Cell Membrane Structure + Movement

Cell Membrane Structure

Made up of phospholipids, which possess hydrophobic tails and hydrophilic heads. Heads interact with the interior and exterior of the cell. The tails congregate in the middle away from the liquid. The membrane contains unsaturated and saturated fatty acids.

Factors that affect membrane fluidity

1. In animals, cholesterol is slotted between the phospholipids and acts as a fluidity buffer. In high temperatures, it keeps the membrane from being too fluid. In low temperatures, it keeps the membrane from freezing.

2. The amount of unsaturated and saturated fatty acid tails in the phospholipids.

Fluid Mosaic Model

Membrane is a flexible, moving patchwork of phospholipids and associated proteins and carbohydrates.

Glycolipids

Lipid with carbohydrate chain. Made in the smooth ER and the carb chain is added in Golgi. Allows cells to recognize each other (cell communication and immune response).

Glycoprotein

Protein with a carbohydrate chain. If it’s a part of the plasma membrane, its was synthesized in the rough ER, and the carb chain was added in the golgi. Allows cells to recognize each other (cell communication and immune response).

True or False: Different sides of the membrane may have a different composition of proteins, glycolipids, etc . . .

TRUE

Lipid rafts

Microdomains (regions) of the cell membrane with a high concentration of proteins and other signaling components.

True or False: Cell membrane components can shift laterally, but very rarely flip from one side to the other

TRUE

Membrane protein types: Integral proteins

Span the membrane. May serve as pumps or channels to move materials in and out of the cell. Also can be receptors and enzymes.

Membrane protein types: Peripheral proteins

Found on the exterior or interior of the membrane, attached to either an integral or to a phospholipid head.

Membrane protein types: Lipid anchored protein

Possess an attache lipid that integrates into the membrane, anchoring the protein.

Passive Transport

Transport of substances across the membrane without using energy.

Two types:

1. Simple diffusion

2. Facilitated diffusion

Simple diffusion

When a substance moves from areas of high concentration to low concentration through the membrane itself. No proteins required.

Brownian motion

movement of particles due to internal energy.

Factors that affect simple diffusion

1. Temperature - high temps have increased energy, so particles move faster than in low temps.

2. Extent of concentration gradient - greater difference in concentration, the more rapid the diffusion

3. Mass of molecules - heavier molecules move slower than lighter ones

4. Solvent density - as the density of the solvent increases, diffusion of substrate decreases

5. Solubility - non polar, hydrophobic lipids can pass through easier than polar, hydrophilic molecules

6. Distance traveled - the greater the distance the substance must travel, the slower the rate of diffusion and vice versa

7. Pressure - the higher the pressure, the faster the diffusion

8. Charge - only neutral atoms ca pass through via simple diffusion

Filtration

Movement of water and solute through a selectively permeable membrane driven by hydrostatic pressure gradient

Semi-permeability

When a membrane allows some molecules to pass but not others. If a substance is permeable to the membrane, it will pass through via simple diffusion moving from high to low concentration until dynamic equilibrium is reached.

Facilitated diffusion

Substances that move down the concentration gradient using integral proteins such as channels or carriers to cross membrane.

Osmosis

Movement of water across the membrane. Solute does not move. Water moves down concentration gradient until dynamic equilibrium is reached (water is moving in and out of cell at equal rates)

Tonicity

A measure of the osmotic pressure and the ability for a solution to make a cell lose or gain water

Isotonic solution

When the cell and the solution have the same concentration of non-permeable solutes. No net water loss.

Hypertonic solution

Water leaves the cell because the solution has more non-permeable solute than the cell interior

Hypotonic solutions

Water enters the cell because the solution has less non permeable than cell interior.

Dialysis

When a semi-permeable membrane separates small molecules from larger ones.

True or False: Animal and plant cells prefer be in isotonic solutions

FALSE. Animal cells prefer to be in isotonic solutions, however plant cells prefer to be in hypotonic solutions so they remain turgid and not go through plasmolysis.

Types of transport proteins: Channel

Integral proteins that are highly specific for the substance being transported. Often have hydrophilic environments, allowing polar compounds to avoid hydrophobic environments. Aquaporins are channel proteins made for water passage.

Types of transport proteins: carrier

Integral proteins that bind a substance, triggering a shape change in the protein. The bound substance is moved through the protein and to the interior of the cell. Also highly specific to the substance. Does not transport as quickly as channel proteins.

Active transport

When substances move against concentration gradient, requiring energy in the form of ATP (adenine triphosphate)

Forms of energy

Energy - capacity to do work

Potential energy: stored energy

Kinetic energy: energy of motion

Primary active transport

Involves a substance moving against concentration gradient. It creates a concentration gradient (a form of potential energy) and that gradient can be used to perform secondary active transport

Secondary active transport

Involves a substance moving down concentration gradient in order to drag another substance against the concentration gradient.

Types of carrier proteins: Uniporters

Carries one specific molecule or ion. *more for passive transport

Types of carrier proteins: Symporters

Carries two different ions or molecules in the same direction, but one moves down and the other against concentration gradient

Types of carrier proteins: Antiporters

Carries two different ions or molecules in opposite directions, and one moves down and the other against concentration gradient.

Bulk Transport

Used to transport large molecules or a large number of molecules at a time.

Types of Bulk Transport: Exocytosis

Vesicles fuse with plasma membrane, releasing its contents to the external environment

Types of Bulk Transport: Endocytosis

Plasma membrane forms a pocket around material and pinches off to form a vesicle around the cell

Types of Endocytosis: Phagocytosis

When a cell engulfs a large molecule or bacteria

Types of Endocytosis: Pinocytosis

The cell engulfs dissolved ions from a solution

Types of Endocytosis: Receptor Mediated

When ligands binds to receptors on cell surface, and the membrane dips inwards and to pinches off to form a vesicle.