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).
  

INTRODUCTION TO BIOLOGY

Biology is the study of living things which are often called organisms. Living organisms have several features or characteristics which make them different from non-living things. The following are characteristics of living organisms:

• They are able to REPRODUCE new offsprings.
• They FEED.
  
• They are able to RESPIRE by releasing energy from their food (This is usually done in the presence of oxygen).
• They GROW.
• They EXCRETE: they get rid of toxic waste substances or substances which are present in the body in excess.
• They MOVE (Not necessarily from one place to another).
• They are SENSITIVE: they can sense and respond to changes in thier surroundings.

RECAP: HOW TO REMEMBER THESE CHARACTERISTICS EASILY?

Remember MRS GREF
Movement
Respiration
Sensitivity
Growth
Reproduction
Excretion
Feeding