A Simple Review – The Plant Cell

I am fascinated by the plant cell…

All life functions must occur within a cell.  This microscopic structure is filled with organelles and tunnels and performs manufacturing of everything needed for life on an unimaginable scale at speeds that cannot be comprehended.

Just how small?  You need to forget about inches.  Plant cells are 10 to 100 micrometers.  For the metric challenged, that’s 0.0004 to 0.004 inches.  They are 3 dimensional.  About 50 can fit on the period at the end of this sentence.  While this size might seem like a limiting factor, it is the perfect size for life.

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The plant cell is surrounded by a cell wall.   Woody plants also have an inner cell wall.  It is called a wall, but it looks more like lattice work.  Water, ions, and gas can easily move through this cellulose lattice to the cell membrane.  This can occur at a rate of 10 million molecules per second.

The cell membrane sits against the cell wall.  There are special proteins on the cell membrane that produce the cellulose and lignin lattice structures that form the cell wall(s).  The cell membrane is fluid and is the consistency of thick motor oil.  It is constantly moving and changing position.  It is a double phospholipid membrane.

The outer cell membrane is called the plasmalemma.  It is a semi-permeable membrane that only allows certain stuff to pass through.  Good stuff in, and good stuff and waste out.  Good stuff out like the exudates that are intended to feed mycorrhizae and beneficial bacteria.   Water and what is dissolved in water can easily pass through.  So can oxygen, carbon dioxide, and nitrous oxide.  Other molecules enter through special transport proteins and carbohydrates embedded in and going through the plasmalemma.

The plant cell membrane has tunnels called plasmodesmata and these tunnels are connected to all adjacent cells.  Every cell in the plant is connected by these tunnels. Once inside the plasmodesmata, water and other molecules of the right size can travel to every cell in the plant.  You could enter one of these tunnels in a root cell and eventually travel all the way to a leaf cell without ever leaving this tunnel system.  An individual plant cell can have from 1,000 to 100,000 of these tiny tunnels.

Aquaporins are proteins embedded in the plasmalemme.  They are so small they can only transport water one water molecule at a time. Aquaporins use no plant energy for this transport and rely on the cohesion property of water.  One at a time, water molecules move through an Aquaporin, 10,000 per second. Wonder what graduate student did the counting?

There are many other types of transport proteins embedded in the cell membrane.  Each allows specific molecules to enter or exit the cell.  A tremendous amount of the energy used by a cell is dedicated to producing these transport proteins.

Pump proteins use energy from sodium and potassium and hydrogen to move other molecules across the cell membrane.  To enter, a molecule must have a charge.  It must be an ion (with the exception of Boron).

The plant cell is very cautious about what it lets in.  Even with all these specific mechanisms for transport, sometimes something is available that is needed and it is too big or there are no specific transport proteins for it.  The cell membrane envelopes it (endocytosis) and moves it across.

Inside the cell membrane, the cell is filled with a liquid cytoplasm.  Within the cytoplasm are organelles.  Also floating in the cytoplasm are proteins and enzymes and amino acids.  It is about the consistency of a thick tomato soup.  Incredibly there are about 1,000 different proteins, maybe a total of 100,000 proteins at one time floating in this soup.  And that’s just in the space that is not occupied by organelles.

The organelle that produces energy from the sugars made by photosynthesis is called mitochondria.  It is surrounded by a double phospholipid membrane embedded with transport proteins.  There are thousands of mitochondria in most plant cells.  Inside, enzymes strip electrons from glucose molecules and use them to add phosphorus atoms to ADP to make ATP.  ATP is the energy source for functions within a cell.  Breaking the phosphate bond of ATP returns it to ADP and releases an electron that supplies that energy.  All life, no matter how small, requires energy.

Ribosomes are organelles that are the factories for protein synthesis.  Ribosomes are produced by the nucleus.  Some are free floating in the cytoplasm and some are attached to the rough endoplasmic reticulum.  The free floating ribosomes produce proteins used by the cell and those embedded in the rough endoplasmic reticulum produce proteins that are moved out of the cell for use in other parts of the plant.  Ribosomes do not have a membrane.

So, DNA in the nucleus replicates small sections of RNA which contain the blueprint for a protein.  Messenger RNA carries this message to a ribosome.  Transport RNA gathers the correct amino acids (there are 20) and delivers them to the ribosome.  The amino acids enter the ribosome, are assembled according to the directions of the RNA, ATP provides some energy, and out comes the requested protein.  The protein is then shipped to the Golgi apparatus, another organelle, for final processing.

The endoplasmic reticulum is a long membrane that is attached to the outer membrane surrounding the nucleus.  It has many folds and attaches to the cell membrane.  The part near the nucleus is called the rough endoplasmic reticulum due to embedded ribosomes.  Proteins are processed here with nucleotides that send the protein out of the cell to a specified destination.  Sugars can be added to the proteins making them into glycoproteins to be used for cellular functions and reactions.  Toxins in the cell are changed into benign substances for transport out of the cell.

The Golgi apparatus is attached to the endoplasmic reticulum.  The Golgi apparatus is surrounded by a membrane and it is the packaging and shipping center of the cell, the FedEx of the cell.  It is surrounded by tiny tubules that extend throughout the cell.  There are only about 5 to 8 Golgi apparatus in a cell and almost all molecules within a cell must pass through one of them.  Each one has a separate set of enzymes.  Molecules are finished, sorted, chemically tagged with a destination code, and then loaded into vesicles for final transport along the tubules.

Taking up the most room inside the cell is the vacuole, another organelle that is surrounded by a membrane embedded with transport proteins.  It maintains a pH of 7 in the cytoplasm.  It can merge with the cell membrane to purge waste.  Waste can also be recycled by Lysosomes and Peroxisomes.

Lysosomes digest proteins and break them into their component parts for recycling in the plant cell.  There are up to 100 in each plant cell.  They have a pH around 5.

Peroxisomes digest fats and lipids.  In seeds, they provide enzymes that start the conversion of stored fatty acids to sugars.  They help with the assimilation of nitrogen and the metabolism of hormones.

The nucleus is where the genome is stored along with instructions for synthesizing proteins.  It is surrounded by 2 phospholipid membranes.  The outer membrane becomes the rough endoplasmic reticulum.  The inner membrane of the nucleus (nuclear lamina) is a mesh network of fibrous proteins.  Inside is DNA, RNA and the nucleus has its own organelle, the nucleolus.  The nucleolus is the organelle that produces ribosomes.

How’s that for complex and small.  And we have not even mentioned chloroplasts.  There are typically 40-50 chloroplasts in each plant cell.  Each has a double membrane.  They contain chlorophyll.  In a leaf, there are about 500,000 chloroplasts per square millimeter.  Inside each chloroplast, thylakoids are stacked with attached chlorophyll molecules.

When light hits a chlorophyll molecule’s electrons, they jump into an outer orbit.  This increases their energy level causing electrons to flow (like electricity).  They flow through a special channel protein, ATPase.  This results in a phosphate being attached to ADP to make ATP, the energy source for biological systems.

The electrons then get a second dose of light and are picked up by NADP (I will mercifully not spell that one out).  The energy from ATP is used to form a sugar molecule.  Each chloroplast can create thousands of sugar molecules per second.  With 500,000 per square millimeter of leaf tissue, a plant can generate an awesome amount of sugar.

The outer cell wall and the plasma membrane are a barrier and regulator of what may enter and exit the cell.  Special membrane proteins allow water and nutrients to enter the cell while keeping unwanted stuff out.  The cytoplasm holds structures and organelles that perform functions using nutrients.  Mitocondria provide power for biological functions.  The nucleus is the command center containing DNA, RNA, and its own organelle.  Cells have transportation and communication infrastructure, protein synthesis areas, and even tunnels that connect every single cell within the plant.  All contained in a body so small that 50 can fit on the period at the end of this sentence. All cells come from other cells.

All functions necessary for life take place within each cell.

I am fascinated by the plant cell…

 

This article was based on Jeff Lowenfels’ Teaming With Nutrients, Chapter 1.