Lesson 2: Cell Membrane characteristics and Biogenesis

Cell Membrane

-thin continuous shee that spreads over entire cell surface

-formed by lipid, proteins and carbohydrates

Functions cell membrane

Barrier: seperates compartments

Communication: substance and information exchange

Lipids

Cell membranes main compartment (50% of the mass)

Types of membrane lipids

Phospholipids: most abundant and form the bilayer. A polar head group towards the surface and two hydrophobic hydrocarbon tails.

Cholestrol: small polar group

Glycolipids: contain oligosaccharrides and are located on Luther side of cell membrane

Lipid structure

Hydrophilic head (polar)

Hydrophobic tail: (nonpolar)

Lipids in contact with water

Bilateral: Polar Head on outside and nonpolar on the inside

Micelles

Types of movement

Cell membranes are highly dynamic systems

-Lateral diffusion: exchange of place sidewards

-Rotation: turn of lipid like globus

-Flexion: movement of tails

-Flip-flop: change of bilayer site

Proteins

-Involved in Structure, Recognition (receptors), adhesion (connection), transport or cell metabolism (enzymes)

-Around 40-50% of cells membrane mass

-More lipid molecules than protein molecules

Classification: Degree of association with membrane

Peripheral: inside or outside the bilayer and easily separated from membrane -Cytosolic or extracellular

Intergal: Strongly attached to the cell membrane -cytosolic, extracellular and transmembrane

Classification: According to their Location in the cell membrane

Transmembrane proteins: Cross entire bilayer and out of both side of bilayer

-Amphiphatic molecules: aminoacids pass are apolar

-can be bound to fatty acids

Cytosolic proteins: completely in cytosol

-Bound to monolayer by: helix, one or more fatty acids, other proteins

Extracellular proteins: located outside of cell membrane

Relation transmembrane proteins and integral proteins

Every transmembrane proteins are integral proteins, but not all integral protein is a transmembrane protein.

Transmembrane proteins

-Always have the same orientation, depending their function (eg. Surface receptors)

-Most highly glycosylated in the outer layer of the cell membrane

-Rotation and lateral movement

Carbohydrates

-Only found on outer surface of bilayer

-Associated with lipids or proteins (glycolipids and glycoproteins)

-Glycocalyx loose layer of oligosaccharide and polysaccharide on outer surface of plasma membrane

-Layer with secreted proteins outside glycocalx → extracellular matrix

Functions of the Glycocalyx

-Protection of cell surface

-viscosity

-Cell recognition: Immunity and cell identification, receptor, cell growth and division, fertilization (sperm recognise and bind to eggs)

Self sealing

Closing edges by itself

-small tear in bilayer: lipids rearrange and close

-larger tear in bilayer: repaired by fusion of vesicles

Fluidity

Lipid molecules are able to diffuse freely

Weak interactions: Van der Waals forces, hydrophobic interactions

Factors influencing membrane fluidity

-Temperature: increases fluidity

-Molecular composition of lipids: Bilayers are more fluid of the hydrocarbon chains of fatty acids are short or double bonded

-Presence of cholesterol

Cholesterol effects and structure

-decreases fluidity and flexibility

-avoids crystallisation of tails at low temperature

-mechanical stability

-polar head group, rigid steroid in central area, nonpolar tail

Asymmetry

-asymmetric distribution of components in each monolayer

-proteins and lipids different in each side

-Carbohydrates only on external surface, forming glycoprotein and glyoclipids

-functionally important, each monolayer performs different functions

Permeability of plasma membrane

- Small polar and nonpolar molecules and water can cross the cell membrane.

- Larger polar molecules and ions are not able to cross the bilayer.

BIOGENESIS

The cell membrane renews constantly

Origin of its components

• The lipids (phospholipids and cholesterol)are synthesized by enzymes of the smooth ER membrane (SER).

• The proteins are synthesized at the ribosomes in the rough ER (RER).

• The oligosaccharides that will bind the lipids and proteins and produced in the ER.

• the terminal glycosylation of lipids and proteins is performed in the Golgi, forming glycolipids and glycoproteins.

Transport

Packaged in vesicles and delivered to cell membrane

Transport

-Nutrients in, waste products out

• According the flow-direction of the transport and the type of substance that is going to cross the membrane:

- Ingestion: molecules needed for the cell metabolism enter into the cytoplasm.

- Excretion: waste products are secreted out of the cell.

- Secretion: when the secreted product is not waste but molecules synthesized for being exported out of the cell.

-Water and small non-polar molecules diffuse rapidly across

Types of Transport depending on energy requirements

Passive Transport:

• It does not require energy; it is driven by a concentration or voltage gradient.

Types:

- Simple (simple diffusion)

- Facilitated

Active Transport:

Against a concentration or chemical gradient; it always requires energy.

• Types:

- Primary active transport

- Secondary active transport

- Transport of macromolecules

Passive Transport: Simple diffusion

Molecules that can diffuse spontaneously

-Gases (CO2, O2)

-Small hydrophobic molecules

Passive Transport: Faciliated diffusion

Carried out by membrane proteins

-Hydrophilic molecules: Amino acids, nucleotides

-Charged molecules: ions

Channel proteins: Channel proteins, Carriers (change conformation and binding molecules cross membrane)

Primary active transport

• Uses energy in the form of ATP

• against electrochemical gradient

• e.g. Na+ - K+ pump

Secondary active transport

• It uses the energy stored in the electrochemical gradient of an ion to transport another solute.

• ATP synthesis

Macromolecules transport

Transport of macromolecules

- Requires energy (active transport).

- Through the deformations in the membrane and the formation of vesicles.

- Necessary to transport large molecules

- Types:

▪ Exocytosis

▪ Endocytosis

▪ Transcytosis

Exocytosis

-Secretion or excretion through the flow of vesicles that leave the Golgi complex and move towards the cell membrane

-Secretion of products to the extracellular matrix and also renewal of the cell membrane

Constitutive Exocytosis

• In all eukaryotic cells.

• there is a continuous flow of vesicles leaving the Golgi Complex and moving towards the cell membrane, where they fuse with the lipid bilayer.

▪ Function: Renewal of the cell membrane and secretion of proteins.

Secreted proteins:

-Incorporate to the extracellular layer of the cell membrane.

▪Form part of the extracellular matrix.

▪Diffuse through the extracellular fluid and serve as nutrients or

signalling molecules to adjacent cells.

Induced

• Found in cells specialized in secretory processes that produce large amounts of a secretory product that is stored in secretory vesicles.

- These vesicles are formed in the Golgi apparatus and accumulate

near the cell membrane.

- In response to an external stimulus, the vesicles fuse with the cell

membrane and discharge their content into the extracellular matrix.

Endocytosis and types

▪ Ingestion of substances by vesicles formed within the cell membrane.

▪ Types:

• Phagocytosis

• Pinocytosis

• Receptor-mediated endocytosis

Phagocytosis

-Ingestion of large solid particles

-after endocytosis, there is a phagocytic vacuole or phagosome in the

cytoplasm.

Functions Phagocytosis

• Function:

• In unicellular organisms it is a form of feeding.

• In multicellular organisms it can have a defensive role.

Specialized cells: Phagocytes

- The ingested membrane must be renewed by exocytosis.

Pinocytosis

• Ingestion of substances dissolved in the form of small liquid drops that will cross the cell membrane when there is an infolding of the membrane.

• Small pinocytic vesicles are formed.

Receptor-mediated endocytosis

• These vesicles are coated with clathrin, a coat protein found on

the cytoplasmic side of the membrane.

• Types:

• Constitutive

• Regulated (receptor-mediated)

Transcytosis

• Transcellular transport.

• Combination of endocytosis and exocytosis.

• Receptors from the surface of certain polarized cells can transfer specific macromolecules from one extracellular space to another, through vesicle formation.

• Example: epithelial cells