IB Bio Test #1: Cells and Plasma Membranes (Q/A)

The Cell Theory

The Three Postulates:

  1. The cell is the smallest unit of life

  2. Cells only arise from pre-existing cells

  3. All living things are composed of one or more cells

Exceptions:

Skeletal Muscle: Contain multiple nuclei

Red Blood Cell: Contain no nucleus

Fungal Hyphae: Lack internal walls, making multiple nuclei

Sieve Elements: Contain no nucleus

Microscopes

Light Microscopy

Used to view living specimens in their natural colors

Glass lenses bend light to magnify the images

Clarity of cellular sub-structures can be increased via fluorescent labeling

  • Synthetic dyes- bind cellular compounds to resolve specific structures

  • Immunofluorescence staining- use antibodies connected to fluorescent probes to target specific cellular components

Electron Microscopy

Generate images at a higher magnification and resolution, but cannot use living specimens in their natural colors

Electromagnets focus electrons and produce monochromatic images

Two types of electron microscopes:

  1. Transmission electron microscopes (TEM)- pass electrons through a specimen to generate a cross-section image

  2. Scanning electron microscopes (SEM)- scatter electrons over a surface to differentiate depth/map in 3D

Cryogenic microscopy- freezing samples prior to viewing in order to generate images of a comparable standard to X-ray crystallography

  • Allows for determination of molecular structures at a near-atomic resolution without the crystallization of a specimen

  • Freeze fracturing- a frozen specimen is cracked along a plane so internal cellular structures can be studied

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cell Characteristics

Single-celled organisms without compartmentalized organelles (not membrane-bound)

Two domains:

  • Bacteria- namely includes disease-causing pathogens

  • Archaea- extremophiles; one of the oldest classifications of organisms

Main strand of DNA (the genophore) is found within the nucleoid; additional DNA molecules (plasmids) can be exchanged via bacterial conjugation

  • Pili help with this plasmid exchange

Commonly Found Organelles:

Ribosomes (70S)- smaller than those of eukaryotic cells (usually 80S)

Cell Wall

Glycocalyx- a slime capsule exterior (not necessary but commonly found)

Pili- Adhesion Pili aid with adhesion to surfaces and S Pili with transfer of genetic material

Flagella- facilitate movement

Eukaryotic Cell Characteristics

Organisms whose cells contain a nucleus and membrane-bound/compartmentalized organelles

Four domains:

  • Plant- cell wall made of cellulose, autotrophic via photosynthesis

  • Animal- no cell wall, heterotrophic

  • Fungus- cell wall made of chitin, heterotrophic via absorption

  • Protist- any organism that does not fit into the plant, animal, and fungus domains

DNA is found within a double-membrane-bound nucleus

Commonly Found Organelles

Ribosomes (80S)- larger than those of prokaryotic cells (70S)

Nucleus- double membrane organelle with pores that stores genetic information

Membrane-bound organelles

Chloroplast (plants only)- aid with photosynthesis

Filamentous hyphae (fungi only)- enable nutrient absorption, separated by septa (internal walls)

Key Differences Between Prokaryotes and Eukaryotes

  1. Eukaryotes store their DNA in a nucleus

Allows cells to separate transcription (nucleus) and translation (cytoplasm/ribosomes)

  • Transcription: genes —> mRna

  • Translation: mRna —-> proteins/polypeptide chains

Separating transcription and translation allows for the modification fo mRNA before it is turned into proteins

  • Stabilizes mRNA transcript and removes unnecessary non-coding sequences (introns)

  • Improves efficiency of protein synthesis and allows greater control over gene expression

  1. Eukaryotes have membrane-bound organelles

Enables organelles to maintain internal homeostasis, which can be different from cytoplasm

Keeps important enzymes and metabolites concentrated where they are needed most

Key example: Lysosomes and phagocytic vacuoles

  • If not contained, these organelles would digest everything in the cell (autophagy)

Organelles and Their Functions

Subunits of cells that are adapted to perform a specific, important function

In eukaryotic cells, many organelles are membrane-bound, which allows for further specialization and compartmentalization

Types of Organelles

Nucleus- double membrane structure that stores genetic information

  • Nucleolus found inside makes ribosomes

Mitochondria- responsible for ATP production via aerobic respiration

  • Inner membrane is folded to increase SA:Vol ratio

Endoplasmic Reticulum- membraneous network responsible for transporting materials via vesicles

  • Smooth ER- synthesizes lipids

  • Rough ER- synthesizes proteins

Golgi Complex- assembly of folded membranes where materials are stored, modified, and exported from the cell

Vesicles- membrane sacs that transport/store materials

  • Peroxisomes- digestion of toxic metabolites

  • Lysosomes- breaking down of cell waste

  • Vacuoles- comparatively larger, store excess fluids

Chloroplast- responsible for photosynthesis, employing chlorophyll to absorb/utilize sunlight

Centrosome (Animal)- Microtubule-organising centre aiding in mitotic cell division

Exceptions

Cell walls are not considered organelles because they are extracellular

Cytoskeletons/cytosol are categorized as extracellular components, not membrane bound

Stem Cells

Properties

Unspecialized cells that have two key qualities:

  1. Self renewal- can continually divide and replicate

  2. Potency- have the capacity to differentiate into different cell types

Once a stem cell specializes, it can no longer form alternative cell types

Types of Stem Cells

  1. Totipotent- can form any type of cell and develop entirely new organisms

  2. Pluripotent- can form any type of cell from the top three germ layers

  3. Multipotent- can only form a number of closely related cell types

Totipotent and pluripotent cells are considered embryonic stem cells while multipotent are considered adult stem cells

While embryonic stem cells are more potent than adult stem cells, there are greater ethical concerns for obtaining them, while adult stem cells have less graft rejection and ethical issues. This means adult stem cells are used for often to treat diseases

Induced pluripotent stem cells made from adult stem cells are difficult and expensive to produce

Location/Function

Stem cell niches- pools of adult stem cells that are maintained in preparation of future differentiation/proliferation

Bone Marrow - haemopoietic stem cells creates different types of blood cells

  • Erythrocytes, leucocytes, and thrombocytes

  • Bone marrow transplants are often used to cure leukemia

Hair Follicles- contain various epidermal stem cells used for hair growth, skin innervation, and wound repair

  • Can be potentially harvested to repair scar tissue on burn victims

Cell Membrane

Function to enclose contents of the cell and maintain homeostasis

Possess two key qualities to promote homeostatic regulation:

  1. Semipermeability- only certain materials can cross freely

  2. Selectivity- can control the passage of any materials that cannot go freely

Membrane as a Barrier

Comprised of two main elements: phospholipids and proteins

Phospholipid Bilayer- acts as a barrier to some materials

  • Hydrocarbon chains at the core have low permeability and are hydrophobic to large and charged substances

  • Large compounds/hydrophilic substances cannot cross

Membrane Proteins- embedded within phospholipid bilayer; act as points of transport for large and charged substances

  • Makes lipid bilayer a selective barrier because hydrophilic materials can be transported as needed

Phospholipids

Polar head that is hydrophilic and composed of a glycerol and phosphate molecule; two non-polar tails that are hydrophobic composed of fatty acid (hydrocarbon) chains

  • Because regions are both hydrophilic and hydrophobic, it is classified as amphipathic

Arranged into a bilayer (two parallel layers)

  • Hydrophobic tails face inwards, shielded from polar fluids

  • Hydrophilic heads face outwards to associate with cytosolic/extracellular fluids

  • Presence of both restricts passage of many substances

Bilayer is held together by weak hydrophobic interactions between tails

Individual phospholipids can move within the bilayer, allowing for fluidity/flexibility

  • Allows for breaking and reforming of membranes (exocytosis/endocytosis)

Proteins

Integral Proteins- penetrate the phospholipid bilayer to remain permanently attached to the membrane

  • Cannot be readily isolated without disrupting the bilayer

  • Ex. protein pumps, carrier proteins, glycoproteins, ion channels

Peripheral Proteins- temporarily associated with one side of a membrane

  • Attached to integral proteins, linked to polar heads of the bilayer, or held in place by cytoskeleton/extracellular matrix

  • Ex. receptor proteins involved in cell signaling

Non-polar amino acids often associate with the lipid bilayer while polar amino acids will face aqueous solutions

The inner surface of a protein channel will be lined with polar amino acids to help with passage of polar and charged molecules.

Functions:

Junctions- join/connect two cells together
Enzymes- localize metabolic pathways
Transport- facilitated diffusion and active transport
Recognition- markers for cellular identification
Anchorage- attachment points for cytoskeleton/extracellular matrix
Transduction- receptors for peptide hormones

Glycocsylation

A carbohydrate chain is attached to phospholipids/proteins, resulting in glycolipids and glycoproteins respectively

Carbohydrate chains are found on the extracellular side of a membrane

  • Serve as attachment points for other cells and act as a point of recognition between cells

Glycolipids and glycoproteins play important roles in maintaining structural integrity of extracellular matrix, which provides structure and biochemical support to surrounding cells

  • Link extracellular molecules together to make a cohesive matrix

Fluid Mosaic Model

Cell membranes are:

  1. Fluid- viscous, allows individual phospholipids to move around

  2. Mosaic- embedded with proteins

Cholesterol may be embedded in animal cell membranes

Transportation Through the Cell Membrane

Passive vs. Active Transport

Passive transport involves the movement of materials along a concentration gradient, which does not require energy/ATP

  • Three types of passive transport: simple diffusion, osmosis, and facilitated diffusion

Diffusion is the net movement of molecules from a region of high to low concentration until equilibrium is reached

  • Temperature, molecular size, and steepness of gradient affect speed of diffusion

Active transport involves the movement of materials against a concentration gradient, which requires the expenditure of energy/ATP

  • Two types of active transport: primary active transport and secondary active transport

Simple Diffusion

Small, lipophilic molecules freely pass between phospholipids to cross the bilayer

  • Small, non-polar gases such as oxygen and carbon use simple diffusion

  • Lipophilic molecules like non-polar steroids (testosterone and oestradiol) can do so as well

Large, charged molecules can not cross the membrane via simple diffusion

Facilitated Diffusion

The passive movement of molecules across the membrane with the help of a membrane protein along a concentration gradient

  • Utilized by large, polar, charged molecules

Channel proteins- integral lipoproteins that contain a hydrophilic pore so ions may cross the membrane

  • Ion-selective; may be gated to regulate passage of ions to certain stimuli

  • Faster rate of carriage than carrier proteins

Carrier proteins- integral glycoproteins that bind a solute, undergo a conformational change, and then translocate the solute across a membrane

  • Only bind specific molecules similar to an enzyme-substrate interaction

  • Called protein pumps in active transport

  • Have a much slower rate of carriage

Active Transport

Uses energy to move molecules against a concentration gradient

Primary active transport- direct hydrolysis of ATP

Secondary active transport- coupling transport with another molecule that is moving against the gradient

Involves the use of protein pumps (membrane proteins using energy)

  • A solute binds to the pump on one side, ATP hydrolysis causes conformational change, solute is translocated across membrane and released

Osmosis

Net movement of water molecules from areas of low solute concentration to areas of high solute concentration

  • Water is polar but small enough to pass between phospholipids in the bilayer

  • It is attracted to polar molecules and ions

Water is a solvent that can dissolve any polar or charged molecule to create a solution

Integral proteins called aquaporins function as water channels in the bilayer

  • Facilitate faster rates of transportation and regulate solute concentrations

Tonicity

Hypertonic- high solute concentration

Hypotonic- low solute concentration

Isotonic- equivalent solute concentrations

Osmolarity- measure of a solute concentration as defined by the number of osmoles per litre