Cells:- the building blocks Animal cell Plant cell

The secret of life lies within the cell!

Outline of the cell theoryall living things are made of one or more cells the cell is the smallest functional unit of life all cells come from pre-existing cells through

cell divisionCell Theory is attributed to Schleiden and

Schwann (1838).the term 'cell' was first coined by Robert

Hooke after observing the structure of cork in 1655.

What is a theory?a theory is a general system of ideas used to

explain or interpret observations theories provide predictive power by generating

hypothesesa hypothesis is a specific prediction than can be

tested through observation or experiment each aspect of cell theory is based on evidence

obtained from observations and experiments

Evidences for the cell theory1. All living things are made of one or

more cells ☺microscopes allow us to observe that all

living things are either unicellular (made up of one cell) or multicellular (made up of several cells)

☻exceptions: a)skeletal muscle, some fungal hyphae, and

some algae have multinucleate cytoplasm, lacking normal cell separations

b)connective tissue, such as bone, blood and cartilage, is composed of both cellular and extracellular structures, especially extracellular proteins and fluids; however, these extracellular structures are products of cellular activities

2. The cell is the smallest unit of life☺nothing smaller than a cell can survive

independently ☺ sub-cellular structures (i.e. organelles)

such as nuclei, ER, Golgi apparatus, chloroplasts, mitochondria e.t.c. cannot survive independently

☺ the lower limit on cell size is about 200nm, large enough for DNA, ribosomes, and membranes

3. All cells come from pre-existing cells☺ this seems to imply that life has always

existed, which is incompatible with geological evidence about the age of Earth

☺ therefore, an exception is made for the origin of life, when cells must have arisen from non-living substances

☺ since the conditions of early Earth were anaerobic, they allowed for cells to form from non-living substances

☺ the conditions of present Earth are aerobic, precluding the formation of cells from non-living substances

Unicellular organisms

Amoeba Rhizobium bacteria

Functions of life carried out by unicellular organisms metabolism: chemical reactions inside the

cell, including cell respiration to release energy

sensitivity: perceiving and responding to changes in the environment

homeostasis: keeping conditions inside the organisms within tolerable limits

growth: an irreversible increase in size reproduction: producing offspring either

sexually or asexually nutrition: obtaining food, to provide energy

and the materials needed for growth

Relative sizes of structures in SI units1 Centimeter(cm) = 10-2 m

1 Millimeter(mm) = 10-3 m

1 Micrometer(μm) = 10-6 m

1 Nanometer(nm) = 10-9 m

Structure Relative size

SI unit

molecules 1 nm

cell membrane

10 nm

Viruses 100 nm

bacteria 1 μm

organelles 10 μm

cells 100 μm

Linear magnification of drawings☺Magnification shows how much larger or

smaller the image is compared to the real specimen.

☺ Magnification = measured length of the image /measured length of the specimen

☺ Magnification could be stated (for example, ×250)or indicated by means of a scale bar, for example:

1 μm

☺Is the actual leaf bigger or smaller than the image shown above? Explain your answer.

☺ If the image length = 4.2cm, calculate the actual size of the leaf.

☺Magnification of the image:

X 1:- image same size as the specimen

X<1:- image is smaller than the specimen

X > 1:- image is bigger than the specimen

Importance of the surface area to volume ratio as a factor limiting cell size☺surface area (SA) •SA = f(x2) i.e. it increases two fold as the size

of the cell increasesthe rate of exchange of materials and energy

(heat) is a function of its surface area.☺ Volume (Vol)Vol., = f(x3) i.e. it increases three folds as the

size of the cell increases rate of heat production/waste

production/resource consumption of a cell is a function of its volume

Cuboids Length (cm)

SA (cm2) Vol(cm3) SA:Vol ratio

A 1 6 1 6:1

B 2 24 8 3:1

C 3 54 27 2:1

D 4 96 64 3:2

E 5 150 125 6:5

Which of the two animals has a bigger SA: Vol ratio?African Elephant (Loxodonta africana) Pygmy Shrew

(Sorex minutus)

Biological Consequences:

As the size of an organism increases the surface area to volume ratio decreases.

For elephant heat exchange is quite slow which is a problem for elephants living in a warm region.

Elephant has evolved large ears to increase the surface area for heat exchange allowing them to remain cool

Small organisms have the opposite problem of a rapid rate of exchange with the environment.

Shrew looses body heat so fast that it consumes food at a furious rate simply to produce heat from respiration.

Emergent properties in multicellular organismsMulticellular organisms show emergent

properties i.e. occurrence of unexpected characteristics or properties in a complex system.

Emergent properties arise from the interaction of the component parts - the whole organism is greater than the sum of the parts.

Example: organisms that can regulate their body temperature using their hairs, sweating, changing blood flow’ altering rate of metabolism etc can exploit more habitats than ones that cannot.

Differentiation of cells in multicellular organisms

multicellular organisms can differentiate into a variety of interdependent cell types each specialized to carry out a subset of functions thereby achieving a greater efficiency through division of labor among a multicellular cooperative

cellular differentiation is achieved through differential gene expression

all cells in an organism have identical DNA = genome

different cell types make different proteins usually as a result of transcriptional regulation

each cell type expresses a closely regulated subset of its genome through “turning on” some genes and “turning off” others

Therapeutic use of stem cells stem cells are undifferentiated cells that

retain the capacity to divide and have the ability to differentiate along different pathways i.e. they are multipotent.

Stem cells can be obtained from: bone marrow, umbilical cord blood, inner cell mass of embryo (embryonic stem cells)

Stem cells can be used to treat: leukaemia, heart disease, diabetes, graft new skin for people with severe burns, stroke due to loss of neurons

Patients’ differentiated cells i.e. cells causing the condition are killed by chemotherapy and then replaced with stem cells through transplant

Revision Questions Q1.(a) State two characteristics of stem cells that can be used to distinguish them from other body cells. [2](b) Outline one therapeutic use of stem cells. [3](c) Describe the importance of stem cells in differentiation. [3]

Q2. (a) Outline the cell theory . [3] (b) Discuss the evidence for the cell theory . [5]

Q3.(a) Explain the importance of the surface area to volume ratio as a factor limiting cell size . [4]

(b)The scanning electron micrograph below shows the surface of the nuclear envelope with numerous nuclear pores.

(i) Calculate the power of magnification of the image. [1](ii) State the diameter of the pore labelled X. [1](c) Compare the relative sizes of viruses and bacteria to this cell. [2]

Q4. Explain how multicellular organisms develop specialized tissues. [4]