Chapter 3: Cell Structure & Function
Chapter 3: Cell Structure & Function
I. Cell Theory
All living things are composed of cells.
A single cell is the smallest unit that exhibits all the characteristics of life.
All cells come only from pre-existing cells.
II. Cell Classification Based on Internal Organization
Cells are classified into two general types based on their internal structure.
All cells possess a plasma membrane that encloses the cell.
A. Prokaryotic Cells
Considered more "primitive."
The internal environment of the cell is not divided into membrane-bound compartments.
Lack a Nucleus and Organelles:
Includes all organisms in the domains Bacteria and Archaea.
Their structure typically consists of:
A plasma membrane.
A rigid cell wall that covers the plasma membrane.
Cytoplasm.
Genetic material that is not enclosed by a membrane.
Absence of membrane-bound organelles.
B. Eukaryotic Cells
The internal environment is divided into membrane-bound compartments called organelles.
Have a Nucleus, Cytoplasm, and Organelles:
Eukaryotes include humans, all other animals, plants, fungi, and protists.
III. Why Cells Remain Small to Stay Efficient
The total metabolic activities of a cell are directly proportional to its volume of cytoplasm.
As cells increase in size, their volume increases at a faster rate than their surface area.
Small cells maintain a higher surface-to-volume ratio, which is crucial for efficient exchange of substances with their environment.
Microvilli: Small, finger-like projections of the cytoplasmic membrane that serve to increase the surface area, enhancing absorption and exchange.
IV. Internal Structures of Eukaryotic Cells and Their Specific Functions
Cells contain both membrane-bound and non-membrane-bound internal structures that carry out specific functions.
A. Nucleus — Information Storage and Control Center
Contains the genetic information (DNA) of the cell.
Controls all activities of the cell.
Structural features:
Chromosomes: Structures composed of DNA, which house the cell's genetic information.
Nucleolus: The site within the nucleus where components of ribosomes are synthesized.
B. Ribosomes — Protein Factories
Particles composed of ribosomal RNA (rRNA) and protein.
Carry out protein synthesis in two primary locations:
In the cytosol (referred to as free ribosomes).
On the outside of the Endoplasmic Reticulum (ER) or the nuclear envelope (referred to as bound ribosomes).
Diagrams typically show ribosomes composed of a large subunit and a small subunit.
C. Endoplasmic Reticulum (ER) — Biosynthetic Factory
Accounts for more than half of the total membrane in many eukaryotic cells.
Its membrane is continuous with the nuclear envelope.
Divided into two distinct regions:
Smooth ER:
Lacks ribosomes on its surface.
Functions:
Synthesizes lipids.
Metabolizes carbohydrates.
Detoxifies drugs and poisons.
Stores calcium ions (e.g., in muscle cells).
Rough ER:
Covered with ribosomes on its surface.
Functions:
Has bound ribosomes that synthesize proteins, particularly glycoproteins (proteins with attached carbohydrate chains), which are then secreted.
Distributes transport vesicles, which are membrane-bound sacs containing proteins.
Serves as a "membrane factory" for the cell, synthesizing new membrane phospholipids and proteins.
D. Golgi Apparatus — Shipping/Receiving Center
Refines and modifies synthesized products received from the ER.
Serves as the cell's packaging and shipping center.
Products are packaged into vesicles and shipped to other locations within the cell or to the plasma membrane for export (secretion).
E. Vesicles: Membrane-Bound Storage and Shipping Containers
Several types with specialized functions:
Secretory vesicles: Contain products destined for export from the cell.
Endocytic vesicles: Contain substances imported from the external environment.
Peroxisomes: Contain enzymes that detoxify various wastes produced by the cell, often producing hydrogen peroxide (\text{H}2\text{O}2) as a byproduct, which is then converted by other enzymes into water and oxygen.
Lysosomes: Contain digestive enzymes.
F. Lysosomes — Digestive Compartments
Membranous sacs that contain a variety of hydrolytic enzymes capable of digesting macromolecules.
These enzymes hydrolyze proteins, fats, polysaccharides, and nucleic acids.
Lysosomal enzymes function optimally in the acidic environment maintained inside the lysosome (a \text{pH} of approximately 4.5-5.0).
G. Endomembrane System — A Functional Network
A system of membranes inside and around a eukaryotic cell, related either through direct physical continuity or by the transfer of membrane segments as vesicles.
Includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.
These components work together to synthesize, modify, package, and transport proteins and lipids.
H. Mitochondria — The Powerhouse of the Cell
Mitochondria are the sites of cellular respiration, a metabolic process that utilizes oxygen to generate adenosine triphosphate (ATP), the cell's main energy currency.
I. Fat and Glycogen: Sources of Energy Storage
These energy sources may be stored within the cell but are not enclosed by any membrane.
Fat (Triglycerides):
Serves as a long-term energy storage in animals.
Stored primarily in the cytoplasm of fat cells (adipocytes).
Glycogen:
A carbohydrate storage polymer.
Serves as short-term energy storage in animals.
Stored in the cytoplasm of muscle cells and liver cells.
V. Cell Structures for Support and Movement
Cells possess specialized structures for maintaining shape, facilitating internal transport, and enabling movement.
A. Cytoskeleton — Internal Scaffolding
Provides internal scaffolding that helps maintain cell shape and provides mechanical support.
Composed of:
Microtubules: Tiny hollow tubes made of tubulin protein. They are involved in maintaining cell shape, organelle movement, and cell division.
Microfilaments: Thin, solid fibers made of actin protein. They are involved in muscle contraction, cell division, and cell shape changes.
Microtubules and microfilaments together form a dynamic framework that supports the cell and helps anchor other cellular structures.
B. Cilia and Flagella — Specialized for Movement
Motile appendages that extend from the cell surface.
Flagella:
Are typically long and few in number (often single).
Enable cells, such as spermatozoa, to swim.
Cilia:
Are typically short and numerous.
Found in ducts and tubes (e.g., lining the respiratory tract, where they help move mucus).
C. Centrioles
Short, rod-like microtubular structures usually located near the nucleus.
Play an important role in cell division, particularly in the formation of the spindle fibers.
VI. The Plasma Membrane — The Cell's Boundary
Separates a cell from its external environment.
Exhibits selective permeability, meaning it controls which substances enter and exit the cell.
Enables the transfer of information and signals between the environment and the cell.
A. Structure of the Plasma Membrane — A Lipid Bilayer
The plasma membrane is described by the fluid mosaic model.
Phospholipids:
Form the fundamental two layers (bilayer) of the membrane.
Each phospholipid has a hydrophilic head (water-loving) and a hydrophobic tail (water-fearing).
The hydrophilic heads face the aqueous environment inside and outside the cell, while the hydrophobic tails face inward, away from water.
Cholesterol:
Embedded within the lipid bilayer, it increases the mechanical strength and modulates membrane fluidity.
Proteins:
Integrated into or associated with the lipid bilayer, they serve various functions, including transport of products across the membrane, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.
Nonrigid and Fluid Mosaic:
The membrane is nonrigid; phospholipids and proteins are not static but are able to drift and move laterally relative to each other.
B. Fluidity of Membranes
Membrane fluidity is crucial for proper function.
As temperatures cool, membranes undergo a phase transition from a fluid state to a solid (gel-like) state.
Membranes rich in unsaturated fatty acids (with kinks in their hydrocarbon tails) are more fluid than those rich in saturated fatty acids because the kinks prevent tight packing.
C. Why Cholesterol is So Important for Membrane Fluidity
Membranes must maintain a certain level of fluidity to function properly.
Cholesterol has varying effects on membrane fluidity at different temperatures:
At warm temperatures (e.g., 37°C in humans), cholesterol restrains the movement of phospholipids, thereby reducing fluidity.
At cool temperatures, cholesterol maintains fluidity by preventing the phospholipids from packing too closely together and solidifying.
VII. How Molecules Cross the Plasma Membrane
Molecules cross the plasma membrane via various transport mechanisms.
A. Passive Transport — No Energy Required
Movement of substances across the membrane without the cell expending metabolic energy.
Substances move down their concentration gradient.
The diffusion of a substance across a biological membrane is passive transport.
Diffusion:
Molecules inherently tend to spread out evenly into an available space.
Each molecule moves randomly, but the diffusion of a population of molecules may be directional (net diffusion) from an area of higher concentration to an area of lower concentration until equilibrium is reached.
Substances diffuse down their concentration gradient.
Osmosis:
A specific type of diffusion involving water.
Water diffuses across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration until the solute concentration is equal on both sides.
Tonicity refers to the ability of a surrounding solution to cause a cell to gain or lose water.
Isotonic solution: The solute concentration is the same as that inside the cell; there is no net water movement across the plasma membrane. Animal cells are normal in this state.
Hypertonic solution: The solute concentration is greater than that inside the cell; the cell loses water. Animal cells shrivel, and plant cells become plasmolyzed (plasma membrane pulls away from the cell wall).
Hypotonic solution: The solute concentration is less than that inside the cell; the cell gains water. Animal cells may lyse (burst), while plant cells become turgid (normal and firm) due to the cell wall preventing lysis.
Facilitated Diffusion:
Still passive (no energy), but involves transport proteins to help molecules cross the membrane more quickly.
Channel proteins: Provide corridors that allow a specific molecule or ion to cross the membrane.
Examples include aquaporins for facilitated diffusion of water and ion channels that can open or close in response to a stimulus (gated channels).
Carrier proteins: Bind to molecules and undergo a subtle change in shape that translocates the solute-binding site across the membrane, releasing the solute on the other side.
B. Active Transport — Energy Required
Movement of substances against their concentration gradients (from an area of lower concentration to an area of higher concentration).
Requires the cell to expend metabolic energy, typically in the form of ATP.
Sodium-Potassium Pump: A well-known example of an active transport system.
It pumps 3 Na^+ ions out of the cell and 2 K^+ ions into the cell for each molecule of ATP consumed.
Mechanism:
1. Three Na^+ ions from the cytoplasm bind to the pump.
2. ATP phosphorylates the pump, causing a conformational change.
3. The pump releases the three Na^+ ions outside the cell.
4. Two K^+ ions from the extracellular fluid bind to the pump.
5. The phosphate group is released, and the pump reverts to its original shape.
6. The pump releases the two K^+ ions into the cytoplasm.
VIII. Bulk Transport via Vesicles
Used for transporting large molecules (like polysaccharides and proteins) across the membrane, which is impossible through the lipid bilayer or via small transport proteins.
Involves the formation of membranous vesicles or vacuoles and requires energy.
A. Exocytosis
Mechanism: Transport vesicles migrate to the plasma membrane, fuse with it, and then release their contents to the outside of the cell.
Many secretory cells use exocytosis to export their products (e.g., hormones, neurotransmitters, enzymes).
B. Endocytosis
Mechanism: The cell takes in macromolecules by forming vesicles from the plasma membrane.
This process is essentially the reverse of exocytosis but involves different proteins.
There are three main types:
Phagocytosis ("Cellular Eating"):
A cell engulfs a particle (e.g., bacteria, cellular debris) in a large vacuole (phagosome or food vacuole).
The vacuole then typically fuses with a lysosome to digest the particle (e.g., observed in amoebas or macrophages).
Pinocytosis ("Cellular Drinking"):
The cell "gulps" droplets of extracellular fluid into tiny vesicles.
It is a non-specific process, taking in any solutes dissolved in the fluid.
Receptor-Mediated Endocytosis:
A more specific type of endocytosis that allows the cell to acquire bulk quantities of specific substances.
Extracellular substances (ligands) bind to specific receptor proteins on the outer surface of the membrane.
These receptors are clustered in coated pits that then invaginate to form a coated vesicle containing the bound ligands.