many types of membrane-based organelles:
The nucleus is considered a type of membrane-based
organelle, surrounded as it is by the doubled membrane called a
nuclear envelope. The outer membrane is generally
considered continuous with the endoplasmic reticulum,
described below. There is some evidence that the endoplasmic
reticulum "grows" from the nuclear envelope, but the reverse also
has some experimental support. Inside a nucleus, the local
cytoskeleton, the nuclear matrix, is fairly dense,
holding the nucleus in a fairly permanent shape and interacting with
the processes going on in there.
All about the cell nucleus.
More on the nuclear envelope.
More on the
Vesicles, vacuoles, and other fairly simple sacs.
The inside of a cell may have many bubble-like membrane structures.
They can do simple work, like storing materials or carrying them
from place to place: the small versions of these bubbles would be
vesicles, the large ones vacuoles. Oddly
enough for biological terms, there seems to be no set size range for
either: the smallest are definitely vesicles, the largest are
vacuoles, but across the middle range either term may be used.
A few ones get special
Food Vacuoles. When items must be taken into a cell
but are either too large, as when large cells eat smaller ones, or
for which there is no other entrance, as for some molecules which
have no carriers in the membrane, the items are surrounded by a
membrane sphere "growing" out from the cell surface that buds into
the cell interior in a process generally called
endocytosis. Proteins called clathrins
are involved in closing a bit of membrane around a particular space.
More specifically, there is phagocytosis
(Latin for "cell eating") when there is an obviously visible item
taken in, and
pinocytosis ("cell drinking," named because no items were
seen in the vacuoles), sometimes called potocytosis.
Different types of endocytosis.
Lysosomes. These are often associated with food
vacuoles. They are tiny vesicles filled with digestive
enzymes; the enzymes are "dumped" into the food vacuoles,
then activated to
break down the contents for absorption. Lysosomes may also be
involved in a sequence by which cells kill themselves purposely, a
apoptosis. Apoptosis (the second "p" may or may not be
pronounced) is very important in multicellular organisms:
lysosomes respond to a signal sent to, say, its worn-out cell by
breaking and releasing digestive enzymes into the cell
interior, killing it before it becomes
dangerously defective. It can happen when it shouldn't, too,
leading to some degenerative diseases. There also seems
to be a wide range of lysosomes that are in the secretion business
(called, logically, secretory lysosomes), an area just
currently being researched.
More on lysosomes.
More on apoptosis.
Apoptosis role in disease.
Secretory lysosomes (abstract).
are used in some plant cells to sustain stiffness, being filled with
water under pressure (turgor pressure). In
plants that wilt when they have been too long without water, it is central vacuoles rather than
the network of cell walls that keep them
Central vacuole (image).
Role in plants.
Peroxisomes are generally sites of
some sort of complex metabolic function that just needs an isolated
chamber of enzymes to work properly. A lot of molecule
breakdown for recycling occurs in peroxisomes.
More on peroxisomes.
are used to pump out water that
floods into a cell by osmosis,
a process covered later. They are found mostly in unicellular
animal-like fresh-water organisms.
Video of contractile vacuole.
This is a network of membrane passages and outcroppings which may be
integrated with sacs. ER,
as it's thankfully called most of the time, has a variety of functions,
most of which should make sense:
It can provide a way of
getting materials quickly from one part of the cell to another.
A cell seems like a small place and a fast move for any molecule
that has to get from here to there, but there is a lot of stuff
potentially in the way. Materials move through these channels or
in tiny vesicles
that bud off the ER.
It can store materials
It can be a surface for
enzyme-based reactions that seal off areas of activity or send
materials on to where they will be used.
It can be a source of
lipid-based materials, including new membranes in the cell and
lipid-based hormones. Its lipid nature makes it a logical
place for this.
All of the above
functions are done on what appear to be plain membranes and
the ER involved is called
Smooth Endoplasmic Reticulum (SER). ER can also be
used as a site of protein synthesis, from ribosomes embedded
in the membrane - this type of ER is called
Rough Endoplasmic Reticulum (RER). Proteins that need
to be moved to particular places, or confined for a while, are built
and dumped into the ER spaces or along the ER membrane.
More on endoplasmic reticulum (image shows connection to nucleus).
More on smooth endoplasmic reticulum.
More on rough endoplasmic reticulum.
Golgi Complex. Also called
Golgi Apparatus and
Golgi Bodies. These membrane-enclosed chambers take in
materials and process them for export (a process called
secretion) from the cell. They often take the form of
stacks of membrane discs, progressively smaller from those that
start processing to those that end it by budding off and moving to
the cell surface, where secretion-filled vesicles "flow" into the
cell membrane and what was inside them gets released from the cell
Secretions may be fully released into the environment, or be integrated into some
cell-surrounding matrix, such as cell walls.
3 names, same thing -
Usually there would be more than one in a cell, so knowing the plural
form is useful. They are the main site of
aerobic respiration. They come in a variety of shapes, the most common being a stumpy
cylinder. Mitochondria have an external membrane and a
highly-folded inner membrane (the folds are called cristae)
embedded with enzymes upon which most of the aerobic reactions of
respiration take place. A mitochondrion (and the chloroplast
discussed next) also has its own independent loop-shaped chromosome
(with just a few locally-used specialty genes, not enough to make a
whole mitochondrion) and its own ribosomes.
Mitochondria are also significant participants in many versions of
apoptosis, and altered mitochondrial function appears to be
associated with various cancerous changes in cells.
More on the mitochondrion.
Connection to ER (abstract).
Mitochondrial diseases: modifying human embryos.
Plastids are chambers found in plant cells. There are
Leucoplasts used for storage of energy-source molecules such as
which contain colored pigments often involved in some light-capturing
phase of photosynthesis or special coloring; and Chloroplasts, where the
takes place. Proplastids, a kind of preliminary structure, may
considered yet another type, and there are a few more derived from leukoplasts.
During photosynthesis, light
energy is used to produce ATP molecules which are then used to construct
sugar molecules from carbon dioxide and hydrogen obtained from water.
Chloroplasts have two outer membranes and several stacks of small
disc-shaped membranes called
(also spelled thylacoids) inside. Thylakoids are where light is
used to make ATP, and the thick fluid (or unstacked membranes, there
seems to be some disagreement about this) around them, called
is where the ATP is used to build sugars.
mitochondria, have prokaryote-like chromosomes and contain ribosomes.
This is a significant piece of evidence in support of the
More on chloroplasts.
High-level structural determination (article).