AS BIOLOGY UNITS. Cell division is a fundamental process, necessary for reproduction, growth and repair.

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1 AS BIOLOGY UNITS Unit F211 Cells, Exchange and Transport Module 1 Cells Cells are the basic units of all living things. Organisms function because of communication and co-operation between specialised cells. Cell division is a fundamental process, necessary for reproduction, growth and repair Cell structure The cell is the basic unit of all living things. An understanding of how to use a light microscope is developed along with an understanding of why electron microscopes are so important in biology. Careful observation using microscopes reveals details of cell structure and ultrastructure and provides evidence to support hypotheses regarding the roles of cells and organelles. Students should be able to: (a) state the resolution and magnification that can be achieved by a light microscope, a transmission electron microscope and a scanning electron microscope: AS Biology p. 4-5; 8-9 MICROSCOPE TYPE MAXIMUM MAXIMUM MAGNIFICATION RESOLUTION LIGHT nm* TRANSMISSION ELECTRON MICROSCOPE (TEM) SCANNING ELECTRON MICROSCOPE (SEM) 500, nm 100, nm *1nm = mm (b) explain the difference between magnification and resolution: AS Biology p.4 Magnification: the degree to which the image of an object is larger than the object itself. Resolution : smallest distance between two points which can be seen using an optical instrument.

2 Limits to the resolution of optical microscopes are due to the long wavelength of light. Resolution is inversely proportional to wavelength. Wavelength of electrons c nm; wavelength of light c. 500 nm. (c) explain the need for staining samples for use in light microscopy and electron microscopy: AS Biology p.5,9 Staining is required to ensure that the details of the biological material are visible. Light microscopes: the material is stained with coloured chemicals that binds to chemicals in or on the specimen. Electron microscopes: the biological material is coated with, or impregnated with, electron-dense metals. (d) calculate the linear magnification of an image: AS Biology p.6-7 Actual size = image size/magnification Magnification = image size/actual size (e) describe and interpret drawings and photographs of eukaryotic cells as seen under an electron microscope and be able to recognise the following structures: nucleus, nucleolus, nuclear envelope, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, ribosomes, mitochondria, lysosomes, chloroplasts, plasma (cell surface) membrane, centrioles, flagella and cilia: AS Biology p

3 (f) outline the functions of the structures listed in (e): AS Biology p NUCLEUS ORGANELLE NUCLEOLUS NUCLEAR ENVELOPE ROUGH ENDOPLASMIC RETICULUM SMOOTH ENDOPLASMIC RETICULUM GOLGI APPARATUS MITOCHONDRIA CHLOROPLASTS LYSOSOMES RIBOSOMES CENTRIOLES CELL SURFACE MEMBRANE CILIA FLAGELLA FUNCTION Contains the cell s genetic material, which codes for the synthesis of proteins The site of the synthesis of ribosomes The membrane enclosing the nucleus Synthesis of proteins by attached ribosomes; transport of proteins for modification or secretion Synthesis and transport of lipids Modification and packaging of proteins into vesicles for storage or secretion The site of synthesis of ATP The site of photosynthesis, which manufactures of carbohydrates Contain LYSOZYME, which break down materials inside the cell Large RNA/protein complexes that synthesise polypeptides from amino acids, using the base sequence code of messenger RNA (mrna) Organise the spindle during cell division (not found in plant cells) Selectively permeable, regulating the transport of materials into and out of the cell Cell recognition and signalling Movement of liquids outside the cell Locomotion of the cell (g) outline the interrelationship between the organelles involved in the production and secretion of proteins (no detail of protein synthesis is required): AS Biology p.14 The coded information for making a protein is the base sequence of a specific gene This DNA base sequence is transcribed into a base sequence in messenger RNA (mrna) The mrna leaves the nucleus via nuclear pores The mrna binds to ribosomes, most of which are attached to RER The ribosome translates the mrna base sequence, assembling the protein from its constituent amino acids The protein molecules are enclosed in vesicles, which bud off the RER, and are transported to the Golgi apparatus In the Golgi apparatus, the protein may be modified; it is then packaged into vesicles for storage or secretion

4 (h) explain the importance of the cytoskeleton in providing mechanical strength to cells, aiding transport within cells and enabling cell movement: AS Biology p The cytoskeleton is the internal framework of a cell. It is made of microtubules of the protein tubulin. Some microtubules move chromosome during mitosis (the spindle) Some microtubules have other proteins associated, which move organelles and other cell contents along the fibres (i) compare and contrast, with the aid of diagrams and electron micrographs, the structure of prokaryotic cells and eukaryotic cells: AS Biology p Smaller: µm Prokaryotes Cell walls not cellulose or chitin; peptidoglycan etc. No membrane-bound organelles. Mesosomes etc. Circular DNA - no histones Cilia / flagella made of flagellin, no 9+2 structure Store glycogen and volutin Ribosomes 70S Very seldom multicellular Eukaryotes Larger : c. 20µm (mean) Cell wall cellulose (plants) or chitin (fungi). Membrane-bound organelles. No mesosomes Linear DNA with histones Cilia / flagella made of tubulin, 9+2 structure. Store lipids, starch (plants),glycogen (animals). Ribosomes 80S Many multicellular. (j) compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of plant cells and animal cells: AS Biology p.10

5 1.1.2 Cell membranes Membranes are a fundamental part of the cell. The structure of the cell surface membrane allows cells to communicate with each other. Understanding this ability to communicate is important as scientists increasingly make use of membrane-bound receptors as sites for the action of medicinal drugs. Understanding how different substances enter cells is also crucial to the development of mechanisms for the administration of drugs. Students should be able to: (a) outline the roles of membranes within cells and at the surface of cells; AS Biology p.16 As well as the cell surface membrane, membranes are also found in many organelles. Functions: Separate cell contents from the outside environment Separate cell contents from the cytoplasm Cell recognition and signalling Holding the components of some metabolic pathways in place Regulating the transport of materials in and out of cells (b) state that plasma (cell surface) membranes are partially permeable barriers; AS Biology p.17 Cell membranes are partially permeable: The phospholipid bilayer is a barrier to water and all water soluble molecules, but some of these are able to leak through, eg water, O 2, CO 2 Other chemicals have to pass through channels in order to penetrate the membrane (c) describe, with the aid of diagrams, the fluid mosaic model of membrane structure; AS Biology p The term fluid mosaic refers to the arrangement of the molecules within cell membranes. It is fluid because the component molecules are able to move within it, and mosaic because it is composed of more than one type of molecule.

6 The main features of the fluid mosaic model: A phospholipid bilayer forms the basic structure Protein molecules embedded in the phospholipid bilayer may be free, or bound to other components within the structure The phospholipids within the bilayer are able to move, and may flip between the layers (d) describe the roles of the components of the cell membrane; phospholipids, cholesterol, glycolipids, proteins and glycoproteins; AS Biology p Phospholipids: Comprised of glycerol, two fatty acids, and a phosphate The phosphate heads are hydrophilic, therefore located on the outside of the bilayer The hydrophobic fatty acid chains are located on the inside of the bilayer They form a barrier to water and water-soluble molecules Cholesterol: Molecules slot between fatty acid chains of phospholipids Provides mechanical stability to membrane Reduces permeability of membrane to water and water-soluble molecules Proteins: Channel proteins allow the movement of larger, hydrophilic molecules/ions eg glucose, Na +, through the membrane Carrier proteins actively transport substances across the membrane, using energy from ATP, eg the uptake of magnesium ions by roots of plants Enzymes and coenzymes are embedded in both cell surface and organelle membranes, many of which have an increased surface area eg adenyl cyclase in cell surface membranes, ATP synthase in mitochondrial membranes Receptors for hormones eg adrenaline Glycoproteins: Proteins with attached carbohydrate groups Adhesive filaments that bind cells together Hormone receptors Antigens in cell surface membranes Glycolipids: Lipids with attached carbohydrate groups

7 Hormone receptors Antigens in cell surface membranes (e) outline the effect of changing temperature on membrane structure and permeability; AS Biology p.19 As the temperature increases, the molecules in the membrane move faster, and the membranes become leaky, allowing the passage of chemicals through more easily. Organisms living in environments with temperature extremes have adapted the cholesterol content of their membranes to compensate. (f) explain the term cell signaling; AS Biology p Cell signalling: communication between cells, or within cells, involving chemicals. Examples: Hormones Cytokines Both of these types of signalling chemicals are known as ligands. (g) explain the role of membrane-bound receptors as sites where hormones and drugs can bind; Signal molecules fit into complementary receptors in the membranes or cytoplasm of target cells. This triggers a response in the target cells, usually the result of a complex series of chemical reactions within the cell, eg the binding of insulin to its receptor increases the permeability of the cell membrane to glucose, and the rate of conversion of glucose to glycogen Some medicinal drugs fit into receptors, blocking the effects of the ligand eg β-blockers prevent the sino-atrial note from increasing heart rate, so lowering blood pressure Viruses bind to receptors on the cell surface membrane in order to penetrate into the cell Some toxins also bind to receptors eg the Clostridium botulinum toxin binds to receptors in the membranes of muscle cells, preventing them contracting

8 (h) explain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis; AS Biology p Passive transport: Diffusion the movement of molecules from a region of high concentration to a region of low concentration, down a concentration gradient, eg the exchange of respiratory gases. Facilitated diffusion the movement of molecules down a concentration gradient, using channel or carrier proteins eg the uptake of glucose by liver cells. Active transport: The movement of molecules from a region of low concentration to a region of high concentration, against a concentration gradient, using the energy from ATP to drive protein pumps eg the Na + /K + pump in the membranes of neurones. Bulk transport: Endocytosis: The import into the cell of material from the outside, by the formation of vesicles budding in from the cell surface membrane. Phagocytosis the uptake of large lumps of material eg bacteria by phagocytes. Pinocytosis the uptake of droplets of liquid eg the products of digestion by the epithelial cells of villi. Exocytosis the fusion of vesicles with the cell surface membrane to excrete or secrete chemicals from the cell eg the secretion of insulin from the β cells in the Islets of Langerhans of the pancreas. (i) explain what is meant by osmosis, in terms of water potential. (No calculations of water potential will be required); AS Biology p Osmosis: A special case of diffusion. The movement of water molecules from a region of high water potential to a region of low water potential across a partially permeable membrane. Water potential (symbol ψ): The concentration of free water molecules ie water molecules that are able to diffuse. Pure water has a water potential of 0kPa. Any solute lowers the water potential of a solution ie makes it more negative.

9 Isotonic solutions that have the same ψ. Hypertonic a solution that has a lower ψ than a second solution. Hypotonic a solution that has a higher ψ than a second solution. (j) recognise and explain the effects that solutions of different water potentials can have upon plant and animal cells (HSW3). AS Biology p.27 Animal cells: In a hypotonic solution, animal cells will gain water by osmosis because the ψ of their cytoplasm is lower than that of the surrounding medium (ψ i <ψ e ); eventually, the cell will burst (lysis). In a hypertonic solution, animal cells will lose water by osmosis because the ψ of their cytoplasm is higher than that of the surrounding medium (ψ i >ψ e ); the cell will shrink (crenation). Plant cells: In a hypotonic solution, plant cells will gain water by osmosis because the ψ of their cytoplasm is lower than that of the surrounding medium (ψ i <ψ e ). This continues until the pressure of the cell wall prevents the further uptake of water, and the cell is described as turgid. In a hypertonic solution, plant cells will lose water by osmosis because the ψ of their cytoplasm is higher than that of the surrounding medium (ψ i >ψ e ); the cell s cytoplasm and vacuole shrink until the cell surface membrane pulls away from the cell wall (plasmolysis).

10 1.1.3 Cell division, cell diversity and cellular organisation During the cell cycle, genetic information is copied and passed to daughter cells. Microscopes can be used to view the different stages of the cycle. In multicellular organisms, stem cells are modified to produce many different types of specialised cell. Understanding how stems cells can be modified has huge potential in medicine. To understand how a whole organism functions, it is essential to understand the importance of cooperation between cells, tissues, organs and organ systems. Students should be able to: (a) state that mitosis occupies only a small percentage of the cell cycle and that the remaining percentage includes the copying and checking of genetic information; AS Biology p Cell cycle the sequence of events by which a parent cell divides to produce two daughter cells. The cell cycle can be as short as 30 minutes, or take several days. The cell cycle is divided into four stages: Interphase, during which the DNA replicates, and is checked for copying errors (gene mutations) Mitosis the nuclear division Cytokinesis the division (cleavage) of the cytoplasm A growth phase The nuclear division only occupies 5-10% of the entire cell cycle. (b) describe, with the aid of diagrams and photographs, the main stages of mitosis (behaviour of the chromosomes, nuclear envelope, cell membrane and centrioles); AS Biology p (Hint: the names of the stages can be remembered using the acronym IPMAT) Interphase: 90-95% of cycle organelle synthesis/replication DNA replication and histone synthesis Duplication of centrioles Increased ATP synthesis Cell growth

11 Prophase: Condensation of replicated chromosomes, each of which is comprised of two sister chromatids (the products of DNA replication) held together by the centromere Disappearance of nucleolus Centrioles move to opposite poles of cell Nuclear membrane breaks into vesicles and disperses through the cytoplasm Spindle forms from microtubules in the cytoplasm Metaphase: Chromosomes line up on equator of spindle Held in place by kinetochore microtubules radiating from the centromere Anaphase: Centromeres divide spindle fibres pull chromatids to opposite poles of spindle each chromatid now becomes a chromosome Telophase: Chromsomes decondense Spindle disintegrates Nuclear envelopes reforms around each pole, forming two new nuclei Nucleolus reappears Cytokinesis: Division of cytoplasm Organelles become evenly distributed Differs in plants and animals: Animals: Cell membrane invaginates, forming a cleavage furrow Cytoplasm pinches in two Plants: Spindle fibres remain across centre of cell Vesicles from Golgi coalesce to form the cell plate New cell wall is laid down inside the cell plate The membrane of the cell plate forms the new cell membrane

12 Differences between mitosis in animal and plant cells: ANIMALS centrioles no cell plate cleavage furrow most somatic cells (exceptions: muscle, nerve, red blood cells) PLANTS no centrioles cell plate no cleavage furrow usually only MERISTEMATIC CELLS (c) explain the meaning of the term homologous pair of chromosomes; AS Biology p.33, 261 Homologous pair of chromosomes: the paternal and maternal copies of each chromosome in a diploid organism homologous chromosomes carry the same genes in the same position, or locus (although not necessarily the same alleles) Diploid an organism or cell in which there are 2 copies of each chromosome, one from the mother, and one from the father (symbol 2n). Haploid an organism or cell in which there is only one copy of each chromosome (symbol n). (d) explain the significance of mitosis for growth, repair and asexual reproduction in plants and animals; AS Biology p.30 Mitosis produces cells that are genetically identical to the parent cell. Mitosis is used to produce more body (somatic) cells to allow growth, repair of damaged tissues, and replacement of dead cells. Unicellular eukaryotes (not prokaryotes) use mitosis to reproduce asexually by binary fission eg Amoeba or by budding eg yeast. Some multicellular organisms use mitosis to produce structures used for asexual reproduction, eg budding in Hydra, yeast, Bryophyllum, runners in strawberry and spider plants. (e) outline, with the aid of diagrams and photographs, the process of cell division by budding in yeast;

13 AS Biology p.33 Cytokinesis produces smaller cells, which bud off the parent cells. (f) state that cells produced as a result of meiosis are not genetically identical (details of meiosis are not required); AS Biology p.33 Meiosis: A type of cell division that produces four haploid cells from a diploid parent cell (germ cell). Used by organisms to produce gametes or spores (plants), therefore linked to sexual reproduction. During meiosis, the alleles on the homologous pairs of chromosomes are recombined, producing chromosomes that are different in allele sequence to those of the parent cell. The products of meiosis are genetically different to the parent cell, and to one another. An important source of genetic variation for populations. (g) define the term stem cell; AS Biology p.32 Stem cells: Undifferentiated cells potentially capable of becoming any of the different types of cells found in an organism. Found only in small numbers in adults, but found in large numbers in embryos and umbilical cord blood. Totipotent/omnipotent stem cells capable of differentiating into any kind of cell eg embryonic stem cells. Pluripotent stem cells capable of differentiating into a narrower range of cell types eg cord blood stem cells. (h) define the term differentiation, with reference to the production of erythrocytes (red blood cells) and neutrophils derived from stem cells in bone marrow, and the production of xylem vessels and phloem sieve tubes from cambium; AS Biology p.34-35; Differentiation - the changes occurring in the cells of a multicellular organism as they become specialised. Specialisation the acquisition of certain features that adapts the cell to perform a certain function within the organism.

14 Specialisation involves: Changes in the number of certain organelles Changes in the shape of the cell Changes in the contents of the cell often resulting from the switching on/off of specific genes within the differentiating cell. Red blood cells (erythrocytes): Produced from stem cells in the bone marrow Loss of the nucleus, mitochondria, Golgi apparatus, RER Become packed full of haemoglobin Assume a biconcave disc shape (large SA) Transport of respiratory gases Neutrophils: Produced from stem cells in the bone marrow Nucleus retained Cytoplasm appears granular because of presence of many lysosomes Ingest invading microorganisms by phagocytosis Xylem: Produced from cambium cells in vascular bundles Cell walls become impregnated with waterproof lignin Cell contents die, leaving a hollow tube with a wide lumen End walls break down Transport of water and minerals from roots to the rest of the plant Phloem: Produced from cambium cells in vascular bundles Sieve tube elements lose most of their cytoplasm and organelles Transport of glucose and amino acids from photosynthesising cells to the rest of the plant (i) describe and explain, with the aid of diagrams and photographs, how cells of multicellular organisms are specialised for particular functions, with reference to erythrocytes (red blood cells), neutrophils, epithelial cells, sperm cells, palisade cells, root hair cells and guard cells; AS Biology p Erythrocytes and neutrophils see (h) above Epithelial cells - form layers and linings; underlain by mesh of collagen and glycoprotein fibres called the basement membrane.

15 Squamous epithelium thin, flattened cells which often line tubes eg blood vessels, alveoli Ciliated epithelium column-shaped cells with cilia in their outer membrane line tubes such as trachea and bronchioles, oviducts and uterus often coated with mucus, which moved by beating cilia Sperm cells: Haploid Super-condensed DNA in very small nucleus Acrosome containing lysozyme for penetrating halo of cells around the egg, and the egg membrane Many mitochondria packed into the middle section provide ATP for movement of the flagellum Elongated flagellum provides propulsion Palisade cells: Elongated leaf cells located under the upper epidermis - provide maximum SA for absorbing light and CO 2 Packed with chloroplasts Root hair cells hair-like projection from their outer cell walls provides a large SA for absorbing water and minerals. Guard cells: Found in lower epidermis of leaves Contain chloroplasts Inner cell walls thickened - when the cells absorb water, they bend, opening the stomata between them (j) explain the meaning of the terms tissue, organ and organ system; AS Biology p.35 Tissue - a collection of similar cells that are specialised to perform a particular function eg ciliated epithelium, xylem. Organ collection of tissues working together to perform a function eg leaves, liver. Organ system made of of a number of organs working together to perform a function eg reproductive and excretory systems. (k) explain, with the aid of diagrams and photographs, how cells are organised into tissues, using squamous and ciliated epithelia, xylem and phloem as examples; AS Biology p See section (i) above.

16 (l) discuss the importance of cooperation between cells, tissues, organs and organ systems; Each cell needs to play its part in the body of a multicellular organism, so its response to internal and external stimuli must be coordinated with that of other cells to ensure that the growth, development and metabolic processes of the organism promotes its survival.

17 FURTHER READING: general Biological Sciences 1&2 (1997) D.J. Taylor, N.P.O. Green, G.W. Stout Cambridge University Press Chapter 5 p Cells Chapter 23 p Chromosomes and Mitosis Chapter 6 p Histology (tissues) Cell Biology and Biochemistry (2006) T. Greenwood, L. Shepherd, R. Allan, D. Butler Biozone Learning Media Section 2 p Cells and cellular organisation Section 3 p Membranes and transport Section 5 p Mitosis and cell differentiation OCR Biology AS (2008) T. Greenwood, K. Pryor, L. Bainbridge-Smith, R. Allan Biozone Learning Media Section 2 p Cell Structure Section 3 p Cell Membranes and Transport Section 3 p Cell Division and Organisation An Atlas of Histology (1978) W.H. Freeman, B. Bracegirdle Heineman Educational Books CELL STRUCTURE A Cloistered Existence (1991) S. Fry Biological Sciences Review September 1991 p.8-11 The Golgi Apparatus (1991) G. Warren Biological Sciences Review November 1991 p Cell Division, Cell Diversity and Cellular Organisation Understanding Division (1996) D. Gull Biological Sciences Review May 1996 Out of Control (1999) J. Itzhaki Biological Sciences Review January 1999 p To Divide or Not to Divide? (1999) P. Nurse Biological Sciences Review March 1999 p.2-5

18 EXAM QUESTIONS: cell structure membranes and transport June q.1 January q.1 January q.2 June q.3 June q.6 June q.3 January q.2 January q.1 January q.4 June q.1 June q.6 June q.1 June 2009 F211 q.2 January q.1 June 2010 F211 q.3 January q.1 January 2011 F211 q.6 January 2009 F211 q.1 June 2011 F211 q.2 January 2010 F211 q.3 June q.5 June 2010 F211 q.1 January 2012 F211 q.6 January 2011 F211 q.5 January 2012 F211 q.4 June 2012 F211 q.2 Mitosis, tissues and organs January q.1 mitosis June q.6 mitosis January q.5 mitosis and cancer June q.7 June q.1 tissues June q.2 mitosis January q.1 tissues June q.2 mitosis January q.5 cancer January q.3 stem cells June q.5 mitosis January 2010 F211 q.2 tissues June 2010 F211 q.4 tissues January 2011 F211 q.1 mitosis June 2011 F211 q. 4 mitosis January 2012 F211 q.2 levels of organisation June 2012 F211 q.1 mitosis

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