Bio Ch 7

Membrane Stucture Function

All cells have a…

plasma membrane

Plasma membrane functions

To contain the cell’s contents (cytosol, organelles)

To regulate what enters and exits the cell

(Selectively Permeable)

Selectively Permeable

Only certain molecules may pass through the

membrane

Plasma membrane structure

Phospholipid Bilayer

Cholesterol

Glycocalyx

Proteins

Phospholipids structure

Polar, hydrophilic heads:

• Glycerol + phosphate group

Nonpolar, hydrophobic fatty acid tails:

• Saturated and unsaturated

The Phospholipid Bilayer

Selective Permeability

Prevents large molecules from crossing the membrane

Hydrophobic region keeps out polar and charged molecules

Sugars, ions

What can pass the Phospholipid Bilayer

Prevents large molecules

from crossing the

membrane

• Hydrophobic region

keeps out polar and

charged molecules

• Sugars, ions

Importance of Selective Permeability

Intake of nutrients

Intake of signals

Excretion of waste

Intake of oxygen/excretion of carbon dioxide

Regulation of ion concentration

***Transport Proteins are needed to allow polar molecules into the cell***

Membrane Fluidity

Membranes with more unsaturated fatty acids remain fluid at

lower temperatures than those with more saturated fatty acids

Plants increase percentage of unsaturated fatty acid tails in…

the fall

Cholesterol

Acts as a patching substance: blocks small molecules from passing

The membrane “fluidity buffer”

Resists changes in membrane fluidity

At low temperatures cholesterol:

Prevents phospholipids from packing too tightly

At high temperatures cholesterol:

Prevents phospholipids from separating

Glycocalyx functions:

Cell-Cell Recognition

Embryonic tissue sorting

Immune system:

• Transplant rejection

• ABO blood type

Glycocalyx membrane carbohydrates

Glycolipids, Glycoproteins

Glycolipids

carbohydrate bonded to lipid

Glycoproteins

carbohydrate bonded to protein

Plasma Membrane Proteins 2 major types

Integral Proteins, Peripheral Proteins

Integral Proteins

Embedded in the hydrophobic region of membrane. Transmembrane proteins

Peripheral Proteins

Bound on the surface of the membrane or the exposed portion of integral proteins

Transmembrane proteins

Span the entire membrane. Many are channel proteins allowing passage of polar molecules

Plasma Membrane Proteins functions

Transport: Allow passage of molecules that can’t cross the membrane

Enzymatic Activity: function in metabolic pathways

Signal Transduction: Receive signals from outside the cell

Cell-Cell Recognition: Important in immunity (Why our bodies reject transplants)

Intercellular junctions: Cells attach in tissue formation or for communication

Attachment to cytoskeleton and ECM: for structure/communication

Transport Proteins

Allow certain polar molecules to cross the membrane (They are all specific)

Channel Proteins (no energy needed)

form channels for sugars and certain ions to pass

lined with hydrophilic amino acids

Aquaporins

a channel protein for water

Carrier proteins (some may need energy)

Bind ions/molecules and change shape to bring them across

Cell-Cell Recognition

Cells must have compatible cellular proteins for recognition (Often Glycoproteins)

Membrane proteins are important in the immune system bc:

Antibodies and T-cells recognize cells as “self”

Viruses have capability of infecting human cells if viral proteins are compatible with cell protein receptors

Molecular Movement

Molecules are constantly in motion

Speed determines state of matter

Molecules moving fast = Gas

Molecules moving very slow = solid

Molecular movement can be…

directional

Diffusion

movement of molecules from a high concentration to a low concentration

Passive Transport

Transport of molecules across a membrane requiring no energy

3 types of passive transport

Simple Diffusion

Facilitated Diffusion

Osmosis

Simple Diffusion

movement of molecules across a membrane from a high concentration to a low concentration

Molecules move along their own concentration gradient, independent of others (T/F)

True. (Difference between the high and low concentrations)

Drives diffusion

concentration gradient

Diffusion Across a Membrane

Facilitated Diffusion

Facilitated Diffusion

Diffusion of a molecule through a channel or carrier protein

Facilitated diffusion examples

Aquaporins: 3 billion water molecules per second

Glucose Transporters

Osmosis

Diffusion of WATER across a semipermeable membrane

Solute

Osmotic Activating Substance (OAS)

Water moves when the solute cannot…

diffuse through the membrane

Water moves in the direction of the…

high solute concentration

At a high solute concentration:

less free water b/c ***Water dilutes the solute***

At a low solute concentration:

more free water b/c ***Water dilutes the solute***

Water moves toward…

the direction of the solute

Water diffuses from high to low concentration…

of itself

Tonicity

The ability of a solution to cause a cell to gain or lose water

Tonicity depends on…

relative amounts of nonpenetrating solutes outside the cell vs. inside the cell

Three types of Tonicity solutions

Isotonic

Hypertonic

Hypotonic

Isotonic solution

Equal concentrations of solutes on either side of the membrane (No net flow of water)

Hypertonic solution

Solution with higher solute concentration

Hypotonic solution

Solution with lower solute concentration

Water moves from a Hyp__tonic to a Hyp__tonic

solution

Hypotonic, Hypertonic

In a Hypertonic Environment:

Animal cells: crenate (shrivel due to water loss)

Plant cells: plasmolyze (Plasma membrane pulls away from cell wall, and the plant wilts)

In a Hypotonic Environment:

Animal cells: lyse (burst due to too much water entering cell

Plant cells: become turgid. Firm and healthy-plant is upright due to pressure on cell wall

In an Isotonic Environment

Animal cells: appear normal. (No net movement of water)

Plant cells: appear flaccid (No net movement of water, so not enough pressure on cell walls to keep plant upright)

Osmoregulation

The control of water balance

Paramecium

contractile vacuole to rid cell of excess water

Organisms which lack cell walls have adaptations to maintain water balance if they live in…

Use of osmosis

Preserving foods! How?

Sugar and salt draw water out of bacteria.

Prevent growth and food spoilage

Active Transport

Transport that requires ENERGY

Why does active transport require energy?

•Carrier proteins move molecules against their concentration gradients

From a low concentration to a high concentration

• Requires ATP

Active transport ATP

ATP transfers its terminal phosphate group to power the carrier protein

Membrane Potential

Voltage across a membrane

Electrochemical gradient

Concentration gradient combined with membrane potential

Membrane Potential maintained by

on pumps (electrogenic pumps)

Cytoplasm is negative

-Cations are favored to cross into the cell

Extracellular fluid is positive

-Anions are favored to cross out of the cell

Pumps maintain the charge differential

Cotransport

Diffusion of one solute drives the active transport of another solute

How do molecules too big to fit through a channel protein get across the membrane?

Exocytosis and Endocytosis

Exocytosis

Movement of molecules OUT of the cell by fusion of a vesicle with plasma membrane

Endocytosis

Movement of molecules INTO the cell by creation of a new vesicle from the plasma membrane

Exocytosis

Transport vesicles fuse with the plasma membrane and release contents from cell

Endocytosis Three Types

Three Types:

Phagocytosis

Pinocytosis

Receptor-Mediated endocytosis

Phagocytosis: “cell eating”

Pseudopodia extend outward to enclose particles into a vesicle

Pinocytosis: “cell drinking”

Plasma membrane invaginates, enclosing extracellular fluid into a vesicle

Receptor-Mediated endocytosis

Receptor proteins bind their ligand, congregate, and are brought into the cell via a coated vesicle