Cells and organisation

The nucleus

提示It is the headquarters that gives the orders.

为什么The nucleus stores the DNA and uses it to direct everything the cell does, like a control centre issuing instructions.

The mitochondria

提示These tiny powerhouses supply the whole cell with usable energy.

为什么Mitochondria carry out aerobic respiration, transferring energy from glucose so the cell can use it. Busy cells such as muscle cells have many of them.

The cell membrane

提示It is the gatekeeper at the edge, letting some things through.

为什么The membrane is a thin barrier surrounding the cell that lets useful substances in and waste out, controlling the cell's exchange with its surroundings.

The cell wall

提示Made of tough fibres, it stops the plant flopping over.

为什么The cell wall is made of cellulose and lies outside the membrane. It gives the cell a firm shape and helps hold the whole plant upright.

The chloroplast

提示Its green pigment traps sunlight so the plant can make food.

为什么Chloroplasts contain the green pigment chlorophyll, which soaks up light energy for photosynthesis. That is why they sit in leaf cells, not roots.

vacuole

提示This fluid-filled sac swells to press outward and keep the plant rigid.

为什么The vacuole stores cell sap and, when full, presses out against the wall to keep the cell turgid — which is why a well-watered plant stands up straight.

An organ

提示The same level as your heart or a kidney: several tissue types combined for one job.

为什么The ladder runs cell → tissue → organ → organ system. A tissue is many cells of one type doing one job; the next rung combines several tissues for a bigger job. Plants climb the same ladder — leaf, root, stem and flower — with transport and reproductive systems above them.

Image size ÷ real size

提示Compare how big it looks with how big it truly is.

为什么Magnification tells you how many times bigger the image is than the object. Rearranged, real size = image size ÷ magnification.

To make more room to carry oxygen

提示Losing its control centre frees up space for its cargo.

为什么Without a nucleus taking up space, a red blood cell can pack in more haemoglobin, so each cell carries as much oxygen as possible.

Its long hair-like extension

提示It juts out as a thin finger reaching into the soil for moisture.

为什么The long, thin projection gives the cell a much larger surface area, so it can absorb more water from the soil by osmosis.

From a high concentration to a low concentration

提示They drift towards wherever there is more room, leaving the crowded region behind.

为什么Diffusion moves particles down a concentration gradient until they are evenly spread. Cells use it to take in oxygen and get rid of carbon dioxide.

Water

提示It is the substance a thirsty root is trying to soak up.

为什么Osmosis is the movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one. Only water moves, not the dissolved solute.