The Cell Membrane

Cell membranes are composed of two phospholipid layers.

The cell membrane, or the plasma membrane, forms a boundary between the cell and the outside environment.It controls the passage

of materials into and out of the cell.

The cell consists of a double layer of phospholipids interspersed with a variety of other molecules.

PhospholipidsA phospholipid is a

molecule composed of three basic parts:A charged phosphate

(PO4-) head.GlycerolTwo fatty acid chains

Together, the glycerol and the phosphate group form the “head” of a phospholipid.

The fatty acids form the tail.

Polarity of PhospholipidsThe head of the phospholipid is polar because it bears

a charge.Water molecules are also polar.

Polar molecules love polar molecules, so water and the polar head of the phospholipid form a hydrogen bond.

Polarity of PhospholipidsThe fatty acid tails are non-polar.Polar molecules (like water) and non-polar

molecules repel each other.Therefore, non-polar tails are attracted to

each other and repel water.

Sandwich MembraneThe polar (or non-polar)

properties of the phospholipids causes them to arrange themselves in layers, like a sandwich.Polar heads are like bread.

They form the outer surfaces of the membrane, where they interact with the watery environment both outside and inside a cell.

The non-polar tails are like the filling.The are sandwiched between the

layers of polar heads, where they are protected from the watery environments.

Molecules in the membrane.Molecules are embedded

throughout the cell membrane.Cholesterol molecules

strengthen the cell membrane.

Some proteins help materials cross through them membrane, others are components of the cytoskeleton.

Carbohydrates attached to membrane proteins serve as identification tags, enabling cells to distinguish one cell type from another.

Fluid Mosaic ModelThe cell membrane is

flexible, not rigid.The phospholipids in each

layer can move from side to side and slide past each other.

This makes the cell membrane act like oil on the surface of water.The proteins that stud the

membrane act like an arrangement of colorful tiles with different textures and patterns.

Selective PermeabilityThe cell membrane

allows some, but not all, materials to cross.Selective permeability

allows a cell to maintain homeostasis.Because a cell need to

maintain certain conditions to carry out its functions, it must control the import and export of certain molecules and ions.

Crossing the membrane.Molecules cross the membrane in several ways.

Some methods require the cell to expend energy, some do not.How a particular molecule crosses the membrane depends on the

molecules size, polarity, and concentration inside versus outside the cell.Small non-polar molecules easily pass through the membrane.Small polar molecules are transported via proteins.Large molecules are moved in vesicles.

oxygen, carbon dioxide, and other small, nonpolar molecules; some water molecules

glucose and other large, polar, water-soluble molecules; ions (e.g.,H+, Na+, K+, Ca++, Cl–); water molecules

Chemical signals are transmitted across the cell membrane

A receptor is a protein that detects a signal molecule and performs an action in response.It recognizes and binds to

only certain molecules, which ensures that the right cell gets the right signal at the right time.

When receptors are activated (they combine with a hormone or neurotransmitter), they change shape. This relays information to the

cell.

Types of ProteinsThe cell has many different types of

proteins embedded inside it.Adhesion proteins-Help the cell adhere to

neighboring cells.Communication proteins-chemical and

electrical signals flow from one cell to the next.

Receptor proteins-gets a message from neighboring cells and communicates with the nucleus.

Recognition proteins-identify the cell as “self”Passive transporters-have an open channel

through which material may cross.Active transporters-have a pump that can pull

substances across against concentration gradients.

Adhesion

communication

receptor

recognition

passive

active

Intracellular ReceptorIntracellular

receptors are inside the cell.These generally

react to hormones, which can pass through the cell membrane easily.

Each intracellular receptor only responds to certain chemicals, they are specific.

Intracellular receptors

Membrane ReceptorWhen a molecule cannot pass through the cell membrane,

they need a membrane receptor to help them get the message through.When the membrane receptor binds to a signal molecule, the

entire membrane receptor changes shape.As a result, it causes the molecules inside the cell to respond.

Example:Band 3 protein binds to a membrane receptor. The receptor changes

shape. This triggers processes that carry carbon dioxide from body tissues to the lungs.

Voyage into the Cell MembraneVoyage inside the Cell: Membrane - YouTub

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