Ocr A Level Biology. Organelles

Front 1

The structure of the nucleus, nuclear envelope and nucleolus.

Back 1

· The nuclear envelope separates the contents of the nucleus from the rest of the cell.

· In some regions the outer and inner nuclear membranes fuse together. At these points some dissolved substances and ribosomes can pass through.

· The pores enable larger substances, such as messenger RNA to leave the nucleus. Substances, such as some steroid hormones, may enter the nucleus, from the cytoplasm, via these pores.

· The nucleolus is where ribosomes are made

· Chromosomes contain the organism’s genes.

In summary, the nucleus

· Is the control centre of the cell

· Stores the organism’s genome

· Transmits genetic information

· Provides the instructions for protein synthesis

Front 2

Function of the nucleus, nuclear envelope and nucleolus.

Back 2

· The nuclear envelope separates the contents of the nucleus from the rest of the cell.

· In some regions the outer and inner nuclear membranes fuse together. At these points some dissolved substances and ribosomes can pass through.

· The pores enable larger substances, such as messenger RNA to leave the nucleus. Substances, such as some steroid hormones, may enter the nucleus, from the cytoplasm, via these pores.

· The nucleolus is where ribosomes are made

· Chromosomes contain the organism’s genes.

In summary, the nucleus

· Is the control centre of the cell

· Stores the organism’s genome

· Transmits genetic information

· Provides the instructions for protein synthesis

Front 3

Structure of the rough endoplasmic reticulum (RER)

Back 3

· This is a system of membranes, containing fluid filled cavities (cisternae) that are continuous with the nuclear membrane

· It is coated with ribosomes

Front 4

Function of the rough endoplasmic reticulum (RER)

Back 4

· RER is the intracellular transport system: the cisternae form channels for transporting substances from one area of a cell to another

· It provides a large surface area for ribosomes, with assemble amino acids into protein. These proteins then actively pass through the membrane into the cisternae and are transported to the Golgi apparatus for modification and packaging.

Front 5

Structure of the smooth endoplasmic reticulum (SER)

Back 5

· This is a system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane

· There are no ribosomes on its surfaces

Front 6

Function of the smooth endoplasmic reticulum (SER)

Back 6

· SER contains enzymes that catalyse reactions involved with lipid metabolism, such as:

Synthesis of cholesterol

synthesis of lipids/phospholipids needed by the cell

synthesis of steroid hormones

· It is involved with absorption, synthesis and transport of lipids (from the gut)

Front 7

Structure of the Golgi apparatus

Back 7

· This consists of a stack of membrane-bound flattened sacs.

· Secretory vesicles bring materials to and from the Golgi apparatus

Front 8

Function of the Golgi apparatus

Back 8

· Proteins are modified for example by:

Adding sugar molecules to make glycoproteins

Adding lipid molecules to make lipoproteins

Being folded into their 3D shape

· The proteins are packaged into vesicles that are pinched off and then

Stored in the cell or

Moved to the plasma membrane, either to be incorporated into the plasma membrane, or exported outside the cell

Front 9

Structure of the mitochondria (single mitochondrion)

Back 9

· These may be spherical, rod-shaped or branched, and are 2-5 micrometres long

· They are surrounded by two membranes with a fluid-filled space between them. The inner membrane is highly folded into cristae

· The inner part of the mitochondrion is a fluid-filled matrix

Front 10

Function of the mitochondria

Back 10

· Mitochondria are the site of ATP (energy currency) production during aerobic respiration.

· They are self-replicating, so more can be made if the cell’s energy needs increase.

· They are abundant in cells where much metabolic activity takes place, for example in liver cells and at synapses between neurones where neurotransmitter is synthesised and released

Front 11

Structure of the chloroplasts

Back 11

· These are large organelles, 4-10 micrometres long

· They are found only in plant cells and in some protoctists

· They are surrounded by a double membrane or envelope. The inner membrane is continuous with stacks of flattened membrane sacs called thylakoids (resembling piles of plates), which contain chlorophyll. Each stack or pile of thylakoids is called granum (plural: grana). The fluid filled matrix is called the stroma.

· Chloroplasts contain loops of DNA and starch grains

Front 12

Function of the chloroplasts

Back 12

· Chloroplasts are the site of photosynthesis.

· The first stage of photosynthesis, when light energy is trapped by chlorophyll and used to make ATP, occurs in the grana. Water is also split to supply hydrogen ions.

· The second stage, when hydrogen reduces carbon dioxide, using energy from ATP, to make carbohydrates, occurs in the stroma. Chloroplasts are abundant in leaf cells, particularly the palisade mesophyll layer.

Front 13

Structure of the vacuole

Back 13

· The vacuole is surrounded by a membrane called the tonoplast, and contains fluid.

Front 14

Function of the vacuole

Back 14

· Only plant cells have a large permanent vacuole.

· It is filled with water and solutes and maintains the cell stability, because when full it pushes against the cell wall, making the cell turgid.

· If all the plant cells are turgid then this helps to support the plant, especially in non-woody plants.

Front 15

Structure of the Lysosomes

Back 15

· These are small bags, formed from the Golgi apparatus. Each is surrounded by a single membrane.

· They contain powerful hydrolytic (digestive) enzymes.

· They are abundant in phagocytic cells such as neutrophils and macrophages (types of which blood cell) that can ingest and digest invading pathogens such as bacteria

Front 16

Function of the Lysosomes

Back 16

· Lysosomes keep the powerful hydrolytic enzymes separate from the rest of the cell

· Lysosomes can engulf old cell organelles and foreign matter, digest the, and return the digested components to the cell for reuse.

Front 17

Structure of cilia and undulipodia

Back 17

· These are protrusions from the cell and are surrounded by the cell surface membrane.

· Each contain microtubules

· They are formed from centrioles

Front 18

Function of the cilia and undulipodia

Back 18

· The epithelial cells lining your airways each have many hundreds of cilia that beat and move the band of mucus.

· Nearly all the cell types in the body have one cilium that acts as an antenna. It contains receptors and allows the cell to detect signals about its immediate environment.

· The only type of human cell to have an undulipodium (a longer type of cilium) is a spermatozoon. The undulipodium enables the spermatozoon to move.

Front 19

Structure of the ribosomes

Back 19

· Small spherical organelles, about 20nm in diameter.

· Made of ribosomal RNA

· Made in the nucleolus, as two separate subunits, which pass through the nuclear envelope into the cell cytoplasm and then combine.

· Some remain free in the cytoplasm and some attach to the endoplasmic reticulum.

Front 20

Function of the ribosomes

Back 20

· Ribosomes bound to the exterior RER are mainly for synthesising proteins that will be exported outside the cell.

· Ribosomes that are free in the cytoplasm, either singly or in clusters, are primarily the site of assembly of proteins that will be used inside the cell.

Front 21

Structure of the centrioles

Back 21

· The centrioles consist of two bundles of microtubules at right angles to each other. The microtubules are made of tubulin protein subunits, and are arranged to form a cylinder.

Front 22

Function of the centrioles

Back 22

· Before a cell divides, the spindle, made of threads of tubulin, forms from the centrioles.

· Chromosomes attach to the middle part of the spindle and motor proteins walk along the tubulin threads, pulling the chromosomes to opposite ends of the cell.

Centrioles are involved in the formation of cilia and undulipodia:

· Before the cilia form, the centrioles multiply and line up beneath the cell surface membrane

· Microtubules the sprout outwards from each centriole, forming a cilium or undulipodium.

· Centrioles are usually absent from cells of (higher) plants but may be present in some unicellular green algae such as Chlamydomonas.

Front 23

Structure of the cytoskeleton

Back 23

A network of protein structures within the cytoplasm.

It consists of:

· Rod-like microfilaments made up of the protein actin; they are polymers of actin and each microfilament is about 7nm in diameter.

· Intermediate filaments about 10 nm in diameter

· Straight, cylindrical microtubules, made of protein subunits called tubulin; about 18-30nm in diameter.

· The cytoskeletal motor proteins, myosins, kinesins and dyneins, are molecular motors. They are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy source.

Front 24

Function of the cytoskeleton

Back 24

The protein microfilaments within the cytoplasm give support and mechanical strength, keep the cell’s shape stable and allow cell movement.

Microtubules also provide shape and support to cells, and help substances and organelles to move through the cytoplasm within the cell.

· They form the track along which motor proteins (dynein and kinesin) walk and drag organelles from one part of the cell to another.

· They form the spindle before a cell divides. These spindle threads enable chromosomes to be moved within the cell.

· Microtubules also make up the cilia, undulipodia and centrioles.

Intermediate proteins are made of a variety of proteins. They:

· Anchor the nucleus within the cytoplasm

· Extend between cells in some tissues, between special junctions, enabling cell-cell signalling and allowing cells to adhere to a basement membrane, therefore stabilising tissues.

Front 25

Structure of the cellulose cell wall

Back 25

The cell wall of plants is on the outside of the plasma membrane. It is made from bundles of cellulose fibres.

Front 26

Function of the cellulose cell wall

Back 26

Absent from animal cells, the cell wall is strong and can prevent plant cells from bursting when turgid.

The cell walls of plant cells:

· Provide strength and support

· Maintain the cell’s shape

· Contribute to the strength and support of the whole plant

· Are permeable and allow solutions (solute and solvent) to pass through

Fungi have cell walls that contain chitin, not cellulose