As you've learned already, cells are the basic unit of life.
And whether you're hoping to ace your middle school or high school biology tests or are looking for a refresher before college biology, knowledge eukaryotic cell structure is a must-have.
Read on for a general overview that'll cover everything you need to know for (most) middle school and high school biology courses. Follow the links for detailed guides to each cell organelle to ace your courses.
Overview of Eukaryotic Cells
What exactly are eukaryotic cells? They're one of two major classifications of cells – eukaryotic and prokaryotic. They're also the more complex of the two. Eukaryotic cells include animal cells – including human cells – plant cells, fungal cells and algae.
Eukaryotic cells are characterized by a membrane-bound nucleus. That's distinct from prokaryotic cells, which have a nucleoid – a region that's dense with cellular DNA – but don't actually have a separate membrane-bound compartment like the nucleus.
Eukaryotic cells also have organelles, which are membrane-bound structures found within the cell. If you looked at eukaryotic cells under a microscope, you'd see distinct structures of all shapes and sizes. Prokaryotic cells, on the other hand, would look more uniform because they don't have those membrane-bound structures to break up the cell.
So why do organelles make eukaryotic cells special?
Think of organelles like rooms in your home: your living room, bedrooms, bathrooms and so on. They're all separated by walls – in the cell, these would be the cell membranes – and each type of room has its own distinct use that, overall, make your home a comfy place to live. Organelles work a similar way; they all have distinct roles that help your cells function.
All those organelles help eukaryotic cells carry out more complex functions. So, organisms with eukaryotic cells – like humans – are more complex than prokaryotic organisms, like bacteria.
The Nucleus: The Control Center of the Cell
Let's chat about the the "brain" of the cell: the nucleus, which holds most of the cell's genetic material. Most of your cell's DNA is located in the nucleus, organized into chromosomes. In humans, that means 23 pairs of two chromosomes, or 26 chromosomes overall.
The nucleus is where your cell makes decisions about which genes will be more active (or "expressed") and which genes will be less active (or "suppressed"). It's the site of transcription, which is the first step toward protein synthesis and expressing a gene into a protein.
The nucleus is surrounded by a bilayer nuclear membrane called the nuclear envelope. The envelope contains several nuclear pores, which allow substances, including genetic material and messenger RNA or mRNA, to pass into and out of the nucleus.
And, finally, the nucleus houses the nucleolus, which is the largest structure in the nucleus. The nucleolus helps your cells produce ribosomes – more on those in a second – and also plays a role in the cell's stress response.
In cell biology, each eukaryotic cell is separated into two categories: the nucleus, which we just described above, and the cytoplasm, which is, well, everything else.
The cytoplasm in eukaryotic cells contains the other membrane-bound organelles we'll discuss below. It also contains a gel-like substance called cytosol – a mix of water, dissolved substances and structural proteins – that makes up about 70 percent of the cell's volume.
The Plasma Membrane: The Outer Boundary
Every eukaryotic cell – animal cells, plant cells, you name it – is enveloped by a plasma membrane. The plasma membrane structure is made up of several components, depending on the type of cell you're looking at, but they all share one major component: a phospholipid bilayer.
Each phospholipid molecule is made up of a hydrophilic (or water-loving) phosphate head, plus two hydrophobic (or water-hating) fatty acids. The double membrane forms when two layers of phospholipids line up tail to tail, with the fatty acids forming the inner layer of the membrane and the phosphate groups on the outside.
This arrangement creates distinct borders for the cell, making each eukaryotic cell its own distinct unit.
There are other components of the plasma membrane, too. Proteins within the plasma membrane help transport materials in and out of the cell, and they also receive chemical signals from the environment to which your cells can react.
Some of the proteins in the plasma membrane (a group called glycoproteins) also have carbohydrates attached. Glycoproteins act as "identification" for your cells, and they play an important role in immunity.
The Cytoskeleton: The Cellular Support
If a cell membrane doesn't sound all that strong and secure, you're right – it's not! So your cells need a cytoskeleton underneath to help maintain the cell's shape. The cytoskeleton is made up of structural proteins that are strong enough to support the cell, and that can even help the cell grow and move.
There are three major types of filaments that make up the eukaryotic cell cytoskeleton:
- Microtubules: These are the largest filaments in the cytoskeleton, and they're made of a protein called tubulin. They're extremely strong and resistant to compression, so they're key to keeping your cells in the proper shape. They also play a role in cell motility or mobility, and they also help transport material within the cell.
- Intermediate filaments: These medium-sized filaments are made of keratin (which, FYI, is also the main protein found in your skin, nails and hair). They work together with the microtubules to help maintain the cell's shape.
- Microfilaments: The smallest class of filaments in the cytoskeleton, microfilaments are made of a protein called actin. Actin is highly dynamic – actin fibers can easily get shorter or longer, depending what your cell needs. Actin filaments are especially important for cytokinesis (when one cell splits into two at the end of mitosis) and also plays a key role in cell transport and mobility.
The cytoskeleton is the reason eukaryotic cells can take on very complex shapes (check out this crazy nerve shape!) without, well, collapsing in on themselves.
Look at an animal cell on the microscope and you'll find another organelle, the centrosome, that's closely related to the cytoskeleton.
The centrosome functions as the main microtubule organizing center (or MTOC) of the cell. The centrosome plays a crucial role in mitosis – so much that defects in the centrosome are linked to cell growth diseases, like cancer.
You'll find the centrosome only in animal cells. Plant and fungal cells use different mechanisms to organize their microtubules.
The Cell Wall: The Protector
While all eukaryotic cells contain a cytoskeleton, some types of cells – like plant cells – have a cell wall for even more protection. Unlike the cell membrane, which is relatively fluid, the cell wall is a rigid structure that helps maintain the shape of the cell.
The exact makeup of the cell wall depends on what type of organism you're looking at (algae, fungi and plant cells all have distinct cell walls). But they're generally made of polysaccharides, which are complex carbohydrates, as well as structural proteins for support.
The plant cell wall is part of what helps plants stand up straight (at least, until they're so deprived of water that they start to wilt) and stand up to environmental factors like wind. It also functions as a semi-permeable membrane, allowing certain substances to pass into and out of the cell.
The Endoplasmic Reticulum: The Manufacturer
Those ribosomes produced in the nucleolus?
You'll find a bunch of 'em in the endoplasmic reticulum, or ER. Specifically you'll find them in the rough endoplasmic reticulum (or RER), which gets its name from the "rough" appearance it has thanks to all those ribosomes.
In general, the ER is the manufacturing plant of the cell, and it's responsible for producing substances your cells need to grow. In the RER, ribosomes work hard to help your cells produce the thousands and thousands of different proteins that your cells need to survive.
There's also a portion of the ER not covered with ribosomes, called the smooth endoplasmic reticulum (or SER). The SER helps your cells produce lipids, including the lipids that form the plasma membrane and organelle membranes. It also helps produce certain hormones, like estrogen and testosterone.
The Golgi Apparatus: The Packing Plant
While the ER is the manufacturing plant of the cell, the Golgi apparatus, sometimes called Golgi body, is the packing plant of the cell.
The Golgi apparatus takes proteins newly produced in the ER and "packages" them so they can function properly in the cell. It also packages substances into small membrane-bound units called vesicles, and then they're shipped off to their proper place in the cell.
The Golgi apparatus is made up of small sacs called cisternae (they look like a stack of pancakes under a microscope) that help process materials. The cis face of the golgi apparatus is the incoming side that accepts new materials, and the trans face is the outgoing side that releases them.
Lysosomes: The "Stomachs" of the Cell
Lysosomes also play a key role in processing proteins, fats and other substances. They're small, membrane-bound organelles, and they're highly acidic, which helps them function like the "stomach" of your cell.
The lysosomes' job is to digest materials, breaking down unwanted proteins, carbohydrates and lipids so they can be removed from the cell. Lysosomes are an especially important part of your immune cells because they can digest pathogens – and keep them from harming you overall.
The Mitochondria: The Powerhouse
So where does your cell get the energy for all that manufacturing and shipping? The mitochondria, sometimes called the powerhouse or battery of the cell. The singular of mitochondria is mitochondrion.
As you've probably guessed, the mitochondria are the main sites of energy production. Specifically, they're where the last two phases of cellular respiration take place – and the location where the cell produces most of its usable energy, in the form of ATP.
Like most organelles, they're surrounded by a lipid bilayer. But the mitochondria actually have two membranes (an inner and outer membrane). The inner membrane is closely folded in on itself for more surface area, which gives each mitochondrion more space to carry out chemical reactions and produce more fuel for the cell.
Different cell types have different numbers of mitochondria. Liver and muscle cells, for instance, are particularly rich in them.
While the mitochondria might be the powerhouse of the cell, the peroxisome is a central part of the cell's metabolism.
That's because peroxisomes help absorb nutrients within your cells and come packed with digestive enzymes to break them down. Peroxisomes also contain and neutralize hydrogen peroxide – which could otherwise harm your DNA or cell membranes – to promote the long-term health of your cells.
The Chloroplast: The Greenhouse
Not every cell contains chloroplasts – they're not found in plant or fungal cells, but they are found in plant cells and some algae – but those that do put them to good use. Chloroplasts are the site of photosynthesis, the set of chemical reactions that help some organisms produce usable energy from sunlight and also help remove carbon dioxide from the atmosphere.
Chloroplasts are packed with green pigments called chlorophyll, which capture certain wavelengths of light and set off the chemical reactions that make up photosynthesis. Look inside a chloroplast and you'll find pancake-like stacks of material called thylakoids, surrounded by open space (called the stroma).
Each thylakoid has its own membrane – the thylakoid membrane – as well.
Check out a plant cell under the microscope and you're likely to see a big bubble taking up plenty of space. That's the central vacuole.
In plants, the central vacuole fills up with water and dissolved substances, and it can become so large that it takes up three-quarters of the cell. It applies turgor pressure to the cell wall to help "inflate" the cell so that the plant can stand up straight.
Other types of eukaryotic cells, like animal cells, have smaller vacuoles. Different vacuoles help store nutrients and waste products, so they stay organized within the cell.
Plant Cells vs. Animal Cells
Need a refresher on the biggest differences between plant and animal cells? We've got you covered:
- The vacuole: Plant cells contain at least one large vacuole to maintain the cell's shape, while animal vacuoles are smaller in size.
- The centriole: Animal cells have one; plant cells don't.
- Chloroplasts: Plant cells have them; animal cells don't.
- The cell wall: Plant cells have an outer cell wall; animal cells simply have the plasma membrane.