Cell Biology

Key Principles cell theory:

- living organisms are made of one or more cells

- cells are the smallest units of life

- all cells come from pre-existing cells

= one cell can carry out all the functions of life and anything which cannot is not considered a cell: e.g. viruses --> not made of cells --> not livings organisms

- An exception to cell theory: red blood cells --> don't have a nucleus, but nuclei are present as they form + red blood cells of other animals do have nuclei

-Other exceptions to cell theory:

• fungal hyphae have many nuclei in their long threads
• Skeletal muscle is made of fibres that are much larger than normal cells + contain many nuclei
• Giant algae are unicellular but associate with other cells in a matrix

= these few exceptions mean there is no new theory

surface area important for breathing and absorption of food

• as cell gets larger => proportionately less surface area available => can't obtain all the materials it needs through its surface and to dispose of waste 
• rate of exchanging materials becomes limiting and cannot keep up with needs of cell => so beyond a certain size= cell cannot survive

Solution to problem (able to become larger): 

• living things develop structures, e.g. shape( long/thin), folds or villi, on cell surface => single cell size still limited
• => cell must divide => that's why many organisms have become multicellular to overcome problems of the limited size of the cell
• Multicellular organism= many advantages: 1) it can grow to a larger size 2) its cells can differentiate so that different cells do different jobs

Definition: new properties that appear in multicellular organisms as a result of interactions of the components of their cells

Unicellular organisms: carry out all functions of cells

Cell in group with others: interact to perform a range of more complicated tasks= emergent tasks

e.g. cells form tissues and organs which carry out functions such as 1) breathing 2) reproduction in a different way

analogy musical group: 1 instrument play simple tune but group of instruments produce wider variety of sounds and effects

• stem cells retain the ability to turn into a great many different cell types and they are:
  • unspecialised
  • can divide repeatedly to make large numbers of new cells
  • can differentiate into several cell types

• Embryonic stem cells: come from a blastocyst (ball of cells from a fertilized egg, which are all alike)
• Adult stem cells: e.g. found in bone marrow, are different and can only differentiate into a limited number of cell types

Scientists must consider ethics of any research involving living cells. Some people consider all stem cell research as unethical but different sources of stem cells have different properties and should be considered properly

Medical Use of stem cells

1) stem cells from umbilical cord blood to treat certain types of leukaemia

2) embryonic stem cells used to treat Stargardts disease (leads to macular degeneration and blindness)

3) Stem cells from bone marrow from living donors used to treat leukaemia 

• a model used to explain the understanding of membrane structure

• based on Singer and Nicolson's model (1972)
• membrane mosaic formed of many small separate units => phospholipids
• each can appear in any area of membrane => said to be fluid
• a membrane can fold and form vesicles => can rejoin main structure at any point because phospholipid units can fit into a new area anywhere in its structure
• phospholipids form 2 layers: hydrophilic heads= outside; hydrophobic tails= inside

• Integral proteins: embedded in bilayer and form protein channels for transport
• peripheral proteins: attached to surface; some have carbs attached and act as hormone binding sites or for cell-to-cell communication
• Some proteins in a membrane: enzymes

• Cholesterol molecules: embedded between non-polar fatty acid chains; make membrane more rigid => can function effectively at a wider range of temperatures

• plant cells: no cholesterol => instead, un/saturated fatty acids maintain proper membrane fluidity

Endocytosis: infolding of plasma membrane to form small vesicle within cell; vesicle contains either liquid or solid items that cell takes in from its external environment: e.g. white blood cells will engulf a bacterium by endocytosis, so it can be destroyed inside cell

Exocytosis: method cell uses to export something from within a cell; e.g. enzyme for digestion that the cell has made on the RER//waste product e.g. remains of a bacterium

Vesicles formed inside cell move towards membrane and fuse with it, opening up so they can release their contents outside

! both these processes work because the membrane is a fluid mosaic, so vesicles can break away or rejoin main membrane at any position

1) A specific protein binds to 3 intracellular sodium ions

2) the binding of sodium ions causes phosphorylation by ATP (ATP has 3 attached phosphates//During Phosphorylation 1 phosphate is lost => Result: 2-phosphate compound (ADP))

3) Phosphorylation causes protein to change shape => thus expelling sodium ions to exterior

4) 2 extracellular potassium ions bind  to different regions of proteins =>causes release of phosphate group

5) loss of phosphate group restores the protein's original shape => causes release of potassium ions into intracellular space

• Louis Pasteur used experiments to demonstrate that living cells can't spontaneously generate (appear) and must be produced from existing cells
• first cells appeared approx. 3.5 bio years ago and must have arisen from chemicals present at that time
• Certain steps must have occurred
  • Organic molecules must have formed, and larger molecules been assembled from basic organic molecules
  • some molecules must have been able to reproduce themselves and have formed membrane from mixture of larger molecules

Evidence to support theory: observations of chloroplasts and mitochondria

• contain smaller 70S ribosomes found in prokaryotes
• have 2 membranes on exterior: consistent with engulfing process
• can replicate by binary fission
• contain small circular pieces of DNA like plasmids

theory suggests that long time ago, prokaryotes engulfed by larger cells and remained inside them =symbiotic relationship: big cell = protection and carbon compounds/bacteria: provides ATP

• Interphase: period when cell carries out tasks which it is programmed to do. =Reduce protein and secrete enzymes. DNA replicated -> chromosomes=> 2 sister chromotids= ready for mitosis

• Mitosis: division of nucleus in 4 stages
• Cytokinesis: short period after mitosis when cytoplasm divides

• After mitosis, cytoplasm divides to separate the two new nuclei. In plant cells, a cell plate forms first and this eventually becomes the new cell wall

when studying cells, researchers count proportion of cells undergoing mitosis

mitotic index: number of cells undergoing mitosis divided by the total number of cells in the sample

mitotic index used in cancer studies to predict the likely response of cancer cells to treatment

cells get out of control -> tumour

Primary tumour: tumour occurs at original site of cancer

Secondary tumour: metastasis, a cancerous tumour that has spread  from original part of organism to another e.g. brain tumour -> composed of breast cancer cells

Some cases: metastasis of primary tumour cells so extensive that secondary tumours are found in many locations of organism

How and why does it form? most organisms: sections of genes that may mutate/ may be expressed at abnormally high levels (section of genes called oncogenes; contribute to converting a normal cell into a cancer cell -> may mutate because they are triggered by an outside agent -> mutagen (e.g. cigarette smoke) -> pos. correlation cigarette smoke and cancer