Cell Membrane Structure/ Function and Cell Transport

Ms. KimHonors Biology

Cell membrane

The Cell Membrane• Thin, flexible, surround all cells• Controls the traffic of molecules into and out

of the cell (school doors/fence/security guards)–Takes in substances & takes out the cell’s

waste• Allows substances through by either

passive transport (no energy) or active transport (energy required!)

3

Overview of CellMembrane

• The plasma (cell) membrane –the boundary that separates

living cell from its nonliving surroundings –Also called the phospholipid

bilayer

Structure of Plasma Membrane• Made of phospholipids– Phosphate head and 2 fatty acid chains

are the tail• Arranged in a bilayer– Heads (polar)– hydrophilic– Tails (nonpolar) – hydrophobic

• Roots:– Phospho- polar phosphate group– Lipid- non-polar fat– Bi- two– Layer- sheet or quantity covering a

surface 5

Phospholipids

Proteins

Membrane movement animation

Polar heads love water & dissolve.

Non-polar tails “hide” from water.

Carbohydrate cell markers

Fluid Mosaic Model of the cell membrane

8

• The phospholipid bilayer is a selectively permeable membrane. It is semi-permeable.• Only allows certain substances to cross it more

easily than others.

• Selective Permeability: – Allows some materials,

but not all, to cross through

– Small, non-polar, hydrophobic, neutral molecules can pass easily through membrane

– Although polar, water is SO small that it can move through the cell membrane.

Selective Permeability

• Small polar molecules must pass through proteins• Large molecules must be moved

with vesicles

Membrane Proteins and Their Functions• A membrane– Includes different proteins embedded in the

fluid lipid bilayer• FUNCTION:– Help move molecules and ions across the membrane– They help cells communicate between the cell’s

internal and external environments

2 major types of

membrane proteins

• Integral• Permanently attached to the membrane • Are often transmembrane • found throughout the entire bilayer

• Peripheral • Are appendages loosely bound to the surface of the

membrane• Only found “outside” of the bilayer

Proteins

• Cholesterol stabilizes (stiffens) the membrane– Integrates between phospholipids– Without cholesterol, cell membranes would be too fluid, not firm

enough (very mushy).• Cholesterol also helps maintain the fluidity

– Also helps separate the phospholipids so that the fatty acid chains can’t come together and crystallize.

• Therefore, cholesterol helps prevent extremes—whether too fluid, or too firm—in the consistency of the cell membrane.

• Present in animal cell membranes• Absent in bacteria and most plants where the cell wall provides stability

Cholesterol

Why have Carbohydrates on Cell Membrane proteins?

• Cell-cell recognition–Is a cell’s ability to distinguish one type of

neighboring cell from another–It’s their “ID” tag

• Carbs found on the outside for identification– Glycolipids and Glycoproteins serve as

recognition sites for other cells

Carbohydrates (Oligosaccharides)

Glycoprotein

Glycolipids

2 different types membrane carbohydrates

• Glycolipids – Carbohydrates covalently attached to lipids

• Glycoproteins– Carbohydrates covalently attached to proteins

(most abundant)

Fluid Mosaic Model• Description of arrangement of molecules that make up the

cell membrane– Cell Membrane is flexible, not rigid– Phospholipids can move from side to side and slide past each other

• Variety of molecules studding the membrane is similar to arrangement of tiles with different textures and patterns

Know this picture!

16

http://www.dnatube.com/video/360/Fluid-Mosaic-Model