Topic 1:Cell Biology

The Cell Theory

INTRODUCTION

Until microscopes became powerful enough to view individual cells, no one knew for certain what living organisms were made from.

A scientist called Robert Hooke came up with the term "cells" in the 1660s after examining the structure of cork
Matthias Schleiden and Theodor Schwann were two scientists who studied animal and plant cells
- In 1837, they came up with the idea that all living organisms are made of cells
- This idea is known as ‘cell theory’
- The cell theory is a unifying concept in biology (meaning it is universally accepted)

The cell theory includes three main ideas:

Living organisms are composed of cells (one or more) – that is, cells are the building blocks of organisms.
Cells are the smallest units of life – that is, a cell is the basic unit capable of carrying out all the functions of a living organism.
Cells come from pre-existing cells (omni cellulae e cellula) – that is, cells do not show spontaneous generation.

Although cells vary in size and shape they all:

Are surrounded by a membrane
Contain genetic material
Have chemical reactions occurring within the cell that are catalysed by enzymes.me

Fig 1 Robert Hooke (1635 - 1703)

Exceptions to the Cell Theory

NOS: Looking for trends and discrepancies; although most organisms conform to cell theory, there are exceptions

Science looks for trends and patterns to explain phenomena. Within these trends there are often exceptions, particularly in living systems. The cell theory has been adapted on many occasions since cells were discovered, to include new observations. However, there are some exceptions to the cell theory, such as striated muscle cells, giant algae and aseptate fungal hyphae.

Atypical examples

Striated muscle fibres, aseptate fungal hyphae and giant alga are three examples of cells/tissue with structures that question the integrity of the cell theory

Striated muscle fibres (fused muscle cells) are:

Longer than typical cells (up to 300 mm in length in comparison to a cardiac

muscle cell which has a length of 100 - 150 µm)

Have multiple nuclei surrounded by a single membrane (sarcolemma)

These features question the cell theory because striated muscle cells are formed from multiple cells which have fused together (which is how they have many nuclei rather than one) that work together as a single unit, challenging the concept that cells work independently of each other even in a multicellular organism

Aseptate fungal hyphae

Fungi have many long, narrow branches called hyphae
Hyphae have cell membranes, cell walls and some have septa
Aseptate fungal hyphae do not have septa, thus these cells are multinucleated with continuous cytoplasm
This questions the cell theory because the cells have no end walls making them appear as one cell

Giant Alga (e.g. Acetabularia)

Acetabularia can grow to heights of 100 mm, and yet consist of only one cell with

a single nucleus

Acetabularia have a relatively complex structure. They are divided into three parts:

rhizoid, stalk and cap

The features above question the cell theory because the trend for most unicellular

organisms is to be small in size and simple in structure.

Functions of Life

Unicellular (single-celled) and multicellular (many cells) organisms must carry out the following functions to stay alive:
- Metabolism - all the enzyme-catalysed reactions occurring in a cell, including cell respiration
- Reproduction - the production of offspring. It may be sexual or asexual
- Homeostasis - the ability to maintain and regulate internal conditions within tolerable limits, including temperature
- Growth - the permanent increase in size
- Response - (or sensitivity), the ability to respond to external or internal changes (stimuli) in their environment. Thus improving their chance of survival
- Excretion - the disposal of metabolic waste products, including carbon dioxide from respiration
- Nutrition - the acquisition of energy and nutrients for growth and development, either by, absorbing organic matter or by synthesising organic molecules (e.g. photosynthesis)

Functions of Life: Paramecium & Chlorella

Paramecium

Paramecium are unicellular protozoans commonly found in freshwater.
They range in size from 50 to 320 μm.

Chlorella

Chlorella is a small (2 to 10 μm) unicellular green alga. They are abundant in freshwater and can be found in a symbiotic relationship with Paramecium
As Chlorella are living they carry out all the functions of life, although due to different structures, there are some differences to Paramecium

Septate and aseptate fungal cells_edited

Fig 2 Striated muscle

Fig 3 Fungal hyphae

Fig 4 Acetabularia

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