranged with the hydrophilic phosphate heads facing outwards, and the hydrophobic fatty acid tails (consisting of hydrocarbon chains) facing into the middle of the bilayer. It is a barrier against all molecules except the smallest, CO₂ and
O₂. The phospholipids can change position on the horizontal plane, but not the vertical.
Integral Proteins – These usually span from one side of the phospholipid bilayer to the other. They are usually involved in transporting substances across the membrane.

Peripheral Proteins – These sit on the surfaces. They will slide around the membrane quickly and collide with each other, but will never flip from one side to the other. The ones on the inside of the membrane are often involved in maintaining the cell’s shape or motility. Thesemight also be enzymes, catalyzing reactions in the cytoplasm.
Glycoproteins – These are usually involved in cell recognition which is part of the immune system. They can also act as receptors in cell signaling such as with hormones.
Cholesterol – Binds together lipid in the plasma membrane reducing its fluidity as conferring structural stability.
This is called the fluid mosaic model because it is in a fluid state, and in electron micrographs of the membrane, the proteins form a mosaic pattern.

1.4.2 – Explain how the hydrophobic and hydrophilic properties of phospholipids help to
maintain the structure of cell membranes

The structure of the phospholipid bilayer is very stable, as the hydrophobic hydrocarbon tails are attracted to each other, and the hydrophilic phosphate heads are also attracted to each other. This attraction makes the barrier strong and stable.
The heads are suited to the high water content of the tissue fluid and cytoplasm on either side of the membrane. The tails repel water, creating a barrier between the internal and external water environments of the cell and a barrier to movement of charged molecules.
The charges on the phospholipids attract them to each other, making them fairly stable, though allowing for some movement. The presence of cholesterol molecules increases the stability of the phospholipid.

1.4.3 – List the functions of membrane proteins
Channel Protein – They span the membrane, allowing movement of large molecules across it. Within these are passive and active membrane pumps. They only allow specific ions through.
Receptor Protein – These detect hormones arriving at cells to signal changes in function.
They are also involved in other cell and substance recognition as in the immune system.

Enzymes – Integral proteins in the membrane may be enzymes (i.e. ATP Synthetase, Maltase)
Electron Carriers – These are a chain of peripheral and integral proteins that allow electrons to pass across the membrane. Active pumps use ATP to move specific substances across the membrane.

1.4.4 – Define diffusion and osmosis
Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration.
Diffusion through a cell membrane will occur if the membrane is fully permeable to the solute. In the case of the phospholipid
bilayer, it is permeable to non-polar substances, such as steroids and glycerol, as well as oxygen and carbon dioxide. They will diffuse
quickly via this route. It will also occur if the pores in the membrane are large enough for the solute to enter. Water diffusing through the plasma membrane passes via the protein-lined pores of the membrane, and tiny spaces between the phospholipid molecules.
This will occur more easily if the membrane contains phospholipids with unsaturated hydrocarbon tails, as they are spaced apart more widely. As a result, the membrane is especially leaky to water.


Osmosis is the passive movement of water molecules, across a partially permeable
membrane, from a region of lower solute concentration to a region of higher solute


The water moves through plasma membrane pores called aquaporin. When a solution is separated from water by a membrane permeable to water molecule, water molecules tend to diffuse, while dissolved molecules and their group of water molecules move less.
Osmosis can also be defined as the net movement of water molecules from a region of high concentration of water molecules to a region of lower concentration of lower concentration of water molecules, across a selectively permeable membrane.


1.4.5 – Explain the passive transport across membrane by simple diffusion and facilitated

Passive movement means that no energy (ATP) is used for the movement of molecules from one side of the membrane to the other.
Simple diffusion – The molecules are so small they can simply pass through the phospholipid molecules of the membrane, as it offers little resistance. Examples include O2 and CO2, as well as lipid molecules, even though they are large.
Facilitated diffusion – For larger molecules, there are channel proteins to take the through the membrane. These have complex shapes, which provide a channel through the protein, or the pore. It acts as a shield against the non-charged regions of the membrane for the molecule. These channels only allow a specific type of substance through, but there is no control over the direction of movement.

1.4.6 – Explain the role of protein pumps and ATP in active transport across membranes

Active transport is necessary as cells may have a higher concentration than the outside, thus diffusion is not possible. As a result, the particles are moving against the concentration.

In active transport, ATP is used to provide the energy necessary. It is hydrolysed into ADP. Protein pumps use this energy to pump molecules across the cell membrane, moving from a low concentration to a high concentration. They are called active because they need energy to function.
This energy causes the shape of the protein to change, allowing it to move the molecule across the membrane. Active transport is also highly selective, tending to absorb ions that reflect the cells needs. Sodium-Potassium pump creates an electro-chemical gradient across the membrane of all cells. The inside of the cell has a negative charge compared to the outside. In nerve cells, this pump is modified to create electrochemical phenomena.


1.4.7 – Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi, apparatus, and the plasma membrane
Cells manufacture molecules to be secreted outside the cell, which are sometimes a complex combination of proteins, carbohydrates and lipids. A gene is coded in the base protein, and the expression of it starts the process.


  • A protein that has already been synthesises is present in the rER
  • As the protein moves through the rER, it is modified
  • At the end of the rER, a vesicle is formed containing the protein
  • The vesicle then migrates to the Golgi Apparatus
  • Vesicle and Golgi membranes fuse, and the protein enters the lumen of the Golgi. The Golgi further modifies the protein.
  • Another vesicle is formed from the Golgi membrane and breaks away. It is transported to the plasma membrane, which fuse together, then secrete the protein. This is exocytosis.

1.4.8 – Describe how the fluidity of the membrane allows it to change shape, break and reform during endocytosis and exocytosis
Exocytosis – The vesicle membrane fuses with the plasma membrane, and its contents are secreted. The vesicle fuses with the plasma membrane, and its contents are expelled.
Endocytosis – A vesicle is formed when the plasma membrane infolds, then breaks off. Part of the membrane is pulled inwards, and a droplet of fluid is enclosed when it is pinched off.
They can then move the contents through the cytoplasm. The continuity of the plasma membrane is not disrupted.

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