IB Notes

Introduction to cells

 

* ACCORDING TO THE CELL THEORY, LIVING ORGANISMS ARE COMPOSED OF CELLS.

Cell theory:

- Cells are the fundamental building blocks of all organisms.

- All organisms are composed of one cell (unicellular) or many cells (multicellular).

- All cells come from existing cells.

 

Commonalities in cells:

- Cells have an energy release system that powers all of the cell's metabolism.

- Cells are surrounded by plasma membranes that divide them from each other.

- Cells contain genetic information either in the cytoplasm or in a nucleus.

- Cells contain enzymes which help catalyse the cells reactions.

- Cells are the smallest form of life, nothing smaller can live.

 

* LOOKING FOR TRENDS & DISCREPENCIES: ALTHOUGH MOST ORGANISMS CONFORM TO THE CELL THEORY, THERE ARE EXCEPTIONS.

- It is unlikely that the cell theory will be discarded because so many tissues do consist of cells.

 

* QUESTIONING THE CELL THEORY USING ATYPICAL EXAMPLES.

Striated muscle

- These muscle cells are found in your skeletal muscles.

- Made up of very long cells- average length is 30mm whereas other cells are around 0.03mm.

- Multinucleted cells.

 

Giant algae

- Giant algae are unicellular organisms.

- However, their cells are huge- up to 100mm. Such huge organisms should be multicellular.

 

Aseptate fungal hyphae

- These are long extensions from fungi.

- Has long fibre made of cells. However, they share the same cytoplasm, forming one long strand.

- Multinucleated cells.

 

Unicellular organisms

* ORGANISMS CONSISTING OF ONLY ONE CELL CARRY OUT ALL FUNCTIONS OF LIFE IN THAT CELL.

 

Functions of life:

- Nutrition: Obtaining food to provide the cell with the energy and material for growth.

- Metabolism: Chemical reactions inside the cell including cell respiration to release energy. 

- Growth: An irreversible increase in size. 

- Response: The ability to react to changes in the environment.

- Excretion: Getting rid of the waste products of metabolism.

- Homeostasis: Keeping conditions inside the organism within tolerable limits.

- Reproduction: Producing offspring either sexually or asexually.

 

- Viruses lack the properties of living things: they have no energy metabolism, they do not grow, they don't excrete, and they do not respond to stimuli. They also don't reproduce independently but must replicate by invading living cells.

 

* INVESTIGATE THE FUNCTIONS OF LIFE IN PARAMECIUM.

A unicellular protozoan easily found in water sources.

 

- Nutrition: The food is taken in through the oral groove and into the food vacuole. It is gradually digested by enzymes and absorbed into the cytoplasm. 

- Metabolism: Reactions such as cellular respiration occur in the cytoplasm. Enzymes, located inside, speed up the processes. 

- Growth: Absorbed nutrients in the cytoplasm provide material and energy needed for growth.

- Response: The cilia can beat which helps to propel the Paramecium in any direction.

- Excretion: Waste can leave by diffusing through the membrane or the anal pore.

- Homeostasis: The contractile vacuole fill up with water and expel it through the plasma membrane to keep the water content at a tolerable level.

- Reproduction: Done asexually (usually) where the parent cell splits into two daughter cells. Nucleus (which can divide to make a new nucleus) controls this. If done sexually, the nucleus will fuse with another nucleus.

 

 

 

 

 

 

 

 

 

 

 

 

* INVESTIGATE THE FUCTIONS OF LIFE IN CHLAMYDOMONAS.

A photosynthetic organism found in damp soil and stagnant water.

 

- Nutrition: Photosynthesis occurs in the chloroplasts leading to the production of sugars.

- Metabolism: Enzymes speed up chemical reactions in the cytoplasm. Photosynthesis happens in the chloroplasts in the cytoplasm. The mitochondrion produces ATP through cellular respiration.

- Growth: Carbon dioxide can be converted into compounds for growth. Carbon compounds can be absorbed from other organisms in the dark.

- Response: The flagella can beat to propel the organism through the water. The eye spot is a light detector which allows the organism to move to brighter areas.

- Excretion: The permeable cell wall allows waste gases from respiration and photosynthesis to pass through via diffusion. The cell membrane controls what enters and exits the cell.

- Homeostasis: The contractile vacuole can fill up with water to maintain the correct concentration inside.

- Reproduction: The nucleus can divide or fuse with another leading to asexual or sexual reproduction.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

* SURFACE AREA TO VOLUME RATIO IS IMPORTANT IN THE LIMITATION OF CELL SIZE.

- Most chemical reactions take place in the cytoplasm. The rate of reaction is proportional to the volume of the cell.

- If a cell becomes too large, diffusion through the plasma membrane becomes inefficient due to low surface : area volume. Waste product will accumulate as they are made more rapidly than excreted.

- The larger the volume of an organism, the more material needs to be exchanged.

- The change in ratio should be relative the change in surface area. If the organism increases its side length by 5 times, the ratio decreases by 5 times (in a cube).

- If the ratio is too small, cells may overheat because metabolism produces heat faster than it is lost.

- Cells are only able to survive at a small size in order to optimise the exchange of materials.

 

Multicellular organisms

* MULTICELLULAR ORGANISMS HAVE PROPERTIES THAT EMERGE FROM THE INTERACTION OF THEIR CELLULAR COMPONENTS.

- A single mass of cells fused together are multicellular. Humans are an example of this.

- C. elegans (hermaphrodite) feed on bacteria that causes decomposition. They have a mouth, pharynx, intestine and anus. The brain in C. elegans coordinates responses to the environment but doesn't control how individual cells develop.

 

- The cells in C. elegans and other multicellular organisms are working in cooperative groups (with no cell 'leader')

- Individual cells within the group organizes themselves and interacts with each other to form a living organism with distinctive properties.

- The characteristics of the organism along with the fact that it is alive is known as emergent properties.

- Emergent properties arise from the interation of the component parts of a complex structure. The whole is greater than the sum of its parts.

 

* SPECIALIZED TISSUES CAN DEVELOP BY CELL DIFFERENTIATION IN MULTICELLULAR ORGANISMS.

- Different cells perform different functions.

- A tissue is a group of cells specialized in the same way to perform the same function.

- Specialized cells become more efficient in their role as compared to having to do many different tasks. They are able to develop the ideal structure with the right enzymes to speed up chemical reactions.

- Differentiation is the development of cells in different ways. There are 220 different cell types in humans.

 

* DIFFERENTIATION INVOLVES THE EXPRESSION OF SOME GENES AND NOT OTHERS IN A CELL'S GENOME.

- All cells in a multicellular organism have the same set of genes. However, not all genes are used. 

- Genes that are being used are said to be expressed. The information can be used to make a protein or other gene product.

- The development of a cell involves expressing certain genes. Cell differentiation happens because a different sequence of genes is expressed in different cell types.

- An extreme example of differentiation are the cells found in the skin of the nose. The olfactory receptor cells only expresses one the genes to make one of the receptors in order to distinguish between different smells.

 

Stem cells

* THE CAPACITY OF STEM CELLS TO DIVIDE AND DIFFERENTIATE ALONG DIFFERENT PATHWAYS IS NECESSARY IN THE EMBRYONIC DEVELOPMENT. IT ALSO MAKES STEM CELLS SUITABLE FOR THERAPEUTIC USES.

- Stem cells can divide again and again to produce copious quantities of new cells. Commonly used for growth of tissues or the replacement of cells that have been lost or damaged.

- Stem cells can differentiate in different ways to produce different cell types.

- Small amounts of stem cells are present in the adult body. It can be found in bone marrow, skin and liver. 

- They allow some tissues to repair quickly (skin) and others limited repair (brain, kidney, heart).

 

Embryonic:

- Embryonic stem cells are capable of dividing many times to produce large amounts of tissue. They are extremely versatile and can differentiate into any of the cells found in that particular animal. These are the most useful.

- Blood extracted from the umbilical cord of a new born baby and stem cells can be obtained from it. The cells may be stored for future use.

- Can regenerate tissue such as skin for those who have suffered burns. Can heal diabetes where a particular cell type has been lost or malfunctioned.

- As the embryo develops, the cells commit themselves to a pattern of differentiation. They will still be able to divide at this point but all of the cells will differentiate in the same way and they are no longer stem cells.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

* ETHICAL IMPLICATIONS OF RESEARCH: RESEARCH INVOLVING STEM CELLS IS GROWING IN IMPORTANCE AND RAISES ETHICAL ISSUES.

- Some consider stem cell research as unethical as embryos are killed. (But when does life begin? If they can't feel pain then is it ok?)

- It is unethical to create human lives solely for the purpose of obtaining stem cells. 

- Embryos created through IVF involves hormone treatment of women with some associated risk including the removal of eggs from the ovary. If women are paid to do this, many college students may be exploited.

- Some claim that this medical research is carried out on patients without their consent.

 

- However, stem cells have the potential to allow methods of treatment for diseases and disabilities that are currently incurable.

- If successful, suffering in some individuals will be greatly reduced.

 

* 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.

- Embryos can be deliberately created by fertilizing egg cells with sperm and allowing it to divide into 4 - 16 (embryonic stem) cells. 

 

* USE OF STEM CELLS TO TREAT STARGARDT'S DISEASE

- Genetic disease where a recessive mutation of ABCA4 (gene) results in the malfunction of a membrane protein used for active transport in the retina cells.

- Photoreceptive cells in the retina will degenerate so vission worsens (blind in some cases).

- Researchers use embryonic stem cells to develop into retina cells.

- A woman was treated by having 50 000 retina cells derived from embryonic stem cells injected into her eyes. There was an improvement in her vision, none were rejected, no tumours developed and there were no harmful side effects.

 

* USE OF STEM CELLS TO TREAT LEUKEMIA

- Leukemia (cancer) involves the production of abnormally large numbers of white blood cells.

- Cancer cells in the bone marrow that are producing too many white blood cells must be destroyed.

- Dividing cells can be destroyed by treating the patient will chemicals. Stem cells that can produce white blood cells and keep the patient healthy will be destroyed.

 

Chemotherapy:

- Large needle is inserted into a large bone, usually the pelvis and fluid is removed from the bone marrow. 

- Stem cells are extracted from this fluid and are stored by freezing them. Since they are adult stem cells, they can only produce blood cells. 

- High dose of chemotherapy drugs is given to the patient to kill all the cancer cells in the bone marrow. 

- The stem cells are returned back to the patient's bone marrow, multiply and start to produce blood cells.

 

* USE OF A LIGHT MICROSCOPE TO INVESTIGATE THE STRUCTURE OF CELLS AND TISSUES.

- Place the cells on the slide, making sure it is only one cell thick.

- Add a drop of stain to the specimen.

- Lower the cover slip on top of the specimen and carefully press out the air bubbles. 

- Remove excess stain by pressing and dabbing the cover slip with tissue.

- Place the slide onto the stage.

- Turn on the lights and adjust the intensity of it by using the diaphragm.

- Turn to the objective lens with the lowest power.

- Put you eye to the eyepiece lens.

- Adjust the coarse focusing wheel, then the fine focusing wheel until the image is as clear as possible.

- Note down what you see or change to a more powerful objective lens.