Knowt
Unit 2: The Cell
Fundamental Units of Life
Cell Basics: All living things are composed of cells, which are the fundamental units of life. Microscopes are often used to study cell organelles—specialized structures within cells that help them function.
Types of Cells:
Prokaryotic Cells:
Found in bacteria and archaea. These cells have DNA stored in an open area called the nucleoid, without a protective membrane.
Eukaryotic Cells:
Found in plants, animals, fungi, and protists. These cells have DNA enclosed within a nucleus, which is surrounded by a double membrane.
Size Comparison:
Eukaryotic cells are generally larger than prokaryotic cells.
Shared Structures:
All cells have a plasma membrane (selective barrier allowing oxygen, nutrients, and waste exchange) and cytoplasm (a fluid that suspends organelles). Cells rely on a high surface area-to-volume ratio for efficient diffusion, explaining why larger organisms have more cells rather than bigger ones.
Eukaryotic Cell Structure
Internal Membranes:
Eukaryotic cells contain an extensive internal membrane system that divides the cell into compartments, mainly formed by phospholipid bilayers with embedded proteins.
Genetic Control Center (Nucleus):
Structure: The nucleus houses most genes and is enclosed by the nuclear envelope, a double membrane with pores for regulating entry and exit.
DNA Organization: DNA in the nucleus is organized into chromosomes, which are composed of chromatin (DNA + proteins). Humans have 46 chromosomes in most cells.
Nucleolus: The nucleolus synthesizes ribosomal RNA (rRNA) and assembles it with proteins to form ribosomes.
mRNA: Messenger RNA (mRNA) carries genetic information from the nucleus to ribosomes for protein synthesis.
Protein Synthesis Centers (Ribosomes)
Composition and Function: Ribosomes are composed of rRNA and proteins. They synthesize proteins by linking amino acids into polypeptides.
Types of Ribosomes:
Free Ribosomes: Located in the cytosol; often synthesize proteins that function within the cytosol.
Bound Ribosomes: Attached to the endoplasmic reticulum (ER) or nuclear envelope; synthesize proteins destined for membranes or secretion.
Endomembrane System
Components:
Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane.
Function:
These organelles collaborate to synthesize, modify, transport, and break down cellular materials.
Endoplasmic Reticulum (ER):
Structure: Network of membranous sacs (cisternae) with an internal compartment (ER lumen).
Types:
Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies substances, and stores calcium ions.
Rough ER: Covered with ribosomes; assists in protein synthesis and membrane production, packages proteins into transport vesicles.
Golgi Apparatus:
Structure: Consists of flattened sacs (cisternae) with a cis face (receiving side) and trans face (shipping side).
Function: Modifies ER products, manufactures certain macromolecules, and sorts/packages materials for transport using "molecular tags" for accurate delivery.
Lysosomes:
Membrane-bound sacs filled with enzymes that digest macromolecules through phagocytosis (engulfing particles) and autophagy (recycling cellular components).
Vacuoles:
Types:
Food Vacuoles: Formed by phagocytosis.
Contractile Vacuoles: Pump excess water out of cells.
Central Vacuole (Plants): Stores nutrients, waste, and helps maintain cell structure.
Energy Conversion Organelles
Mitochondria and Chloroplasts:
Mitochondria: Sites of cellular respiration, converting glucose into ATP. Have double membranes and contain their own DNA and ribosomes.
Chloroplasts (Plants): Sites of photosynthesis, converting solar energy into chemical energy (glucose). Contain thylakoids (stacked into grana) and stroma.
Endosymbiont Theory: Suggests that mitochondria and chloroplasts originated as prokaryotic cells engulfed by an ancestral eukaryote.
Cytoskeleton (Cellular Support and Movement)
Function:
Provides structural support, facilitates cell movement, and organizes cell contents.
Components:
Microtubules: Thick filaments that maintain cell shape, help cell division (form the mitotic spindle), and enable movement via cilia and flagella.
Microfilaments: Thin rods made of actin, involved in muscle contraction and cell division.
Intermediate Filaments: Provide mechanical support for the cell, especially in animal cells.
Additional Structures
Cell Walls (Plants):
Provide rigidity, protect cells, and prevent excessive water uptake.
Extracellular Matrix (ECM) (Animals):
Supports tissue structure, composed of glycoproteins like collagen, and connects cells through integrins.
Intercellular Junctions:
Plant Cells: Connected by plasmodesmata (channels allowing communication).
Animal Cells: Tight junctions, desmosomes, and gap junctions connect cells and allow them to communicate.
Membrane Structure and Function
Plasma Membrane:
Semi-permeable barrier made of phospholipids and proteins; controls movement in and out of the cell. Its fluid mosaic model describes how molecules move within the membrane.
Membrane Fluidity:
Phospholipid tails influence fluidity; unsaturated tails make membranes more fluid.
Cholesterol: Stabilizes membrane fluidity in varying temperatures.
Transport Proteins:
Integral and peripheral proteins that help move substances across the membrane, enabling:
Passive Transport: Movement along the concentration gradient without energy use.
Active Transport: Movement against the gradient using energy, e.g., sodium-potassium pump.
Osmosis and Tonicity
Osmosis:
Water movement across the membrane based on solute concentration. The direction of water flow is governed by water potential, which is the potential energy of water in a system. Water potential is influenced by two components:
Solute potential (osmotic potential): The effect of solutes on the direction of water movement.
Pressure potential: The physical pressure exerted on water.
Water moves from areas of higher water potential to lower water potential, and the formula for water potential is:
Ψ=Ψs+Ψp
Where:
Ψ is the water potential.
Ψs is the solute potential.
Ψp is the pressure potential.
Types of Solutions:
Isotonic Solution: Equal solute concentration inside and outside; no net water movement.
Hypertonic Solution: Higher solute concentration outside; water exits the cell, potentially causing it to shrivel.
Hypotonic Solution: Lower solute concentration outside; water enters, causing cell swelling or bursting (lysing).
Osmoregulation:
Mechanism to balance water and solutes, crucial for cells in various environments, especially plant cells which become turgid in hypotonic solutions.
Molarity and its Effect on Osmosis:
Molarity (M) is a measure of the concentration of solute in a solution and plays a critical role in osmosis. The higher the molarity of a solution, the lower its water potential (because the solute potential is more negative), driving water towards it. Osmosis occurs from regions of low solute concentration (higher water potential) to high solute concentration (lower water potential). Understanding molarity helps predict how water will move in and out of cells under different conditions, particularly in laboratory experiments and physiological processes.
