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Homework Help: (Biology) Cell Transport Methods & Mitosis in Animals

  1. Mar 20, 2008 #1
    Hi guys,

    Some background : I'm in the early stages of my first semester of High School Biology and I'm already having some problems coming to terms with the basics. Our original teacher was in car accident over break so we're stuck with a sub, who's a spanish teacher and completely hopeless. It wouldn't be that bad, but someone ordered the wrong textbooks so while we're waiting for the correct ones to arrive we've been using printouts from various sources and they're totally inadequate for what we're expected to do. </end inane rant>

    I've searched around and tried to fill in the gaps where I can before coming here (your search feature was very useful, thanks!), but after more time than I would have thought possible I decided it was time to swallow my pride and ask about the last of the things I don't understand. So I was hoping someone could look at a couple of things I still need to answer and walk me through them/correct my thinking where need be? Thanks **so** much for any help.

    + Match the following 25 substances with the correct transport method needed to get these molecules into a cell.
    2) glucose
    3) sucrose (against concentration gradient)
    8) calcium ion
    21) droplet of liquid

    The options left (provided I didn't screw up one of the others) are: -
    through a transport membrane
    through a transport membrane
    diffusion across the liquid bilayer
    by a carrier protein

    I currently have it as:
    glucose = diffusion across the liquid bilayer
    sucrose = through a transport membrane
    calcium ion = by a carrier protein
    droplet of liquid = through a transport membrane

    How far off am I?


    + The second is:
    Place the following events in animal cell replication in order: -
    a)Re-formation of nuclear envelope
    b)Duplication of centromere
    c)Separation of sister chromatids
    d)Elongation of the
    e)Decondenstion of chromosomes
    f) Attachment of microtubules to centromere region
    g)spindleCondensation of chromosomes
    h)Pinching of cell into two
    i)Alignment of chromosomes on the spindle equator
    j)Separation of centromeres
    k)Breakdown of nuclear envelope

    After digging around I ended up with this:
    →Condensation of chromosomes
    → Breakdown of nuclear envelope
    → Attachment of microtubules to centromere region
    → Alignment of chromosomes on the spindle equator
    → Separation of centromeres
    → Elongation of the spindle
    → Separation of sister chromatids
    → Decondenstion of chromosomes
    → Beginning of the re-formation of nuclear envelope
    → Pinching of cell into two

    The majority of this is just based off what I can gather from the Internet (after searching longer than I'd admit too) so as you can see, I seriously need help. I understand the basics of mitosis and the phases the cell goes through, but the majority of these specific events weren't even eluded to in the information we were given, like when does centromere duplicate? Is this done when the chromosomes start to replicate? If someone could explain when these events occur and how they react in relation to each other I would be so beyond grateful.

    ((I was unsure about putting the questions in seperate threads, sorry if having them in together was the wrong thing to do.))
  2. jcsd
  3. Mar 20, 2008 #2
    Anyhow, to the coursework help itself:

    The transport questions

    As you know, the cellular membrane in eukaryotes and bacteria consists of a phophoslipid bilayer. The bilayer has a polar-nonpolar-polar structure, and since "like dissolves like," the stable nonpolar layer in the middle functions as a solubility barrier that separates the internal polar aqueous environment, (a.k.a. intracellular environment, cytosolic environment, cytoplasmic environment) from the external polar, aqueous environment.

    Embedded within the membrane are proteins which function it is to moderate the aquisition of nutrients and the elimination of toxins which cannot pass through the lipid bilayer unaided.

    All traffic that goes into of the cell has to do one of the following things: a) pass through the membrane itself without assistance, b) pass through the membrane with the assistance of one of these special proteins, or c) the membrane itself has to envelop the particle, bud off from the rest of the cell surface, and travel to the interior of the cell, where the contents are subsequently degraded, a process known as endocytosis.

    a) passing through the membrane unaided

    In general, the things that pass through the membrane itself are all small, nonpolar, and charge-neutral. These include gasses such as molecular oxygen (O2), molecular nitrogen (N2), and a variety of nonpolar chemicals, such as steroid hormones (e.g. androgen, estrogen).Water is the only glaring exception I can think of, and it only passes through because it is present in such high concentration (it is the solvent).

    b) passing through the membrane with protein assistance

    For things that are small but cannot pass through the membrane on their own, cells have evolved specific proteins called carrier proteins which recognize certain target chemicals called substrates and only allow the passage of these substrates. Common substrates for transport proteins include metal ions and small sugars.
    This process of transport involving a protein specifically tailored for a substrate is often further subcategorized depending on where the energy for the transport is coming from.
    Facilitated diffusion, or passive transport, occurs if the substrate is moving down its own concentration gradient, without energy input from the cell.
    Active transport occurs if the cell expends energy in some form to move a substrate against its concentration gradient.
    Active transport is further subcategorized according to the type of energy that is consumed to pump the substrate against its concentration gradient: If the energy is being provided by ATP, (or another cellular chemical energy carrier), then we call it direct active transport. If the energy to transport one substrate against its concentration gradient is being provided by the simultaneous transport of another substrate down its concentration gradient, we call that indirect active transport.

    c) The membrane itself buds off and swallows the particle; endocytosis

    Large macromolecules such as proteins and DNA, as well as other particles too large for a single protein to handle, are assimilated like this. Small neurotransmitters are also taken up en masse by this method after being released from the cell.

    So with this framework we can go back and answer the questions you have:

    glucose - Glucose is a relatively small, polar molecule,which rules out direct diffusion across the bilayer. Since glucose is generally consumed inside the cell, one would expect the chemical potential of glucose to be lower inside the cell, and the concentration gradient of glucose to flow inwards, towards the cell interior. Thus there is no reason for active transport to occur, and glucose transport would probably proceed by facilitated diffusion.
    sucrose - Here we're given the information that transport occurs against the concentration gradient. This usually implies active transport.
    calcium ion - Ions are small and heavily polar, so one would expect a carrier protein to be involved. As a general rule, calcium concentration is lower inside the cell than outside, so one would expect facilitated diffusion.
    large liquid droplet - Too large for a single protein to handle, can't diffuse all at once across the bilayer. Transport must proceed by endocytosis.
    Last edited by a moderator: Mar 29, 2008
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