Cell Size is Limited• Surface to Volume Ratio limits upper size• Larger cells have less surface area relative to volume

Cell Size

Size ranges of cells: Cell Type Diameter Arechaea 0.1 - 1.0 μm Most bacteria 1.0 - 10.0 μm Most eukaryotic cells 10.0 - 100.0 μm

Cell Size

Tour of the Eukaryotic Cell

All organisms are made of cellsThe cell, as a microcosm, illustrates the following principles:

Cells as microcosms

1. Life arises from interactions among cellular components

2. Ordered cellular processes are based upon ordered structures

Examples of cell processes:protein synthesis, respiration, photosynthesis, cell-cell recognition, cellular movement, membrane production and secretion

3. Cells are excitable, responding to environmental stimuli, and exchange materials and energy with their environment.

4. Evolutionary adaptations are the basis for the correlation between structure and function.

Cell size is limited by metabolic requirements

• The lower limits are probably determined by the smallest size with enough:– DNA to program metabolism– cellular components to sustain life and reproduce

• The upper limits of size are imposed by the surface area to volume ratio. – As a cell increases in size, its volume grows

proportionately more than its surface area.

SA/Volume

The surface area of the plasma membrane must be large enough for the cell volume, in order to provide an adequate exchange surface for oxygen, nutrients and wastes.

Why cells have organelles

• Internal membranes compartmentalize the functions of a eukaryotic cell

• Most eukaryotic cells have 1000 X the volume of the average Eubacteria/Archaea cell, but only 100 X the surface area.

• i.e. – eukaryotic cells have a smaller surface area to volume ratio

How Eukaryotic cells compensate for a small SA/Volume ratio

• Eukaryotic cells compensate by having internal membranes which:– Give the cell compartments where

reactions occur without interference from other parts of the cell.

– Allows for specialized functions – cells become specialized – skin cells, nerve cells etc

Eukaryotic Organelles

• Nucleus• Mitochondria• Endoplasmic Reticulum (ER)• Vacuoles• Golgi Bodies (Apparatus)• Chloroplasts• Centrioles

The Nucleus

The nucleus is a large membrane-bound organelle, in all eukaryotic cell

It contains most of the genes that control the entire cell - the genetic library

It is enclosed by a nuclear envelope

Nuclear envelope• A double membrane• Two lipid bilayers separated ~20 to 40 nm.• Each bilayer has it’s own specific proteins.• Attached to the inner membrane is a network

of protein filaments, the nuclear lamina, which stabilizes nuclear shape.

• Is perforated by pores that regulate movement of molecules into and out of the nucleus

• The envelope's inner and outer membranes are fused at the lip of each pore.

DNA

• DNA is organized with proteins into a complex called chromatin.– Chromatin - Complex of DNA and proteins, which

makes up chromosomes• Chromosomes - Long thread-like structures

nucleotides that make up our genes

Nucleolus

• The most visible structure within the non-dividing nucleus

• Makes Ribosomes• Consists of of nucleolar organizers (= genes for

ribosomal RNA synthesis) (thus the nucleolus contains DNA)

• Packages of ribosomal subunits• Ribosomal subunits pass through nuclear pores to the

cytoplasm, where their assembly is completed.

Ribosomes

• A cytoplasmic organelle that make proteins• Are complexes of RNA and protein• Two types

1. free floating2. bound

Free Floating and Bound Ribosomes

• Are structurally identical and interchangeable.• Free floating ribosomes are suspended in the

cytosol– Make proteins for use inside the cell

• Bound ribosomes are attached to the outside of the endoplasmic reticulum.– Make proteins for use in membranes and export

out of cell

Ribosomes – are not membrane bound

Task 1

• Draw a model of a nucleus. Include the following details:

• Lipid bilayer – inner and outer layer and inter-membrane space

• Nuclear pores• Nuclear lamina• Chromatin• Nucleolus• Nucleoplasm

Task 2Complete the size comparison chart (handout)