Unit 2 Overview: Cell Structure and Function

This unit is all about the cell and parts of the cell!

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Subcellular and Organelle Components

Subcellular and organelle components are especially important to know because they pop up throughout the year. All organelles are essential, and students must understand all of them. However, the most critical organelles are: 🍿

• Ribosomes: composed of ribosomal RNA and protein. They help to synthesize proteins. All forms of life have ribosomes, which demonstrates common ancestry 👨‍👩‍👧‍👦.
• Endoplasmic Reticulum (ER): provides mechanical support and plays a role in intracellular transport. There are two kinds of ER, rough and smooth. 🚕
  • Rough ER: helps to compartmentalize the cell and helps to carry out protein synthesis in the ribosomes.
  • Smooth ER: helps in detoxification and lipid production, maintaining cellular homeostasis.
• Golgi Complex: a membrane-bound organelle that is composed of several flattened membrane sacs called cisternae. The Golgi is vital in the final stages of preparing a protein. A newly made protein will get help in correctly folding and modifying as needed. The Golgi also helps in packaging proteins and sorting them before transport. 📦
• Mitochondria: helps with ATP production. It has a small set of its own DNA 🧬 and is a double membrane organelle. The outer membrane is a smooth phospholipid bilayer. The inner membrane is highly convoluted, meaning it is highly folded, which increases the surface area for a growing number of electron transport chains. The increase in surface area facilitates the production of ATP.
  • The mitochondria is the site where cellular respiration occurs. Glycolysis is the first step in cell respiration and occurs with or without oxygen present and shows common ancestry. After the completion of glycolysis, the rest of cellular respiration occurs in the mitochondria, given that oxygen is present. The Citric Acid/Kreb’s Cycle happens in the matrix of the mitochondria, and oxidative phosphorylation, with the help of the electron transport chain, occurs in the inner membrane. 💨

Image from Micromagnet

• Lysosomes: membrane-enclosed sacs that contain hydrolytic enzymes. These enzymes are digestive enzymes that help to break down excess or worn-out cell parts. The lysosomes also help with programmed cell death, known as apoptosis. 🚮☠️
• Vacuole: a membrane-bound sac that has many different roles, including storage and release of macromolecules and waste . Plants have a specialized large central vacuole that also serves many functions. The primary function of the large central vacuole is water retention. Water retention is important in turgor pressure, which helps to maintain the rigidity and function of plant cells. 🌱
• Chloroplasts: photosynthetic algae and plants contain these specialized organelles that can photosynthesize (capture, store, and use solar energy ⚡️) and make simple sugars. Chloroplasts have a double membrane and thylakoids that are flattened sacs with a phospholipid bilayer.
  • Chloroplasts are the site where photosynthesis occurs. The two stages of photosynthesis are the light-dependent ☀️reaction and the light-independent reaction. The light-dependent reaction occurs in the grana and produces the ATP and NADPH necessary for the light-independent reaction. The light-independent reaction is where the Calvin-Benson cycle takes place and carbon is fixed to make simple sugars. 🌓

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Cell Size

Cell size is a concern because surface area-to-volume ratios affect the ability of a biological system to obtain necessary resources, eliminate wastes, pull in or remove heat energy, and exchange materials with the environment. 🤲

Plasma Membrane

The plasma membrane is made of a phospholipid bilayer, composed of two layers of phospholipid molecules, with the hydrophobic tails facing inward and the hydrophilic heads facing outward. The arrangement of the phospholipids in the bilayer creates a semi-permeable barrier that selectively allows certain molecules to pass through while preventing others from entering or leaving the cell.

The plasma membrane is not a static structure and is constantly undergoing changes in response to various signals and stimuli. Many proteins are embedded in the membrane, and these proteins play a number of important roles in the cell. Some proteins are involved in cell recognition, helping the cell to communicate and interact with other cells. Others act as channels or pumps, allowing specific molecules to pass through the membrane. Others help to maintain the structural integrity of the membrane, providing rigidity and support.

The movement of molecules across the plasma membrane is regulated by the hydrophobic and hydrophilic properties of the phospholipids. Small, nonpolar molecules like nitrogen, oxygen, and carbon dioxide can freely pass through the membrane, while larger, polar molecules and ions need the help of proteins to cross the membrane. Water is a small, polar molecule that can move across the membrane in small quantities, but it needs the help of aquaporins and other proteins to cross the membrane in larger amounts.

Image from Biology Dictionary.

Membrane Transport

In order for a cell to function properly, it's important for the internal environment to be maintained in a way that is conducive to the life and function of the cell. To do this, materials must be able to move in and out of the cell as needed, and this movement is facilitated by the plasma membrane. The movement of molecules across the plasma membrane occurs through a process called membrane transport. 

Depending on the type of molecule being transported, the cell may need the help of a transport protein to facilitate its movement across the membrane. Transport proteins are specialized proteins that are embedded in the plasma membrane and are responsible for the movement of specific molecules across the membrane. There are different types of transport proteins, including channels, pumps, and carriers, which work to transport molecules in different ways. There are several different types of membrane transport:

• Passive Transport means that the molecules are moving from an area of high concentration to an area of low concentration. Energy is not required in passive transport. ⚡️
• Active Transport means that the molecules are moving from an area of low concentration to an area of high concentration and will require energy use.
• Endocytosis is the process of taking bulk material into the cell.
• Exocytosis is the process of removing bulk material out of the cell.
  

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Tonicity and Osmoregulation

Water moves by osmosis from high water potential (low osmolarity, low solute concentrations) to low water potential (high osmolarity, high solute concentrations.) Osmoregulation maintains water balance and allows organisms to control the internal environment. Remember, the movement of water in osmosis will always occur in a way that brings the concentration of solute to equilibrium! More on this below. 💦 

Image from wikipedia.

Image from LibreTexts.

Image from apcentral.

Water potential is the tendency for water to move in one direction or another. In this case, osmoregulation allows cells to maintain their internal solute balance. These two equations are used to find the solute concentration inside a cell and the tendency that water will leave the cell. As evolution has occurred, cells have changed and have been altered. One thing that helps cells to be able to take care of different processes inside the cell is the membrane and organelles that have membranes as well. The endosymbiotic theory is where organelles that were once free-living prokaryotic cells became engulfed and serve a purpose now inside the cell. The endosymbiotic theory is summarized below. (Click on the image to see a larger version) 

Image from Wikimedia