All living things are made of cells, and cells are the smallest
units that can be alive. Life on Earth is classified into five
kingdoms, and they each have their own characteristic kind of
cell. However the biggest division is between the cells of the
prokaryote kingdom (the bacteria) and those of the other
four kingdoms (animals, plants, fungi and protoctista), which
are all eukaryotic cells. Prokaryotic cells are smaller
and simpler than eukaryotic cells, and do not have a nucleus.
- Prokaryote = without a nucleus
- Eukaryote = with a nucleus
We'll examine these two kinds of cell in detail, based on structures
seen in electron micrographs (photos taken with an electron
microscope). These show the individual organelles inside
- Cytoplasm (or Cytosol). This is the solution within
the cell membrane. It contains enzymes for metabolic reactions together with sugars,
salts, amino acids, nucleotides and everything else needed for
the cell to function.
- Nucleus. This is the largest organelle. Surrounded
by a nuclear envelope, which is a double membrane with
nuclear pores - large holes containing proteins that control
the exit of substances such as RNA from the nucleus.
The interior is called the nucleoplasm, which is full
of chromatin- a DNA/protein complex containing
the genes. During cell division the chromatin becomes condensed
into discrete observable chromosomes. The nucleolus
is a dark region of chromatin, involved in making ribosomes.
- Mitochondrion (pl. Mitochondria). This is a sausage-shaped
organelle (8µm long), and is where aerobic respiration
takes place in all eukaryotic cells. Mitochondria are surrounded
by a double membrane: the outer membrane is simple, while the inner membrane is highly folded into cristae,
which give it a large surface area. The space enclosed by the
inner membrane is called the matrix, and
contains small circular strands of DNA. The inner membrane is
studded with stalked particles, which are the site of
- Chloroplast. Bigger and fatter than mitochondria,
chloroplasts are where photosynthesis takes place, so are only
found in photosynthetic organisms (plants and algae). Like mitochondria
they are enclosed by a double membrane, but chloroplasts also
have a third membrane called the thylakoid membrane. The
thylakoid membrane is folded into thylakoid disks, which
are then stacked into piles called grana. The space between
the inner membrane and the thylakoid is called the stroma.
The thylakoid membrane contains chlorophyll and stalked
particles, and is the site of photosynthesis and ATP synthesis.
Chloroplasts also contain starch grains, ribosomes and circular
- Ribosomes. These are the smallest and most numerous
of the cell organelles, and are the sites of protein synthesis.
They are composed of protein and RNA, and are manufactured in
the nucleolus of the nucleus. Ribosomes are either found free
in the cytoplasm, where they make proteins for the cell's own
use, or they are found attached to the rough endoplasmic reticulum,
where they make proteins for export from the cell. They are often
found in groups called polysomes. All eukaryotic ribosomes
are of the larger, "80S", type.
- Smooth Endoplasmic Reticulum (SER). Series of membrane
channels involved in synthesising and transporting materials,
mainly lipids, needed by the cell.
- Rough Endoplasmic Reticulum (RER). Similar to the
SER, but studded with numerous ribosomes, which give it its rough
appearance. The ribosomes synthesise proteins, which are processed
in the RER (e.g. by enzymatically modifying the polypeptide chain,
or adding carbohydrates), before being exported from the cell
via the Golgi Body.
- Golgi Body (or Golgi Apparatus). Another series of
flattened membrane vesicles, formed from the endoplasmic
reticulum. Its job is to transport proteins from the RER to the
cell membrane for export. Parts of the RER containing proteins
fuse with one side of the Golgi body membranes, while at the
other side small vesicles bud off and move towards the cell membrane,
where they fuse, releasing their contents by exocytosis.
- Vacuoles. These are membrane-bound sacs containing
water or dilute solutions of salts and other solutes. Most cells
can have small vacuoles that are formed as required, but plant
cells usually have one very large permanent vacuole that fills
most of the cell, so that the cytoplasm (and everything else)
forms a thin layer round the outside. Plant cell vacuoles are
filled with cell sap, and are very important in keeping
the cell rigid, or turgid. Some unicellular protoctists have
feeding vacuoles for digesting food, or contractile
vacuoles for expelling water.
- Lysosomes. These are small membrane-bound vesicles
formed from the RER containing a cocktail of digestive enzymes.
They are used to break down unwanted chemicals, toxins, organelles
or even whole cells, so that the materials may be recycled. They
can also fuse with a feeding vacuole to digest its contents.
- Cytoskeleton. This is a network of protein fibres
extending throughout all eukaryotic cells, used for support,
transport and motility. The cytoskeleton is attached to the cell
membrane and gives the cell its shape, as well as holding all
the organelles in position. There are three types of protein
fibres (microfilaments, intermediate filaments
and microtubules), and each has a corresponding motor
protein that can move along the fibre carrying a cargo such
as organelles, chromosomes or other cytoskeleton fibres. These
motor proteins are responsible for such actions as: chromosome
movement in mitosis, cytoplasm cleavage in cell division, cytoplasmic
streaming in plant cells, cilia and flagella movements, cell
crawling and even muscle contraction in animals.
- Centriole. This is a pair of short microtubules involved
in cell division.
- Cilium and Flagellum. These are
flexible tails present in some cells and used for motility. They are an extension of the cytoplasm, surrounded by the cell membrane,
and are full of microtubules and motor proteins so are capable
of complex swimming movements. There are two kinds: flagella
(pl.) (no relation of the bacterial flagellum) are longer than the
cell, and there are usually only one or two of them, while cilia
(pl.) are identical in structure, but are much smaller and there are
usually very many of them.
- Microvilli. These are small finger-like extensions
of the cell membrane found in certain cells such as in the epithelial
cells of the intestine and kidney, where they increase the surface
area for absorption of materials. They are just visible under
the light microscope as a brush border.
- Cell Membrane (or Plasma Membrane). This is a thin,
flexible layer round the outside of all cells made of phospholipids
and proteins. It separates the contents of the cell from the
outside environment, and controls the entry and exit of materials.
The membrane is examined in detail later.
- Cell Wall. This is a thick layer outside the cell
membrane used to give a cell strength and rigidity. Cell walls
consist of a network of fibres, which give strength but are freely
permeable to solutes (unlike membranes). Plant cell walls are
made mainly of cellulose, but can also contain hemicellulose,
pectin, lignin and other polysaccharides. There are often channels through
plant cell walls called plasmodesmata, which link the
cytoplasms of adjacent cells. Fungal cell walls are made of chitin. Animal cells do not have a cell
- Cytoplasm. Contains all the enzymes needed for all
metabolic reactions, since there are no organelles
- Ribosomes. The smaller (70 S) type.
- Nuclear Zone. The region of the
cytoplasm that contains DNA. It is not surrounded by a nuclear
- DNA. Always circular, and not associated with any
proteins to form chromatin.
- Plasmid. Small circles of DNA, used to exchange DNA
between bacterial cells, and very useful for genetic engineering.
- Cell membrane. made of phospholipids and proteins,
like eukaryotic membranes.
- Mesosome. A tightly-folded region of the cell membrane
containing all the membrane-bound proteins required for respiration
- Cell Wall. Made of murein,
which is a glycoprotein (i.e. a protein/carbohydrate complex). There are two kinds of cell
wall, which can be distinguished by a Gram stain: Gram
positive bacteria have a thick cell wall and stain purple,
while Gram negative bacteria have a thin cell wall with
an outer lipid layer and stain pink.
- Capsule (or Slime Layer). A thick polysaccharide
layer outside of the cell wall.
Used for sticking cells together, as a food reserve, as protection
against desiccation and chemicals, and as protection against
- Flagellum. A rigid rotating helical-shaped tail used
for propulsion. The motor is embedded in the cell membrane and
is driven by a H+ gradient across the membrane. Clockwise
rotation drives the cell forwards, while anticlockwise rotation
causes a chaotic spin. This is an example of a rotating
motor in nature.
Summary of the Differences Between Prokaryotic
and Eukaryotic Cells
small cells (< 5 mm)
larger cells (> 10 mm)
no nucleus or any membrane-bound organelles
always have nucleus and other membrane-bound organelles
DNA is circular, without proteins
DNA is linear and associated with proteins to form
ribosomes are small (70S)
ribosomes are large (80S)
always has a cytoskeleton
cell division is by binary fission
cell division is by mitosis or meiosis
reproduction is always asexual
reproduction is asexual or sexual
Prokaryotic cells are far older and more diverse than eukaryotic
cells. Prokaryotic cells have probably been around for 3.5 billion years - 2.5
billion years longer than eukaryotic cells. It is thought that eukaryotic cell organelles like mitochondria and chloroplasts are derived from prokaryotic cells
that became incorporated inside larger prokaryotic cells. This
idea is called endosymbiosis, and is supported by these
- organelles contain circular DNA, like bacteria cells.
- organelles contain 70S ribosomes, like bacteria cells.
- organelles have double membranes, as though a single-membrane
cell had been engulfed and surrounded by a larger cell.
The cell membrane (or plasma membrane) surrounds all
living cells. It controls
how substances can move in and out of the cell and is responsible
for many other properties of the cell as well. The membranes that
surround the nucleus and other organelles are almost identical
to the cell membrane. Membranes are composed of phospholipids,
proteins and carbohydrates arranged in a fluid mosaic structure,
as shown in this diagram.
The phospholipids form a thin, flexible sheet, while the proteins
"float" in the phospholipid sheet like icebergs, and
the carbohydrates extend out from the proteins.
The phospholipids are arranged in a bilayer,
with their polar, hydrophilic phosphate heads facing outwards,
and their non-polar, hydrophobic fatty acid tails facing each
other in the middle of the bilayer. This hydrophobic layer acts
as a barrier to all but the smallest molecules, effectively isolating
the two sides of the membrane. Different kinds of membranes can
contain phospholipids with different fatty acids, affecting the
strength and flexibility of the membrane, and animal cell membranes
also contain cholesterol linking the fatty acids together and
so stabilising and strengthening the membrane.
The proteins usually span from one side of the phospholipid
bilayer to the other (intrinsic proteins), but can also
sit on one of the surfaces (extrinsic proteins). They
can slide around the membrane very quickly and collide with each
other, but can never flip from one side to the other. The proteins
have hydrophilic amino acids in contact with the water on the
outside of membranes, and hydrophobic amino acids in contact with
the fatty chains inside the membrane. Proteins comprise about
50% of the mass of membranes, and are responsible for most of
the membrane's properties.
- Proteins that span the membrane are usually involved in transporting
substances across the membrane (more details below).
- Proteins on the inside surface of cell membranes are often
attached to the cytoskeleton and are involved in maintaining
the cell's shape, or in cell motility. They may also be enzymes,
catalysing reactions in the cytoplasm.
- Proteins on the outside surface of cell membranes can act
as receptors by having a specific binding site where hormones
or other chemicals can bind. This binding then triggers other
events in the cell. They may also be involved in cell signalling
and cell recognition, or they may be enzymes, such as maltase
in the small intestine (more in digestion).
The carbohydrates are found on the outer surface of
all eukaryotic cell membranes, and are usually attached to the membrane
proteins. Proteins with carbohydrates
attached are called glycoproteins. The
carbohydrates are short polysaccharides composed of a variety
of different monosaccharides, and form a cell coat or glycocalyx
outside the cell membrane. The glycocalyx is involved in protection
and cell recognition, and antigens such as the ABO antigens on
blood cells are usually cell-surface glycoproteins.
Remember that a membrane is not just a lipid bilayer, but comprises
the lipid, protein and carbohydrate parts.
Cell membranes are a barrier to most substances, and this property
allows materials to be concentrated inside cells, excluded from
cells, or simply separated from the outside environment. This
is compartmentalization is essential for life, as it enables
reactions to take place that would otherwise be impossible. Eukaryotic
cells can also compartmentalize materials inside organelles. Obviously
materials need to be able to enter and leave cells, and there
are five main methods by which substances can move across a cell
- 1. Simple Diffusion
- 2. Osmosis
- 3. Facilitated Diffusion
- 4. Active Transport
- 5. Vesicles
1. Simple Diffusion
A few substances can diffuse directly through the lipid bilayer
part of the membrane. The only substances that can do this are
lipid-soluble molecules such as steroids, or very small molecules,
such as H2O, O2 and CO2. For
these molecules the membrane is no barrier at all. Since lipid
diffusion is (obviously) a passive diffusion process, no energy
is involved and substances can only move down their concentration
gradient. Lipid diffusion cannot be controlled by the cell, in
the sense of being switched on or off.
Osmosis is the diffusion of water across a membrane. It is
in fact just normal lipid diffusion, but since water is so important
and so abundant in cells (its concentration is about 50 M), the
diffusion of water has its own name - osmosis. The contents of
cells are essentially solutions of numerous different solutes,
and the more concentrated the solution, the more solute molecules
there are in a given volume, so the fewer water molecules there
are. Water molecules can diffuse freely across a membrane, but
always down their concentration gradient, so water therefore diffuses
from a dilute to a concentrated solution.
Water Potential. Osmosis can be quantified using water
potential, so we can calculate which way water will move,
and how fast. Water potential (Y,
the Greek letter psi, pronounced "sy") is a measure of the water
molecule potential for movement in a solution. It is measured in units of pressure
(Pa, or usually kPa), and the rule is that water always moves by osmosis from
less negative to more negative water potential (in other words it's
a bit like gravity potential or electrical potential). 100% pure
water has Y = 0,
which is the highest possible water potential, so all solutions
have Y < 0 (i.e. a
and you cannot get Y > 0.
Cells and Osmosis. The concentration (or OP) of the
solution that surrounds a cell will affect the state of the cell,
due to osmosis. There are three possible concentrations of solution
- Isotonic solution a solution of equal OP (or concentration)
to a cell
- Hypertonic solution a solution of higher OP (or concentration)
than a cell
- Hypotonic solution a solution of lower OP (or concentration)
than a cell
- The effects of these solutions on cells are shown in this
The diagram below shows what happens when 2 fresh raw eggs with
their shells removed with acid are placed into sucrose solution (hypertonic) and
distilled water (hypotonic). Water enters the egg in water (endosmosis) causing it to swell
and water leaves the egg in sucrose causing it to shrink (exosmosis).
These are problems that living cells face all the time. For
- Simple animal cells (protozoans) in fresh water habitats
are surrounded by a hypotonic solution and constantly need to
expel water using contractile vacuoles to prevent swelling
- Cells in marine environments are surrounded by a hypertonic
solution, and must actively pump ions into their cells to reduce
their water potential and so reduce water loss by osmosis.
- Young non-woody plants rely on cell turgor for their support,
and without enough water they wilt. Plants take up water through
their root hair cells by osmosis, and must actively pump ions
into their cells to keep them hypertonic compared to the soil.
This is particularly difficult for plants rooted in salt water.
3. Facilitated Diffusion.
Facilitated diffusion is the transport of substances across a membrane
by a trans-membrane protein molecule. The transport proteins tend
to be specific for one molecule (a bit like enzymes), so substances
can only cross a membrane if it contains the appropriate protein.
As the name suggests, this is a passive diffusion process, so
no energy is involved and substances can only move down their
concentration gradient. There are two kinds of transport protein:
- Channel Proteins form a water-filled pore or channel
in the membrane. This allows charged substances (usually ions)
to diffuse across membranes. Most channels can be gated
(opened or closed), allowing the cell to control the entry and
exit of ions.
- Carrier Proteins have a binding site for a specific
solute and constantly flip between two states so that the site
is alternately open to opposite sides of the membrane. The substance
will bind on the side where it at a high concentration and be
released where it is at a low concentration.
The rate of diffusion of a substance across a membrane increases
as its concentration gradient increases, but whereas lipid diffusion
shows a linear relationship, facilitated diffusion has a curved
relationship with a maximum rate. This is due to the rate being
limited by the number of transport proteins.
4. Active Transport (or Pumping).
Active transport is the pumping of substances across a membrane
by a trans-membrane protein pump molecule. The protein
binds a molecule of the substance to be transported on one side
of the membrane, changes shape, and releases it on the other
side. The proteins are highly specific, so there is a different
protein pump for each molecule to be transported. The protein
pumps are also ATPase enzymes, since they catalyse the
splitting of ATP into ADP + phosphate (Pi), and use the energy released to change shape
and pump the molecule. Pumping is therefore an active process,
and is the only transport mechanism that can transport substances
up their concentration gradient.
Pump. This transport protein is present in the cell membranes
of all animal cells and is the most abundant and important of
all membrane pumps. We look at it in more detail in module 4 (A2 course)
The processes described so far only apply to small molecules.
Large molecules (such as proteins, polysaccharides and nucleotides)
and even whole cells are moved in and out of cells by using membrane
Endocytosis is the transport of materials into a cell.
Materials are enclosed by a fold of the cell membrane, which then
pinches shut to form a closed vesicle. Strictly speaking the material
has not yet crossed the membrane, so it is usually digested and
the small product molecules are absorbed by the methods above.
When the materials and the vesicles are small (such as a protein
molecule) the process is known as pinocytosis (cell drinking),
and if the materials are large (such as a white blood cell ingesting
a bacterial cell) the process is known as phagocytosis
Exocytosis is the transport of materials out of a cell.
It is the exact reverse of endocytosis. Materials to be exported
must first be enclosed in a membrane vesicle, usually from the
RER and Golgi Body. Hormones and digestive enzymes are secreted
by exocytosis from the secretory cells of the intestine and endocrine
Sometimes materials can pass straight through cells without
ever making contact with the cytoplasm by being taken in by endocytosis
at one end of a cell and passing out by exocytosis at the other
Summary of Membrane Transport