Plasma membranes

The Structure and Function of the Cell Membrane

The cell membrane is a fluid mosaic of lipids, proteins, and carbohydrates.


cell membrane notes

WATER POTENTIAL

In OSMOSIS, water moves from HIGHER (LESS NEGATIVE) water potential to a LOWER (MORE NEGATIVE) water potential

PURE water has a water potential of ZERO. Increasing the SOLUTE in water LOWERS the water potential (ie. MORE negative)

Plasma Membranes

Lipids

Easily dissolved in organic solvents but not in water

Triglycerides (fats and oils)

o Also called triacylglycerides (TAG)

o Consists of 3 fatty acids linked by ester bonds to glycerol

- Require 3 condensation reactions (but are not polymers!)

- Glycerol contains 3 -OH groups

- One fatty acid contains a -COOH group

o Excess energy available from food is stored as TAG

o Can be broken down to yield energy when needed

o Contain twice as many energy stored per unit of weight as carbohydrates

Saturated fatty acids

o -COOH group without double bonds in the carbohydrate chain

o May cause blockage of arteries which can lead to strokes and heart attacks

o High melting point / solid at room temperature (fats) / typical animal fats

Unsaturated fatty acids

o -COOH group with double bonds in the carbohydrate chain

o Low melting point / liquid at room temperature (oils)

o Found in plants

Phospholipids

o Found in cell membrane

o Formed by replacing one fatty acids in a triglyceride with a phosphate group

o Phosphate is polar / hydrophilic / does mix with H2O

o Fatty acid tails remain non-polar / hydrophobic / insoluble, does not mix with H2O

Fluid-Mosaic Model

- Membranes consist of a phospholipid bilayer studded with proteins, polysaccharides, lipids

- The lipid bilayer is semipermeable - H2O and some small, uncharged, molecules (O2, CO2) can pass through

Phospholipids have two parts

o "Head": hydrophilic → attracts and mixes with H2O

o Two "fatty acid tails": hydrophobic

Diffusion

- Uses energy from moving particles (kinetic energy)

- Substances move down their conc. gradient until the conc. are in equilibrium

Fick's law → rate of diffusion across an exchange surfaces (e.g. membrane, epithelium) depends on

o Surface area across within diffusion occurs (larger)

o Thickness of surface (thinner)

o Difference in conc. gradient (larger)

o (surface area * difference in conc.) / thickness of surface

Microvilli

o Extensions of the plasma membrane

o They increase the surface area of the membrane

o Accelerate the rate of diffusion

Temperature increases rate of diffusion due to increasing K.E. (kinetic energy)

Facillitated Diffusion

Transmembrane proteins form a water-filled ion channel

o Allows the passage of ions (Ca2+, Na+, Cl-) down their conc. Gradient

o NB: this is a passive process → no ATP required

o Some channels use a gate to regulate the flow of ions

o Selective permeability → not all molecules can pass through selective channels

Transport mechanism

o Carrier protein binds to substrate (specific molecule)

o Molecule changes shape

o Release of the diffusing molecule (product) at the other side of the membrane

Example:

o If you want to move a muscle, a nerve impulse is sent to this muscle

o The nerve impulse triggers the release of a neurotransmitter

o Neurotransmitter binds to a specific transmembrane protein

o The protein opens channels that allow the passage of Na+ across the membrane

o In this specific case, this causes muscle contraction

o These Na+ channels can also be opened by a change in voltage

Osmosis

Special term used for the diffusion of water through a differentially permeable cell membrane

Water is polar and able to pass through the lipid bilayer

Transmembrane proteins that form hydrophilic channels accelerate osmosis, but water is still able to get through membrane without them

Osmosis generates pressure called osmotic pressure

o Water moves down its conc. gradient

o When pressure is equal on both sites net flow ceases (equilibrium)

o The pressure is said to be hydrostatic (water-stopping)

Water Potential

Measurement of ability or tendency of water molecules to move

Water potential of distilled water is 0, other solutions have a negative water potential

Hypotonic: solution with a lower conc. of solute / gains water by osmosis

o Solution is more dilute

o Cells placed in a solution which is hypotonic will grow as water moves in

o Red blood cells will swell and burst if it is in a hypotonic solution

o Plant cells are unable to burst due to their strong cellulose wall

Hypertonic: solution with a higher conc. of solutes / loses water by osmosis

o Cells will shrink in hypertonic solutions (eg red blood cells)

Isotonic: solutions being compared have equal conc. of solutes

o Cells which are in an isotonic solution will not change their shape

o The extracellular fluid of the body is an isotonic solution

Molecules collide with membrane / creates pressure, water potential

More free water molecules, greater water potential, less negative

Solute molecules attract water molecules which form a "shell" around them

o water molecules can no longer move freely

o less "free water" which lowers water potential, more negative

Active Transport

Movement of solute against the conc. gradient, from low to high conc.

Involves materials which will not move directly through the bilayer

Molecules bind to specific carrier proteins / intrinsic proteins

Involves ATP by cells (mitochondria) / respiration

o Direct active transport - transporters use hydrolysis to drive active transport

o Indirect active transport - transporters use energy already stored in gradient of a directly-pumped ion

Bilayer protein transports a solute molecule by undergoing a change in shape (induced fit)

Occurs in ion uptake by a plant root; glucose uptake by gut cells

The Absorption of Glucose from the Small Intestine

The chemical digestion of carbohydrates results in the production of monosaccharides such as glucose. These need to be absorbed by the small intestine and passed into the bloodstream for use by the body. The process of diffusion alone would not result in all of the glucose present in the small intestine being absorbed as an equilibrium would be reached and any remaining glucose would pass out of the body in the faeces. Our digestive systems have evolved to absorb all of the glucose produced.

Glucose is therefore absorbed by the small intestine using an active process. It is considered an active process because ATP is required for it to happen. However it uses the ATP indirectly as it is the movement of sodium ions which actually powers the movement of glucose into the cells. It is also an example of CO-TRANSPORT because two molecules (glucose AND sodium) are involved.

1. Sodium ions are actively transported out of the epithelial cell into the blood by the sodium potassium ATPase. This protein pump is present in the membrane of all eukaryotic cells.
2. Sodium ions are now at a lower concentration in the epithelial cell than in the lumen of the small intestine.
3. Sodium ions now diffuse down their concentration gradient through a co-transport protein present in the plasma membrane of the epithelial cell. The energy released as the sodium ions move down their concentration gradient allows glucose molecules to pass through the co-transporter too despite the epithelial cell having a higher concentration of glucose than the lumen of the small intestine.
4. The glucose now passes into the blood via facilitated diffusion.

Cholera Prokaryotic Organisms - Bacteria

Bacteria are prokaryotes

o Nucleus (5µm)

- Contains chromosomes (genes made of DNA which control cell activities)

- Separated from the cytoplasm by a nuclear envelope

- The envelope is made of a double membrane containing small holes

- These small holes are called nuclear pores (100nm)

- Nuclear pores allow the transport of proteins into the nucleus

o Undergo asexually reproduction by binary fission / 2 identical daughter cells

Classification

o Most bacteria require oxygen to survive: aerobic bacteria

o Bacteria that are growing in the absence of oxygen: anaerobic bacteria

Grow best at optimum conditions (human body)

o Constant temperature

o Neutral pH

o Constant supply of food, H2O, O2

o Mechanism removing waste

Only a small number are pathogens. Pathogens cause disease by:

o Damaging our cells; or

o Producing toxins; or

o Directing our immune system against our own cells

Vibrio Cholera

Produces enterotoxins released from bacteria

o Enters enterocytes (cells lining the surface of the intestine) by endocytosis

o Activates the CFTR protein (cystic fibrosis transmembrane regulator)

o Causes secretion of sodium, chloride and bicarbonate ions from enterocytes

o Water follows sodium into the intestinal lumen

Osmotic loss of up to 10L of water per day!

o Results in severe watery diarrhoea of sudden onset

o Dehydration leads to death within hours if untreated

Giving oral sodium would cause more water to be secreted into the intestine, worse!

Giving oral glucose and sodium (oral rehydration therapy)

o Glucose is still absorbed through the intestinal wall

o This is done by a glucose-sodium co-transporter

o Carries one glucose molecule and one sodium ion across the intestine into the blood

o Water always follows sodium

o Diarrhoea is less severe and body becomes rehydrated

Oral rehydration therapy (ORT) also contains potassium and bicarbonate ions

o Prevents electrolyte imbalance

o Prevents metabolic acidosis

Cell Transport

Life - the Science of Biology - Chapter 5

Transport In and Out of Cells

Animation on the effect of the cholera toxin

Cell transport animations