Biology - A2.2 Cell Structure

Cells definition

Cells are the fundamental units of life and the smallest unit of self-sustaining life

Cell theory, three points to summarise

• Cells are the basic structural unit of life

• Living organisms consist of cells

• Cells come from preexisting cells

Unicellular vs. Multicellular

• Organsims that are unicellular only have one cell

• Larger organisms are usually multicellular and have differentiated and specialised cells

Two categories of living organisms

Prokaryotes and eukaryotes. (viruses aren’t living)

Structures common to cells in all living organisms

plasma membrane enclosing the cell composed of lipids, cytoplasm composed mainly of water for metabolism, DNA for genetic material (and ribosomes)

Prokaryote 3 basic structures

Rods (bacili), spheres (cocci), spirals (spirilli)

Prokaryote cell components

• Cytoplasm,

• nucleoid (lighter part of cytoplasm where single naked polymer/molecule of DNA/chromosome is located),

• 70S ribosomes,

• plasma membrane,

• cell wall made from peptidoglycans (sugars and proteins)

• pili are exterior extensions/appendages made from proteins that can transfer genetic material and attach the bacteria to another surface

• flagella are appendages for movement

• plasmids are small circle DNA that is often transfered

• meosome are the powerhouse of the prokaryotes, produce energy/ATP

• capsule is an outer layer that protects the bacteria from different environments and resists it from bursting

Cytoplasm

Contains water and many substances dissolved or suspended in it. There are also enzymes bc the cytoplasm is a key role in metabolism.

Plasma membrane

• Membrane on the outside that encloses the contents of the cell and controls the entry and exit of substances.

• The plasma membrane ensures favorable inner environment that might be different from exterior (e.g. solute concentrations and pressure).

• Made from phospholipids

• Fragile and can burst if interior pressure is too great, lysis

• Some animal cells have projections called microvilli

DNA

DNA is the genetic material for all cells. Can be located either inside the nucleus or in the cytoplasm (nucleoid)

Prokaryote nucleoid

lighter part of cytoplasm where single naked polymer/molecule of DNA/chromosome is located

Pili from prokaryotes

Exterior extensions/appendages made from proteins that can transfer genetic material and attach the bacteria to another surface

Prokaryote flagella

appendages for movement

Prokaryote plasmid

small circle DNA that is often transfered

Prokaryote mesosome

Folded invaginations of the plasma membrane are the powerhouse of the cell. Produce energy/ATP

Prokaryote capsule

Outer layer that protects the prokaryote from exterior environments and form bursting

Compartmentalization of cells

Eukaryotic cells are compartmentalized so some areas are seperated from the rest of the cell/cytoplasm with one or two membranes, organelles such as nucleus are an example

Nucleus

In eukaryotic cells the DNA is associated with histone protein and found in the nucleus.

There is a double membrane with nuclear pores that allows communication with the rest of the cell

There is a nucleolus (dense structure involved in ribosome synthesis)

Common structures for all eukaryotic cells

Nucleus, 80S ribosomes, mitochondria, cytoplasm, plasma membrane, endoplasmic reticulum and Golgi apparatus

Ribosomes

Small structures in all cells that are either free, or associated with the endoplasmic reticulum and synthesise proteins

Vacuoles

In eukaryotic cells (smaller in animal cells), they are structures that contain nutrients

Eukaryotic centrioles

Associated with nuclear division, they are composed of microtubes that can divide the nucleus in cell division

Components of a generalised animal cell

• Ribosomes (80S)

• Vacuoles

• Cytoplasm

• Nucleus

• Mitochondria

• Plasma membrane

• Golgi apparatus

• Endoplasmic reticulum (smooth or rough)

• Centrioles

• Lysosomes

Eukaryotic lysosomes

Sacs bound by a single membrane that contain enzymes and transport them

• Not usually present in plant cells

Golgi apparatus

Structures that can store, modify and package proteins (included in the last stages of making a protein)

Endoplasmic reticulum

structure within the cytoplasm

• Rough ER has ribosomes which are for protein synthesis

• Smooth ER (without ribosomes) is part of lipid synthesis

Components of a generalised plant cell

• Chloroplasts

• Cytoplasm

• Golgi apparatus

• Mitochondria

• Plasma membrane

• Endoplasmic reticulum

• Cell wall

• Large central vacuole

• Starch granules - amyloplasts

• Nucleus

Amyloplasts/Starch granules

Carbohydrates stored in amyloplasts which store and synthesise carbs

Cell wall

A semi-rigid structure composed of mainly cellulose, provides structural support

Chloroplasts

Specialised plastids containing the green pigment chlorophyll. Function: photosynthesis

• Have a double membrane and thylakoids

Eukaryotic cytoskeleton

Microtubules and microfilaments that guide the movement of organelles around the cell

Differences in eukaryotic cells (between animal, plant and fungi)

composition of cell walls, the size and function of vacuoles, presence of chloroplasts/plastids, general size, presence of centrioles, cilia and flagella

Differences in eukaryotic cells (between animal, plant and fungi) in terms of cell wall

Aren’t present for animals, made out of cellulose for plant cells and made of chitin for fungi

Size and function of vacuoles differing for eukaryotic cells

Small and temporary for animal cells, large and permanent for plant and fungi cells.

Function is to store substances and pressurise the cells for both plant cells and fungi cells.

Presence of plastids/chloroplasts differs for animal, plant and fungi eukaryotic cells

In both fungi and animal they aren’t present whilst plant cells have both chloroplasts and amylosplasts

Presence of centrioles differs for animal, plant and fungi eukaryotic cells

Absent for plant and fungi cells

Used in animal cells for cell reproduction

Compare and contrast size of different eukaryotic cells

Animal cells: 10-30 micrometres

Plant cells: 40-100 micrometres

Fungi cells: 2-10 micrometres

Presence of cilia/flagella (undulipodia) for animal, plant and fungi cells

Can be present for animal cells (e.g. sperm cells)

Usually absent for plant and fungi cells

Name four examples of atypical eukaryotic cells

Red blood cells, phloem sieve tube elements, skeletal muscles, aseptate fungal hyphae

Why are red blood cells atypical eukaryotic cells?

They have no nucleus so that they can carry more haemoglobin but they’re lifespan is shorter because they cannot repair themselves

Why are phloem sieve tube elements atypical eukaryotic cells?

These cells only have plasma membrane and a smaller amount of cytoplasm so that the liquid can flow easily.

An adjacent cell (a companion cell) is associated with the sieve tubes so that they remain alive.

Why are skeletal muscle cells atypical eukaryotic cells?

They are multinucleate and very long because groups of cells fuse together to form the skeletal muscle cells.

Why are aseptate fungal hyphae atypical eukaryotic cells?

They are multinucleate (coenocytes), as when the cells grow and the nucleus divides, there is no septa created (hence aseptate)

Tips for drawing from electron micrographs

• Label

• Use straight lines for labels

• Draw with a well-sharpened pencil

• No sketching

• The cell wall is the inside layer between the two lines whilst the plasma membrane is the inner-most line (for plant cells and fungi cells)

Mitochondria

Organelles that carry out respiration to produce ATP

• Have a double membrane and the inner membrane goes in towards the center

Formula to calculate magnification

Size of image (what it measures with a ruler, usually in mm) divided by actual size of structure (usually given)

• remember, both should be in the same units (micrometres or millimetres)

Definition magnification

How much an object is enlarged from its true size

written: x 400 e.g.

Stage micrometer + Eyepiece graticule

A special microscope slide that has divisions on it that are usually 100 micrometres apart.

An eyepiece graticule is a graticule with a ruler

You line them up and find out how many units of the eyepiece are equivalent to 100 micrometres.

Then divide the 100 by the units and you get what 1 eyepiece granule unit is equivalent to.

What are the 8 essential life processes that all (most) living organisms carry out?

• Nutrition (supplying nutrients for energy, growth and repair)

• Metabolism (all biochemical reactions including catabolic and anabolic)

• Growth (increase the number of cells)

• Response to stimuli (appropriate action after perceiving stimuli)

• Excretion (removal of waste products)

• Homeostasis (maintaining a constant internal environment)

• Reproduction (producing offspring)

• Movement (ability to change position)

Unicellular organisms

The cell has to carry out all life processes. E.g. Chlamydomonas and Paramecium

(In multicellular organisms, life processes can be done by different cells)

Chlamydomonas and all life processes in one cell

• Excretion through freely permeable cell wall

• Metabolic reactions occur in cytoplasm

• Nutrition through storing starch in chloroplasts and food vacuoles

• Reproduction and growth as the nucleus can divide to produce genetically identical nuclei

• Movement with two flagella

• Response to stimuli as the light sensitive ‘eyespot’ can sense brightest light and swim towards it

• Homeostasis as the water level is kept constant with the contractile vacuoles

Paramecium and unicellular life processes

• Movement with whip-like cillia

• Response to stimulia as the cilia can move the cell in direction of changes in environment

• Nutrition through food vacuoles

• Metabolic reactions take place in the cytoplasm

• Homeostasis by regulating water contents with contractile vacuoles

• Excretion happens by waste products diffusing out through the plasma membrane

• Reproduction and growth as the nucleus can divide

Electron microscope

Instead of using light, they have a beam of electrons. Can be transmission (goes through the specimen) or scanning (bounces of specimen)

+higher resolution

-no living tissue

-only black and white images

Freeze fracture electron microscopy

1) Sample is frozen

2) Sample is fractured at weakest point

3) Some ice is removed through etching

4) Vapour of carbon fired to obtain a replica (the replica is then observed with an electron microscope)

+can observe surfaces inside cells

Cryogenic electron microscopy (cryo-EM)

Once a sample is flash frozen the electron beams can go through computer software to get a 3D image of the structure of proteins

+3D image

+Less intense electron beams than traditional electron microscopy

Fluorescent stains

Stains used that under UV or violet/blue light re-emit light

• the chemicals bind to some to highlight those areas

Immunofluorescent stains

The stains attach to some antibodies to attach to proteins and antigens

If the right proteins are targeted, the amount of infected cells can be visually monitored through immunofluorescent staining.

Endosymbiotic theory

Evidence suggests all eukaryotic cells originated from a common unicellular ancestor (prokaryotic cell)

1) 2 billion years ago a large prokaryote with a nucleus and that was sexually reproducing engulfed (through endocytosis) a smaller prokaryote that did aerobic respiration

2) They had a symbiotic (mutually beneficial) relationship and after evolving the smaller prokaryote became mitochondria.

3) The cell could then engulf a prokaryotic cell that could photosynthesise which could evolve into a chloroplast.

Evidence for endosymbiotic theory

Both chloroplasts and mitochondria have:

• double membranes (one from the outside of the larger prokaryote and one from the smaller one itself)

• Their own naked and circular DNA (hence prokaryote bc of plasmid)

• 70S ribosomes (as prokaryotes do)

• Similar size to prokaryotes

• Independent division

Multicellular organisms main advantage

Cells can be specialised for specific and unique functions

• Therefore, the organisms become larger, more complex, and have greater diversity.

Cell differentiation definition and process

The process for developing specialized tissues in multicellular organisms

• Done by switching genes on or off according to the function of that cell

• Gene expression changes if there are changes in the environment

Evolution of multicellularity

All plants and animals are multicellular and so are many fungi and eukaryotic algae

Multicellularity evolves repeatedly