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Cell Structure and Function
Definition of a cell:
 basic structural and functional unit of life
 the smallest units that display the
characteristics of life, i.e. reproduction,
metabolism, response to stimuli
General Subdivisions of a Cell
• Plasma Membrane = selectively permeable
boundary between the cell and the
environment
• Nucleus = regulatory center of the cell
• Cytoplasm = everything between the plasma
membrane and the nucleus (fluid + organelles)
Plasma Membrane
1. Structure = phospholipd bilayer with proteins
embedded in, and attached to, the inner
(intracellular) and outer (extracellular) surfaces
2. Function
a. Selectively permeable barrier: controls what
enters and leaves the cell
b. Phospholipids are liquid at body temperature, so
proteins float around in the membrane
-functions as a Fluid Mosaic
Plasma Membrane
Plasma Membrane (2)
2. Function – continued
c. Main responsibility: ensure the composition of
extracellular fluid is not the same as the
composition of the intracellular fluid
d. Water-soluble substances (salts, nutrients) cross
membrane with aid of protein channels, which are
selective about what can pass through
e. Lipids can pass directly through bilayer by
diffusion (the random walk of molecules)
f. Attachment site of cytoskeleton, the internal
support of the cell
Plasma Membrane (3)
3. Glycocalyx = protein and carbohydrate coat
covering the extracellular surface of the
plasma membrane
a. Allows attachment to other cells
b. Allows the cell to interact with the environment
c. Gives each person’s cell a distinctive surface;
allows your body to recognize foreign tissues
as different from your own (i.e., blood type,
transplant rejection)
Plasma Membrane
General Subdivisions of a Cell
• Plasma Membrane = selectively permeable
boundary between the cell and the
environment
• Nucleus = regulatory center of the cell
• Cytoplasm = everything between the plasma
membrane and the nuclear compartment
Cell Structure Overview
Nucleus
1. Nuclear Envelope (membrane)
a. Phospholipid bilayer with nuclear pores
b. Controls what enters/leaves the nucleus
-- things only go in or out by passing through
protein channels, which are selective
c. Encloses all the chromosomes
Nucleus
2. Chromatin = all the chromosomes, which are
long strands of the molecule DNA
-- DNA regulates all cell activities, yet never
leaves the nucleus; how is this possible?
produces RNA, short messenger molecules
that exit through nuclear pores
RNA carries instructions out into the cytoplasm
Nucleus
3. Nucleolus – site of ribosome synthesis
a. compartment in the nucleus where ribsomes
are assembled
b. ribosomes are then moved out into cytoplasm
through nuclear pores
c. ribosomes and RNA work together outside
the nucleus, to build all the proteins in the cell
Nuclear
Compartment
Ribosomes
Ribosomes = site of protein synthesis
--assembled in the nucleolus
--exported into the cytoplasm
a. Free – unbound in the fluid cytoplasm, produce
proteins for use in the cell
b. Bound – attached to the endoplasmic reticulum (ER),
produce proteins for export, or for the plasma
membrane
Cytoplasm + Organelles
1. Fluid portion = “cytoplasm”
a. Water with dissolved salts, nutrients and
enzymes (= proteins that do a particular job)
b. Site of many metabolic reactions
2. Cytoskeleton = network of protein fibers
extending throughout the fluid cytoplasm
a. Support and structure for the cell
b. Very dynamic, always remodeling itself
c. Critical for cells to divide and copy themselves
Cell Structure
Organelles
1. Separate compartments within the cytoplasm
formed by membranes
2. Mitochondrion = “thread granule”, major source
of cell’s energy
a. energy is taken from sugar, stored in molecule
called ATP (adenosine triphosphate)
b. requires oxygen to make this exchange
(aerobic metabolism)
c. contained within double membrane
Mitochondrion
Organelles (2)
3. The Cytomembrane System = system of tubes
and chambers formed by membranes
a. extensively distributed throughout the fluid
cytoplasm
b. involved in synthesis, modification,
processing & packaging of cellular lipids
and proteins
Cell Structure
Cytomembrane System
1. Endoplasmic Reticulum (ER) = “within the
cytoplasm network”, a system of tubes and sacs
formed by membranes (an enclosed space)
a) Rough = with bound ribosomes
--modifies proteins produced by the ribosomes
b) Smooth = without bound ribosomes
-doesn’t modify proteins
-functions in lipid synthesis, drug
detoxification, carbohydrate metabolism
Cytomembrane System (2)
2.
Golgi Apparatus = series of flattened sacs formed by
membranes, functions in final protein processing prior
to use by the cell
a. proteins get shuttled from the ER to one end
of the Golgi
b. in each sac, different modifications are made
(proteins get individually tailored)
c. proteins get sorted and shipped off to their
destination (like the post office of the cell)
Cytomembrane System (2)
3. Vesicles = small membrane-bound structures
that transport proteins and lipids around the cell
a. little transporters that shuttle their
contents from one organelle to another
b. when they contact the appropriate organelle,
they fuse with its outer membrane and dump
their contents inside
c. same for plasma membrane, allowing the
export of materials from the cell (exocytosis)
Vesicles move from ER to Golgi
Cytomembrane System (2)
4. Lysosome = membrane-bound vesicle that
contains digestive enzymes or toxic chemicals
a. merges with vesicles containing food
particles, invading bacteria
b. harsh chemicals and enzymes degrade the
food or bacteria, without harming rest of cell
Lysosomes
• Garbage disposal of
the cell
• Contain digestive
enzymes that break
down wastes
Chloroplast
• Found only in plant
cells
• Contains the green
pigment chlorophyll
• Site of food (glucose)
production
• Bound by a double
membrane
Cell Wall
• Found in plant and
bacterial cells
• Rigid, protective barrier
• Located outside of the
cell membrane
• Made of cellulose
(fiber)
Vacuoles
• Large central vacuole
usually in plant cells
• Many smaller vacuoles
in animal cells
• Storage container for
water, food, enzymes,
wastes, pigments, etc.
Centriole
• Aids in cell division
• Usually found only in
animal cells
• Made of microtubules
Where else have we talked
about microtubules?