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  • Created on: 16-05-14 09:58


Describe how assimilates are loaded into the phloem.

  • Hydrogen ions (H+) are actively transported out of the companion cells of the phloem.
  • This creates a concentration gradient of hydrogen ions.
  • The hydrogen ions move back into the companion cells by facilitated diffusion.
  • Sucrose/assimilate molecules move into the companion cells along with the hydrogen ions through special co-transporter proteins.
  • These sucrose/assimilate molecules are then free to diffuse from the companion cells into the sieve tube elements via the plasmodesmata.
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Sieve tubes

The sap in phloem sieve tubes is moved by mass flow. State two adaptions of sieve tubes that enable mass flow to occur.

  • Sieve plates (pores in end walls).
  • No nucleus and few organelles.
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SA:V ratio

Explain why a single-called organism, such as Euglena, does not need a specialised area to carry out gaseous exchange.

  • Single-celled organisms have a large surface area to volume ratio.
  • This means they have a low demand for oxygen.
  • Simple diffusion is adequate to meet their needs.
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Before the division of a nucleus of a cell, the genetic material must replicate. Explain why this is essential.

  • Mitosis produces two genetically identical daughter cells.
  • In order to be genetically identical, the daughter cells need to contain the same DNA.
  • So the genetic material replicates so both daughter cells can receive a full copy of DNA.
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Homologous chromosomes

Unlike yeast, the nuclei of most eukaryotic organisms contain homologous pairs of chromosomes. Explain what is meant by homologous pair of chromosomes.

  • Homologous pairs of chromosomes carry the same genes, but one is maternal and one is paternal.
  • They pair up during meiosis.
  • They have their centromeres in the same position and have the same banding pattern.
  • Homologous pairs of chromosomes are usually similar in length.
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Pressure in the aorta

Pressure fluctuates as blood flows along the aorta. Explain what causes this fluctuation.

  • The spikes in pressure are caused by ventricular systole.
  • The drops in pressure occur because of diastole.
  • The term used to refer to the number of fluctuations per minute is heart rate.
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Blood pressure

Explain what causes the overall change in pressure as blood flows from the aorta to the arteries and from the arteries to the capillaries.

  • Pressure decreases because the blood flows into a larger number of vessels.
  • The total cross-sectional area of the capillaries > t.c-s.a of arteries > t.c-s.a of aorta.
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Pressure in the capillaries

Explain why it is important that the pressure changes as blood flows from the aorta to the capillaries.

  • The capillary wall is only one cell thick.
  • It is important that pressure is low in the capillaries because high pressure would burst or damage the capillary walls.
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Plant cell turgidity

A student placed a small sample of blood onto a microscope slide and added a drop of distiller water. When viewed at a high power, the student observed that the red blood cells had burst. In a similar procedure using plant epidermis, the student observed that the plant cells did not burst. Explain these observations.

  • Cell cytoplasm has a lower water potential than distilled water.
  • Water moves down a water potential gradient.
  • It entered the cells by osmosis.
  • The plasma membrane of animal cells/the red blood cell is weak and burst (haemolysed) under pressure when enough water moved into it.
  • Plant cells have a strong cell wall that provides support and can withstand pressure.
  • The plant cell would have become turgid, which decreases water uptake.
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Describe the function of glycoproteins in the cell surface membrane.

  • Glycoproteins act as antigens in antigen presenting cells. They 'present' the antigens of foreign/invading pathogens to allow recognition by other cells.
  • They act as receptor sites where hormones and other medicinal drugs can bind, because they are shaped complementary to the specific hormone molecule they can recieve.
  • Glycoproteins allow recognition of 'self' cells.
  • They are used in cell adhesion to bind the cells of a tissue together.
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