Bacterial cell structure & function: Plasma Membrane

Written By: Michelle
MB0902
091715M

Scope:

1. Functions and composition of cell membrane.
2. Selective permeability.
3. Types of movements - Passive and Active.

1. Functions and Composition of Cell Membrane.
The cell membrane (also called the plasma membrane or plasmalemma) is the biological membrane separating the interior of a cell from the outside environment.

Roles of cell membrane:

- It encloses the cytoplasm.
- The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell.

Fig 1.

The cell membrane is composed of:

- Phospholipid (PL) Bilayer, both hydrophilic and hydrophobic.
- Proteins - interspersed between the phospholipid bilayer and acts as pores/channels in movement of materials in/out of the cell.

Phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophobic; its polar head hydrophilic.
As the plasma membrane faces watery solutions on both sides, its phospholipids accommodate this by forming a phospholipid bilayer with the hydrophobic tails facing each other.

Fig 2.

2. Selective Permeability

- Allows only certain molecules and ions in and out of the cell.
- Permeability depends on size and its permeability in lipids. See Fig 3.

Fig 3.

3. Types of Movements: Passive

Passive: Movements of materials from an area of high concentration to low concentration.

- No expenditure of Adenosine Triphosphate, ATP (energy).
- Further broken down into 3 types of movements:
A. Simple Diffusion.
B. Facilitated Diffusion.
C. Osmosis.

A. Simple Diffusion

Definition: Movement of substance from an area of higher concentration to lower concentration.
Movement will cease when the molecules or ions are equally distributed (equilibrium).

B. Facilitated Diffusion

Definition: Substances transported by transporter proteins across membranes from areas of high to low concentration.

Fig 4.

*Note: Always remember for simple and facilitated diffusion, molecules always move from HIGH to LOW. It's just remembering that for simple, it's the movement of substance whereas for facilitated, it's the transportation of substances. (:

C. Osmosis

Definition: It's the movement of water across a selective permeable membrane until equilibrium is reached.
Water molecules move from diluted to more concentrated area.

There are 3 types of osmotic solutions that bacteria may encounter:

Isotonic: equal number of water molecules
Hypotonic: water molecules entering the bacteria
Hypertonic: water molecules moving out of the bacteria

Fig 5.

3. Types of Movements: Active

Active: Movement of materials from an area of low concentration to high concentration.

- Expenditure of Adenosine Triphosphate ATP (energy).
- Further broken down into 2 types of movements:
A. Active Transport
B. Group Translocation

A. Active Transport

Definition: Substances transported by transporter proteins from low concentration to high concentration.
ATP is used during the process.

Fig 6.

B. Group Translocation

Definition: It is the use of energy such as phosphoenolpyruvic acid (PEP) to modify chemicals and transport them across the cell membrane.
Modified substances cannot leave the cells, this is useful when the cell is in an environment where nutrients are limited.

Stage 1: When bacteria use the process of group translocation to transport glucose across their membrane, a high-energy phosphate group from phosphoenolpyruvate (PEP) is transferred to the glucose molecule to form glucose-6-phosphate.