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Unit 2

2.0 Unit 2 Overview: Cell Structure and Function

5 min readjune 28, 2021

tejas

Tejas


Watch: AP Biology - Review of Unit 2

Resources:

Unit 2: Cell Structure and Function

This unit is all about the cell and parts of the cell!
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Image from howstuffworks

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, he 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.
  • Golgi Complex: a membrane-bound organelle that is composed of several flattened membrane sacs. They are 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.
  • 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. Glycolysis does not happen in the mitochondria. 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.
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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 ☠️, 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.
  • 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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Image from biologydictionary.

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. 
🎥 Watch: AP Bio - Cell Size

Plasma Membrane

The plasma membrane is composed of the phospholipid bilayer. Each phospholipid has a hydrophilic head 🐵 (water-loving) and a hydrophobic tail 🐒 (water-fearing.) Many proteins are embedded into the plasma membrane to help with cell recognition, allow molecules to pass through, or aid in membrane rigidity. Since the membrane has hydrophilic heads and hydrophobic tails, this limits what can and cannot come into and out of the cell. Small nonpolar molecules like N2, O2, CO2 can freely pass through the membrane 😏. Large, polar molecules and ions can’t pass through unaided and need help to move across the membrane 😞. Water is small, polar, and uncharged. It can move across in small quantities 😊. Since water 💦 is a molecule that needs to mass move most of the time, aquaporins aid in the water movement across the cell membrane. Aquaporins and other proteins embedded in the cell membrane allow molecules in a variety of ways to move across the membrane.
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Image from biology dictionary.

🎥 Watch: AP Bio - Plasma Membrane

Membrane Transport

To maintain an internal environment conducive to the cell, materials will need to pass in both directions across the membrane. There is a concentration gradient that is established to ensure the movement of molecules occurs where required. There will be movement across the membrane in all membrane transport 🚕, and depending on the type of molecule, the cell membrane may need a transport protein.
  • 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 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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Image from Wikibooks

🎥Watch: AP Bio - Membrane Transport

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.
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Image from wikipedia.

🎥Watch: AP Bio - Tonicity and Osmoregulation
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Image from Libretexts.

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Image from apcentral.

Water potential is the tendency for water to move in one direction or another. Osmoregulation maintains water balance and 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)
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Image from Wikimedia

🎥Watch: AP Bio - Review of Unit 2
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