The cell is the basic unit of life. All living things-whether unicellular or multicellular- are made up of cells.
In 1655, Robert Hooke looked at cork tissue under a microscope. He noticed that there were structures that looked like “rooms” and he termed them cells.
Since then there have been major advances in microbiology and microscopy and so we have learned much more about cells and the structures within them.
The human body has trillions of cells. These cells vary in shape, size, and function. To appreciate the differences in shapes of cells think of egg cells, sperm cells, macrophages, and muscle cells- they all have very different shapes. These shapes are based on their function.
In this article, we will go over the structure and function of the different organelles (organelle means “little organ”) within an animal cell.
The plasma membrane is also called the cell membrane, or the phospholipid bilayer. It is called a phospholipid bilayer because it is made up of two layers (“bi” means two) of phospholipids. The phospholipids have a head and a tail. The tail is composed of two fatty acid chains, and the head has a phosphate group in it. The head is hydrophilic (hydro-water, philia-loving) and the tail is hydrophobic (phobic-hating). This makes the plasma memberane overall water-proof. Water can enter the cell, however, it needs special pores to do so.
The plasma membrane is selectively permeable which means it is very picky; it decides what substances can enter and exit the cell.
The nucleus is a darkly staining round structure in the cell. It houses DNA. It is surrounded by a nuclear envelope and has nuclear pores. Nuclear pores are small openings within the nuclear envelope. These openings exist because ions need to enter the nucleus and mRNA needs to leave the nucleus. (mRNA is involved in protein synthesis).
Within the nucleus is a darker staining region called the nucleolus. This is where ribosomal units are made.
The cytoplasm is the fluid filled area between the nucleus and the plasma membrane. The gel-like fluid in the cytoplasm is referred to as the cytosol. So essentially, cytoplasm consists of the cytosol and the organelles of the cell.
Ribosomes are small organelles that make proteins. They are made up of two subunits thate are made in the nucleolis. Ribosomes can exist as clusters (called polyribosomes) or as single ribosomes. They can be found freely floating in the cytoplasm or attached to the endorplasmic reticulum.
They are more abundant in cells that make proteins. For example, the pancreas makes digestive enzymes and so there are lots of ribosomes in pancreatic cells. Remember enzymes are proteins.
Mitochondria are called the “powerhouse” of the cell because they make ATP. (Mitochondria is plural, mitochondrion is singular). ATP is the battery of the cell and holds energy. Whenever the cell need energy to carry out any of its processes it breaks an ATP molecule and uses the released energy.
Mitochondria are double-membraned; they have an inner membrane and an outer membrane. The inner membrane forms folds on the inside of the mitochondria, these folds are called cristae. The inside of the mitochondria is called the matrix.
Mitochondria use oxygen to make ATP in a process called cellular respiration. Carbon dioxide is a byproduct of this reaction. When he exhale we breathe out this carbon dioxide.
Muscle cells use a lot of energy to keep us moving and to help us maintain our posture and for this reason muscle cells have a lot of mitochondria in them.
Peroxisomes are small round, membrane-bound organelles that break down fatty acids, amino acids, other poisons and hydrogen peroxide. Hydrogen peroxide is a byproduct of the reactions that occur in the cell. It is toxic to the cell and so peroxisomes break it down to water and oxygen which are harmless to the cell. Peroxisomes contain an enzyme called catalase that breaks down hydrogen peroxide. The peroxisomes get their name from their function i.e. breaking down hydrogen peroxide.
Liver cells detoxify alcohol and contain a large number of peroxisomes.
Lysosomes and peroxisomes are often confused with each other because they look very similar and have similar functions as well. However, lysosomes are not found in plant cells, whereas, peroxisomes are. Lysosomes are membrane-bound organelles and get their name from the word “lyse” which means to break apart. Lysosomes contain digestive juices that break polymers down. (A polymer is a chain made up of individual units called monomers). Lysosomes break down proteins, polysaccharides, lipids, nucleic acids down to their monomers. They also break down organelles that are no longer working. Infact, when the cell needs to undergo apoptosis (apoptosis- programmed cell death) the lysosomes inside the cell break open releasing the digestive enzymes and killing the cell.
We can think of both peroxisomes and lysosomes as the janitorial staff of the cell as they are involved in getting rid of material that is not needed, or is harmful to the cell.
Vacuoles are membrane-bound sacs that store materials. In plant cells vacuoles store mostly water, in animal cells vacuoles store waste until it is expelled out of the cell.
In plant cells, a central vacuole holds and stores water. When the plant does not receive water it becomes flacid, that is because the vacuole is empty. When the plant is watered the vacuole becomes turgid making the plant upright again.
The endoplasmic reticulum (ER) is a series of interconnected membraneous sacs and tubules. These are found right next to the nucleus. Some of the ER have ribosomes attached at their surface giving them a rough appearance under the microscope. These are called rough endoplasmic reticulum or simply rER. The ER that does not have ribosomes appear smooth and are called smooth endoplasmic reticulum or sER.
Rough Endoplasmic Reticulum
Since rER have ribosomes they are involved in making and modifying proteins.
Liver cells make lots of digestive enzymes and so they contain a lot of rER.
Smooth Endoplasmic Reticulum
sER do not have ribosomes. The smooth endoplasmic reticulum is involved in making carbohydrates, lipids, steroid hormones and it also detoxifies medications, poisons. Muscle cells have a special function for the sER; it stores Calcium ions and it is called the sarcoplasmic reticulum and not the smooth endoplasmic reticulum.
The Golgi apparatus receives proteins and lipids form the endoplasmic reticulum and sorts, tags, and packages them for their destination. It adds short sugar molecules or phosphate groups to the proteins or lipids. It tags them so that the cell knows there the product is headed. This is much like the post office receiving packages and sorting them, labelling them for their destination.
From the Golgi apparatus, the proteins or lipids pinch off in a vesicle and go on their way to wherever they re meant for- sometimes they are meant to remain in the cell and sometimes they are meant to go outside the cell like hormones.
Much like our skeleton, the cell also has a skeleton. It is composed of three components: microfilaments, intermediate filaments, and microtubules.
The cytoskeleton helps the cell maintain it’s shape, anchor organelles in place and help the cell move.
Microfilaments are the narrowest of the three and are 7nm in diameter. They are made up of two twisted strands of actin subunits. Actin is a round protein. Microfilaments can assemble and disassemble on demand.
Microfilaments are involved in helping the cell move. Sometimes this means that the entire cell moves for example our white blood cells patrol around looking for bacteria and other pathogens. They engulf a pathogen by surrounding it. Microfilaments are what help the white blood cells move.
Other times microfilaments help change the shape of a cell as it is undergoing cell division. As a cell is about to divide it momentarily looks like the number 8 right before it splits into two. Microfilaments allow the changing of this shape.
Intermediate filaments are 8 to 10nm in diameter. They are made up of several fibrous proteins. Fibrous proteins (like keratin) give toughness to whatever they are part of.
They are not involved in cell movement, they have a purely structural role. They bear compression so that the cell can maintain its shape. They are spread out throughout the cell and anchor the nucleus and the organelles in place. (If you think about it, the cell’s organelles would be floating around here and there if there wasn’t a cytoskeletal network holding them down).
Microtubules are the largest among the three components of the cytoskeleton. They are about 25nm in diameter and are made up of two glorbular proteins: α-tubulin and β-tubulin. They are hollow and can assemble and disassemble as needed (like microfilaments).
They resist compression (like the springs of a mattress) and provide a track for vesicles to move through (like railroad tracks). They also help during cell division by pulling the replicated chromosomes away during anaphase.
Flagella, cilia and centrioles are made up of microtubules.
The centrosome refers to the area where the two centrioles exist. They lie parallel to each other and are the source of microtubules in animal cells. Plant cells don’t have the centrosome. Prior to cell division the centrosome replicates and migrates to either ends of the cell. As cell division proceeds the centrioles release microtubules that eventually pull chromosomes apart. As the cell divides each new cell has a centrosome.