2A: Cell Structure and Division


Eukaryotic Cells and Organelles

Eukaryotic cells are complex (Animal, plant, algal, fungal). Prokaryotic cells are smaller and simpler. 

Animal cells: Cell-surface plasma membrane, lysosomes, rough endoplasmic reticulum, smooth endoplasmic reticulum, ribosomes, nucleus, nucleolus, nuclear envelope, golgi apparatus, cytoplasm, mitochondria

Plant cells: Same as animal but extra organelles: Cellulose cell wall with plasmodesmata ("channels" for exchanging substances between adjacent cells), vacuole, chloroplasts

Algal cells: Same as organelles as plants. Unlike plant, can be unicellular or multicellular. Chloroplasts are different sizes and shapes to plant chloroplasts.

Fungal cells: Also can be multicellular or unicellular. Same as plant, but cell walls made of chitin, and no chloroplasts because they don't need to photosynthesise.

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Functions of Organelles (1)

Cell-surface membrane: Found on surface of animal cells and inside the cell wall of other cells. Made of lipids and proteins. Regulates movement of substances in and out of cell. Has receptor molecules, allows it to respond to chemicals like hormones.

Nucleus: Large organelle surrounded by double membrane (contains many pores). Contains chromosomes called nucleolus. Controls cell activities. Contains instructions to make proteins. Pores allow substances to move between nucleus and cytoplasm. Nucleolus makes ribosomes.

Mitochondrion: Oval shaped, has double membrane, inner one is folded to form structures - cristae. Matrix inside, contains enzymes involved in respiration. Site of aerobic respiration. AR produces ATP (common energy source in cell). Mitochondria found in large numbers in cells that are very active and require a lot of energy.

Chloroplast: Small, flattened structure in plant cells and algal cells. Double membrane, has membranes inside - thylakoid membranes. These are stacked up to form grana. Grana linked by lamellae - thin, flat pieces of thylakoid membrane. Where photosynthesis takes place. Sometimes takes place in grana, sometimes in stroma (thick fluid found in chloroplasts)

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Functions of Organelles (2)

Golgi apparatus: Group of fluid-filled membrane-bound flattened sacs. Vesicles at edges of sacs. Processes and packages new lipids and proteins. Also makes lysosomes.

Golgi vesicle: Small fluid filled sac in cytoplasm, surround by membrane and produced by GA. Stores lipids and proteins made by GA, transports them out of cell via cell surface membrane.

Lysosome: Round organelle surrounded by membrane, no clear internal structure. Type of GV. Contains digestive enzymes called lysozymes, kept separate from cytoplasm by membrane. Can be used to digest invading cells or break down worn out components of cell.

Ribosome: Very small organelle, floats free in cytoplasm or is attached to RER. Made of proteins and RNA. Site where proteins are made

RER: System of membranes enclosing a fluid-filled space. Surface covered with ribosomes. Folds and processes proteins that have been made at the ribosomes.

SER: RER but no ribosomes. Synthesises and processes lipids

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Functions of Organelles (3)

Cell wall: Rigid structure that surrounds cells in plants, algae, fungi. Plants and algae made of carbohydrate cellulose. Fungi - chitin. Supports cells and prevents them from changing shape.

Cell vacuole: Membrane bound, found in cytoplasm. Contains cell sap (weak solution of sugars and salts). Surrounding membrane is tonoplast. Maintains pressure in cell and keep cell rigid. Stops plants wilting. Involved in isolation of unwanted chemicals inside the cell.

In multicellular eukaryotic organisms, cells become specialised to carry out specific functions. Cells structure helps it do that.

Epithelial cells: Adapted to absorb food efficiently. Walls have lots of villi, increase surface area for absorption. Epithelial cells on surface of villi have folds in their cell surface membranes - microvilli - increase surface area further. Lots of mitochondria provides energy for for the transport of digested food molecules into the cell.

Red blood cells: Adapted to carry oxygen around body. No nucleus to make room for haemoglobin.

Sperm cells: Contain a lot of mitochondria to provide large amounts of energy needed to propel themselves towards an egg.

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Cell Organisation

In multicellular eukaryotic organisms, specialised cells form tissues.

Tissues: Group of cells working together to perform a particular function. Form organs. Then these form organ system

Epithelial cells > Epithelial tissue, muscular tissue, glandular tissue > Stomach > Digestive system

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Prokaryotic Cell Structure

Cytoplasm: Contains ribosomes smaller than in eukaryotic

Cell-surface membrane: Made of lipids and proteins. Controls movements in and out of cell.

Cell Wall: Supports cell, prevents shape change. Made of polymer murein (glycoprotein, protein with carbohydrate attached)

Flagellum: Rotates to make cell move. Not all have. Some have more than one.

DNA: Floats free in cytoplasm. Circular, one coiled strand. Not attached to histone proteins.

Plasmid: Small loops of DNA. Contain genes for antibiotic resistance, can be passed between prokaryotes. Not always present. Some have several.

Capsule: Secreted slime. Helps protect from attack of immune system cells.

<2um in diameter (0.002mm). Eukaryotic up to 50x bigger

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Prokaryotic Cell Replication + Viruses

Binary fission: Replicates genetic material then physically splits into 2 daughter cells.

1: Circular DNA and plasmids replicate. Main DNA loop once, plasmids can be multiple times.

2: Cell gets bigger and DNA loops move to opposite poles of cell.

3: Cytoplasm begins to divide and new cell walls begin to form.

4: Cytoplasm divides and 2 daughter cells produced. Each has 1 copy of circular DNA, but can have variable number of copies of plasmids.

Viruses: Acellular. Nucleic acids surrounded by protein. Invade and reproduce inside cells of other organisms (host cells). Have protein coat (capsid), attachment proteins, core of RNA/DNA. Attachment proteins let virus cling to host cell. Smaller than bacteria. HIV is 0.1um. 

Replication: Virus attachment proteins bind to complementary receptor proteins on cell-surface membrane of host cells. DNA/RNA inserted into host cell. Uses cells own machinery (ribosomes, enzymes) to replicate viral particles. Different virus = different attachement proteins = requires different receptor proteins on host cell.

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Analysis of Cell Components

Magnification: How much the image is than the specimen. Mag = Size of Image/Size of Real Object. mm > um > nm = multiply/divide by 1000.

Resolution: How detailed image is. How well microscope distinguishes between 2 points close together.

Optical: Use light to form image. Max res 0.2um. Can't see ribosomes, ER, lysosomes. Can see mitochondria, nucleus. Max mag x1500.

Electron: Electrons form image. Higher res than optical. Max res 0.0002um, x1000 higher than optical. Max mag x1500000. Produce B&W image, coloured by computer. Expensive.

Transmission Electron Microscope (TEM): Electromagnets to focus beam of electrons, then transmitted through specimen. Denser parts absorb more electrons, looks darker on image. High res images, can see internal structure of chloroplasts i.e, but vacuum means specimens are dead. Can only be used on thin specimens.

Scanning Electron Microscopes (SEM): Scan beam of electrons across specimen. Knocks off electrons from specimen, gathered in cathode ray tube to form image. Shows surface of specimen, can be 3D. Can be used on thick specimens, but lower res than TEM.

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Preparing Microscope Slides

Look at specimen with optical microscope needs microscope slide. Done using temporary mount. Specimen is suspended in drop of liquid on slide.

1. Pipette small drop of water onto centre of slide.

2. Tweezers place thin section of specimen on top of water drop. Need to let light through to see clearly under microscope.

3. Add drop of stain, highlights objects in cell. Eosin for cytoplasm. Iodine for starch grains.

4. Stand cover slip upright on slide, next to water droplet. Tilt and lower so covers specimen. No air bubbles, obstruct view of specimen.

Artefacts: Things seen down microscope not part of cell or specimen. Dust, air bubbles, fingerprints, squashed/stained sample. Common in EM because specimens need lots of preparation. 1st scientists could only tell difference by repeatedly preparing specimens in different ways. If can be object seen in 1 prep technique but not another, more likely artefact.

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Cell Fractionation

Look at organelles under EM, need to separate first.

Homogenisation - break up cells: Vibrating cells or grinding in blender. Breaks plasma membrane, releases organelles in solution. Ice-cold reduces activity of enzymes that break down organelles. Isotonic: Same concentration of chemicals as cells broken down, prevent damage to organelles via osmosis. Buffer maintains pH.

Filtration - get rid of big bits: Filtered through gauze to separate cell/tissue debris from organelles. Organelles smaller than debris so pass through gauze.

Ultracentrifugation - separating the organelles: Cell fragments poured into tube. Put in centrifuge, spun at low speed. Heavy organelles get flung to bottom. Form thick sediment (pellet). Rest stay suspended in fluid above (supernatant).

Supernatant drained off, poured in tube, spun higher speed centrifuge. Heaviest organelles form pellet at bottom of tube. Supernatant containing rest of organelles is drained off and spun in centrifuge at higher speed.

Process repeated at higher and higher speeds, until all organelles sorted out. Nuclei, Chloroplasts, Mitochondria, Lysosomes, ER, Ribosomes.

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Cell cycle starts when cell is produced by cell division, ends with cell dividing to produce 2 identical cells. Is made of Interphase and Mitosis. Interphase split into G1, S, G2.

G1: Cell grows, new organelles and proteins made. S: Cell replicates DNA. G2: Cell keeps growing, proteins needed for cell division made. Interphase: DNA unravelled and replicated, double genetic content. Organelles replicated as spares. ATP content increased to provide energy. Chromosomes: 2 chromatids joined by centromere. 2 strands on same chromosome sister chromatids. 2 because made identical copy during interphase. Back to 1 after mitosis.

Prophase: C's condense, shorter and fatter. Bundles of protein (centrioles) move to opposite poles, form network of protein fibres called spindle. Nuclear envelope breaks down and C's lie free.

Metaphase: C's line up along middle, become attached to spindle by centromere.

Anaphase: Centromeres divide, separates each pair of sister chromatids. Spindles contract, pulling chromatids to opposite poles of spindle, centromere first. Chromatids V-shaped.

Telophase: Chromatids reach opposite poles on spindle. Uncoil, long and thin again. C's again. Nuclear envelope forms, 2 nuclei. Cytokinesis, 2 daughter cells genetically identical. Repeat.

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If mutation in gene that controls cell division, they can grow out of control. Keep dividing to make more and more cells, forms tumour. Cancer is tumour that invades surrounding tissue.

Treatment designed to control rate of cell division in tumour cells by disrupting cell cycle. Kills tumour cells. Don't distinguish tumour cells from normal cells, kill normal body cells dividing. Tumour cells divide more frequently than normal cells, so treatment more likely to kill.

G1: Chemotherapy prevents synthesis of enzymes needed for DNA replication. If not produced, cell unable to enter S phase, disrupting cycle, forces cell to kill itself.

S Phase; Radiation and some drugs damage DNA. At several points in cycle, DNA in cell is checked for damage. If severe DNA damage detected, cell kills itself - prevents further tumour growth.

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Investigating Mitosis - Required Practical 2

1. Add 1M HCl to boiling tube, enough to cover root tip. Put in water bath at 60C. 2. Cut 1cm from growing root tip with scalpel, as thats where growth occurs and mitosis takes place. 3. Transfer root tip into boiling tube of acid and incubate for 5mins. 4. Remove root tip using tweezers and rinse using pipette in cold water. Leave to dry on towel. 5. Place root tip on microscope slide, cut 2mm from tip. 6. Use mounted needle to break tip open and spread out cells thinly. 7. Add stain and leave for few minutes, makes easier to see chromosomes (toluidine blue O, ethano-orcein, feulgen stain needs extra rinse). 8. Cover slip over cells, put folded filter paper on top. Push down firmly to squash tissue, and allow light through. Dont smear cover slip sideways or might damage chromosome. 9. Look.

1. Clip slide on stage. 2. Select lowest powered objective lens. 3. Use course adjustment knob to bring up stage just below lens. 4. Look down eyepiece (ocular lens) Use course adjustment knob to move downwards until focussed. 5. Adjust with fine adjustment knob until clear image. 6. if need to see with greater magnification, swap to higher powered objective lens and refocus.

Mitotic Index = No. of cells with visible chromosomes/total no. of cells observed.

High could mean growing/tissue repair/cancerous growth

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Calculating Actual Size of Cells

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