2 Transport across cell membranes Flashcards
Cell Surface Membrane
Membrane that surrounds cell and forms boundary to the outside.
Controls the movement of substances in and out of cell.
Features of Phospholipid in CSM
Hydrophilic head outside CSM - attracts water
Hydrophobic tail inside CSM - repels water
Function of Phospholipid in CSM
- allow lipid-soluble molecules to enter and leave
- prevent water-soluble molecules entering and leaving
- make the membrane flexible and self-sealing
Extrinsic Proteins
Embedded in the surface of bilayer
Either used for mechanical support or in conjunction with glycolipids
Intrinsic Proteins
Embedded completely in the bilayer
1) Protein channels - allow water soluble ions through
2) Carrier Proteins - used for facilitated diffusion
Functions of Proteins in Membranes
- provide structural support
- act as channels for water-soluble substances
- allow active transport across membrane
- form cell surface receptors
- help cells adhere together
Cholesterol Molecules
Occur throughout bilayer
Very hydrophobic
Function of Cholesterol Molecules
- add strength to membranes
- reduce movement of molecules
- make membranes less fluid at high temperatures
- prevent loss of water and ions
Glycolipid
Carbohydrate covalently bonded with a lipid
Carbohydrate section extends out of bilayer into watery environment outside (acts as a receptor)
Function of Glycolipids
- act as recognition sites for hormones
- helps maintain stability
- helps cells attach to one another and form tissues
Glycoprotein
Carbohydrate chains attached to many extrinsic proteins on outer surface
Function of Glycoproteins
- act as recognition sites for hormones and neurotransmitters
- helps cells attach to one another and form tissues
- allows cells to recognise each other (e.g. lymphocytes)
Why can’t some molecules diffuse through the membrane?
- not lipid-soluble
- too large
- same charge as channel protein (they are repelled)
- polar
Fluid-Mosaic Model
Fluid - phospholipid molecules can move around - gives a flexible structure
Mosaic - proteins embedded in bilayer are varied in shape and size
Simple Diffusion
Net movement of particles from high concentration to a low concentration, until evenly distributed.
Facilitated Diffusion
Movement of charged ions and polar molecules through the CSM
Passive process - relies on kinetic energy from molecules
Facilitated Diffusion - Protein Channels
- form hydrophilic channels
- allow specific water-soluble ions through
Facilitated Diffusion - Carrier Proteins
- molecule binds to carrier protein outside
- causes change in shape
- molecule is released on inside
Osmosis
Movement of water molecules from high water potential to low water potential through a partially permeable membrane
Solute
Substance that is dissolved in solvent
Water Potential
Pressure created by water molecules
Water Potential of Pure Water
Zero (this is the highest water potential everything else is negative)
Addition of Solute to Pure Water
Will lower water potential (be negative)
Osmosis in Animal Cells
Less negative water potential = water entering cell, cell swells and bursts (turgid)
Equal water potential = no change (incipient plasmolysis)
More negative water potential = water leaving cell, cell shrinks (plasmolysed)
Active Transport
movement of molecules from low concentration to high concentration using ATP and carrier proteins
ATP in Active Transport
- directly move molecules
- in co-transport
Process of Direct Active Transport
- molecule binds to carrier protein
- on the inside ATP binds to carrier protein causing it to split into ADP + inorganic phosphate
- protein changes shape and molecule released on the inside
- phosphate is released and protein changes back
Sodium-Potassium Pump
Sodium ions are actively removed from cell while potassium ions are actively taken in
Microvilli
Finger-like projections that line the ileum, provide more surface area for transport
Role of Diffusion in Absorption
Greater concentration of glucose and amino acids in ileum than the blood
Concentration gradient formed
Glucose/amino acids transported into blood by facilitated diffusion
Role of Active Transport in Absorption
Not all glucose and amino acids can be absorbed by diffusion
Example of co-transport
Co-transport of Glucose Molecule
1) Sodium ions are actively transported out of epithelial cells by sodium-potassium pump (protein carrier molecule)
2) higher concentration of sodium ions in lumen of ileum
3) sodium ions diffuse into epithelial cells (by co-transport protein) carrying glucose molecule
4) glucose passes into blood by facilitated diffusion
What factors affect the rate of facilitated diffusion?
- the concentration gradient
- the number of channel and carrier proteins
What factors affect the rate of osmosis?
- water potential gradient
- thickness of the exchange surface
- surface area of the exchange surface
What is a co-transporter?
type of carrier protein
they bind two molecules at a time
concentration gradient of one of the molecules is used to move the other molecule against its concentration gradient
How is rate of active transport increased?
- having microvilli
- increased number of protein channels and carrier proteins
What factors affect the rate of active transport?
- speed of individual carrier proteins
- number of carrier proteins present
- rate of respiration in the cell and availability of ATP
