Cell Structure (OCR GCSE Combined Science A (Gateway))

Cell Structure

• 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

Prokaryote Cell Structure

• 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

Prokaryotic cells do not have a nucleus, and are much smaller than eukaryotic cells

Eukaryote Cell Structure

• 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

A typical animal cell showing some key sub-cellular structures

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