2.1. cell structure 2 Flashcards
Describe the structure of the nucleus
- Nuclear envelope contains nuclear pores
- Contains chromatin (the genetic material), consisting of DNA wrapped around histone proteins- this is spread out/extended when cell is not dividing. Coils/condenses into chromosomes when about to divide
Describe the function of the nucleus
- The nuclear envelope separates the contents of the nucleus from the rest of the cell
- In some regions the outer and inner nuclear membranes fused together- at these points, some dissolved substances and ribosome’s can pass through
- the pores enable larger substances such as messenger RNA [mRNA] 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
Describe the structure of the rough endoplasmic reticulum [RER]
- a system of membranes, containing fluid-filled cavities [cisternae] that are continuous with the nuclear membrane
- coated with ribosome’s
Describe the function of the rough endoplasmic reticulum [RER]
- the intracellular transport system: the cisternae form channels for transporting substances from one area of a cell to another
- provides a large surface area for ribosome’s, which assemble amino acids into proteins. These proteins then actively pass through the membrane into the cisternae and are transported to the golgi apparatus for modification and packaging
Describe the structure of the smooth endoplasmic reticulum [SER]
- a system of membranes, containing fluid-filled cavities [cisternae] that are continuous with the nuclear membrane
- there are no ribosome’s on its surface
Describe the function of the smooth endoplasmic reticulum [SER]
- contains enzymes that catalyse reactions involved with lipid metabolism such as:
- synthesis of cholesterol
- synthesis of lipid’s/ phospholipids needed by the cell
- synthesis of steroid hormones
- involved with absorption, synthesis and transport of lipid’s from the gut
Describe the structure of the Golgi apparatus
- consist of a stack of membrane-bound flattened sacks
- secretory vesicles bring materials to and from the golgi apparatus
Describe the function of the Golgi apparatus
- proteins are modified, for example by:
- adding sugar molecules to make glycoproteins
- adding lipid molecules to make lipoproteins
- being folded into their 3D shape
- then, 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 of the cell
Describe the structure of mitochondria (a mitochondrion)
- These may be spherical, rod-shaped or branched, and are 2-5 micrometers long
- They are surrounded by 2 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
Describe the function of mitochondria (a mitochondrion)
- Mitochondria are the site of ATP (energy currency) production during aerobic respiration
- They are self-replicating, so more can be made if the cells energy needs increase
- They are abundant in cells where much metabolic activity takes place, for example in liver cells and at synapses between neurones and where neurotransmitter is synthesised and released
Describe the structure of chloroplasts
- They are large organelles- 4-10 micrometres long
- They are found only in plant cells and some protoctists
- They are surrounded by a double membrane (envelope)
- The inner membrane is continuous with stacks of flattened membrane sacks called thylakoids which contain chlorophyll
- each sack or pile of thylakoids is called a granum (plural grana)
- The fluid-filled matrix is the stroma
- Chloroplasts contain loops of DNA and starch grains
Describe the function of chloroplasts
- Site of protein synthesis
- The first stage of photosynthesis, where 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
Describe the structure of the vacuole
- surrounded by a membrane called the tonoplast
- Contains fluid
Describe the function of the vacuole
- Only plant cells have a large, permanent vacuole
- It is filled with water and solutes and maintains cell stability, because when full it pushes against the cell wall, making it turgid
- if all the plat cells are turgid, this heps to support the plant, especially in non-woody plants
Describe the structure of lysosomes
- small bags, formed by the Golgi apparatus
- each is surrounded by a single membrane
- Contain powerful hydrolytic (digestive) enzymes
- Abundant in phagocytic cells such as neutrophils and macrophages (types of white blood cells) that can ingest and digest invading pathogens such as bacteria
Describe the function of lysosomes
- Lysosomes keep the powerful hydrolytic enzymes separate from the rest of the cell
- Lysosomes can engulf old cell organelles and foreign matter, digest them and return the digested components to the cell to reuse
Describe the structure of cilia and undulipodia
- Protrusions from the cell and are surrounded by the cell surface membrane
- Each contains microtubules
- Formed from centrioles
Describe the function of cilia and undulipodia
- The epithelial cells lining your airways each have many hundreds of cilia that beat and move the band of mucus
- Nearly all cell types in the body have one cilium that acts as an antenna- it contains receptors and allows the cells to detect signals about its immediate environment
- The only type of human cell to have an undulipodium (a long cilium) is a spermatozoon- it allows it to move
Which organelles don’t have membranes
- ribosomes
- centrioles
- cytoskeleton
- cellulose cell wall
Describe the structure of ribosomes
- Small, spherical organelles, about 20nm in diameter
- made of ribosomal RNA
- Made in the nucleolus, as 2 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
Describe the function of ribosomes
- ones bound to the exterior of RER are mainly for synthesising proteins that will be exported outside the cell
- ones free in the cytoplasm for proteins used inside cell
Describe the structure of centrioles
- Consist of 2 bundles of microtubules at right angles to each other
- the microtubules are made of tubulin protein subunits, and are arranged to form a cylinder
Describe the function of centrioles
- Before a cell divides, the spindle (made of threads of tubulin) forms the centrioles
- Chromosomes attach to the middle part of the spindle and motor proteins walk along 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 membrane
- Microtubules then sprout outwards from each centriole, forming a cilium or undulipodium
Usually absent from cells of higher plants but bay be present in some unicellular green algae, such as Chlamydomonas
Describe the structure of the cytoskeleton
A network of protein structures within the cytoplasm, consisting of:
- rod-like microfilaments made of subunits 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 soure
Describe the function of the cytoskeleton
The protein microfilaments within the cytoplasm give support and mechanical strength, keep the cells chape stable, and allow cell movement
Microtubules also provide shape and support to cells and help substances move through the cytoplasm within a cell
- they form the track along which motor proteins (dynein and kinesin) walk and drag organelles from one part of a cell to another
- they form the spindle before a cell divides- these spindle threads allow chromosomes to be loved up within the cell
- microtubules also make up the cilia, undulipodia and centrioles
Intermediate filaments 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
Describe the structure of the cellulose cell wall
- cell wall of plants is made out of plasma membrane- it is made from cellulose fibres
Describe the function of the cellulose cell wall
Absent from animal cells, the cell all is strong and can prevent plant cells from bursting when turgid (swollen)
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
