cells

• eukaryotic cells:

  animal cells, plant cells, algal and fungal cells
• Plasma membrane:

  • Found on the surface of animal cells and inside cell walls of plants
  • Made of lipids and proteins
  • Regulates movement of substances in and out of the cell
  • Contains receptor molecules that respond to chemicals like hormones
• Nucleus:

  • Surrounded by a nuclear envelope (double membrane-bound)
  • Controls what goes in and out of the nucleus
  • Reactions take place within
  • Contains chromosomes, which are protein-bound histones and linear DNA
  • Nucleolus is a region within the nucleoplasm, creating ribosomal RNA and assembling ribosomes
• Mitochondrion:

  • Double membraned, with the inner membrane folded to form cristae
  • Cristae provide a large surface area for attachment of enzymes and proteins involved in respiration
  • Matrix contains enzymes, proteins, lipids, ribosomes, and DNA
  • Site of aerobic respiration, where ATP is produced
• Chloroplast:

  • Found in plant and algal cells
  • Surrounded by a double membrane
  • Contains thylakoid membranes stacked up to form grana
  • Grana are linked together by lamellae
  • Site of photosynthesis: grana for the light-dependent reaction and stroma for the light-independent reaction
• Golgi apparatus:

  • Fluid-filled, membrane-bound flattened sacs (cisternae)
  • Processes and packages new lipids and proteins, and makes lysosomes
  • Vesicles are often found at the edge of the Golgi apparatus
• Golgi vesicle:

  • Fluid-filled sac in the cytoplasm surrounded by a membrane
  • Produced by the Golgi apparatus
  • Stores lipids made by the Golgi apparatus and transports them out of the cell
• Lysosome:

  • Round organelle surrounded by a membrane
  • Contains digestive enzymes called lysozymes
  • Used to digest invading cells or break down worn-out components of cells
• Ribosomes:

  • Small organelles that float free in the cytoplasm or are attached to the rough endoplasmic reticulum
  • Made of proteins and RNA
  • Site of protein synthesis
• Rough endoplasmic reticulum (RER):

  • Membranes enclosing fluid-filled space, with ribosomes covering the surface
  • Folds and processes proteins made at ribosomes
• Smooth endoplasmic reticulum:

  • No ribosomes, similar structure to RER
  • Synthesizes and processes lipids
• Cell wall:

  • Rigid structure surrounding plants, algae, and fungi
  • In plants and algae, made of carbohydrate cellulose; in fungi, made of chitin
  • Supports cells and prevents them from changing shape
• Cell vacuole:

  • Membrane-bound organelle in plant cells containing cell sap
  • Maintains pressure inside the cell, keeping it rigid and preventing wilting
  • Involved in isolating unwanted chemicals inside cells
• multicellular eukaryotic organisms:

  • cell become specialised, help to carry out specific function
  • e.g. epithelial cells in small intestine specialised to absorb food efficiently
  • walls of small intestine have villi: increase sa for absorption
  • epithelial cells on surface of membrane have fold in cell-surface membrane called microvilli- which increase SA more
  • have lots of mitochondria- provides energy for transport of digested food molecules into cell
• eukaryotic:

  • specialised cells grouped to form tissues
  • tissues are groups in cells working together perform particular function
  • different tissue work together to form organs
  • different organs make organ system
• Prokaryotic cell structure:

  • Cytoplasm: no membrane-bound organelles, has ribosomes smaller than eukaryotic cells
  • Plasma membrane: made of lipids and proteins, controls movement of substances in and out of the cell
  • Cell wall: supports cell, prevents change of shape, made of polymer murein (glycoprotein)
  • Some have a capsule made of slime that helps protect from attacks by cells of the immune system
  • Plasmids: small loops of DNA containing genes for antibiotic resistance, can be passed between prokaryotes, not always present
  • Circular DNA floats freely in the cytoplasm
  • Flagellum: rotates to make prokaryotic cells move, not all cells have flagella, some have more than one
• viruses:

  • acellular
  • nucleic acids, surrounded by protein
  • smaller than bacteria
  • no plasma membrne, no cytoplasm, no ribosomes
  • all virus invade and reproduce inside cells of other organisms. cells are called host cells
  • structure:
  • contain core of genetic material (DNA/ RNA)
  • protein coat around core called caspid
  • attachment proteins stick out from edge of caspid. let virus cling onto suitable host cell
• prokaryotic cells replication:

  • by binary fission

  1. circular DNA and plasmid replicat, main DNA loop only replicated once plasmids replicated many times
  2. cells get bigger and DNA loops move to opposite ends of cells
  3. cytoplasm begin to divide, new cell walls begin forming
  4. cytoplasm divide and two daughter cells produced, each daughter cell has a copy of circular DNA, but can have many plasmids
• viruses replication

  • use attachment proteins to bind to complementary receptor proteins on host cells
  • different viruses have different attachent proteins and require differeny receptor cells on host cells, can only infect one type of cell
  • not alive, don't undergo cell division, injects DNA/ RNA into host cell- enzymes, ribosomes in cell replicate viral particles
• Microscopy:

  produce magnified image of an object

  long wavelength of light rays so can distinguish between two objects

  limitation: is it can only distinguish two objects in a short distance, which can overcome by beam of electrons

  electrons have shorter wavelength and can distinguish two objects
• Magnification:

  how much bigger image is than specimen

  equation: magnification=size of image/ real size of object
• resolution:

  the ability to distinguish two objects that are close together from two objects that are far apart
• optical microscopes:

  • use light to form an image of a specimen
  • max resolution of 0.2 micrometres
  • can't view smaller organelles: ribosomes, ER, lysosomes
  • can see bigger organelles: nucleus, and mitochondria (not in clear detail)
  • have max magnification of x1500
• electron microscopes

  • have a higher magnification than light microscopes and can see structures that are too small to be seen with light microscopes
  • electron beam of short wavelength and can resolve objects well- high resolving power
  • electrons are negatively charged beam can be focused using electromagnets
• electron microscopes are either scanning or transmission
• transmission electron microscopes (TEMs)

  • use electromagnets to focus a beam of electrons, which is transmitted through specimen
  • denser parts of specimen absorb more electrons, makes them look darker on final image
  • give high resolution, can see internal structure of organelles, like chloroplasts
  • only used on thin specimens
  • whole system must be in a vacuum, living specimen can not be examined
  • staining process needed image produced not in colour
• scanning electron microscope (SEMs):

  • scan a beam of electrons across specimen, knocks off electrons from specimen, gathered in a cathode ray tube and form an image
  • images show the surface of the specimen and can be 3D
  • are lower resolution than TEMs
• cell fractionation: separates cell components into different fractions based on size and density

  • to study function of organelles, need to be isolated
  • cell fractionation breaks cells and then organelles can be separated
  • before cell fractionation, tissue needs to be placed into:
  • cold solution: to reduce enzyme activity that may break down organelles
  • same water potential as tissue: prevent organelles bursting or shrinking due to osmosis
  • buffered: pH will not change, pH change could change structure of organelles altering structure of organelles or affect function of enzymes
• Homogenation

  Cells are broken down in a homogeniser (blender), releasing organelles from the cell
• Filtration


  Homogenate is filtered through gauze, removing whole cells and large pieces of debris
• Ultracentrifugation


  Fragments in the filtered homogenate are separated by centrifuge
• Cell fractionation

  1. Homogenation

  2. Filtration

  3. Ultracentrifugation
• Ultracentrifugation

  1. Cell fragments are poured into a tube, placed into a centrifuge and spun at a slow speed

  2. Heaviest organelles, like nuclei, are forced to the bottom of the tube, forming a thin sediment or pellet

  3. Fluid at the top of the tube (supernatant) is removed, leaving the sediment of nuclei

  4. Supernatant is transferred to another tube and spun in the centrifuge at a faster speed

  5. Next heaviest organelles, like mitochondria, are forced to the bottom of the tube, forming another pellet
• Order of separation

  • Nuclei
  • Chloroplast (if plant tissue)
  • Mitochondria
  • Lysosomes
  • Endoplasmic reticulum
  • Ribosomes
• Specialised cells

  • Root hair cells
  • Epithelial cells in small intestine
  • Kidney cells
  • Neurones & muscle cells
• Root hair cells

  • Shape to increase surface area rate of water uptake
  • Thinner walls
  • Permanent vacuole: cell sap, more concentrated than soil, ensures high water potential gradient maintained
• Epithelial cells in small intestine

  • Microvilli increase cell surface area
  • Villi has constant blood supply, constant transport of products of digestion, maintains high concentration gradient across epithelial cell exchange surface
• Kidney cells

  • Many aquaporins allow facilitated diffusion through cell membrane
  • Can reabsorb water, stop it from being excreted
• Neurones & muscle cells

  • Cell membrane contain channel proteins (NA+, K+, Ca+)
  • Opens and closes play important role in speed of electrical impulse, along membrane of neurones and in muscle cells