5 - Plasma Membranes

Factors Affecting Plasma Membranes

• temperature

• solvents

How does temperature affect plasma membranes?

• phospholipids are fluid

• they gain more kinetic energy

• increases fluidity of the membrane and causes it to lose its structure

• if temperature increases too much, the cell will break down completely

• loss of structure increases the permeability of the membrane

• carrier and channel proteins will be denatured at higher temperatures

Solvents

• hydrophobic tails are positioned away from the water on the inside of the membrane

• many organic solvents are less polar than water (e.g. alcohols) or non-polar (benzene)

• these will dissolve membranes and disrupt cells

• non-polar molecules can enter the cell membrane which disrupts the membrane

• the membrane becomes more fluid and more permeable

• can disrupt neuronal transmissions

Membrane structure

• plasma membrane

• formed from a phospholipid bilayer

• formed from phospholipids with the hydrophilic head on the outer surface and the hydrophobic tails on the inner surface

• cells are usually in aqueous environments

• hydrophilic phosphate heads can interact with water

Fluid Mosaic Model

• phospholipids are free to move within the layer

• gives flexibility

• vary in shape, size and position but fit together like a mosaic

Glycoprotein

• have branching carbohydrate chains

• acts as a recognition site for chemicals

• play a role in cell adhesion

• receptors for neurotransmitters (e.g. acetylcholine) and peptide hormones (e.g. insulin & glucagon)

Glycolipid

• acts as a recognition site (e.g. in immune responses

• lipids with attached carbohydrate chains

• ‘cell markers’

• can be self or non-self

Cholesterol

• stability

• flexibility

• lipid with hydrophilic and hydrophobic ends

• regulates fluidity

• prevent membranes becoming too solid by stopping the phospholipids from grouping

Extrinsic protein

• partially embedded in the membrane

• lying on the surface

• usually have hydrophilic R-groups on their outer surfaces

Intrinsic protein (integral proteins)

• spanning the phospholipid bilayer

• transmembrane proteins

• amino acids with hydrophobic R-groups on the external surfaces

• these interact with the hydrophobic core of the membrane

Types of intrinsic proteins

Channel proteins

• provide hydrophilic channel

• allows passive movement of polar molecules and ions down conc gradient

• held in place by hydrophobic core & R-groups

Carrier proteins

• passive and active transport

• involves the shape of the protein changing

Site of chemical reactions

• proteins have to be in particular positions for chemical reactions to take place

• examples

  • electron carriers and ATP synthase have to be in correct positions within the cristae

  • enzymes of photosynthesis are found on membrane stacks within the chloroplasts

Diffusion

• The net movement of particles from a region of higher concentration to a region of lower concentration

• passive process

• will continue until equilibrium

• random

• the shorter the diffusion distance, the faster the rate of diffusion (less collision can take place)

Factors affecting the rate of diffusion

• temperature

  • higher temp = higher rate

  • kinetic energy increases

• concentration difference

  • greater conc difference = faster rate

  • overall movement will be larger

Diffusion across membranes

• particles pass through the phospholipid bilayer

• membrane must be permeable to the particles (non-polar diffuse freely)

• hydrophobic core repels charged particles

• polar molecules can diffuse through at a very slow rate

• small molecules faster than large

• membranes are partially permeable

Factors affecting diffusion rate across membranes

• surface area

  • larger area of exchange surface = higher rate

• thickness of membrane

  • thinner = higher rate

Facilitated diffusion

• diffusion through channel proteins

• protein channels are selectively permeable

• can also involve carrier proteins

• rate is also affected by the number of channel proteins present

Active transport

• movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration against the concentration gradient

• requires energy and carrier proteins

• act as pumps

Stages of active transport

1. molecule binds to receptors in the channel of the carrier protein

2. ATP binds to the carrier protein and is hydrolysed into ADP and phosphate

3. phosphate molecule binds to the carrier protein which changes shape

4. molecule is released into the cell

5. phosphate molecule released from carrier protein and combines with ADP —> ATP

6. carrier protein returns to original shape

Bulk transport

• large molecules (e.g. enzymes)

• whole cells (bacteria)

• too large so move in by bulk transport

• endocytosis

• exocytosis

Endocytosis

• bulk transport into cells

• phagocytosis (solids)

• pinocytosis (liquids)

• cell-surface membrane invaginates when it comes into contact with material

• membrane enfolds the material

• membrane fuses, forming a vesicle

• pinches off and moves into the cytoplasm

• moves towards lysosomes

Exocytosis

• bulk transport out of cells

• reverse of endocytosis

• vesicles (usually formed by Golgi) move towards and fuse with cell surface membrane

• contents released outside of the cell

Water potential

• pressure exerted by water molecules until they collide with a membrane or container

• measured in Pa or kPa

• pure water = 0kPa

• presence of a solute lowers the water potential below 0

• more conc solution = more negative Ψ

Cytolysis

• animal cells

• if a solution has a higher Ψ than the cytoplasm, water will move into the cell by osmosis

• increases hydrostatic pressure

• cell membrane can’t stretch so will burst

Hydrostatic pressure

• pressure of solution in a closed system

Crenation

• animal cell

• if solution has a lower Ψ the cytoplasm will lose water to the solution

• darker cell - conc haemoglobin

Preventing cytolysis and crenation

• control mechanisms

• cells continuously surrounded by aqueous solution with equal Ψ

Turgid

• water enters by osmosis

• Ψ higher outside the cell

• hydrostatic pressure increases pressure against the cell wall (turgor pressure)

Plasmolysed

• Ψ lower outside the cell

• water is lost by osmosis

• reduction in the volume of cytoplasm

• cell-surface membrane is pulled away from the cell wall