Cell Structure (OCR Gateway GCSE Biology: Combined Science)

• Organisms are made of cells which are the smallest unit with the basic properties of life
• Organisms can be classified as eukaryotes or prokaryotes 
  • Eukaryotes can be unicellular (consist of one cell) or multicellular (consist of many cells) such as animals and plants
  • Prokaryotes (e.g. bacteria) are much smaller than eukaryotes and are always unicellular
• Both types of cells contain sub-cellular structures (parts of the cell) that each have specific functions
• But the internal cell structure of eukaryotic and prokaryotic cells are very different

• Prokaryotic cells are much smaller in comparison to eukaryotic cells, often measuring ~ 1 µm in size
  • Their small size allows them to have a much simpler structure than eukaryotic cells
• The inside of the cell contains a gel-like substance called the cytoplasm
  •  Most of the chemical reactions take place here
• The cell membrane encloses the cytoplasm
  • It is permeable to some substances and provides a selective barrier, controlling what goes in and out of the cell e.g. nutrients, waste
  • Receptors in the membrane help prokaryotes sense their environment  e.g detect nutrients
• The outside of the cell is surrounded by a cell wall
  • Helps provide strength and maintain the shape of the cell
• A defining feature of prokaryotic cells is that their chromosomal DNA (genetic material) consists of one long circular chromosome
  • It floats free in the cytoplasm (not enclosed within a nucleus)
  • It controls the cells activities and replication
• Extra small loops of extra DNA called plasmids may also be present
  • These can be exchanged between bacteria and give additional benefits to the bacteria
    • e.g. antibiotic resistance or ability to use a different energy source

• Eukaryotic cells are larger and more complex than prokaryotic cells
• They have many more sub-cellular structures to help efficiently carry out specific functions within the cell

Animal Cells

• Like prokaryotic cells, animal cells also contain a cytoplasm and cell-membrane
• The cytoplasm is the gel like substance where most of the chemical reactions take place
• The cell membrane helps to hold the cell together
  • Provides a selective barrier, controlling what goes in and out 
  • Contains receptor molecules used for cell communication e.g. by hormones
• Unlike prokaryotic cells, the DNA (genetic material) which controls the cell's activities is found in the nucleus
  • DNA is packaged into multiple linear chromosomes 
• Animal cells contain specialised structures called mitochondria where cellular aerobic respiration occurs
  • Mitochondria contain all the enzymes required for the reactions involved
  • This allows cells to derive energy from food in the presence of oxygen

Table of animal cell structures

Plant Cells

• Plant cells contain the same sub-cellular structures as animal cells and additional structures to help its function:
• A rigid cell wall made of cellulose to give support for the cell
• Plant cells found in the leaf and stem contain chloroplasts
  • Photosynthesis takes place in chloroplasts
  • They contain a green substance called chlorophyll to help absorb energy from sunlight

A typical plant cell showing some key sub-cellular structures (you don't need to know about ribosomes, endoplasmic reticulum or permanent vacuole)

Table of plant cell structures and functions