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.
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.
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.
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.
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.
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.
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.
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.
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.
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.