1.1: Cells In Our Body

Cell Definition

• Membrane bound organelles that contain macromolecules

• Phospholipid membrane sorrounds all cells

Cell Membrane Composition

• made up of lipid macromolecules called phospholipids

• each molecules has hydrophillic/hydrophobic properties, allowing them to form stacked bilayers of lipid molecules

• the core of these molecules (made up of fatty tails) is hydrophobic (water-hating)

• the polar head, which is faced towards the outside to interact with external environements is a hydrophillic (water-loving) surface

• only 5 - 10 nm thick

4 types of macromolecules in cells:

1) nucleic acid
2) proteins
3) carbohydrates/polysaccharides
40 phospholipids

Nucleic Acids

• make up hereditary material of cells called DNA (chromosomes).

• nucleic acids (DNA) encodes for functional products of cells, especially proteins

Active Transport

• movement of molecules against concentration gradient

• requires protein trasnporters and Adenosine Triphosphate (ATP)

• charged particles often use this method of transport - i.e. sodium, potassium, calcium, etc.

diffusion

• small molecules and ions travelling across the cell membrane naturally from areas of high concentration to areas of low concentration (along the concentration gradient)

• small, non-polar/hydrophobic molecules -> pass through cell membrane with great ease

• charged, large, and/or polar substance cross with difficulty if they can even cross at all

Faciliatated Diffusion

• movement of small molecules with concentration gradient through a transmembrane transport protein, but no need for energy.

• for ions and hydrophillic molecules

• most water also passes via facillitated diffusion

Passive Transport

the movement of small molecules in the direction of a concentration gradient.

The process involves the molecule coming into contact with and crossing the phospholipid bilayer into or out of the cell.

Aquaporins

• water moves much faster across cell membrane in the presence of aquaporin transmembrane proteins

• such protein channels move water through osmosis (passive trasnport), without requiring energy and along concentration gradient

• moves 10 times faster

Fluid Mosaic Model

• cell membrane is made up of a mosaic of lipids, proteins, & carbs

• proteins and carb are embedded within the phospholipid bilayer

• transmembrane proteins/protein channels: allow hydrophillic molecules across the cell membrane

Factors Affecting Membrane Fluidity:

1. Hydrocarbon Chain Length: Longer chains = packing together more tightly = less membrane fluidity

2. Double Bonds: double bonds in hydrocarbon chain = unsaturated = bends and kinks in chain = phosphlipids being pushes further apart = more membrane fluidity

3. Temperature: Higher temp = more fluid (cold-adapted creature -> unsaturated chains to maintain fluidity)

4. Cholestrol: found in membranes of all cells in the body - constrain membrane fluidity by packing closely with neighbouring phospholipids. (also helpful in lower temp environemnts when phospholipids start to solidify and cholestrol helps keep them apart from one another to maintain fluidity)

Lipid Micelles 

• structures important for absoprtion of fat-soluble vitamins and complex lipids in the body

• these are a phsopholipid aggregate because they form spontaneously and without energy.

Lipid Raft

• a region of lower fluidity in the cell membrane

• hold together the macromolecules within the membrane

• involved in processes regarding collecting or gathering proteins and receptors required for a metabolic pathway.

Phospholiipid Movements

• the bilayer is fluid

• phsophlipids can move laterally in any direction (L, R, F, B) within the layer of lipid bilayer, but have trouble moving to other layers without using a great deal of energy

Phospholipid composition

• made up of a glycerol molecule linkes to a phosphate group of one side and 2 fatty acids on the other (fatty acid tails).

• Amphipathic molecules because they contain both a hydrophobic and hydrophillic part.

• tails are made up of hydrocarbons (16-18 per chain)

• If hydrocarbon chains = single bond : saturated

• If hydrocarbon chains = double bonds = unsaturated

Polysaccharides/Carbohydrates

• important structural component in many cells -> bacterial capsules, plant cells walls

• source of stored energy in the body (i.e wheat, rice)

Proteins

• make up structural elements of the cells

• perform metabolic activities

• eg. flagellum of bacteria is a structural component made of proteins

• enzymes, ribosomes are all proteins that are involved in metabolic processes.

Reasons Why Cells Need Membranes

• separate internal environment from external environment

• membrane protects cells from toxins

• ensures that important compounds can easily enter cells

• allows for removal of metabolic wastes

Selective Permeability

ability of cell membranes to control the traffic entering and leaving the cell and its organelles

Steroids

• cholestrol is a type of steroid

• also contain both hydrophillic and hydrophobic regions

Transmembrane Movement 

• movement of asubstances from outside of cell to inside of cell or vice versa

Variation of fluidity factors

• longer phospholipid tails = taller regions = less fluidity such as in lipid raft regions

• single bonds of hydrocarbons in tails = saturates tails = straight tails = pack more closely together = lower fluidity -> helps hold macromolecules within lipid rafts

• Higher cholestrol amount = lower fuidity 

• Higher number of unsaturated fats + lower cholestrol = more fluidity = more permeable membrane

• lower number of unsaturated fats + higher cholestrol amount = less fluid = less permeability