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Cells  [all] Cells [all] Presentation Transcript

  • Syllabus Requirements: 3.2 Cell structure and function 3.2.1 The cell as the basic unit of living things. Comparison of the principal features of prokaryotic and eukaryotic cells. The structure of a generalised plant and animal cell as revealed by both light and electron microscopy. Organelles should include the nucleus and nuclear envelope, nucleolus, centrioles, basal bodies, eukaryotic flagella (undulipodia), endoplasmic reticulum, ribosomes, Golgi apparatus, lysosomes, peroxisomes, mitochondria, chloroplasts and cytoskeleton.
  • The cell theory has three key points: Amoeba 1. All living things are made up of one or more cells. 2. Each cell is a basic living unit. 3. A new cell arises only from cells that already exist.
  • Cells can be cultured to produce more cells: In vivo In vitro / Ex-vivo – outside organism – inside organism Cell line: cells grown in tissue culture and representing generations of a primary culture.
  • Cells can be: Eukaryotic A eukaryotic cell is generally 10x greater Prokaryotic 10 m
  • i) Prokaryotic  very primitive  include the bacteria  lack:  a nucleus  membrane-enclosed internal compartments ii) Eukaryotic  more advanced  include protists, plants, fungi and animals
  • What about viruses? Viruses have no cellular structure but however, depend upon cells for reproduction Tobacco mosaic virus causes disease
  • All cells have 4 features in common: 1. Genetic material (DNA) 2. Cytoplasm:  a semifluid matrix containing organelles 3. Ribosomes  synthesise proteins 4. A plasma membrane DNA
  • The Kingdom Prokaryotae: Includes the bacteria  Diplo - a prefix used with the shape name to indicate pairing of cells.  Strepto - a prefix used with the shape name to indicate chains.  Staphylo - a prefix used with the shape name to indicate clusters
  • Prokaryote Cell Structure
  • Prokaryote Cell Structure
  • Cell wall  is strong and rigid due to peptidoglycan /murein
  • Peptidoglycan is:  unique to prokaryotes  made of polysaccharide chains cross-linked at regular intervals by short chains of amino acids
  • Functions of the cell wall: 1. Protects the cell 2. Maintains shape 3. Prevents the cell from bursting when the cell absorbs water
  • Two natural groups of bacteria  due to differences in their wall structure depending on whether they take up Gram’s stain: Gram positive [purple] Gram negative [red]
  • Gram positive bacteria  take up the stain  have a thick cell wall Gram negative bacteria  do not take up the stain  have a thin cell wall  a membrane covers peptidoglycan
  • Hans Christian Gram developed the stain in 1883
  • Gram positive bacteria:  e.g. Staphylococcus, Bacillus, Lactobacillus  attacked by:  penicillin  lysozyme (enzyme in tear fluid) Gram negative bacteria:  e.g. Salmonella, E. coli, Azotobacter  not attacked by penicillin & lysozyme
  • How does penicillin kill Gram positive bacteria? Penicillin:  interferes with the cross-linking in the peptidoglycan of growing cells  makes the walls:  weaker  more likely to burst when water enters by osmosis
  • Lysozyme attacks bonds in peptidoglycan wall
  • Explain why penicillin and lysozyme have no effect on gram-negative bacteria. Attacked by penicillin & lysozyme G+ G- Not attacked by penicillin & lysozyme The outer membrane protects bacteria from lysozyme and attack by penicillin
  • Penicillin has no effect on plant and animal cells. Why? No peptidoglycan wall present. Animal cell Plant cell
  • Plasma membrane in bacteria regulates movement of materials into and out of the cell
  • Cytoplasm is an aqueous substance containing  organelles  various substances
  • Cytoplasm & Cytosol compared consists of:  organelles  cytosol  makes up of about 70% of the cell volume  composed of water, salts & some macromolecules such as protein enzymes
  • Functions of infoldings of the cell membrane mesosomes aerobic respiration nitrogen fixation contain photosynthetic pigments e.g. bacteriochlorophyll No infoldings in eukaryotes.
  • Genetic material bacterial DNA is:  is a single circular molecule, 1mm long  usually coiled  attached to the plasma membrane
  • Bacterial DNA is:  concentrated in a region of the cell called nucleoid  not physically separated from the cytoplasm by a membrane Nucleoid
  • Give TWO advantages of the bacterial DNA being haploid: 1. More efficient; grows quicker 2. Mutations allow adaptation to environment quicker Haploid prokaryote Diploid eukaryote
  • How do bacteria reproduce? Asexually by Binary Fission
  • Ribosomes (70S) are the site of protein synthesis are complexes of :  RNA & proteins smaller than those of eukaryotes A svedberg unit (symbol S) is:  a unit for sedimentation rate  technically a measure of time, and is defined as exactly 10−13 seconds
  • Capsules  are slimy or gummy secretions of certain bacteria  function: 1. unite bacteria into colonies 2. offer protection against white blood cells 3. prevent cells from drying out Slime layer Electron micrograph of a colony of Staphylococcus aureus Rigid capsule
  • Spores some bacteria form endospores (spores produced inside cells) are:  thick- walled  long lived  resistant to particularly:  heat  drought  short-wave radiations
  • Flagella (singular flagellum) one or more are present for motility some are rigid, though shaped into a wave a single flagellum is a made of a protein called flagellin
  • Differences between a flagellum in prokaryotes and in eukaryotes: much simpler in structure not made of microtubules Flagellum in a prokaryote: rotates Flagellum in a eukaryote: beats
  • Flagella propel the cell along by:  rotating at the base, providing a corkscrew-like motion rather than a beat
  • Pili (singular pilus)  are fine protein rods:  shorter & thinner than flagella  project from the walls of Gram negative bacteria  are concerned with adherence Bacterial pili have long been recognised as mediators of initial host–pathogen interactions important for the progression of Gram-negative bacterial diseases.
  • Diagram shows how bacteria: with pili may resist being flushed out with urine without pili are flushed out with urine
  • Sex-pili  help bacteria join to each other to exchange genetic material  ‘sex’ in genetic sense NOT directly linked to reproduction as in most eukaryotes A pilus projecting from surface of a Gram negative bacterium Transfer of plasmid
  • Fimbriae similar to pili but shorter help cells to adhere to surfaces e.g. animal cells for: food protection
  • Plasmids  tiny circles of self-replicating DNA found in some species of bacteria in addition to the circular DNA  a bacterium may contain:  dozens or  hundreds of copies of a plasmid
  • A plasmid possess a few genes which give extra survival advantage plasmid genes are known to: 1. produce an enzyme which breaks down penicillin, thus bacterium is resistant to penicillin 2. confer resistance to disinfectants 3. confer ability to use complex chemicals as food, such as hydrocarbons, with potential applications in clearing oil spills
  • When a bacterium dies, the plasmids liberated into the environment may transform other bacteria Bacterial transformation is the process by which bacterial cells take up naked DNA molecules.
  • Transformation occurs: Naturally in some species of bacteria By artificial means in other cells [by genetic engineering]
  • QUESTION: [SEP, 2002] Which type of cell is evolutionarily more primitive, eukaryotic or prokaryotic? Briefly give reasons for your answer. (2) Prokaryotic. Lacks membrane-bound organelles. DNA is not wound around histones – not organised.
  • As size increases, SA:Vol ratio decreases
  • Why are cells small? because they need a high surface area-tovolume ratio. Volume determines the amount of chemical activity in the cell per unit time. Surface area determines the amount of substances that can pass the cell boundary per unit time.
  • Why do large organisms consist of many cells rather than one large cell?  exchange would be limited by the: 1. distance from the centre of the cell to its surface 2. surface area of the cell
  • Eukaryotic cells have membranebound organelles the evolution of compartments was an important development:  enabled eukaryotic cells to specialise, forming the organs and tissues of complex multicellular organisms
  • Compartamentalisation  makes it possible to have an environment within the organelle which is biochemically distinct from the cytoplasm  metabolic pathways & processes are contained  by having the enzymes & cofactors together:  it makes for a more energy efficient process  by keeping them within a membrane it can ensure that metabolic processes can occur safely that would otherwise be harmful or interfere with the activity within the cytosol
  • If there were no compartments the free-floating molecules would basically wander around the cell until they bumped into the right reactants works only for small cells as cells get bigger, the chance of two particular molecules meeting goes way down BUT
  • protoplasm = nucleus + cytoplasm nucleus cytoplasm Plasma membrane is a transparent semisolid or gelatinous fluid contains: 1. organelles 2. cytosol
  • Venn Diagrams Animal Cells Centrioles Lysosomes Plant Cells Cell membrane Ribosomes Nucleus Endoplasmic reticulum Golgi apparatus Vacuoles Mitochondria Cytoskeleton Peroxisomes Cell Wall Chloroplasts Central Vacuole
  • Animal Cell
  • Microvilli: only in animal cells are finger-like extensions of the cell surface membrane form a fringe called a brush border
  • Role of microvilli: increase the surface area by as much as 25 x absorption or secretion at base of microvillus microvillus
  • Microvilli can contract as they have actin filaments within them What is the importance of microvilli being able to contract? To facilitate absorption
  • An is: a specialised subunit within a cell that has a specific function, and it is usually separately enclosed although certain structures e.g. ribosomes do not have a membrane, they are still considered as organelles
  • Organelles with membranes 1. Nucleus 2. Endoplasmic reticulum 3. Golgi apparatus [Golgi complex] 4. Lysosomes 5. Mitochondria 6. Chloroplasts 7. Peroxisomes 8. Vacuoles Non-membranous organelles 1. Ribosomes 2. Cytoskeleton 3. Centrioles
  • QUESTION: [MAY, 2003] List three advantages of eukaryotic organisation over prokaryotic organisation. (3) a) Can reach larger sizes thus provides protection from predators; b) Division of labour (each organ has its own role and each cell can perform a different function) leads to greater efficiency; c) Allows compartamentalisation for example membranes surrounding different organelles allow each organelle to have its own set of chemicals and chemical reactions.
  • Organelles with membranes 1. Nucleus 2. 3. 4. 5. 6. 7. 8. Endoplasmic reticulum Golgi apparatus [Golgi complex] Lysosomes Mitochondria Chloroplasts Peroxisomes Vacuoles
  • Nucleus: largest organelle controls the activities of the cell as it contains DNA found in all eukaryotic cells  except: red blood cells phloem sieve element
  • The nucleus is surrounded by: a nuclear envelope  has pores to allow communication between the nucleus & the cytoplasm
  • The outer membrane of the nuclear envelope is continuous with: the endoplasmic reticulum (ER) may be covered with ribosomes
  • The nucleus contains: NUCLEUS chromatin nucleolus [one or more nucleoli]
  • What does ‘chromatin’ mean? “coloured material” & refers to the fact that it is easily stained Neutrophils in a blood smear
  • What is chromatin composed of? nucleolus chromatin Chromatin is composed mainly of: 1. coils of DNA bound to 2. basic proteins called histones
  • DNA is wound around the histones which form: nucleosomes [bead-like structures]  are in turn regularly packed in the chromatin nucleosome How much DNA in each nucleus?
  • Chromatin organisation of eukaryotes
  •  stains lightly  located towards the centre of the nucleus  stains intensely  is seen as dark patches near the nuclear envelope
  • Euchromatin:  loosely coiled chromatin Heterochromatin:  tightly coiled chromatin
  • During nuclear division, chromatin stains more intensely becomes more conspicuous WHY? Because it condenses into more tightly coiled threads called chromosomes
  • Nucleolus appears as a rounded, darkly stained structure inside the nucleus manufactures ribosomal RNA (rRNA)  stains intensely as it contains large amounts of: DNA & RNA
  • Organelles with membranes 1. Nucleus 2. Endoplasmic reticulum 3. 4. 5. 6. 7. 8. Golgi apparatus [Golgi complex] Lysosomes Mitochondria Chloroplasts Peroxisomes Vacuoles
  • ENDOPLASMIC RETICULUM (ER) consists of flattened cavities – cisternae: made up of parallel membranes two types of ER: Rough ER lined with ribosomes Smooth ER no ribosomes
  • Role of the rough ER RER is concerned with the transport of proteins which are made by the ribosomes on its surface
  • Proteins: enter into the ER via a channel are transported through the cisternae are usually being modified en route
  • Proteins from the RER commonly travel to the Golgi apparatus:
  • Flow of proteins :
  • Proteins from the Golgi apparatus can be: 1. secreted from the cell 2. passed on to other organelles in the same cell
  • Smooth ER is made up of tubular cavities occupies a smaller area compared to the rough ER Rough ER Smooth ER [occurs away from the nucleus]
  • Functions of the smooth ER: 1. lipid synthesis  e.g. in the epithelium of the intestine, lipids are made from fatty acids & glycerol absorbed from the gut and passes them to the Golgi apparatus for export
  • Functions of the smooth ER: 2. makes steroids (a type of lipid)  some steroids are hormones e.g. testosterone
  • 3. site for the hydrolysis of glycogen 4. chemically modifies small molecules taken in by the cell  this is especially true for drugs and pesticides 5. proteins synthesised on the rough ER are chemically modified within the lumen of the smooth ER
  • Organelles with membranes 1. Nucleus 2. Endoplasmic reticulum 3. Golgi apparatus 4. 5. 6. 7. 8. Lysosomes Mitochondria Chloroplasts Peroxisomes Vacuoles
  • The Golgi apparatus consists of two components: Lumen 1. Cisternae 2. Golgi vesicles [stack of flattened, membranebound sacs]
  • Functions of the Golgi apparatus: 1) it receives proteins from the ER and chemically modifies them e.g.: carbohydrate is added to proteins to form glycoproteins the slime, wax, gum & mucilage secretions of many cells are released by the Golgi apparatus
  • 1) it receives proteins from the ER and chemically modifies them e.g. in hormone secretion
  • New cisternae are constantly formed at one end & of the Golgi
  • Functions of the Golgi apparatus: 2) proteins are concentrated, packaged, and sorted before being sent to their cellular or extracellular destinations 3) forms lysosomes
  • Organelles with membranes 1. Nucleus 2. Endoplasmic reticulum 3. Golgi apparatus [Golgi complex] 4. Lysosomes 5. 6. 7. 8. Mitochondria Chloroplasts Peroxisomes Vacuoles
  • Lysosomes originate from the Golgi apparatus: LYSOSOME Golgi apparatus A lysosome is surrounded by a single membrane
  • Lysosomes are: simple sacs that contain digestive enzymes such as: Proteases Nucleases Lipases
  • Enzymes in the lysosome:  carry out hydrolysis reactions (splitting by adding water)  work best in an acid environment
  • Question: SEP, 2006 Explain the following observations regarding cell organelles. The interior of a lysosome (pH 4.8) is more acidic than the surrounding cytosol (pH 7). (2) A lysosome is full of degrading enzymes whose optimum pH is acidic.
  • Four Functions of lysosomes: 1. phagocytosis 2. autophagy – unwanted structures within the cell are digested 3. release of enzymes outside the cell (exocytosis) 3 1 2
  • Four Functions of lysosomes: autophagy 2 phagocytosis 3 4 1 LYSOSOME Golgi apparatus exocytosis autolysis – self-digestion of a cell
  • Name process A and B. A: Phagocytosis B: Autophagy A B
  • Organelles with membranes 1. 2. 3. 4. Nucleus Endoplasmic reticulum Golgi apparatus [Golgi complex] Lysosomes 5. Mitochondria 6. Chloroplasts 7. Peroxisomes 8. Vacuoles
  • Mitochondria occur in all eukaryotic cells  large organelles with a double membrane  function: aerobic respiration crista matrix inner membrane is folded forming cristae inner outer membrane membrane
  • MITOCHONDRIA have :  DNA  70S ribosomes ATP synthase
  • Organelles with membranes 1. 2. 3. 4. 5. Nucleus Endoplasmic reticulum Golgi apparatus [Golgi complex] Lysosomes Mitochondria 6. Chloroplasts 7. Peroxisome 8. Vacuoles
  • CHLOROPLASTS contain chlorophyll and carotenoid pigments function: carry out photosynthesis
  • What is a ‘granum’ [plural: grana]? a stack of thylakoid membranes thylakoids : an internal membrane system consisting of flattened sacs granum
  • Grana under the electron microscope
  • Chloroplasts contain: DNA 70S ribosomes
  • QUESTION: [DEC, 1986] List four similarities between a mitochondrion and a chloroplast. (4) Both have: i) a double membrane ii) 70S ribosomes iii) their own DNA iv) electron transport chains / ATP synthase
  • Plastids have a double membrane occur only in plants Proplastids are simple, generally colorless undifferentiated plastids e.g. Amyloplasts store starch
  • Plastids can change from one type to another  Function: the site of manufacture & storage of important chemical compounds used by the cell
  • Various types of plastid in plants 1. Chloroplasts – for photosynthesis 2. Leucoplasts – colourless; for storage 3. Amyloplasts – contain starch 4. Chromoplasts – are red, orange or yellow plastids; in fruit & flowers
  • Organelles with membranes 1. 2. 3. 4. 5. Nucleus Endoplasmic reticulum Golgi apparatus [Golgi complex] Lysosomes Mitochondria 6. Chloroplasts 7. Peroxisome 8. Vacuoles
  • Peroxisomes: also called microbodies spherical organelles bounded by a single membrane contain catalase Peroxisomes in a liver cell.
  • What is the function of catalase? An enzyme that catalyses the decomposition of hydrogen peroxide to the harmless products water and oxygen
  • H2O2  is a by-product of certain reactions of metabolism e.g. lactic acid breakdown in liver cells Catalase in potato breaks H2O2.
  • Question: SEP, 2006 Explain the following observations regarding cell organelles. Peroxisomes contain oxidative enzymes. (2) Peroxisomes contain enzymes that degrade fatty acids and amino acids, producing hydrogen peroxide. They contain catalase which brings about oxidation reactions.
  • Organelles with membranes 1. 2. 3. 4. 5. Nucleus Endoplasmic reticulum Golgi apparatus [Golgi complex] Lysosomes Mitochondria 6. Chloroplasts 7. Peroxisome 8. Vacuoles
  • 8. VACUOLES a vacuole is a fluid-filled sac bounded by a single membrane animal cells contain:  relatively small vacuoles e.g.  food vacuoles  phagocytic vacuoles
  • Plant cells have:  a large central vacuole bounded by a membrane: tonoplast What is ‘cell sap’? The fluid present in plant cell vacuoles.
  • Cell sap is a concentrated solution of: mineral salts sugars organic acids oxygen carbon dioxide pigments some waste products
  • Organelles without a membrane 1. Ribosomes 2. Cytoskeleton 3. Centrioles
  • 1. RIBOSOMES  the sites of protein synthesis  each ribosome consists of two subunits:  a large  a small one
  • Ribosomes are made up of roughly equal amounts of: protein + rRNA (ribosomal RNA) rRNA is made in the nucleolus
  • A ribosome builds a protein
  • Two types of ribosomes: 70S in:  prokaryotes  mitochondria  chloroplasts 80S in:  eukaryotes
  • Ribosomes are located : Cell membrane bound to endoplasmic reticulum Microtubule Microfilament free in cytoplasm Mitochondrion
  • 2. The cytoskeleton
  • 2. The cytoskeleton is in the form of a network of fibres extending throughout the cytoplasm  was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton
  • Why don’t organelles collect at base of a cell by gravity? kept in place by the cytoskeleton
  • Functions of the cytoskeleton: 1. intracellular transport of organelles 2. establishing cell shape 3. providing mechanical strength 4. chromosome separation in mitosis and meiosis
  • The cytoskeleton is made up of three kinds of protein filaments: 1. Microfilament 2. Intermediate filament 3. Microtubule
  • Microfilaments: Structure:  thin filament made up of globular protein actin
  • Function of microfilaments:  play a major role in: • muscle contraction • cytoplasmic streaming movement in plants • amoeboid movement
  • Actin is required to split the cytoplasm
  • Intermediate filaments: Structure:  made up of the fibrous protein keratin  highly stable Function:  resist pulling  anchorage of: nucleus other organelles
  • Microtubules: Structure: straight, unbranched hollow cylinders, 25 nm wide & usually quite short in length made of the protein tubulin
  • Microtubules are constantly being built up & broken down
  • Functions of microtubules:  determine the overall shape of the cell  form mitotic spindle  intracellular transport [e.g. mitochondria & lysosomes]
  • 3. CENTRIOLES small hollow cylinders occur in pairs usually located at 90 to each other near the nuclear membrane Found in:  animal cells  most protists Absent in:  plant cells  fungi
  • Centrioles are composed of nine triplets of microtubules arranged in a 9+0 arrangement microtubules triplet
  • What is the ‘centrosome’?  the region surrounding the pair of centrioles in all animal cells No centrosomes in plant cells
  • Function of centrioles: help organise the spindle fibres spindle itself is made of microtubules
  • Centrioles :  replicate themselves at the beginning of nuclear division  the two pairs migrate to opposite poles of the spindle  spindle: the structure on which the chromosomes line up
  • Label peroxisome smooth ER plasma membrane ribosome mitochondrion rough ER nucleolus cytoplasm nuclear membrane
  • Flagella & cilia: whiplike structures  push or pull the cell through its aquatic environment e.g. cilia move a Paramecium; human sperm moves by a flagellum  move surrounding liquid over the surface of the cell e.g. cilia move mucus in trachea; flagella in sponges beat to create a water current for respiration sponge
  • Direction of swimming (a) Motion of flagella: snake-like 5 µm Direction of organism’s movement Power stroke (b) Motion of cilia Recovery stroke 15 µm
  • Cilia in Paramecium
  • Differences between cilia & flagella CILIA FLAGELLA Short Longer Usually many present Usually one or two present Move with stiff power Movement is snakestroke and flexible like recovery stroke
  • Flagella & cilia are enclosed by a plasma membrane: made of microtubules in “9 + 2” array Axoneme: the central strand of a cilium or flagellum, composed of an array of microtubules, typically in 9+2 arrangement Basal body / kinetosome: Connects cilium or flagellum just below the plasma membrane
  • Model of axoneme showing microtubules in “9 + 2” array
  • Model of axoneme The outer nine sets are often referred to as doublet microtubules.
  • Microtubules in the axoneme & basal body 9+2 array of microtubules Outer microtubule doublet Central microtubules Cross section of cilium Longitudinal section of cilium Cross section of basal body Triplet 9+0 array of microtubules
  • The basal body: is derived from a centriole controls the direction of the movement of cilia Axoneme Basal body
  • What microtubules are found in the basal body? the nine microtubule doublets [9+0]  each doublet is accompanied by another microtubule, making nine sets of three microtubules  the central, unfused microtubules do not extend into the basal body
  • Microtubule doublets in cilia & flagella are linked by proteins Nexin Inner-arm dynein Outer-arm dynein
  • Roles of nexin & dynein: Nexin: inter-doublet linkage that prevents microtubules in the outer layer of axonemes from movement with respect to each other. Dynein arms: are motor complexes which produce the force needed for bending.
  • What is the ‘radial spoke’?
  • The radial spoke is another protein complex:  the radial spoke projects from each set of outer doublets toward the central microtubules  thought to be important in regulating the motion of the axoneme Radial spokes
  • What causes the motion of cilia and flagella? Motion results from the sliding of the microtubules past each other driven by a motor protein called dynein which can undergo changes in its shape driven by energy from ATP Nexin cross-links the doublets preventing them from sliding: thus cilium bends
  • Question: [MAY, 2005] 1. What is a flagellum? (1) A flagellum is a whip-like organelle that pulls or pushes the cell through its aqueous environment.
  • Question: [MAY, 2005] 2. Briefly describe the structure of a eukaryotic flagellum. (3) A eukaryotic flagellum is surrounded by the plasma membrane and contains a 9+2 array of microtubules. Nine fused pairs of microtubules, called doublets, form an outer cylinder and one pair of unfused microtubules runs up the centre. A spoke radiates from one microtubule of each pair and connects the doublet at the centre of the structure. In the cytoplasm at the base of each flagellum is an organelle called a basal body. The nine microtubule doublets extend into the basal body. The protein called dynein is permanently attached to one microtubule and moves it with respect to a neighbouring one.
  • Role of dynein dynein molecules attached to one microtubule bind to a neighbouring microtubule as the dynein molecules change shape, they move the microtubule past its neighbour
  • Motor proteins drive vesicles along microtubules dynein & another motor protein, kinesin, are responsible for carrying protein-laden vesicles from one part of the cell to another
  • CELL WALLS: extracellular structures
  • A cell wall is found in: 1. plant cells 2. prokaryotes 3. fungi chemical composition is different is a relatively rigid wall:  surrounds the cell  is secreted by the living cell
  • Composition of cell wall:
  • Two types of cell wall laid down during cell division may be laid down later in life when cell expansion is complete
  • Primary & secondary cell walls
  • Thin primary wall: No support Thick secondary wall: Provides support In some cells, e.g. mesophyll cells, the primary wall remains the only wall
  • Middle lamella: holds adjacent cells together composed of sticky, gel-like magnesium & calcium pectate [pectins]
  • Plasmodesmata: cytoplasmic connections that form when the new wall is laid down
  • Pits form where cell wall is not thickened further
  • Composition of: Cellulose Pectates Hemicelluloses Often impregnated with other substances:  Lignin is deposited in wood cells  Suberin makes cells waterproof
  • Polysaccharides in primary cell wall: Cellulose
  • Cellulose is built from glucose 1 cellulose molecule = about 3000 glucose molecules
  • Microfibrils run in all directions allowing for considerable stretching during cell growth Microfibrils
  • Lignin replaces pectins in the secondary cell wall Lignin:  adds strength to cell walls  makes cell walls inflexible
  • Waxy suberin in cork tissue (tree bark) Lignin in wood
  • Lignin in secondary cell walls  is the main supporting material of trees  cements & anchors cellulose fibres together  acts as a very hard & rigid matrix, giving the cell wall extra tensile strength and particularly compressional strength which prevents buckling  protects the cells from physical and chemical damage
  • Three major roles of the cell wall Provides support for the cell and limits its volume by remaining rigid. Acts as a barrier to infection by fungi and other organisms that can cause plant diseases. It contributes to plant form by growing as plant cells expand.
  • Junctions between cells
  • What are ‘Junctions? structures that allow cells to connect together occur in multicellular organisms In plant cells Plasmodesmata (singular = plasmodesma) In animal cells Tight junctions Desmosomes Gap junctions
  • Plasmodesmata  are living connections between neighbouring plant cells which run through very fine pores in the walls
  • How do plasmodesmata make communication & coordination between plant cells easier? Molecules & ions do not have to cross a cell surface membrane
  • Junctions in animal cells
  • Tight junctions: are barriers that prevent or reduce fluid movements in the spaces between cells  e.g. in bladder prevent urine from leaking out  at the tight junction the outer parts of adjacent membranes are fused
  • Desmosomes  hold cells together e.g.epithelial cells  are equivalent to spot welding in metal engineering  dense fibrous material loops in and out of the desmosome region
  • Gap junctions: tiny open channels in the plasma membrane through which small molecules and ions may pass  occur in a wide variety of cells, including certain muscle and nerve cells
  • How did eukaryotic cells originate? eukaryotic cells appeared about 1.5 billion years ago Endosymbiosis theory explains how eukaryotes could evolve from prokaryotes
  • The Endosymbiotic Theory proposes that: some of today’s eukaryotic organelles evolved by a symbiosis arising between two cells that were each free-living
  • Symbiosis is a close relationship between organisms of different species that live together
  • A small prokaryote:was ingested but not digested divided at the same rate as the larger one successive generations continued to be inhabited by the smaller one this is called ENDOSYMBIOSIS – ‘living within’ another cell or organism
  • What happened to the cells engulfed? Became mitochondria & chloroplasts
  • What happened to most of the genes of the mitochondria over the billion and a half years in which they have existed as endosymbionts? Most of their genes have been transferred to the chromosomes of the host cells – but not all
  • Evidence supporting Endosymbiosis Theory Mitochondria and chloroplasts:  have two membranes  possess circular DNA  possess 70S ribosomes  are about the size of a prokaryotic cell  divide by binary fission as bacteria not mitosis
  • Essay Titles 1. Organelles in cells are regarded as analogous to organs in multicellular organisms. Comment on the validity of this statement. [MAY, 2004] 2. Are the cells of unicellular eukaryotes any different from those found in multicellular eukaryotes? Discuss. [SEP, 2004] 3. Compare and contrast the structure of prokaryotic and eukaryotic cells. [SEP, 2007]