Professor Dave here, let’s learn about plant cells.
We learned a lot about animal cells and all of the organelles within them.
Plant cells, which are also eukaryotic, have a lot of the same components, but there are
a few key differences that we will want to be aware of, so let’s go over these now.
Looking at the cell as a whole, we see the familiar nucleus and nucleolus, extending
to the rough and smooth ER that we are familiar with.
We also see the Golgi apparatus, and mitochondria.
But in addition, we see a new organelle, the chloroplast.
This is the photosynthetic organelle that produces simple sugars from sunlight, water,
and carbon dioxide, which are then metabolized in the mitochondria.
So whereas animals ingest food and then convert the chemical energy in the food to other forms
of energy for cellular function, plants convert solar energy into chemical energy, in essence
making their own food, which is then converted into energy for cellular function.
We talked about photosynthesis in the biochemistry course, but let’s take another look at the
chloroplast now.
This organelle, like the mitochondrion, is proposed to have once been a separate organism
by endosymbiotic theory.
At some point, an ancestor of the chloroplast was ingested by a larger host cell but not
digested, rather a mutually beneficial or symbiotic relationship was established, and
over the course of evolution these merged into a single organism.
Just like mitochondria, chloroplasts have two membranes, ribosomes, and their own DNA.
There is a narrow intermembrane space between the two membranes, and all the action happens
on the inside.
Here, within a fluid called stroma, floats the DNA, ribosomes, and various enzymes.
In addition, there is a system of interconnected sacs called thylakoids.
These are arranged in grana, which are little stacks, and these thylakoids are where photosynthesis
takes place within any plant cell.
Chlorophyll, the pigment molecule that gives leaves their green color, is present in the
membrane of the thylakoids, and these are the molecules that initially absorb the energy
from sunlight to set photosynthesis into motion.
Chloroplasts can grow and reproduce on their own, and they are also mobile within the cell,
moving around on the cytoskeleton.
In addition to the chloroplast, plant cells have a large central vacuole that contains
a fluid called cell sap.
This is where the cell stores inorganic ions like potassium and chloride.
A plant cell also has many smaller vacuoles with a variety of functions.
Some store important proteins.
Others contain poisons that discourage animals from eating them.
Others contain pigment molecules to attract pollinating insects.
It is the expansion and contraction of this vacuole that controls the size of the cell.
And lastly, unlike animal cells, which possess only a plasma membrane to separate the intracellular
space from the extracellular space, plants possess a cell wall in addition to the plasma
membrane.
This wall, which is much thicker than the membrane, protects the cell, maintains its
shape, and further regulates what goes in and out.
The wall is made of a matrix of polysaccharides like cellulose, as well as some proteins.
When many plant cells are adjacent to one another, each has its own primary cell wall,
and once mature, a secondary cell wall between the primary cell wall and the plasma membrane.
Between the primary cell walls of adjacent cells sits the middle lamella, made of polysaccharides
called pectins.
This acts as a glue to keep all the cells together.
In addition, tiny channels called plasmodesmata connect adjacent cells, allowing for the movement
of cytosol and small molecules from one cell to another, which essentially turns the whole
plant into one huge continuum.
Plants are absolutely fascinating organisms, and we will learn about a variety of plant
species in my botany series one day, but for now, let’s get back to animal cells and
learn more about the processes they undergo.