IB DP Biology Unit 1 1 Cell theory

IB DP Biology Unit 1. 1 Cell theory

1. 1. U 1 According to the cell theory, living organisms are composed of cells.

Cell Theory 1. Cells are the smallest units of life 2. Living organisms are composed of cells 3. Cells come from pre-existing cells

Cell Theory A theory is a general system of ideas used to explain or interpret observations Theories provide predictive power by generating hypotheses A hypothesis is a specific prediction that can be tested through observation or experiment

TOK QUESTION: Just theories? How much evidence is needed for a hypothesis to become a theory? Should a theory be abandoned when there is evidence that it does not offer a full explanation?

Cell Theory Each aspect of cell theory is based on evidence obtained through observations and experiments

Cell Theory Evidence Cells are the smallest units of life Nothing smaller than a cell can survive independently ◦ Subcellular structures (nuclei, ribosomes, etc. ) Lower limit on cell size is about 200 nm ◦ Large enough for DNA, membranes, etc.

Cell Theory Evidence All living things are made of one or more cells Microscopes allow us to observe that all living things are composed of cells ◦ Either unicellular or multicellular

Cell Theory Evidence All cells come from pre-existing cells ◦ Prokaryotes reproduce by binary fission ◦ Eukaryotes reproduce by mitosis All cells then have a common ancestor ◦ Implies that all cells are related

Cell Theory Evidence All cells come from pre-existing cells Geology implies life hasn’t always existed ◦ Life originated 1 billion years after earth

1. 1. A 1 Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae.

Cells What is a cell? A functional unit of cytoplasm surrounded by a membrane and containing genetic material ◦ Not all cells meet this definition

Cells What is a cell? Exceptions: ◦ Striated muscle, fungal hyphae, and some algae have cytoplasms that are multinucleated Challenges idea that cells have one nucleus ◦ Giant algae have one nucleus, but are huge! 5 -100 mm Challenges idea that cells are small

Cells What is a cell? Exceptions: ◦ Connective tissue is composed of both cellular and extracellular structures But they are a product of cellular activity Organisms are made up of cells and modified cells and cell products ◦ This modification is called Organismal Theory

1. 1. U 2 Organisms consisting of only one cell carry out all functions of life in that cell.

Cells Unicellular organisms carry out all the functions of life: ◦ Growth An irreversable increase in size ◦ Homeostasis Keeping conditions inside the organisms within tolerable limits ◦ Metabolism Chemical reactions inside the cell Including cell respiration to release energy

Cells Unicellular organisms carry out all the functions of life: ◦ Nutrition and Excretion Obtaining food to provide energy and the materials to grow, then getting rid of waste products ◦ Reproduction Producing offspring either sexually or asexually ◦ Response Perceiving and responding to changes in the environment

Cells MR H GREN

1. 1. A 2 Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.

Cells

Cells

1. 1. S 1 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)

Cells Units of size ◦ mm = millimeter = 10 -3 meters ◦ μm = micrometer = 10 -3 millimeters ◦ nm = nanometer = 10 -3 micrometers

Cells

Cells Important ◦ ◦ ◦ scales 1 nm = molecules 10 nm = cell membrane thickness 100 nm = viruses 1 μm = bacteria 1 – 10 μm = organelles 10 – 100 μm = eukaryotic cells MTV Beats Other Channels!

Cells Calculating magnification Magnification = size of image / real size Drawing of microscopic structures must include: ◦ Scale bars: |--------| = 10 nm ◦ Magnification: x 500

TOK QUESTION: So small…. Nearly all these biological entities are beyond our ability to perceive directly. One must use technology to ‘see’ them. Should a distinction be made between knowledge claims dependent upon observations made directly with the senses and knowledge claims dependent on technology or maths?

1. 1. U 3 Surface area to volume ratio is important in the limitation of cell size.

Cells Surface Area: Volume ratio Surface Area ◦ (SA) = f(x 2) ◦ Rate of exchange = f(SA) Volume ◦ (V) = f(x 3) ◦ Metabolism = f(V) Includes heat and waste production and resource consumption

Cells Surface Area: Volume ratio Therefore: ◦ V increases faster than SA when the dimensions of a cell increase ◦ SA: V ratio decreases with growing cell size

Cells Surface Area: Volume ratio As the organism gets bigger, its SA: V ratio decreases ◦ This rule is a limiting factor for cell size ◦ As the cell gets bigger the ratio decreases ◦ If the ratio decreases the rate of exchange decreases

Cells Surface Area: Volume ratio Example: Oxygen for respiration ◦ A cell respiring aerobically needs oxygen Oxygen is obtained from the surrounding environment Such as water or blood (depends on the cell) ◦ Oxygen diffuses across the cell membrane. More membrane allows more diffusion

Cells Surface Area: Volume ratio Example: Oxygen for respiration ◦ Ratio of SA: V decreases in bigger cells Volume of oxygen obtained for each unit of cell volume is actually decreasing ◦ Cells must not get too big because they cannot obtain sufficient oxygen to satisfy the demands of the cell.

Cells Surface Area: Volume ratio Therefore: ◦ Sets an upper limit on cell size Low SA reduces rate of exchange High V increases metabolic demands Oxygen, nutrients, wastes, etc. ◦ Low SA: V means rate of exchange can not meet metabolic demands

1. 1. U 4 Multicellular organisms have properties that emerge from the interaction of their cellular components.

Cells are emergent Whole is greater than the sum of its parts ◦ Arises from the interaction of component parts ◦ Life itself can be viewed as an emergent property

Cells are emergent Example: Light bulb ◦ Bulb made of several parts ◦ We can individually study the properties of : Tungsten Metal cup Glass container ◦ Does not allow the prediction of the lightbulb Only combine them can this property be found

TOK QUESTION: Emergent-cy? The emergent nature of life raises many questions… When should death be defined? Most human cells are still alive long after the heart stops beating.

1. 1. U 5 Specialized tissues can develop by cell differentiation in multicellular organisms.

Cells Differentiation Unicellular organisms ◦ Must solve all of life’s challenges in a single cell Multicellular organisms ◦ Can differentiate into different types of cells Each type can be specialized for certain functions, increasing efficiency Division of labor

1. 1. U 6 Differentiation involves the expression of some genes and not others in a cell’s genome.

Cells Differentiation Achieved through differential gene expression ◦ All cells in an organism have identical DNA ◦ Different cell types make different proteins ◦ A result of transcription regulation Each cell type expresses a subset of its genome “turning on” some genes and “turning off” others

1. 1. U 7 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.

Cells Stem cells can 1. Divide 2. Differentiate 3. Specialize

1. 1. A 3 Use of stem cells to treat Stargardt’s disease and one other named condition.

Cells In Stargardt’s disease: ◦ Causes blindness due to dying retina cells 1. Embryonic stem cells obtained 2. These are injected into the eye of patient 3. Cells attach to patient’s retina and differentiate into healthy cells.

Cells In lymphoma or leukemia: 1. Cells are removed from the bone marrow of the patient 2. High doses of chemotherapy drugs kill dividing cells in patient Both cancerous and normal cells die 3. HS Cells from the marrow are transplanted back into the patient Fully restores the production of healthy blood cells and marrow

Cells Stem cells: Therapeutic use Bone marrow transplants use hematopoietic stem cells (HS cells) ◦ Found in bone marrow and produce red and white blood cells 100 HS cells can completely replace the blood system of mice after all their marrow has been destroyed

1. 1. A 4 Ethics of therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a newborn baby and from an adult’s own tissues.

Cells Stem cells: Ethical issues Use involves death of early-stage embryos Therapeutic cloning could reduce suffering in a wide variety of conditions

TOK QUESTION: Stem Cells Do the potential medical benefits of research on human embryos justify their use? What can we use them for and how can we get them? How old can they be? Should stem cells be used even though they can potentially turn into tumors?

MAJOR SOURCES Thank you to my favorite sources of information when making these lectures! John Burrell (Bangkok, TH) www. click 4 biology. info Dave Ferguson (Kobe, JA) http: //canada. canacad. ac. jp/High/49 Stephen Taylor (Bandung, IN) www. i-biology. net Andrew Allott – Biology for the IB Diploma C. J. Clegg – Biology for the IB Diploma Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate Howard Hugh’s Medical Institute – www. hhmi. org/biointeractive Mr. Hoye’s TOK Website – http: //mrhoyestokwebsite. com And all the contributors at www. You. Tube. com