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
Function of Cholesterol Molecules in CSM
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
extends out into environment and acts 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
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
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
Water Potential
Pressure created by water molecules
Pure Water: Zero
Add Solute: more negative wp/ lower wp
Add water: less negative wp/ higher wp
Osmosis in Animal Cells (Plant Cells)
Less negative water potential outside of cell = water entering cell, cell swells and bursts (turgid)
Equal water potential = no change
More negative water potential outside of cell = water leaving cell, cell shrinks (plasmolysed)
Active Transport
movement of molecules from low concentration to high concentration using ATP and carrier proteins
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
Microvilli
Finger-like projections that line the ileum, provide more surface area for transport.
Causes faster rate of active 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 via Sodium -Potassium Pump
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
-temperature
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
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
