Cell Structure and Function · ⁨מבנה ותפקוד התא⁩

Big Idea 4 — Systems Interactions (SYI): Biological systems interact, and these systems and their interactions exhibit complex properties.

Learning Objective 2.1.A: Explain how the structure and function of subcellular components and organelles contribute to the function of cells.

• 2.1.A.1 Ribosomes are comprised of ribosomal RNA (rRNA) and protein. These non-membrane, subcellular structures are found in cells in all forms of life and reflect the common ancestry in all known life. Ribosomes synthesize proteins according to messenger RNA (mRNA) sequences.
• 2.1.A.2 The endomembrane system consists of a group of membrane-bound organelles and subcellular components (endoplasmic reticulum (ER), Golgi complex, lysosomes, vacuoles and transport vesicles, the nuclear envelope, and the plasma membrane) that work together to modify, package, and transport polysaccharides, lipids, and proteins intercellularly.
• 2.1.A.3 Endoplasmic reticulum provides mechanical support by helping cells maintain shape and plays a role in intracellular transport.
  • i. Rough ER is associated with membrane-bound ribosomes, allows for the compartmentalization of cells, and helps carry out protein synthesis.
  • ii. Smooth ER functions include the detoxification of cells and lipid synthesis.
  • Exclusion statement: Knowledge of the specific functions of smooth ER in specialized cells is beyond the scope of the AP Exam.
• 2.1.A.4 The Golgi complex is a membrane-bound structure that consists of a series of flattened membrane sacs. Functions of the Golgi include:
  • i. Correctly folding and chemically modifying newly synthesized cellular products
  • ii. Packaging proteins for trafficking
  • Exclusion statement: Knowledge of the role of Golgi in the synthesis of specific phospholipids and packaging of specific enzymes for lysosomes, peroxisomes, and secretory vesicles is beyond the scope of the AP Exam.
  • Illustrative examples for 2.1.A.4:
    • Glycosylation and other chemical modifications of proteins that take place within the Golgi and determine protein function or targeting
• 2.1.A.5 Mitochondria have a double membrane that provides compartments for different metabolic reactions involved in aerobic cellular respiration. The outer membrane is smooth, while the inner membrane is highly convoluted, forming folds that enable ATP to be synthesized more efficiently.
• 2.1.A.6 Lysosomes are membrane-enclosed sacs that contain hydrolytic enzymes that digest material. Lysosomes also play a role in programmed cell death (apoptosis).
• 2.1.A.7 Vacuoles are membrane-bound sacs that play many different roles.
  • i. In plant cells, a specialized large vacuole maintains turgor pressure through nutrient and water storage.
  • ii. In animal cells, vacuoles are smaller in size, are more plentiful than in plant cells, and store cellular materials.
• 2.1.A.8 Chloroplasts are specialized organelles that are found in plants and photosynthetic algae. Chloroplasts contain a double membrane and serve as the location for photosynthesis.

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.2.A: Explain the effect of surface area-to-volume ratios on the exchange of materials between cells or organisms and the environment.

• 2.2.A.1 Surface area-to-volume ratios affect the ability of a biological system to obtain necessary nutrients, eliminate waste products, acquire or dissipate thermal energy, and otherwise exchange chemicals and energy with the environment.
  • Relevant equations:
    • Volume of a Sphere: $V = \dfrac{4}{3}\pi r^3$
    • Volume of a Cube: $V = s^3$
    • Volume of a Rectangular Solid: $V = lwh$
    • Volume of a Cylinder: $V = \pi r^2 h$
    • Surface Area of a Sphere: $SA = 4\pi r^2$
    • Surface Area of a Cube: $SA = 6s^2$
    • Surface Area of a Rectangular Solid: $SA = 2lh + 2lw + 2wh$
    • Surface Area of a Cylinder: $SA = 2\pi rh + 2\pi r^2$
    • $r$ = radius
    • $l$ = length
    • $h$ = height
    • $w$ = width
    • $s$ = length of one side of a cube
  • Illustrative examples for 2.2.A.1:
    • SA/V Ratios and Exchanges
      • Root hairs
      • Guard cells
      • Gut epithelial cells
    • Cilia
    • Stomata
• 2.2.A.2 The surface area of the plasma membrane must be large enough to adequately exchange materials.
  • i. The surface area-to-volume ratio can restrict cell size and shape. Smaller cells typically have a higher surface area-to-volume ratio as well as a more efficient exchange of materials with the environment than do larger cells.
  • ii. As cells increase in volume, the surface area-to-volume ratio decreases and the demand for internal resources increases.
  • iii. More complex cellular structures (e.g., membrane folds) are necessary to adequately exchange materials with the environment.
  • iv. As organisms increase in size, their surface area-to-volume ratio decreases, affecting properties like rate of heat exchange with the environment. Smaller amounts of mass exchange proportionally more heat with the ambient environment than do larger masses. As mass increases, both the surface area-to-volume ratio and the rate of heat exchange decrease.
  • v. There is a relationship between metabolic rate per unit body mass and the size of multicellular organisms; typically, the smaller the organism, the higher the metabolic rate per unit body mass.

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.3.A: Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.

• 2.3.A.1 Phospholipids have both hydrophilic and hydrophobic regions. The polar hydrophilic phosphate regions of the phospholipids are oriented toward the aqueous external or internal environment, while the nonpolar hydrophobic fatty acid regions face each other within the interior of the membrane.
• 2.3.A.2 Embedded proteins can be hydrophilic (with charged and polar side groups), hydrophobic (with nonpolar side groups), or both.
  • i. Hydrophilic regions of the proteins are either inside the interior of the protein or exposed to the cytosol (cytoplasm).
  • ii. Hydrophobic regions of proteins make up the protein surface that interacts with the fatty acids in the interior membrane.

Learning Objective 2.3.B: Describe the fluid mosaic model of cell membranes.

• 2.3.B.1 Plasma membranes consist of a structural framework of phospholipid molecules embedded with proteins, steroids (such as cholesterol in vertebrate animals), glycoproteins, and glycolipids. All of these can move around the surface of the cell within the membrane, as illustrated by the fluid mosaic model.

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.4.A: Explain how the structure of biological membranes influences selective permeability.

• 2.4.A.1 Plasma membranes separate the internal environment of the cell from the external environment. Selective permeability is the result of the plasma membrane having a hydrophobic interior.
• 2.4.A.2 Small nonpolar molecules, including $\mathrm{N_2}$, $\mathrm{O_2}$, and $\mathrm{CO_2}$, freely pass across the membrane. Hydrophilic substances, such as large polar molecules and ions, move across the membrane through embedded channels and transport proteins.
• 2.4.A.3 The nonpolar hydrocarbon tails of phospholipids prevent the movement of ions and polar molecules across the membrane. Small polar, uncharged molecules, like $\mathrm{H_2O}$ or $\mathrm{NH_3}$ (ammonia), pass through the membrane in small amounts.

Learning Objective 2.4.B: Describe the role of the cell wall in maintaining cell structure and function.

• 2.4.B.1 Cell walls of Bacteria, Archaea, Fungi, and plants provide a structural boundary as well as a permeability barrier for some substances to the internal or external cellular environments and protection from osmotic lysis.

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.5.A: Describe the mechanisms that organisms use to maintain solute and water balance.

• 2.5.A.1 The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane.
• 2.5.A.2 Passive transport is the net movement of molecules from regions of high concentration to regions of low concentration without the direct input of metabolic energy.
• 2.5.A.3 Active transport requires the direct input of energy to move molecules. In some cases, active transport is utilized to move molecules from regions of low concentration to regions of high concentration.

Learning Objective 2.5.B: Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.

• 2.5.B.1 The processes of endocytosis and exocytosis require energy to move large substances or large amounts of substances into and out of cells.
  • i. In endocytosis, the cell takes in large molecules and particulate matter by folding the plasma membrane in on itself and forming new (small) vesicles that engulf material from the external environment.
  • ii. In exocytosis, internal vesicles release material from cells by fusing with the plasma membrane and secreting large molecules from the cell.

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.6.A: Explain how the structure of a molecule affects its ability to pass through the plasma membrane.

• 2.6.A.1 Facilitated diffusion requires transport or channel proteins to enable the movement of charged ions across the membrane.
  • i. Membranes may become polarized by the movement of ions across the membrane.
  • ii. Charged ions, including $\mathrm{Na^+}$ (sodium) and $\mathrm{K^+}$ (potassium), require channel proteins to move through the membrane.
• 2.6.A.2 Facilitated diffusion enables the movement of large polar molecules through membranes with no energy input. In this type of diffusion, substances move down the concentration gradient.
• 2.6.A.3 Aquaporins transport large quantities of water across membranes.

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.7.A: Explain how concentration gradients affect the movement of molecules across membranes.

• 2.7.A.1 External environments can be hypotonic, hypertonic, or isotonic to internal environments of cells. Movement of water can also be described as moving from hypotonic to hypertonic regions. Water moves by osmosis from regions of high water potential to regions of low water potential.
  • Equation (Water Potential): $\psi = \psi_p + \psi_s$
    • where:
    • $\psi_p$ = pressure potential
    • $\psi_s$ = solute potential
  • Illustrative examples for 2.7.A.1:
    • Contractile vacuole in protists
    • Central vacuole in plant cells

Learning Objective 2.7.B: Explain how osmoregulatory mechanisms contribute to the health and survival of organisms.

• 2.7.B.1 Growth and homeostasis are maintained by the constant movement of molecules across membranes.
• 2.7.B.2 Osmoregulation maintains water balance and allows organisms to control their internal solute composition and water potential. Water moves from regions of low osmolarity or solute concentration to regions of high osmolarity or solute concentration.
  • Equation (Solute Potential of a Solution): $\psi_s = -iCRT$
    • where:
    • $i$ = ionization constant
    • $C$ = molar concentration
    • $R$ = pressure constant $\left(R = 0.0831 \dfrac{L \cdot bars}{mol \cdot K}\right)$
    • $T$ = temperature in Kelvin (°C + 273)

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.8.A: Describe the processes that allow ions and other molecules to move across membranes.

• 2.8.A.1 Metabolic energy (such as that from ATP) is required for active transport of molecules across the membrane and to establish and maintain electrochemical gradients.
  • i. Membrane proteins are necessary for active transport.
  • ii. The $\mathrm{Na^+}/\mathrm{K^+}$ pump and ATPase contribute to the maintenance of the membrane potential.

Big Idea 2 — Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

Learning Objective 2.9.A: Describe the membrane-bound structures of the eukaryotic cell.

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

Learning Objective 2.9.B: Explain how internal membranes and membrane-bound organelles contribute to compartmentalization of eukaryotic cell functions.

• 2.9.B.1 Internal membranes facilitate cellular processes by minimizing competing interactions and by increasing the surface area where reactions can occur.

Big Idea 1 — Evolution (EVO): The process of evolution drives the diversity and unity of life.

Learning Objective 2.10.A: Describe similarities and/or differences in compartmentalization between prokaryotic and eukaryotic cells.

• 2.10.A.1 Membrane-bound organelles such as mitochondria and chloroplasts evolved from once free-living prokaryotic cells via endosymbiosis.
• 2.10.A.2 Prokaryotes typically lack internal membrane-bound organelles but have internal regions with specialized structures and functions.
• 2.10.A.3 Eukaryotic cells maintain internal membranes that partition the cell into specialized regions.