Topic 4 – Fluid Movement in Cells

Cell Membrane  Allows some substances to enter or leave the cell  Because it allows only certain materials to cross it, it is said to be selectively permeable  A membrane that lets anything across is called permeable and one that lets nothing across is called impermeable

Diffusion  The structure of the cell membrane controls what can move in and out of the cell, but what causes substances to move in the first place?  The particle model tells us that there is constant movement of particles in all liquids and gases.  Particles move in all directions, bumping into one another.  Therefore, particles that are concentrated in one area will spread apart into areas where there are fewer particles  Eventually the particles will become evenly distributed throughout the area  This spreading out process is called diffusion

Diffusion In Cells  Diffusion plays a major part in moving substances in and out of cells  Example: an amoeba living in water the concentration of dissolved CO 2 in the water is the same as in the cytoplasm of the amoeba. - therefore, CO 2 particles move into and out of the cell at the same rate, passing through small openings in the cell membrane - therefore, CO 2 particles move into and out of the cell at the same rate, passing through small openings in the cell membrane  So, who cares and what does this mean?

 Imagine if the amoeba produced carbon dioxide gas as a waste product  The concentration of the CO 2 in the cytoplasm is now more concentrated than the CO 2 in the water  Therefore, more CO 2 will move out of the cell by diffusion

Osmosis  The most common substance found within a cell is water (about 70%!!)  Water particles can easily move in and out of cells by diffusion The diffusion of water through a selectively permeable membrane is called osmosis

Fluid Movement in Plants Fluid Movement in Plants  Most plants need a large supply of water (to make sugar in photosynthesis)  Inside plants there are two types of tissues, called vascular tissues, that connect the roots to the leaves  Phloem tissue transports sugars manufactured in the leaves to the rest of the plant.  Xylem tissue conduct water and minerals absorbed by the roots to every cell in the plant

Transpiration  The loss of water in plants is not a problem if more water enters the plant through the roots.  Loss of water from a plant through evaporation is called transpiration  If the plant loses too much water through transpiration without replacement of water from the roots, the plant can be in serious trouble