Understanding Cell Biology: Structure, Function, and Processes

Posted on Aug 24, 2024 in Biology

General Properties and Types of Cells

Cell Theory

1. All organisms are made of cells.
2. The cell is the fundamental unit of life.
3. Cells come from pre-existing cells.

What is a Cell?

1. Contains a stable blueprint of information (DNA).
2. Has a discrete boundary separating the cell interior from the environment (cell membrane).
3. Ability to harness material and energy from the environment.
4. Able to read and interpret instructions (DNA).

The Central Dogma of Molecular Biology

1. Describes the basic flow of information in a cell.
2. Pathway from DNA → Transcription → RNA → Translation → Protein.

Purpose of Cellular Membrane

1. Controls the movement of material in and out of the cell.
2. Internal membranes divide cells into discrete compartments – each specialized for a particular function (e.g., the nucleus houses DNA).

Why Prokaryotes (Bacteria) are Successful

1. Small size.
2. Reproduce rapidly.
3. Ability to obtain energy from diverse sources.

Why are Bacteria Small Compared to Eukaryotes?

1. Small cell = higher surface area to volume ratio.
2. Diffusing molecules do not have to travel as far.

Similarities & Differences in Bacteria (Prokaryote) vs. Eukaryote Cell

The Three Domains of Life

Parts of the Cell

Nucleus

1. Stores the cell’s genetic information (DNA).
2. Site for RNA synthesis.

Nucleoid

Discrete region of the cell where DNA is concentrated in prokaryotes.

Plasmids

Small circular molecules of DNA that carry additional genes in bacteria.

Plasma/Cell Membrane

1. Composed of phospholipids and proteins.
2. Regulates material flow in & out of the cell.

Cell Wall

1. Only in plants.
2. Rigid barrier.
3. Composed of polysaccharides.

Mitochondria

Produces most of the ATP (serves as the energy currency of the cell).

Chloroplasts

1. Enables plant cells to harness the sun’s energy.
2. Synthesizes sugars.

Metabolism

Chemical reactions by which cells convert energy from one form to another.

Transcription

The synthesis of RNA from a DNA template – existing proteins create a copy of DNA’s information via RNA (step 1).

Translation

When specialized molecular structures in the cell “read” the RNA molecule to determine what building blocks to use to create a protein (step 2).

Lipids

Components of Cell Membrane

Made of PHOSPHOLIPIDS → made of:

1. Glycerol backbone attached to a phosphate group and 2 fatty acids.
2. Phosphate head → polar.
3. Fatty acid tails → hydrophobic & no hydrogen bond.

Why Phospholipids Arrange in an Aqueous Environment

Phospholipids are amphipathic = a molecule with both hydrophilic and hydrophobic regions. In water:

1. Polar heads go on the outside & interact with water.
2. Nonpolar tails go inside away from water.
3. Forms LIPOSOME, BILAYER (less bulky head & 2 tails), or MICELLE (bulky head & single tail).

Reasons for Fluid Nature of Membranes

1. Extensive van der Waals forces (LDF & DPDP) between fatty acids.
2. Can be easily broken & reformed → allow lipids to move within the plane of the membrane → therefore FLUID.

Types of Non-Covalent Interactions Amongst Various Parts of Membrane Components

1. Van der Waals forces amongst lipids → break & reform to move along the plane of the membrane.
2. Cholesterol (amphipathic attaching to lipid):
   1. Cholesterol head (hydrophilic OH group) interacts with the hydrophilic phospholipid head.
   2. Cholesterol ring (hydrophobic) has van der Waals interactions with lipid fatty acid chains.

Factors Affecting Fluidity of Cell Membrane

1. Length of fatty acid tail: long fatty acid tail → more van der Waals interactions → tighter packing → reduced lipid mobility & fluidity.
2. Presence of double bonds between neighboring carbon atoms: no double bonds → straight & tight packed → reduced mobility.

Role of Cholesterol in Maintaining Membrane Fluidity

1. Decrease fluidity in warm/most temperatures: interaction of the rigid ring of cholesterol with the fatty acid tail reduces the mobility of phospholipids (kinks).
2. Increase fluidity in low temperatures: cholesterol prevents phospholipids from packing tightly with other phospholipids → prevent transition from fluid to solid-state.

Different Types of Proteins Found in Cell Membrane

• Transporter: Moves ions & other molecules across the membrane.
• Receptor: Allows the cell to receive signals from the environment.
• Enzyme: Catalyzes chemical reactions.
• Anchor: Attaches to other proteins and helps maintain cell structure & shape.

Integral Membrane Protein

PERMANENTLY associated with the cell membrane → cannot separate from the membrane without destroying the membrane (typically transmembrane proteins).

Peripheral Membrane Proteins

TEMPORARILY associated with the lipid bilayer → easily separated from the membrane.

Transmembrane Proteins

(Most integral membrane proteins are this) composed of 3 regions:

• 2 hydrophilic regions on each side of the membrane.
• 1 hydrophobic region through the membrane.

The Fluid Mosaic Model

The idea that lipids & proteins coexist in the membrane and both are able to move in the plane of the membrane.

Membrane Permeability

What Chemical Properties Make the Lipid Bilayer Selectively Permeable?

1. Hydrophobic interior of the bilayer: prevents ions, charged, or polar molecules from diffusing across the plasma membrane.
2. Packed bilayer: prevents macromolecules (e.g., proteins & polysaccharides) from crossing the membrane alone → too large.
3. Gas, lipids, and small polar molecules can freely move across the bilayer.

Hydrophobic Effect

It is energetically favorable for a molecule to make the strongest type of intermolecular interaction available – higher entropy molecules can make more arrangements → spontaneous bilayer.

Why Membrane is Necessary as Cell Size Increases

1. Not enough surface area: volume ratio to complete cellular reactions (e.g., transport essential nutrients), the membrane = more surface area for diffusion.
2. Diffusion travel is long.
3. Nutrients/chemicals necessary for reactions are easily diluted in large cells → membrane-bound organelles concentrate diffused solutes in areas.

Ways that Cells Avoid Cell Shrinkage and Lyses (Bursts) When Not in Isotonic Environment

1. Contractile Vacuole – compartments that take up excess water from inside the cell then expel it via contraction when hypotonic:
   • Take in water by aquaporin.
   • Take water with a proton through pumps, with water following by osmosis.
2. Cell Wall: Allows turgor pressure to build up → opposes the driving force of water to enter.
3. Vacuole: Absorbs water and contributes to turgor pressure.

Proteins

Characteristics of an Amino Acid

1. Has a central carbon – called the alpha carbon.
2. Covalent bonds to the amino group.
3. Covalent bond to the carboxyl group.
4. Covalent bond to the R group – AKA side chain.

Primary Structure

Peptide bond links α-amino with the next α-carboxyl – the sequence of amino acids in a protein (primary) determines how a protein folds.

Secondary Structures Result from Hydrogen Bonds

1. α-helix: Polypeptide backbone is twisted tight in a RIGHT-handed coil → 3.6 amino acids per complete turn → helix is stabilized by hydrogen bonds between carbonyl & amide groups → R groups project out which DETERMINE where the α-helix is located in the protein.
2. β-sheet: Polypeptide folds back and forth on itself → forming a PLEATED sheet → R groups project alternately above & below the plane of the β-sheet → 4 to 10 polypeptide chains side by side → amides in each chain are hydrogen-bonded to the carbonyls on either side.

Tertiary Structures Result from Interactions Between R Groups

1. Structure of the tertiary structure is determined by:
   • Spatial distribution of hydrophilic & hydrophobic R groups.
   • Different types of bonds (ionic, hydrogen, van der Waals) between R groups.
2. Tertiary is made of several secondary structures.
3. Tertiary shape is defined by the PRIMARY amino acid sequence.

Protein Denaturation

Protein unfolds because of chemical treatment, high temperature, pH change → hydrogen and ionic bonds are disrupted & protein loses functional activity.

Gibbs Free Energy

ΔG = ΔH + TΔS → entropy = motional freedom of a system & enthalpy = measure of how strongly bonded a system is.

Endosymbiosis Steps

1. Aerobic bacteria were internalized by a protoeukaryotic host.
2. Developed an endosymbiotic relationship.
3. Transferred many bacterial genes into the host nucleus over time.
4. Now semi-autonomous intracellular (mitochondrion).