Plasma Membrane and Water Potential

Plasma Membrane

A barrier composed of phospholipids and proteins.

Phospholipid Bilayer

Two layers of phospholipids with hydrophilic heads.

Fluid Mosaic Model (Singer + Nicolson)

Phospholipid bilayer with proteins partially or fully imbedded, electron micrographs of freeze-fractured membrane

Robert Hooke

First to describe cells in 1665.

Charles Overton

Proposed membrane composed of lipids in 1895.

E. Gorter and G. Grendel

Identified phospholipid bilayer structure in 1925.

Sandwich Model (Danielli + Davson)

Plasma membrane model where 2 layers of globular proteins with phospholipid inside to make a layer and then join 2 layers together to make a channel for molecules to pass

Unit Membrane Model (Robertson)

Outer layer of protein with phospholipid bilayer inside, believed that all cells are the same composition, does not explain how some molecules pass through or use the proteins with nonpolar parts, used transmission electron microscopy

Cholesterol

Regulates membrane fluidity at varying temperatures.

Transport Proteins

Facilitate movement of substances across the membrane.

Isotonic Solution

Equal solute concentration inside and outside the cell.

Hypotonic Solution

Lower solute concentration outside, causing cell swelling.

Hypertonic Solution

Higher solute concentration outside, causing cell shrinkage.

Plasmolysis

Cell shrinkage due to water loss in hypertonic solution.

A plant wilts (sags) in a hypertonic environment, since the water in the cells diffuses out and turgor pressure is lost

Exocytosis

Movement of large molecules bound in the vesicles out of the cell with the aid of ATP energy

Vesicle fuses with the plasma membrane to eject macromolecules

Endocytosis

Movement of large molecules bound in the vesicles into the cell with the aid of ATP energy

Phagocytosis

Cellular eating; engulfing large particles or cells.

Pinocytosis

Cellular drinking; engulfing liquids and small solutes.

Receptor-Mediated Endocytosis

Specific uptake of molecules via receptor binding.

Cotransport

Active transport of one solute drives another's transport.

Active Transport

Molecules from from low to high concentration gradient using ATP energy

Membrane Potential

Voltage difference across a cell membrane.

Water Potential (ΨW)

Potential energy of water; quantitative description of free energy in water

Pressure Potential (ΨP)

Pressure exerted by water in plant cells.

Osmotic Potential (ΨS)

Effect of solutes on water potential, always negative.

Membrane Potential (ΨM)

Influence of membranes on water potential.

tendency to bump into other things that depresses movement

Sandwich model

Unit Membrane model

Fluid Mosaic model

What are the 3 plasma membrane models?

Basic Tenets of the Fluid Mosaic Model

• Hydrophilic lipid heads face outwards and hydrophobic lipid tails face inward

• The “fluid” part is attributed to the sideway movements of phospholipids

• Membrane proteins define the “mosaic” part

• Biological model by Singer-Nicolson that describes the structure of the plasma membrane as a mosaic of components that gives the membrane a fluid character.

• The cell membrane is heterogeneous and patchy due to the nature of proteins which diffuse or remain attached to its neighbors.

Phospholipid bilayer

Cholesterol

Proteins

Cytoskeletal filaments

Carbohydrate chain

The plasma membrane contains what?

Transmembrane/Intrinsic/Integral: fully embedded in bilayer

Peripheral/Extrinsic: outer of inner surface of the bilayer

What are the two types of proteins?

Transport proteins

Receptor proteins

Enzymatic proteins

Cell Recognition proteins

Attachment proteins

Intercellular Junction proteins

What are the 6 membrane functions of the proteins in the plasma membrane?

• Carrier Protein - possess configurations for certain

  molecules to pass through the membrane

• Channel Protein - allows materials to pass through the

  membrane; acts as a tunnel

2 types of transport proteins

Receptor proteins

Recognize and receive chemical signals

Enzymatic proteins

Serve as enzymes or the venue for certain cellular reactions

Cell recognition proteins

Possess structures (e.g., carbohydrate chain) that can be used to interact with other cells

Attachment proteins

Have cytoskeletal structures attached to them, allowing the protein to attach to other structures

Intercellular Junction Proteins

Serve as a junction between other cells

Through the plasma membrane, some move between the phospholipids or through the proteins

How do materials move into and out of the cell?

No, because proteins are randomly distributed in each phospholipid bilayer where some are organized into lipid drafts or microdomains

Do all plasma membranes look the same?

Plasma/cell membrane

What cell structure is common to all living things that acts as a selective barrier to passage of materials into and out of the cell?

Plasmodesmata vs pits

Plasmodesmata: gaps

Pits: depression only

But these both serve the same purpose

Thin areas on the secondary cell walls of plants. They look like depressions on the walls. Also, they aid in the transport of minerals and water between the cells

What are pits?

Diffusion

Facilitated Diffusion

Osmosis

3 membrane transport mechanisms of passive transport

Diffusion

Molecules move directly through the lipid bilayer

Net movement of molecules from a high concentration to a low concentration until equilibrium is reached

Gases (O, C, D), water molecules (slow rate due to polarity), lipids (steroid hormones), lipid-soluble molecules (hydrocarbons, alcohols, some vitamins), small uncharged molecules (NH3)

What materials pass through the plasma membrane by diffusion?

temperature, pressure, state of matter, size of concentration gradient, and surface area of membrane

Diffusion rate is related to…

Facilitated Diffusion

Net movement of molecules from a high concentration to a low concentration with the aid of a channel of carrier proteins

Ions, sugars, amino acids, small water-soluble molecules, water (faster than diffusion)

What materials pass through the plasma membrane by facilitated diffusion?

Channel proteins: ions, small solutes, and water to pass

Carrier proteins: glucose and amino acids

How does facilitated diffusion work?

Osmosis

movement of water through the plasma membrane, form high to low concentration gradient

Tonicity

Measure of the effective osmotic pressure gradient

Total solute concentration of the solution outside the cell

Isotonic

■ Concentration: outside = inside

Hypertonic

■ Concentration: outside > inside

■ net movement of water will be out of the body and into the solution

Hypotonic

■ Concentration: outside < inside
■ net movement of water will be into the body from the solution

Differentiate isotonic, hypertonic, and hypotonic from each other

Plants use osmosis in hypotonic soil

How to maintain rigidity

plasmolyzed/shriveled

Water leaving the cell will make the cells

Turgid

Water entering the cell will make it

Turgor pressure

Pressure of water molecules against the cell wall; cell swelling stops when the expanding membrane hits the cell wall

Cell wall

What keeps plant cells from bursting?

Active transport

Exocytosis

Endocytosis

3 types of active transport

Pumps

Protein carriers in active transport are called

Needed for the constant supply of raw materials and to get rid of unwanted molecules

Maintains internal conditions different from the environment

Regulates the volume of cells by controlling osmotic potential

Functions of active transport

Class of cellular structures consisting of multi-protein complexes that provide contact or adhesion between neighboring cells or between the cell and extracellular matrix of animals

Define cell junctions

Plasmodesmata, like xylem and phloem

Cell junctions in plants

Ѱs = the more solute, the less the water potential; always negative

Ѱp = the more water flowing inside denotes higher pressure potential, thus higher Ѱw

Ѱm = the higher the membrane potential, the lesser the water potential

Ѱe and Ѱg can be ignored

What is the full equation of water potential?

Ѱs = negative

Ѱm = negative

Ѱp = positive

Pure water means Ѱw = 0

Conditions in water potential

negative

Plant cells usually have a _______ Ѱw

How to calculate Ѱs?

Pressure

Concentration

Electrical

Gravity

Ѱw can be affected by:

In the case of a tree, the soil outside the roots must have greater water potential. How does this happen?

If there is higher water potential inside, water will leave which will make it wilt. To revive it, just water it again

Why does a plant become wilted?