The Plasma Membrane
http://en.wikipedia.org/wiki/Cell_membrane
The plasma membrane organizes chemical activities of the cell.
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The plasma membrane is selectively semi-permeable, ie. allows only certain substances to enter
within the cell.
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The plasma membrane is composed of mainly a phospholipid bilayer. (The lipid components are
hydrophobic and point inwards, whereas the phosphate components are hydrophilic and point outwards).
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The plasma membrane is a fluid mosaic barrier, composed of mainly phospholipids but also of
proteins, carbohydrates and cholesterol.
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Its functions are: protection to the cell and its components and transport
The phospholipids bilayer is largely hydrophobic. Like dissolves like therefore the membrane
gives preference hydrophobic solutes verses hydrophilic solutes. Still small molecules dissolved in water
often to gain entry in the cell via diffusion. Water readily enters the cell.
Proteins and carbohydrates are often on the surface of the cell. They can be receptors or tags
identifying the cell.
Cholesterol in the plasmamembrane affects the viscosity of the membrane.
Illustration of a cell membrane
Transport includes entry into and exit out of cell. The following are methods of transport.
1. Passive Transport requires no input of energy
Passive transport involves the movement of molecules down the concentration gradient (ie. From high
concentration to low concentration). Includes: Simple Diffusion, Osmosis and Facilitated Diffusion
Simple Diffusion is passive since it requires no energy input.
Movement of solute molecules through a semi permeable membrane, down a concentration gradient until
equilibrium is reached.
Example: Oxygen enters and carbon dioxide exits the red blood cells via diffusion.
Osmosis is the passive movement of water or other solvent molecule across a semi-permeable
membrane from a hypotonic (less concentrated solution with regards to the solute) to a hypertonic
solution (more concentrated solution with regards to the solute). Transport continues until the concentration
of both become equal / isotonic.
Facilitated Diffusion is also a form of passive transport. Protein molecules on the plasma
membrane facilitate diffusion of some bulky solute particles down the concentration gradient without
expenditure of energy.
2. Active transport requires input of energy
Active transport involves the movement of molecules against the concentration gradient (ie. From
low concentration to high concentration) . Active transport involves the transport of molecules against the
concentration gradient by the expenditure of energy (ATP hydrolysis coupled reaction). i.e.
Sodium/Potasium (Na/K) Pump
Visit the following website to read more:
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Vesicle Mediated Transport
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Exocytosis – Export / excretion of bulky materials outside the cell against its concentration
gradient with the expenditure of energy.
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Endocytosis: Import / ingestion of molecules from outside the cell to inside the cell against its
concentration gradient, with the expenditure of energy.
Types of Endocytosis
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There are three types of endocytosis:
different forms of Endocytosis
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1) Phagocytosis – ‘cellular eating’.
Example: Ingestion of food by amoeba
2) Pinocytosis – Non-specific ‘cellular drinking’ which may include any solutes dissolved in the
water / solvent droplets.
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3) Receptor-mediated endocytosis – Highly specific process which involves the import of
specific materials that are recognized by and bind to the receptors on a plasma membrane.
CoTransport: Active transport of one ion to form a gradient. A second solute travels across the membrane
passively by diffusion using the concentration gradient formed by the first ion.
Water Potential predicts the movement of water into and through the plant by the osmotic potential and the pressure
potential. (Remember pressure restricts water movement into the cell and osmotic (solute) potential enhances
movement in).
WP = PP and OP
PP is always positive OP is either 0 or negative
If Water potential is positive water moves out of the cell (shrinks)
If Water potential is 0 no net movement of water (equilibrium)
If Water potential is negative water move into the cell (expands)
Study this for understanding Water potential, solute potential and pressure potential
When WPa< WPb water moves into cell “a”
Water move into increasingly negative cells.
For water to move up the tree the WP must be increasingly negative.
Transpiration is the Transport water and Minerals through the Xylem.
Xylem is made of tracheids and vessels
These dead cells transport water and minerals
Rates of transpiration determine water needs
Crop plants: 0.5 gal a day (per acre is 16K gal a day)
Large tree in the summer 200-900 gal a day
Factors involved in transpiration
Physical properties of water (Cohesion (water to water), Adhesion (water to minerals)
Tension (negative pressure on water or solutions. Derived from water evaporating from stomata ( large neg
WP of air) sucking on water inside plant.
Transpiration called “Cohesion-Tension Theory” describes the mechanism
WP soil = -0.3 MPa;
WProot hairs =-0.6 MPa;
WPmidway up the tree =-0.7MPa;
WPleaf = -3.0 MPa;
WPair = -5.0 to -100 MPa