What are Cells?

Teeny-tiny bits of you. That’s what I call cells. They work twenty-four hours a day, seven days a week to keep you alive.

Introductions: the Prokaryotes and the Eukaryotes

There are two main kinds of living cells: eukaryotic (pronounced you-carry-otic) and prokaryotic (pronounced pro-carry-otic).

These cells have little internal machines called organelles.

Eukaryotic cells are the kinds of cells we have. These cells have nuclei. Because of this, the word eukaryotic means “true nucleus.” The nucleus is a type of organelle that has all the DNA stored inside it. Eukaryotic cells have membrane-bound organelles, which mean they have an organic boundary between them and the rest of the cell.

However, unlike mushrooms or plants (which have eukaryotic cells), we don’t have cell walls. Otherwise, we would be all rigid all the time!

Prokaryotes (means “before nucleus”) cells are different from eukaryotes (pronounced you-carry -oat). Prokaryotes were around before eukaryotes. A prokaryotic cell is a small simple cell but don’t be fooled by this. Some of the most harmful prokaryotes give humans disease.

Bacterial or archaeal cells are prokaryotic.

A plant would have a chloroplast (that takes the energy from the sunlight and transforms it into food or sugar), a water vacuole (keeps a good reservoir of water so that the plant stays turgid), and a cell wall.

These organelles are “membrane-bound,” which mean there is an organic boundary between that organelle and the inside of the cell.

The Prokaryotes

Prokaryotes were around before us eukaryotes. A prokaryotic cell is a small simple cell but don’t be fooled by this. Some of the most harmful prokaryotes give humans disease.

Most prokaryotes have fimbriae or ligaments that help the cell move around. A place inside the cell called a nucleoid is where the DNA hangs out. Different from eukaryotes, prokaryotes don’t have membrane-bound organelles. Their DNA is free-floating in the nucleoid. Some prokaryotes do have gelatin-like capsules that surround the their body to keep things in or out.

Neat, huh?

There are some similarities between prokaryotes and eukaryotes. Both have ribosomes, cytosol (gelatin-like matter surrounding the inner-parts of the cell), DNA, and plasma membranes. Depending on which eukaryote you’re talking about, cell walls. Both eukaryotes and prokaryotes have a plasma membrane covering their cells to allow nutrients in and excrete wastes.

Plasma Membranes

Unlike cell walls, plasma membranes are found in all cells. It is semi-permeable, meaning that certain things can get in and certain things can’t. Small molecules and non-polar molecules like water, lipids, and gases can get in, while, large molecules, ionic molecules, and polar molecules cannot. For big and ionic molecules, the plasma membrane has integrated proteins that act as a channel. Plasma membranes are made up of the phospholipid bilayer (some archaea have a lipid monolayer), which are 50% phospholipid and 50% proteins.

The Eukaryotes

Eukaryotic cells have nuclei. In the eukaryotic cell, there are other types of organelles in the cell like mitochondria (create the energy for the cell), the endoplasmic reticulum (depending on which kind – smooth ER or rough ER – these organelles do a lot of things), ribosomes (create proteins), lysosomes (digests wastes in the cell), Golgi apparatus (packages and ships proteins), cytoskeletal structure (not technically an organelle but contributes to structure of the cell).

In eukaryotes, not only are organelles amazing at what they do, but you they do it 24/7 nonstop. In the nucleus, stored DNA stays. On the outside of the nucleus, there is the nuclear envelope. The endomembrane system consists of the nuclear envelope, endoplasmic reticulum, the golgi apparatus, lysosomes, different types of vesicles (tiny pouches made from the plasma membrane that carry nutrients in the cell or carry wastes outside) and vacuoles (in plants, the vacuole stores and takes care of wastes through hydrolysis*), ribosomes, and the plasma membrane (lays outside the cell to protect it and allow certain molecules in or out of the cell).

Ribosomes are pretty interesting. They’re complicated molecules made up of ribosomal RNA (RNA is ribonucleic acid) and protein. From amino acids, ribosomes construct proteins for the cell. Sometimes they like to hang out the cytosol (area inside the cell) and other times, they are on the outside of the nucleus and the rough endoplasmic reticulum (because of the ribosomes, it’s called “rough”). However because they don’t have membranes, they aren’t considered organelles.

The smooth endoplasmic reticulum can make lipids, create energy from carbohydrates, detoxes the cell, and keeps calcium ions (Cl-). The rough endoplasmic reticulum makes proteins and glycoproteins (proteins with carbohydrates on them). For the pancreas, the rough ER can make the protein insulin. From the help of transport vesicles, proteins make their way to the Golgi apparatus.

The Golgi apparatus fixes proteins before they’re sent off to wherever they’re needed. This organelle also makes large molecules that leave inside vesicles.

Lysosomes act like the cell’s waste disposal. With hydrolytic (a process that splits bonds) enzymes (enzymes are proteins that speed up chemical reactions) inside them. They engulf the cell’s trash and break it down.

You recall that plants have vacuoles? Other organisms have vacuoles too, however, they don’t necessarily fill them up with water. There are food vacuoles that help the (usually single-celled) organisms digest their prey. Contractile vacuoles (usually in single-celled organisms) push out excess water. This keeps the cell balanced and happy with just the right amount of nutrients!

Thus ends our journey through the endomembrane system.

The mitochondrion and the chloroplast are most interesting of the organelles. Both of these guys make energy. Mitochondria perform what’s called cellular respiration. This helps our cell do the things it needs to do by making an invaluable molecule called adenosine triphosphate (C10H16N5O13P3). Mitochondria use oxygen to create this awesome molecule that powers everything to our brains to the muscles in our feet.

The chloroplast organelle, with the help of carbon dioxide and water, in plants and some single-celled organisms can allow them to take the energy from the sun and turn this energy into sugar. This is photosynthesis.

Eukaryotes have a cytoplasm (don’t confuse this with cytosol) that holds and surrounds the nucleus.

It also has something that holds the cell firm called the cytoskeleton, which is a bit like your skeleton. The cytoskeleton is a structure that has three parts: microfilaments (the fine structures of the cell), intermediate filaments (a bit smaller than microtubules, but larger than microfilaments), and microtubules (the stockiest of the bunch).

Early Life

In the beginning, cells were just RNA strands in liposomes. This is RNA world.

Speaking of early cells, there is this cool theory (meaning it’s a scientifically proven explanation to a natural event) called the endosymbiont theory. The theory explains how eukaryotic cells (that’s the kind we have) got their mitochondria (for animal and plants) and chloroplasts (just for plants). The theory says that the mitochondrion and the chloroplast were a bacterium before engulfed – not eaten mind you – by an early cell. The bacterium are kept in the cell because it gave the cell energy. This makes sense because mitochondria are the energy creator in eukaryotic cells (both plant and animal cells) and chloroplasts convert sunlight into sugar (this is not in animal cells).

There are other reasons this theory makes sense. Mitochondria and chloroplasts have a double-membrane. It’s as if they were engulfed by another cell and got to keep their plasma membranes! They also have their own ribosomes and DNA!


Reference:

Reece, Jane B. Urry, Lisa A. Cain, Michael L. Minorsky, Peter V. Jackson, Robert B (2013). Campbell Biology. (10th ed.) Boston, MA: Pearson.

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