Transcript Higher unit 1 - Nairn Academy

Cell Membrane
and
Transport of Materials
Lesson Aims
• To study the structure and function of the
plasma membrane
• To examine the transport of substances
across the membrane by diffusion, active
transport, endocytosis, exocytosis,
pinocytosis, phagocytosis,
Functions of Cell Membrane
• Controls entry/exit of materials including
water and soluble molecules
• Communicates with other cells e.g.
hormones binding with receptors
• Important for the connections between
cells
Danielli and Davson 1930's-40's
Danielli and Davson’s
early model of the cell
membrane.
• Studied triglyceride lipid
bilayers over a water surface.
• Lipids arranged themselves
with the polar heads facing
outward.
• Formed droplets (oil in water)
and the surface tension was
much higher than that of cells
• Added proteins, the surface
tension was reduced and the
membranes flattened out
Fluid Mosaic Model
Fluid mosaic model
• Membrane is composed of a number of proteins
(mosaic) with spaces between them being filled
with fluid-like phospholipids
• Proteins act as:
channel
receptors
support
carriers
enzymes
antigens
• Phospholipids form a bilayer that acts like a
barrier between the cell and the environment.
Phospholipid bi-layer
Hydrophilic
phosphate
head
Hyrdophobic
lipid tail
Phospholipid Structure
Phospholipid consists of:
• hydrophilic head which points towards the
outside environment and the cytoplasm
• hydrophobic tail which repels water and point
in
Freeze-fracture of Plasma
Membrane
Freeze-fracture of Plasma
Membrane
• This electron
micrograph shows the
inside of a membrane
and bumps, grooves,
ridges. These were
later found to be
proteins.
Membrane composition
Membrane Type
% Lipid
% Protein
Myelin
18
16
Mitochondrion
24
76
Red blood cell
50
50
Passive Transport
• molecules move down the
concentration gradient
• no energy is required
• Diffusion - molecules moves
from high concentration to low
concentration e.g. minerals,
oxygen and carbon dioxide
Channel-forming proteins
• Osmosis - water molecules
through a selectivelypermeable cell membrane from
HWC to LWC
Osmosis in Red Blood Cells
HWC inside and LWC
outside. Water moves
out of cells and cells
shrink
Osmosis in Red Blood Cells
water concentration inside
= water concentration
outside. No net gain or loss
of water
Osmosis in Red Blood Cells
LWC inside and HWC
outside. Water moves
into cells and cells burst
Active Transport
• Molecules move against the
concentration gradient
• Energy (ATP) required
• Proteins act as carrier
molecules
• Proteins are specific
• Rate affected by
temperature, respiratory
substrate, oxygen
concentration
Sodium/potassium pump
Endocytosis
• Cells absorb material e.g. proteins from the outside by
engulfing it with cell membrane
• It is used by all cells of the body because most
substances important to them are large polar molecules
so cannot pass through the hydrophobic plasma
membrane.
Phagocytosis
• “Cell-eating” – large solid particles engulfed by
cell
• e.g. white blood cell eating fungus Candida
albicans
Pinocytosis
• “Cell-drinking” –
liquid-filled vesicles
formed
• e.g. Amoeba showing
pinocytosis
Exocytosis
• Cell directs secretory
vesicles to the cell
membrane
• Vesicles contain:
soluble proteins to be
secreted to the
extracellular
environment
membrane proteins and
lipids that are sent to
become components of
the cell membrane
The Facts You Need To Know
• page 3-4
• from “membranes are composed of….”
• to “as active transport depends…”