Bio Chapter 3 (movement of substances across the plasma membrane)

Plasma membrane

Composes of phospholipids and proteins

Phospholipid bilayer

A double layer of phospholipids where hydrophilic heads face the fluid (extracellular fluid or cytoplasm) and hydrophobic tails face each other.

Hydrophillic head

The polar, water-loving phosphate part of a phospholipid that can form hydrogen bonds with water.

Hydrophobic tail

The non-polar, water-fearing fatty acid part of a phospholipid that repels water.

Channel protein

A protein that forms open channels or pores allowing specific substances to pass through the membrane.

Carrier protein

A membrane protein that changes shape to transport specific substances across the plasma membrane.

Glycoprotein and Glycolipids

Proteins or phospholipids with carbohydrate chains attached that act as receptors for hormones, help stabilize the membrane and act as antigens for cell recognition.

Cholestrol

A molecule that makes the plasma membrane more stable and flexible and less permeable to water-soluble substances.

Permeability of membrane

Selectively-permeable membrane that allows certain molecules to move across

organelles with selectively-permeable membranes

(1) Mitochondria (2) Chloroplast (3) Nucleus

E.g of objects with selectively-permeable membranes

(1) Visking tubing (2) Egg membrane (3) Cellophane tape

If there is no cholesterol in the plasma membrane:

(1) PM becomes more permeable to water-soluble substances / less flexible / too fluid.

(2) PM may break easily (become unstable).

3 factors that determine the permeability of a substance

(1) Molecule size (2) Polar molecule (3) Ionic charge

Give an example of a substance which moves across the plasma membrane by simple

diffusion and state the characteristics of the substance.

(1) Vitamin

(2) Vitamin A is small in size

(3) and is lipid-soluble.

Explain how vitamin A is transported across the plasma membrane.

(1) Vitamin A is transported across the plasma membrane by simple diffusion

(2) down the concentration gradient

(3) across the phospholipid bilayer.

Concentration

The amount of a substance in a given volume.

Concentration Gradient

The difference in concentration of a substance between two regions.

Definition Simple diffusion

The net movements of substance from a region of higher concentration to a region of lower concentration down the concentration gradient.

Definiton Osmosis

The net movement of water molecules from a region of high water potential to low water potential through a selectively permeable membrane.

Definition Active transport

The movement of substances from a region of lower concentration to higher concentration against the concentration gradient across a plasma membrane.

Facilitated diffusion

Involves either channel proteins or carrier proteins.

Simple diffusion

• Simple diffusion occurs until a dynamic equilibrium is achieved.

• Simple diffusion can occur with or without a PM

Higher rate of diffusion

Larger surface area and smaller molecules

Selectively permeable membrane

Permeable to water but impermeable to some solutes

Similarities of simple diffusion and osmosis

Both do not need ATP and moves down concentration gradient

Differences of simple diffusion and osmosis

Involves movement of any/water molecules molecules (SD/O)

Does not involve/involves a selectively permeable membrane

Characteristics of channel protein

1) Always open to allow small ions to pass through

2) Specific sites for specific charge for specific ions

Characteristics of carrier protein

(1) Have specific sites - to bind with specific molecules / ions and transport it

across the PM.

(e.g - carrier proteins that transport glucose cannot transport amino acids) • E.g - Mechanism of transport of glucose across the PM into the cell :

Active Transport

Involves carrier proteins(pumps) only .

• Active transport results in the accumulation or excretion of substances in the cell.

Examples of active transport

Sodium-potassium pump and proton pump

Simple diffusion in Animals

• Plants •: Gaseous exchange through the stomata of leaves.

• Humans : Gaseous exchange between the alveoli and blood capillaries. Osmosis

Osmosis in living organisms

Plants : Absorption of water from soil by plant root hair cells.

• Humans : Reabsorption of water through the renal tubule in the kidney.

Active Transport in living organisms

Plants: Absorption of mineral ions by a plant root hair cell

Humans: accumulation of iodine in the cells of the thyroid gland

Isotonic Solution

Solution A and B have the same concentrations of solutions.

Isotonic toward each other = no net movement of water

Hypotonic Solution

Solution A has low solutes concentration and high water potential.

Water diffuses from solution A to B by osmosis.

Hypertonic solution

Solution A has a high solutes concentration and low water potential

Solution A is hypertonic to B. Water diffuses from solution B to A by osmosis.

• When red blood cell is placed in a hypotonic solution - e.g : distilled water :

(1) The solution is hypotonic to red blood cells.

(2) Water will diffuse into red blood cells by osmosis.

(3) Red blood cells will undergo haemolysis (burst). • When placed in an isotonic solution - e.g : 0.9% sodium

• When red blood cell is placed in an isotonic solution - e.g : 0.9% sodium chloride (NaCl) solution

(1) The solution is isotonic to red blood cells.

(2) Water will diffuse in and out of red blood cells at equal rate by osmosis

(3) Red blood cells maintain their normal appearance.

(3) • When red blood cell is placed in a hypertonic solution - e.g : 20% NaCl solution :

(1) The solution is hypertonic to red blood cells.

(2) Water will diffuse out of red blood cells by osmosis.

(3) Red blood cells undergo crenation(shrinkage). Plant cell • When placed in a hypotonic solution :

• When plant cell is placed in a hypotonic solution :

(1) The solution is less concentrated to the (vacuole / cell sap of the) plant cell.

(2) Water diffuse into the plant cell by osmosis.

(3) The vacuole expands.

(4) Cytoplasm and plasma membrane are pushed against the cell wall.

(5) Plant cell becomes turgid. • When placed in an isotonic solution :

When plant cell is placed in an isotonic solution :

(1) The solution is isotonic to the vacuole of the plant cell.

(2) Water diffuses in and out of plant cell at equal rates by osmosis.

(3) The plant cell does not change in appearance. • When placed in a hypertonic solution :

• When plant cell is placed in a hypertonic solution :

(1) The solution is more concentrated than the vacuole of the plant cell.

(2) Water moves out of the plant cell by osmosis.

(3) The vacuole shrinks.

(4) Plasma membrane pulls away from the cell wall.

(5) Plant cell undergoes plasmolysis.

Wilting of plants (excessive fertilisers)

• 1) Soil water hypertonic to vacuole of plant cell

• 2) Water diffuses out by osmosis

• 3) Vacuole shrinks

• 4) Plasma membrane pulls away from cell wall

• 5) Plasmolysis undergo plasmolysis

• 6) Plant cell – flaccid (wilts)

Water supplied immediately

• undergo deplasmolysis

• plant cell again become turgid

If water is not supplied immediately

• the total surface area of leaves exposed by sunlight decreases → the rate of photosynthesis decreases

• Treatments (solute)

Food Preservation with salt / sugar (e.g - salted fish / canned fruits)

(1) Salt solution is hypertonic the fish tissues.

(2) Water diffuses out of the fish tissues by osmosis.

(3) Fish tissues becomes dehydrated.

(4) This prevents bacteria growth.

(5) Bacteria cells undergo crenation.

(6) Fish will last longer.

Preservation with vinegar (e.g - pickled mangoes/cucumber)

(1) Vinegar has a low PH.

(2) Vinegar diffuses into the mango tissues.

(3) Mango tissues becomes acidic.

(4) This low pH prevents bacteria growth.

(5) Mangoes will last longer.

Isotonic drinks, Saline solutions and Oral rehydration salts

(1) Taken to replenish water loss from the body

(2) and balance electrolytes in the body.

(3) - This prevents dehydration. Saline solution

Liposomes

vesicle the contain aqueous solution surrounded by phospholipid bilayer membrane. Used to protect drugs or active substances taken orally from being destroyed by gastric juices.

Desalination of Seawater

Uses reverse osmosis technology

Pressure is applied to push the seawater through a semi-permeable membrane.

Only allows water molecules to pass through but no foreign particles.

Thus, only pure fresh water is released