IGCSE Biology Chapter 2: Organisation of Organisms - Notes

Organisation of Organisms

Cell: The Basic Unit of Life

A cell is the basic structural and functional unit of life.

Animal Cells

Main features of animal cells:
- Multicellular
- Contain a nucleus with a distinct membrane
- Lack cellulose cell walls
- Lack chloroplasts and cannot perform photosynthesis
- Feed on organic substances from other living things
- Often store carbohydrates as glycogen
- Usually have nervous coordination
- Able to move from place to place

Plant Cells

Main features of plant cells:
- Multicellular
- Contain a nucleus with a distinct membrane
- Have cell walls made of cellulose
- Contain chloroplasts and can perform photosynthesis
- Feed by photosynthesis
- Store carbohydrates as starch or sucrose
- Lack nervous coordination

Cell Structures Found in Both Animal and Plant Cells

Nucleus
- Contains the genetic material (DNA) which controls the activities of the cell.
Cytoplasm
- A gel-like substance composed of water and dissolved solutes.
- Supports internal cell structures.
- Site of many chemical reactions, including anaerobic respiration.
Cell Membrane
- Holds the cell together, separating the inside from the outside.
- Controls which substances can enter and leave the cell.
Ribosomes
- Found in the cytoplasm.
- Site of protein synthesis.
Mitochondria
- Site of most of the reactions involved in aerobic respiration, where energy is released to fuel cellular processes.
- Cells with high metabolic rates have more mitochondria.

Cell Structures Found Only in Plant Cells

Cell Wall
- Made of cellulose (a polymer of glucose).
- Gives the cell extra support, defining its shape.
Chloroplasts
- Contain green chlorophyll pigments (to absorb light energy) and the enzymes needed for photosynthesis.
Permanent Vacuole
- Contains cell sap; a solution of sugars and salts dissolved in water.
- Used for storage of certain materials.
- Helps support the shape of the cell.

Bacteria Cells

Characteristics of bacteria cells:
- Microscopic single-celled organisms.
- Cell wall (made of peptidoglycan, not cellulose), cell membrane, cytoplasm and ribosomes.
- Lack a nucleus, but contain a circular chromosome of DNA.
- Plasmids are sometimes present - these are small rings of DNA that contain extra genes to those found in the chromosomal DNA.
- Lack mitochondria, chloroplasts and other membrane-bound organelles found in animal and plant cells.
- Some bacteria have a flagellum for movement.
Examples of bacteria:
- Lactobacillus
- Pneumococcus

Identifying Cell Structures & Function

Using higher magnification (electron microscope), the following organelles are visible:
- Mitochondria - organelles found throughout the cytoplasm.
- Ribosomes are tiny structures that can be free within the cytoplasm or attached to a system of membranes within the cell known as Endoplasmic Reticulum.
Rough Endoplasmic Reticulum (R.E.R.) is endoplasmic reticulum studded with ribosomes.
Vesicles are small circular structures found moving throughout the cytoplasm (visible at higher magnification).

Producing New Cells

Cells divide to help the body grow and repair itself.
New cells are produced by the division of existing cells.

Specialised Cells

Specialised cells have developed certain characteristics to perform particular functions.
These differences are controlled by genes in the nucleus.
Cells specialise by undergoing differentiation: this is a process by which cells develop the structure and characteristics needed to be able to carry out their functions.

Specialised Cells in Animals

Ciliated Cell
- Function: Movement of mucus in the trachea and bronchi.
- Adaptations:
  - Extensions of the cytoplasm at the surface of the cell form hair-like structures called cilia which beat to move mucus and trapped particles up to the throat
Nerve Cell
- Function: Conduction of impulses.
- Adaptations:
  - Long so that nerves can run to and from different parts of the body to the central nervous system.
  - The cell has extensions and branches, so that it can communicate with other nerve cells, muscles and glands.
  - The axon (extension of cytoplasm away from the cell body) is covered with a fatty sheath, which insulates the nerve cell and speeds up the nerve impulse.
Red Blood Cell
- Function: Transport of oxygen
- Adaptations:
  - Biconcave disc shape increases surface area for more efficient diffusion of oxygen.
  - Contains haemoglobin which joins with oxygen to transport it.
  - Contains no nucleus to increase the amount of space available for haemoglobin inside the cell.
Sperm Cell
- Function: Reproduction
- Adaptations:
  - The head contains the genetic material for fertilisation in a haploid nucleus (containing half the normal number of chromosomes).
  - The acrosome in the head contains digestive enzymes so that a sperm can penetrate an egg.
  - The mid-piece is packed with mitochondria to release energy needed to swim and fertilise the egg.
  - The tail enables the sperm to swim.
Egg Cell (Ovum)
- Function: Reproduction
- Adaptations:
  - Contains a lot of cytoplasm which has nutrients for the growth of the early embryo.
  - Haploid nucleus contains the genetic material for fertilisation.
  - Cell membrane changes after fertilisation by a single sperm so that no more sperm can enter.

Specialised Cells in Plants

Root Hair Cell
- Function: Absorption of water and mineral ions from soil.
- Adaptations:
  - Root hair increases surface area of cell to ensure maximum absorption of water and mineral ions.
  - Walls are thin to ensure water moves through quickly.
  - No chloroplasts present.
Xylem Vessel
- Function: Conduction of water through the plant, support of the plant.
- Adaptations:
  - No top and bottom walls between xylem vessels, so there is a continuous column of water running through them.
  - Cells are dead without organelles or cytoplasm to allow free passage of water
  - Their walls become thickened with a substance called lignin which means they are able to help support the plant
Palisade Mesophyll Cell
- Function: Photosynthesis.
- Adaptations:
  - Column shaped to maximize absorption of sunlight and fit as many in a layer under the upper epidermis of the leaf as possible.
  - Contains many chloroplasts for maximum photosynthesis.

Levels of Organisation in an Organism

Cells: Basic functional and structural units in a living organism.
Tissues: Groups of cells of similar structure working together to perform the same function.
Organs: Made from different tissues working together to perform specific functions.
Organ Systems: Groups of organs with related functions, working together to perform body functions.
Examples:
- Shoot System: Leaf, stem, flower, fruit (Epidermis, mesophyll, xylem, pholem)
- Root System: Root, tuber (Xylem, pholem, ground tissue)
- Digestive System: Oesophagus, stomach, small intestine, large intestine (Muscle, Connective, Nerve, Epithelial)
- Circulatory System: Heart, veins, arteries (Muscle, Connective)
- Immune System: Thymus, Spleen (Connective, Nerve, Epithelial, Bone Marrow)
- Repiratory System: Trachea, Bronchi, Lungs (Muscle, Epithelial)
- Excretory System: Liver, Kidney, Skin, Lungs (Connective, Muscle, Epithelial)
- Nervous System: Brain, Spinal Cord (Nerve)
- Reproductive System: Ovary, Cervix, Uterus, Vagina, Testes, Penis (Muscle, Connective, Nervous, Erectile)

Magnification Formula

Magnification = Image size / Actual size
$Magnification = \frac{Image \,size}{Actual \,size}$
Actual size = Image size / Magnification
$Actual \, size = \frac{Image \,size}{Magnification}$
Image size = Magnification x Actual size
$Image \, size = Magnification \times Actual \, size$
Magnification does not have any units.

Worked Example

An image of an animal cell is 30 mm in size and it has been magnified by a factor of x 3000. What is the actual size of the cell?
To find the actual size of the cell:
* $Actual \, size = \frac{30 \, mm}{3000} = 0.01 \, mm$