Section 5: plasma membrane

What is the function of the membrane on the surface of the cell?

• A barrier between the cell and it’s environment

• Controls what moves in and out of the cell

• Allow for recognition by other cells

• Allows for communication

What is the function of the membrane within a cell on organelles?

• Divides the cell into compartments (barrier between cytoplasm and organelle, function is more efficient)

• Controls what enters and leaves the organelle I.e. RNA

• Forms vesicles- substance transport

• Within organelles e.g. thylakoid membrane

• Provides a site for chemical reactions I.e. mitochondria inner membrane

What type of pattern does a membrane follow?

Mosaic

What components make up the membrane?

• Phospholipid bilayer

• Protein pore

• Cholesterol

• Glycoprotein

• Extrinsic protein

• Lipids

• Carbohydrate tail of the glycoprotein

• Hydrophilic head and hydrophobic tails

What is the function and features of the phospholipid bilayer?

• Forms a barrier to dissolve substances

• Separates inside and outside of the cell

• Impermeable to water but soluble to charged solutes e.g. charged ions

What are the two types/components of protein pores?

1. Channel proteins

2. Carrier proteins

What is the function of channel proteins?

Allows for small/charged(polar) molecules

Allows for passive movement- diffusion

What is the function of carrier proteins?

Facilitated diffusion/active transport

• Transports larger molecules + ions

• Can change shape to fit substance

What is the function of cholesterol?

• Type of lipid

• Binds to the hydrophobic tail and helps stability of the membrane (temp high=more fluid+ less rigid)(cold temp=makes it less rigid and more fluid)

What is the function of glycoprotein?

Cell signalling via neurotransmitters sent by nerves of hormones such as insulin

what Is the function of the extrinsic protein?

Hydrophilic and binds to the heads of the membrane

What is the function of the lipids in the membrane?

Cell marker (antigen)

Allows for cell recognition i.e. by immune cells

ID as self

What makes the hydrophilic heads attracted to water?

Their negative charges

What do hydrophobic tails do and what are they made from?

Repel water

Made from fatty acids but attract fat soluble substances I.e. vitamins

Why does a cell surface membrane need to maintain its fluidity?

If fluidity is not maintained the cell would not be able to function

What processes need a fluid membrane?

• The diffusion of substances across the membrane

• Membranes to fuse e.g. a vesicle fusing with the cell membrane during exocytosis

• Cells to move and change shape e.g. macrophages during phagocytosis

What effect does heating have on the membrane?

• As temp increases the kinetic energy of the phospholipid increases

• This increased movement creates gaps In the bilayers

• Molecules can pass through the gaps; the permeability of the membrane increases

When temperature increases, the phospholipid membrane becomes more and more leaky(permeable). Why is this?

• The phospholipid bilayer becomes more fluid

• The weak hydrogen bonds of the proteins break, the tertiary structure changes, the protein becomes denatured forming a hole through the membrane

What happens to the membrane when temp decreases? (Below 0 degrees)

• As temp decreases, the kinetic energy of the phospholipids decreases

• This decreases movement and phospholipids pack closely together becoming rigid

• Below 0 degrees, channel proteins denature

• Increased permeability

• Ice crystals form and pierce the membrane

(Gel temp)

What is the membrane like between 0-45 degrees?

• Phospholipids move and are not closely packed

• Membrane is partially permeable

(Normal temp)

What is the membrane like at high temps?

• As temp increases, the kinetic energy of the phospholipids increases

• This increased movement creates gaps in the bilayer

• Molecules can pass through the gaps; the permeability of the membrane increases

• Above 45degrees- channel proteins and carrier proteins denature(no control of what enters or leaves the cell)

(Fluid temp)

What three factors affect membrane permability?

1. Temperature

2. Solvent

3. Types of lipids

What effects do solvents have on the membrane?

• Water is a polar molecule and it’s important in creating membrane stability with the phospholipids

• Other solvents like alcohol are less polar; benzene is non-polar

• This can move into the bilayer disrupting the structure and dissolving the lipids(increased membrane permeability)

• This is why pure, or very strong alcohol solutions are toxic to cells

• Many cells( like neurones) rely on intact surface membranes for their specific function

What is diffusion?

The net, passive movement of substances down their concentration gradient until a concentration equilibrium is achieved

What type of substances diffuse?

Small, non-polar substances

E.g. carbon dioxide, oxygen, lipids and water

What factors affect the rate of diffusion?

• Concentration gradient → bigger difference in concentration gradient= faster diffusion

• Thickness of exchange surface → thinner the exchange surface= faster rate of diffusion

• Temperature → warmer temp=more kinetic energy=faster diffusion

• Surface area → larger surface area=faster the rate of diffusion

What is facilitated diffusion?

Large molecules and polar molecules can enter the cell by facilitated diffusion down their concentration gradient with the aid of proteins

• Doesn’t require metabolic energy

• Uses carrier proteins and channel proteins to move from a high to low conc

• Slightly polar molecules like water and some slightly larger molecules diffuse through hydrophilic channel proteins

What are the features of active transport?

• Requires metabolic energy or ATP

• Moves materials from low to high concentration against the concentration gradient

• E.g. glucose and iodine

How does ATP provide energy for active transport ?

Binds to carrier proteins

What does energy from the ATP cause to happen to the carrier proteins?

Energy from ATP cause the conformational change in shape of the carrier

What is bulk transport ?

Taking in big substances/molecules (ATP needed)

What is endocytosis?

Takes in substances (engulf)

What are the two types of endocytosis?

• Pinocytosis

• Phagocytosis

What is pinocytosis?

Liquids taken in

• Most common form of endocytosis

• Cells form an invagination-materials dissolve in water to be brought into cells

• “Cell drinking”

What is phagocytosis?

Solids taken in

• Used to engulf large particles such as food, bacteria etc into vesicles

• “Cell eating”

What is exocytosis?

secreting substances out of the cell

How does exocytosis work?

The substances to be released such as enzymes or hormones

• Packaged into secretary vesicles formed from the Golgi body

• Vesicles travel to the cell surface membrane

• Fuse of vesicle with the cell membrane and release their contents outside of the cell

What processes are active?

• Active transport

• Bulk transport

What are the processes that are passive?

• Diffusion

• Facilitated diffusion

• Osmosis

What does the solvent do?

dissolve the solute

What is the solute?

dissolved by the solvent

What is osmosis?

Osmosis is the net, passive movement of water molecules from a region of high water potential to a region of low water potential across a partially permeable membrane

What is water potential?

The tendency of water molecules in a system to move

What is water potential measured in?

KiloPascals (kPa)

What is a hypotonic solution?

Solution with a higher water potential than the cell

What is a isotonic solution?

Solution with the same water potential as the cell

What is a hypertonic solution?

Solution with a lower water potential than the cell