Membranes : Biology WJEC AS

Cell (Plasma) Membrane Width

7nm/0.007um

External Membranes
FUNCTIONS

PLASMA MEMBRANE
•Separates cell contents from environment.
•Regulates transport of nutrients- selectively permeable membrane.
•Receptor sites. (neurotransmission etc)
•Cell Recognition (antigens)

Internal Membranes FUNCTIONS

•Site of chemical reactions (ATP production on inner mitochondria membrane + light dependent reactions on thylakoid membrane).
•Separates cell contents from cytoplasm.

[ribosomes are attached to RER membrane + nuclear pores allow mRNA and ribosomes to leave nucleus]

Fluid-Mosaic Model

1) Proteins and phospholipids are in constant movement.
2) Scattered protein arrangement (size + shape).

Phospholipids: Component

70%
Form a bilayer (7nm)
-Hydrophobic fatty acid tails & hydrophilic phosphate head.
BARRIER:
-allows gases & lipid-soluble molecules
-prevents hydrophilic substances.

Cholesterol: Component

Between fatty acid tails.
-Rigidity.
-Controls fluidity (no freezing in the cold).

Intrinsic Protein: Component

Non-polar so next to uncharged fatty acid tails.

Extrinsic Protein: Component

Polar so attracted to negative phosphate head.

Transmembrane Protein/Channel Protein
(INTRINSIC PROTEIN)

-Type of intrinsic protein.
-Spans to each end of membrane.
-Transports hydrophilic molecules by facilitated diffusion.

Carrier Proteins (INTRINSIC PROTEIN)

Molecule binds. Energy needed to change carrier shape. Molecule released on opposite side of membrane.

•Facilitated diffusion: uses molecules KE
(High to low)

•Active transport: uses ATP energy
(Low to high)

Glycolipid: Component

A carbohydrate chain attached to phosphate head.
•Antenna-> for cellular recognition + communication.

Glycoprotein: Component

A carbohydrate chain attached to an extrinsic protein.
•Antenna-> for cellular recognition + communication.
•Receptors/binding site for hormones and other molecules.

Rate of Diffusion Equation

SA x Difference in CONC //// Length of diffusion path

Factors Affecting Rate of Diffusion

•Temp
•Conc gradient (# of molecules on one side).
•SA
•Membrane Thickness/Diffusion Distance.
•Molecule Size
•Lipid Solubility

Osmosis

Movement of water from an area of high water potential to an area of low water potential. Down a water potential gradient across a partially permeable membrane.

Highest Water Potential

Distilled water = 0
(No solutes)

Low water potential

Negative number
(Many solutes)

Hypotonic Solution

Low amount of solutes = higher water potential than cell.
(Water moves into cell)

Hypertonic Solution

High amount of solutes = lower water potential than cell. (Water moves out of cell into solution)

Isotonic Solution

Same water potential as the cell. No net movement of water.

Why is saline solution used?

An isotonic solution to human cells/tissues.

Animal Cell Placed in Hypotonic Solution

Water moves into cell.
Pressure potential increases so it swells and bursts cell membrane.
=LYSIS (RBC=HAEMOLYSIS)

Animal Cell Placed in Hypertonic Solution

Water moves out of cell.
Pressure potential decreases, cell shrinks.
= CRENATED

Animal Cell Placed in Isotonic Solution

Same water potential as cell.
No net movement of water.
=MAINTAINS SHAPE

Plant Cell Placed in Hypotonic Solution

Water moves into cell's cytoplasm and central vacuole.
Cell wall maintains shape so doesn't burst.
=TURGID CELL

Plant Cell Placed in Hypertonic Solution

Water moves out of cell. So cytoplasm and central vacuole shrink.
Cell membrane pulls away from cell wall.
=PLASMOLYSED/FLACCID
The space between is filled with the solution as the cell wall is 100% permeable (not selective).

Plant Cell in Isotonic Solution

Not all plant cells have the same water potential.
So 50% remain the same & 50% undergo incipient plasmolysis (just beginning to pull away from cell wall).

Active Transport

Movement of ions/other molecules from low->high conc across membrane VIA CARRIER PROTEINS.

•Requires ATP-the phosphate binds to make carrier protein change shape.
-ATP provided by aerobic respiration.
•Carrier proteins are intrinsic proteins.

Active Transport Examples

•Absorption of mineral ions from soil, via plant roots.
•Sodium-potassium pump. (2x Na, 3x K)

•Protein Synthesis
•Muscle Contraction
•Nerve Impulse Transmission

Active Transport Limiting Factor

•The number of carrier proteins (they all become used up).
•Oxygen- no O2 means no aerobic respiration.
•Cyanide- a non comp inhibitor of resp enzymes so no ATP is produced.

Endocytosis

Bulk transport of material.
-Cell engulfs a material and the cell membrane fuses around it. The substance is in a vesicle made from the cell membrane.

[phagocytosis] -a bacteria cell in a vesicle is called a PHAGOSOME (engulfed by a phagocyte)

Phagocyte

A WBC that engulfs bacteria. (by endocytosis)

Pinocytosis

Bulk transport of liquids.

Example of molecule moved by...
SIMPLE DIFFUSION

Vitamin A
Vitamin D

Example of molecule moved by...
FACILITATED DIFFUSION

Potassium Ions
Sugars
Amino Acids

Example of molecule moved by...
OSMOSIS

Water (duhhh)

Example of molecule moved by...
ACTIVE TRANSPORT

Glucose

Why is initial rate the maximum rate?

1) The substrate conc. is at it's maximum.
2) All active sites are OCCUPIED.