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Absorption and Secretion of Materials

Learning outcomes: • Structure of the plasma membrane • The characteristics of the plasma membrane • Introduction to the role of proteins in the membrane

Structure of the Plasma Membrane The plasma membrane is composed of phospholipid molecules with watersoluble hydrophilic heads and waterinsoluble hydrophobic tails that naturally arrange themselves in a bilayer

This model of the plasma membrane structure is called the Fluid-Mosiac Model.

Protein molecule Protein molecule spanning membrane

Partly embedded Channel-forming protein

See also Fig 2.4 Page 7 Torrance

Proteins • Make notes on the proteins in the plasma membrane from page 8 of the Torrance textbook

Learning Outcomes: • Understand the effect of heat and solvent on the plasma membrane • Explain why these effects take place • Define the terms diffusion and osmosis and relate these to the plasma membrane

Chemical nature of the plasma membrane Conclusions:

The proteins in the plasma membrane are denatured by excessive heat. The phospholipid bilayer is emulsified by solvents such as ethanol.

Diffusion and Osmosis Diffusion is the net movement of molecules or ions from an area of high concentration to an area of low concentration of that molecule or ion This is a passive process and does not require any energy

Osmosis is net movement of water molecules from an area of high water concentration (HWC) to an area of lower water concentration (LWC) through a selectively permeable membrane. This process is also passive.

A solution with a higher water concentration is said to be hypotonic. A solution with a lower water concentration is said to be hypertonic.

Osmosis in a red blood cell • Copy figure 6.5 page 46 Torrance

Active Transport Active transport works in the opposite way to the passive process of diffusion and requires energy. • Copy figure 6.6 page 46 of Torrance

Conditions required for active transport. Factors such as temperature, availability of oxygen and concentration of respiratory substrate (which all affect the rate of respiration) affect the rate of active transport since active transport relies on the energy produced by respiration to take place. Top Tip & Handy Hint: More More More! Rachel...

Gross movements of the cell membrane Endocytosis Endocytosis is the process by which a cell engulfs and takes in large particles or large quantities of materials. The plasma membrane folds inwards and engulfs the particle or material and forms a pouch. The pouch becomes closed off and detached from the cell membrane, forming an intracellular vesicle.

Endocytosis animation

There are two types of endocytsis: 1. Phagocytosis: (“cell-eating�) the engulfing of large, solid particles such as a white blood cells engulfing bacteria. The contents of the vesicle are usually then digested. Phagocytosis animation

2. Pinocytosis: (“cell-drinking�) The formation of small, liquid-filled vesicles by the cell membrane Pinocytosis animation

Exocytosis Exocytosis is the reverse of endocytosis where intracellular vesicles fuse with the cell membrane allowing their contents to be expelled. Enzymes and hormones are secreted in this way

Exocytosis animation

Plant Cell Walls and Osmosis Plant cell walls are composed of cellulose fibres, arranged in a basketweave arrangement. They are, therefore, fully-permeable to water whilst still providing structural support.

2. Absorption and Secretion of Materials  
2. Absorption and Secretion of Materials