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P A R T 2  Chemotherapeutic agents

into the area. Exocytosis is the opposite of endocyto- sis and involves removing substances from a cell by pushing them through the cell membrane. Hormones,

neurotransmitters, enzymes and other substances produced within a cell are excreted into the body by this process (see Figure 7.3). Homeostasis The main goal of a cell is to maintain homeostasis, which means keeping the cytoplasm stable within the cell membrane. Each cell uses a series of active and passive transport systems to achieve homeostasis; the exact system used depends on the type of cell and its reactions with the immediate environment. For a cell to produce the energy needed to carry out cellular metab- olism and other processes, the cell must have a means to obtain necessary elements from the outside environ­ ment. In addition, it must have a way to dispose of waste products that could be toxic to its cytoplasm. To accomplish this, the cell moves substances across the cell membrane, either by passive transport or by active (energy-requiring) transport (see Figure 7.4). Passive transport Passive transport happens without the expenditure of energy and can occur across any semipermeable membrane. There are essentially three types of passive transport: diffusion, osmosis and facilitated diffusion. Diffusion Diffusion is the movement of a substance from a region of higher concentration to a region of lower concentra- tion. The difference between the concentrations of the substance in the two regions is called the concentra- tion gradient of the substance; usually, the greater the concentration gradient, the faster the substance moves. Movement into and out of a cell is regulated by the cell membrane. Some substances move through channels or pores in the cell membrane. Small substances and

Release of stored material

Extracellular fluid

P l a s m a m e m b r a n e

Cytoplasm

Vesicle

Fusion with plasma membrane

Stored material

A

Particle

Extracellular fluid

P l a s m a

Cytoplasm

Phagocytic vesicle

B

FIGURE 7.3  Schematic representation of endocytosis and exocytosis. A. Exocytosis is the movement of substances (waste products, hormones, neurotransmitters) out of the cell. B. Endocytosis involves the destruction of engulfed proteins or bacteria.

Extracellular fluid

Higher solute concentration

Solute particle

Transporter

P l a s m a m e m b r a n e

Cytoplasm

A

Concentration gradient

C

B

Lower solute concentration

FIGURE 7.4  Schematic representation of transport across a cell membrane (A) , which includes diffusion through the cell membrane (B) and pore diffusion through a protein channel (C) .