Cell Anatomy

Relative cell size

Before we even begin to consider what is inside a cell, it’s important to think about the relative size of cellsThe basic structural and functional units of life..  To the left on this diagram are things you can see with the unaided eye, or macroscopic items.  In this course, whether in person or online, we are going to use a microscope to view cells.  There are many websites, two of them listed at the top right of this slide, that walk you through a relative scale.  These are fun websites, take a look if you can.  You get a sense of the size of cells, their components, and maybe some things you might be familiar with like yeast, DNA, and carbon atomsThe smallest units of matter that retain the properties of an element..

Surface Area to Volume Ratio

Another aspect of the external anatomyThe study of the structure of the human body. of cells is their surface area to volume ratio.  This is the comparison of how much covering there is to how much insides.  Your cutaneous membrane, or your skinThe body’s largest organ, providing protection and regulation., is equivalent to your surface area, and it is quite large.  Your insides, your squishy organs, your muscles, your bones, are your volume.  You want this ratio to be large most of the time, so that exchange with the external environment is efficient.  I like to think of my cats.  On cold days they curl up, and one of them burrows into blankets.  Their volume is fixed, so they reduce their surface area to reduce their heat loss to the environment.  On hot days, or when we have the wood stove going, they stretch out trying to increase their heat loss and prevent a cooked kitty.

Cell Comparison

There are so many kinds of cells.  One way we create large categories of cells is to compare simple cells, called prokaryotes, to complex cells called eukaryotes.  Prokaryotes are things like bacteria.  Other fascinating organisms called archaea are also simple celled organisms and live in extreme environments like volcanoes and hot springs and really salty lakes.  We are going to consider the more complex cells called eukaryotes or eukaryotic cells.  These cells have what are called organellestructures within a cell that perform specialized functions..  Yes, organelles are to cells what organs are to your body.  We explore plant and animal cells like those pictured here, but I want to remind you that there are other eukaryotic cells of fungi and baffling organisms called protists, which are on the outside of this picture.  We discuss these organisms in the next part of this course.

Nucleus

So, let’s zoom in and look at the components of a eukaryotic animal cell.  This is like zooming in on your body and looking at your liverA large organ that produces bile, detoxifies blood, and stores nutrients. that detoxifies your blood, your brain that controlsVariables that remain constant to ensure a fair test. your voluntary and involuntary muscles, among other things, and your digestive system that breaks down moleculesGroups of atoms bonded together..  Whenever we start looking at these little components called organelles, we usually look at the nucleusThe control center of the cell that contains DNA and directs cellular activities. first.  There’s reason behind this: eukaryotic cells have a nucleus, prokaryotic cells don’t.  If you are doing a microscope exercise, you know you have  eukaryotic cell if you have a nucleus. I like this picture here because it shows you a drawing representation of the nucleus, but there’s this cut out of a microscopic picture of it too.  The nucleus houses the DNA of the cell.  DNA comes in many forms, chromatinDNA and associated proteins in a loose, uncondensed form inside the nucleus., chromosomes, and other smaller divisions, but it’s all found in the nucleus.  The nucleus is double wrapped with what is called a nuclear envelopeA double membrane that surrounds the nucleus and separates it from the cytoplasm..  It’s like a plasma membraneThe outer boundary of a cell that controls what enters and exits. with only these openings or pores, none of that other stuff, like the proteinsLarge molecules made of amino acids with various functions in the body. and cholesterolA lipid molecule that is a key component of cell membranes and a precursor for bile acids and steroi.  The DNA holds the instructions for life, or the recipes of proteins, which carry out most of the functions of life.  It’s like the recipe book, and you are the collection of recipes it can make. 

Ribosomes

While the recipes or instructions for proteins are locked up in the nucleus, the machinery for making them is in the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol., or the gooey substance of the cell that’s inside the cell membrane, but outside the nuclear envelope.  You have to think of the nucleus like the reserve materials in a library.  Reserve materials can be used in the library, but they can’t be removed from the library, so things like encyclopedias and dictionaries.  You can copy the information in the reserve materials, and thus get the information out of the library, but you can’t remove the books.  The copy is called mRNA, which you can see here in the bottom right corner as the pink ribbon.  It is being read by the ribosome and the ribosome is making the polypeptide according to the instructions.  We go into this process in much more detail in later modules.  Ribosomes can be free floating in the cytoplasm or they can be bound to this internal network called the endoplasmic reticulum.  Free ribosomesSmall structures responsible for protein synthesis, either free-floating or attached to the rough ER usually make proteins that will be inserted into the cell’s membrane whereas bound ribosomes usually make proteins that will be shipped from the cell to be used elsewhere.

Endoplasmic Reticulum

What is the endoplasmic reticulum other than what looks like a maze of tubules?  Let’s so some root word exercises here and dissect this name.  Endo…you know what it means, it mean inside.  Plasmic, well, we must be referring to something that is inside the plasmaThe liquid component of blood. membrane.  Reticulum is a work that means network or netting, and that’s what this organelle looks like.  We consider the rough and the smooth endoplasmic reticulumA membrane network involved in lipid synthesis, detoxification, and calcium storage. or ER as separate organelles even though they look alike and are attached.  The smooth ER is a lipid making organelle.  It’s main role is in detoxification.  It makes lipidsOrganic molecules including fats, oils, and steroids. that surround toxins, allowing their removal.  Your liver cells, called hepatocytesLiver cells responsible for detoxification, metabolism, and bile production., filter toxins from your blood and tend to have lots of smooth ER.  Alcoholics with a tolerance to alcohol have tons of smooth ER in their hepatocytes trying to eliminate all the alcohol.  Chirrosis of the liver can result from this overabundance of smooth ER in cells.  The rough ER has many jobs, but most have to do with the building of a protein.  The rough ER plays a role in folding the polypeptide chain and it can also make a little bubble of phospholipids, called a transport vesicle, that contains the protein so that it can be transported to the next organelle in the process of making and shipping a protein.  But the main difference between these organelles is that the smooth ER gets all the girls.

Golgi Apparatus

The Golgi apparatus has been called many different things: the Golgi body, the Golgi complex, but it’s all the same organelle.  I like this organelle because its looks like staked pita bread if you cut it and look at if from the side.  Think of this organelle like a conveyor belt.  A protein, contained in a vesicle from the rough ER enters on the receiving side, then gets kinda labeled for its final destination and then gets pooped out the other side in a new transport vesicle.  The Golgi makes vesicles for other reasons, too.  Some vesicles, not containing proteins, leave the Golgi and get inserted into the lipid bilayer for repair.  Some of them turn into our next organelle, lysosomes.

Lysosomes

Lysosomes are basically little stomachs that float around the inside of a cell and digest things like broken down organelles.  The term “lyso” mean ‘To split.”  Rumors have it that it is the origin of the brand name Lysol.  Lysosomes originate from the Golgi apparatus and contain digestive enzymesProteins that speed up chemical reactions in the body..  An interesting, but upsetting disease called Tay-sacs comes from a malfunction of lysosomes.  In Tay-sacs disease, the lysosomes do not contain an enzyme for lipid breakdown.  This is a genetic disease in which the DNA code for the enzyme is not present.  The lysosomes become engorged with lipids from the inability to break them down.  As the lysosomes fill and fill, they crowd out other organelles of cells and essentially becomes fatal.

It is important to note that lysosomes are present only in animal cells.

Vacuoles

Vacuoles, on the other hand, are present only in plant cells.  This is a bubble for storage of mineralsInorganic elements essential for body function. and waterThe universal solvent essential for life..  On the left here is a protist with a vacuole that contracts and expands to create pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. differences for movementA fundamental property of life involving motion of the body or its parts..  In the plant cell on the right, the vacuole is filled with water when the plant in well watered, and the vacuole empties when the plant is wilted. 

Chloroplasts

Another organelle that is present only in plants and protists is the chloroplast.  Mention of this organelle usually makes students think of photosynthesis.  Yes, this is where photosynthesis occurs in two steps, the first using light and the second using carbon dioxide.  Chloroplasts have this inner and out membrane surrounding what looks like stacks of coins.  The spaces between these structures are used to create gradients that harness energyThe capacity to do work or cause change..  The green color we associate with chloroplasts is called chlorophyll, but we are going to learn about other pigments in later modules.  Chloroplasts are one of the two energy-converting organelles in a cell.  Chloroplasts, unlike the ER and Golgi, don’t contribute to the making of proteins.

Mitochondria

The other energy converting organelle is mitochondria.  It’s a common mistake to think that chloroplasts are present in plants and mitochondria are present in animals, but mitochondria are present in both plants and animals.  Once the plants make glucoseA simple sugar that is the main source of energy for cells. from photosynthesis, it is then used by mitochondria to make ATPThe energy currency of cells used for muscle contraction..  For animals, we eat plants to provide the glucose to make ATP.  Either way, it’s a mitochondrion that makes ATP.  Much like chloroplasts, there is a set up of membranes that allows the creation of gradients.  The energy in these gradients are harnessed to make ATP.  Kind of like the Hoover damn, that converts running water into electricity.  While creating ATP, mitochondria consume oxygen and glucose, producing carbon dioxide and water as waste.  If you’re a plant, awesome, because you can use these things to harness the energy of the sun to create glucose.  It’s a cycle!

Random organelles

Totally random slide here.  It’s like the kitchen drawer of organelles.  OK, so there are these little barrel like structures called centriolesCylindrical structures involved in organizing microtubules and cell division..  These are used to make other organelles, like the centrosome that directs mitosis, and ciliaHair-like projections on the surface of some cells that move fluids or particles. and flagellaLong, whip-like structures used by some cells (e.g., sperm) for movement., which are rotational organelles.  A flagellum, present on human sperm, is not a tail, swishing side to side.  It rotates with a smooth motion around and around.  Cilia, which are present on your trachea or windpipe beat in unison to trap particles and move them upward.  Cilia look like feathery appendages on cell surfaces.

Cytoskeleton & Extracellular Matrix

You can see an enhanced view of a cell membrane here.  On the top is the outside of the cell, and on the bottom is the inside.  Inside a cell is this cytoskeleton A network of protein filaments that provide structure, shape, and movement to cells., which seems kind of boring at first.  Yeah, yeah, it like our skeleton, providing support.  But the cytoskeleton in individual cells is an intricate network of filaments along which proteins are transported.  It’s like a highway.   Check out the video link once you’re done with this powerpoint.  The link is a talk with some projected views of the cytoskeleton.  The outside of a cell, especially in your body, is also and intricate network of proteins like collagenA structural protein in the dermis that provides strength and elasticity. with cells suspended throughout.  Your lymph nodes are a network of fibers that your white blood cells move through while they scan other cells that are also moving through the matrix.

Types of
Organelles

Take a look back on the groups of organelles that we’ve talked about.  We had organelles like the nucleus, ER, and Golgi that took part in protein manufacturing.  Mitochondria and chloroplast converted energy.  We has some appendages and an inner highway.  Can you label each on this diagram?  Can you tell me what each does?  Get organized for the quiz!