Page 23 - 84 human physiolofy part-1

Cell Physiology

membrane and exit the cell. The Golgi apparatus is the central delivery system for the cell. It is a
group of flattened sacs arranged much like a stack of bowls. They function to modify and package
proteins and lipids into vesicles, small spherically shaped sacs that bud from the ends of a Golgi
apparatus. Vesicles often migrate to and merge with the plasma membrane, releasing their contents
outside the cell. The Golgi apparatus also transports lipids and creates lysosomes and organelles
involved in digestion.

• Vacuoles

Spaces in the cytoplasm that sometimes serve to carry materials to the cell membrane for discharge
to the outside of the cell. Vacuoles are formed during endocytosis when portions of the cell membrane
are pinched off.

• Lysosomes

Lysosomes are sac-like compartments that contain a number of powerful degradative enzymes.
They are built in the Golgi apparatus. They break down harmful cell products and waste materials,
cellular debris, and foreign invaders such as bacteria, and then force them out of the cell. Tay-Sachs
disease and Pompe's disease are just two of the malfunctions of lysosomes or their digestive proteins.

• Peroxisomes

Organelles in which oxygen is used to oxidize substances, breaking down lipids and detoxifying
certain chemicals. Peroxisomes self replicate by enlarging and then dividing. They are common in liver
and kidney cells that break down potentially harmful substances. Peroxisomes can convert hydrogen
peroxide, a toxin made of H O to H O.
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Extracellular structures

• Extracellular matrix Human cells, like other animal cells, do not have a rigid cell wall.
Human cells do have an important and variable structure outside of their cell membrane called
the extracellular matrix. Sometimes this matrix can be extensive and solid (examples = calcified
bone matrix, cartilage matrix), while other times it consists of a layer of extracellular proteins
and carbohydrates. This matrix is responsible for cells binding to each other and is incredibly
important in how cells physically and physiologically interact with each other.

• Flagella Many prokaryotes have flagella, allowing, for example, an E. coli bacteria to
propel its way up the urethra to cause a UTI (Urinary Tract Infection). Human cells, however
(and in fact most eukaryotic cells) lack flagella. This makes sense since humans are
multicellular, and individual cells do not need to swim around. The obvious exception to this is
with sperm, and indeed each sperm is propelled by a single flagellum. The flagellum of sperm is
composed of microtubules.

• Cilia Cilia are especially notable on the single-celled protozoans, where they beat in
synchrony to move the cells nimbly through the water. They are composed of extensions of the
cell membrane that contain microtubules. When present in humans they are typically found in
large numbers on a single surface of the cells, where rather than moving cells, they move
materials. The mucociliary escalator of the respiratory system consists of mucus-secreting cells

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