Science Focus Topic 4 Notes: Fluid Movement in Cells | Print |
The Cell Membrane

- A cell membrane allows some substances to enter or leave the cell, while stopping other substances. It is a selectively permeable membrane. (A permeable membrane allows all materials through, while an impermeable membrane does not allow anything through)


- The structure of the cell membrane controls what moves in or out of a cell.  Particles - moving in all directions, bumping into each other, eventually spread out evenly throughout the cell (diffusion). 
- Diffusion plays a part in moving substances into and out of a cell.  Concentration determines the direction that a substance takes through the cell membrane ? particles move from higher concentration areas to lower concentration areas (equal concentration allows the movement of particles in and out equally ? whereas, a higher concentration of particles on the inside of the cell will move to an area of lower concentration on the outside (so movement will only occur from inside to outside) ? until there is a balance.


- The diffusion of water through a selectively permeable membrane is called Osmosis.
- Water helps to dissolve many of the substances involved in cell processes.
  When water is lost (moves out of the cell) it leaves behind a high concentration of the dissolved substances ? when water moves back into the cell, the substances become more diluted and can be used by the cell for it?s life functions.

Fluid Movement in Plants

- Plants require a large supply of water to make sugars in the process of photosynthesis. A group of cells, that perform similar functions, are called tissue.  The transportation of nutrients is the role of plant tissues.

Vascular tissues connect the roots to the leaves.
Phloem Tissue transports sugars manufactured in the leaves to the rest of the plant.
Xylem tissue conducts water and minerals, absorbed by the root cells, to every cell in the plant.
* Xylem and Phloem tissue usually occur together, along the length of plant stems and roots

From Root to Leaf

- The root system contains fine ? root hairs ?. These hairs are extensions of epidermal cells (which protect the outside of the plant)
- When the concentration of water is greater on the outside of these ?
root hairs ? then water can pass through the membrane by osmosis ? which continues from cell to cell, until it reaches the xylem tissue.

- The tube-shaped xylem cells then move the water by a build up of water pressure in the root hairs (high pressure to low pressure) forcing the water up the xylem tissue, like water up a straw, into the stems and leaves.

Leaves are the plant?s food-producing organs (this is where photosynthesis takes place).  Photosynthesis takes place in the layer of cells that contain chloroplasts (these cells are called palisade cells).  They are thin, allowing a large amount of light in (large surface area), and enabling the gases (in the air) to diffuse into the leaf cells. (see figure 2-19)

- The tiny openings, called stomata, allow air to enter the leaf (supplying oxygen for respiration and carbon dioxide for photosynthesis).  The spaces between leaf cells allow the air to flow and the guard cells open and close the stomata.


- The loss of water (in a plant) happens through evaporation and is called transpiration.  It is not a problem, unless, the plant loses too much water and doesn?t replace it by the roots.

- The movement of water throughout the plant happens because of the differences in pressure ? high pressure in the root hairs to lower pressure in the leaves ? (pushing and pulling water throughout the whole plant)

Topic 4 Review p.137