bio cellstructure L1 eukaryiotic cells.

Cell theory - definition

The theory that all living organisms are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells

Nucleus - structure

Surrounded by a nuclear envelope (double membrane) containing nuclear pores; contains chromatin (protein-bound, linear DNA) and one or more nucleoli

Nucleus - function

Contains the genetic material (DNA) that controls the cell's activities via protein synthesis; site of DNA replication and transcription (mRNA production)

Chromatin

Protein-bound, linear DNA found within the nucleus; condenses to form chromosomes during cell division

Nuclear pores - function

Allow the passage of large molecules (e.g. mRNA and ribosomal subunits) between the nucleoplasm and the cytoplasm

Nucleolus - function

Produces ribosomal RNA (rRNA) and assembles ribosomes

Nucleoplasm

The granular, jelly-like material that fills the nucleus

Why the nucleus is described as containing "protein-bound, linear DNA"*

The DNA in eukaryotic cells is associated with histone proteins and exists as long, linear chromosomes, unlike the short circular DNA of prokaryotes which is not protein-bound

Rough endoplasmic reticulum (RER) - structure

A system of membrane-bound, flattened sacs with ribosomes attached to its surface

Rough endoplasmic reticulum (RER) - function

Folds and processes proteins that have been synthesised by the attached ribosomes, and transports them (e.g. to the Golgi apparatus)

Smooth endoplasmic reticulum (SER) - structure

A system of membrane-bound, flattened sacs with no ribosomes attached

Smooth endoplasmic reticulum (SER) - function

Synthesises, processes and stores lipids and carbohydrates

Why a cell that secretes large amounts of protein (e.g. an enzyme-secreting gland cell) would contain a lot of RER*

RER is the site where ribosome-synthesised proteins are folded, processed and transported onward, so cells with high rates of protein secretion need more RER to process this greater output

Ribosome - function

The site of protein synthesis (translation)

Golgi apparatus - structure

A stack of membrane-bound, flattened sacs

Golgi apparatus - function

Modifies proteins and lipids (e.g. by adding carbohydrate chains) and packages them into vesicles, often for secretion

Golgi vesicle - function

Membrane-bound sacs, pinched off from the Golgi apparatus, that transport and/or store substances (e.g. modified proteins) within the cell, or fuse with the cell membrane to release their contents outside the cell

Vesicle - general definition

A small, membrane-bound sac used to transport or store substances within a cell

Lysosome - structure

A membrane-bound organelle (a type of vesicle)

Lysosome - function

Contains and releases hydrolytic (digestive) enzymes, used to break down/digest worn-out organelles, debris, or pathogens engulfed by the cell

Mitochondrion - structure

Surrounded by a double membrane; the inner membrane is folded to form cristae, which enclose the matrix

Mitochondrion - function

The site of the later stages of aerobic respiration, producing ATP; the matrix contains the enzymes involved in respiration

Why the inner membrane of a mitochondrion is folded into cristae*

Folding increases the surface area available for the attachment of enzymes and proteins involved in aerobic respiration, increasing the rate of ATP production

Centriole - structure

Small hollow cylinders made of microtubules, found in animal cells (and only some other cells)

Cell-surface (plasma) membrane - structure

A phospholipid bilayer with embedded proteins

Cell-surface (plasma) membrane - function

Controls the movement of substances into and out of the cell; some embedded proteins are involved in cell signalling or act as antigens for immune recognition

Cell wall (plants and algae) - composition

Cellulose

Cell wall (fungi) - composition

Chitin

Why plant, algal and fungal cell walls are needed even though animal cells do not have one*

Cell walls provide structural support and prevent the cell from bursting due to osmotic water uptake, which is necessary because these organisms don't have another way of resisting the internal (turgor) pressure generated

Explain why a muscle cell would need a large number of mitochondria (2 marks)

Muscle cells have a high demand for ATP to power muscle contraction [1]; mitochondria are the site of the later stages of aerobic respiration, which produces most of the cell's ATP [1]

Describe the structure and function of the nucleus (4 marks)

Surrounded by a nuclear envelope containing pores [1]; contains chromatin, which is protein-bound, linear DNA [1]; contains one or more nucleoli, which produce ribosomes [1]; the nucleus controls the cell's activities by controlling protein synthesis (via transcription of DNA into mRNA) [1]

Explain the role of nuclear pores in protein synthesis (2 marks)

They allow mRNA to pass out of the nucleus and into the cytoplasm [1]; where it can then be translated at ribosomes to synthesise proteins [1]

A cell contains an unusually large nucleolus. Suggest what this indicates about the cell's activity (2 marks)

The nucleolus produces ribosomes/rRNA [1]; a larger nucleolus suggests the cell is producing more ribosomes, likely because it has a high rate of protein synthesis [1]

Describe the roles of the rough and smooth endoplasmic reticulum in a cell (4 marks)

Rough ER has ribosomes attached to its surface [1]; it folds/processes proteins synthesised by these ribosomes and transports them [1]; smooth ER has no ribosomes attached [1]; it synthesises, processes and stores lipids and carbohydrates [1]

A student views an organelle under an electron microscope and sees a flattened membrane sac studded with small dots. Identify the organelle and explain your reasoning (2 marks)

Rough endoplasmic reticulum [1]; the small dots are ribosomes attached to the surface of the membrane, which distinguishes it from smooth ER [1]

Describe how the Golgi apparatus processes a protein before it leaves the cell (3 marks)

The protein arrives at the Golgi apparatus (from the RER) [1]; the Golgi apparatus modifies the protein, e.g. by adding carbohydrate chains [1]; the modified protein is packaged into a (Golgi) vesicle, which can move to and fuse with the cell-surface membrane to release its contents [1]

Explain why lysosomes must be membrane-bound (2 marks)

Lysosomes contain hydrolytic (digestive) enzymes [1]; the membrane keeps these enzymes separated from the rest of the cell's contents, preventing them from digesting/damaging the cell's own organelles and molecules [1]

Describe the structure of a mitochondrion and explain how its structure relates to its function (4 marks)

Surrounded by a double membrane [1]; the inner membrane is folded to form cristae [1]; this increases the surface area for attachment of enzymes/proteins involved in respiration [1]; the matrix (enclosed by the inner membrane) contains enzymes needed for the reactions of aerobic respiration [1]

State the type of cell wall found in fungi and in plants, and explain why both organisms need a cell wall (3 marks)

Fungi = chitin; plants = cellulose [1]; the cell wall provides structural support/strength [1]; and prevents the cell bursting from the osmotic uptake of water (resists turgor pressure) [1]

Chloroplast - structure

Surrounded by a double membrane; contains a system of membranes called thylakoids, which are stacked into structures called grana; grana are linked by lamellae; also contains the stroma (fluid-filled matrix)

Chloroplast - function

The site of photosynthesis; thylakoids/grana are the site of the light-dependent reaction (contain chlorophyll); the stroma is the site of the light-independent reaction

Chloroplast - found in

Plant cells and algal cells (not animal or fungal cells)

Cell vacuole (plant) - structure

A membrane-bound sac (the surrounding membrane is called the tonoplast) filled with cell sap

Cell vacuole (plant) - function

Stores cell sap (water, sugars, ions and waste products) and helps maintain the pressure inside the cell (turgor pressure), keeping the cell rigid/supporting the plant

Why a plant cell's vacuole is important for keeping the plant upright*

The vacuole fills with water, generating turgor pressure that pushes the cytoplasm against the cell wall, keeping cells (and therefore the plant) rigid and supported

Describe the structure and function of a chloroplast (4 marks)

Surrounded by a double membrane [1]; contains thylakoids stacked into grana, which are the site of the light-dependent reaction [1]; contains the stroma, which is the site of the light-independent reaction [1]; overall function is to carry out photosynthesis [1]

Explain why chloroplasts, but not mitochondria, are found only in certain types of eukaryotic cell (2 marks)

Chloroplasts are the site of photosynthesis [1]; only plant and algal cells photosynthesise, so only they contain chloroplasts, whereas mitochondria (needed for aerobic respiration) are required by almost all eukaryotic cells [1]

Explain how the cell vacuole helps to support a plant (3 marks)

The vacuole contains cell sap, largely water [1]; water enters the vacuole (e.g. by osmosis), generating turgor pressure [1]; this pressure pushes the cytoplasm against the cell wall, keeping the cell turgid/rigid and supporting the plant [1]

Cytoplasm - what it's made of

A jelly-like/aqueous substance containing dissolved nutrients, ions and enzymes, in which the organelles are suspended

Where cytoplasm is found in the cell

Fills the cell between the plasma membrane and the nucleus, surrounding all the other organelles

Starch granule - function

Stores starch as an energy reserve in plant cells (often found within chloroplasts)

Organelles visible with a LIGHT microscope (list)

Cell wall, plasma membrane (no detail), nucleus, chloroplast, permanent vacuole

Organelles NOT visible with a light microscope, requiring an electron microscope (list)

Ribosomes, rough ER, smooth ER, Golgi apparatus, mitochondria (no internal detail)

Why ribosomes, ER and Golgi apparatus can't be seen with a light microscope*

They are too small — below the resolution limit of a light microscope (~200 nm); an electron microscope, with much higher resolution, is needed to see them

Features that identify an electron micrograph as showing a PLANT cell (3 features)*

Presence of a cell wall, chloroplasts, and a large (permanent) vacuole — none of these are found in animal cells

Describe two organelles in a plant cell associated with storage (2 marks)

The (permanent) vacuole stores cell sap — water, sugars, ions and waste products [1]; starch granules store starch as an energy reserve, often within chloroplasts [1]

[Analogy] The cell as a factory

Nucleus = the office/control room (holds the instructions, decides what's made). Ribosomes = the workers on the factory floor (build the product — proteins). Rough ER = the assembly line right next to the workers (folds/finishes what they just built). Golgi apparatus = the packaging and shipping department (wraps products, labels them, sends them out). Mitochondria = the power plant (generates the energy/ATP the whole factory runs on). Lysosomes = the waste disposal/recycling unit. (Intuition aid only — do not use in exam answers)

[Analogy] Why the nucleus controls the cell

Like a head office that keeps the master blueprint (DNA) locked away safely, and only sends out copies of individual instructions (mRNA) through the nuclear pores when needed, rather than letting the original blueprint leave the building. (Intuition aid only)