CELL STRUCTURES AND ORGANISATION

Chapter 2 Cell Structure & Organization - Lecture Notes

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CELL SPECIALISATION

Analogy First OK!:
Current form five students all over Brunei are generally between the age of 15 to 17. In another way to say they are generally the same in terms of their age. Apart from age, they all are going to sit for their 'O' level examination sometimes November this year.

What happens next year after the examination result is released? It all depends on their results and qualifications, right? Some will further their studies locally or abroad. Some will have to leave school and seek employment. Those who opt for employment will go to various sectors depending on their qualifications. So in this case, we can say that the students are being "specialized" after doing their 'O' level examination. So in this analogy we can assume that the students before the examination are just like cells before being specialized, and the students after the examination (furthering their studies or working) are just like cells after being specialized to suit their functions.

SO, WHAT IS CELL SPECIALIZATION?
Cell specialization (or modification or differentiation) is actually a process that occurs after cell division where the newly formed cells are structurally modified so that they can perform their function efficiently and effectively.

EXAMPLES OF SPECIALIZED CELLS
A. RED BLOOD CELL (ERYTHROCYTE)

A red blood cell is a tiny, disc-like cell (biconcave shape) which has no nucleus. In the cytoplasm of a red blood cell, there is a red pigment called haemoglobin (a protein combined with iron). Each red blood cell lives for about 4 months, after which it breaks down. The red haemoglobin changes to a yellow pigment, bilirubin which is excreted in the bile. The iron from the haemoglobin is stored in the liver (hence liver is a very rich source of iron!!!). Red blood cells are made by the bone marrow of certain bones in the skeleton (the ribs, vertebrae and breastbone)

Function of red blood cell: To transport oxygen to all respiring cells all over the body.

Adaptations to function (The way a cell is structurally modified in such a way it can perform its function efficiently):

• It contains haemoglobin that can combine with oxygen to form oxyhaemoglobin.
• It has no nucleus so that more haemoglobin can be accomodated (hence more oxygen can be transported). By having no nucleus, enable a red blood cell to squeeze through small blood capillaries.
  
• It has biconcave shape for increasing its surface area thus diffusion of oxygen in and out of the red blood cell becomes easy.

B. ROOT HAIR CELL

Root hair cells are actually modified epidermal cells of the roots. A root hair cell has a long and narrow protrusion (may also be referred to as hair-like structure). A root hair cell has a large vacuole with lots of mitochondria in the cytoplasm. Now, unlike any typical plant cells, root hair cells have no chloroplasts! (Is there a point in having them? Think, root hair cells are in the soil right? And sunlight cannot penetrate the soil, right? So photosynthesis is impossible in the soil. Hence there is no point for a root hair cell to have chloroplasts).

Function of root hair cell: To absorb water and mineral salts by osmosis and active transport respectively.

ADAPTATIONS TO FUNCTION:

• The hair-like structure helps to increase the surface area of the root hair cell, thus helps the root hair cell to absorb more water and mineral salts.
• The hair-like structure which is long and narrow helps the root hair cell to penetrate in between soil particles in search of water and mineral salts.
• The presence of mitochondria in large number in the cytoplasm of the root hair cell, helps for more absorption of mineral salts by active transport (Remember, active transport will only occur in the presence of energy provided by the mitochondria).
• The large vacuole enable more water and mineral salts to be stored after being absorbed. (Note: the absence of chloroplasts actually helps the vacuole to stretch out)

C. THE XYLEM VESSELS

A xylem vessel is made up of long cells joined end to end. Once a region of the plant has ceased growing, the end walls of these cells are digested away to form a continuous fine tube. At the same time, the cell walls are thickened and impregnated with a substance called lignin which makes the cell wall very strong and impermeable (The process is called lignification). Since these lignified cell walls prevent free passage of water and nutrients, the cytoplasm dies (However, the passage of water in the vessel itself is not affected). So a xylem is just like a water pipe which is hollow with no living materials (cytoplasm, nucleus and other organelles) in it.

Function of xylem vessel: To transport water and minerals from the roots to other parts of a plant as well as to provide support to the whole plant hence enable a plant to stand erect.

ADAPTATIONS TO FUNCTIONS:

• The cell walls of the xylem vessel being lignified adds or provides strength. This in turn provides support to the whole plant. The xylem in the leaf for example, helps the leaf to be positioned horizontally on the plant towards the sun. This helps the leaf to absorb as much light energy as possible to be used for photosynthesis.
• The absence of protoplasm and cross-wall in the xylem vessel provides no obstacle for water to flow up the xylem vessel.
• Being very narrow helps water to move up the xylem vessel by means of capillary action.

OTHER EXAMPLES OF SPECIALIZED CELLS
D. GUARD CELLS

Guard cells are cells in between a stoma. They are specialized in such a way that the cell wall in the inner side of the guard cells are thicker than the outer side. This feature helps the guard cells to bend outward when they become turgid. This results in the opening of the stoma. If the guard cells become flaccid, the guard cells will bend inward resulting in the closing of the stoma.


E. MUSCLE CELL

A muscle cell is generally elongated and elastic containing mitochondria in large number. The elongated and elastic feature helps muscle tissues to contract and relax. Contraction and relaxation of muscle tissues help in movement. The large number of mitochondria is very important in tissue respiration in the muscle cells (Remember, energy is required for muscle to contract)

F. SPERM CELL

A sperm cell is specialised in such a way that it has a tail, a large number of mitochondria and a small vacuole containing enzyme. The tail is very important for mobility when the sperm cell needs to swim towards an egg cell before fertilisation can take place. The large number of mitochondria provides energy for mobility and the enzyme in the small vacuole helps the sperm to digest the wall of the egg cell so that the nucleus can enter the egg cell for fertilisation to take place (Fertilisation is fusion of the nucleus of the sperm with the nucleus of the egg cell to form a zygote)

Note: There is an assumption that the sperm cell enters the egg cell. This is a big misconception. Remember: Only the nucleus of the sperm cell enters the egg cell and later the nucleus of the sperm cell and the nucleus egg cell fuse to form a zygote - FERTILIZATION)

CELLS ORGANISATION

Remember the typical plant and animals cells in the previous post? You may think that all the cells in your body and the cells in a plant are all alike. Well, that is not the case. A large organism like yourself may contain many millions of cells which are totally different structurally and functionally. Almost all of these cells can perform the activities which are characteristics of living things, but many of them specialise in doing some of these better than other cells do. Lets see how cells are specialise to form tissues, organs and organ systems.

TISSUE

• A tissue is a group of specialised cells having the same structures, which perform a particular function.
• Example 1: The layer of cells lining your stomach. The function of this tissue is to make digestive enzymes which helps digestion of protein in the stomach.
• Example 2: The palisade mesophyll layer of the leaf consists of palisade cells. This layer of cells (tissue) contains chloroplasts in high concentration. Because of this, more photosynthesis occurs in this layer.
  

ORGAN

• An organ is a group of tissues working together to perform a particular function (although each of these tissues have their own function to do).
• Examples: Stomach, Heart, Kidney, Lung, Leaf and Flower
  
• Consider a leaf: A leaf consists of the epidermal layers (note: the stomata are found in the lower epidermal layer), palisade mesophyll layer, spongy mesophyll layer and the vascular tissues. Each of these tissues has its own function but they work together to make food in photosynthesis.
  

ORGAN SYSTEM

• An organ system is a group of organs working together to perform a particular function.
• The stomach is only one of the organs which help in the digestion of food. The mouth, the intestines and the stomach are all part of the digestive system.
• The heart and the blood vessels are parts of the circulatory system.
  

SUMMARY:

ORGANELLES --> CELLS --> TISSUES --> ORGANS --> ORGAN SYSTEM --> ORGANISM

CELL STRUCTURES

WHAT ARE CELLS?
Cells are the basic units of all living organisms (just like bricks are to buildings). Cells cannot be seen by using your naked eyes. So in order to see cells clearly, you need to use a light microscope. A very good light microscope can magnify about 1500 times and can show all the structures as shown in the following figures:

The following figure shows a typical animal cell

The following figure shows a typical plant cell

The following figure shows how a mitochondrion looks like:

COMPARISON BETWEEN A TYPICAL ANIMAL CELL AND A TYPICAL PLANT CELL

A. SIMILARITIES:

• Both have a cell membrane (plasma membrane) surrounding the cell.
• Both have cytoplasm.
• Both have a nucleus (one per cell).
• Both contain mitochondria (note: mitochondrion, if singular).

B. DIFFERENCES:

• A plant cell has a cellulose cell wall outside the cell membrane but an animal cell has no cell wall.
• Most plant cells have chloroplasts containing chlorophyll in the cytoplasm but animal cells have not chloroplasts.
• Plant cells have a very large vacuole containing cell sap (some plant cells have two large vacuoles) but animal cells have non-permanent small vacuoles (meaning, even if they are present, they are usually very small in size and temporary in nature).
• Plant cells are usually larger in size compared to animal cells.
• Plant cells are usually regular in shape (due to the presence of cell wall) but animal cells are usually irregular in shape (due to the absence of cell wall).
• Plant cells often have starch granules (starch granules store some of the food made during photosynthesis) but starch granules are absent in animal cells (some animal cells have glycogen granules instead).

FUNCTIONS OF CELL PARTS:

• Nucleus: It controls the activities of the cells. It contains genetic materials in the form of chromosomes.
  
• Cytoplasm: It is a jelly-like substance in which other cell parts (organelles) are found and it is also a site where most chemical reactions take place.
  
• Cell membrane: It is selectively permeable made of a very thin layer of protein and fat. Hence it controls the passage of materials in and out of the cell. Selectively permeable means that it can allow smaller molecules like water molecules to pass through but cannot allow larger molecules like starch molecules to pass through.
  
• Cell wall: It is made of cellulose which is a kind of carbohydrate which makes the cell wall tough hence maintains the shape of plant cells. It is fully permeable (meaning, large and small molecules can pass through it).
  
• Mitochondrion: It is a site for respiration where energy from food (glucose) is released.
  
• Chloroplast: It is a site for photosynthesis. It contains a green pigment, chlorophyll which absorbs energy from sunlight.
  
• Vacuole: It is a space in a cell (surrounded by a membrane called tonoplast) containing a solution of sugars and other substances (cell sap).