This is a thin semi-permeable covering that envelopes a cell. It allows certain atoms, ions and molecules into the cell but not others. The cell membrane (also known as a plasma membrane) is mostly comprised of a phospholipid bilayer, this is a double layer of lipid molecules combined with phosphorus. Within this membrane there are various proteins which act as channels or pumps. Particles may diffuse across channels or be pumped into the cell by the protein pumps via active transport. Membranes are very thin, usually around 7 millionths of a millimeter thick and require the use of an electron microscope in order to see, unlike many other components of cells which may be seen through a light microscope. The cell membrane is involved in other processes such as cell adhesion, ion conductivity and signalling with other cells. It allows cells to attach to one another in order to form tissues.
There are 4 ways in which substances may move across the membrane of a cell:
1) Passive movement (diffusion or osmosis): this describes the movement of substances without the need for expending energy. The substances simply move into the cell due to differences in concentration of molecules inside and outside of the cell. If there is a high concentration of molecules in the extracellular fluid then these molecules will move into the cell and vice versa. Molecules that are capable of moving across the plasma membrane without cellular energy expenditure must be small enough to fit through it's gaps. Examples of such molecules are carbon dioxide, oxygen and water.
2) Transmembrane protein channels and transport proteins: substances which are necessary for the cell in order to survive, (e.g. molecules which are used for respiration such as glucose) are pumped into the cell by protein pumps. This requires energy as it can sometime involve movement of the substances against the concentration gradient (here meaning that substances may move from a region of relative low concentration to a region of relative high concentration). The proteins that carry out this process are quite specific but can be fooled.
3) Endocytosis: this describes the process by which cells take in molecules by engulfment. First the cell moves closer to the substance and then the plasma membrane pulls inward toward the centre of the cell to form a C-shape with the substance to be captured becoming trapped within the membrane. This is called invagination. This invagination is then removed from the membrane and forms a vesicle which is used to trap the molecule into a package. This is essentially the way in which cells 'eat'.
4) Exocytosis: pretty much the exact opposite of endocytosis. The molecules to be removed are first packaged into a vesicle which then attaches itself to the inner cell membrane causing their membranes to fuse together and opening the vesicle. This causes the molecules previously contained within the vesicle to be released into the extracellular medium. This is essentially cellular excretion.
Vacuoles, lysosomes, and autophagy
Vacuoles are used to contain harmful materials, waste products, and small molecules. They also maintain pressure and an acidic pH within the cell. While most, if not all of plant cells have at least one vacuole, not all animal cells have one. In animal cells which contain vacuoles, they aid the processes of exocytosis and endocytosis and play a critical role in a process known as autophagy.
Autophagy, ('self-eating) is a catabolic process in which cell parts that are no longer functioning correctly or are simply no longer necessary are broken down. This occurs by first isolating the targeted components / organelles and then fusing lysosomes to them. Lysosomes are organelles that contain acid hydrolase enzymes. This process is especially beneficial during periods of starvation in which the cell may digest its own components for survival.