Cells and functions of cells (BIOL 112)

Cells and functions of cells
Cells and functions of cells
Welcome to the tutoring wiki for BIOL 112. Here is where you'll find learning objectives for BIOL 112 and some detailed explanations for slightly more challenging topics.
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Cell theory

Describe the current version of cell theory

All organisms are made of one or more cells

Cells are the smallest units of life

Cells come from pre-existing cells

All cells are based on the same chemistry

Define a cell and describe the minimal structures and functions required for a cell

A cell is a membrane-bound unit capable of carrying out essential life processes

Metabolism: ability to assimilate and transform energy

Anabolism: gathering raw materials and gathering or making parts

Response to a change in the environment

Homeostasis maintain a stable internal environment with concentrations of molecules and ions that differ from those outside

Growth: For a unicellular organism means an increase in cell size and volume. For a eukaryotic organism means increasing number of cells.

Reproduction: For the unicellular organism is largely asexual through cell division to form a clone

Nutrition: either the synthesis of organic molecules or the absorption of organic matter.

Cells and organisms

Identify the major macromolecular molecular constituents of cells

Polymer	Monomer
Proteins	Amino acids
DNA	Deoxyribonucleotides
RNA	Ribonucleotides
Carbohydrates	Monosaccharides
Lipids	Glycerol and fatty acids

Describe the typical size range for viruses, prokaryotic cells and eukaryotic cells

Viruses: 10-100nm

Prokaryotic cells: 100nm-1µm

Eukaryotic cells: 10µm-100µm

Describe the characteristics to distinguish between prokaryotic and archaeal and eukaryotic cells

Characteristics	Archeal	Prokaryote	Eukaryote
Intial amino acid in translation	Methionine	N-Formylmethionine	Methionine
Membrane-bound organelles	Absent	Absent	Present
Diphtheria toxin	Sensitive	Insensitive	Sensitive
Streptomycin	Insensitive	Sensitive	Insensitive
Presence of nucleus, chromosome, and mitochondria	Absent	Absent	Present
Cell membrane lipids	Ether lipids	Phospholipids	Phospholipids
Coupling of transcription and translation	Yes	Yes	No
Introns in genes	Absent	Absent	Present
Size of ribosomes (in svedberg unit)	70S	70S	80S

Cell growth

Describe what is meant by growth in a unicellular organism

The formation of colonies by producing daughter cells usually via binary fission

Calculate the approximate number of cells in a growing population based upon the initial number of cells

Coming soon

Identify the following phases on a growth curve graph: lag phase, exponential phase, stationary phase, death phase

Contrast lag phase, exponential phase, stationary phase, and death phase in terms of division rate, cell composition, and cell viability

Coming soon

Predict the response in cell growth to environmental conditions

Coming soon

Membranes

Remember the pattern of electronegativity of O, N, C S P H atoms

O > N > S = C > P = H

Describe why water is termed a polar molecule

Due to the difference in electronegativities of oxygen and hydrogen atoms, the O-H bond is polar. These 2 polar bonds do not cancel out in the V-shaped structure of water, hence the water molecule has an overall dipole moment. Thus it is a polar molecule.

Describe the process of diffusion and the consequences of diffusion for cells

Random movement of molecules due to kinetic energy, from an area of high solute concentration to an area of lower solute concentration.

Consequence: equilibrium is reached- there is equal movement of particles in both directions across the membrane. This does not mean that the molecules stop moving completely; just an equal rate of molecules moving in both directions.

Recognize the types of interactions that can form between polar and non-polar of molecular groups on molecules

Polar: Permanent dipole, hydrogen bonding (H-bonding)

Non-polar: Van de Waal's forces, hydrophobic interaction

Describe why phospholipids are amphipathic

A molecule is amphipathic if it has both hydrophilic and hydrophobic character. The phospholipid molecule has a charged phosphate head giving it hydrophilic character. It also has either one or two fatty acid chains which give it the hydrophobic character.

Describe why it is favourable to exclude hydrophobic molecules from water (the hydrophobic effect)

When a hydrophobic molecule is dropped into water, water molecules tend to form a stiff cage around this hydrophobic molecule and cannot make as many hydrogen bond as it would be without these hydrophobic molecules

This restricts the number of configuration/random states that the water molecules can take. This is not favorable because according to the 2nd Law of Thermodynamics, systems attempt to maximize its entropy.

Hence, the hydrophobic effect is an entropy-driven process that cause hydrophobic molecules to aggregate, being less exposed to water molecules so that these water molecules are more free to make hydrogen bonding with other neighboring water molecules.

Predict the structures that will form when phospholipids are mixed with water and provide a rationale

Phospholipids usually have diglycerides- 2 hydrophobic 'tails'. When they are mixed with water, they automatically form a bilayer that can contain an aquaeous cavity.

This is the structure that creates a water-free region where the hydrophobic tails are in contact with one another and let the hydrophilic phosphate heads be exposed to the aquaeous external environment. This is the most thermodynamically favourable, thus stable arrangement of phospholipids when mixed with water.

For phospholipids that have single hydrocarbon tail, they form micelles lacking an aquaeous cavity

Describe how phospholipid bilayers form a barrier to diffusing molecules

The phospholipids are closely packed because small distance facilitates the van der Waals' interaction between the hydrophobic fatty acid tails. This prevents the entry of large molecules.

The phospholipids bilayer form mainly due to the hydrophobic interaction between the fatty acid chains which create a water-free region. This prevents the entry of polar/ charged molecules because they cannot form a stable interaction with this hydrophobic region and enter cross the bilayer.

Predict the membrane permeability of various types of molecules based on size and charge

In order of decreasing permeability: Small uncharged (oxygen)-> large uncharged (some small proteins)-> small polar (water) -> large polar (carbohydrates)-> charged (ions such as Na⁺, K⁺)

Transport

Describe the different mechanisms of transport across cell membranes

Passive diffusion: random movement of molecules from a region of higher solute concentration to a region of lower solute concentration

Osmosis: movement of water molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration

Facilitated diffusion: passive movement of molecules down the concentration gradient across membranes through protein channels

Active transport: molecules being pumped across the membrane against their concentration gradient by proteins using ATP Bulk transport: molecules are transported using vesicles formed by invagination of membranes

Contrast the different types of transport in terms of energy requirements, proteins, and concentration dependence

Type of transport	Energy requirement	Protein requirement	Concentration gradient
Passive diffusion	None	None	Down
Facilitated diffusion	None	Protein channels	Down
Active transport	ATP	Protein pumps	Against

Describe the Fluid Mosaic Model of membranes and the benefits such a membrane provides to cells

The fluid mosaic model of membranes describe the structure of the biological membrane as a bilayer of amphipathic phospholipids. These phospholipids can exchange place within the same leaflet giving the membrane lateral fluidity. Biological membranes also contain peripheral and integral proteins which are distributed asymmetrically.

Such a membrane provides protection of the organelles against minor mechanical damages, an effective barrier to control uptake of molecules and ions and also play a role in cell-cell communication.

Describe the characteristics of the cell membrane that lead to the problem of osmosis and osmotic pressure

Plasma membranes are more permeable to water molecules than almost other ions, small molecules and macromolecules due to the presence of protein channels called aquaporins that selectively allow the passage of water.

Cells generally contain higher solute concentration in the cytosol compared to the surrounding environment (in a hypotonic solution). This causes water to move into cells, increasing the osmotic pressure inside the cells causing lysis

Explain how the external environment changes the osmotic pressure experienced by cells

In a hypotonic solution (concentration of solute inside cell is higher than outside), water moves into cells creating an outward osmotic pressure, causing cells to lyse

In an isotonic solution (concentrations of solute inside and outside cells are the same), cells neither gain nor lose water.

In a hypertonic solution (concentration of solute inside cells is lower than outside), water leaves the cells causing them to shrink

Describe the mechanisms eukaryotic and prokaryotic cells use to prevent damage due osmosis

In bacteria and plants, a rigid cell wall surrounds the membrane preventing osmotic lysis

Some protists have a contractile vacuole that pumps water out of the cells

Multicellular organisms try to increase osmolarity of blood plasma and interstitial fluid surrounding cells with albumin and other proteins

Cells also actively pump out ions to increase the osmolarity of their surroundings

Describe the basic differences between the bacterial structures of Gram negative and Gram positive cell walls and how they relate to osmotic pressure

Characteristics	Gram-positive	Gram-negative
Peptidoglycan layer	Thick	Thin
Periplasmic space	Absent	Present
Outer membrane	Absent	Present
Liposaccharide content	Virtually none	High
Resistance to physical disruption	High	Low
Resistance to sodium azide/ drying	High	Low
Inhibition by basic dyes	High	Low

Peptidoglycan layer prevents lysis of the bacteria. Gram-positive bacteria has a thicker peptidoglycan layer than Gram-negative; however Gram-negative bacteria has an outer membrane that protects peptidoglycan.

Other challenging topics in First Year Biology:

Membranes

Protein and enzymes

Transcription in prokaryotes

Transcription in eukaryotes

Gene regulation

lac/mal operon

Electron transport chain (ETC) and chemiosmosis

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