Cell structure and function

What is cell ?     

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Define cell in simple term?

“A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes.”

Who discovered cells?

Robert hookie discovered the cell in 1665.

The cell structure and its components.

The cell structure comprises several individual components which perform specific functions essential to carry out life processes.
The components of the cell are as follows:

• Cell membrane
• Cytoplasm
• Nucleus
• Cell organelles

• • Nucleolus
  • Nuclear membrane
  • Endoplasmic reticulum
  • Golgi Bodies
  • Ribosome
  • Mitochondria
  • Lysosomes
  • Chloroplast
  • Vacuoles

 

Cell membrane                                      

The cell membrane, also called the plasma membrane, is found in all cells and separates the interior of the cell from the outside environment. The cell membrane consists of a lipid bilayer that is semipermeable. The cell membrane regulates the transport of materials entering and exiting the cell.

Cytoplasm

The part of a cell outside the nucleus that includes a fluidlike substance and organelles (as chloroplasts and mitochondria)

An analogy for the cytoplasm is that the cytoplasm is like the jelly inside a donut. The cytoplasm contains nutrients and salts, as well as water. It is a thick gel-like substance that also holds all the organelles inside the cell.

Nucleus

The structure in a cell that contains the chromosomes. The nucleus has a membrane around it.

Cell organelles

Cells are composed of various cell organelles that perform certain specific functions to carry out life’s processes. The different cell organelles, along with its principal functions, are as follows:

Cell Organelles and their Functions

Nucleolus

The nucleolus is the site of ribosome synthesis. Also, it is involved in controlling cellular activities and cellular reproduction.

Nuclear membrane

The nuclear membrane protects the nucleus by forming a boundary between the nucleus and other cell organelles.

Chromosomes

Chromosomes play a crucial role in determining the sex of an individual. Each human cells contain 23 pairs of chromosomes.

Endoplasmic reticulum

The endoplasmic reticulum is involved in the transportation of substances throughout the cell. It plays a primary role in the metabolism of carbohydrates, synthesis of lipids, steroids and proteins.

Golgi Bodies

Golgi bodies are called the cell’s post office as it is involved in the transportation of materials within the cell.

Ribosome

Ribosomes are the protein synthesis of the cell.

Mitochondria

The mitochondrion is called “the powerhouse of the cell.” It is called so because it produces ATP – the cell’s energy currency.

Lysosomes

Lysosomes protect the cell by engulfing the foreign bodies entering the cell and help in cell renewal. Therefore, they are known as the cell’s suicide bags.

Chloroplast

Chloroplasts are the primary organelles for photosynthesis. It contains the pigment called chlorophyll.

Vacuoles

Vacuoles store food, water, and other waste materials in the cell.

Function of cell

The essential functions of the cell include:

• The cell provides support and structure to the body.
• It facilitates growth by mitosis.
• It helps in reproduction.
• Provides energy and allows the transport of substances.

Characteristics of Cells

Following are the various essential characteristics of cells:

• Cells provide structure and support to the body of an organism.
• The cell interior is organised into different individual organelles surrounded by a separate membrane.
• The nucleus (major organelle) holds genetic information necessary for reproduction and cell growth.
• Every cell has one nucleus and membrane-bound organelles in the cytoplasm.
• Mitochondria, a double membrane-bound organelle is mainly responsible for the energy transactions vital for the survival of the cell.
• Lysosomes digest unwanted materials in the cell.
• Endoplasmic reticulum plays a significant role in the internal organisation of the cell by synthesising selective molecules and processing, directing and sorting them to their appropriate locations.

Animals

• The main features of animals:
  • They are multicellular
  • Their cells contain a nucleus with a distinct membrane
  • Their cells do not have cellulose cell walls
  • Their cells do not contain chloroplasts (so they are unable to carry out photosynthesis)
  • They feed on organic substances made by other living things
  • They often store carbohydrates as glycogen
  • They usually have nervous coordination
  • They are able to move from place to place

Plants

• The main features of plants:
  • They are multicellular
  • Their cells contain a nucleus with a distinct membrane
  • Their cells have cell walls made out of cellulose
  • Their cells contain chloroplasts (so they can carry out photosynthesis)
  • They feed by photosynthesis
  • They store carbohydrates as starch or sucrose
  • They do not have nervous coordination

• Cell Structures Found in Both Animal and Plant Cells Table

Structure	Function
Nucleus	Contains genetic material (DNA) which controls the activities of the cell
Cytoplasm	A gel like substance composed of water and cell solutes. It supports the internal cell structures and is the site for many chemical reactions
Cell membrane	Holds the cell together, separating the inside of the cell from the outside. Controls which substances leave and enter the cell
Ribosomes	Found in the cytoplasms, these are the site of protein synthesis
Mitochondria	The site of aerobic respiration where energy is released to the cell. Large numbers of mitochondria are found in cells that are very metabolically active, such as muscle cells

Cell Structures Found Only in Plant Cells Table

Structure	Function
Cell wall	Made of cellulose, gives extra support to the cell
Chloroplasts	Contain green chlorophyll pigments which absorb light energy during photosynthesis
Permanent vacuole	Contains cell sap which is a solution of dissolved sugars and ions. It is used for storage and support of the cell structure

 Examining Cells Under the Microscope

• Many biological structures are too small to be seen by the naked eye
• Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and larger organelles to be seen and studied
• Light is directed through a thin layer of biological material that is supported on a glass slide
• This light is focused through several lenses so that an image is visible through the eyepiece
• Getting a visible image requires a very thin sample of biological tissue because light has to pass through the sample and into the lenses of the microscope

• The most common specimens to observe under a light microscope are cheek cells(animal cells) and onion cells (plant cells)
• A stain is often used to ensure cell structures are clearly visible under the microscope

General method

• Specimens must be prepared on a microscope slide to be observed under a light microscope
• This must be done carefully to avoid damaging the biological specimen and the structures within it

• Preparing a slide using a liquid specimen:
  • Add a few drops of the sample to the slide using a pipette
  • Cover the liquid/smear with a coverslip and gently press down to remove air bubbles
    • Wear gloves to ensure there is no cross-contamination of foreign cells
• Preparing a slide using a solid specimen:
  • Use scissors to cut a small sample of the tissue
  • Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps)
  • Some tissue samples need to be treated with chemicals to kill/make the tissue rigid
  • Gently place a coverslip on top and press down to remove any air bubbles
  • A stain may be required to make the structures visible depending on the type of tissue being examined
    • Commonly used stains include methylene blue to stain cheek cells and iodine to stain onion cells
  • Take care when using sharp objects and wear gloves to prevent the stain from dying your skin
• When using an optical microscope always start with the lowest power objective lens:
  • It is easier to find what you are looking for in the field of view
    • This helps to prevent damage to the lens or coverslip in case the stage has been raised too high
• Preventing the dehydration of tissue:
  • The thin layers of material placed on slides can dry up rapidly
  • Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration
• Unclear or blurry images:
  • Switch to the lower power objective lens and try using the coarse focus to get a clearer image
  • Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly
  • There could be cross-contamination with foreign cells or bodies.

Viewing plant tissue

• An ideal tissue is the onion epidermis (found between the layers of onions) because it forms a layer just one cell thick
  • Being a non-photosynthetic tissue, onion epidermis is not green as it does not contain any chloroplasts

Apparatus

• The key components of an optical microscope you will need to use are:
  • The eyepiece lens
  • The objective lenses
    • The stage
    • The light source
    • The coarse and fine focus
• Other apparatus used:
  • Forceps
  • Scissors
  • Scalpel
  • Coverslip
  • Slides
  • Pipette
  • Iodine solution 

Viewing animal tissue

• Human cheek cells are a good choice for examination under the light microscope because they are:
  • Plentiful
  • Easy to obtain safely
  • Can be obtained without an overly intrusive process
  • Relatively undifferentiated and so will display the main cell structures

Safety considerations

• Do not perform the sampling on a person who has a cold, cough, throat infection etc.
  • To avoid spreading the infection to others
• Concentrated methylene blue is toxic if ingested
  • Wear gloves and do NOT allow children to handle methylene blue solution or have access to the bottle of solution

• Apparatus glass microscope slides
• Cover slips
• Paper towels or tissue
• Staining solution
  • Methylene blue solution
    • 0.5% to 1%
    • Dilute according to concentration of the stock solution
• Plastic pipette or dropper
• Sterile, individually packed cotton wool buds or swabs

Method

• Brush teeth thoroughly with normal toothbrush and toothpaste
  • This removes bacteria from teeth so they don’t obscure the view of the cheek cell
• Take a clean, sterile cotton swab and gently scrape the inside cheek surface of the mouth for 5-10 seconds
• Smear the cotton swab on the centre of the microscope slide for 2 to 3 seconds
• Add a drop of methylene blue solution
• Place a coverslip on top
  • Lay the coverslip down at one edge and then tilt it down flat
    • This reduces bubble formation under the coverslip
• Absorb any excess solution by allowing a paper towel to touch one side of the coverslip.
• Place the slide on the microscope, with 4 x or 10 objective in position and find a cell
• Then view at higher magnification to reveal more detail
• Methylene blue stains negatively charged molecules in the cell, including DNA and RNA
  • This causes the nucleus and mitochondria appear darker than their surroundings
• The cells seen are squamous epithelial cells from the outer epithelial layer of the mouth

Parts of the cell that can be seen with a light microscope

• Nucleus
• Mitochondria
• Cell membrane
• Cytoplasm

Parts of the cell that cannot be seen with a light microscope

• Ribosomes*
• Endoplasmic reticulum*
• Golgi*
• Details of the nucleus, mitochondria and cell membrane

* these are parts of the cell that you don’t need to know the names of, but they are included here as examples of very small structures within the cell

Bacteria Cells

• Bacteria, which have a wide variety of shapes and sizes, all share the following biological characteristics:
  • They are microscopic single-celled organisms
  • Possess a cell wall (made of peptidoglycan, not cellulose), cell membrane, cytoplasm and ribosomes
  • Lack a nucleus but contain a circular chromosome of DNA that floats in the cytoplasm
  • Plasmids are sometimes present – these are small rings of DNA (also floating in the cytoplasm) that contain extra genes to those found in the chromosomal DNA
  • They lack mitochondria, chloroplasts and other membrane-bound organelles found in animal and plant cells

•  Some bacteria also have a flagellum (singular) or several flagella (plural). These are long, thin, whip-like tails attached to bacteria that allow them to move
• Examples of bacteria include:
  • Lactobacillus (a rod-shaped bacterium used in the production of yoghurt from milk)
  • Pneumococcus (a spherical bacterium that acts as the pathogen causing pneumonia)

Level of organization in biology 

• The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems.
• Finally, groups of organs make up organ systems that work together to carry out vital processes. For example, the animal’s cardiovascular system is comprised of the heart, vessels, and blood. In plants, a diverse set of tissues create the root and shoot system that provides different functions.
• Individual Organisms Create Populations and Communities Several organisms of the same species that exist at the same place and time form populations.
• Communities are composed of individuals of different species at the same location and time. Communities can include multiple species.
• Ecosystems are comprised of not just the living (biotic) community but also the non-living (abiotic) environmental factors that influence the community. For example, an oasis is an ecosystem that may contain populations of date palms, fig trees, and small lizards found at the same location and time.
• This community lives and interacts with an environment of fertile soil and fresh water in an otherwise arid environment. The biosphere comprises all areas (air, soil, and water) on Earth that harbors living organisms. As such, the biosphere is composed of many ecosystems.

Magnification Formula

Calculating magnification and specimen size using milli meters as units

• Magnification is calculated using the following equation:

Magnification = Image size ÷ Actual size

• A better way to remember the equation is using an equation triangle:

• Rearranging the equation to find things other than the magnification becomes easy when you remember the triangle – whatever you are trying to find, place your finger over it and whatever is left is what you do, so:
  • Magnification = image size / actual size
  • Actual size = image size / magnification
  • Image size = magnification x actual size

Remember magnification does not have any units and is just written as ‘x 10’ or ‘x 5000’