Unit 2

Topic 2 (Cells)

2.1- Cell theory• 2.1.1 Outline cell theory: • 2.1.2 Evidence for cell theory• 2.1.3 Unicellular organisms• 2.1.4 Relative sizes of cells• 2.1.5 Magnification.• 2.1.6 Surface area: Volume ratios and cell size.• 2.1.7 Emergent properties• 2.1.8 Cell differentiation• 2.1.9 Stem cells• 2.1.10 Therapeutic uses of stem cells

2.1.1 Outline the cell theory

1) All living things are made of cells. 2) Cells are the smallest unit of life.3) Existing cells have come from other cells. • Stated in this way Cell Theory might be

attributed to Schleiden and Schwann (1838).

• Robert Hooke first coined the term 'cell' after observing the structure of cork in 1655.

• The first observation of living cells was by Anton van Leeuwenhoek in 1674.

2.1.2 Discuss the evidence for the cell theory

• Microscopes:• Microscopes have increased man's ability to visualise tiny objects• All living things when viewed under a microscope have been found to be made

of cells and cell products (e.g. hair)• Note: Certain types of cells do not conform to the standard notion of what

constitutes a cell – Muscle cells contain multiple nuclei– Fungal hyphae consist of multiple cells that share a continuous cytoplasm

Types of MicroscopesSimple – low magnification with sunlight as main source of energy, 1 lens pieceCompound – up to 2000x the actual size, has multiple lensElectron – up to 500,000x the actual size, beam of electrons hits object and

produces a computer-generated image

• http://www.telegraph.co.uk/science/picture-galleries/7606811/Hayfever-sufferers-know-your-enemy-Scanning-Electron-Microscope-pictures-of-grains-of-pollen.html?image=1

• http://www.environmentalgraffiti.com/featured/images-inside-human-body-images/8292

• Experimental Evidence:• Cells removed from tissues can survive

independently for short periods of time• Nothing smaller than a cell has been found to

be able to live independently• Experiments by Francesco Redi and Louis

Pasteur have demonstrated that cells cannot grow in sealed and sterile conditions

2.1.3 State that unicellular organisms carry out all the functions of life

• Unicellular organisms (such as amoeba, paramecium, euglena and bacterium) are the smallest organisms capable of independent life.

• All living things share 7 basic characteristics:– Movement: Living things show movement, either externally or internally– Reproduction: Living things produce offspring, either sexually or asexually– Sensitivity: Living things can respond to and interact with the environment– Growth: Living things can grow or change size / shape– Nutrition: Living things use substances from the environment to make energy– Excretion: Living things exhibit the removal of wastes– Respiration: Living things exchange materials and gases with the environment

2.1.4 Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using the

appropriate SI unit

• Relative sizes:1. molecules (1nm). 2. cell membrane thickness (10nm).3. virus (100nm).4. bacteria (1um).5. organelles (less 10um).6. cells (<100 um).7. generally plant cells are larger than animal cells.

• Scale of the universe - http://htwins.net/scale2/

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

magnification

• To calculate the linear magnification of a drawing the following equation should be used:– Magnification = Size of image (with ruler) ÷ Actual

size of object (according to scale bar)

• To calculate the actual size of a magnified specimen the equation is simply re-arranged:– Actual size = Size of image (with ruler) ÷

Magnification

2.16 Explain the importance of the surface area to volume ratio as a factor limiting cell size.

• The rate of metabolism of a cell is a function of its mass / volume• The rate of material exchange in and out of a cell is a function of

its surface area• As the cell grows, volume increases faster than surface area

(leading to a decreased SA:Vol ratio)• If the metabolic rate is greater than the rate of exchange of vital

materials and wastes, the cell will eventually die • Hence the cell must consequently divide in order to restore a

viable SA:Vol ratio and survive• Cells and tissues specialised for gas or material exchange (e.g.

alveoli) will increase their surface area to optimise the transfer of materials

Microvilli increase surface area allowing for a more efficient exchange of materials / heat

2.1.7 State that multicellular organisms show emergent properties

• Emergent properties arise from the interaction of component parts: the whole is greater than the sum of its parts

• Multicellular organisms are capable of completing functions that individual cells could not undertake - this is due to the interaction between cells producing new functions

• In multicellular organisms:– Cells may group together to form tissues– Organs are then formed from the functional grouping of multiple tissues– Organs that interact may form organ systems capable of carrying out specific body functions– Organ systems carry out the life functions required by an organism

2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their

genes but not others.• All cells of an individual organisms share an identical

genome - each cell contains the entire set of genetic instructions for that organism

• The activation of different instructions (genes) within a given cell by chemical signals will cause it to differentiate from other cells like it.

• Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature

• Active genes are usually packaged in an expanded and accessible form (euchromatin), while inactive genes are mainly packaged in a condensed form (heterochromatin)

• Differentiated cells will have different regions of DNA packaged as heterochromatin and euchromatin depending on their function

2.1.9 State that stem cells retain the capacity to divide and have the ability to differentiate along different

pathways.

• Stem cells are unspecialised cells that have two key qualities:

• 1. Self renewal: They can continuously divide and replicate

• 2. Potency: They have the capacity to differentiate into specialised cell types

2 types a. Adultb. Embryonic - controversial

1. To differentiate cells – only certain genes are turned “ON”

2. Goal of research = take stem cell & turn it into any type of cell a person may need to cure/treat diseases

2.1.10 Outline one therapeutic use of stem cells.

• Stem cells can be derived from embryos or the placenta / umbilical cord of the mother; also minimal amounts can be harvested from some adult tissue

• Stem cells can be used to replace damaged or diseased cells with healthy, functioning ones

• This process requires:– The use of biochemical solutions to trigger differentiation into

desired cell type– Surgical implantation of cells into patient's own tissue– Suppression of host immune system to prevent rejection of cells– Careful monitoring of new cells to ensure they do not become

cancerous

• Examples of therapeutic uses of stem cells:– 1. Retinal cells: Replace dead cells in retina to

cure diseases like glaucoma and macular degeneration

– 2. Skin cells: Graft new skin cells to replace damaged cells in severe burn victims

– 3. Nerve cells: Repair damage caused by spinal injuries to enable paralysed victims to regain movement

– 4. Blood cells: Bone marrow transplants for cancer patients who are immuno-compromised as a result of chemotherapy

videos• What is a stem cell?

http://www.youtube.com/watch?v=6bM5HeBGf9c• 2005 Stem cell video - http://www.pbs.org/wgbh/nova/body/ste

m-cells-research.html

• 2008 stem cell breakthrough - http://www.pbs.org/wgbh/nova/body/stem-cells-breakthrough.html

• Future of stem cells - http://www.youtube.com/watch?v=zz2bZQFZgRc&feature=relmfu

• Newest - http://news.discovery.com/videos/first-cloned-human-embryos-yield-stem-cells.htm

2.2 – Prokaryotic Cells

• 2.2.1 Structure of a prokaryotic cell• 2.2.2 Function of the prokaryotic cell parts • 2.2.3 Electron micrograph study of E. coli• 2.2.4 Binary fission in prokaryotes

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

2D 3D

2.2.2 Annotate the diagram from 2.2.1 with the functions of each named structure.

• Cell Wall: A rigid outer layer made of peptidoglycan (polysaccharide) that maintains shape and protects the cell from damage or bursting if internal pressure is high

• Cell Membrane: Semi-permeable barrier that controls the entry and exit of substances

• Cytoplasm: Fluid component which contains the enzymes needed for all metabolic reactions

• Nucleoid: Region of the cytoplasm which contains the genophore (the prokaryotic DNA)

• Plasmid: Additional DNA molecule that can exist and replicate independently of the genophore - it can be transmitted between bacterial species

• Ribosome: Complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryotic ribosomes are smaller than eukaryotes - 70S)

• Slime Capsule: A thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis (getting eaten by other cells)

• Flagella (singular flagellum): Long, slender projection containing a motor protein which spins the flagella like a propeller, enabling movement

• Pili (singular pilus): Hair-like extensions found on bacteria which can serve one of two roles– Attachment pili: Shorter in length, they allow bacteria to

adhere to one another or to available surfaces– Sex pili: Longer in length, they allow for the exchange of

genetic material (plasmids) via a process called bacterial conjugation

2.2.3 Identify structures from 2.2.1 in electron micrographs of E. coli.

2.2.4 State that prokaryotic cells divide by binary fission.

• Binary fission is a form of asexual reproduction and cell division used by prokaryotic organisms

• It is not the same as mitosis, there is no condensation of genetic material and no spindle formation

• In the process of binary fission:– The circular DNA is copied in response to a replication

signal– The two DNA loops attach to the membrane– The membrane elongates and pinches off (cytokinesis)

forming two separate cells

• What is bacteria? - http://www.youtube.com/watch?v=pcXdfofLoj0

• Growing bacteria - http://www.youtube.com/watch?v=LSZE6WofLAs

2.3- Eukaryotic Cells

• 2.3.1 Diagram of the eukaryotic liver cell.• 2.3.2 Functions of the cell parts.• 2.3.3 Electron micrographs of the liver cell.• 2.3.4 Compare the prokaryotic and eukaryotic cell.• 2.3.5

Three differences between a plant and an animal cell.

• 2.3.6 Outline two roles of extracellular components.

2.3.1.Draw and label a diagram of the ultrastructure of a liver cell as an example of an animal cell.

2D 3D

2.3.2 Annotate the diagram from 2.3.1 with the functions of each named structure.

• Cell Membrane: Semi-permeable barrier that controls the entry and exit of substances

• Cytosol: The fluid portion of the cytoplasm (does not include the organelles or other insoluble materials)

• Nucleus: Contains hereditary material (DNA) and thus controls cell activities (via transcription) and mitosis (via DNA replication)

• Nucleolus: Site of the production and assembly of ribosome components

• Ribosome: Complexes of RNA and protein that are responsible for polypeptide synthesis (eukaryotic ribosomes are larger than prokaryotes - 80S)

• Mitochondria: Site of aerobic respiration, which produces large quantities of chemical energy (ATP) from organic compounds

• Golgi Apparatus: An assembly of vesicles and folded membranes involved in the sorting, storing and modification of secretory products

• Lysosome: Site of hydrolysis / digestion / breakdown of macromolecules

• Peroxisome: Catalyses breakdown of toxic substances like hydrogen peroxide and other metabolites

• Centrioles: Microtubule-organizing centers involved in cell division (mitosis / meiosis and cytokinesis)

• Endoplasmic Reticulum: A system of membranes involved in the transport of materials between organelles– Rough ER: Studded with ribosomes and involved in the synthesis and

transport of proteins destined for secretion– Smooth ER: Involved in the synthesis and transport of lipids and

steroids, as well as metabolism of carbohydrates

2.3.3 Identify structures from 2.3.1 in electron micrographs of liver cells.

2.3.4 Comparison of prokaryotic and eukaryotic cells.

• Similarities:• Both have a cell membrane• Both contain ribosomes• Both have DNA and cytoplasm

• Differences

2.3.5 State three differences between plant and animal cells.

Plant Cell

2.3.6 Outline two roles of extracellular components.

• Plants• The cell wall in plants is made from cellulose secreted from the cell,

which serves the following functions:– Provides support and mechanical strength for the cell (maintains cell shape)– Prevents excessive water uptake by maintaining a stable, turgid state– Serves as a barrier against infection by pathogens

• Animals• The extracellular matrix (ECM) is made from glycoproteins secreted

from the cell, which serve the following functions:– Provides support and anchorage for cells– Segregates tissues from one another– Regulates intercellular communication by sequestering growth factors

2.4 - Membranes• 2.4.1 Structure of the membrane.• 2.4.2 Properties of the membrane phospholipids • 2.4.3 Functions of membrane proteins . • 2.4.4 Definitions of diffusion and osmosis.• 2.4.5 Passive transport across membranes.• 2.4.6 Active transport across the membrane.• 2.4.7 Vesicle transport within the cell.• 2.4.8

Membrane fluidity and transport across membrane by endocytosis and exocytosis.

2.4.1 Draw and label a diagram to show the structure of membranes.

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of the cell

membranes.• Structure of Phospholipids• Consist of a polar head (hydrophilic) made from glycerol and phosphate• Consist of two non-polar fatty acid tails (hydrophobic)

• Arrangement in Membrane• Phospholipids spontaneously arrange in a bilayer• Hydrophobic tail regions face inwards and are shielded from the surrounding polar fluid while the two

hydrophilic head regions associate with the cytosolic and extracellular environments respectively

• Structural Properties of Phospholipid Bilayer• Phospholipids are held together in a bilayer by hydrophobic interactions (weak associations)• Hydrophilic / hydrophobic layers restrict entry and exit of substances • Phospholipids allow for membrane fluidity / flexibility (important for functionality)• Phospholipids with short or unsaturated fatty acids are more fluid• Phospholipids can move horizontally or occasionally laterally to increase fluidity• Fluidity allows for the breaking / remaking of membranes (exocytosis / endocytosis)

2.4.3 List the functions of membrane proteins.

• Transport: Protein channels (facilitated) and protein pumps (active)

• Receptors: Peptide-based hormones (insulin, glucagon, etc.)• Anchorage: Cytoskeleton attachments and extracellular matrix• Cell recognition: MHC proteins and antigens• Intercellular joinings: Tight junctions and plasmodesmata• Enzymatic activity: Metabolic pathways (e.g. electron

transport chain)

2.4.4 Define diffusion and osmosis.

• Diffusion: • The net movement of particles from a region of high

concentration to a region of low concentration (along the gradient) until equilibrium

• Osmosis: • The net movement of water molecules across a semi-

permeable membrane from a region of low solute concentration to a region of high solute concentration until equilibrium is reached

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

2.4.5 Explain passive transport across membranes by simple diffusion and facilitated

diffusion.• The plasma membrane is semi-permeable and selective in what can cross• Substances that move along the concentration gradient (high to low) undergo

passive transport and do not require the expenditure of energy (ATP)

• Simple diffusion:• Small, non-polar (lipophilic) molecules can freely diffuse across the membrane

• Facilitated diffusion:• Larger, polar substances (ions,

macromolecules) cannot freely diffuse and require the assistance of transport proteins (carrier proteins and channel proteins) to facilitate their movement (facilitated diffusion)

2.4.6 Explain the role of protein pumps and ATP in active transport across membranes.

• Active transport is the passage of materials against a concentration gradient (from low to high)

• This process requires the use of protein pumps which use the energy from ATP to translocate the molecules against the concentration gradient

• The hydrolysis of ATP causes a conformational change in the protein pump resulting in the forced movement of the substance

• Protein pumps are specific for a given molecule, allowing for movement to be regulated (e.g. to maintain chemical or electrical gradients)

• An example of an active transport mechanism is the Na+/K+ pump which is involved in the generation of nerve impulses

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum , Golgi

apparatus and plasma membrane.

• Polypeptides destined for secretion contain an initial target sequence (a signal recognition peptide) which directs the ribosome to the endoplasmic reticulum

• The polypeptide continues to be synthesised by the ribosome into the lumen of the ER, where the signal sequence is removed from the nascent chain

• The polypeptide within the rough ER is transferred to the golgi apparatus via a vesicle, which forms from the budding of the membrane

• The polypeptide moves via vesicles from the cis face of the golgi to the trans face and may be modified along the way (e.g. glycosylated, truncated, etc.)

• The polypeptide is finally transferred via a vesicle to the plasma membrane, whereby it is either immediately released (constitutive secretion) or stored for a delayed release in response to some cellular signal (regulatory secretion = for a more concentrated and more sustained effect)

• Vesicle discovery - http://www.bbc.co.uk/news/health-24427951

2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and

exocytosis.

• The membrane is principally held together by the relatively weak hydrophobic associations between phospholipids

• This association allows for membrane fluidity and flexibility, as the phospholipids (and to a lesser extent the proteins) can move about to some extent

• This allows for the breaking and remaking of membranes, allowing larger substances access into and out of the cell (this is an active process)

• Endocytosis• The process by which large substances (or bulk amounts of smaller

substances) enter the cell without travelling across the plasma membrane• An invagination of the membrane forms a flask-like depression which

envelopes the material; the invagination is then sealed off forming a vesicle• There are two main types of endocytosis:

1. Phagocytosis– The process by which solid substances (e.g. food particles, foreign pathogens) are

ingested (usually to be transported to the lysosome for break down)

2. Pinocytosis– The process by which liquids / solutions (e.g. dissolved substances) are ingested by

the cell (allows quick entry for large amounts of substance)

• Exocytosis• The process by which large substances exit the cell

without travelling across the plasma membrane• Vesicles (usually derived from the golgi) fuse with the

plasma membrane expelling their contents into the extracellular environment

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis

2.5 – Cell division

• 2.5.1 Stages of the cell cycle• 2.5.2 Tumors• 2.5.3 Interphase• 2.5.4 Stages of mitosis• 2.5.5 Nuclei of daughter cells• 2.5.6 Uses of mitosis

2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G 2), mitosis and cytokinesis.

• The cell cycle is an ordered set of events that culminates in cell growth and division into two daughter cells• It can roughly be divided into two main stages:

• Interphase• The stage in the development of the cell between two successive M phases• This phase of the cell cycle is a continuum of 3 distinct stages (G1, S, G2), whereby the cell grows and

matures (G1), copies its DNA (S) and prepares for division (G2)• Sometimes cells will leave the cell cycle and enter into a quiescent state (G0), whereby it becomes amitotic

and no longer divides

• M phase• The periods of nuclear division (mitosis) and cytoplasmic division (cytokinesis)

2.5.2 State that tumors (cancers) are the result of uncontrolled cell division and that these can occur in

any organ or tissue.• The cell cycle is controlled by a complex chemical control system

that responds to signals both inside and outside of the cell• Tumor suppressor genes produce proteins which inhibit cell

division, while proto-oncogenes produce proteins that promote growth and division

• Mutations to these genes result in uncontrolled cell division, resulting in the formation of a tumor

• Tumors can grow in size which causes damage local tissue; they may also spread to other parts of the body (malignant tumors)

• Diseases caused by the growth of tumors are collectively known as cancers

2.5.3 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.

• Interphase is an active period in the life of a cell - many events need to occur before a cell can successfully undergo division:

• Protein synthesis: The cell needs to synthesise key proteins and enzymes to enable it to grow, copy its contents and then divide

• ATP production: The cell will need to generate sufficient quantities of ATP in order to successfully divide• Increase number of organelles: The cell needs to ensure both daughter cells will have the necessary numbers of

organelles needed to survive• DNA replication: The genetic material must be faithfully duplicated before division (this occurs during the S phase)

• As none of these processes can occur during the M phase, interphase contains growth checkpoints to ensure division is viable

• G1: A checkpoint stage before DNA replication during which the cell grows, duplicates organelles, synthesises proteins and produces ATP

• S: The stage during which DNA is replicated• G2: A checkpoint stage before division during which the copied DNA is checked for fidelity (mutations) and final

metabolic reactions occur

2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and

telophase)

• Prophase• DNA supercoils, causing chromosomes to condense and become visible under a

light microscope• As DNA was replicated during interphase, the chromosomes are each

comprised of two genetically identical sister chromatids joined at a centromere• The centrosomes move to opposite poles of the cell and spindle fibres begin to

form between them (in animals, each centrosome contains 2 centrioles)• The nuclear membrane is broken down and disappears

• Metaphase• Spindle fibers from the two centrosomes

attach to the centromere of each chromosome• Contraction of the microtubule spindle fibres

cause the chromosomes to line up separately along the centre of the cell (equatorial plane)

• Anaphase• Continued contraction of the spindle fibres cause

the two sister chromatids to separate and move to the opposite poles of the cell

• Once the two chromatids in a single chromosome separate, each constitutes a chromosome in its own right

• Telophase• Once the two sets of identical chromosomes arrive at

the poles, the spindle fibres dissolve and a new nuclear membrane reforms around each set of chromosomes

• The chromosomes decondense and are no longer visible under a light microscope

• The division of the cell into two daughter cells (cytokinesis) occurs concurrently with telophase

2.5.5 Explain how mitosis produces two genetically identical nuclei.

• During interphase (the S phase) the DNA was replicated to produce two copies of genetic material

• These two identical DNA molecules are identified as sister chromatids and are held together by a single centromere

• During the events of mitosis (as described in 2.5.4), the sister chromatids are separated and drawn to opposite poles of the cell

• When the cell divides (cytokinesis), the two resulting nuclei will each contain one of each chromatid pair and thus be genetically identical

2.5.6 State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.

• Growth: Multicellular organisms increase their size by increasing their number of cells through mitosis

• Asexual reproduction: Certain eukaryotic organisms may reproduce asexually by mitosis (e.g. vegetative reproduction)

• Tissue Repair: Damaged tissue can recover by replacing dead or damaged cells

• Embryonic development: A fertilized egg (zygote) will undergo mitosis and differentiation in order to develop into an embryo