Cell structure

Cell Structure

Cells

The cell theory has 3 tenets:

1. All living things are made up of cells

2. The cell is the basic unit of life

3. All cells come from pre-existing cells

Unicellular = made up of one cell

Multicellular = made up of multiple cells

Eukaryote and Prokaryote

Eukaryotes are organisms (uni or multicellular) composed of cells which contain membrane-bound organelles.

• They are divided into four kingdoms of life

1. Animals

2. Plants

3. Fungi

4. Protists

Prokaryotes are unicellular organisms which do not have membrane-bound organelles.

• They are divided into two kingdoms

1. Bacteria

2. Archaea

Drawing a scaled diagram

1. Determine the actual size of the specimen

2. Choose a size that you will use to draw your diagram

3. Draw and label

4. Determine the scale bar using

Scale = actual size of image/size of drawing

5. Draw scale bar onto its diagram

Organelles

Organelles are substructures found inside cells. They help keep the cell alive and perform specialised functions.

• It can be found in membrane-bound or non-membrane-bound

  • Membrane-bound organelles can only be found in eukaryotes.

Cell Membrane

Structure: A sac made of phospholipids, proteins, and carbohydrates that surrounds the cell

Function: Protects the cell, holds the cell in shape, and allows for selective permeability.

Cytosol

Structure: A gel-like aqueous solution inside the cell membrane.

Function: Substances are dissolved or suspended in it, houses all organelles

Nucleus

Structure: large, spherical, porous membrane containing the organism's DNA.

Function: regulates the expression of the genetic material of the organism

Nucleolus

Structure: A small spherical organelle found inside the nucleus

Function: Produces some specialised RNA, such as ribosomal structures

Ribosomes

Structure: A two-subunit structure made of protein and RNA found either in the cytoplasm or attached to the endoplasmic reticulum.

Function: Produces proteins

Endoplasmic Reticulum

Structure: A network of membranes and canals which connects the nuclear membrane with the cell membrane.

• May have ribosomes (rough ER) or not (smooth ER)

Function: Transports a variety of substances around the cell

Mitochondria

Structure: A bean-shaped structure with a double membrane - the inner membrane is highly folded

Function: carries out cellular respiration to produce energy

Golgi Apparatus

Structure: A stack of membrane sacs found closely associated with the endoplasmic reticulum.

Function: Receives, sorts and dispatches substances in small vesicles

Lysosomes

Structure: A small sac filled with hydrolytic enzymes

Function: Allows cells to digest large molecules (proteins, fats, carbs, etc.) or foreign invaders (bacteria).

• It can rupture to allow the cell to kill itself 

  • Apoptosis = Natural killing of the cell

  • Necrosis = Abnormal killing of the cell

Centrioles

Structure: A cylindrical structure which produces tubules

Function: Assists in cell division (mitosis and meiosis)

Vacuoles

Structure: A membrane-bound space containing water, dissolved minerals and sometimes, nutrients.

Function: Stores water, salts and food. Contributes to the structural support of the cell

Cell wall 

Structure: A rigid structure made of cellulose which surrounds the cell membrane

Function: Protects and supports the cell, allows neighbouring cells to adhere (stick) to each other

Chloroplast

Structure: A double-membraned structure with stacks of grana inside, each containing chlorophyll

Function: Site of photosynthesis, storage of starch.

Cell Wall

The cell wall exists surrounding the Cell membrane in some types of organisms:

• Plants

• Fungi

• Bacteria

• Archaea

Usually, the cell wall is a tough layer which helps to protect the cell from damage and the environment. 

Cell Wall in plants

In plants, the cell wall is made of cellulose

• Cellulose is a type of polysaccharide (multiple sugars) which is rigid, hard, and highly protective. 

  • Note: A polysaccharide is a chemical made of multiple, covalently bonded units of any sugar

Cellulose is composed of repeated subunits of B-glucose covalently bonded together.

Cell Wall - Bacteria

Bacteria are a very diverse kingdom, and this leads to a large diversity in the structure of cell walls.

• Bacterial cell walls are made of peptidoglycan (a type of polysaccharide that can also be called murein)

  • The cell wall is essential for the survival of most bacteria - removing the cell wall or preventing its construction will usually kill the bacterium.

Bacteria can be divided into 2 major groups, depending on the structure of their cell walls.

• Gram-positive

Has one cell, and a thick peptidoglycan layer surrounding it

• Gram-negative

Has two cell membranes (inner and outer) with a thin peptidoglycan layer sandwiched in between.

To differentiate these two classes of bacteria, a gram stain is performed. 

• A gram stain stains the bacteria with crystal violet, a chemical which will only bind to peptidoglycan. This turns gram-positive bacteria dark purple under a light microscope.

  • Gram-negative bacteria, with their outer cell membrane, will not be stained with crystal violet. A counter-stain is added to stain their outer cell membrane so it becomes visible. It will usually stain light pink.

DNA - Bacteria and Archaea

Eukaryotes carry their DNA in the nucleus

• DNA is linear, forms multiple paired chromosomes

• Bacteria and archaea do not have nuclei. Instead, their DNA simply aggregates in the centre of the cell, forming a nucleoid (defined space in a cell)

Prokaryotic DNA is circular, and usually only forms a single circular chromosome.

• There can be smaller, free-floating circular DNA molecules in the cytoplasm called plasmids.