Download Topic guide 14.2: Biological cell membranes

Unit 14: Cell biology
.
14 2
Biological cell
membranes
The cell surface membrane surrounds the cell and acts as a barrier between
the cell’s contents and the environment. The cell membrane has multiple
functions that include controlling materials moving in and out of the cell,
separating cell structures from the cytoplasm and cell recognition and
signalling.
On successful completion of this topic you will:
•• understand the structural features of eukaryotic and prokaryotic
cells (LO1).
To achieve a Pass in this unit you will need to show that you can:
•• relate the structure of membranes to their function (1.3)
•• explain the interdependence of subcellular organelles (1.4).
1
Unit 14: Cell biology
1 Cell membranes
Cell membranes are formed from a double layer (bilayer) of phospholipids
usually 7–10 nm thick (see Figure 14.2.1). Each phospholipid has a water soluble
(hydrophilic) head and an oil soluble (hydrophobic) tail. This means that the
molecules naturally form bilayers with the heads pointing outwards, exposed
to the aqueous environment, and the tails inside. Bilayers also have proteins,
cholesterol and polysaccharides embedded into the membrane.
In 1972 scientists Singer and Nicolson introduced the Fluid Mosaic Model to
describe the structure of biological membranes. The cell membrane bilayer is very
fluid-like and the phospholipids are constantly moving.
Key terms
Phospholipid: A phosphate head
and two fatty acid tails.
Fluid Mosaic Model: A model in
which biological membranes are
considered as a two-dimensional
liquid, allowing other embedded
molecules such as proteins and
carbohydrates to diffuse freely.
Diffusion: The movement of
molecules from a high concentration
to a low concentration down a
concentration gradient.
Biological membranes are selectively permeable. Although it is the role of the
membrane to separate the contents from the environment, some substances
do need to get into and out of cells. Some molecules can diffuse directly across
membranes (e.g. oxygen), whereas others move through specific channels or
are transported across by proteins. Proteins embedded in the membranes act as
channels that allow specific water-soluble molecules to pass through and they
span the width of the membrane. Also, carrier proteins, present in membranes
actively move molecules across the membrane but they require ATP.
Some biological molecules only span half of the membrane; cholesterol present in
membranes fits between fatty acid tails and provides stability. Some proteins have
small polysaccharide branches attached to them – these are called glycoproteins.
Phospholipids with polysaccharide branches attached are called glycolipids.
Both glycoproteins and glycolipids help in cell recognition and can identify the
difference between foreign cells and body cells.
Carbohydrate chains
Protein molecule
Lipids (bilayer)
Protein channel
Figure 14.2.1: Mosaic model of phospholipid membrane.
14.2: Biological cell membranes
2
Unit 14: Cell biology
2 Membrane transport
Diffusion
Diffusion is the movement of molecules down a concentration gradient, from a
high concentration to a low concentration, until they are spread evenly. There are
many factors that affect the rate of diffusion. These include: the distance that the
molecules are going to move; the concentration gradient; the size of the particles;
and the temperature.
•• The rate of diffusion will be slower if the molecules have to travel a greater
distance.
•• The higher the concentration gradient, the faster the rate of diffusion.
•• The smaller the particles, the quicker the rate of diffusion.
•• The higher the temperature, the more kinetic energy the molecules will have
to diffuse, so this will speed up the rate of diffusion.
Facilitated diffusion
Key terms
Osmosis: The movement of water
from a high water potential to a
low water potential down a water
potential gradient across a biological
membrane.
Active transport: The movement
of molecules across a biological
membrane against a concentration
gradient using energy.
Facilitated diffusion uses carrier proteins and ion channels that facilitate the
movement of molecules across a membrane. Carrier proteins bind with the
molecule, and the protein changes shape so that the molecule makes its way to
the other side of the membrane. Ion channels facilitate the diffusion of charged
particles such as Na+ and K+. Facilitated diffusion occurs down a concentration
gradient and does not require any energy.
Osmosis
Osmosis is the diffusion of water across a partially permeable membrane, from a
high concentration to a low concentration down a concentration gradient.
Active transport
Active transport is the movement of molecules against a concentration gradient.
Molecules can move from a low concentration to a high concentration but it requires
energy in the form of ATP. Carrier proteins present in the cell membrane require
energy from respiration to move molecules against a concentration gradient.
Link
Activity
The structure of phospholipids is
discussed in Unit 1: Biochemistry
of macromolecules and metabolic
pathways.
Use the Internet to research and source molecular diagrams to illustrate diffusion, facilitated
diffusion, osmosis and active transport.
Checklist
In this topic you should now be familiar with the following ideas about cell membranes:
 cell membranes consist of phospholipids, proteins, cholesterol and polysaccharides
 cell membranes allow the movement of molecules because they are partially permeable
 the Fluid Mosaic Model refers to the fluidity and mosaic appearance of cell membranes.
14.2: Biological cell membranes
3
Unit 14: Cell biology
3 Interdependence of organelles
Protein sorting
Protein sorting initially starts at the ribosomes where the amino acid sequence is
formed from the triplet codons on the mRNA. The newly synthesised protein and
the ribosome are transported to the endoplasmic reticulum (ER) where protein
synthesis is completed. The polypeptide chain is then transported to the Golgi
body where it may need slight modification before being packaged into vesicles.
This is known as the secretory pathway. Some of these proteins are secreted out of
the cell and some are used in the cell.
Vesicular transport
Transport vesicles are membrane-bound sacs found in eukaryotic cells. They
move many different substances around the cell and to the cell’s surface as
part of the secretory pathway. Vesicles are very important for the organisation
and functioning of cells; for example, lysosomal enzymes must be transported
from the Golgi body to the lysosomes. It would be detrimental if these digestive
enzymes were transported to the membrane or mitochondria.
Ingestion pathways
Key terms
Endocytosis: The process of bringing
materials into the cell.
Exocytosis: The process of
transporting molecules out of the
cell.
Endocytosis is the process of bringing materials into the cell in small vesicles and
exocytosis is the process of transporting molecules out of the cell – the vesicle
fuses with the cellular membrane and expels the material. Phagocytosis is the
transport of solid material (bacteria or dead red blood cells) into a cell through
a vesicle, for example, in order to destroy them. When the cell membrane fuses
around the molecule it produces a food vacuole and lysosomes use their enzymes
to digest the material inside. Pinocytosis is similar to phagocytosis but it is the
transport of fluids into the cell.
Checklist
In this topic you should now be familiar with the following ideas about cells:
 newly synthesised proteins and ribosomes are transported to the endoplasmic reticulum (ER)
Link
Protein synthesis, mRNA and triplet
codons are discussed in Unit 7:
Molecular biology and genetics.
 polypeptide chains are transported to the Golgi body where they are modified and packaged
into vesicles
 transport vesicles move many different substances around the cell and to the cell’s surface
 endocytosis is the process of bringing materials into the cell
 exocytosis is the process of transporting molecules out of the cell.
Further reading
Kennedy, P., Sochacki, F. and Hocking, S. (2008) OCR Biology AS, Heinemann (Pearson Education Ltd.)
14.2: Biological cell membranes
4
Unit 14: Cell biology
Acknowledgements
The publisher would like to thank the following for their kind permission to reproduce their
photographs:
Corbis: Steve Gschmeissner / Science Photo Library
All other images © Pearson Education
Every effort has been made to trace the copyright holders and we apologise in advance for any
unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any
subsequent edition of this publication.
14.2: Biological cell membranes
5