Essentil Knowledge for Unit 2 Cells

2.1 Cell Structure and Function

• Ribosomes comprise ribosomal RNA (rRNA) and protein. Ribosomes synthesize protein according to mRNA sequence.

•  Ribosomes are found in all forms of life, reflecting the common ancestry of all known life.

• Endoplasmic reticulum (ER) occurs in two forms—smooth and rough. Rough ER is associated with membrane-bound ribosomes - Rough ER compartmentalizes the cell.  Smooth ER functions include detoxification

• The Golgi complex is a membrane-bound structure that consists of a series of flattened membrane sacs

  • Functions of the Golgi include the correct folding and chemical modification of newly synthesized proteins and packaging for protein trafficking.

• Mitochondria have a double membrane. The outer membrane is smooth, but the inner membrane is highly convoluted, forming folds.

• Lysosomes are membrane-enclosed sacs that contain hydrolytic enzymes.

• A vacuole is a membrane-bound sac that plays many and differing roles. 

  • In plants, a specialized large vacuole (central vacuole) serves multiple functions. 

• Chloroplasts are specialized organelles that are found in photosynthetic algae and plants. Chloroplasts have a double outer membrane.

• Organelles and subcellular structures, and the interactions among them, support cellular function

  • Endoplasmic reticulum provides mechanical support, carries out protein synthesis on membrane-bound ribosomes, and plays a role in intracellular transport.

  •  Mitochondrial double membrane provides compartments for different metabolic reactions. 

  • Lysosomes contain hydrolytic enzymes, which are important in intracellular digestion, the recycling of a cell’s organic materials, and programmed cell death (apoptosis). 

  • Vacuoles have many roles, including storage and release of macromolecules and cellular waste products. In plants, it aids in retention of water for turgor pressure.

• The folding of the inner membrane increases the surface area, which allows for more ATP to be synthesized.

  • Within the chloroplast are thylakoids and the stroma.

  • The thylakoids are organized in stacks, called grana.

  • Membranes contain chlorophyll pigments and electron transport proteins that comprise the photosystems.

  • The light-dependent reactions of photosynthesis occur in the grana.

  • The stroma is the fluid within the inner chloroplast membrane and outside of the thylakoid. 

  • The carbon fixation (Calvin-Benson cycle) reactions of photosynthesis occur in the stroma.

  • The Krebs cycle (citric acid cycle) reactions occur in the matrix of the mitochondria.

  • Electron transport and ATP synthesis occur on the inner mitochondrial membrane.

2.2 Cell Size

• • Surface area-to-volume ratios affect the ability of a biological system to obtain necessary resources, eliminate waste products, acquire or dissipate thermal energy, and otherwise exchange chemicals and energy with the environment.

(See calculations for surface area and volume on the formula sheet.) 

• The surface area of the plasma membrane must be large enough to adequately exchange materials

  • These limitations can restrict cell size and shape. Smaller cells typically have a higher surface area-to-volume ratio and more efficient exchange of materials with the environment. 

  • As cells increase in volume, the relative  surface area decreases and the demand for internal resources increases. 

  • More complex cellular structures (e.g., membrane folds) are necessary to adequately exchange materials with he environment. 

  • As organisms increase in size, their surface area-to-volume ratio decreases, affecting properties like rate of heat exchange with the environment.

• Organisms have evolved highly efficient strategies to obtain nutrients and eliminate wastes. Cells and organisms use specialized exchange surfaces to obtain and release molecules from or into the surrounding environment.

2.3 Plasma Membranes

1. Phospholipids have both hydrophilic and hydrophobic regions. The hydrophilic phosphate regions of the phospholipids are oriented toward the aqueous external or internal environments, while the hydrophobic fatty acid regions face each other within the interior of the membrane.

2. Embedded proteins can be hydrophilic, with charged and polar side groups, or hydrophobic, with nonpolar side groups.

3. Cell membranes consist of a structural framework of phospholipid molecules that is embedded with proteins, steroids (such as cholesterol in eukaryotes), glycoproteins, and glycolipids that can flow around the surface of the cell within the membrane.

2.4 Membrane Permeability

1. The structure of cell membranes results in selective permeability.

2. Cell membranes separate the internal environment of the cell from the external environment. 

3. Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model. 

4. Small nonpolar molecules, including N₂, O₂, and CO₂, freely pass across the membrane. Hydrophilic substances, such as large polar molecules and ions, move across the membrane through embedded channel and transport proteins.

5. Polar uncharged molecules, including H₂O, pass through the membrane in small amounts.

6. Cell walls provide a structural boundary, as well as a permeability barrier for some substances to the internal environments.

7. Cell walls of plants, prokaryotes, and fungi are composed of complex carbohydrates.

2.5 Membrane Transport

1. Passive transport is the net movement of molecules from high concentration to low concentration without the direct input of metabolic energy.

2. Passive transport plays a primary role in the import of materials and the export of wastes.

3. Active transport requires the direct input of energy to move molecules from regions of low concentration to regions of high concentration.

4. The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane.

5. The processes of endocytosis and exocytosis require energy to move large molecules into and out of cells

   1. In exocytosis, internal vesicles fuse with the plasma membrane and secrete large macromolecules out of the cell.  

   2. In endocytosis, the cell takes in macromolecules and particulate matter by forming new vesicles derived from the plasma membrane.

2.6 Facilitated Diffusion

1. Membrane proteins are required for facilitated diffusion of charged and large polar molecules through a membrane

   1. Large quantities of water pass through aquaporins. b. Charged ions, including Na+ and K+, require channel proteins to move through the membrane. 

   2. membranes may become polarized by movement of ions across the membrane.

2. Membrane proteins are necessary for active transport.

   1. Metabolic energy (such as from ATP) is required for active transport of molecules and/ or ions across the membrane and to establish and maintain concentration gradients.

   2. The Na+/K+ ATPase contributes to the maintenance of the membrane potential.

2.7 Tonicity & Osmoregulation

1. External environments can be hypotonic, hypertonic or isotonic to internal environments of cells

   1. Water moves by osmosis from areas of high water potential/low osmolarity/ low solute concentration to areas of low water potential/high osmolarity/high solute concentration. 

   2. Growth and homeostasis are maintained by the constant movement of molecules across membranes.

   3. Osmoregulation maintains water balance and allows organisms to control their internal solute composition/water potential.

2.8 Mechanism of Transport

• A variety of processes allow for the movement of ions and other molecules across membranes, including passive and active transport, endocytosis and exocytosis.

2.9 Compartmentalization

1. Membranes and membrane-bound organelles in eukaryotic cells compartmentalize intracellular metabolic processes and specific enzymatic reactions.

2. Internal membranes facilitate cellular processes by minimizing competing interactions and by increasing surface areas where reactions can occur.

2.10 Origins of Cell Compartmentalization

1.  Membrane-bound organelles evolved from once free-living prokaryotic cells via endosymbiosis.

2. Prokaryotes generally lack internal membrane bound organelles but have internal regions with specialized structures and functions.

3. Eukaryotic cells maintain internal membranes that partition the cell into specialized regions.

4. Membrane-bound organelles evolved from previously free-living prokaryotic cells via endosymbiosis.