“skin,” its surface membrane. 
This membrane not only keeps 
the cell intact, it also provides 
channels that open and close 
to allow selected molecules 
into and out of the cell. 
Scientists are seeking to learn 
more about the ways cells 
respond to outside signals, 
which are often conveyed when 
molecules bind to special 
receptors in cell membranes. 
Because the shape of a mole- 
cule plays a large part in 
determining its function, 
scientists are also keenly 
interested in determining the 
shape of important molecules 
and the rules that cause a 
string of chemicals to fold into 
a specific molecular shape. 
To understand cellular function, 
most scientists study parts of 
specific biochemical pathways, 
such as the cell cycle, that 
involve individual molecules, 
cells, groups of cells, and 
whole organisms. The goal is, 
of course, to be able to put 
all the parts together to 
understand normal cellular 
activities and how they 
malfunction in disease. 
Nuclear membrane 
Rough 
endoplasmic 
reticulum 
Ribosomes 
Cytoplasm 
Centrioles 
Golgi apparatus 
Cell membrane 
Nuclear pore 
Rough 
endoplasmic 
reticulum 
Smooth 
endoplasmic 
reticulum 
Peroxisome 
Nucleolus 
Nucleus 
Mitochondrion 
Secretory vesicle 
Lysosome 
This drawing of an idealized 
animal cell is based on 
photographs taken with 
powerful electron microscopes. 
Within the cell’s membrane 
are such organelles as the 
mitochondria (energy 
producers), the rough 
endoplasmic reticulum 
(a site of protein production), 
the Golgi apparatus 
(a protein sorter), and the 
largest organelle, the 
nucleus (which contains the 
hereditary material DNA). In 
addition to these organelles, 
cells also contain an 
elaborate network of protein 
filaments called the 
cytoskeleton (not shown here) 
that anchor the organelles , 
maintain the cell’s shape, and 
direct intracellular traffic. 
Free ribosomes 
