Cell Structure

Cell

The basic unit of all living organisms; it is surrounded by a cell surface membrane and contains genetic material (DNA) and cytoplasm with organelles suspended within it

Organelle

A functionally and structurally distinct part of a cell (eg. a ribosome or mitochondrion)

Nucleus

A relatively large organelle in eukaryotic cells but not prokaryotes

Contains DNA arranged into chromosomes and is surrounded by a double membrane called the nuclear envelope which contains many pores

5-20 micrometers

Cell surface membrane

A very thin, partially permeable membrane formed a phospholipid bilayer and spans a diameter of around 10nm surrounding all cells;

Controls the exchange of materials between the cell's cytoplasm and its external environment Separates cell contents from the surrounding environment

Enables cells to receive hormone messages as part of cell signalling

Enables cells adhesion

Chromatin

The material of which chromosomes are made, consisting of DNA, proteins (such as histones) and some RNA

Chromosome

Structure made of tightly coiled chromatin found in the nuclei of eukaryotic cells, visible during cell division

Nucleolus

A small structure found inside the nucleus (some nuclei have one nucleolus, some have several), visible as a densely stained body

Function is to make ribosomes using its own DNA

Contains a core of DNA from one or more chromosomes which contain genes that code for rRNA

Protoplasm

All of the living material inside a cell (nucleus and cytoplasm combined)

Cytoplasm

The contents of a cell bound by the cell surface membrane, excluding the material inside the nucleus

Mitochondria

The organelle in eukaryotes in which aerobic respiration takes place, and energy is released from energy- rich molecules - most of which is transferred to ATP

1 micrometer in size

Double membrane structure: outer membrane responsible for movement of substances in and out of cell, inner membrane folds to form Cristae increasing SA for energy production

Mitochondrial matrix houses enzymes, mitochondrial DNA and ribosomes for breaking down food

Circular DNA codes for proteins vital for mitochondrial function

Reactions occur in solution in matrix and cristae

Once made, ATP leaves mitochondrion and spreads rapidly to all parts of the cell requiring energy

Energy released: ATP —> ADP + Pi

Mitochondrial matrix

Contains enzymes, mitochondrial RNA, and ribsomes needed for aerobic respiration

Ribosomes

Found in cytoplasm of all cells in the cytoplasm or in the RER of eukaryotic cells

Each one is a complex of ribosomal RNA and proteins (1:1 ratio)

80S ribosomes are found in eukaryotic cells

70S ribosomes are found in prokaryotes, mitochondria and chloroplasts

Site of translation during protein synthesis, allows all interacting molecules involved in this process to gather in one place

Cell Wall

A rigid structure that surrounds the cell membrane in prokaryotes, plant cells, and fungal cells

Provides structural support to the cell and prevents it from bursting due to osmosis if the cell is surrounded by a hypotonic solution

Made from polysaccharide cellulose in plants and chitin in fungi

relatively rigid - parallel fibres of polysaccharide cellulose running through a matrix of other polysaccharides make it inelastic and increase its tensile strength

Plasmodesmata

A pore through the cell walls of neighbouring plant cells that allows the cytoplasm to be continuous to enable the controlled passage of molecules such as sucrose, amino acids, water, and plant hormones between each cell

Pores contain ER and are lined with cell surface membrane

Large Permanent Vacuole

An organelle in eukaryotic plant cells that stores materials such as cell sap, water, salts, proteins, pigments and sugars (such as sucrose)

May also contain hydrolysis and act as lysosomes

It also provides additionally structural support to cells as water enters it via osmosis inflating it and causing a build-up of pressure

Surrounded by tonoplast - membrane that controls exchange between vacuole and cytoplasm

Up to 40 micrometers in diameter

They are sometimes found in animal cells, but these will be small and temporary

Tonoplast

The partially permeable membrane enclosing the central vacuole in plant cells

Chloroplast

The double-membrane bound organelle in eukaryotes where photosynthesis takes place

Membrane-bound compartments called thykaloids stack together to form structures called grana

Thykaloids are the site of light dependent reactions during photosynthesis

Grana are joined together by lamellae

Chlorophyll and other photosynthetic pigments are found in the membranes of the thylakoids

Chloroplasts contains circular pieces of DNA and 70S ribosomes used to synthesize proteins needed in chloroplast replication and photosynthesis

Photosynthesis

The production of organic molecules from inorganic ones using energy from light

Magnification

The number of times larger an image of an object is than the real size of the object (M = I/A)

Eyepiece graticule

Small scale placed in a microscope eyepiece

Stage micrometer

A very small, accurately drawn scale of known dimensions, engraved on a microscope slide used to calibrate the eyepiece graticule

Micrograph

A picture taken through a microscope, a photomicrograph is taken through a light microscope and an electron micrograph is taken through an electron microscope

Microvilli

Small, finger-like protrusions in the cell's surface membrane which increases the surface area for more efficient absorption or secretion

Found in lining of small intestine and the kidney tubules

Nuclear envelope

The two membranes surrounding the nucleus perforated with pores

Controls entrance and exit of substances between the cytoplasm and the nucleus

Connected to the Endoplasmic Reticulum

Nuclear pores

Channels in the nuclear envelope which allow mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes and signalling molecules to move in from the cytoplasm

Endoplasmic reticulum (ER)

Series of membranes that form flattened sacs running through the cytoplasm of eukaryotic cells; molecules (particularly proteins) are transported through the cell inside these sacs

6-8nm thick

Continuous with nuclear envelope

Rough Endoplasmic Reticulum (RER)

Continuous folds of membrane that are linked with the nuclear envelope

Surface of the RER is covered in ribosomes which are the sites of protein synthesis

Role is to process secretory and membrane proteins that are produced on the ribosomes

Polypeptide chains are threaded into cisternal space of ER to fold into their native conformation

Packages proteins into vesicles to be transported by the Golgi body

Smooth Endoplasmic Reticulum (SER)

Does not have ribosomes on the surface

Involved in the production of lipids and of steroid hormones such as oestrogen and testosterone

Major storage site for calcium ions - therefore abundant in muscles

Involved in drug metabolism in the liver

Golgi body

An organelle found in eukaryotic cells consisting of a stack of flattened sacs called cisternae that constantly form at one end and break up into vesicles at the other

Chemically modifies molecules from the RER (eg. adding sugars to proteins to make glycoproteins) using enzymes and packages them into vesicles to be transported to other parts of the cell

Vesicles

Small membrane-bound sacs used by cells for transport and storage

Can be pinched off at the ends of the Golgi body (Golgi vesicles)

GVs carry their contents to other parts of the cell, often to the cell surface membrane for secretion or to be embedded in the cell surface membrane

Vesicles can fuse with the cell surface membrane to allow exocystosis or bud from the membrane during endocytosis

Lysosome

A spherical specialized vesicle found in eukaryotic cells containing hydrolytic enzymes to break down a variety of substances:

Eg. getting rid of unwanted cell components, endocytosis, exocytosis, self-digestion

Single membrane, usually 0.1-1.5 micrometers in diameter

Optimum pH conditions are pH 4-5

Enzymes synthesized on RER then delivered to lysosomes via Golgi Apparatus

Cristae

Folds of the inner mitochondrial membrane to increase its surface area so there's more space for proteins (such at electron carrier proteins and ATP synthase) to embed

ATP

The nucleotide molecule that is the universal energy currency - used to transfer energy in all energy-requiring processes in cells Energy is released from ATP when its is broken down to ADP and inorganic phosphate Process is reversed during respiration to make ATP and maintain a supply of energy

Microtubules

Tiny, long, rigid, hollow tubes made of tubulin protein and found in most eukaryotic cells, microtubules make up cytoskeleton of the cell together w/ actin and intermediate filaments

25nm in diameter, 15nm lumen, up to 25micrometers in length

Alpha + beta tubulin molecules combine to form dimers which are then joined end-to-end to form protofilaments

13 protofilaments line up to form a hollow cylinder = 1 microtubule

Secretory vesicles and other organelles can be moved alongside the outside surface of the microtubules

During nuclear divisions, spindles made of microtubules are used to separate chromosomes

Cytoskeleton

Used to provide support and movement of the cell

Centriole

250nm diameter

Small cylindrical structures arranged perpendicular to each other

made from 9 triplets of microtubules found in animal cells ONLY and at the base of cilia and flagella

Involved in cell division - interphase and mitosis

Centrosome

The main microtubule organising centre in animal cells, made of two centrioles at right angles to each other, which organises the spindle fibres which allow during cell division

Cilia

Hair-like projections made from microtubules found on the surface of many animal cells and prokaryotic cells; they beat causing locomotion and/or movement of substances across the cell's surface.

10 micrometers long, 2 micrometers wide

Flagella

Longer than cilia, but otherwise the same, these are whip-like structures projecting from the surface of many animal cells and unicellular organisms; they beat causing locomotion and/or movement of substances across the cell's surface

Bacteria

A group of single-celled prokaryotes with a peptidoglycan cell wall, circular DNA, and the ability to make spores

Generally 1-5 micrometers diameter

Peptidoglycan

A protein-carbohydrate compound that makes the cell walls of bacteria rigid (sometimes also referred to as murein)

Plasmid

A small circular loop of DNA in a bacterium that is additional to its main circular chromosome, often contain antibiotic resistance genes

Virus

Non-cellular particles that infect living cells

Much smaller than prokaryotic cells, with a diameter of 20-300nm

Have a nucleic acid core made of either DNA or RNA and a protein coat called a capsid

Some have an outer envelope formed from the membrane phospholipids of the host cell in which they were produced

Can only reproduce by infecting living cells + using protein-building machinery to produce new virus particles

They use attachment proteins on their surface to bind to and infect their host cells

Phospholipid

A glycerol molecule bonded to two fatty acid tails and a phosphate group

Structure of a prokaryote

Eveolved 3.5 billion yrs ago

Typical diameter 1-5 micrometers

Circular DNA not surrounded by a double membrane (nucleoid)

70S ribosomes

No membrane-bound organelles: only contain ribosomes, cell membrane, cytoplasm, cell wall, flagella and pili

Cell wall made of peptidoglycan in bacteria

Flagella are simple and lack microtubules, project outside cell surface membrane (extracellular)

Cell division occurs by binary fission, does not involve a spindle

Structure of a eukaryote

Thought to have evolved 1.5 bn yrs ago

Up 40 micrometers in diameter and 1000 x volume of prokaryotic cells

Linear DNA contained in a nucleus surrounded by double membrane (nuclear envelope)

80S ribosomes

Contains organelles:

• surrounded by single membrane - lysosomes, Golgi apparatus, vacuoles, ER

• Double membrane - nucleus, mitochondria, chloroplasts

• No membrane - ribosomes, centrioles, microtubules

Cell wall present in plants and fungi - contains cellulose or lignin in plants and chitin in fungi

Flagella (and cilia) are complex with ‘9 + 2’ arrangement of microtubules, surrounded by cell surface membrane (intercellular)

Cell division takes place by mitosis or meiosis and involves a spindle

None carry out nitrogen fixation