Cell Physiology and Transport Across the Cell Membrane

1. Structure of the Cell Membrane:

The cell membrane is a structure that encloses the cell, playing a vital role in protecting, exchanging substances, and maintaining the cell’s life. The cell membrane is composed of:

• Lipids (42%):

• Phospholipids (25%): The primary component of the lipid bilayer. Phospholipid molecules have a hydrophilic head (phosphate group) and a hydrophobic tail (fatty acid chain). The hydrophilic heads of the phospholipids face outwards towards the cell’s exterior and interior, while the hydrophobic tails face inwards, forming the lipid bilayer.

• Cholesterol (13%): A type of lipid with a structure consisting of a hydrophilic OH group and a hydrophobic steroid core. Cholesterol is interspersed between phospholipid molecules, helping maintain the stability and fluidity of the cell membrane.

• Proteins (55%): There are two types of membrane proteins:

• Integral proteins: Span the entire thickness of the cell membrane, including:

• Channel proteins: Form small channels allowing ions to pass through the membrane along their concentration gradient.

• Carrier proteins: Bind to the substances to be transported and facilitate their movement across the membrane.

• Peripheral proteins: Only attach to one side of the membrane, often possessing enzymatic activity, playing a role in regulating and controlling the cell membrane’s activity.

• Carbohydrates (3%): Typically exist as combinations with proteins or lipids, forming glycoproteins or glycolipids.

2. Glycocalyx:

Glycocalyx is a carbohydrate layer covering the outer surface of the cell membrane, formed by the sugar chains of glycoproteins and glycolipids. Glycocalyx has important functions:

• Negative Charge: Helps retain water and ions, creating a favorable environment for chemical reactions occurring on the cell membrane.

• Adhesive Substance: Allows cells to connect to each other and other structures within the body.

• Receptor: Recognizes signals from the external environment, enabling the cell to respond to environmental changes.

• Immune Response Involvement: Antigens on the cell surface are recognized by the immune system through the glycocalyx.

3. Transport of Substances Across the Cell Membrane:

Transport of substances across the cell membrane occurs in two main ways:

a. Passive Transport:

• Does not require energy.

• Transport follows the concentration gradient or electrical potential.

• Includes these forms:

• Simple diffusion: Substances move across the membrane following the concentration gradient, from a region of high concentration to a region of low concentration. Example: O2, CO2, alcohol, nitrogen.

• Channel diffusion: Ions move through specialized protein channels. Example: Na+, K+, Ca2+, Cl-.

• Facilitated diffusion: Substances require the assistance of carrier proteins to move across the membrane. Example: Glucose, amino acids.

b. Active Transport:

• Requires energy.

• Transport occurs against the concentration gradient or electrical potential.

• Includes two types:

• Primary active transport: Directly uses energy from ATP hydrolysis. Example: Na-K-ATPase pump.

• Secondary active transport: Uses energy indirectly from the concentration gradient established by previous primary active transport. Example: Glucose co-transport with Na ions.

4. Special Transport Forms:

• Phagocytosis: The process of a cell engulfing large solid particles. Example: Macrophages engulf bacteria.

• Endocytosis: The process of a cell enclosing liquids or small solid particles inside the cell.

• Indirect endocytosis: Substances are enclosed within small vesicles (endosomes) before being brought into the cell. Example: Cholesterol, cholesterol metabolites, viruses, toxins.

• Exocytosis: The process of a cell releasing substances outside. Example: Proteins, hormones.

5. Factors Affecting Transport Across the Cell Membrane:

• Concentration: The greater the concentration difference between the two sides of the membrane, the faster the diffusion rate.

• Temperature: Higher temperatures increase the rate of diffusion.

• Molecular Size: Smaller molecules diffuse faster than larger ones.

• Lipid Solubility: Lipids readily diffuse across the cell membrane.

• Membrane Permeability: The more permeable the membrane, the faster the diffusion rate.

• Presence of Carrier Proteins and Channels: Carrier proteins and channels help increase the transport rate of substances.

6. Note:

• The cell membrane is flexible and dynamic, capable of changing structure and function over time and under different environmental conditions.

• Transport of substances across the cell membrane is a complex process governed by numerous factors and mechanisms.

• Understanding the mechanisms of transport across the cell membrane is crucial for explaining cell functions, biological processes, and diseases related to the cell membrane.

7. Additional Information:

• Membrane Potential: The difference in electrical potential between the two sides of the cell membrane, mainly created by the Na-K-ATPase pump.

• Action Potential: A rapid and temporary change in membrane potential that occurs when a nerve cell is stimulated.

• Gated Channels: Protein channels that can open and close under certain environmental conditions, such as changes in membrane potential or the presence of chemical substances.

8. Summary Table:

Transport Type	Mechanism	Energy	Example
Simple diffusion	Following the concentration gradient	Not required	O2, CO2, alcohol, nitrogen
Channel diffusion	Through protein channels	Not required	Na+, K+, Ca2+, Cl-
Facilitated diffusion	Through carrier proteins	Not required	Glucose, amino acids
Primary active transport	Uses ATP	Required	Na-K-ATPase pump
Secondary active transport	Uses concentration gradient	Required	Glucose co-transport with Na ions
Phagocytosis	Engulfing large solid particles	Required	Macrophages engulf bacteria
Endocytosis	Enclosing liquids or small solid particles	Required	Cholesterol, viruses, toxins
Exocytosis	Releasing substances outside	Required	Proteins, hormones

9. Exercises:

• Explain the mechanism of glucose transport across the intestinal cell membrane.

• List the factors affecting the rate of simple diffusion.

• Compare primary active transport and secondary active transport.

• Explain the role of channel proteins in transporting substances across the cell membrane.

10. References:

• Textbook of Physiology – University of Medical and Pharmaceutical Sciences

• Campbell Biology, 11th Edition

• Principles of Anatomy and Physiology, 16th Edition

11. Notes:

• This article is just an introduction to cell physiology and transport across the cell membrane. To understand more, you should consult specialized materials.

• Concepts and terminology in the article can be explained in greater detail in other articles.

12. Conclusion:

The cell membrane is a complex and dynamic structure that plays a crucial role in protecting, exchanging substances, and maintaining the cell’s life. Transport of substances across the cell membrane is a complex process governed by numerous factors and mechanisms. Understanding these mechanisms helps us better understand cell functions and biological processes within the body.