Tour of a Typical Cell

Anatomy and Physiology- Anusha Murali

• The plasma membrane is the boundary that separates the living cell from its nonliving surroundings

• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

ECF and ECM

Cells of the human body are surrounded by Extracellular Fluid.(mostly water) andExtra cellular matrix.( may be solid or gel like) - ECM would depend on the type of tissue.

Plasma Membrane

• Helps define the outer shell of a cell.• Inside the membrane is the cytoplasm which

contains everything except the nucleus.• Functions of a Plasma Membrane:• ------ keeps the inside of the cell in and outside

of the cell out.• ------Controls what passes through the cell.

Plasma membrane

Cellular membranes are fluid mosaics of lipids and proteins

• Phospholipids are the most abundant lipid in the plasma membrane

• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it.

http://www.youtube.com/watch?v=Rl5EmUQdkuI

Phospholipid bilayer

The Fluidity of Membranes

• Phospholipids in the plasma membrane can move within the bilayer

• Most of the lipids, and some proteins, drift laterally

• Rarely does a molecule flip-flop transversely across the membrane

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Movement of phospholipids

• As temperatures cool, membranes switch from a fluid state to a solid state

• The temperature at which a membrane solidifies depends on the types of lipids

• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

• Membranes must be fluid to work properly; they are usually about as fluid as salad oil

ViscousFluid

Unsaturated hydrocarbontails with kinks

Membrane fluidity

Saturated hydro-carbon tails

Proteins in Cell Membrane

• Structural Proteins• Glycoproteins• Channels

Structural Proteins

• Examples are Cadherin, Integrin, Spectrin Functions: Connect the cell to 1.neighboring cells2.To the extracellular matrix3.To the cytoskeleton inside the cell. These structural proteins form a link between

the inside and outside the cell.

Structural Proteins

• Structural proteins are responsible for creating cell junctions.

• Cell adhesion molecules (CAMs) are proteins located on the cell surface involved in binding with other cells or with the extracellular matrix (ECM) in the process called cell adhesion.

Types of Proteins

• Cadherin- Calcium dependent adhesion molecule- important in fetal development.

• Integrin - act as adhesion receptors, transporting signals across the plasma membrane in multiple directions.

• Spectrin- is a cytoskeletal protein that lines the intracellular side of the plasma membrane.

Channel Proteins

Channel Proteins

• Have a hollow central pore or channel that allows water or small charged particles such as sodium, potassium, calcium and chloride to pass into or out of the cell.

• These substances cannot enter or leave the cell without the channel because of their polarity or charge makes them unable to penetrate a lipid bilayer.

• http://www.youtube.com/watch v=doxfuV3tqDA

Selective Membrane

• Channels are very selective about which substances are allowed to cross the membrane and when.

• Example • Aquaporins : Will let water cross but will not

let H+ or OH- to cross.• This selective property is important in

controlling the cell’s ph.

Glycoproteins

proteins with carbohydrate attached to it.•These proteins are usually found in the plasma membrane with the carbohydrate group projecting into the ECF.

Glycocalyx

• The layer of carbohydrate groups surrounding a cell is called Glycocalyx.

• Helps distinguish own healthy cells from diseased cells, transplanted cells.

• Helps cells binds to extracellular substances which is important for cell recognition

Structure of the typical lipid bilayer of the Plasma Membrane.

• Integral proteins are those that pass through the bilayer.

• Peripheral proteins are associated with the inner surface of the plasma membrane.

• Most integral proteins are modified by carbohydrate addition to their extracellular domains.

• Membranes also contain carbohydrate modified lipids (glycolipids) in addition to the more common phospholipids and cholesterol that constitute the bulk of the lipid content of the membrane.