Plant Cells
Plants are not only made up of modules, but each module is made up of organs (such as stem, leaves, buds,
flowers etc.) and tissues. Plant tissues, like animal tissues are made up of cells. However, plant cells differ in
certain respects from animal cells although both have certain features in common. See the section on
animal
cells to learn more about cells. Plant cells tend to be slightly larger than animal cells and vary from 10
micrometres (one hundredth of a millimetre) to one millimetre in length. The diagram above shows a 'typical'
plant cell whose main features are discussed below.

As the body is made up of organs, so cells are made up of organelles. Like animal cells, planet cells consist of
cell membrane enclosing the
protoplast which is divided into the nucleus and cytoplasm. Those organelles
common to both plant and animal cells include the
mitochondria (sing. mitochondrion) which are the power
generators of the cell, using chemical fuel (sugars, fats, amino acids) to generate electrical energy which is
then stored as a chemical called
ATP (adenosine trisphosphate). ATP is the universal energy currency of the
cell and almost all of the cells machinery uses ATP as an energy source. ATP is the energy currency of animal
and plant cells (and the cells of other organisms too).

The
nucleus is also common to both plant and animal cells and is the central computer and database of the
cells, storing all the genetic information as DNA (deoxyribose nucleic acid) and performing computations to
determine how the cell should respond to changes in its environment or to signals from other cells.

The
endoplasmic reticulum is a network of branching membranous tubules and sacs that transport and
manufacture certain materials for the cell.
Smooth endoplasmic reticulum (SER) metabolises fats, amongst
other roles, while
rough endoplasmic reticulum (RER) transports proteins manufactured by attached ribosomes.
Ribosomes may be attached to the SER or free in the cytoplasm and are the factories that manufacture
proteins. Proteins are one of the principal building materials that the cell uses. Some proteins are enzymes -
tiny machines that perform necessary chemical reactions within the cell. The
Golgi apparatus is found in both
animal and plant cells and takes proteins from the RER and steroids from the SER and packages them into tiny
spherical membranous spheres, called vesicles, for export to other parts of the cell or even for excretion into
the cell's surroundings. In plants the Golgi apparatus is typically very distinct and made up of a series of
membranous discs stacked together and is sometimes called the dictyosome (a name given to this distinct type
of Golgi apparatus). The ribosomes are tiny particles, and as they attach to the rough endoplasmic reticulum it
is these ribosomes that give it its rough appearance.

Not shown in the diagram below is the
cytoskeleton. For a long time little was known of the plant cytoskeleton
compared to that of the animal cell, but it consists of similar networks of filamentous and tubular proteins that
form a branching network throughout the cytoplasm. The cytoskeleton forms a scaffold for the nucleus and a
'monorail' system of tracks that transport endoplasmic reticulum, ribosomes, mitochondria, vesicles and other
components from one part of the cell to another part where they are needed. It also mixes the cytoplasm,
ensuring that nutrients are well dispersed throughout the cell - in short it acts as a transport system within the
cell.

Mitochondria, endoplasmic reticulum, vesicles and ribosomes are all part of the cytoplasm, and are bathed by
the watery part of the cytoplasm, which is called the
cytosol. The cytoplasm is enclosed by the cell membrane.
As in animal cells, the
cell membrane is a phospholipid bilayer and serves as a selective barrier, only allowing
substances into or out of the cell as needed whilst keeping everything else in or out. It is also responsible for
much of the sensing that the cell does - responding to chemicals in the environment, such as signals from other
cells, which may include growth hormones signalling a bud to grow and open, for example.

The plant cell also has a number of unique organelles which are not found in animal cells. These include the
choloroplasts. The chloroplasts contain the green chlorophyll that absorbs light energy and it is here in the
chloroplast that this light energy is harvested, they are the solar engines of the cell. Some chloroplasts are
modified and lack green chlorophyll (and are called
leucoplasts, leuco- meaning 'white'). Leucoplasts may store
starch in which case they are called
amyloplasts. Starch is the main food reserve of plant cells (animal cells
store glycogen). Green chloroplasts do not occur in root cells, but leucoplasts and amyloplasts are abundant in
roots. In petals, the chloroplasts may be modified with additional coloured pigments and so appear red, orange
or yellow, given the petal its colour; these coloured plastids are called
chromoplasts.

In the centre of many plant cells is a large
central vacuole - a membrane (rather like the cell membrane) called
the tonoplast (vacuolar membrane) enclosing a fluid-filled region. This fluid (or
cell sap) performs various
functions, it pressurises the cell (rather like an inflatable filled with water) and keeps it
turgid (stiff), and it also
acts as a water reservoir. When cells in a green shoot are pressurised in this way, the whole shoot is stiff and
turgid, but when the plant loses water its vacuole shrinks and the plant wilts. The cell vacuole also serves to
degrade unwanted materials within the cell and acts as a store of certain materials, including pigments that
provide the colour of petals in some flowers. The vacuole also removes toxins from the cell.

Outside the cell membrane of the plant cell is a structure that animal cells do not have, called the
cell wall
(analogous to the extracellular matrix in animal tissues). The plant cell wall is the main determinant of the
mechanical properties (such as stiffness, elasticity and strength) of plant tissues. When cells are connected
together to form a tissue, their cell walls form a honeycomb-like structure of interconnected struts and beams
that strengthen the tissue. Cells in wood have very different cell walls to those in a leaf, which is the main
reason why the leaf is soft and flexible whilst the wood is stronger, harder and stiffer. In young and 'fleshy'
(non-woody) parts of the plant the cell walls contain primarily cellulose. Cellulose is made from sugar molecules
joined together into long rope-like chains and woven into fibrils (fibril = tiny fibre). This material is stronger than
steel but much lighter and forms the main mechanical support of such tissues along with the
turgor pressure
from the vacuoles which pushes out against the cytoplasm, which in turn pushes out against the elastic but
immensely strong cellulose cell wall, pressurising or 'inflating' the cell and supporting the plant body. This type
of tissue is called
parenchyma. In woody plants the system of support is very different - click here to learn about
wood.

Between the cell walls of neighbouring plant cells is a 'glue' that binds the cells together. This glue forms a layer
called the
middle lamella (literally 'middle sheet' as it is sandwiched between the two cell walls of the adjacent
cells).

Plasmodesmata (sing. plasmodesma) are channels that cross the cell membrane and cell wall and line up with
similar channels in neighbouring cells - thus they serve as ports allowing signals and materials to be
transported from one cell to the next. (Animal cells have similar connections called gap junctions).


Plants are composite organisms

In addition to the nucleus, mitochondria and chloroplasts also contain DNA. Chloroplasts and mitochondria also
have detailed structures resembling bacteria with many of the unique characteristics of bacteria. It appears that
long ago an ancestral cell (long before animals or plants evolved) ingested bacteria (either to eat them, or the
bacteria invaded it!) but without destroying them. The two cells - one of them bacterial - lived and evolved
together until they became so dependent on one another that neither can exist alone now. This is the
endosymbiotic theory (symbiosis means 'living together' and endo- means 'within'). The bacterial
endosymbionts became mitochondria. They produced energy for the cell (bacteria pioneered the most efficient
energy-producing systems) and received nutrients in return. In this way both animals and plants are really
composite organisms - more than one organism closely fused together into a single organisms. Plants are three
such organisms fused together, since their chloroplasts are undoubtedly evolved from photosynthetic bacteria.
The unique similarities between chloroplasts, mitochondria and bacteria are too close to be ascribed to any
known cause other than endosymbiosis. Whereas animals must eat food to provide the fuel for their
mitochondria to produce ATP, plants use the chloroplasts to make their own fuel which the mitochondria then
burn for the energy to make ATP. In this way plants are more self-sufficient.

Symbiosis does not stop there however. Animals and plants contain much DNA that appears to be of 'external'
origin - DNA incorporated from viruses and other organisms that has become part of the hosts's own genetic
make-up. Bacteria and viruses are particularly good at carrying genes from one organism to another in what is
known as horizontal genetic transfer (vertical transfer occurs from parent to offspring via the gametes (egg and
sperm)). Animals are also dependent upon the bacteria living in their guts for vital functions, for example, the
cow's stomach nurtures bacteria that produce the enzyme cellulase that breaks down the cellulose in the plant
cell walls of the grass the cow eats. Without these bacteria cows could not live off grass! Most plants are also
dependent on fungi in the soil. These fungi interface with the host plant's roots, forming a so-called mycorrhiza
('fungus-root') - a hybrid of plant and fungus. The fungus is much better at taking up certain minerals, such as
phosphorus, from the soil than the plants own roots, and in exchange the plant gives the fungus sugars that the
plant has made by photosynthesis - thus both partners benefit. Without these fungi most plants grow poorly, if
at all. Some plants can only form these mycorrhizae with certain types of fungus and when planting trees in
their non-native habitats, it is sometimes essential to provide native soil to innoculate the new habitat with the
right fungus.

Finally, the roots of neighbouring plants, particularly trees (?) may fuse together beneath the ground,
especially if the trees are of the same species. Nutrients have been seen to flow through these root grafts from
stronger trees into weaker trees, so in this way trees can help one another out. The point of this section is that
what we call an individual organisms is often a false distinction, in reality organisms may interact closely, merge
or even fuse together as all living things fit in to a complex living web.

The 'typical' plant cell we have considered here is a
parenchyma cell - the most basic cell type that packs fleshy
organs, such as fruit, leaves, green non-woody stems and shoots and non-woody roots. These cells may
change dramatically as they develop into other cell types, forming tissues such as wood. To see how plant cells
are put together to make tissues, click one of the following links:
The model above is a simplified representation of a typical plant cell with its top removed to show the internal
structures that may be seen with a light microscope. The whole cell is about 50-100 micrometres (or 0.05 to 0.1
millimetres) long. The plant cell is surrounded by a cellulose
cell wall, forming a rigid sheath stronger than steel.
The
middle lamella is a 'middle layer' of biological glue that joins the cell to neighbouring cells (and so forms a
middle layer sandwiched between the cell walls of neighbouring cells). Inside the cell wall is the
protoplast,
surrounded by a typical (and very thin)
cell membrane which encloses the cytoplasm and nucleus. The
cytoplasm has the ability to change its consistency from a watery fluid to a gelatinous solid, and vice versa, as
needed. It contains various organelles, such as the green and often disc-shaped
chloroplasts, the mitochondria
and the large central fluid-filled
vacuole (which is bounded by a typical membrane very similar to the cell
membrane). The
cytosol is the fluid matrix of the cytoplasm. The nucleus contains the DNA. The plasmodesma
are channels containing cytoplasm that connect the cell to its neighbours and act as ports along which certain
substances, water and signals can pass.

It is important to realise that living cells are very dynamic. Mitochondria move around and constantly change
shape, branching, dividing and then fusing back together again. Chloroplasts turn to face the light. The nucleus
moves and changes shape. The cytoplasm contains numerous tiny granules and vesicles (membranous
spheres) that can be seen to be in constant motion, following well-defined channels of fluid cytoplasm that flow
around the cell, circulating nutrients and oxygen and carbon dioxide around the cell.

The diagram below shows a more complicated version of a plant cell, showing additional structures that may be
seen under the more powerful electron microscope.
Plant cell
Plant cell labeled
plant cell detailed
Plant cells        Plant cell types          Multicellularity           Modularity
Plant cell diagram
  • Plants, like animal cells, are eukaryotes – they have a true nucleus in which their DNA is enclosed by a nuclear
    envelope.
  • Plant cells have no centrosomes – they have different MTOCs for mitosis, but still produce a mitotic spindle.
  • Plant cells have cellulose cell walls. Cellulose is a glucose polymer (a chain of glucose molecules joined end-to-end)
    and these chains are bundled into cellulose microfibrils which are deposited in layers with the microfibrils in one
    layer more-or-less pointing in the same direction, but with adjacent layers oriented at angles to one another to form a
    porous mesh. In some tissues other substances may be added to the cell wall, e.g. the cell walls of xylem vessels are
    impregnated with lignin which strengthens and waterproofs them. Cell walls of cork cells (in bark) contain suberin
    which waterproofs them. [Suberin also forms the Casparian strips of root endodermal cells.]
  • The middle lamella is the layer of ‘biological glue’ between adjacent plant cells and is composed of pectin.

  • Plasmodesmata (sing. plasmodesma) – channels that cross the cell-walls of adjacent cells and join them together.
    these channels are lined by cell surface membrane and contain SER – the cytoplasms of adjacent plant cells are
    continuous through the plasmodesmata. They allow plant cells to exchange materials, especially water. They also
    allow electric currents to pass from cell to cell, e.g. when the Venus flytrap closes the eclls send electrical signals to
    one another to bring about the rapid movement.

  • Plant cells rarely have lysosomes as this function is taken over by the large central vacuole.
  • Vacuole – stores water and other materials (e.g. pigments to give the plant colour), acts as a lysosome, gives the
    cell turgor when full of water (cell sap). The vacuole is bounded by its own phospholipid bilayer membrane – the
    tonoplast.

  • Plastids – a group of related organelles:
  • Chloroplast – specialised for photosynthesis, contain chlorophyll;
  • Chromoplast – coloured, e.g. purple leaves of copper beech (protect against UV light); petals of flowers;
  • Amyloplast – a type of leucoplast (colourless or ‘white’ plastid) specialised for starch storage
  • Plastids that store large amounts of starch lose their chlorophyll and become amyloplasts (starch grains).
  • Plastids containing the green pigment chlorophyll give plants their green colour and are called chloroplasts.
  • Chloroplasts are found in leaves and stems exposed to sunlight.
  • ‘Chloroplasts’ in roots have no chlorophyll and so are called leucoplasts and they may store starch, in which case
    they are called 'starch grains' or amyloplasts.
Chloroplast
Chloroplasts are flattened discs about 5 micrometres long, 2 micrometres wide and 1 micrometre deep.

DNA            a circular molecule of dsDNA (usual double-stranded helical DNA)
G                Granum (a stack of granal thylakoids)
IM               Inner membrane
IL                Intergranal lamella (intergranal thylakoid, stromal thylakoid) –
                  a thylakoid which traverses the stroma to connect two grana together
IS                Intermembrane ‘space’
LG              Lipid globule (lipid droplet) – fat storage
OM             Outer membrane
R                 Ribosome – similar to the prokaryotic type
S                 Stroma
SG              Starch grain – glucose reserve/storage (see starch grain in the plant cell)
Thylakoid – a cisterna (flattened membranous sac)
TL               Thylakoid lumen (fluid-filled)
TM               Thylakoid membrane – contains chlorophyll

Chlorophyll – the green photosynthetic pigment that gives plants their colour and captures light-energy for
photosynthesis.