A2.2 Cell Structure PDF

' Guiding questions What are the features common to all cells and the features that differ? How is microscopy used to investigate cell structure? This chapter cov...

' Guiding questions What are the features common to all cells and the features that differ? How is microscopy used to investigate cell structure? This chapter covers the fo llowing syllabus content: ► A2.2.1 Cells as the basic structural unit of all living organisms ► A2.2.2 Mfcroscopy skills ► A2.2.3 Developments in microscopy ► A2.2.4 Structures common to cells in all living organisms ► A2.2.5 Prokaryote cell structure ► A2.2.6 Eukaryote cell structure ► A2.2.7 Processes of life in unicellular organisms ► A2.2.8 Differences in eukaryotic cell structure between animals, fungi and plants ► A2.2.9 Atypical cell structure in eukaryotes ► A2.2.10 Cell types and cell structures viewed in light and electron micrographs ► A2.2.11 Drawing and annotation based on electron micrographs ► A2.2.12 Origin of eukaryotic cells by endosymbiosis (HL only) ► A2.2.13 Cell differentiation as the process for developing specialized tissues in multicellular organisms (H L only) ► A2.2.14 Evolution of multicell ularity (HL only) Introduction to cells {e Common The cell is the basic structural unit of all living organisms - it is the smallest part of an organism that ,,ye can say is alive. lt is cells that carry out the essential processes of life. We chink of then, as self- mistake contained units of structure and function. Students sometimes Cells are extremely small - most are only visible as distinct structures \,vhen \~,e use a 1nicroscope use t he terms 'eel I' and (although a fe1.v types of cell are just large enough to be seen by the naked eye). 'ti'.>sue' as if t hey are synonymous (i.e. are Observations of cells ,vere first reported over 300 years ago, following the early development of the same thing) - this microscopes. You ,nay have already used a light microscope to vie\v living cells, such as the single- is not the case. Cells celled organism Arnoeba, shown in Figure A2.2,l. are the basic structural unlt of all living organisms, whereas All living organisms are made from cells. Some organisms are made of single cells (such t issues are a collection as protists and bacteria) and others are multicellular (animals, plants and most fungi). of cells of similar In multicellular organisms, cells are the building blocks for tissues. structure and fu nction. A2.2 Cell structure 49  Chlamydamanas - a motile, unicellular alga Amoeba -a protozoan of freshwater habl1ats of fresh water habitats rlch in ammonium ions cytoplasm endoplasm clear ectoplasm pseudopodia plasma ' < - - - flagella - --,/ contracti le membrane vacuole cytoplasm - - ~ food vacuoles length 400 µm llgh1-sens1t1ve --,'-#--,""1 spot Escherichia coli - a bacterium found in the intestines of animals, e.g. humans 0 -+-- ~>---- nucleus cell wall 0 0 (oligosaccharides plasma cytoplasm plasmid + am11;0 acids) membrane. ·. o.·... length 30µm. ' 0.. 0 ·.. 0 : p1li circular Figure A2.2.1 Introducing unicellular organiz-ation DNA ribosomes length 2.0 µm ♦ Unicellular: consisting Unicellular and multicellular organisms ot a sing le cell (e.g. Sotne organis1ns are made of a single cell and are kno,vn as unicellular. Examples of unicellular prokaryotes, protists and some fungi), organisn,s are introduced in Figure A2.2.l. There are vast numbers of different unicellular organis1ns ♦ Protists: eukaryotes in the living ,vorlcl, many \Vith very long evolucionary histories. One cype of unicellular organism is in consisting of single-celled a kingdom called the Protoctista (such as Chlan1ydon1onas andA1noeba in FigureA2.2.l - organisn1s in organisms. this kingdom are referred LO as protists) and another are the bacteria (Escherichia coli, Figure A2.2.l). ♦ Multicellular: Other organisms are made of many cells and are known as multicellular organisms. Examples or consisting of many cells (e.g. animals, plants and 1nulticellular organisms are 1nammals and flov. ering plants, 1 most fungi). Tnn 1- nl ,. Much of the biology in this book 1s about multicellular organisms and the processes that go on in these organisms. But remember, single-celled organisms carry out all the essential functions of life too, within a single cell. Cell theory All organ isms are cornposed of one or n1ore cells. Cell theory includes the idea that cells are the u11it of structure and function in living organisms. The cell theory states that: cells can only arise from pre-existing cells living organisms are composed of cells, ,vhich are Lhe smallest unit of life organis1ns consisting of only one cell carry out all functions of life in that cell; cells perform life functions at some point in their existence. AlthoL1gh most organisms conform to cell theory, there are exceptions (see page 78). "'~ Many biologists contributed ro the development of the cell theory. This concept evolved gradually ~~,, in ,.vestern Europe during the nineteenth century because of the steadily accelerating pace of developments in microscopy and biochemistry. Theme A: Unity and diversity- Cells  Table A2.2.1 Units of length used 1n microscopy Cell size 1 metre (m) = Since cells are so smaU, \Ve need appropriate units to n1easure then,. The metre {syrnbol 111) is the 1000 millimetres (mm) standard unit of length used in science (ir is an inrernationally agreed uni1, or 51 llnit) 1 mm (10-3 m) = Looi, at Table A2.2.l , showing the subdivisions of the n,etre that are used to ,neasurt: cells and their contents. 1000 micrometres (µm) (or microns) These units are listed in descending order of size. You ,1/ill see that each subdivision is one 1 µm (1 0-6 m) = thousandth of rhe unit above il The smallest units are probably quite ne\v to you; Lhey may tal,l,,_~;r,,.. endoplasmic reticulum........ (RER) with.... ribosomes H-+-- cell surface attached.... membrane... - l---\:------ r----... cell surface --\.... 1-1--- cellulose membrane '--------/:==f=:::!F====i======:;::: cell wall ♦ Nuclear envelope: a chromatin pe11rianen1 vacuole lipid bilayer surrounding temporary vacuoles ,nuclear envelop~~---'n-'u""cl..c.eo"-l""'u~ formed by intucking the genetic material ol plasma membrane nucleus of the cell, containing nuclear pores that Figure A2.2.18 The u ltrastructure of the eukaryotic animal and plant ce ll control the movement of molecules between the We next consider the structure and function of the organelles. inside of the nucleus and Our kno,vledge of organelles has been built up by e,-xatnining electron 1nicrographs ol many different the cytoplasm. cells. The outcome, a detailed picture of the ultrastructure of animal and plant cells, is represented ♦ Nuclear pore: a protein-lined channel in the diagrammatically in a generalized cell in Figure A2.2.18. nuclear envelope; regulates the transport of molecules Introducing the organelles present in all eukaryotic cells between the nucleus and the cytoplasm. Nucleus ♦ Chromosome: length The appearance of the nucleus in electron micrographs is shov,rn in Figure A2.2.13 (page 62). of DNA that carries The nucleus is the largest organelle in the eukaryotic cell, typically 10-20µm in dian1eter. It is specific genes in a linear surrounded by a double-layered 1ne1nbrane, the nuclear envelope. This contains many nuclear sequence. pores formed by specific proteins. These pores are tiny, about 100nm in dia111eter. However, the ♦ Chromatin: nuclear material comprised pores are so numerous that they n1ake up about one-third of the nuclear 1nen1brane's surface area. of DNA and histone This suggests that communications bet\veen nucleus and cytoplasm are i111portant. proteins in the nucleus The nucleus contains the chromosomes. -rhese thread-like structures are visible at the ti1ne the of eukaryotic cells at interphase; forms into nucleus divides (page 651). Chron1osomes are made of DNA bound to histone proteins (page 26). chromosomes during Histones help to pack the DNA into condensed structures so chat they can be moved within the cell mitosis and meiosis. during division. At other times, the chro1nosomes appear as a diffuse network called chromatin. Theme A: Unity and diversity- Cells (e Common (e Common mistake mistake A common misconception is to regard a chromosome solely as a condensed structure, visible Do not confuse the with a light microscope, implying that chromosomes are not present unless a cell is carrying out terms 'nucleus' and nuclear division. Chromosomes are continuously present in the nucleus of a eukaryotic cell; the 'nucleolus'. The uncondensed state should be seen as normal as this form persists longest and is when the genes nucleolus is a sub- can be transcribed into proteins. organelle located inside the nucleus, A nucleolus (plural , nucleoli) is a tiny, rounded, darkly-staining body. One or more nucleoli are whereas the nucleus present in the nucleus. It is the site ,~,here the sub-u nits of ribosomes (see belo,v) are synthesized. is a membrane-bound Chromatin, chro1noson1es and the nucleolus are visible only if stained with certain dyes. The organelle in the cell. everyday role of the nucleus in cell management, and irs behaviour ,vhen the cell divides, are the The nucleolus produces subject of Chapter D2.1 (page 648). ribosomes. and t he ~1ost cells contain one nucleus but there are interesting exceptions. For example, both the red blood nucleus contains the cells of mam1nals (page 78) and the sieve tube element of the phloem of flo,vering plants (page 79) genetic material of do not have a nucleus. Both lose their nucleus as they mature. the cell. Mitochondria ♦ Nucleolus: compact Mitochondria appear mostly as rod-shaped or cylindrical organelles in electron micrographs (Figure reg ion of nucleus A2.2.19). Occasionally their shape is 1nore variable. They are relatively large organelles, typically where ribosomes are 0.5- 1.5µm ,vide and 3.0-10.0µ m long. Mitochondria are found in all cells and are usually present synthesized. in very large nun1bers. Very metabolically active cells contain thousands of the1n in their cytoplas1n, ♦Mitochondrion [or example, muscle fibres and hormone-secreting cells. Human liver cells can individually have (plural, mitochondria): organelle in eukaryotic 2000 mitochondria. cells, site of Krebs The milochondrion also has a double membrane. The outer membrane is a smooth boundary, ,vhile cycle and the electron- the inner membrane is folded to forn1 cristae. The interior of the n1itochondrion contains an aqueous transport pathway. solution or metabolites and enzymes. This is called rhe matrix. The mitochondrion is rhe site o[ the ♦ Cristae: folds in the inner membrane of aerobic stages of respiration (see Chapter Cl.2, page 416). mitochondria. a mitochondrion, cut open ♦ Matrix: the fluid to show the inner membrane and cristae that is surrounded by the inner membrane of a mitochondrion, containing enzymes, inner membrane - - ~ ribosomes and matrix - - - -~ ~~-//; mitochondrial DNA. In the mitochondrion many of t he enzymes of respiration are housed, and the 'energy currency' molecules adenosine triphosphate (ATP) are formed. small subunit Figure A2.2.19 The mitochondrion Ribosomes Ribosomes are tiny structures, approximately 25 nm in diameter. They are built of t,vo subunits and do not have 1uembranes as part of their structures. Chen1ically, they consist of protein and the nucleic acid RNA. Riboso1nes are found free in the cytoplasm and bound to endoplasmic large subunit reticulum (rough endoplasmic reticulum - RER, see belo\v). They also occur ,~rithin milochonclria and in chloroplasts. The. sizes of tiny objects like ribosomes are recorded in Svedberg units (S). Figure A2.2.20 The ribosome This is a measure of their rate of sedimentation in centrifugation, rather than of their actual size. A2.2 Cell structure 69  Riboson1es of mitochondria and chloroplasts are slightly s1naller (705) than those iJ1 the rest of the 10 Explain why the cell (80S). As we have already seen, prokaryotes contain 705 riboso1nes (page 66). nucleus in a human cheek cell (Figure Ribosomes are the sites \1/here proteins are made in cells. The structure of a ribosome is sho\.vn in A2.2.15, page 64) figure A2.2.20. Many differen t types of cell contain vast numbers of ribosomes. Some of the cell may be viewed by proteins produced in the ribosomes have structural roles. Collagen is an example (Chapter Bl.2, light microscopy in page 219). Most cell proteins are enzymes. These are biological catalysts: they cause rhe reactions of an appropriately metabolism to occur more quickly under the conditions found \.vithin the cytoplasm. stained cell, but Endoplasmic reticulum the ribosomes The endoplasmic reticulum consists of a network of folded membranes formed into sheets, tubes or cannot. sacs that are extensively mterconnected. Endoplasmic reticulum is contiguous with (connected to) the membrane of the nuclear envelope (figure A2.1 21). The cytoplasm of metabolically active cells is commonly packed with endoplasmic reticulum. In figure A2.2.21 \1/e can see there are t\1/0 distinct types of endoplasmic reticulum. SER and RER in cytoplasm, showing origin from outer membrane of nucleus electron micrograph of RER ribosomes rough. -------- vesicles pinched off 1"" - endoplasmic reticulum 0 0..-------- with pro teins/enzymes for export from cell nucleus 0 ' 0 C) 0 0 '. 0 0 0. "!" ◄,-. ' 0 E 0 0 -... :::, :::, O 0 0 vesicles with...... Cl) 0 steroid hormones ·-E... 0 0 0 0 -"' Ill 0.. electron micrograph of SER ~-..;.,- 0 0 0 "t:I. 0 ~ C: Cl) I7nthetic pigmenrs such as carotene, and occurring in flower petals and the root tissue of space in the cytoplasm, carrots, and anthocyanin ,vhich shades and protects the photosynthetic apparatus by absorbing especially large and permanent in plant cells. excess visible and UV lighL ♦ Tonoplast: membrane Permanent vacuole around the plant cell vacuole. A vacuole is a fluid-filled space vvithin the cyroplasrn (see Figures A2.2.15 and A2.2.18, pages 64 and 68), surrounded by a single membrane (called a tonoplast). The tonoplasr separates rhe contents of the vacuole from the cell's cytoplasm. The main Liolc: functions of vacuoles include maintaining cell turgor pressure For more on photosynthesis, and regulating waste. As the vacuole fills \Vith \Vater, it pushes see Chapter C1.3, the cytoplasm against the cell wall, creating turgor pressure page 425. Figure A2.2.29 The chloroplast in the cells which maintains the rigidity of plant tissue. Theme A: Unity and diversity- Cells  Ions in the sap ,vithin the vacuole provide the negative 0s1notic pressure (or potential) thal d 1-a,vs ,.vater 15 Discuss, in tabular tnoleculcs into the vacuole (see Chapter D2.3, page 692) Che1nicals can be pennanently stored in form, the location the vacuole, such as pigments (for exa111ple, bccalains, the red pigment in beetroot} Carbohydrates, of membranes proteins and fats arc stored in the vacuoles of storage cells in seeds. In addition, the vacuole functions and t heir function in a sin1ilar way to lysosomes in animal cells - ,.vaste material is n1oved there for disposal. 1-he within plant and environment inside a vacuole is slightly acid (about pl-l 5.0), allowing hydrolase enzymes to break animal cells. do~vn large molecules. The products o[ the breakdo,.vn process are retained ,vi thin the vacuole. 16 Outline the functions of the cell Tnn tinl wall in a plant cell. Plant cells frequently have a large, permanent vacuole present. By contrast, animal cells may have 17 Describe how a small vacuoles, but these are mostly temporary. typical plant cell differs from a typical animal cell. Extracellular components We have noted that the contents of cells are contamed ,1/lch1n the plastna membrane. 1-:lovvever, cells may secrete n1aterial outside the plas1na membrane; for exatnple, plant cells have an external ,vall, (e Common ,vhile n1any animal cells secrete glycoproteins. mistake The cell wall Cell walls are not only The plant cell differs from an animal cell in char it IS surrounded by a \Vall. This \.vaU is completely found in plant cells - external to the cell; it is not an organelle. Plant cell walls are prin1arily constructed of cellulose - a prokaryote cell walls polysaccharide and an extremely strong material. Cellulose n1olecules are very long and are an·anged exist as well, and cell in bundles called microfibrils (discussed in Chapter Bl.I, page 193). walls are also part of fu ngal cell structure. Cell ~valls 111ake the boundaries of plant cells easy to see ,vhen plant tissues are exa111ined by n1icroscopy. The presence of this strong structure allo,.vs the plant cell to develop high internal pressure due to 'Nater uptake, \Vithout danger of the cell bursting. This is a n1ajor difference between ♦ Glycoprotein: membrane protein with the cell water relations of plants and ani111als. a carbohydrate chain Cell \Valls are also found in fungi (see more belo1v) and prokaryotic cells, alLhough Lhe chemicals attached. used in Lhe consLruction of Lhe 1valls are cliITerenL in each case. cell wall Extracellular glycoproteins around animal cells (made of chi tin)-..,...;:;;;;;:;;;;~~::;:: Nlany ani.J11al cells can adhere to one other. Th.is property enables cells to form compact tissues and organs. Other anin1al cells occur in sin1ple sheets or layers, attached to a basement men1brane belo\v then,. These cases of adhesion are brought cytoplasm +H-- about by glycoproteins that the cells have secreted. Glycoproteins are large molecules of protein to which a s1nall nun1ber of sugar 111olecules bonded together vacuole - ; + - - - - 1. - (called oligosaccharides) are attached by covalent bonding. Fungal cells Fungi are a large group or eukaryoLic organisms that obtatn their rood by absorbing nuLrients from their external environment. They cannot photosynthesize as they 1 µm do not contain chloroplasts. Fungal cells have a cell ,vall buL are made of a different Figure A2.2.30 Diagram of a yeast maLerial con1parecl lO plants and bacteria - chitin. The n1ajoriry are 1nulticellular, cell; yeast is a single-cel led fungus although some, such as yeasL, are unicellular. 18 Compare and contrast the The cell su·ucture of yeast is shovro in Figure l\2.2.30. structure of plant, animal and fungal cells. A2.2 Cell structure 77  c:: T,,.n fi I Some bacteria have flagella for movement, although the structure is different to those in animals. ♦ Multinucleate: a cell that has two or Atypical cell structure in eukaryotes more nuclei. ♦ Hypha: the tubular Although most organisms conform to the cell theory (page 50), there are exceptions. In addition to the filament 'plant' body familiar unicellular and multicellular organization of living things, there are a few multinucleate organs of a fungus, whfch in and organisms that are not divided into separate cells. These are known as atypical cells. An example of certain species is divided by cross walls into either an atypical organism is the 1nould Mucor, in ,vhich the main body of the organism consists of fine, multicellular or unicellular thread-like stn1crures called hyphae (Figure A2.2.31). An example of an atypical organ is the striped compartments. muscle fibres that 1nake up the skeletal n111Scles or mam1nals (Figure A2 2.32). fungal hyphae diagram of the cell structure at the tip of one hypha cell wall mitochondrion 11--- sporangiophore nucleus hyphae stolon vacuole ribosome cel l membrane Figure A2.2.31 Atypica l structure in Mucor skeletal musde cut to show the bundles of fibres Another example of atypical cell struc1ure i.s demonstrated by red QV bundle of thousands of muscle fibres blood cells (Figure A2.2.ll, page 60). These cells have no nucleus. Th·is adaptation allo\vs more roon, for haemoglobin - the red pig1nent that carries oxygen in the blood and gives the cell it.s reel colour. The lack of nucleus also creates a bicon