Cell Structure

Cell Structure (1st Topic)

Learning Outcomes:
• Explain why most cells fall within the size range 1 to 100 um (micrometer)
•  Describe the general structure of a prokaryotic cell
• Describe the general structure of a eukaryotic cell
• Distinguish between prokaryotic and eukaryotic cells andgive examples of both.
• Draw a labelled diagram of the structure of a generalised animal cell and the same for a plant cell. Why?
• Describe the role of the eukaryotic cell organelles

What is a cell? The Cell Theory
• All organisms consist of one or more cells
• The cell is the basic unit of structure for all organisms
• All cells arise only from pre-existing cells

The Unit of Biological Organisation: The Cell
• All living organisms are composed of cells
• Most cells fall within the size range 1-100um (micrometre)



Factors limiting Cell size:
• Requirement for adequate surface area/volume ratio
• The rates at which molecules diffuse - to maximise efficiency
• The need to maintain adequate concentrations of reactants and catalysts




Prokaryotic vs. Eukaryotic Cells



All cells share four common components:
• A plasma membrane
• An outer covering that separates the cell’s interior from its surrounding environment
• Cytoplasm, consisting of a jelly-like cytosol within the cell in which there are other cellular
• components
• DNA, the cell's genetic material
• Ribosomes, which synthesize proteins
•
However, prokaryotes differ from eukaryotic cells in several ways





The main differences between prokaryotic and Eukaryotic cells:

• Prokaryotes have no membrane bound nucleus
• Prokaryotic cells are unicellular

• Eukaryotes have a membrane bound nucleus
• Eukaryotes are not unicellular, they are usually multicellular.
• Eukaryotic cells have a more complex structure
• Eukaryotes are bigger than prokaryotes

UNICELLULAR ORGANISMS:
Consist of a single cell that performs all cellular activities on it own

MULTICELLULAR ORGANISMS:
Are made up of multiple cells, with different cells performing specific jobs


The Prokaryotic Cell:

• Prokaryotic cells are unicellular (single celled)
• The cells are not compartmentalised (seperated into parts)
• Has no membrane-enclosed organelles
• Prokaryotes have no nucleus
• DNA is in a circular strand
• Prokaryotes have NO cytoskeleton
• Cells divide by binary fission

BINARY FISSION:
How a single cell divides to make two new cells. It is a type of asexual reproduction where a single cell grows bigger, then spilts to form 2 identical daughter cells, each with the same DNA.






Eukaryotic Cells:

• Eukaryotes have membrane enclosed organelles.
• Has a 'true' nucleus
• DNA in eukaryotes are in multipe chromosomes
• They have a complex cytoskeleton
• Eukaryotes are bigger than prokaryotes
• The cells in eukaryotes divide by mitosis

MITOSIS:
A cells way of making a copy of itself. The cell gets ready by making a copy of its DNA, which is the genetic material that tells the cell how to work. The copied DNA lines up in the middle of the cell. The cell then pulls apart the copied DNA so that each side has a complete set. Finally, the cell splits into two new cells, each with the same DNA as the original. This process helps organisms grow and repair themselves.
The two cells are genetically identical to the original parent cell.

MEIOSIS:
The cell divides in 4, making 4 new cells, with each cell containing only half of the genetic information (23). The cells are not genetically identical to the original parent cell.







Cellular Structure

Secretory Vesicle:
Vesicle fusing with the plasma membrane, releasing materials ti ve secreted from the cell.

Mitochondrion:
• Organelle in which energy is extracted from food during oxidative metabolism.
• Have inner and outer membranes
• Enzymes are embedded in the cristae (foldings) of the inner membrane
• Energy generation - metabolic pathways take place in the matrix of the mitochdria
• Food molecules are broken down to release energy
• Cardiac muscle cells contain a lot of mitochondria because they are cells that are constantly working and therefore require a huge amount of energy, and mitochondria produces energy.

Cytoplasm:
The cells entire region between the plasma membrane and the nuclear envelope. It is compromised of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals. Semifluid substance/matrix. Everything is suspended in the cytoplasm. Contains nucleus and other organelles.

Centriole:
Complex assembly of microtubules that occurs in pairs.

Cytoskeleton:
The collective name for the protein filaments that supports organelles and cell shape, and also plays a roll in cell motion. It is also involved with the movements of the cytoplasm.

• Microtubules/Actin filament: Long, hollow cylinders made up of many molecules of the protein tubulin. Tubulin consists of two subunits.  Tube of protein molecules present in cytoplasm, centrioles, cilia, and flagella.

• Intermediate filaments: Intertwined protein fibres that provide support and stregth. Made up of fibrous proteins organised into tough, ropelike assemblages that stabilise a cell's structure and help maintain its shape.

• Microfilaments: Twisted protein fibres that are responsible for cell movement. Made up of strands of the protein actin; often interacts with strands of other proteins.

Nucleus:
• The biggest membrane bound organelle
• Houses the cell's DNA and directs the sythesis of ribosomes and proteins
• Command centre of the cell/controls activity of the cell
• Inner and outer nuclear membrane = nuclear envelope
• Pores (opens like a door) in nuclear envelope for transport of water-soluble compounds, mRNA, chromosomal poteins, etc.
• Contains chromosomes with genes which control our characteristics (46 chromosomes in the nucleus of human cells)
• Poteins are synthesised by ribosomes
•
How does it's structure enable it's function?
To make ribosomal RNA and proteins. The place where nucleic acid is assembled.

CHROMATIN:
A complex of DNA and proteins found in eukaryotic cells. It's main functions are:
• To package long DNA molecules into more compact, denser structures
• To compress DNA into a compact unit that fits within the nucleus
• Form chromosomes during cell division




Nucleolus:
The site where ribosomes are produced

Nuclear envelope:
Double membrane between the nucleus and the cytoplasm

Nuclear pore:
Opening embedded with proteins that regulates passage into and out of the nucleus

Endoplasmic reticulum:
• Coming out of nuclear envelope
• A membrane system that forms a series of flattened sacs within the cytoplasm
• It's main function: An internal transfer system for the cells
• Synthesis of proteins in ribosomes
• Glycosylation of proteins in the ER interior
• Lots of folds and folded membranes

The rough endoplasmic reticulum: (internal membranes)
• It is continuous with the nuclear envelope
• Has a rough appearance because it has ribosomes attached to it's outer surface
• It lies immediately beside the nucleus, It's membrane is continuois with the outer membrane of the nucleus.
• It's function is the synthesis, folding, and transport of proteins,

Smooth endoplasmic reticulum:
• Has no ribosomes, therefore has a smooth outer surface.
• It is involved in the synthesis of lipids (which are used in the production of new cell membranes)

Golgi complex:
• Collects, packages, and distributes molecules manufactured in the cell.
• Packages, modifies and transports protein and lipids to different locations inside and outside of the cell.
• Vesicles breaking off
• Vesicles (membrane bound space)
• Vescile fuses with plasma membrane
• Secretory vesicle- secretes whatever it makes
• Extracellular - outside the cell
• Transport vesicle from rough ER - proteins are synthesised in ribosomes around the rough ER, brought to golgi for final process
• Stack of flattened vesicles
• Vesicles bud off the ER fuse with membranes of the Golgi Complex
• New vesicles formed and released
• Lots of golgi complex- cells that are making hormones and other chemical messengers for secretion - making something that needs to be secreted from cell - eg - beta cells that make insulin

Function of golgi complex:
• Processing of secretory proteins
• Synthesis of polysaccharides
• Processes, packages, aand distributes (sends) lipid and protein molecules

Lysosomes:  (only found in animal cells)
• Gets rid of worn out organelles, molecules that are no longer fit for purpose
• Circular in shape
• They contain enzymes to break down old cells and worn out cell parts
• Keeps out things that will destroy the cell
• They are highly acidic
• Hydrolytic enzymes (breaking down) breaks down proteins, lipids or carboydrates, depending what you're looking at
• Phagocytes - macrophages
• Phagocytes have lots of lysosomes because they are taking stuff in like a bacteria

Structure:
• Single membrane
• Contain hydrolyases (enzyme)  (ase indicates enzyme)
• Built in the Golgi complex
• Fuse with vacuoles, digesting their contents
• Acidic pH
Function:
• Receive and degrade macromolecules and worn out organelles
• Release the digestion products --> recycling
• Lysis - to burst


Peroxisomes (found in all eukaryotes)
• Found in eukaryotes
• Vesicle that contains enzymes that carry out particular reactions, such as detoxifying potentially harmful molecules
• Contain oxidative enzymes that break down H202 (hydrogen peroxide)
• Break down toxic substances within the cell
• Resembles lysosomes but smaller
• Hepatocytes (liver cells) - lots of lysosomes and peroxisomes found in liver cells and they detoxify alcohol and other harmful compunds
• Role in oxidative breakdown of long-chain fatty acids (FAs)

Antioxidants - reduce the amount of work that peroxisomes have to do in the cell

Vacuoles:
Mostly associated with plant cells
• Many plant cells have a large, single central vacuole that typically takes up most of the space in the cell
• It stores water and dissolves sugars, salts, vitamins and minerals, but functions vary according to the type of cell
• Temporary storage and transport
• Maintenance of turgor pressure