Define a cell: The smallest living structures that serve as the basic unit of structure and function for an organism, composed of atoms and molecules, and exhibiting widely varying structures based on their functions.
Structures and Functions of Cells
Common features of cells:
Plasma membrane: Forms the outer, limiting barrier between the inside of the cell and the outside environment.
Nucleus: The largest structure in a cell that contains genetic material (DNA).
Cytoplasm: All cellular components between the nucleus and plasma membrane, consisting of:
Cytosol: The liquid component of the cytoplasm.
Organelles: Organized structures with unique shapes and functions.
Inclusions: Clusters of a single type of molecule, e.g., pigments.
Overview of Generalized Cell Structures
Non-membrane-bound organelles:
Ribosomes: Free ribosomes and bound ribosomes, both essential for protein synthesis.
Centrosome: Key for organizing microtubules.
Proteasomes: Involved in degradation of unneeded proteins.
Cytoskeleton: Provides structural support.
Membrane-bound organelles:
Nucleus
Endoplasmic reticulum (ER): Rough ER (dotted with ribosomes) and Smooth ER (without ribosomes).
Golgi apparatus: Modifies, packages, and sorts proteins.
Lysosome: Contains digestive enzymes.
Peroxisome: Involved in fatty acid oxidation and detoxification.
Mitochondrion: Site for ATP production.
Vesicles: Small membrane-bound transport structures.
Plasma membrane: Defined by modifications such as microvilli, cilia, and flagellum.
Plasma Membrane
Structure and Components
Composition: Fluid matrix made of lipids and proteins.
Lipid bilayer:
Phospholipids arranged with hydrophilic heads facing the cytoplasm and interstitial fluid, hydrophobic tails facing inward.
Cholesterol present for membrane strengthening.
Membrane proteins:
Integral proteins: Span the membrane, often serving as transporters or receptors.
Peripheral proteins: Attached to the outer or inner surface of the membrane.
Functions include:
Transporters
Receptors
Identity markers
Enzymatic activity
Anchoring sites
Cell adhesion proteins
Functions of the Plasma Membrane
Physical barrier: Protects cellular contents and supports cell structure through the phospholipid bilayer.
Selectively permeable boundary: Regulates entry and exit of ions, nutrients, and waste.
Establishes electrochemical gradients across the membrane, important for cellular function.
Communication: Contains receptors that respond to molecular signals.
Membrane Transport Processes
Passive Transport
Definition: Movement across the plasma membrane that requires no energy.
Types of passive transport:
Diffusion: Movement of solutes down their concentration gradient.
Simple diffusion: No transport protein required for small, nonpolar molecules.
Facilitated diffusion: Requires transport proteins:
Channel-mediated: Uses a membrane channel.
Carrier-mediated: Uses a carrier protein that changes shape.
Osmosis: Movement of water across a selectively permeable membrane.
Active Transport
Definition: Movement of solutes against their concentration gradient, requiring energy.
Types of active transport:
Primary active transport: ATP energy directly moves substances.
Secondary active transport: Uses energy from the movement of another substance.
Vesicular transport: Involves vesicles for transport.
Exocytosis: Vesicle release to the outside of the cell.
Endocytosis: Vesicle formation to bring materials into a cell.
Phagocytosis: Cellular eating.
Pinocytosis: Cellular drinking.
Receptor-mediated endocytosis: Requires binding of specific ligands.
Details on Membrane Transport Processes
Osmosis
Importance: Water moves from regions of higher water concentration to lower water concentration, through phospholipid bilayer or aquaporins.
Effects of Solutions on Cells
Isotonic solution: Equal concentration inside and outside the cell → no net movement of water.
Hypotonic solution: Lower solute concentration outside the cell → water enters the cell.
Hypertonic solution: Higher solute concentration outside the cell → water leaves the cell.
Summarization of Membrane Transport Processes
Passive Processes:
No energy expenditure.
Substances move down concentration gradients.
Examples: diffusion, osmosis.
Active Processes:
Energy expenditure required.
Involves substance movement against gradients or vesicle formation.
Examples: primary & secondary active transport, exocytosis, endocytosis.
Cellular Organelles and Their Functions
Nucleus
Largest cell structure: Often regarded as control center.
Contains DNA for genetic instructions.
Manages ribosomal subunit production in nucleolus.
Enclosed by the nuclear envelope (double membrane).
Endoplasmic Reticulum (ER)
Composition: Extensive, interconnected membrane network with one continuous lumen, constituting about ½ of cellular membrane.
Rough ER:
Dotted with ribosomes.
Functions: Synthesizes, modifies proteins; forms transport vesicles.
Smooth ER:
Lacks ribosomes.
Functions: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, forms transport vesicles.
Golgi Apparatus
Structure: Composed of stacked cisternae.
Functions:
Modifies proteins from rough ER.
Packages and sorts proteins for secretion or distribution.