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The Human Cell 101 - Simple to understand


The Human Cell 101


On this page:

   Cell parts

   Cell membrane – The Cell’s “Castle Guard”

   How do substances cross the cell membrane?


Cell Parts


      Differing in size, shape, and function, the cell is the smallest functional unit of tissues and organs - Each cell is somewhat self-contained and self-maintaining:


-       It can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce


-       Human cells have a surrounding protective membrane - enclosing a salty, watery cytoplasm in which is floating a set of membrane-bound organelles ("little organs"), which perform a number of vital functions for the cell.


For example:


         The cell nucleus houses DNA - the cell’s genetic blueprint containing the building instructions for various protein molecules, such as enzymes;


         The mitochondrion - is the cell’s “Power Plant”;


         The endoplasmic reticulum - is the transport network for active molecules;


         Ribosome – primary site of protein synthesis


         The Golgi complex  /apparatus  - is the cell’s central delivery system and a site for protein processing, packaging, and transport.


         Lysosome - contain enzymes that break down waste materials and cellular debris


         Centriole – two per cell;  help with mitosis and meisosis when cell divides.







The Cell Membrane – The Cell’s “Castle Guard”



      The cell membrane is made mostly of a double layer of phospholipids, which envelopes the cell and separates its interior from its environment - A phospholipid molecule has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty-acid tails. By forming a bilayer with their tails intertwining, they are able to form a separation between the watery inside and outside of the cell.     At body temperature, membranes are a liquid with a consistency similar to cooking oil.


Cell Membrane



      All human cells acquire the nutritional molecules and ions they need from their surrounding extracellular fluid - which is also where the cell “dumps” its waste. Thus, there is an unceasing traffic of molecules and ions in and out of the cell through the cell membrane (E.g. glucose, Na+, Ca2+), and also through its organelle membranes (E.g. proteins, mRNA, Ca2+, ATP). Typically, the cells are located in close proximity to the blood vessels, which supply nutrients and oxygen and pick up waste to and from the cells (substances are transferred by diffusion between the capillaries and the extracellular fluid).


      The cell membrane serves as both a gateway and a barrier for the cell and is said to be 'semi-permeable' - It can either allow a molecule or ion from the extra-cellular fluid to pass through freely, pass through to a limited extent or not pass through at all, thus regulating its interaction with its environment. The cell membrane also contains cholesterol, which serves to ‘waterproof” the cell.




How do Substances Cross the Cell Membrane


Substances can cross the cell or organelle membrane

 by either a passive or active transport mechanism.



Passive Transport (By Diffusion)

 – Does not require energy


      Simple Diffusion (through membrane lipid bi-layer) a substance moves freely across  the membrane lipid bilayer down a concentration gradient (i.e. diffuses from an area of high concentration to an area of low concentration )


-       Generally limited to small, non-polar substances - Molecule dissolves in phospholipid bilayer, diffuses across it, then dissolves in the aqueous solution on the other side. E.g. Lipids; steroids; lipid-soluble molecules; Uncharged molecules, such as O2, N2;; Alcohol; Anaesthetic gases (ether); Pesticides; and non-polar solutes, such as CO2, urea.


-       Water (polarized) can also diffuse slowly through the lipid by-layer - Water is a special case, passing through the membrane by osmosis.






      Facilitated Diffusion (carrier or channel mediated) – Substances are transported down an electrical or chemical concentration gradient via a carrier-protein  or ion-channel spanning the membrane. Solutes, such as charged ions in solution (E.g. Na+, K+ and Cl- as solutes) require a carrier or channel to cross the membrane to avoid coming into contact with the water-hating core of the membrane lipid-bilayer. (A carrier or channel is made of protein molecules with a water hating, lipid loving exterior which “happily” spans the membrane, and a water loving center through which water and small water soluble molecules can pass). Some substances, such as glucose, which are too large to pass directly through the membrane, are able to cross when provided a carrier large enough to accommodate them. Some molecules, such as starch and proteins are just simply too large to cross the cell membrane. 




-       A carrier-protein - binds a selective substance on one side of the membrane, and then following a change of shape, the protein releases the substance on the other side of the membrane. Each carrier protein is designed to respond to only one substance. Used to transport small organic molecules (E.g. glucose, sucrose, amino acids) and some inorganic ions (E.g. K+, Na+).



-       An ion-channel – is a tiny pore generally used to transport only inorganic ions (E.g. K+, Na+ Cl-, Ca2+), but 1000x faster than by carrier protein (Ions move single file at  108 /second). Most ion-channels are selective of the type of ion they allow to pass.




         “Switch-Operated” GATES for Ion-Channels  - most ion-channels contain a “switch-operated” GATE that may be opened or closed, such that a channel only transports when the gate is opened in response to an external chemical, electrical or mechanical stimulus or by conditions within the cell.


There are 4 types of gating mechanisms:


(1)    Chemically-gated (also called ligand-gated) – Opened/closed by certain binding molecules  (e.g.  neurotransmitters, hormones).









(2)    Voltage-gated – opened/closed by a change in the cell membrane voltage - K+ and Na+ ion-channels are voltage-gated. The voltage is mainly determined by the balance of K+ and Na+ ions either side of the membrane. The transferral of ions further changes the cell membrane voltage.


(3)    Electromagnetic Gate – Opened/closed by specific-frequency electromagnetic signals produced by the body or received from the external environment. 


(4)    Mechanical-Gate –  E.g. Opened / closed by sound waves bending the hair cells of the inner ear leading to the creation of nerve impulses, which the brain translates into sound.



Active Transport (Using a pump)

-  ATP energy is needed to run a “Switch-Operated” Enzyme Pump to transport through a protein channel against a gradient  



      Direct Active Transport Mechanism - When there is an electrical or chemical gradient  (called an electrochemical gradient) opposing the movement of an ion or molecule across the membrane, a special transport protein pump is needed to convey the ion or molecule.


-       Active transport enables the cell to receive essential nutrients from the extracellular fluid - even when the nutrients are more concentrated inside the cell


-       Active transport enables the cell to remove waste products from the inside of the cell -  into the extracellular fluid.


-       Active transport also supports the vital imbalance of ions - such as K+, Na+, Ca++ and H+ across the membrane.


The most important example of direct active transport

 is the sodium/potassium (Na/K) pump


The Na/K pump pumps sodium ions (Na+) out of the cell and potassium ions (K+) into the cell, across the membrane against the electrochemical forces, in order to maintain the cell’s “battery” voltage.


      Indirect Active Transport Mechanism (E.g. used to transport glucose) - involves using energy to establish a gradient across the cell membrane, and then utilizing that gradient to transport a molecule against its concentration gradient.   The Na+/glucose indirect active transport mechanism, in particular, uses the Na/K pump as the first step, generating a strong Na+ gradient across the cell membrane. Then the glucose/Na+ symport protein uses that Na+ gradient to transport glucose into the cell. (Symport means binding two molecules at a time and using the gradient of one solute’s concentration to force the other molecule against its gradient). 




Related Links


Cell Parts  

Cell Membrane“The Cell’s Castle Guard”

-  How do substances cross cell membrane  


Cellular Respiration