Cell Boundaries

I. Cell Membrane• A. All cells are surrounded by a thin flexible barrier

known as the cell membrane• B. Cell membrane regulates what goes in and out of

a cell and provides protection and support.

• C. The cell membrane is composed of a lipid bilayer layer that gives it its flexible structure. – 1. In addition the membrane is

composed of a mosaic of molecules (carbohydrates, proteins & cholesterols)

– 2. The proteins can either be peripheral located along the surface of the membrane or integral proteins which are amphipathic(have both hydrophilic & hydrophobic regions) found throughout the entire membrane bilayer

Fluid Mosaic Model

II. Cell Walls

• A. Cell walls are present in many organisms including plants, algae, fungi, and many prokaryotes.

• B. Most cell walls are porous enough to allow water, oxygen, carbon dioxide, and other substances to pass through easily.

• C. Cell walls are composed of thick fibers made from carbohydrates known as cellulose or chitin(fungi) or peptidoglycan(bacteria)

III. Transport

• A. Cell transport across a membrane is dependent on 2 things– 1. Semipermeability of the membrane– 2. Size & charge of particles

• B. Plasma membrane is composed of lipids so substances that are lipid-soluble will cross easily through the only exception that also moves through is water. – Water uses proteins (aquaporins) to help regulate water flow

IV. Diffusion

• A. Every living cell lives in a liquid environment to survive.

• B. The cytoplasm in the cell is a solution of many different substances in water– 1. Each solution in different types of cells have

distinct concentrations– Ex. 12g/3L = – 12g/6L =

IV. Diffusion

• C. Particles of a solution are in constant motion• D. Diffusion is when particles of a solution move

from an area of high concentration to an area of low concentration; moves down conc. gradient– Think perfume bottle or a fart (random motion)

• E. When the concentration is equal on both sides the system is said to be in equilibrium– 1. Diffusion does not require the cell to use any

energy Ex. Dialysis, kidney

Countercurrent exchange

V. Osmosis

What if the molecule is to large to move across the membrane??

• A. The cell membrane is said to be selectively permeable meaning only certain molecules can pass; molecule that are small and have no charge can pass easily in and out of a cell.

• B. Osmosis is the diffusion of water across a selectively permeable membrane

VI. How Osmosis Works

• A. Water will always move across a membrane from a high conc. to a low conc. (with conc. gradient) until equilibrium is reached.– 1. When two solution are equal the solutions are

isotonic to one another or “same strength”– 2. When a solution is of higher concentration to

another it is hypertonic or “above strength”– 3. A solution that is very diluted to another is

hypotonic or “below strength”

• Based on Water potential (Ψ)

VII. Facilitated Diffusion

• A. Molecules that are too large (lipid-insoluble) gain the help of proteins to move easily across a cell membrane this is called facilitated diffusion

• B. These channel proteins are very specific and only allow one type of molecule to move across

• C. Facilitated diffusion still does not take the use of the cells energy since it only occurs down a concentration gradient

VIII. Active Transport

• A. Active transport moves materials in the opposite direction or against the concentration gradient– 1. Active transport requires the cell to

use energy

• B. The materials are usually moved across the membrane through the use of protein pumps– 1. Calcium, potassium, sodium ions

move across this way

IX. Large Molecule Transport

• A. Movement of large molecules inside the cell by pinching of cell membrane into a vesicle is called endocytosis.– 1. 3 examples of endocytosis

• a. Phagocytosis – take in food• b. Pinocytosis – take in liquid• c. Receptor-mediated – particle binds to receptor (cholesterol)

• B. Movement of large molecules outside a cell is called exocytosis (neurons)

• C. Bulk flow is one-way movement of fluids brought about by pressure.

X. Cell Junctions

• A. When cells are in close contact they sometimes form intercellular junctions that allow cells to form strong connections, prevent passage of materials or establish rapid communication. – Types of junctions:– 1. Desmosomes – hold cells tightly together– 2. Gap Junctions – protein complex form channels allow

for communication & transfer of small molecules/ions– 3. Tight Junctions – extremely tight connection no space

between cells