I. Plasma Membrane Structure

Plasma membrane

– Boundary that separates living cells

from their nonliving surroundings.

- Apprx. 8 nm thick

- Composed chiefly of lipids and proteins

- Surrounds the cell and controls chemical traffic in/out of cell

- Is semi-permeable

Enables cells to maintain internal environment different from external environment

Phospholipid bilayer

-Composed of 2 layers of phospholipids

-Heads are hydrophillic

-Tails are hydrophobic

Membrane Structure (Fluid Mosaic Model)

• Membrane proteins embedded in phospholipid bilayer

• Give membrane ‘fluidity’ similar to salad oil

• Phospholipids & proteins can drift laterally (2 um / sec)

- solidification causes changes in permeability and enzyme deactivation

Membranes must be fluid to work properly !

How do cells control membrane fluidity ?

1. Unsaturated hydrocarbon tails

Decreases fluidity at low temps by restraining phospholipid movement

Increases fluidity at high temps by preventing close packing of phospholipids

2. Adding cholesterol makes membrane:

• Increase the percentage of cholesterol in phospholipids

• Prevents membrane from solidifying in cold weather

winter wheat

So, how do the plant overcome the winter?

Proteins in Plasma Membrane

- Mosaic of proteins ‘bobbing’ in a fluid lipid bilayer

- Proteins determine a membrane’s specific function:

Two types

1. Integral proteins (‘transmembrane’, or embedded)

2. Peripheral proteins (bound to surface of membrane)

Transport – protein provides channel across membrane for particular solutes

Enzymatic activity – proteins may be enzymes that catalyze steps in metabolic pathway

Signal transduction – protein is a receptor for chemical messenger (hormone). Conformational change in protein relays message to inside of cell

Intercellular joining – membrane proteins of adjacent cells join together for strength (epithelium)

Cell-cell recognition – glycoproteins act as I.D. tags that are recognized by other cells (e.g. RBCs)

Some Functions of Membrane Proteins

Regulating Traffic Across Membranes

II. Passive Transport: Diffusion and Facilitated diffusion

Diffusion : net movement

of a substance down

a concentration gradient.

• Solutes diffuse from high to low concentration.

• Continues until a dynamic equilibrium is reached.

• No requirement for energy expense (passive)

• Examples:

Uptake of O2 by cell performing respiration

Elimination of CO2 from cell

Diffusion of solutes across a membrane

Each dye diffuses down its own concentration gradient.

Facilitated diffusion

a) Channel protein : aquaporins, ion channels

b) Carrier protein

• Passive transport• Transport proteins speed the movement of molecules

across the plasma membrane.• Channel protein and Carrier protein required

Osmosis• Diffusion (passive transport) of

water across a selectively permeable membrane

• Direction of water movement is determined by the difference in total solute concentration, regardless of type or diversity of solutes.

• Water moves always from high concentration solution to low concentration solution.

Water balance of living cells

• Tonicity : the ability of a solution to cause a cell to gain or lose water Isotonic: no net movement of water across the membrane (normal). Hypertonic : the cell loses water to its environment (crenation). Hypotonic : the cell gains water from its environment (lysis).

QuestionsAn artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to sucrose.

1. Glucose?2. Fructose?3. Hypotonic/

Hypertonic?4. Water?

Active Transport• Requires the cell to expend energy: ATP

• Transport proteins pump molecules across a membrane against their concentration gradient.

• “Uphill” transport

• Maintain steep ionic gradients across the cell membrane (Na+ , K+ , Ca++ , Mg++ , Cl-)

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

inside outside

Na+

An Example of Active Transport: The Sodium-Potassium Pump

Passive and Active Transport

More examples of active transport

• Exocytosis– Removing large particles out of the cell with a vesicle

• Endocytosis– Ingesting large particles – Pinocytosis: “Cell drinking”– Phagocytosis: “Cell eating”

Protein Synthesis

• The process of using DNA to form proteins

• Involves two steps:– Transcription – Translation

Genetic Information• Uses 2 main forms of genetic information:

– DNA Deoxyribonucleic Acid• Double stranded• Sugar: Deoxyribose• Stays in the nucleus• Bases: A T G C

– RNA Ribonucleic Acid• Single stranded• Sugar: Ribose• Can leave the nucleus• Bases: A U G C

Transcription

• DNA unwinds

• One strand of the double helix is used as a template

• Nucleotides line up along the DNA and form a copy, called mRNA

• Once completed, DNA winds back up and mRNA leaves

• mRNA must be spliced before it leaves the nucleus ( immature RNA)– Enzymes remove noncoding areas called

introns, and coding regions called exons are spliced back together

– The result is a shorter, coding strand of mRNA– Every 3 bases on mRNA is a codon

Codons

• Codes for amino acids

• 64 codons can code for 20 different amino acids

Translation

• mRNA binds to a ribosome• tRNA binds to ribosome along the codon and

reads which amino acid it codes for• tRNA finds the specific amino acids • For every codon, the tRNA brings the amino acids• Amino acids link together forming a proteins• Peptide bonds link each amino acid together.