1. The Plasma Membrane

2. Cell Membranes
The cell membrane/plasma membrane represents the barrier that separates the cell’s contents from the surrounding environment and controls what moves in and out
In eukaryotic cells, membranes are also used to generate compartments within the cell, each with a specialised function e.g. golgi apparatus, endoplasmic reticulum, lysosomes etc.

3. Membrane functions Provides selectively permeable barriers
Compartmentalisation
Localises reactions in the cell
Transport of solutes (active transport)
Signal transduction – receptor proteins on the membrane surface recognise and respond to different stimulating molecules, enabling specific responses to be generated
Cell to cell recognition – the external surface of the membrane represents the cell’s biochemical “personality”

4. Hydrophobic and hydrophilic interactions
Cellular proteins influenced by hydrophobic or hydrophilic
nature of protein structure
R groups determine protein location in the cell
Hydrophilic R groups will occur predominately at the surface of a soluble protein found in the cytoplasm
In these proteins hydrophobic R groups may cluster to from a
globular structure.
Regions of hydrophobic R groups allow strong hydrophobic
Interactions that hold integral proteins within the phospholipid
bilayer

5. Types of Membrane Proteins
Proteins approx. 50% of the mass of the plasma membrane and can be classified into different groups depending on their arrangement in the membrane and/or their function
INTRINSIC - proteins may be embedded
TRANSMEMBRANE – proteins run through completely
EXTRINSIC – proteins may be on surface
Glycoproteins – proteins may have carbohydrates attached

6. Functions of Membrane Proteins
The main functions of these membrane proteins are as follows: Transport Cell recognition Receptor sites Enzymes Intercellular Junctions

7. TRANSPORT PROTEINS
Transport non-diffusable Channel proteins – provide a ‘pore’ across the membrane through which molecules (usually small and charged) can diffuse Carrier proteins – these are more specific with binding sites for only one solute Both these proteins permit passive transport (with a concentration gradient this is called facilitated diffusion) To transport molecules against the concentration gradient, special types of the carrier proteins are needed. These harness energy to drive the transport process during active transport e.g. sodium- potassium pump

8. CELL RECOGNITION PROTEINS
usually glycoproteins
the carbohydrate chain of the glycoprotein projects out of the cell
the immune system can recognise it’s own cells and organs e.g. ABO blood group antigens:
A = glycoprotein antigen A
O = no glycoprotein antigens

9. RECEPTOR PROTEINS
These have a specific conformation (shape) that allows binding of a particular molecule (ligand)
The binding of the ligand will then trigger a response in the cell

10. ENZYMES A protein that catalyses a specific reaction
Some receptor proteins have enzymatic activity; the cytoplasmic portion of the protein catalyses a reaction in response to binding a ligand

11. The Cytoskeleton
The eukaryotic cell is a 3D structure. It has a cytoskeleton anchored to proteins in the plasma membrane
These proteins both maintain shape and allow movement
The cytoskeleton is a dynamic structure, as the microfilaments and microtubules can depolymerise and repolymerise very easily
MICROFILAMENTS
INTERMEDIATE
FILAMENTS
MICROTUBULES

12. The Cytoskeleton 3 components, in order of increasing diameter.
They are …
1) Actin filaments/microfilaments
2) Intermediate filaments
3) Microtubules

13. Microfilaments These are composed of actin (protein)
2 strands of protein molecules twisted together about 7nm in diameter
These are present throughout the cell but are most highly concentrated just inside the plasma membrane
They are important in all cell movement and contraction
Actin fibres in a cell stained with a fluorescent strain specific for actin

14. Intermediate Filaments
tough fibrous protein strands twisted together about 10nm in diameter
very stable structures provide mechanical strength to animal cells which lack the strong cell walls of plants
The nucleus in epithelial cells is held within the cell by a basket like network of intermediate filaments made of keratins which have been stained here using a fluorescent stain

15. Microtubules growing in vitro from an isolated centrosome
hollow tubes (like straws)
tubulin protein (a globular protein)
cylindrical arrangement
a relatively rigid structure
Microtubules only form around a centrosome (organising centre)
The centrosome provides a “place” from which the microtubules form.
important in cell division as part of the spindle fibre network
can move components within the cell
Microtubules growing in vitro from an isolated centrosome

16. Functions
All of these components give mechanical support and shape to the cell
Primary importance of the cytoskeleton is in cell motility. The cytoskeleton extends throughout the cytoplasm determines the internal movement of cell organelles, cell locomotion and muscle fibre contraction